JP4563468B2 - Optical pickup - Google Patents

Description

  The present invention relates to an optical pickup for recording information on a recording medium or reproducing information from a recording medium, and in particular, the thickness of a light transmission layer from a light incident surface to an information recording surface, and the optimum light beam to be used. The present invention relates to an optical pickup suitable for recording or reproducing information on recording media having different wavelengths.

  2. Description of the Related Art Conventionally, there is an optical disc player (optical recording / reproducing device) that can read recorded information from an optical disc (optical recording medium) such as a DVD (digital video disc) or a CD (compact disc). Recently, a DVD with a capacity of 4.7 GB has been introduced into the market, but there is a strong demand for a higher-density optical disc, and studies for realizing it are in progress. To improve the recording density, as is well known, it is effective to shorten the wavelength of light used for reading and to increase the NA of the objective lens.

  At present, in an optical pickup for reproducing a next-generation high-density optical disk, the numerical aperture (NA) of an objective lens is increased from 0.6 to 0.85 in a conventional DVD, and the wavelength of light to be used is increased. By shortening the wavelength from 650 nm to 405 nm in the conventional DVD, the narrowed spot is miniaturized and the recording density is increased.

  By the way, when the NA of the objective lens is increased, the coma aberration generated when the optical disk is tilted increases rapidly, so that there is a problem that the condensing characteristic of the aperture spot is likely to deteriorate. In this specification, the coma aberration refers to an aberration that occurs when focusing on a point other than on the optical axis. Since the coma generated by the tilt of the optical disk is proportional to the thickness of the light transmission layer from the light incident surface to the information recording surface of the optical disk, the NA of the objective lens is increased by making the light transmission layer of the optical disk thinner. Accordingly, it is possible to suppress an increase in coma aberration. Based on this concept, it has been proposed to reduce the thickness of the light transmission layer from 0.6 mm to 0.1 mm in the conventional DVD in the next generation high density optical disc.

  Here, next-generation high-density optical discs are required to ensure compatibility with conventional DVDs and CDs that are currently widely used. That is, an optical disc player for reproducing the next generation high density optical disc needs to be able to reproduce the current DVD and CD.

  However, if the wavelength of light to be used and the thickness of the light transmission layer of the optical disk are different, there is a problem that it is difficult to ensure compatibility of the optical disk. Usually, the objective lens is designed assuming the light transmission layer thickness of a specific optical disk and the wavelength of light to be used. This is because spherical aberration occurs in the aperture spot and the light condensing characteristic of the aperture spot deteriorates. In this specification, the spherical aberration refers to a difference between the focal position of a paraxial ray that is a ray close to the center of the ray and the focal position of a peripheral ray that is a ray away from the ray center.

  For this reason, there has been proposed an optical pickup that includes a plurality of laser light sources according to the wavelength used, and converges the laser light with the necessary numerical aperture on the information recording surface with the same objective lens (for example, Patent Documents 1 to 3 and Non-patent document 1).

  In Patent Document 1, an optical disk having a working wavelength of 400 nm and a light transmitting layer of 0.6 mm, an optical disk having a working wavelength of 650 nm and a light transmitting layer of 0.6 mm, a working wavelength of 780 nm and a light transmitting layer of 1.2 mm. A technique for recording / reproducing an optical disk of the above type using an optical system in which a diffractive surface is provided on the curved surface of an objective lens is disclosed. At this time, the objective lens provided with the diffractive surface is designed to use the first-order diffracted light for each wavelength.

  Patent Document 2 discloses an optical pickup device including a first light source, a second light source, a condensing optical system, and a correction optical system. In this optical pickup device, the first light source emits a light beam having a wavelength of 650 nm. The second light source emits a light beam having a wavelength of 780 nm. This condensing optical system is configured so that the light beam from the first light source can be focused on the information recording surface of the DVD with reduced spherical aberration. The correction optical system is disposed between the second light source and the condensing optical system. This correction optical system reduces spherical aberration that occurs when the light beam from the second light source is condensed on the information recording surface of the CD by the condensing optical system.

    Patent Document 3 discloses, for example, Patent Document 1 a technique relating to an optical pickup for reproducing an optical disc having different wavelengths of light to be used and having the same thickness of a light transmission layer. In this technology, two light sources that emit light having wavelengths of 405 nm and 650 nm (blue light and red light, respectively), and diffraction that can collect blue light on an optical disc having a light transmission layer thickness of 0.6 mm. An optical element and an objective lens are provided. In addition, light of any wavelength is configured to be incident on the diffractive optical element as parallel light, and second-order diffracted light by the diffractive optical element is used for blue light, and first-order diffracted light is used for red light. Therefore, sufficient diffraction efficiency can be obtained for light of two different wavelengths, and spherical aberration generated for red light can be corrected.

Non-Patent Document 1 discloses that an optical disk having a use wavelength of 405 nm and a light transmission layer of 0.1 mm, an optical disk having a use wavelength of 655 nm and a light transmission layer of 0.6 mm, a use wavelength of 785 nm and a light transmission layer of 1. A technique for recording and reproducing a 2 mm optical disk by an optical system provided with an objective lens and a hologram (diffraction element) that functions as a concave lens only for a light beam having a wavelength of 785 nm is disclosed.
JP 2002-197717 A (publication date: July 12, 2002) JP 2000-306261 A (publication date November 2, 2002) JP 2001-93179 A (publication date April 6, 2001) Naoki Kaibo, 6 others, "DVD / CD compatible technology on Blu-ray Disc", No. 63 Proceedings of the 63rd Joint Lecture on Applied Physics in Autumn, No. 3, 1008 Lecture number (27p-YD-5)

  The technique disclosed in Patent Document 1 described above is an optical disc having light transmission layers with different thicknesses, that is, a next-generation high-density optical disc (use wavelength 400 nm, light transmission layer 0.1 mm) and DVD (use wavelength 650 nm, The following are problems when adapted to a light transmission layer (0.6 mm) and CD (use wavelength: 780 nm, light transmission layer: 1.2 mm).

  In general, in an optical pickup (compatible optical pickup) that can handle optical disks having different recording densities, an objective lens in which aberration is corrected is used for an optical disk having a larger recording capacity. Therefore, in the compatible optical pickup that can handle the next-generation high-density optical disc, the conventional DVD, and the conventional CD, the aberration of the objective lens is corrected with respect to the next-generation high-density optical disc. If this objective lens is used as it is for a conventional DVD or CD having a light transmission layer thickness different from that of the next-generation high-density optical disc, spherical aberration increases and recording / reproduction cannot be performed.

  In order to solve this problem, when recording and reproducing a conventional DVD, divergent light is incident on the objective lens, and an aberration opposite to the spherical aberration that occurs due to the thick light transmission layer is generated. It is possible to correct.

  In other words, in order to record / reproduce optical disks having light transmission layers having different thicknesses, the convergence / divergence degree of light of the corresponding wavelength is changed and incident on the objective lens.

  When blue light (light having a wavelength of 400 nm) is incident on parallel light and focused on a next-generation optical disk having a light-transmitting layer thickness of 0.1 mm with respect to an objective lens having an effective diameter of 3 mm, the thickness of the light-transmitting layer In order to correct the spherical aberration in the DVD generated due to the large value, the divergence degree of the red light (light having a wavelength of 650 nm) needs to be about −0.03, and the thickness of the light transmission layer is large. In order to correct the spherical aberration in the CD, it is necessary to increase the degree of divergence of infrared light (light having a wavelength of 780 nm) to about −0.07. Here, the degree of convergence / divergence is the reciprocal of the focal length. When the value is negative, the divergent light is indicated. When the value is positive, the convergent light is indicated.

  At this time, with respect to red light and infrared light, the degree of divergence of the light incident on the objective lens is large, so the radial direction of the objective lens by tracking or the like (direction substantially orthogonal to the optical axis of the light beam incident on the objective lens) As a result of this shift, coma aberration added to the aperture spot on the optical disk increases, resulting in a problem that the condensing characteristic is greatly deteriorated. In particular, in the case of a CD, since the degree of divergence of light incident on the objective lens is larger, the deterioration of the condensing characteristic due to the shift of the objective lens in the radial direction becomes large.

  Further, in the objective lens provided with a diffractive surface disclosed in Patent Document 1, when the diffraction efficiency of the first-order light at one wavelength is approximately 100%, the first-order diffracted light at other wavelengths is reduced. There is a limit to the diffraction efficiency, and a desired high diffraction efficiency cannot be obtained. For this reason, a loss of the light amount occurs, causing a problem that the light use efficiency is deteriorated. When a loss of light intensity occurs, a higher power laser is required, especially when recording information. Further, unnecessary light other than the used diffraction order may enter the detector as stray light during reproduction, which may cause signal degradation.

  When the technique disclosed in Patent Document 2 is applied to an optical pickup device for reading information from a next-generation high-density optical disc and DVD, an objective lens having a high numerical aperture is provided. Since this objective lens is made of a glass material having a high refractive index, it has a high wavelength dependency. Since the objective lens is highly wavelength-dependent in this way, there arises a problem that the focal position is greatly deviated with respect to wavelength fluctuation that the actuator cannot follow, such as wavelength fluctuation due to mode hop phenomenon and wavelength fluctuation due to high frequency superposition. .

  The technique disclosed in Patent Document 3 is applied to an optical disc having light transmission layers having different thicknesses, that is, a next-generation high-density optical disc (light transmission layer thickness 0.1 mm), and a conventional DVD (light transmission layer thickness). Is necessary to correct the spherical aberration that occurs due to the large difference in the thickness of the light transmission layer when parallel light is incident on the diffractive optical element. It is necessary to increase the angle difference between the diffraction angle of blue light and the diffraction angle of red light to about 2 to 3 °. Here, the relationship between the pitch of the diffraction grating and the angle difference in the diffractive optical element is as shown in the graph of FIG. From FIG. 35, it is understood that the pitch of the diffraction grating needs to be a fine width of 3.5 to 4.5 μm in order to make the angle difference about 2 to 3 °.

  In general, since the objective lens (infinite objective lens) is optimized for blue light coming from an infinite distance, the light emitted from the diffractive optical element needs to be converted into parallel light. Therefore, in the diffractive optical element, it is necessary to convert the blue light bent at the diffractive surface into parallel light at the refracting surface (the surface on the objective lens side of the diffractive optical element). By doing so, it is also possible to prevent the occurrence of aberration due to the positional deviation between the diffractive optical element and the objective lens.

  FIG. 36 shows the relationship between the pitch of the diffraction grating and the curvature of the refracting surface of the diffractive optical element when blue light incident on the diffractive optical element as parallel light becomes parallel light after passing through the diffractive optical element. Show. The relationship shown in FIG. 36 is for a diffractive optical element in an optical pickup that uses an objective lens having an effective radius of 2 mm. The refractive surface of the diffractive optical element is a spherical surface. FIG. 36 shows that the radius of curvature of the refractive surface of the diffractive optical element needs to be 2.2 mm or less in order to set the pitch of the diffraction grating to 3.5 to 4.5 μm.

  However, since the effective radius of the objective lens is 2 mm, the effective diameter of the diffractive optical element is also 2 mm. Therefore, the refractive surface with a radius of curvature of 2.2 mm or less is almost hemispherical and cannot be manufactured or practically used. Is impossible. Further, even if the refracting surface is aspherical, the curvature is extremely small, making it difficult to manufacture, and even if it can be manufactured, the axial condensing characteristic is 0.018λ ( rms).

  By the way, generally, the diffraction efficiency of the utilized wavelength of the hologram (diffraction element) is determined by the depth of the diffraction grating. FIG. 2 is a graph showing the relationship between the depth of the diffraction grating and the diffraction efficiency for each diffraction order of each used wavelength when a diffraction grating is formed on a PC (polycarbonate) substrate. In FIG. 2, B0, B1, B2, R0, R1, Ir0, and Ir1 are the zero-order diffracted light, the first-order diffracted light, the second-order diffracted light, It shows the diffraction efficiencies of the 0th-order diffracted light and the 1st-order diffracted light of the light beam having the used wavelength (650 nm) and the 0th-order diffracted light and the 1st-order diffracted light of the light beam having the used wavelength (780 nm) of CD.

  When the technique disclosed in Non-Patent Document 1 is used for the optical pickup, as can be seen from the graph shown in FIG. 2 used in the embodiment of the present invention, the first-order diffracted light at a wavelength of 780 nm If the depth of the diffraction grating is set so as to be higher than the other orders of diffracted light, the diffraction efficiency of diffracted light of a predetermined diffraction order at other wavelengths (0th order diffracted light at wavelengths of 405 nm and 650 nm) is approximately 10 Conversely, when the depth of the diffraction grating is set so that the 0th-order diffracted light at the wavelength of 405 nm or 655 nm is higher than the diffracted light of the other orders at the respective wavelengths, the wavelength at the wavelength of 785 nm The diffraction efficiency of the first-order diffracted light is approximately 10% or less. For this reason, it is practically impossible to set the depth of the diffraction grating so that the light use efficiency is high for light of any wavelength.

  In particular, for a light beam with a wavelength of 405 nm, which makes it difficult to produce a higher-power laser, the diffraction efficiency, that is, the light utilization efficiency is increased, for example, the diffraction efficiency is 80% or more. When the depth of the diffraction grating is set, the utilization efficiency of the first-order diffracted light having a wavelength of 780 nm is 5% or less. Therefore, when the optical pickup is used, there arises a problem that a sufficient amount of light cannot be obtained when recording or reproducing information on a CD.

  The present invention has been made in view of the above problems, and its purpose is to record or reproduce information on recording media having different thicknesses of light transmission layers and different light beam wavelengths suitable for reproduction. Provided is an optical pickup capable of suppressing the focal position degradation caused by wavelength fluctuations, and capable of suppressing the deterioration of the light collection characteristics due to the radial shift of the objective lens. It is. In particular, first, second, and third light beams having different wavelengths are used to provide first, second, and third light transmission layers having thicknesses t1, t2, and t3 (t1 <t2 <t3) on the information recording surface, respectively. An object of the present invention is to provide an optical pickup capable of suppressing deterioration of light collecting characteristics due to radial movement of an objective lens unit (objective shift) when recording or reproducing the second and third recording media.

The optical pickup of the present invention includes the information recording surfaces of the first, second, and third recording media each having a light transmission layer having thicknesses t1, t2, and t3 (t1 <t2 <t3) on the information recording surface. In contrast, an optical pickup capable of recording or reproducing information by condensing first, second, and third light beams having wavelengths λ1, λ2, and λ3 (λ1 <λ2 <λ3), respectively, In order to solve the problem, the first, second, and third light beams can be moved in a direction substantially orthogonal to the optical axes of the incident first, second, and third light beams. An objective lens that focuses light on each information recording surface of the second and third recording media, and is integrated with the objective lens on the incident side of the first, second, and third light beams with respect to the objective lens. And diffracting the first, second and third light beams, and A diffractive optical element that refracts and makes the first, second, and third light beams of a predetermined diffraction order incident on the objective lens, and the diffractive optical element has the second and third light beams with respect to the objective lens. Are incident on the diffractive optical element as Φinr and ΦinIr, and the divergence degrees incident on the objective lens are Φoutr and ΦoutIr, respectively, for the second and third light beams. ,
| Φinr | <| Φoutr | and | ΦinIr | <| ΦoutIr |
It is characterized by satisfying.

  According to the above configuration, in the optical pickup that records or reproduces the first, second, and third recording media using the first, second, and third light beams having different wavelengths, respectively, the shortest wavelength is obtained. When the first light beam is focused on the first recording medium, an objective lens with corrected aberration is used.

  On the other hand, if the objective lens is used as it is to focus the second and third light beams on the second and third recording media having different light transmission layer thicknesses from the first recording medium, And the spherical aberration of the third light beam increases. In order to suppress the increase in spherical aberration, the second and third light beams may be incident on the objective lens as divergent light so as to generate aberrations in the opposite directions for correction.

  Here, in order to sufficiently obtain the effect of suppressing the increase in spherical aberration, it is necessary to increase the degree of divergence of the second and third light beams incident on the objective lens. However, when the degree of divergence of the light beam incident on the objective lens is increased, the radial direction of the objective lens by tracking or the like (direction substantially orthogonal to the optical axes of the first, second, and third light beams incident on the objective lens). Due to the movement, coma added to the aperture spot on the recording medium is increased, which causes a problem that the condensing characteristic is greatly deteriorated.

Therefore, in the above configuration, a diffractive optical element provided so as to be movable integrally with the objective lens is used when the second and third light beams are incident on the objective lens as divergent light. And this diffractive optical element is
| Φinr | <| Φoutr | and | ΦinIr | <| ΦoutIr |
Act to satisfy. That is, the absolute value of the degree of convergence / divergence is made larger in the second and third light beams emitted from the diffractive optical element than in the second and third light beams incident on the diffractive optical element.

  Thus, the absolute value of the degree of convergence / divergence of the second and third light beams incident on the unit (objective lens unit) made up of the objective lens and the diffractive optical element while sufficiently obtaining the effect of suppressing the increase in spherical aberration. Can be made small, that is, close to parallel light. As a result, in the above-described configuration, it is possible to suppress the deterioration of the condensing characteristic due to the radial movement (objective shift) of the objective lens unit as compared with the case where no diffractive optical element is used.

  As described above, in the above-described configuration, a single focused lens is used to form a good condensing spot on a recording medium having a different light transmission layer thickness, and information can be recorded or reproduced. Even if the unit moves in the radial direction, it is possible to prevent the light condensing characteristics from greatly deteriorating.

  In the optical pickup according to the aspect of the invention described above, it is preferable that the diffractive optical element causes the first light beam to enter the objective lens as parallel light.

  According to the above configuration, the first light beam having a short wavelength that requires the most stringent condensing characteristics is incident as parallel light on the objective lens, whereby the diffractive optical element and the objective lens in the use of the first light beam It is possible to suppress the occurrence of aberration due to the positional deviation.

  In the optical pickup of the present invention having the above-described configuration, the diffractive optical element supplies the second-order diffracted light of the first light beam, the first-order diffracted light of the second light beam, and the first-order diffracted light of the third light beam to the objective lens, respectively. In the diffractive optical element, the diffraction efficiency of the second-order diffracted light is the highest for the first light beam, the diffraction efficiency of the first-order diffracted light is the highest for the second light beam, and the first-order diffracted light for the third light beam. The diffraction efficiency is preferably the highest.

  According to the above configuration, the diffraction efficiency of all of the first, second, and third light beams can be improved. Thereby, since the output of the light source of each light beam can be made small, the power consumption in a light source can be suppressed. The above configuration is particularly effective for an optical pickup that records and erases information that requires a high-power beam. In particular, in the diffractive optical element, the diffraction efficiency of the second-order diffracted light is preferably 90% or more for the first light beam.

  In the optical pickup of the present invention, in the above configuration, it is preferable that the third light beam is incident on the diffractive optical element as divergent light.

  According to the above configuration, in order to suppress the increase in spherical aberration, the third light beam that needs to have the greatest degree of divergence is incident on the diffractive optical element by being incident on the diffractive optical element as divergent light. An increase in the degree of divergence of the first and second light beams can be prevented.

  In the optical pickup of the present invention, in the above configuration, it is preferable that the first and second light beams are incident on the diffractive optical element as parallel light and divergent light, respectively.

  According to the above configuration, the divergence degree of the second and third light beams incident on the diffractive optical element can be made relatively small while the first light beam is incident on the diffractive optical element as parallel light. (See FIG. 20). As a result, it is possible to more effectively suppress the deterioration of the light collection characteristics due to the movement of the objective lens unit in the radial direction.

  In the optical pickup of the present invention, in the above configuration, it is preferable that the first light beam is incident on the diffractive optical element as convergent light, and the second light beam is incident as convergent light, parallel light, or divergent light.

  According to the above configuration, the absolute values of the degree of convergence / divergence of the first, second, and third light beams incident on the diffractive optical element can be made relatively small (see FIG. 20). As a result, it is possible to more effectively suppress the deterioration of the light collection characteristics due to the movement of the objective lens unit in the radial direction.

For example, when the effective diameter of the objective lens with respect to the first light beam is φ, the convergence / divergence degree of the first, second, and third light beams incident on the diffractive optical element is
0 ≦ φ × Φinb ≦ 0.11,
−0.048 ≦ φ × Φinr ≦ 0.04, and
−0.18 ≦ φ × ΦinIr ≦ −0.1
It is preferable to satisfy.

In other words, if the power of the diffractive optical element with respect to the first, second and third light beams is Φb, Φr and ΦIr, respectively,
−0.11 ≦ φ × Φb ≦ 0,
−0.2 ≦ φ × Φr ≦ −0.002, and
−0.16 ≦ φ × ΦIr ≦ 0.03
It is preferable to satisfy.

  In the optical pickup of the present invention, the diffractive optical element preferably includes a converging diffractive surface and a concave refracting surface.

  According to the above configuration, the usable wavelength range is wider than that of the optical pickup dedicated to the first recording medium, and the wavelength dependence characteristics can be improved as compared with the case where the objective lens dedicated to the first recording medium is used alone. Therefore, in the above configuration, even when wavelength variation occurs due to mode hopping or the like, it is possible to maintain good light collection characteristics. In addition, the minimum pitch of the diffractive surface of the diffractive optical element can be increased, and the creation of the diffractive optical element can be facilitated.

  In the optical pickup of the present invention having the above-described configuration, it is preferable that the refractive surface of the diffractive optical element is a spherical surface.

  According to the above configuration, it becomes easy to create a diffractive optical element, and an inexpensive optical pickup can be provided.

  In the optical pickup of the present invention, in the above configuration, the power of the refractive surface of the diffractive optical element with respect to the first light beam is preferably −0.1 or more.

  According to the above configuration, it is possible to more effectively suppress the deterioration of the light collection characteristics due to the movement of the objective lens unit in the radial direction.

As described above, the optical pickup of the present invention can move in a direction substantially perpendicular to the optical axes of the incident first, second, and third light beams, and the first, second, and third light beams can be moved. An objective lens that condenses the information recording surfaces of the first, second, and third recording media, respectively, and is integrated with the objective lens on the incident side of the first, second, and third light beams with respect to the objective lens. The first, second, and third light beams are diffracted and refracted so that the first, second, and third light beams having a predetermined diffraction order are incident on the objective lens. A diffractive optical element, and the diffractive optical element causes the second and third light beams to be incident on the objective lens as divergent light, and the second and third light beams are incident on the diffractive optical element to converge / diverge. Degrees are Φinr and ΦinIr respectively, and enter the objective lens. When divergence respectively and Φoutr and ΦoutIr that,
| Φinr | <| Φoutr | and | ΦinIr | <| ΦoutIr |
It is the composition which satisfies.

  In the above configuration, the diffractive optical element provided so as to be movable integrally with the objective lens is used when the second and third light beams are incident on the objective lens as divergent light. In this diffractive optical element, the absolute value of the degree of convergence / divergence is larger in the second and third light beams emitted from the diffractive optical element than in the second and third light beams incident on the diffractive optical element. To be.

  Thus, the absolute value of the degree of convergence / divergence of the second and third light beams incident on the unit (objective lens unit) made up of the objective lens and the diffractive optical element while sufficiently obtaining the effect of suppressing the increase in spherical aberration. Can be made small, that is, close to parallel light. As a result, in the above-described configuration, it is possible to suppress the deterioration of the condensing characteristic due to the radial movement (objective shift) of the objective lens unit as compared with the case where no diffractive optical element is used.

  As described above, in the above-described configuration, a single focused lens is used to form a good condensing spot on a recording medium having a different light transmission layer thickness, and information can be recorded or reproduced. Even if the unit moves in the radial direction, it is possible to prevent the light collecting characteristic from being greatly deteriorated.

  In the optical pickup of the present invention having the above configuration, it is preferable that the diffractive optical element causes the first light beam to enter the objective lens as parallel light.

  In the above-described configuration, the first light beam having a short wavelength that requires the most stringent light condensing characteristics is incident on the objective lens as parallel light, so that the position of the diffractive optical element and the objective lens in the use of the first light beam. Occurrence of aberration due to deviation can be suppressed.

  In the optical pickup of the present invention having the above configuration, the diffractive optical element causes the second-order diffracted light of the first light beam, the first-order diffracted light of the second light beam, and the first-order diffracted light of the third light beam to enter the objective lens, respectively. In the diffractive optical element, the first light beam has the highest diffraction efficiency of the second-order diffracted light, the second light beam has the highest diffraction efficiency of the first-order diffracted light, and the third light beam has the highest diffraction efficiency of the first-order diffracted light. Is preferably the highest.

  With the above configuration, the diffraction efficiency of all of the first, second, and third light beams can be improved. Thereby, since the output of the light source of each light beam can be made small, the power consumption in a light source can be suppressed. The above configuration is particularly effective for an optical pickup that records and erases information that requires a high-power beam. In particular, in the diffractive optical element, it is preferable that the diffraction efficiency of the second-order diffracted light is 90% or more for the first light beam.

  In the optical pickup of the present invention, in the above configuration, it is preferable that the third light beam is incident on the diffractive optical element as divergent light.

  In the above configuration, in order to suppress the increase in spherical aberration, the first light beam that needs to have the greatest degree of divergence is incident on the diffractive optical element by being incident on the diffractive optical element by diverging light. In addition, it is possible to prevent the degree of divergence of the second light beam from increasing.

  In the optical pickup of the present invention, in the above configuration, it is preferable that the first and second light beams are incident on the diffractive optical element as parallel light and divergent light, respectively.

  With the above-described configuration, the first light beam is incident on the diffractive optical element as parallel light, and the divergence degree of the second and third light beams incident on the diffractive optical element can be made relatively small. As a result, it is possible to more effectively suppress the deterioration of the light collection characteristics due to the movement of the objective lens unit in the radial direction.

  In the optical pickup of the present invention, in the above configuration, it is preferable that the first light beam is incident on the diffractive optical element as convergent light, and the second light beam is incident as convergent light, parallel light, or divergent light.

  With the above configuration, the absolute value of the degree of convergence / divergence of the first, second, and third light beams incident on the diffractive optical element can be made relatively small. As a result, it is possible to more effectively suppress the deterioration of the light collection characteristics due to the movement of the objective lens unit in the radial direction.

For example, when the effective diameter of the objective lens with respect to the first light beam is φ, the convergence / divergence degree of the first, second, and third light beams incident on the diffractive optical element is
0 ≦ φ × Φinb ≦ 0.11,
−0.048 ≦ φ × Φinr ≦ 0.04, and
−0.18 ≦ φ × ΦinIr ≦ −0.1
It is preferable to satisfy.

In other words, if the power of the diffractive optical element with respect to the first, second and third light beams is Φb, Φr and ΦIr, respectively,
−0.11 ≦ φ × Φb ≦ 0,
−0.2 ≦ φ × Φr ≦ −0.002, and
−0.16 ≦ φ × ΦIr ≦ 0.03
It is preferable to satisfy.

  In the optical pickup of the present invention, in the above configuration, the diffractive optical element preferably includes a converging diffractive surface and a concave refracting surface.

  In the above configuration, the usable wavelength range is wider than that of the optical pickup dedicated to the first recording medium, and the wavelength dependence characteristics can be improved as compared with the case where the objective lens dedicated to the first recording medium is used alone. Therefore, in the above configuration, even when wavelength variation occurs due to mode hopping or the like, it is possible to maintain good light collection characteristics. In addition, the minimum pitch of the diffractive surface of the diffractive optical element can be increased, and the creation of the diffractive optical element can be facilitated.

  In the optical pickup of the present invention having the above-described configuration, the refracting surface of the diffractive optical element is preferably a spherical surface.

  With the above configuration, it is easy to create a diffractive optical element, and an inexpensive optical pickup can be provided.

  In the optical pickup of the present invention, in the above configuration, the power of the refractive surface of the diffractive optical element with respect to the first light beam is preferably −0.1 or more.

  With the above configuration, it is possible to more effectively suppress the deterioration of the light collection characteristic due to the movement of the objective lens unit in the radial direction.

  In the optical pickup of the present invention having the above-described configuration, the diffractive surface of the diffractive optical element is preferably formed on a refracting surface.

  In the above configuration, it is not necessary to align the refractive surface of the diffractive optical element with the diffractive surface, and the creation of the diffractive optical element can be facilitated.

  In the optical pickup according to the present invention, in the above-described configuration, it is preferable that a diffractive surface of the diffractive optical element is formed with a sawtooth or stepped diffraction grating.

  With the above configuration, the diffraction efficiency of each light beam in the diffractive optical element can be improved. Thereby, since the output of the light source of each light beam can be made small, the power consumption in a light source can be suppressed. The above configuration is particularly effective for an optical pickup that records and erases information that requires a high-power beam.

  Each embodiment of the present invention will be described below.

Embodiment 1
FIG. 1 shows a schematic configuration of an optical pickup 100 according to the present embodiment. In the present embodiment, an optical pickup 100 that is compatible with a next-generation high-density optical disc 14a (first optical disc, first recording medium) and a conventional DVD 14b (second optical disc, second recording medium) will be described.

  The first optical disc has a light (first light beam L1) having a short wavelength of blue light having a wavelength (first wavelength λ1) of about 405 nm and a light transmission layer thickness t1 of 0.1 mm. In the second optical disk, the light (second light beam L2) to be used is a long wavelength red light having a wavelength (second wavelength λ2) near 650 nm, and the thickness t2 of the light transmission layer is 0.6 mm.

  The optical pickup 100 also includes a semiconductor laser 1a that emits a first light beam L1 having a first wavelength λ1, and a semiconductor laser 1b that emits a second light beam L2 having a second wavelength λ2 that is longer than the first wavelength λ1. It has. The semiconductor laser 1a and the semiconductor laser 1b are switched on and turned on according to the target optical disk.

  The optical pickup 100 further includes collimator lenses 2a and 2b that make the first and second light beams L2 emitted from the semiconductor lasers 1a and 1b substantially parallel to each other, and first and second light beams having an elliptical intensity distribution. Shaping optical systems 3a and 3b such as shaping prisms for shaping L2 into a substantially circular intensity distribution, and beam splitters 4a and 4b for transmitting the first and second light beams L2 from the shaping optical systems 3a and 3b, respectively. ing.

  The shaping optical systems 3a and 3b are constituted by a known optical system such as one triangular prism, a bonded triangular prism, or two triangular prisms arranged independently. Note that the optical pickup 1 may not include the shaping optical systems 3a and 3b.

  The first optical system 16a is configured by the semiconductor laser 1a, the collimator lens 2a, the shaping optical system 3a, and the beam splitter 4a. The second optical system 16b is formed by the semiconductor laser 1b, the collimator lens 2b, the shaping optical system 3b, and the beam splitter 4b. Is configured.

  The first and second light beams L2 emitted from the first and second optical systems 16a and 16b are aligned with each other by the dichroic prism 5, and thereafter go through the common optical system.

  In the common optical system, the first and second light beams L2 pass through the spherical aberration compensation system 6 and the quarter wavelength plate 8, are reflected by the mirror 9, and then enter the objective lens unit 13.

  The first and second light beams L2 entering the objective lens unit 13 sequentially pass through the wavelength-selective aperture filter 10, the diffractive optical element 11, and the objective lens 12, and light on the information recording surface of the first optical disc 14a. A minute light spot is formed on the spot or on the information recording surface of the second optical disk 14b.

  Here, the spherical aberration compensation system 6 corrects spherical aberration caused by uneven thickness of the light transmission layer in the first and second optical discs 14a and 14b, and is composed of, for example, a beam expander or a liquid crystal correction element. ing.

  If the optical pickup 1 has a configuration that does not include the shaping optical systems 3a and 3b, the spherical aberration compensation system 6 may not be used. The wavelength selective aperture filter 10 works so that the numerical aperture is NA1 (specifically 0.85) for the light beam having the first wavelength λ1, and the wavelength is the second wavelength λ2. The aperture of the beam is controlled so that the numerical aperture is NA2 (specifically, 0.6). Here, the wavelength-selective aperture filter 10 is disposed between the mirror 9 and the diffractive optical element 11, but may be anywhere as long as it can operate integrally with the diffractive optical element 11 and the objective lens 12. . Further, any filter other than the wavelength selective aperture filter 10 may be used as long as it has the same function as the above aperture control.

  The diffractive optical element 11 and the objective lens 12 are held by a holder 17 (holding body) as shown in FIG. As a result, the diffractive optical element 11 and the objective lens 12 are held by the holder, so that the relative positions of the diffractive optical element and the objective lens can be prevented from shifting. Therefore, deterioration of the condensing characteristics due to the relative displacement of the diffractive optical element and the objective lens can be prevented.

  The objective lens unit 13 includes a drive unit (drive means) (not shown), and adjusts the focal point of light applied to the optical discs 14a and 14b. That is, the drive unit is adapted to follow the surface deflection and rotational eccentricity of the optical disks 14a and 14b.

  The wavelength selective aperture filter 10, the diffractive optical element 11, and the objective lens 12 are integrated as an objective lens unit 13.

  In addition to the above light irradiation optical system, the optical pickup 100 further includes reproduction signal detection optical systems 15a and 15b. In the reproduction signal detection optical systems 15a and 15b, light spot control signals such as autofocus and track following and information signals recorded on the optical disk are reproduced by various conventionally known optical systems.

The diffractive optical element 11 is made of glass or plastic. Further, the diffraction grating of the diffractive optical element 11 is formed of a circular ring groove cut or a convex ring zone laminated by photolithography concentrically around the optical axis. This diffraction grating is formed so that a cross-sectional shape appearing on a plane including the optical axis is a blazed shape, that is, a sawtooth shape or a stepped shape. A diffraction grating with a sawtooth or stepped cross section (especially a sawtooth cross section) is advantageous because of its higher diffraction efficiency.
Further, the diffraction efficiency ηm of such a blazed diffraction grating can be obtained by the following Expression 7.

In the equation, A (x) represents the transmission amplitude distribution, φ (x) represents the phase distribution, and T represents the pitch of the grating.

  Further, FIG. 2 shows diffraction efficiency in the case of a diffractive optical element in which a diffraction grating is specifically formed on a PC (polycarbonate) substrate. In FIG. 2, B0, B1, B2, R0, R1, and R2 are the 0th order diffracted light of the first light beam L1, the 1st order diffracted light, the 2nd order diffracted light, the 0th order diffracted light of the second light beam L2, the 1st order diffracted light, It represents the diffraction efficiency of the second-order diffracted light.

  Here, the utilization efficiency of each diffraction order is determined by the depth of the diffraction grating, and a light beam sufficient for recording and reproduction can be obtained by setting an appropriate diffraction order and grating depth. Therefore, an optical pickup for recording and erasing information that requires high output can be realized, and furthermore, since laser output can be reduced, power consumption can be suppressed.

  Further, the spherical aberration generated by the thickness of the light transmission layer in the optical discs 14a and 14b is generated in the opposite direction to the spherical aberration generated when divergent light is incident on the objective lens 12. Therefore, in order to focus the light beam on both the recording layer of the next-generation high-density optical disc and the recording layer of the DVD disc using the objective lens 12, the objective lens 12 is collimated with the blue light beam. It is desirable that the red light beam is incident on the objective lens 11 as divergent light.

  Therefore, in the optical pickup 100 of the present invention, for example, the diffractive optical element 11 becomes parallel light after the blue first light beam L1 having a wavelength of 405 nm passes, and after the red second light beam L2 having a wavelength of 650 nm passes. It is preferably designed to be diverging light.

  Here, the objective lens unit 13 in the optical pickup 100 satisfying the above conditions will be described in more detail with reference to FIG. In the objective lens unit 13, the light beam that has passed through the diffractive optical element 11 in which the diffraction grating 11 a is formed on the diffraction surface of the lens portion 11 b having the refractive index n is condensed on the recording layer of the optical disk 14 by the objective lens 12. It is like that. The distance in the optical axis direction between the apex of the lens portion 11b and the diffraction surface is a.

  In FIG. 3, a broken line focused on the optical disk 14a through the diffraction grating 11a and the objective lens 12 is defined as a first light beam L1, and a thick line focused on the optical disk 14a is defined as a second light beam L2.

  Here, the outermost circumference of the diffractive optical element 11 when the light beam (second light beam L2) having a wavelength of 650 nm corresponding to NA2 (specifically 0.6) of the numerical aperture of the objective lens 12 passes through the diffractive surface. The radius is R (that is, the radius of the second light beam L2 is R). At this time, the angle of the first light beam L1 and the second light beam L2 having the respective wavelengths passing through the radius R in the diffractive optical element 11 with respect to the optical axis after passing through the diffractive optical element 11 is favorable for the recording layer of the optical disc. What is necessary is just to think so that it may become an angle converged by. That is, when the light beam of 405 nm is diffracted at the diffraction order of m1 and the light beam of 650 nm is diffracted at the diffraction order of m2, the combination of the diffraction orders enabling the above design is Satisfies the conditional expression.

Where r is the radius of curvature, and f (d, mX) is a function as shown in Equation 9.

  Here, X is 1 or 2, and the radius at the diffraction surface of the second light beam L2 corresponding to the numerical aperture NA2 of the objective lens 12 is R. Further, in the diffraction grating 11a of the diffractive optical element 11, the first light beam L1 is diffracted by the order of m1, and the second light beam L2 is diffracted by the diffraction order of m2, and the diffraction angle at that time is α1, Let α2. Further, the angles between the light beam and the optical axis after passing through the lens portion 11b when it becomes a good condensing spot on the optical disk (recording medium) are β1 and β2, and the pitch of the diffraction grating 11a at the radius R is d. .

  Specifically, the divergence angles β1 and β2 for canceling the spherical aberration vary depending on the shape of the objective lens 12, but in the case of the next-generation high-density optical disc and DVD disc, the lens portion 11b (substrate) of the diffractive optical element 11 is used. Polycarbonate (PC) was used, and a lens shape described later was used that was designed to be aberration-free for the next-generation high-density optical disc when blue light (first light beam L1) was incident on the objective lens 12 as parallel light. In this case, β1 is 0 ° and β2 is about 2.5 °. The relationship between the curvature radius r of the spherical surface at this time and the pitch d of the diffraction grating 11a at the radius R = 1.1 is shown in FIGS. FIG. 4 shows the calculation result when the diffraction order of blue light (405 nm) is the first order diffraction, and FIG. 5 shows the calculation result when the diffraction order of blue light (405 nm) is the second order diffraction. Here, the combination of the diffraction orders where there is no intersection means that the diffractive optical element 11 for condensing both the next-generation high-density optical disc and the DVD disc cannot be produced. The possible combinations of diffraction orders are shown in Table 1 below.

  In the above, the divergence angle β2 for canceling the spherical aberration is set to 2.5 °. However, if the incident angle is between 1 and 5 °, the next generation high in the same diffraction order combination as in the case of 2.5 °. The diffractive optical element 11 for condensing both the density optical disc and the DVD disc can be created.

  Examples of the optical pickup according to the present embodiment will be described below.

Example 1
In this example, in the optical pickup 100 described in the first embodiment, as shown in FIGS. 6A and 6B, the diffractive optical element 11 includes a lens 11b having a diffraction grating 11a and a convex spherical surface 11c. The diffraction grating 11a is provided on the light source side. Then, using PC as the substrate material (lens) of the diffractive optical element 11, the first-order diffracted light in the first light beam L1 having a wavelength of 405 nm was used, and the first-order diffracted light in the second light beam L2 having a wavelength of 650 nm was used. The details will be described.

  As shown in FIG. 6A, when a 405 nm laser beam (first light beam L1) enters the diffractive optical element 11 as a parallel light beam, the first light beam L1 is 1 on the surface of the diffraction grating 11a. The light is diffracted in the next diffraction direction (divergence direction), refracted by the convex spherical surface 11c, and emitted from the diffractive optical element 11 as parallel light. The parallel light is collected by the objective lens 12 onto a recording layer of an optical disc (next generation high density optical disc) 14a having a light transmission layer having a thickness of 0.1 mm. Thereby, in the said optical pick-up 100, a favorable condensing characteristic is acquired.

  As shown in FIG. 6B, when a 650 nm laser beam (second light beam L2) is incident on the diffractive optical element 11 as a parallel light beam, the second light beam L2 is incident on the surface of the diffraction grating 11a. The light is diffracted in the first-order diffraction direction (divergence direction), refracted by the convex spherical surface 11c, and emitted from the diffractive optical element 11 as divergent light. The diverging light is condensed by the objective lens 12 onto the recording layer of the optical disc 14b (DVD disc) having a light transmission layer having a thickness of 0.6 mm. Thereby, in the said optical pick-up 100, a favorable condensing characteristic is acquired.

  At this time, spherical aberration that occurs due to the thickness of the light transmission layer in the optical disk 14 b can be suppressed by making divergent light incident on the objective lens 12. Other spherical aberrations that cannot be corrected can be corrected by the aspheric term of the diffraction grating 11a. Of course, the convex spherical surface 11c may be aspherical so as to suppress aberrations.

  Here, light having a wavelength of 405 nm is transmitted and light having a wavelength of 650 nm is transmitted so that light outside the luminous flux corresponding to the numerical aperture of 0.6 is not collected on the optical disk 14b having a light transmission layer thickness of 0.6 mm. The numerical aperture of the objective lens 12 is switched by using the wavelength selective filter 10 that does not transmit the light.

  In this example, the RMS wavefront aberration when the laser beam (blue light) having a wavelength of 405 nm, which is the first light beam L1, is applied to an optical disc having a light transmission layer having a thickness of 0.1 mm is 0.002λ. A sufficiently small amount of aberration. The RMS wavefront aberration when the laser beam (red light) having a wavelength of 650 nm, which is the second light beam L2, is applied to an optical disc having a light transmission layer having a thickness of 0.6 mm is sufficiently small at 0.002λ. Amount. Therefore, the optical pickup 100 can sufficiently read out information signals from the optical disks 14a and 14b.

In the present embodiment, the diffractive optical element 11 having a convex surface on the objective lens 12 side with the diffraction grating 11a is provided on the light source side, but not limited thereto, the diffractive optical element 11 has a convex surface on the light source side and the diffraction grating on the objective lens 12 side. You may make it the structure which becomes 11a.
Here, data automatically designed for the shape of the diffraction grating 11a of the diffractive optical element 11 and the radius of curvature of the refractive surface are shown in Tables 2-4. The values shown in Tables 2 to 4 are the calculation of wavefront aberration in the next-generation high-density optical disc (wavelength 405 nm, light transmission layer thickness 0.1 mm) and DVD (wavelength 650 nm, light transmission layer thickness 0.6 mm). It is a result.

  Here, data automatically designed for the shape of the diffraction grating 11a of the diffractive optical element 11 and the radius of curvature of the refractive surface are shown in Tables 2-4. The values shown in Tables 2 to 4 are the calculation of wavefront aberration in the next-generation high-density optical disc (wavelength 405 nm, light transmission layer thickness 0.1 mm) and DVD (wavelength 650 nm, light transmission layer thickness 0.6 mm). It is a result. Here, the objective lenses in Tables 2 to 4 are optimally designed so as to collect blue light onto an optical disk having a light transmission layer having a thickness of 0.1 mm.

  In Tables 2 to 4, surface numbers 1 and 2 are the entrance surface and exit surface of the diffractive optical element 11, surface numbers 3 and 4 are the entrance surface and exit surface of the objective lens 23, and surface numbers 5 and 6 are the respective optical disks. The surface and the information recording surface. In addition, the surface interval described in the row of each surface number means the distance on the optical axis between the surface with the surface number and the surface with the surface number next to the surface number.

  Table 3 shows the aspheric coefficient of each surface.

  Further, the phase function Φ (r) is expressed by the following formula 10.

(Where m: diffraction order, λ: wavelength, r: radius from the optical axis, DF1 to DF5: coefficient)
In this example (when the diffractive optical element 11 in Tables 2 to 4 is used), laser light (blue light) having a wavelength of 405 nm is applied to an optical disc having a light transmission layer having a thickness of 0.1 mm. The RMS wavefront aberration is a sufficiently small aberration amount of 0.002λ. The RMS wavefront aberration when laser light (red light) having a wavelength of 650 nm is applied to an optical disc having a light transmission layer with a thickness of 0.6 mm is 0.002λ, which is a sufficiently small aberration amount. Therefore, the optical pickup can sufficiently read out information signals from the optical disks 14a and 14b.

(Example 2)
In this example, the optical pickup 100 described in the first embodiment includes an objective lens unit 113 instead of the objective lens unit 13 of the first example, as shown in FIGS. It has become. That is, the diffractive optical element 111 includes a lens 111b having a concave surface 111c and a diffraction grating 111a, and the diffraction grating 111a is provided on the light source side. PC is used as the substrate material (lens) of the diffractive optical element 111, and the diffraction grating 111a surface of the diffractive optical element 111 has a diffraction order of the first diffracted light as m1, a diffraction order of the second diffracted light as m2, and a diffraction grating. When the groove interval of 111a is d,

When satisfying the above, here, the diffraction order m1 of the first diffracted light M1 is +1, and the diffraction order m2 of the second diffracted light M2 is 0, that is, the first-order diffracted light in the first light beam L1 having a wavelength of 405 nm is used. The case where the 0th-order diffracted light in the second light beam L2 having a wavelength of 650 nm is used will be described in detail.

  As shown in FIG. 7A, when a 405 nm laser beam (first light beam L1) enters the diffractive optical element 111 as a parallel light beam, the first light beam L1 is 1 on the surface of the diffraction grating 111a. The light is diffracted in the next diffraction direction (convergence direction), refracted in the diverging direction by the concave surface 111c, and emitted as parallel light. The parallel light is condensed by the objective lens 112 onto the recording layer of the optical disc 14a having a light transmission layer having a thickness of 0.1 mm. Thereby, in the said optical pick-up 100, a favorable condensing characteristic is acquired.

  As shown in FIG. 7B, when a 650 nm laser beam (second light beam L2) is incident on the diffractive optical element 111 as a parallel light beam, the second light beam L2 is incident on the surface of the diffraction grating 111a. It is not diffracted (0th-order diffraction), and is refracted in the diverging direction by the concave surface 111c and emitted from the diffractive optical element 11 as diverging light. The divergent light is collected by the objective lens 112 onto the optical disc 14b having a light transmission layer having a thickness of 0.6 mm. Thereby, in the said optical pick-up 100, a favorable condensing characteristic is acquired.

  At this time, the spherical aberration generated due to the thickness of the light transmission layer in the optical disk 14b can be suppressed to some extent by making the diverging light incident on the objective lens 112. Further, other spherical aberrations that cannot be corrected can be corrected by the aspheric term of the diffraction grating. Of course, you may correct | amend by making the concave surface 111c into an aspherical surface.

  Here, light having a wavelength of 405 nm is transmitted and light having a wavelength of 650 nm is transmitted so that light outside the luminous flux corresponding to the numerical aperture of 0.6 is not collected on the optical disk 14b having a light transmission layer thickness of 0.6 mm. The numerical aperture of the objective lens 12 is switched using a wavelength-selective filter that does not transmit the light.

  In this example, the RMS wavefront aberration when a laser beam (blue light) having a wavelength of 405 nm, which is the first light beam L1, is applied to an optical disc having a light transmission layer having a thickness of 0.1 mm is 0.002λ. A sufficiently small amount of aberration. In addition, when the laser beam (red light) having a wavelength of 650 nm, which is the second light beam L2, is applied to an optical disc having a light transmission layer having a thickness of 0.6 mm, the RMS wavefront aberration is 0.002λ, which is a spherical aberration. Corrected and sufficiently small aberration. Therefore, the optical pickup 100 can sufficiently read out information signals from the optical disks 14a and 14b.

  In the present embodiment, as described above, an example in which the diffraction grating 111a of the diffractive optical element 111 faces the light source side is shown, but the same effect can be obtained even if the concave surface 111c faces the light source side. For example, as shown in FIGS. 41A and 41B, the concave surface 111c of the diffractive optical element 111 may be provided on the light source side.

  In this case, as shown in FIG. 41A, when a 405 nm laser beam (first light beam) is incident on the diffractive optical element 111 as a parallel light beam, the first light beam is diverged by the concave surface 111c. The light is refracted and emitted as parallel light in the first-order diffraction direction (convergence direction) on the surface of the diffraction grating 111a. The parallel light is condensed by the objective lens 112 onto the recording layer of the optical disc 14a having a light transmission layer having a thickness of 0.1 mm. Thereby, in the said optical pick-up, a favorable condensing characteristic is acquired.

  As shown in FIG. 41B, when a 650 nm laser beam (second light beam) is incident on the diffractive optical element 111 as a parallel light beam, the second light beam is refracted in the divergence direction by the concave surface 111c. The light is not diffracted by the surface of the diffraction grating 111a (0th order diffraction) and is emitted from the diffractive optical element 111 as divergent light. The divergent light is collected by the objective lens 112 onto the optical disc 14b having a light transmission layer having a thickness of 0.6 mm. Thereby, in the said optical pick-up, a favorable condensing characteristic is acquired.

  At this time, spherical aberration generated due to the thickness of the light transmission layer in the optical disk 14b can be suppressed to some extent by making divergent light incident on the objective lens. Further, other spherical aberrations that cannot be corrected can be corrected by the aspheric term of the diffraction grating. Of course, the concave surface may be aspherical.

  Here, light having a wavelength of 405 nm is transmitted and light having a wavelength of 650 nm is transmitted so that light outside the luminous flux corresponding to the numerical aperture of 0.6 is not collected on the optical disk 14b having a light transmission layer thickness of 0.6 mm. The numerical aperture of the objective lens 112 is switched using a wavelength-selective filter that does not transmit the light.

  Here, Tables 5 to 7 show data in which the shape of the diffractive surface of the diffractive optical element 111 shown in FIGS. 41A and 41B, the radius of curvature of the refracting surface, and the aspheric term are automatically designed. The values shown in Tables 5 to 7 are the calculation of wavefront aberration in the next-generation high-density optical disc (wavelength 405 nm, light transmission layer thickness 0.1 mm) and DVD (wavelength 650 nm, light transmission layer thickness 0.6 mm). It is a result. Here, the objective lenses in Tables 5 to 7 are optimally designed so as to collect blue light onto an optical disc having a light transmission layer having a thickness of 0.1 mm. The concave surface has an aspherical shape in order to correct aberration caused by light transmission layer in the optical disk.

  In Tables 5 to 7, surface numbers 1 and 2 are the entrance surface and exit surface of the diffractive optical element 111, surface numbers 3 and 4 are the entrance surface and exit surface of the objective lens 23, and surface numbers 5 and 6 are the respective optical disks. The surface and the information recording surface. In addition, the surface interval described in the row of each surface number means the distance on the optical axis between the surface with the surface number and the surface with the surface number next to the surface number.

  Table 6 shows the aspheric coefficient of each surface.

  In this example (when the diffractive optical element 111 of Tables 5 to 7 is used), a laser beam having a wavelength of 405 nm (blue light) is applied to an optical disc having a light transmission layer having a thickness of 0.1 mm. The RMS wavefront aberration is a sufficiently small aberration amount of 0.002λ. The RMS wavefront aberration when laser light (red light) having a wavelength of 650 nm is applied to an optical disc having a light transmission layer with a thickness of 0.6 mm is sufficiently small with spherical aberration corrected to 0.002λ. . Therefore, the optical pickup can sufficiently read out information signals from the optical disks 14a and 14b.

  In the present embodiment, a diffractive optical element having a concave surface on the light source side and a diffraction grating on the objective lens side is provided. However, the present invention is not limited to this, and the present invention may be configured to be a diffraction grating on the light source side and concave on the objective lens side. It does not impair the effect.

  In the embodiments of the present invention described above, as optical disks, next-generation high-density optical disks (light transmission layer thickness 0.1 mm, using blue light (405 nm)) and DVDs (light transmission layer thickness 0.6 mm, red light) However, the present invention is not limited to the thickness of the light transmission layer in the optical disk and the wavelength of light used in the optical disk, and the light transmission layer in various optical disks. It is applicable to the thickness and wavelength of light.

  As described above, the optical pickup of the present invention performs recording or recording using the condensing spot formed by the first light source that emits the first light beam L1 having the first wavelength λ1 and the objective lens having the numerical aperture NA1. A first recording medium having a light transmission layer thickness t1 from the reproducible light incident surface to the information recording surface, and a second light beam L2 having a second wavelength λ2 longer than the first wavelength. The thickness of the light transmission layer from the light incident surface to the information recording surface, which can be recorded or reproduced using a condensing spot formed using only the light source and the numerical aperture NA2 (NA2 <NA1) region of the objective lens In a compatible optical pickup that forms a focused spot using a common objective lens for both of the second optical recording media having a length of t2 (t2> t1), the first and second light sources Arranged in the optical path to the objective lens And a lens having a refractive index n, the distance between the diffraction element surface and the apex of the lens surface is a, and the radius of the second light beam L2 corresponding to the numerical aperture NA2 of the objective lens is R, the diffraction element diffracts with the order of m1 with respect to the first light beam L1, diffracts with the diffraction order of m2 with respect to the second light beam L2, and has diffraction angles α1, α2, Formulas 12 and 13, which show the following conditions at the diffraction element pitch d at the radius R, where β1 and β2 are the angles between the light beam and the optical axis after passing through the lens when a good focusing spot is obtained,

However, f (d, m) is a function as follows, and X is 1 or 2.

It can be said that a combination of diffraction orders m1 and m2 satisfying the above is used.

  Thus, even when an objective lens having a large numerical aperture is used with a light source having a significantly different wavelength, a diffraction element and a lens using a diffraction order satisfying the conditional expression of Formula 1 are used on an optical disk having a different light transmission layer thickness. Thus, it is possible to realize an optical pickup capable of recording / reproducing optical discs having different optical wavelengths different from each other by forming light spots that converge to the diffraction limit and having different optical transmission layer thicknesses and optimum reproduction wavelengths.

  In FIG. 8, the wavelength dependency of the wavefront aberration λrms in the optical pickup 100 of the present embodiment is indicated by a solid line A, and the wavelength dependency of the wavefront aberration λrms in another optical pickup device is indicated by a broken line B. Here, the wavefront aberration λrms at each wavelength is the wavefront aberration λrms at the position of the best image point at the wavelength at which the wavefront aberration λrms is minimum.

  The wavefront aberration λrms gradually increases as the first wavelength λ1 of the first light beam L1 moves away from 405 nm. According to FIG. 8, it can be seen that the increase in the wavefront aberration λrms is smaller in the optical pickup 100 of the present embodiment than in the other optical pickup devices. Therefore, it can be seen that the optical pickup 100 of this embodiment has low wavelength dependency.

  Since the first and second light sources 1a and 2b are realized by the semiconductor laser device as described above, wavelength variation occurs due to a mode hop phenomenon or the like. Also, wavelength fluctuations occur due to high frequency superposition. The shift of the focal position due to such wavelength fluctuation cannot be followed by the actuator that drives the objective lens 12. The optical pickup 100 of this embodiment has a smaller change in wavefront aberration with respect to wavelength fluctuations such as wavelength fluctuations due to mode hopping and high-frequency superposition, compared to the other optical pickup devices. Accordingly, the shift of the focal position due to the wavelength variation is suppressed, and a good spot light can be formed regardless of the wavelength variation.

  Further, in the present embodiment, the suppression of the focal position shift caused by the wavelength variation in the first light beam L1 is effective even in the case of a pickup dedicated for the next generation high density optical disc. That is, the effect is not impaired even when the component is configured with only components corresponding to the first light beam L1.

At this time, when the power of the diffractive optical element is Φ, the power of the diffractive surface of the diffracted photon is ΦD, and the power of the refracting surface is ΦL,
Φ = ΦD + ΦL = 0
If so, the first light beam L1 emitted as parallel light by the first optical system 16a can be emitted as parallel light even after passing through the diffractive optical element, and deterioration of aberration due to misalignment with the objective lens 12 can be suppressed. . In this case, the diffractive optical element can be inserted anywhere between the first light beam L116a and the objective lens.

  In this embodiment, a diffractive optical element having a concave surface on the light source side and a diffraction grating on the objective lens side can increase the pitch of the diffraction grating and the curvature of the concave surface, which is advantageous for production. However, the present invention is not limited thereto, and even if the light source side is a diffraction grating and the objective lens 12 is concave, the effect of the present invention is not impaired.

  Further, in the present embodiment, the effect is not impaired even when it is configured by only parts corresponding to the next generation high density optical disc.

  The present embodiment can be applied to, for example, the optical pickup shown in FIG. 40 in addition to the optical pickup having the configuration shown in FIG. The optical pickup shown in FIG. 40 includes the positions of the quarter wavelength plate 8 and the spherical aberration correction system 6 in the optical pickup shown in FIG. 1, the diffractive optical element 11 in the objective lens unit 13, and the wavelength selective filter 10. The other configurations are the same as those of the optical pickup shown in FIG. 1 except that the positions are reversed.

[Embodiment 2]
FIG. 9 shows a schematic configuration of the optical pickup 200 of the present embodiment. In the present embodiment, an optical pickup 200 that is compatible with a next-generation high-density optical disc (first optical disc) and a conventional DVD (second optical disc) will be described.

  The first optical disc has a light (first light beam L1) having a short wavelength of blue light having a wavelength (first wavelength λ1) of about 405 nm and a light transmission layer thickness t1 of 0.1 mm. In the second optical disk, the light (second light beam L2) to be used is a long wavelength red light having a wavelength (second wavelength λ2) near 650 nm, and the thickness t2 of the light transmission layer is 0.6 mm.

  The optical pickup 200 also includes a semiconductor laser 1a that emits a first light beam L1 having a first wavelength λ1, and a semiconductor laser 1b that emits a second light beam L2 having a second wavelength λ2 that is longer than the first wavelength λ1. It has. The semiconductor laser 1a and the semiconductor laser 1b are switched on and turned on according to the target optical disk.

  The optical pickup 200 further includes collimator lenses 2a and 2b that make the first and second light beams L2 emitted from the semiconductor lasers 1a and 1b substantially parallel to each other, and first and second light beams having an elliptical intensity distribution. Shaping optical systems 3a and 3b such as shaping prisms for shaping L2 into a substantially circular intensity distribution, and beam splitters 4a and 4b for transmitting the first and second light beams L2 from the shaping optical systems 3a and 3b, respectively. ing.

  The shaping optical systems 3a and 3b are configured by a known optical system such as one triangular prism, a bonded triangular prism, or two triangular prisms arranged independently. The optical pickup 2 may not include the shaping optical systems 3a and 3b.

  The first optical system 16a is configured by the semiconductor laser 1a, the collimator lens 2a, the shaping optical system 3a, and the beam splitter 4a. The second optical system 16b is formed by the semiconductor laser 1b, the collimator lens 2b, the shaping optical system 3b, and the beam splitter 4b. Is configured.

  The first and second light beams L2 emitted from the first and second optical systems 16a and 16b are aligned with each other by the dichroic prism 5, and thereafter go through the common optical system.

  In the common optical system, the degree of convergence / divergence of the first and second light beams L2 is changed by the spherical aberration compensation system 6 according to the type of light beam (first light beam L1 or second light beam L2). / 4 After being reflected by the wavelength plate 8 and the mirror 9, it enters the objective lens unit 213.

  The first and second light beams L2 entering the objective lens unit 213 sequentially pass through the wavelength selective aperture filter 210, the diffractive optical element 211, and the objective lens 212, and a minute amount of light on the information recording surface of the first optical disc. A small light spot is formed on the spot or on the information recording surface of the second optical disk.

  Here, the spherical aberration compensation system 6 is a beam expander that corrects spherical aberration caused by uneven thickness of the light transmission layer in the first and second optical discs, and the first and second light beams as described above. It has a function as a light beam control means for changing the degree of convergence / divergence.

  If the optical pickup 200 is configured not to include the shaping optical systems 3a and 3b, the first and second light beams L2 are converged using the collimator lenses 2a and 2b without using the spherical aberration compensation system 6.・ The degree of divergence may be changed. Furthermore, the degree of convergence / divergence of the first and second light beams L2 may be changed using other elements.

  The wavelength-selective aperture filter 210 works so that the numerical aperture is NA1 (specifically 0.85) for the light beam having the first wavelength λ1, and the wavelength is the second wavelength λ2. The aperture of the beam is controlled so that the numerical aperture is NA2 (specifically, 0.6). Here, the wavelength-selective aperture filter 210 is disposed between the mirror 9 and the diffractive optical element 211, but may be anywhere as long as it can operate integrally with the diffractive optical element 211 and the objective lens 212. . In addition, other than the wavelength selective aperture filter 210 may be used as long as it has the same function as the aperture control as described above.

  The wavelength-selective aperture filter 210, the diffractive optical element 211, and the objective lens 212 are integrated as an objective lens unit 213 and can move with respect to the other optical systems of the optical pickup 2. As a result, the focused spot can be made to follow well with respect to surface deflection of the information recording surfaces of the first and second optical disks and rotational eccentricity of the information tracks of the first and second optical disks.

  In addition to the above light irradiation optical system, the optical pickup 200 further includes reproduction signal detection optical systems 15a and 15b. In the reproduction signal detection optical systems 15a and 15b, light spot control signals such as autofocus and track following and information signals recorded on the optical disk are reproduced by various conventionally known optical systems.

  By configuring the diffractive optical element 211 to include a converging diffraction grating 211a and a divergent plano-concave lens 211b, it is possible to suppress the deterioration of wavefront aberration with respect to wavelength variation, which is also favorable for wavelength variation. Condensing characteristics can be obtained.

  Here, in order to reduce the number of components, the diffraction grating 211a is formed on the plane of the plano-concave lens 211b to constitute the diffractive optical element 211. However, the diffractive optical element 211 is formed by combining the two optical elements of the diffractive element and the lens. May be configured.

  Further, the objective lens unit 213 may be configured by using a diffractive optical element 211 having a diffraction grating on the refractive surface opposite to the plane of the translucent lens.

  The plano-concave lens 211b of the diffractive optical element 211 is made of glass or plastic. Further, the diffraction grating 211a of the diffractive optical element 211 is formed by a circular ring groove or a convex ring zone laminated by photolithography concentrically around the optical axis on the plane of the plano-concave lens 211b. The diffraction grating 211a is formed such that a cross-sectional shape appearing on a plane including the optical axis is a blazed shape, that is, a sawtooth shape or a stepped shape. A diffraction grating with a sawtooth or stepped cross section (especially a sawtooth cross section) is advantageous because of its higher diffraction efficiency.

  As the diffractive optical element 211, the same effect can be obtained even if the diffraction grating 211a is formed on the concave surface of the plano-concave lens 211b as described above. In this case, the concave surface and the diffraction grating 211a can be easily aligned, and the concave surface and the diffraction grating 211a can be easily formed.

  Here, out of the diffracted light by the diffraction grating 211a of the diffractive optical element 211, the diffraction order of the diffracted light used for recording / reproduction of the optical pickup 200 is about the first light beam L1 (wavelength 405 nm) so that the efficiency is highest. Uses the second-order diffracted light, and uses the first-order diffracted light for the second light beam L2 (wavelength 650 nm).

  In the optical pickup 200, at least one of the first and second light beams L2 is incident on the diffractive optical element 211 as convergent light or divergent light, that is, non-parallel light. That is, an angle difference is given to the convergence / divergence angle between the first light beam L1 and the second light beam L2 when entering the diffractive optical element 211. As a result, the diffraction angle of the first light beam L1 and the diffraction angle of the second light beam L2 at the diffraction grating, which are necessary for correcting the spherical aberration caused by the large difference in the thickness of the light transmission layer, Can be set to about 0.5 to 1.5 °. As a result, the pitch of the diffraction grating can be increased, and accordingly, the radius of curvature of the concave surface can be increased. Thereby, the production of the diffractive optical element 211 is facilitated.

  In this specification, the “convergence / divergence angle” refers to an angle formed by the optical axis and the outermost periphery of the light beam in a cross section including the optical axis of the light beam. The convergence / divergence angle is a negative value, and when the light beam is diverging light, the convergence / divergence angle is a positive value.

  Further, as shown in FIG. 42 (a), a diffractive optical element 122 ′ having a diffraction grating 122b ′ on a refractive surface (concave surface) S2 opposite to the plane S1 of the translucent plano-concave lens 122a ′ is used. The objective lens unit 120 ′ may be configured.

  The plano-concave lens 122a 'of the diffractive optical element 122' is made of glass or plastic. In addition, the diffraction grating 122b ′ of the diffractive optical element 122 ′ is a convex annular zone laminated on the plane S1 of the plano-concave lens 122a ′ concentrically around the optical axis by a cut annular groove or photolithography. Consists of. As shown in FIG. 3, the diffraction grating 122b 'preferably has a blazed shape, that is, a sawtooth shape, in the cross-sectional shape that appears on the plane including the optical axis. Sawtooth shaped diffraction gratings are advantageous because of their higher diffraction efficiency. Further, as shown in FIG. 42B, the diffraction grating 122b ″ may be an objective lens unit 120 ″ formed so that the cross-sectional shape appearing on the plane including the optical axis is stepped. The step-like cross section diffraction grating 122b "is advantageous because it has the second highest diffraction efficiency after the sawtooth cross section diffraction grating 122b '.

  Note that, as the diffractive optical element 122 ″ shown in FIG. 42B, the same effect can be obtained even if the diffraction grating 122b ″ is formed on the concave surface S2 of the plano-concave lens 122a ″ as shown in FIG. In this case, the concave surface S2 and the diffraction grating 122b ″ can be easily aligned, and the concave surface S2 and the diffraction grating 122b ″ can be easily formed.

  Examples of the optical pickup 200 according to the present embodiment will be described below.

(Example 3)
In this embodiment, as shown in FIGS. 10A and 10B, the first light beam L1 (wavelength 405 nm) is parallel to the objective lens unit 213 (that is, the convergence / divergence angle θ is The second light beam L2 (wavelength of 650 nm) is diverging with respect to the objective lens unit 213 (specifically, the incident light is incident on the diffractive optical element 211). The objective lens unit 213 using the first-order diffracted light by the diffractive optical element 211 was manufactured by making the light incident so that the convergence / divergence angle θ was 1.5 °. The objective lens 212 is an aspheric lens, and the concave surface of the diffractive optical element 211 is also aspheric.

  When the first light beam L1 enters the diffractive optical element 211 as parallel light, the diffracted light diffracted in the second-order diffraction direction (convergence direction with respect to the optical axis) by the diffraction grating of the diffractive optical element 211 is changed. By being refracted by the plano-concave lens in the divergence direction and emitted, it enters the objective lens 212 as parallel light. The diffracted light is further collected by the objective lens 212, transmitted through the light transmission layer (thickness 0.1 mm), and a fine light spot is formed on the information recording surface, thereby obtaining good light collection characteristics. It is done.

  When the second light beam L2 is incident on the diffractive optical element 211 as divergent light, the diffracted light diffracted in the first-order diffraction direction (convergence direction with respect to the optical axis) by the diffraction grating of the diffractive optical element 211 is diffracted optically. The light is refracted in the divergence direction by the plano-concave lens of the element 211 and emitted, and is incident on the objective lens 212 as divergent light. The diffracted light is further collected by the objective lens 212, transmitted through the light transmission layer (thickness 0.6 mm), and a fine light spot is formed on the information recording surface, thereby obtaining good light collection characteristics. It is done.

  In this way, the first light beam L1 is incident on the diffractive optical element 211 as parallel light, and the second light beam L2 is incident on the diffractive optical element 211 as divergent light, so that the second time of the first light beam L1. The convergence / divergence angle θ when the folded light enters the objective lens 212 is set to 0 °, and the convergence / divergence angle when the first-order diffracted light of the second light beam L2 enters the objective lens 212 is set to 2 ° to 3 °. (Ie, the angle difference between the convergence / divergence angle of the second-order diffracted light of the first light beam L1 and the convergence / divergence angle of the first-order diffracted light of the second light beam L2 is changed from 2 ° to 3 °). It has become possible to design the pitch of the diffraction grating and the shape of the concave surface of the plano-concave lens.

  Tables 8 to 11 show the surface data and the like of the designed objective lens 212 and diffractive optical element 211. In Tables 8 to 11, surface number 0 represents a virtual light source, surface numbers 1 and 2 represent the entrance surface and exit surface of the diffractive optical element 211, and surface numbers 3 and 4 represent the entrance surface and exit surface of the objective lens 23, surface number. Reference numerals 5 and 6 denote the surface of the optical disk and the information recording surface. In addition, the surface interval described in the row of each surface number means the distance on the optical axis between the surface with the surface number and the surface with the surface number next to the surface number.

  Table 10 shows the aspheric coefficients of each surface, and Table 11 shows the coefficients of each term when the phase difference function Φ (r) of the diffractive surface is expressed by Equation 14. In Equation 14, m is the diffraction order, λ is the wavelength, r is the distance from the optical axis, and DF1 to DF5 are coefficients. In Tables 10 and 11, for example, “−2.2E-03” means “−2.2-3”.

  Changes in the wavefront aberration λrms on the image plane with respect to the shift amount in the optical axis direction of the objective lens unit 213 when the optical pickup 200 manufactured as the present embodiment is condensed on the optical disk are shown in FIG. This is indicated by a solid line in (b). Further, the case of using a conventional optical pickup produced for comparison is shown by broken lines in FIGS. 11 (a) and 11 (b). FIG. 11A shows the result of focusing on the first optical disk, and FIG. 11B shows the result of focusing on the second optical disk.

  In the conventional optical pickup manufactured for comparison, both the first and second light beams L2 are incident on the objective lens unit as parallel light, and the second-order diffracted light is used as the second light for the first light beam L1. The beam L2 is produced so that the wavefront aberration is optimized when the first-order diffracted light is used.

  As can be seen from FIG. 11A, by using the optical pickup 200, the wavefront aberration with respect to the first optical disc can be reduced as compared with the conventional optical pickup, and a good condensing spot can be formed. .

  Further, as can be seen from FIG. 11B, in the optical pickup 200, the influence of the shift of the objective lens unit 213, which is caused by making the second light beam L2 incident on the objective lens unit 213 as divergent light, is also second. Wavefront aberration with respect to the optical disk can be reduced as compared with a conventional optical pickup if the shift amount of the objective lens unit 213 is in the range of 0.15 mm or less.

  As described above, the optical pickup 200 can reduce the wavefront aberration with respect to the first and second optical disks as compared with the conventional optical pickup.

  In addition, the change of the wavefront aberration λrms with respect to the change of the wavelength of the first light beam L1 when focused on the first optical disk using the optical pickup 200 manufactured as the present embodiment is shown by a solid line in FIG. In addition, a case where an optical pickup dedicated to the first optical disk manufactured for comparison is used is indicated by a broken line in FIG.

  The optical pickup dedicated to the first optical disk manufactured for comparison is an objective lens unit that is composed of a single objective lens 212 (objective lens dedicated to the first light beam L1) in the optical pickup 200 manufactured according to the present embodiment. is there.

  As can be seen from FIG. 12, the optical pickup 200 has a wider usable wavelength range than the optical pickup dedicated to the first optical disk. This is because the diffractive optical element 211 of the optical pickup 200 of the present embodiment is composed of a converging diffraction grating and a plano-concave lens, which makes it possible to use an objective lens dedicated to the first light beam L1 alone. Can also improve the wavelength-dependent characteristics. Therefore, the optical pickup 200 can form a good condensing spot even if wavelength variation occurs due to mode hopping or the like.

Example 4
In this embodiment, as shown in FIGS. 13A and 13B, the first light beam L1 (wavelength 405 nm) is focused on the objective lens unit 213 (specifically, the convergence / divergence angle). The second light beam L2 (wavelength 650 nm) is incident on the objective lens unit 213 in parallel light (with θ being −1.5 °) and using the second-order diffracted light by the diffractive optical element 211 ( That is, the objective lens unit 213 using the first-order diffracted light by the diffractive optical element 211 was manufactured by making the light incident so that the convergence / divergence angle θ was 0 °. The objective lens 212 is an aspheric lens, and the concave surface of the diffractive optical element 211 is also aspheric.

  When the first light beam L1 enters the diffractive optical element 211 as convergent light, the diffracted light diffracted in the second-order diffraction direction (convergence direction with respect to the optical axis) by the diffraction grating 211a of the diffractive optical element 211 is changed. By being refracted in the diverging direction by the plano-concave lens 211b 211 and emitted, it enters the objective lens 212 as parallel light. The diffracted light is further collected by the objective lens 212, transmitted through the light transmission layer (thickness 0.1 mm), and a fine light spot is formed on the information recording surface, thereby obtaining good light collection characteristics. It is done.

  When the second light beam L2 enters the diffractive optical element 211 as parallel light, the diffracted light diffracted in the first-order diffraction direction (convergence direction with respect to the optical axis) by the diffraction grating 211a of the diffractive optical element 211 is diffracted. The light is refracted in the diverging direction by the plano-concave lens 211b of the optical element 211 and is emitted to enter the objective lens 212 as divergent light. The diffracted light is further collected by the objective lens 212, transmitted through the light transmission layer (thickness 0.6 mm), and a fine light spot is formed on the information recording surface, thereby obtaining good light collection characteristics. It is done.

  In this way, the first light beam L1 is incident on the diffractive optical element 211 with convergent light, and the second light beam L2 is incident on the diffractive optical element 211 with parallel light, whereby the second time of the first light beam L1. The convergence / divergence angle θ when the folded light enters the objective lens 212 is set to 0 °, and the convergence / divergence angle θ when the first-order diffracted light of the second light beam L2 enters the objective lens 212 is set from 2 °. (Ie, the angle difference between the convergence / divergence angle of the second-order diffracted light of the first light beam L1 and the convergence / divergence angle of the first-order diffracted light of the second light beam L2 is set to 2 ° to 3 °). It is now possible to design the pitch of the diffraction grating and the shape of the concave surface of the plano-concave lens.

  Tables 12 to 15 show the surface data and the like of the designed objective lens 212 and diffractive optical element 211. The symbols and the like in Tables 12 to 15 are the same as those in Tables 8 to 11.

  Changes in wavefront aberration λrms on the image plane with respect to the shift amount in the optical axis direction of the objective lens unit 213 when the optical pickup 200 manufactured as the present embodiment is condensed on the optical disk are shown in FIG. This is indicated by a solid line in (b). Further, the case of using a conventional optical pickup produced for comparison is shown by broken lines in FIGS. 14 (a) and 14 (b). FIG. 14A shows the result of focusing on the first optical disk, and FIG. 14B shows the result of focusing on the second optical disk.

  In the conventional optical pickup manufactured for comparison, as in the case of the third embodiment, both the first and second light beams L2 are incident on the objective lens unit as parallel light, and the first light beam L1 is used. Is produced so that the wavefront aberration is optimal when the second-order diffracted light is used and the first light diffracted light is used for the second light beam L2.

  As can be seen from FIG. 14A, in the optical pickup 200, the influence of the shift of the objective lens unit 213, which is caused by making the first light beam L1 incident on the objective lens unit 213 as convergent light, is also affected by the first optical disc. Wavefront aberration can be reduced as compared with a conventional optical pickup if the shift amount of the objective lens unit 213 is in a range of 0.12 mm or less.

  Further, as can be seen from FIG. 14B, by using the optical pickup 200, the wavefront aberration with respect to the second optical disk can be reduced as compared with the conventional optical pickup, and a good condensing spot is formed. Can do.

  As described above, the optical pickup 200 can reduce the wavefront aberration with respect to the first and second optical disks as compared with the conventional optical pickup.

(Example 5)
In this embodiment, as shown in FIGS. 15A and 15B, the first light beam L1 (wavelength 405 nm) is focused on the objective lens unit 213 (specifically, the convergence / divergence angle). The second light beam L2 (wavelength of 650 nm) is diverged with respect to the objective lens unit 213 by using the second-order diffracted light by the diffractive optical element 211 (so that θ is −0.8 °). Specifically, the objective lens unit 213 using the first-order diffracted light by the diffractive optical element 211 was manufactured by making it incident so that the convergence / divergence angle θ was 0.8 °. The objective lens 212 is an aspheric lens, and the concave surface of the diffractive optical element 211 is also aspheric.

  When the first light beam L1 enters the diffractive optical element 211 with convergent light, the diffracted light diffracted in the second-order diffraction direction (convergence direction with respect to the optical axis) by the diffraction grating of the diffractive optical element 211 is diffracted optical element 211. By being refracted by the plano-concave lens in the divergence direction and emitted, it enters the objective lens 212 as parallel light. The diffracted light is further collected by the objective lens 212, transmitted through the light transmission layer (thickness 0.1 mm), and a fine light spot is formed on the information recording surface, thereby obtaining good light collection characteristics. It is done.

  When the second light beam L2 is incident on the diffractive optical element 211 as divergent light, the diffracted light diffracted in the first-order diffraction direction (convergence direction with respect to the optical axis) by the diffraction grating of the diffractive optical element 211 is diffracted optically. The light is refracted in the divergence direction by the plano-concave lens of the element 211 and emitted, and is incident on the objective lens 212 as divergent light. The diffracted light is further collected by the objective lens 212, transmitted through the light transmission layer (thickness 0.6 mm), and a fine light spot is formed on the information recording surface, thereby obtaining good light collection characteristics. It is done.

  In this way, the first light beam L1 is incident on the diffractive optical element 211 with convergent light, and the second light beam L2 is incident on the diffractive optical element 211 with divergent light, so that the second time of the first light beam L1. The convergence / divergence angle θ when the folded light enters the objective lens 212 is set to 0 °, and the convergence / divergence angle θ when the first-order diffracted light of the second light beam L2 enters the objective lens 212 is set from 2 °. (Ie, the angle difference between the convergence / divergence angle of the second-order diffracted light of the first light beam L1 and the convergence / divergence angle of the first-order diffracted light of the second light beam L2 is set to 2 ° to 3 °). It is now possible to design the pitch of the diffraction grating and the shape of the concave surface of the plano-concave lens.

  Tables 16 to 19 show surface data and the like of the designed objective lens 212 and diffractive optical element 211. The symbols in Tables 16 to 19 have the same contents as those in Tables 8 to 11.

  Changes in the wavefront aberration λrms on the image plane with respect to the shift amount in the optical axis direction of the objective lens unit 213 when the optical pickup 200 manufactured as the present embodiment is condensed on the optical disk are shown in FIG. This is indicated by a solid line in (b). Further, the case of using a conventional optical pickup produced for comparison is shown by broken lines in FIGS. 16 (a) and 16 (b). 16A shows the result of focusing on the first optical disc, and FIG. 16B shows the result of focusing on the second optical disc.

  As in the case of the fourth embodiment, the conventional optical pickup manufactured for comparison makes both the first and second light beams L2 incident on the objective lens unit as parallel light, and the first light beam L1. Is produced so that the wavefront aberration is optimal when the second-order diffracted light is used and the first light diffracted light is used for the second light beam L2.

  As can be seen from FIGS. 16A and 16B, by using the optical pickup 200 of this embodiment, the wavefront aberration for the first and second optical disks can be reduced as compared with the conventional optical pickup, A good condensing spot can be formed.

  Further, in the optical pickup 200, the influence of the shift of the objective lens unit 213, which is caused by causing the first and second light beams L2 to enter the objective lens unit 213 as convergent light and divergent light, respectively, is also a wavefront with respect to the first optical disc. As far as aberrations are concerned, if the shift amount of the objective lens unit 213 is in the range of 0.23 mm or less, and if the shift amount of the objective lens unit 213 is in the range of 0.3 mm or less with respect to the wavefront aberration for the second optical disc, it is compared with the conventional optical pickup. Can be reduced.

  Thus, the objective lens unit 213 causes the first and second light beams to enter the objective lens unit 213 by causing the first light beam L1 to be incident as convergent light and the second light beam L2 to be incident as divergent light. Even when shifted in the direction of the optical axis of L2, the degradation of wavefront aberration can be kept low in a relatively wide shift range.

  In Examples 3 to 5 described above, the concave surface of the diffractive optical element 211 is made of an aspherical surface, but the same effect can be obtained even if it is made of a spherical surface. In the case of manufacturing with a spherical surface, the diffractive optical element 211 can be easily manufactured as compared with the case of manufacturing with an aspherical surface, and an inexpensive diffractive optical element 211 can be provided, thereby reducing the cost.

  In the optical pickup 200, the first light beam L1 and the second light beam L2 are incident on the diffractive optical element 211 as light beams having different degrees of convergence or divergence, so that the angle difference can be easily increased. it can. Thereby, the restrictions on the diffraction and refraction characteristics required for the diffractive optical element 211 can be relaxed, and the design restrictions of the diffractive optical element 211 can be relaxed. As a result, it is possible to realize the optical pickup 200 that can sufficiently reduce the wavefront aberration of the diffracted light to be collected by using the diffractive optical element 211 that is easy to manufacture.

  The first light beam L1 and the second light beam L2 are incident on the diffractive optical element 211 as light beams having different degrees of convergence or divergence, as described in the third to fifth embodiments. When the other is incident as divergent light, or when one is parallel light and the other is incident as convergent or divergent light, both are convergent or divergent light, and the degree of convergence or divergence is different It may be.

  In order to realize compatibility between the next-generation high-density optical disk and the conventional DVD, when parallel light is incident on the diffractive optical element 211 for both red light and blue light, the difference in thickness of the light transmission layer As described above, it is necessary to increase the angle difference between the diffraction angle of the blue light and the diffraction angle of the red light from about 2 ° to 3 °, which is necessary for correcting the spherical aberration generated due to the large value of is there. Here, the relationship between the pitch of the diffraction grating 211a in the diffractive optical element 211 and the angle difference is as shown in the graph of FIG. From FIG. 43, it is understood that the pitch of the diffraction grating needs to be a fine width of 3.5 to 4.5 μm in order to make the angle difference about 2 ° to 3 °.

  In general, since the objective lens (infinite objective lens) is optimized for blue light coming from an infinite distance, the light emitted from the diffractive optical element needs to be converted into parallel light. Therefore, in the diffractive optical element, it is necessary to convert the blue light bent at the diffractive surface into parallel light at the refracting surface (the surface on the objective lens side of the diffractive optical element). By doing so, it is also possible to prevent the occurrence of aberration due to the positional deviation between the diffractive optical element and the objective lens.

  Therefore, FIG. 44 shows the relationship between the pitch of the diffraction grating and the radius of curvature of the refractive surface of the diffractive optical element when blue light incident on the diffractive optical element as parallel light becomes parallel light after passing through the diffractive optical element. Shown in The relationship shown in FIG. 44 is for a diffractive optical element in an optical pickup that uses an objective lens having an effective radius of 2 mm. The refractive surface of the diffractive optical element is a spherical surface. 44 that the radius of curvature of the refracting surface of the diffractive optical element needs to be 2.2 mm or less in order to make the pitch of the diffraction grating 3.5 to 4.5 μm.

  However, since the effective radius of a general objective lens is 2 mm, the effective diameter of the diffractive optical element is also 2 mm. Therefore, a refracting surface with a radius of curvature of 2.2 mm or less is almost a hemisphere, and cannot be manufactured. It becomes virtually impossible to use. Even if the refracting surface is an aspherical surface, the radius of curvature is extremely small, making it difficult to manufacture, and even if it can be manufactured, the on-axis light condensing characteristic is 0.018λ in any optical disc. (Rms).

  However, as described above, according to the optical pickup 200 according to the present embodiment, such a problem can be solved as described above. That is, according to the present optical pickup 200, information can be recorded or reproduced on a recording medium having a different transmission layer thickness and optimum wavelength for reproduction, and is easy to manufacture and collects light. There is an effect that the aberration of the beam can be sufficiently reduced.

  In this embodiment, in addition to the optical pickup shown in FIG. 9, for example, the present invention can also be applied to the optical pickup shown in FIG. The optical pickup shown in FIG. 45 is the same as that shown in FIG. 9 except that the positions of the quarter-wave plate 8 and the spherical aberration correction system 6 in the optical pickup 200 shown in FIG. 9 are reversed. This is the same as the optical pickup 200.

[Embodiment 3]
FIG. 17 shows a schematic configuration of the optical pickup 300 of the present embodiment. In the present embodiment, a next-generation high-density optical disc (first optical disc 14a, first recording medium), a conventional DVD (second optical disc 14b, second recording medium), and a conventional CD (third optical disc 14c, third recording medium). An optical pickup 300 that can handle a medium) will be described.

  In the first optical disc 14a, the light (first light beam L1) used is blue light having a short wavelength near the wavelength (first wavelength λ1) of 405 nm, and the thickness t1 of the light transmission layer is 0.1 mm. In the second optical disk 14b, the light to be used (second light beam L2) is long wavelength red light having a wavelength (second wavelength λ2) near 650 nm, and the thickness t2 of the light transmission layer is 0.6 mm. In the third optical disk 14c, the light to be used (third light beam L3) is infrared light having a long wavelength near the wavelength (third wavelength λ3) of 780 nm, and the thickness t3 of the light transmission layer is 1.2 mm.

  The optical pickup 300 includes a semiconductor laser 1a that emits a first light beam L1 having a first wavelength λ1, a semiconductor laser 1b that emits a second light beam L2 having a second wavelength λ2 longer than the first wavelength λ1, and a second laser beam 1b. A semiconductor laser 1c that emits a third light beam L3 having a third wavelength λ3 longer than the wavelength λ2 is provided. The semiconductor laser 1a, the semiconductor laser 1b, and the semiconductor laser 1c (light source) are switched on and turned on according to the target optical disk.

  The optical pickup 300 further includes collimator lenses 2a and 2b that make the first and second light beams L2 emitted from the semiconductor lasers 1a and 1b substantially parallel light, and first and second lights having an elliptical intensity distribution. Shaping optical systems 3a and 3b such as a shaping prism for shaping the beam L2 into a substantially circular intensity distribution, and beam splitters 4a and 4b for transmitting the first and second light beams L2 from the shaping optical systems 3a and 3b, respectively. ing.

  The shaping optical systems 3a and 3b are configured by a known optical system such as one triangular prism, a bonded triangular prism, or two triangular prisms arranged independently. The optical pickup 300 may not include the shaping optical systems 3a and 3b.

  The optical pickup 300 further includes a correction lens 2c that makes the third light beam L3 emitted from the semiconductor laser 1c a predetermined divergent light, and a beam splitter 4c that transmits the third light beam L3 that has passed through the correction lens 2c. . Here, the correction lens 2c is an aspherical lens inserted for the purpose of reducing the influence of the shift in the radial direction of the objective lens unit 313.

  The first optical system 16a is configured by the semiconductor laser 1a, the collimator lens 2a, the shaping optical system 3a, and the beam splitter 4a. The second optical system 16b is formed by the semiconductor laser 1b, the collimator lens 2b, the shaping optical system 3b, and the beam splitter 4b. The third optical system 16c is configured by the semiconductor laser 1c, the correction lens 2c, and the beam splitter 4c.

  The first and second light beams L2 emitted from the first and second optical systems 16a and 16b are aligned with each other by the dichroic mirror 5, and the spherical aberration compensation system 6 uses the type of light beam (first light). The degree of convergence / divergence is changed according to the beam L1 or the second light beam L2). Thereafter, the first and second light beams L2 are aligned with each other by the third light beam L3 emitted from the third optical system 16c and the dichroic mirror 7, and thereafter, pass through the common optical system. .

  Here, the spherical aberration compensation system 6 is a beam expander that corrects spherical aberration generated due to uneven thickness of the light transmission layer in the first and second optical discs 14a and 14b, and the first and second as described above. It has a function as a light beam control means for changing the degree of convergence / divergence of the light beam L2. The correction lens 2c has a function as a light beam control means for changing the degree of convergence / divergence of the third light beam L3.

  When the optical pickup 300 is configured not to include the shaping optical systems 3a and 3b, the first and second light beams L2 are converged using the collimator lenses 12a and 12b without using the spherical aberration compensation system 6. / The degree of divergence may be changed. Furthermore, the degree of convergence / divergence of the first and second light beams L2 may be changed using other elements.

  In the common optical system, the first, second, and third light beams pass through the quarter-wave plate 8, are reflected by the mirror 9, and then enter the objective lens unit 313.

  The first, second, and third light beams entering the objective lens unit 313 sequentially pass through the wavelength selective aperture filter 310, the diffractive optical element 311, and the objective lens 312, and the first optical disc 14a, the second optical disc 14b, or A minute light spot is formed on the information recording surface of the third optical disk 14c.

  The wavelength selective aperture filter 310 works so that the numerical aperture is NA1 (specifically 0.85) for the first light beam L1 having the first wavelength λ1, and the wavelength is the second wavelength. For the second light beam L2 with λ2, the numerical aperture is NA2 (specifically 0.6), and for the third light beam L3 with the third wavelength λ3, the numerical aperture is NA3. The aperture is controlled by working so as to be (specifically 0.45). Here, the wavelength-selective aperture filter 310 is disposed between the mirror 9 and the diffractive optical element 311, but can operate integrally with the diffractive optical element 311 and the objective lens 312, and the objective lens. Any location between the light source 312 and the light source may be used. In addition, other than the wavelength selective aperture filter 310 may be used as long as it has the same function as the aperture control as described above.

  The wavelength-selective aperture filter 310, the diffractive optical element 311 and the objective lens 312 are integrated as an objective lens unit 313, and in the optical axis direction, that is, in the direction of arrow Z in FIG. It can be moved. Thereby, with respect to the runout of the information recording surfaces of the first, second and third optical discs 14a, 14b and 14c and the rotational eccentricity of the information tracks of the first, second and third optical discs 14a, 14b and 14c, The focused spot can be made to follow well.

  In addition to the above light irradiation optical system, the optical pickup 300 further includes reproduction signal detection optical systems 15a, 15b, and 15c. In the reproduction signal detection optical systems 15a, 15b, and 15c, light point control signals such as autofocus and track following and information signals recorded on the optical disk are reproduced by various known optical systems.

  The objective lens unit 313 includes an objective lens 312 for condensing the first, second, and third light beams on the information recording surfaces of the first, second, and third optical disks 14a, 14b, and 14c, respectively, and a translucent divergence. This is an assembly in which a diffractive optical element 311 having a plano-concave lens 311b and a converging diffraction grating 311a formed on the plane of the plano-concave lens 311b and a wavelength selective aperture filter 310 are integrated.

  By making the diffractive optical element 311 have a converging type diffraction grating 311a and a divergent plano-concave lens 311b, it is possible to suppress the deterioration of wavefront aberration with respect to wavelength fluctuations, which is also good for wavelength fluctuations. Condensing characteristics can be obtained. In the present specification, wavefront aberration is used as a term representing the amount of aberration.

  Here, in order to reduce the number of parts, the diffraction grating 311a is formed on the plane of the plano-concave lens 311b to constitute the diffractive optical element 311. However, the diffractive optical element is formed by combining the two optical elements of the diffractive element and the lens. May be configured. In any case, a surface having a diffractive action in the diffractive optical element 311 is called a diffractive surface, and a surface having a refracting action is called a refracting surface.

  In this optical pickup 300, the diffraction grating 311a of the diffractive optical element 311 is on the wavelength selective aperture filter 310 (light source side), but the same effect can be obtained even if the diffraction grating 311a is on the objective lens 312 side. it can.

  Further, the objective lens unit 313 may be configured using a diffractive optical element having a diffraction grating on a concave surface (refractive surface) opposite to the plane of the translucent lens. In this case, the concave surface and the diffraction grating can be easily aligned.

  The plano-concave lens 311b of the diffractive optical element 311 is made of glass or plastic. Further, the diffraction grating 311a of the diffractive optical element 11 is formed by a circular ring groove or a convex ring zone that is laminated concentrically around the optical axis on the plane of the plano-concave lens 311b. Alternatively, it is made concentrically around the optical axis by glass molding or resin molding.

  The diffraction grating 311a is preferably formed in a blazed shape, that is, a sawtooth shape, in a cross-sectional shape that appears on a plane including the optical axis. The sawtooth diffraction grating 311a is advantageous because it has higher diffraction efficiency than others. In addition, the diffraction grating 311a may be formed so that the cross-sectional shape appearing on a plane including the optical axis is stepped. The diffraction grating 311a having a stepped cross section is advantageous because it has the second highest diffraction efficiency after the diffraction grating 311a having a sawtooth cross section.

  Here, the objective lens 312 has a first light beam L1 (blue light: wavelength 405 nm) with respect to the information recording surface of the first optical disk 14a (next-generation high-density optical disk: wavelength used 405 nm, light transmission layer 0.1 mm). Design so that aberrations are corrected.

  Convergence / divergence degrees Φoutb, Φoutr, and ΦoutIr of the first, second, and third light beams after passing through the diffractive optical element 311 can be expressed by Equation 15, respectively.

  Where Φinb, Φinr, and ΦinIr represent the degree of convergence / divergence of the first, second, and third light beams incident on the diffractive optical element 311, respectively, and ΦHOEb, ΦHOEr, and ΦHOEIr represent the first, second, and third, respectively. The power at the diffractive surface of the diffractive optical element 311 with respect to the light beam is represented, and ΦLb, ΦLr, and ΦLIr represent the power at the refracting surface of the diffractive optical element 311 with respect to the first, second, and third light beams.

  Here, the degree of convergence / divergence in this specification and the definition of power on each surface will be described with reference to FIG. FIG. 18 is a cross-sectional view of the state of the light beam passing through the diffractive optical element 311 made of a member having a refractive index n as viewed from the direction perpendicular to the optical axis. The state of being diffracted and refracted by the refracting surface S is shown.

  The convergence / divergence degree (incidence convergence / divergence degree) Φin incident on the diffractive optical element 311 represents the reciprocal of the distance s between the object point O and the point A where the optical axis intersects the diffraction surface T. That is, the degree of convergence / divergence of light rays incident at an angle u on the diffraction surface T that is a distance h from the intersection A with the optical axis of the diffraction surface T can be expressed as Equation 16.

  Further, the convergence / divergence degree (outgoing convergence / divergence degree) Φout emitted from the diffractive optical element 311 is the distance s ′ between the point O ′ where the light beam after passing through the diffractive optical element 311 and the optical axis intersect with the refractive surface S. Reciprocal of That is, the degree of convergence / divergence of the light beam emitted from the refracting surface S that is a distance h ′ away from the vertex B of the refracting surface S at an angle of u ′ can be expressed as Equation 17.

  In this specification, when the convergence / divergence degree is a negative value, divergent light is indicated, and when the degree of convergence / divergence is a positive value, the convergent light is indicated. In this specification, the degree of convergence / divergence may be expressed by φ × Φin or φ × Φout obtained by multiplying Φin or Φout by the effective diameter φ of the objective lens 312 with respect to the first light beam L1.

  Next, the powers ΦLb, ΦLr, and ΦLIr of the refracting surface S can be expressed as Equation 18. In the present specification, when the power is a negative value, the power of divergence is represented, and when the power is a positive value, the power of convergence is represented.

  Here, nb, nr, and nIr are the refractive indexes of the diffractive optical element 311 for the first, second, and third light beams, respectively, and R is the radius of curvature of the refractive surface S. Further, the power ΦHOE of the diffractive surface T can be obtained from an optical path difference function representing the shape of the diffractive surface T. The power of the refracting surface S and the power ΦHOE of the diffractive surface T have the same meaning as values representing the ability to refract incident light.

  When the first light beam L1 is incident on the objective lens 312 as parallel light and has no aberration with respect to the first optical disk 14a, the difference in the thickness of the light transmission layer with respect to the second and third optical disks 14b and 14c. The degree of convergence / divergence (degree of convergence / divergence incident on the objective lens) after passing through the diffractive optical element 311 of the second and third light beams necessary for correcting the generated spherical aberration is generally expressed by Equation 19 below. Any range is acceptable. In Expression 19, φ represents the effective diameter of the objective lens 312 with respect to the first light beam L1.

  The optical pickup 300 is designed to satisfy Formula 20 by using the diffractive optical element 311.

  Thereby, even when the second and third light beams having a relatively small absolute value of the divergence degree are incident on the objective lens unit 313, it is sufficient to correct the spherical aberration caused by the difference in the thickness of the light transmission layer. A light beam having a divergence degree can be incident on the objective lens 312. Thus, since the second and third light beams having a relatively small absolute value of the divergence degree can be made incident on the objective lens unit 313, the radial direction of the objective lens unit 313 accompanying the tracking or the like (the first, The influence of the shift in the direction substantially perpendicular to the optical axes of the second and third light beams (in the direction of arrow Z in FIG. 17) can be reduced.

  In order to satisfy Equation 20, the optical path difference function representing the shape of the diffractive surface, the diffraction orders used by the first, second, and third light beams, the first, second, and second light incident on the diffractive optical element 311 are used. It is determined from the degree of convergence / divergence of the three light beams, the radius of curvature of the refractive surface, and the refractive index of the diffractive optical element 311.

  In the optical pickup 300, it is preferable that Φoutb = 0. Accordingly, the first light beam L1 having a short wavelength with the highest accuracy of wavefront aberration can be incident on the objective lens 312 as parallel light. As a result, when the first light beam L1 is used, it is possible to suppress the occurrence of aberration due to the positional deviation between the diffractive optical element 311 and the objective lens 312.

  Next, among the light beams diffracted by the diffraction surface of the diffractive optical element 311, the diffraction order of the light beam to be used will be described. The diffraction efficiency of the blazed diffraction grating can be obtained by Equation 21.

  Here, m is the diffraction order, A (x) is the transmission amplitude distribution, φ (x) is the phase distribution, and T is the length of the period in the x-axis direction. In the following calculation, A (x) = 1 is standardized. FIG. 19 shows the result of concrete calculation of the diffraction efficiency when a diffraction grating is formed on a PC (polycarbonate) substrate using the mathematical formula (5).

  In FIG. 19, B0, B1, B2, R0, R1, R2, Ir0, Ir1, and Ir2 are the 0th order diffracted light of the first light beam L1, the 1st order diffracted light, the 2nd order diffracted light, and the 0th next time of the second light beam L2, respectively. The diffraction efficiencies of the folded light, the first-order diffracted light, the second-order diffracted light, and the zeroth-order diffracted light, the first-order diffracted light, and the second-order diffracted light of the third light beam are shown.

  In the optical pickup 300, the first-order diffracted light is used for the first light beam, and the first-order diffracted light is used for the second and third light beams. Thereby, the utilization efficiency of each of the first, second and third light beams can be increased.

  Specifically, in the case of PC as the material of the diffractive optical element, from FIG. 19, the first light beam L1 can be used almost 100% by setting the depth of the diffraction grating to about 1.3 μm. The second and third light beams can be used at 90% or more. As a result, it is possible to easily realize an optical pickup for recording and erasing information that requires a high output light beam. Moreover, since the output of a light source can be made small, power consumption can be suppressed. Furthermore, since unnecessary light other than the diffracted light is hardly generated, stray light can be prevented from entering a detector such as the reproduction signal detection optical system 15a during reproduction, and deterioration of the reproduction signal can be suppressed.

  Next, FIG. 20 shows the relationship between the incident convergence / divergence degree of the first, second, and third light beams to the diffractive optical element 311. FIG. 20 shows the incident convergence / divergence degrees (φ × Φinr and φ × ΦinIr) of the second and third light beams when the incident convergence / divergence degree (φ × Φinb) of the first light beam L1 is changed. It shows a change.

  Here, the incident light (emitted light from the diffractive optical element 311) of the first light beam L1 to the objective lens 312 is parallel light (φ × Φoutb = 0).

  The incident light (emitted light from the diffractive optical element 311) of the second light beam L2 to the objective lens 312 is divergent light φ × Φoutr = −0.1, and the incident light of the third light beam to the objective lens 312 is obtained. The (emitted light from the diffractive optical element 311) is divergent light φ × ΦoutIr = −0.2. The degree of divergence is the spherical aberration generated due to the difference in thickness of the light transmission layer with respect to the first optical disk 14a when the second and third light beams are focused on the second and third optical disks 14b and 14c, respectively. The above-described range of the general divergence degree necessary for correction is approximately the center value of Equation 19, and is the divergence degree that is most effective for correction.

  Here, the diffraction orders used on the diffractive surface of the diffractive optical element 311 are the second order diffracted light, the second and third light beams are the first order diffracted light, and the curvature of the refractive surface of the diffractive optical element 311 is used. The radius is 5 mm and the material of the diffractive optical element 311 is PC. And the incident convergence / divergence degree of the 1st light beam L1 is changed by changing the shape of the diffraction surface of the diffractive optical element 311. FIG.

  As can be seen from FIG. 20, the incident convergence / divergence degree of the first light beam L1 is substantially proportional to the incident convergence / divergence degrees of the second and third light beams. In addition, by making the incident convergence / divergence degree of the third light beam L3 to the diffractive optical element 311 negative, that is, by causing the third light beam L3 to be incident on the diffractive optical element 311 with diverging light, the first and second The absolute value of the degree of convergence / divergence of the light beam L2 on the diffractive optical element 311 can be kept relatively small. In particular, when the incident light convergence / divergence degree of the first light beam L1 is in a range of 0 or more, that is, when the first light beam L1 is incident on the diffractive optical element 311 with parallel light or convergent light, the second and third lights. The absolute value of the incident convergence / divergence of the beam becomes small.

  Therefore, for example, it is preferable to set so that the first and second light beams are incident on the diffractive optical element 311 as parallel light and divergent light, respectively. As a result, the first and second light beams L1 that are required to have the strictest condensing characteristics are incident on the diffractive optical element 311 as parallel light, and the divergence of the second and third light beams incident on the diffractive optical element 311 is achieved. The degree can be made relatively small. As a result, it is possible to more effectively suppress the deterioration of the condensing characteristic due to the shift of the objective lens unit 313 in the radial direction.

  Alternatively, it is preferable that the first light beam L1 is incident on the diffractive optical element 311 as convergent light and the second light beam L2 is incident on convergent light, parallel light, or divergent light. Thereby, the absolute value of the convergence / divergence degree of the first, second and third light beams incident on the diffractive optical element 311 can be made relatively small. As a result, it is possible to more effectively suppress the deterioration of the condensing characteristic due to the shift of the objective lens unit 313 in the radial direction.

  The result of concrete verification of this is shown in FIG. FIG. 21 shows the relationship between the degree of incident divergence of the first light beam L1 to the diffractive optical element 311 and the wavefront aberration when the shift amount of the objective lens unit 313 in the radial direction is 200 μm. .

  In the present optical pickup 300, as shown in FIG. 20, by setting the incident convergence / divergence degree of the first light beam L1 to the diffractive optical element 311 within the range of Expression 22, the second and third light beams are moved to the diffractive optical element 11. The incident convergence / divergence degree of these satisfies the expressions 23 and 24, respectively.

It is in the range. In this case, as shown in FIG. 21, the wavefront aberration can be suppressed to 0.04 λrms or less for all of the first, second, and third light beams.

  Moreover, in this case, as can be seen from FIG. As a result, the degree of convergence / divergence of the second and third light beams incident on the diffractive optical element 311 can be reduced, and the influence of the radial shift of the objective lens unit 313 accompanying tracking or the like can be reduced. .

  Here, the powers Φb, Φr, and ΦIr of the diffractive optical element 311 for the first, second, and third light beams are defined as in Expression 25. That is, the power of the diffractive optical element 311 is defined as the sum of the power of the diffractive surface (ΦHOE) and the power of the refracting surface (ΦL).

  From Equation 15 and Equation 25, Equation 26 holds.

  Therefore, from Formulas 22, 23, and 24 indicating the range of Φin, Φoutb = 0 indicating the range of Φout, and Formula 19, the power of the diffractive optical element 311 satisfies Formula 27 in this optical pickup 300, respectively. Will be.

Thereby, a favorable wavefront aberration can be obtained regardless of whether the objective lens unit 313 is shifted in the radial direction.

  The powers of the diffractive surface and the refracting surface of the diffractive optical element 311 can be set as appropriate as long as the power of the diffractive optical element 311 falls within the above range. It has a positive power (convergent diffractive surface), and the refractive surface has a negative power (concave surface). As a result, as will be described later, it is possible to reduce the increase in wavefront aberration that occurs when the wavelength of the light source is shifted.

  Here, by setting the incident convergence / divergence degree φ × Φinb = 0 of the first light beam L1 to the diffractive optical element 311 (the first light beam L1 is incident on the diffractive optical element 311 as parallel light), Even for the first light beam L1, whose focusing characteristics are more severe because of the short wavelength, it is possible to reduce the deterioration of the wavefront aberration due to the shift of the objective lens unit 313 in the radial direction. Further, when φ × Φoutb = 0 (the first light beam L1 is incident on the objective lens 312 as parallel light), the power of the diffractive optical element 311 of the first light beam L1 becomes Φb = 0. Is set as follows. Details of this configuration will be described in a sixth embodiment.

  As another embodiment, the incident divergence degree of the first light beam L1 to the diffractive optical element 311 is about φ × Φinb = 0.06 (the first light beam L1 is incident on the diffractive optical element 311 as convergent light). ) Is approximately φ × Φinr = 0 (approximately parallel light incidence), and approximately φ × ΦinIr = −0.15 (divergent light incidence) (see FIG. 20). In this case, the deterioration of the wavefront aberration due to the radial shift of the objective lens unit 313 can be reduced with respect to the second light beam L2 (see FIG. 21). Details of this configuration will be described in a seventh embodiment.

  In addition, when the first, second, and third light beams having a predetermined degree of convergence / divergence are incident on the diffractive optical element 311 as described above, the wavefront aberration is deteriorated even when the objective lens unit 313 is shifted in the radial direction. However, when diverging light is incident on the diffractive optical element 311, the aspherical lens is placed between the light source and the diffractive optical element 311 so as to reduce coma generated by the objective lens unit 313 shifting in the radial direction. A better wavefront aberration can be obtained by inserting it into.

(Example 6)
The embodiment of the present invention will be described below with reference to FIGS. 22 (a) (a) (c), FIG. 23 (a) (b) (c), and FIG.

  In the present embodiment, the radial shift of the objective lens unit 313 can be almost eliminated from the first light beam L1 whose condensing characteristics are more severe. As shown in c), the convergence / divergence degree φ × Φinb = 0 when the first light beam L1 is incident on the diffractive optical element 311. Here, an objective lens 312 having an effective diameter φ = 3 mm for the first light beam L1 is used.

  In this embodiment, the first light beam L1 is incident on the diffractive optical element with parallel light φ × Φinb = 0, and second-order diffracted light is used on the diffractive surface of the diffractive optical element 311 to obtain the second light beam L2. Is incident on the diffractive optical element with divergent light φ × Φinr = −0.048, uses the first-order diffracted light on the diffractive surface of the diffractive optical element 311, and further diverges φ with respect to the third light beam L 3. An optical pickup 300 that is incident on the diffractive optical element at xΦinIr = −0.18 and uses the first-order diffracted light on the diffractive surface of the diffractive optical element 311 is manufactured.

  The diffractive optical element 311 includes a concave surface 311c and a diffraction grating 311a, and is disposed on the light source side of the aspheric objective lens 312. The concave surface is spherical and has a radius of curvature of 5 mm.

  Here, the concave surface 311c can be easily formed by making the concave surface 311c spherical. Further, the shift characteristic in the radial direction of the objective lens unit 313 can be further improved by adopting the aspherical shape.

  In the optical pickup 300 of this embodiment, as shown in FIG. 22A, the first optical beam 14 is incident on the diffractive optical element 311 with parallel light φ × Φinb = 0 on the first optical disk 14a. The light beam diffracted in the diffraction grating 311a by the diffraction surface in the second-order diffraction direction (convergence direction with respect to the optical axis) is refracted in the divergence direction by the concave surface 311c, and is incident on the objective lens 312 as parallel light. Light is condensed on the first optical disk 14a having a light transmission layer thickness of 0.1 mm, and good light condensing characteristics are obtained.

  As shown in FIG. 22B, the second light beam L2 is incident on the diffractive optical element 311 with a divergent light beam φ × Φinr = −0.048 to the second optical disk 14b, and the diffraction surface of the diffraction grating 311a. The light beam diffracted in the first-order diffraction direction (convergence direction with respect to the optical axis) is refracted in the divergence direction by the concave surface 311c, so that a light beam having a predetermined divergence degree (in this embodiment φ × Φoutr = − 0.1), the light enters the objective lens 312 and a good light condensing characteristic can be obtained for the second optical disk 14b having a thick light transmission layer thickness of 0.6 mm.

  At this time, by arranging the diffractive optical element 311, a light beam having a divergence degree smaller than the divergence degree incident on the objective lens 312 can be incident on the objective lens unit 313, and the objective lens unit 313 is shifted in the radial direction. The influence on can be reduced.

  In the case of the third optical disk 14c, as shown in FIG. 22C, when the third light beam L3 is incident on the diffractive optical element 311 with a divergent light beam φ × ΦinIr = −0.18, the diffraction of the diffraction grating 311a is performed. The light beam diffracted in the first-order diffraction direction (convergence direction with respect to the optical axis) by the surface is refracted in the divergence direction by the concave surface 311c, so that a light beam having a predetermined divergence degree (in this embodiment φ × ΦoutIr = −0.2), the light enters the objective lens 312 and a good light condensing characteristic can be obtained with respect to the third optical disk 14c having a light transmission layer thickness of 1.2 mm.

  At this time, by arranging the diffractive optical element 311, a light beam having a divergence degree smaller than the divergence degree incident on the objective lens 312 can be incident on the objective lens unit 313, and the objective lens unit 313 is shifted in the radial direction. The influence on can be reduced.

  The objective lens unit 313 when the first, second, and third light beams are condensed on the first, second, and third optical disks 14a, 14b, and 14c, respectively, using the optical pickup 300 manufactured as the present embodiment. Changes in the wavefront aberration λrms on the image plane with respect to the shift amount in the radial direction (objective shift) are shown by solid lines in FIGS. 23 (a), 23 (b), and 23 (c). A case where an optical pickup (Comparative Example 1a) manufactured for comparison is used is indicated by broken lines in FIGS. 23 (a), 23 (b), and 23 (c). 23A shows the result of condensing the first light beam L1 on the first optical disk 14a, and FIG. 23B shows the result of condensing the second light beam L2 on the second optical disk 14b. FIG. 23C shows the result of focusing the third light beam L3 on the third optical disk 14c.

  In the comparative example 1a, the first light beam L1 is incident on the objective lens as parallel light, and the second and third light beams are irradiated with predetermined divergent light in order to correct spherical aberration due to the difference in the light transmission layer thickness. In order to prevent the shift characteristic of the objective lens from deteriorating in the optical path of the diverging light, it is made incident on the objective lens, and an aspherical lens is inserted so that the wavefront aberration is optimized.

  As can be seen from FIG. 23 (a), by using the optical pickup 300 of this embodiment, it is possible to form a good condensing spot on the first optical disc 14a.

  Further, as can be seen from FIG. 23 (b), by using the optical pickup 300 of this embodiment, it is possible to form a good condensing spot on the second optical disc 14b. Compared with Comparative Example 1a, there is a portion where the influence of the radial shift of the objective lens unit 313 generated by making the second light beam L2 incident on the diffractive optical element 311 as divergent light is large, but the wavefront The degree of aberration deterioration is kept small. Here, in order to suppress the influence of the shift in the radial direction of the objective lens unit 313, the lens surface between the light source and the diffractive optical element 311 is an aspherical surface.

  Further, as can be seen from FIG. 23 (c), by using the optical pickup 300 of this embodiment, it is possible to form a good condensing spot on the third optical disc 14c. In the optical pickup 300 of the present embodiment, the influence of the shift in the radial direction of the objective lens unit 313, which is generated by causing the third light beam L3 to enter the diffractive optical element 311 with diverging light, is compared with Comparative Example 1a. Can be reduced. Here, in order to suppress the influence of the shift in the radial direction of the objective lens unit 313, the lens surface between the light source and the diffractive optical element 311 is an aspherical surface.

  As described above, in the optical pickup 300 of the present embodiment, the first light beam L1 can be equivalent to the comparative example 1a, and the third light beam L3 can have better wavefront aberration than the comparative example 1a. Further, with respect to the second light beam L2, the wavefront aberration may be deteriorated compared to the comparative example 1a depending on the shift amount of the shift of the objective lens unit 313 in the radial direction, but the degree of deterioration can be suppressed to a small level.

  Next, FIG. 24 shows the change of the wavefront aberration λrms with respect to the shift of the wavelength of the first light beam L1 when the first light beam L1 is condensed on the first optical disk 14a using the optical pickup 300 of the present embodiment. Shown in solid line. In addition, a case where an optical pickup (Comparative Example 1b) dedicated to the first optical disk 14a manufactured for comparison is used is indicated by a broken line in FIG. In Comparative Example 1b, an objective lens unit is configured by a single objective lens 312 (objective lens 312 dedicated to the first light beam L1) in the optical pickup 300 of the present embodiment. Here, the value of the wavefront aberration at each wavelength is a value at the position of the best image point at the wavelength at which the wavefront aberration is minimized.

  As can be seen from FIG. 24, in the optical pickup 300 of this example, the usable wavelength range is wider than that of the comparative example 1b. This is because the diffractive optical element 311 of the optical pickup 300 of the present embodiment is composed of a converging diffraction grating and a plano-concave lens, which makes it possible to use an objective lens dedicated to the first light beam L1 alone. Can also improve the wavelength-dependent characteristics.

  In general, high NA objective lenses used in next-generation high-density optical discs use glass materials with a high refractive index, and therefore have a large wavelength dependence. On the other hand, the focal position is greatly deviated and it is difficult to form a good spot. However, if the optical pickup 300 of the present embodiment is used, it is possible to form a good condensing spot even if wavelength variation occurs due to mode hopping or the like.

  Further, as can be seen from FIG. 19, the diffractive surface of the diffractive optical element 311 in the optical pickup 300 of the present embodiment provides the second-order diffracted light for the first light beam L1, and for the second and third light beams. Since the first-order diffracted light is used, it is possible to set the diffraction grating depth at which light can be used with an efficiency of 90% or more for all of the first, second, and third light beams. Therefore, it is possible to easily realize an optical pickup that records and erases information that requires a high-power light beam. Moreover, since the output of a light source can be made small, power consumption can be suppressed. Furthermore, since unnecessary light other than the diffracted light can be prevented from entering the detector, signal deterioration can be suppressed.

(Example 7)
An embodiment of the present invention will be described below with reference to FIGS. 25 (a), (b), (c), FIGS. 26 (a), (b), (c), and FIGS.

  In this embodiment, as shown in FIGS. 25A, 25B and 25C, the degree of convergence when the first light beam L1 is incident on the diffractive optical element 11 is φ × Φinb = 0.06. . As in Example 6, the objective lens 12 having the effective diameter φ = 3 mm of the first light beam L1 was used.

  In this embodiment, the first light beam L1L1 is incident on the diffractive optical element 311 with convergent light φ × Φinb = 0.06, and the second-order diffracted light is used on the diffractive surface of the diffractive optical element 311. The light beam L2 is incident on the diffractive optical element 311 with substantially parallel light φ × Φinr = 0, and the first-order diffracted light is used on the diffractive surface of the diffractive optical element 311. Further, for the third light beam L3, The optical pickup 3 was made incident on the diffractive optical element 311 with a divergence φ × ΦinIr = −0.15 and using the first-order diffracted light on the diffractive surface of the diffractive optical element 311.

  The diffractive optical element 311 includes a concave surface 311c and a diffraction grating 311a. The diffractive optical element 311 is disposed on the light source side of the aspheric objective lens 312. The concave surface 311c has a spherical shape and a radius of curvature of 5 mm.

  Here, the concave surface 311c has a spherical shape that is easy to manufacture. However, in order to further improve the shift characteristic in the radial direction of the objective lens unit 313, an aspheric shape may be used.

  In the optical pickup 300 of the present embodiment, as shown in FIG. 25A, the first light beam L1L1 is applied to the diffractive optical element 311 with the convergent light φ × Φinb = 0.06 for the first optical disc 14a. The light beam incident and diffracted by the diffraction surface of the diffraction grating 311a in the second-order diffraction direction (convergence direction with respect to the optical axis) is refracted in the divergence direction by the concave surface 311c. Incident light is collected on the first optical disk 14a having a light transmission layer thickness of 0.1 mm, and good light collecting characteristics can be obtained.

  As shown in FIG. 25B, the second light beam L2 is incident on the diffractive optical element 311 with substantially parallel light φ × Φinr = 0 and is 1 on the diffraction surface of the diffraction grating 311a. The light beam diffracted in the next diffraction direction (convergence direction with respect to the optical axis) is refracted in the divergence direction by the concave surface 311c, so that a light beam having a predetermined divergence degree (φ × Φoutr = −0. In 1), the light is incident on the objective lens 312 and good condensing characteristics can be obtained with respect to the thick second optical disk 14b having a light transmission layer thickness of 0.6 mm.

  At this time, by disposing the diffractive optical element 311, divergent light can be incident on the objective lens 312, and parallel light can be incident on the objective lens unit 313, so that the objective lens unit 313 can be prevented from shifting in the radial direction. The influence can be reduced.

  In the case of the third optical disk 14c, as shown in FIG. 25C, when the third light beam L3 is incident on the diffractive optical element 311 with a divergent light beam φ × ΦinIr = −0.15, the diffraction of the diffraction grating 311a is performed. The light beam diffracted in the first-order diffraction direction (convergence direction with respect to the optical axis) by the surface is refracted in the divergence direction by the concave surface 311c, so that a light beam having a predetermined divergence degree (in this embodiment φ × ΦoutIr = −0.2), the light enters the objective lens 312 and a good light condensing characteristic can be obtained with respect to the third optical disk 14c having a light transmission layer thickness of 1.2 mm.

  At this time, by arranging the diffractive optical element 311, a light beam having a divergence degree smaller than the divergence degree incident on the objective lens 312 can be incident on the objective lens unit 313, and the objective lens unit 313 is shifted in the radial direction. The influence on can be reduced.

  The objective lens unit 313 when the first, second, and third light beams are condensed on the first, second, and third optical disks 14a, 14b, and 14c, respectively, using the optical pickup 300 manufactured as the present embodiment. Changes in the wavefront aberration λrms on the image plane with respect to the shift amount in the radial direction (objective shift) are indicated by solid lines in FIGS. In addition, the case where an optical pickup (Comparative Example 2a) manufactured for comparison is used is indicated by broken lines in FIGS. 26 (a), (b), and (c). FIG. 26A shows the result of focusing the first light beam L1 on the first optical disk 14a, and FIG. 26B shows the result of focusing the second light beam L2 on the second optical disk 14b. FIG. 26C shows the result of focusing the third light beam L3 on the third optical disk 14c.

  In Comparative Example 2a, the first light beam L1 is incident on the objective lens as parallel light, and the second and third light beams are irradiated with predetermined divergent light in order to correct spherical aberration due to the difference in the light transmission layer thickness. In order to prevent the shift characteristic of the objective lens from deteriorating in the optical path of the diverging light, it is made incident on the objective lens, and an aspherical lens is inserted so that the wavefront aberration is optimized. The comparative example 2a is the same as the comparative example 1a.

  As can be seen from FIG. 26 (a), by using the optical pickup 300 of this embodiment, the influence of the shift of the objective lens unit 313 in the radial direction is suppressed, and a good condensing spot is formed on the first optical disc 14a. Can do.

  In addition, as can be seen from FIG. 26B, since the second light beam L2 is incident on the diffractive optical element 11 as substantially parallel light by using the optical pickup 300 of this embodiment, the radial direction of the objective lens unit 313 is obtained. The influence of the shift can be reduced as compared with Comparative Example 2a.

  Further, as can be seen from FIG. 26C, by using the optical pickup 300 of this embodiment, the objective lens unit 313 is generated by making the third light beam L3 incident on the diffractive optical element 311 with diverging light. The influence due to the shift in the radial direction can be reduced as compared with Comparative Example 2a. Here, in order to suppress the influence of the shift in the radial direction of the objective lens unit 313, the lens surface between the light source and the diffractive optical element 311 is an aspherical surface.

  Thus, in the optical pickup 300 of the present embodiment, the wavefront aberration for the first, second, and third optical disks 14a, 14b, and 14c can be reduced as compared with the comparative example 2a.

  Next, FIG. 27 shows changes in the wavefront aberration λrms with respect to the wavelength shift of the first light beam L1 when the first light beam L1 is condensed on the first optical disk 14a using the optical pickup 300 of the present embodiment. Shown in solid line. In addition, a case where an optical pickup dedicated for the first optical disk 14a manufactured for comparison (same as Comparative Example 2b and Comparative Example 1b) is used is indicated by a broken line in FIG. In Comparative Example 2b, an objective lens unit is configured by a single objective lens 312 (objective lens 312 dedicated to the first light beam L1) in the optical pickup 300 of the present embodiment. Here, the value of the wavefront aberration at each wavelength is a value at the position of the best image point at the wavelength at which the wavefront aberration is minimized.

  As can be seen from FIG. 27, in the optical pickup 300 of this embodiment, the usable wavelength range is wider than that of the comparative example 2b. This is because the diffractive optical element 311 of the optical pickup 300 of the present embodiment is composed of a converging diffraction grating 311a and a plano-concave lens 311b, thereby using a single objective lens dedicated to the first light beam L1. The wavelength dependence characteristics can be improved as compared with the case. Further, even in the optical pickup 300 of the present embodiment, a good condensing spot can be formed even if a wavelength variation occurs due to a mode hop or the like.

  Further, as can be seen from FIG. 19, the diffractive surface of the diffractive optical element 311 in the optical pickup 300 of the present embodiment is the second-order diffracted light for the first light beam L1, and 1 for the second and third light beams. Since the next-order diffracted light is used, the depth of the diffraction grating that enables the use of light with an efficiency of 90% or more can be set for all of the first, second, and third light beams. Therefore, it is possible to easily realize an optical pickup that records and erases information that requires a high-power light beam. Moreover, since the output of a light source can be made small, power consumption can be suppressed. Furthermore, since unnecessary light other than the diffracted light can be prevented from entering the detector, signal deterioration can be suppressed.

  Here, the power of the refractive surface of the diffractive optical element 311 with respect to the first light beam L1 when the first light beam L1 is incident on the diffractive optical element 311 with a degree of convergence / divergence of φ × Φinb = 0.06, FIG. 28 shows the relationship with the minimum grating pitch on the diffraction surface. The case where the refracting surface is concave is indicated by a solid line, and the case where the refracting surface is convex is indicated by a broken line. As shown in FIG. 28, the minimum pitch of the diffractive surface can be increased by making the refracting surface concave and reducing the absolute value of the power of the refracting surface. When the initial pitch of the diffractive surface is increased, the diffractive optical element 311 can be easily created. In addition, it is possible to reduce the aberration caused by the eccentricity between the diffractive surface and the refracting surface.

  The power of the refractive surface of the diffractive optical element 311 with respect to the first light beam L1 when the first light beam L1 is incident on the diffractive optical element with a convergence / divergence of φ × Φinb = 0.06, and the objective lens FIG. 29 shows the relationship with the wavefront aberration when the shift amount of the radial shift (objective shift) of the unit 313 is 200 μm. As can be seen from FIG. 29, when the power of the refracting surface of the diffractive optical element 311 is less than −0.1, the wavefront aberration rapidly deteriorates. For this reason, the power of the refracting surface is preferably set to −0.1 or more.

(Example 8)
The embodiment of the present invention will be described below with reference to FIGS. 30A, 30B, 31C, 31A, 31B, and 32C.

  In this embodiment, as shown in FIGS. 30A, 30B, and 30C, the first light beam L1 is incident on the diffractive optical element 311 as parallel light.

  The optical pickup 300 according to the embodiment has the first light beam so that the influence of the shift in the radial direction of the objective lens unit 313 can be almost eliminated with respect to the first light beam L1 whose focusing characteristics are more severe. L1 is configured to enter the diffractive optical element 311 with parallel light. That is, the convergence / divergence degree φ × Φinb = 0 when the first light beam L1 enters the diffractive optical element 311. Here, an objective lens 312 having an effective diameter φ = 3 mm for the first light beam L1 is used.

  The first light beam L1 is incident on the diffractive optical element 311 as parallel light, and the first-order diffracted light is used on the diffractive surface of the diffractive optical element 311. The second light beam L2 is parallel light. Is incident on the diffractive optical element 311, and the 0th-order diffracted light is used on the diffractive surface of the diffractive optical element 311. An optical pickup 300 using zero-order diffracted light on the diffractive surface was prepared.

  The diffractive optical element 311 includes a concave surface 311c (refractive surface V) and a diffraction grating 311a, and is disposed on the light source side of the objective lens 312. The concave surface 311c has an aspherical shape.

  Here, by making the concave surface 311c an aspherical shape, the spherical aberration caused by the difference in the thickness of the light transmission layer is further corrected with respect to the second and third optical disks 14b and 14c, and the objective lens unit 313 is used. The shift characteristic in the radial direction can be further improved, and a better light collection characteristic can be obtained.

  In the optical pickup 300, as shown in FIG. 30A, the first light beam L1 is incident on the diffractive optical element 11 as parallel light to the first optical disk 14a, and is reflected on the diffraction surface of the diffraction grating 311a for the first time. The light beam diffracted in the folding direction (convergence direction with respect to the optical axis) is refracted in the divergence direction by the concave surface 311c, so that it enters the objective lens 312 with parallel light and has a light transmission thickness of 0.1 mm. Light is condensed on the optical disk 14a, and good light condensing characteristics are obtained.

  For the second optical disc 14b, as shown in FIG. 30B, when the second light beam L2 enters the diffractive optical element 311 as substantially parallel light, the second light that is not diffracted by the diffraction surface of the diffraction grating 311a. The beam L2 is refracted in the divergence direction by the concave surface 311c, so that it enters the objective lens 312 with a light beam having a predetermined divergence degree (φ × Φoutr = −0.03 in this embodiment), and has a light transmission thickness of 0.6 mm. Good condensing characteristics can be obtained for the thick second optical disk 14b. Further, by making the shape of the concave surface 311c an aspherical surface, the remaining spherical aberration can be further corrected, and the deterioration of the shift characteristic of the objective lens unit 313 can be suppressed, and a better condensing characteristic can be obtained. .

  At this time, by arranging the diffractive optical element 311, even if the second light beam L2 is incident on the objective lens unit 313 as substantially parallel light, the light beam is incident on the objective lens 312 with a predetermined degree of divergence. Is possible. Therefore, the influence on the shift in the radial direction of the objective lens unit 313 can be reduced.

  In the case of the third optical disk 14c, when the third light beam L3 is incident on the diffractive optical element 311 as a divergent light beam, the light beam that is not diffracted by the diffraction surface of the diffraction grating 311a is refracted in the divergence direction by the concave surface 311c. A light beam having a predetermined divergence degree (in this embodiment, φ × ΦoutIr = −0.07) is incident on the objective lens 312 and has a good light condensing characteristic with respect to a thick third optical disk 14c having a light transmission thickness of 1.2 mm. Can get. Further, by making the shape of the concave surface 311c an aspherical surface, the remaining spherical aberration can be further corrected, and the deterioration of the shift characteristic of the objective lens unit 313 can be suppressed, and a better condensing characteristic can be obtained. .

  At this time, by arranging the diffractive optical element 311, a light beam having a divergence degree smaller than the divergence degree incident on the objective lens 312 can be incident on the objective lens unit 313, and the objective lens unit 313 is shifted in the radial direction. The influence on can be reduced.

  As described above, the diffractive surface of the diffractive optical element 311 in the optical pickup 300 according to the embodiment uses the first-order diffracted light for the first light beam L1 and the zero-order diffracted light for the second and third light beams. Therefore, an optical pickup for recording and erasing information that requires a high-output light beam can be easily realized. Moreover, since the output of a light source can be made small, power consumption can be suppressed.

  Further, in this embodiment, the diffractive optical element 311 in which the diffraction grating 311a having a converging diffraction surface and the concave surface 311c are arranged, so that only weak divergent light is incident on the objective lens unit 313, and the objective lens 12 is irradiated. The light beam can be incident with a predetermined degree of divergence. Therefore, the influence on the shift of the objective lens unit 313 in the radial direction can be reduced, and the semiconductor lasers 1a, 1b, and 1c can be disposed far from the objective lens unit 313. The semiconductor lasers 1a, 1b, 1c can be easily arranged. Moreover, since the diffractive optical element 311 has a converging diffractive surface and a concave refracting surface, the usable wavelength range is wider than that of the optical pickup dedicated to the first optical disc 14a. The wavelength dependence characteristics can be improved as compared with the case where the objective lens dedicated to the optical disk 14a is used alone. For this reason, according to said structure, even if the wavelength fluctuation by a mode pop etc. arises, a favorable condensing characteristic can be maintained. Moreover, according to said structure, the minimum pitch of the diffraction surface of the diffractive optical element 311 can be expanded, and preparation of the diffractive optical element 311 can be made easy.

  The objective lens unit 313 in the case where the first, second, and third light beams are condensed on the first, second, and third optical disks 14a, 14b, and 14c, respectively, using the optical pickup 300 manufactured in the present embodiment. Changes in the wavefront aberration λrms on the image plane with respect to the shift amount in the radial direction (objective shift) are shown by solid lines a in FIGS. Also, the shift amount of the shift in the radial direction (objective shift) of the objective lens unit 313 when the same operation is performed using an optical pickup manufactured for comparison (hereinafter referred to as a comparative optical pickup (I)). The change of the wavefront aberration λrms on the image plane with respect to is shown by broken lines b in FIGS. FIG. 31A shows the result of focusing the first light beam L1 on the first optical disk 14a, and FIG. 31B shows the result of focusing the second light beam L2 on the second optical disk 14b. FIG. 31C shows the result of focusing the third light beam L3 on the third optical disk 14c. In this specification, wavefront aberration is used as a term representing the amount of aberration as described above.

  The comparative optical pickup causes the first light beam L1 to be incident on the objective lens as parallel light, and the second and third light beams have a predetermined value for correcting spherical aberration due to the difference in the light transmission layer thickness. A divergent light is incident on the objective lens, and an aspherical lens is inserted in the optical path of the divergent light to prevent the shift characteristic of the objective lens from deteriorating, so that the wavefront aberration is optimized.

  As can be seen from FIG. 31A, by using the optical pickup 300 according to the embodiment, the influence of the shift of the objective lens unit 313 in the radial direction is suppressed, and a good condensing spot is formed on the first optical disc 14a. be able to.

  Further, as can be seen from FIG. 31 (b), by using the optical pickup 300 according to the embodiment, the second light beam L2 is incident on the diffractive optical element 311 as substantially parallel light. A light spot can be formed. In particular, in the present embodiment, the influence of the radial shift of the objective lens unit 313 generated by making the second light beam L2 incident on the diffractive optical element 311 as divergent light is suppressed as compared with the comparative optical pickup. It has been.

  Further, as can be seen from FIG. 31 (c), by using the optical pickup 300 according to the embodiment, a good condensing spot can be formed on the third optical disc 14c. According to the optical pickup 300 of the embodiment, the influence of the shift of the objective lens unit 313, which is generated by causing the third light beam L3 to enter the diffractive optical element 311 with divergent light, is reduced as compared with the comparative optical pickup. can do.

  As described above, in the optical pickup 300 of the embodiment, the first light beam L1 is equivalent to the comparative optical pickup, and the second and third optical disks 14b and 14c can obtain better wavefront aberration than the conventional optical pickup. .

  Next, FIG. 32 shows the change of the wavefront aberration λrms with respect to the change (shift) of the wavelength of the first light beam L1 when the first light beam L1 is condensed on the first optical disk 14a using the optical pickup 300 of the embodiment. Is indicated by a solid line c. In addition, a change in wavefront aberration λrms with respect to a change (shift) in the wavelength of the first light beam L1 in the case of using an optical pickup dedicated for the first optical disk 14a manufactured for comparison is shown by a broken line d in FIG. The comparative optical pickup is an objective lens unit that is composed of a single objective lens 312 (the objective lens 312 dedicated to the first light beam L1) in the optical pickup 300 according to the embodiment. Here, the value of the wavefront aberration at each wavelength is a value at the position of the best image point at the wavelength at which the wavefront aberration is minimized.

  As can be seen from FIG. 32, the optical pickup 300 according to the example has a wider usable wavelength range than the comparative optical pickup. Further, in the optical pickup 300 according to the embodiment, the deterioration of the aberration with respect to the wavelength variation that cannot be followed by the actuator such as the mode hop is less than that of the optical pickup dedicated to the first optical disk 14a. This is because the diffractive optical element 311 of the optical pickup 300 according to the example is configured by a converging diffraction grating 311a and a plano-concave lens 311b, and thereby an objective lens dedicated to the first light beam L1 is used alone. The wavelength dependence characteristics can be improved as compared with the case of using.

  In general, high NA objective lenses used in next-generation high-density optical discs use glass materials with a high refractive index, and therefore have a large wavelength dependence. On the other hand, the focal position is greatly deviated and it is difficult to form a good spot. However, if the optical pickup 3 according to the embodiment is used, a good condensing spot can be formed even if the wavelength fluctuates due to a mode hop or the like.

Example 9
An embodiment of the present invention will be described below with reference to FIGS. 33 (a) (b) (c) and FIGS. 34 (a) (b) (c).

  In this embodiment, as shown in FIGS. 33A, 33B, and 33C, the first light beam L1 is incident on the diffractive optical element 311 as parallel light.

  In the optical pickup 300 according to the present embodiment, the first light beam L1 whose focusing characteristics are more severe can be substantially free from the influence of the shift of the objective lens unit 313 in the radial direction. The beam L1 is configured to enter the diffractive optical element 311 as parallel light. That is, the optical pickup 300 according to the present embodiment also has a configuration in which the convergence / divergence degree φ × Φinb = 0 when the first light beam L1 enters the diffractive optical element 311 as in the eighth embodiment. Here, as in Example 8, the objective lens 12 having an effective diameter φ = 3 mm for the first light beam L1 was used.

  In the present embodiment, the first light beam L1 is incident on the diffractive optical element 311 as parallel light, uses the first-order diffracted light on the diffractive surface of the diffractive optical element 311, and is applied to the second light beam L2. Enters the diffractive optical element 311 as parallel light, uses the first-order diffracted light at the diffractive surface of the diffractive optical element 311, and enters the diffractive optical element 311 as a divergent light beam with respect to the third light beam L 3. An optical pickup 300 using first-order diffracted light on the diffraction surface of the element 311 was produced.

  The diffractive optical element 311 includes a convex surface 311d (refractive surface V) and a diffraction grating 311a, and is disposed on the light source side of the objective lens 312. The convex surface 311d has an aspherical shape.

  Here, the shift characteristics in the radial direction of the objective lens unit 313 can be further improved by making the convex surface 311d an aspherical shape.

  In the optical pickup 300 of the present embodiment, as shown in FIG. 33A, the first optical beam 14 enters the diffractive optical element 311 as parallel light with respect to the first optical disc 14a, and the diffraction grating 311a The light beam diffracted in the first-order diffraction direction (divergence direction with respect to the optical axis) by the diffraction surface is refracted in the convergence direction by the convex surface 311d, so that it enters the objective lens 312 with parallel light and has a light transmission thickness of 0. Condensed on the 1 mm first optical disk 14a, and good condensing characteristics can be obtained.

  For the second optical disk 14b, as shown in FIG. 33B, when the second light beam L2 is incident on the diffractive optical element 311 as substantially parallel light, the first-order diffraction direction ( The light beam diffracted in the direction of divergence with respect to the optical axis is refracted in the convergence direction by the convex surface 311d, so that a light beam having a predetermined divergence degree (φ × Φoutr = −0.03 in the ninth embodiment) is used. Good condensing characteristics can be obtained with respect to the second optical disc 14b which is incident on the lens 312 and has a light transmission thickness of 0.6 mm. Further, by making the shape of the convex surface 311d an aspherical surface, the remaining spherical aberration can be further corrected, and the deterioration of the shift characteristic of the objective lens unit 313 can be suppressed, and a better condensing characteristic can be obtained. .

  At this time, by disposing the diffractive optical element 311, even if the second light beam L 2 is incident on the objective lens unit 313 as substantially parallel light, the light beam is incident on the objective lens 312 with a predetermined issuance degree. Is possible. Therefore, the influence on the shift in the radial direction of the objective lens unit 313 can be reduced.

  In the case of the third optical disk 14c, as shown in FIG. 33C, when the third light beam L3 is incident on the diffractive optical element 311 as a divergent light beam, the first diffraction direction (light) on the diffraction surface of the diffraction grating 311a. The light beam diffracted in the direction of divergence with respect to the axis is refracted in the convergence direction by the convex surface 311d, so that the objective lens is a light beam having a predetermined divergence degree (φ × ΦoutIr = −0.07 in the second embodiment). A good light condensing characteristic can be obtained with respect to the third optical disk 14c which is incident on the light 312 and has a light transmission thickness of 1.2 mm.

  Further, by making the shape of the convex surface 311d an aspherical surface, the remaining spherical aberration can be further corrected, and the deterioration of the shift characteristic of the objective lens unit 313 can be suppressed, and a better condensing characteristic can be obtained. .

  At this time, by arranging the diffractive optical element 311, a light beam having a divergence degree smaller than the divergence degree incident on the objective lens 312 can be incident on the objective lens unit 313, and the objective lens unit 313 is shifted in the radial direction. The influence on can be reduced.

  As described above, the diffractive surface of the diffractive optical element 311 in the optical pickup 300 according to the present embodiment uses the first-order diffracted light for each of the first, second, and third light beams, so that high-power light can be obtained. An optical pickup for recording and erasing information that requires a beam can be easily realized. Moreover, since the output of a light source can be made small, power consumption can be suppressed.

  Further, in this embodiment, the diffractive optical element 311 in which the diffraction grating 311a having a diffractive diffraction surface and the convex surface 311d are combined is arranged so that weak divergent light is incident on the objective lens unit 313, and the objective lens 312 is incident. The light beam can be incident with a predetermined degree of divergence. Therefore, the influence on the shift of the objective lens unit 313 in the radial direction can be reduced, and the semiconductor lasers 1a, 1b, and 1c can be disposed far from the objective lens unit 313. The semiconductor lasers 1a, 1b, 1c can be easily arranged.

  The objective lens unit 313 in the case where the first, second, and third light beams are condensed on the first, second, and third optical disks 14a, 14b, and 14c, respectively, using the optical pickup 300 manufactured in the present embodiment. Changes in the wavefront aberration λrms on the image plane with respect to the shift amount in the radial direction are indicated by solid lines e in FIGS. Further, instead of the optical pickup 300, a conventional optical pickup prepared for comparison (shift in the radial direction of the objective lens unit 313 when the same operation is performed using the comparative optical pickup (objective shift)). 34 (a) to 34 (c) shows a change of the wavefront aberration λrms with respect to the shift amount by a broken line f, where Fig. 34 (a) condenses the first light beam L1 on the first optical disk 14a. FIG. 34 (b) shows the result of focusing the second light beam L2 on the second optical disk 14b, and FIG. 34 (c) shows the result of focusing the third light beam L3 on the third optical disk 14c. It is.

  As described above, the optical pickup for comparison makes the first light beam L1 incident on the objective lens as parallel light, and corrects the spherical aberration due to the difference in the thickness of the light transmission layer for the second and third light beams. In order to prevent the deterioration of the shift characteristics of the objective lens in the optical path of the diverging light, an aspherical lens is inserted and the wavefront aberration is optimized. It is.

  As can be seen from FIG. 34 (a), by using the optical pickup 300 according to the embodiment, the influence of the shift of the objective lens unit 313 in the radial direction is suppressed, and a good condensing spot is formed on the first optical disc 14a. be able to.

  Further, as can be seen from FIG. 34 (b), by using the optical pickup 300 according to the embodiment, the second light beam L2 is incident on the diffractive optical element 311 as substantially parallel light, so that the second optical disk has a good concentration. A light spot can be formed. In particular, in the present embodiment, the influence of the radial shift of the objective lens unit 313 generated by making the second light beam L2 incident on the diffractive optical element 311 as divergent light is suppressed as compared with the comparative optical pickup. It has been.

  Also, as can be seen from FIG. 34C, by using the optical pickup 300 according to the embodiment, the objective lens unit 313 is generated by making the third light beam L3 incident on the diffractive optical element 311 with diverging light. The influence of the shift in the radial direction can be reduced as compared with the comparative optical pickup.

  Thus, in the optical pickup 300 according to the embodiment, the wavefront aberration with respect to the second and third optical disks 14b and 14c can be reduced as compared with the comparative optical pickup.

  In each of the embodiments described above, the first light beam is the first-order diffracted light, and the second and third light beams are both the 0th-order diffracted light or the first-order diffracted light. The light beam may be diffracted light having the same or lower diffraction order as the first light beam L1, and the second and third light beams may not have the same diffraction order. .

  For example, in the optical pickup 300, the first and second light beams L2 may be set to use first-order diffracted light, and the third light beam L3 may be set to use zero-order diffracted light. Thereby, the utilization efficiency of each of the first, second and third light beams can be increased.

[Embodiment 4]
The schematic configuration of the optical pickup according to the present embodiment is substantially the same as that of the optical pickup shown in FIG. Therefore, the detailed description of the optical pickup is omitted except for the configuration different from that of the third embodiment. In the present embodiment, as in the third embodiment, a next-generation high-density optical disc (first optical disc 14a, first recording medium), a conventional DVD (second optical disc 14b, second recording medium), and a conventional CD ( An optical pickup that can be applied to the third optical disk 14c and the third recording medium will be described.

  The optical pickup according to this embodiment includes an objective lens unit 413 as shown in FIGS. 37A, 37B, and 37C instead of the objective lens unit 312 of the optical pickup 300 of the third embodiment. Yes.

  In the present embodiment, the first optical disk 14a has a light (first light beam L1) having a short wavelength of blue light having a wavelength (first wavelength λ1) of about 405 nm and a light transmission layer thickness t1 of 0.6 mm. In the second optical disk 14b, the light (second light beam L2) to be used has a long wavelength red light near the wavelength (second wavelength λ2) of 650 nm, and the thickness t2 of the light transmission layer is 0.6 mm. In the third optical disk 14c, the light (third light beam L3) to be used has a long wavelength infrared light near the wavelength (third wavelength λ3) of 780 nm, and the thickness t3 of the light transmission layer is 1.2 mm. It is. Here, the objective lens is a lens optimized for the first optical disk.

  In the present embodiment, the wavelength selective aperture filter 410 works so that the numerical aperture is NA1 (specifically 0.65) for the first light beam L1 having the wavelength of the first wavelength λ1, For the second light beam L2 having the second wavelength λ2, the numerical aperture is NA2 (specifically 0.6), and for the third light beam L3 having the third wavelength λ3. Controls the aperture by working so that the numerical aperture is NA3 (specifically, 0.45).

  Examples of the optical pickup according to the present embodiment will be described below.

(Example 10)
In the present embodiment, as shown in FIGS. 37A, 37B, and 37C, the first light beam L1 is incident on the diffractive optical element 411 with focused light.

  In this embodiment, the first light beam L1 is incident on the diffractive optical element 411 as convergent light, the second-order diffracted light is used on the diffractive surface of the diffractive optical element 411, and the second light beam L2 is applied. The light enters the diffractive optical element 411 with convergent or divergent light, uses the first-order diffracted light at the diffractive surface of the diffractive optical element 411, and further enters the diffractive optical element 411 with divergent light for the third light beam. An optical pickup using first-order diffracted light on the diffractive surface of the optical element 411 was produced.

  The diffractive optical element 411 includes a concave surface 411c and a diffraction grating 411a, is disposed on the light source side of the objective lens 412, and the concave surface has a spherical shape.

  Here, the concave surface 411c has a spherical shape that is easy to manufacture. However, in order to further improve the radial shift characteristics of the objective lens unit 413, an aspheric shape may be used.

  In the optical pickup of this embodiment, as shown in FIG. 37A, the first optical beam 14a is incident on the diffractive optical element 411 with convergent light and is diffracted by the diffraction grating 411a. The light beam diffracted in the second-order diffraction direction (convergence direction with respect to the optical axis) by the surface is refracted in the divergence direction by the concave surface 411c, so that it enters the objective lens 412 as parallel light, and the light transmission layer thickness is 0. The light is condensed on the first optical disk 14a of 6 mm, and good light condensing characteristics can be obtained.

  For the second optical disk 14b, as shown in FIG. 37 (b), the second light beam L2 is incident on the diffractive optical element 411 with convergent or divergent light, and the first diffraction direction (on the diffraction surface of the diffraction grating 411a ( The light beam diffracted in the direction of convergence with respect to the optical axis is refracted in the divergence direction by the concave surface 411c, so that the light beam having a predetermined divergence degree is incident on the objective lens 412 and uses different light transmission layers. Good condensing characteristics can be obtained for the second optical disk 14b of 0.6 mm. Here, the predetermined degree of divergence is the degree of divergence of the light beam incident on the objective lens necessary for correcting the spherical aberration due to the difference in wavelength used.

  In the case of the third optical disk 14c, as shown in FIG. 37C, when the third light beam L3 is incident on the diffractive optical element 11 as a divergent light beam, the first diffraction direction (light) on the diffraction surface of the diffraction grating 411a. The light beam diffracted in the direction of convergence with respect to the axis is refracted in the divergence direction by the concave surface 411c, so that it enters the objective lens 412 with a light beam having a predetermined divergence degree, and the light transmission layer thickness is as thick as 1.2 mm. Good condensing characteristics can be obtained for the third optical disk 14c. Here, the predetermined degree of divergence is the degree of divergence of the light beam incident on the objective lens necessary for correcting the spherical aberration due to the difference in the wavelength used and the light transmission layer thickness of the optical disk.

  At this time, by arranging the diffractive optical element 411, a light beam having a divergence degree smaller than the divergence degree incident on the objective lens 412 can be incident on the objective lens unit 413, and the radial shift of the objective lens unit 413 is achieved. The influence on can be reduced.

  When the first, second, and third light beams are condensed on the first, second, and third optical disks 14a, 14b, and 14c, respectively, using the optical pickup manufactured as the present embodiment, the objective lens unit 413 The change of the wavefront aberration λrms on the image plane with respect to the shift amount in the radial direction is indicated by a solid line A in FIGS. 38 (a), (b) and (c). A case where an optical pickup (comparative example) produced for comparison is used is indicated by a broken line B in FIGS. 38 (a), (b) and (c). FIG. 38A shows the result of focusing the first light beam L1 on the first optical disk 14a, and FIG. 38B shows the result of focusing the second light beam L2 on the second optical disk 14b. FIG. 38C shows the result of focusing the third light beam L3 on the third optical disk 14c.

  In the comparative example, the first light beam L1 is incident on the objective lens as parallel light without using a diffractive optical element, and the second and third light beams are different in wavelength of light to be used and the thickness of the light transmission layer. This is a wavefront aberration in the case where predetermined divergent light necessary for correcting chromatic aberration and spherical aberration due to the difference between these is made incident on the objective lens.

  As can be seen from FIG. 38 (a), by using the optical pickup of this embodiment, it is possible to suppress the influence of the shift of the objective lens unit 413 in the radial direction and to form a good condensing spot on the first optical disc 14a. it can.

  Further, as can be seen from FIG. 38 (b), by using the optical pickup of this embodiment, it is possible to form a good condensing spot on the second optical disc 14b. Compared with the comparative example, there is a portion in which the influence of the shift in the radial direction of the objective lens unit 413, which is generated by making the second light beam L2 incident on the diffractive optical element 411 as divergent light, is large, but the wavefront aberration The degree of degradation of is kept small.

  Further, as can be seen from FIG. 38 (c), by using the optical pickup of the present embodiment, the radial of the objective lens unit 413 generated by making the third light beam L3 incident on the diffractive optical element 411 as divergent light. The influence of the direction shift can be reduced as compared with the comparative example.

  Thus, in the optical pickup of the present embodiment, the wavefront aberration for the first, second, and third optical disks 14a, 14b, and 14c can be reduced as compared with the comparative example.

  Next, the change of the wavefront aberration λrms with respect to the shift of the wavelength of the first light beam L1 when the first light beam L1 is condensed on the first optical disk 14a using the optical pickup of the present embodiment is shown by a solid line in FIG. Indicated by A. A case where an optical pickup (comparative example) dedicated to the first optical disk 14a produced for comparison is used is indicated by a broken line B in FIG. In the comparative example, an objective lens unit is constituted by a single objective lens 412 (an objective lens 412 dedicated to the first light beam L1) in the optical pickup of the present embodiment. Here, the value of the wavefront aberration at each wavelength is a value at the position of the best image point at a wavelength at which the wavefront aberration is minimized (in the embodiment, the wavelength is 405 nm).

  As can be seen from FIG. 39, the optical pickup of this embodiment has a wider usable wavelength range than the comparative example. This is because the diffractive optical element 411 of the optical pickup of the present embodiment is composed of a converging diffraction grating and a plano-concave lens, which makes it possible to use an objective lens dedicated to the first light beam L1 alone. The wavelength dependent characteristic can be improved. In addition, even with the optical pickup of the present embodiment, a good condensing spot can be formed even if wavelength fluctuations due to mode hops or the like occur.

  Further, as can be seen from FIG. 19 of the third embodiment, the diffractive surface of the diffractive optical element 411 in the optical pickup of the present embodiment is changed to second-order diffracted light, second and third light beams with respect to the first light beam L1. On the other hand, since the first-order diffracted light is used, the depth of the diffraction grating that enables the use of light with an efficiency of 90% or more can be set for all of the first, second, and third light beams. Therefore, it is possible to easily realize an optical pickup that records and erases information that requires a high-power light beam. Moreover, since the output of a light source can be made small, power consumption can be suppressed. Furthermore, since unnecessary light other than the diffracted light can be prevented from entering the detector, signal deterioration can be suppressed.

  As described above, the optical pickup according to the present embodiment can record or reproduce information on a recording medium having a different thickness of the light transmission layer and a wavelength of the light beam optimum for reproduction, and can be used as an objective lens. With respect to an optical disc (specifically, a CD) having a thick light transmission layer in which the degree of divergence of incident light is large, it is possible to suppress a significant deterioration of the light collection characteristics due to the shift of the objective lens in the radial direction, and the configuration is improved. A simple optical pickup can be provided.

  The optical pickups shown in the above embodiments can be applied to an information recording / reproducing apparatus as shown in FIG. 46, for example. Here, FIG. 46 shows a schematic block diagram of an information recording / reproducing apparatus using the optical pickup 300 shown in FIG.

  As shown in FIG. 6, the information recording / reproducing apparatus according to the present embodiment has the following configuration in addition to the optical pickup 300 shown in FIG.

  That is, the reproduction signal detection optical systems 15 a, 15 b and 15 c are connected to the demodulation circuit 19 and the error detection circuit 17. The error detection circuit 17 is connected to a drive circuit 18 that drives the tracking control and focus control mechanism of the objective lens unit 313. The photodetector supplies an electrical signal corresponding to the light spot image to the demodulation circuit 19 and the error detection circuit 17. The demodulating circuit 19 generates a recording signal based on the electric signal. The error detection circuit 17 generates a focus error signal, a tracking error signal, and other servo signals based on the electric signal, and supplies each drive signal via the drive circuit 18, and these correspond to each drive signal. Then, the objective lens unit 313 and the like are servo-controlled.

  The optical pickup unit 25 includes semiconductor laser drive circuits 21a, 21b, and 21c that drive the semiconductor lasers to the semiconductor lasers 1a, 1b, and 1c that are light sources that emit the first wavelength, the second wavelength, and the third wavelength, respectively. 2 shows the connected optical pickup 300 shown in FIG.

  The input / output terminal 23 is connected to the modulation / demodulation circuit 19 and inputs recording data from the host device to the modulation / demodulation circuit 19 and outputs reproduction data of the information recording / playback device from the modulation / demodulation circuit 19 to the host device. To do.

  The control signal input / output terminal 24 is connected to the control circuit 20, receives a control signal from the host device, and outputs a control result of the information recording / reproducing device to the host device. The control circuit 20 is connected to the error detection circuit 17, the modulation / demodulation circuit 19, and the switching circuit 22, and controls the recording operation and the reproduction operation by a control signal from the control signal input / output terminal 24.

  The modulation / demodulation circuit 19 supplies recording signals corresponding to the recording data to the semiconductor laser driving circuits 21a, 21b and 21c, and the reproduction signals from the reproduction signal detection optical systems 15a, 15b and 15c are input. The error detection circuit 17 receives the reproduction signals from the reproduction signal detection optical systems 15a, 15b, and 15c, generates a focus error signal, a tracking error signal, and other servo signals based on the reproduction signals and drives them to the drive circuit 18. Output a signal. The drive circuit 18 servo-drives the optical pickup unit 25 according to the drive signal from the error detection circuit 17.

  In response to an instruction from the control circuit 22, the switching circuit 22 includes a semiconductor laser driving circuit 21a and a reproduction signal detection optical system 15a related to the first light beam L1, and a semiconductor laser driving circuit 21b and a reproduction signal detection optical system related to the second light beam L2. 15b, the semiconductor laser drive circuit 21c related to the third light beam L3 and the reproduction signal detection optical system 15c are switched and a semiconductor laser power corresponding to recording or reproduction is supplied. Here, if the stray light incident on the reproduction signal detection optical systems 15a, 15b, and 15c can be substantially ignored, the reproduction signal detection optical systems 15a, 15b, and 15c may not be switched by the switching circuit 22.

  Next, operations during recording and reproduction will be described with reference to FIG.

  During recording, recording data is input from the host device to the input / output terminal 23, and a recording control signal and a light beam switching signal corresponding to the optical disc inserted in the input information recording / reproducing device are input to the control input / output terminal 24. The In the following description, it is assumed that the first light beam L1 is input as the light beam switching signal.

  The switching circuit 22 turns on the semiconductor laser drive circuit 21a and the reproduction signal detection optical system 15a related to the first light beam L1 according to an instruction from the control circuit 20, and the other semiconductor laser drive circuits (21b and 21c) and the reproduction signal detection. The optical system (15b, 15c) is turned off. Further, the semiconductor laser drive circuit 21a is driven so as to drive the semiconductor laser 1a with a recording power stronger than the reproduction level.

  As for the recording data from the input / output terminal 23, the modulation / demodulation circuit 19 outputs a recording signal from the recording data in accordance with an instruction from the control circuit 20, and this recording signal is input to the semiconductor laser driving circuit 1a. The first optical beam L1 is applied to the first optical disk 14a. Further, an output signal corresponding to the light spot image formed on the first optical disk 14a irradiated with the first light beam L1 is input to the error detection circuit 17 via the reproduction signal detection optical system 15a, and based on this output signal. Then, the error detection circuit 17 supplies a drive signal to the drive circuit 18 according to an instruction from the control circuit 17 and servo-controls the optical pickup unit 25. As described above, the information recording / reproducing apparatus of the present embodiment is configured to irradiate the first optical disk 14a with the recording power with the first light beam L1 corresponding to the recording signal while servo-controlling the optical pickup unit 25. Recording data from a higher-level device is recorded on one optical disk 14a.

  At the time of reproduction, a reproduction control signal and a light beam switching signal corresponding to the optical disc inserted in the input information recording / reproducing apparatus are inputted to the control input / output terminal 24. In the following description, it is assumed that the first light beam L1 is input as the light beam switching signal.

  The switching circuit 22 turns on the semiconductor laser drive circuit 21a and the reproduction signal detection optical system 15a related to the first light beam L1 according to an instruction from the control circuit 20, and the other semiconductor laser drive circuits (21b and 21c) and the reproduction signal detection. The optical system (15b, 15c) is turned off. Further, the semiconductor laser drive circuit 21a is driven so as to drive the semiconductor laser 1a with a reproduction power weaker than the recording level.

  When the first light beam L1 emitted from the semiconductor laser 1a irradiates the first optical disk 14a with reproduction power, the reproduction signal detection optical system 15a modulates the output signal corresponding to the light spot image formed on the first optical disk. / Supplied to the demodulation circuit 19 and the error detection circuit 17. In response to an instruction from the control circuit 20, the error detection circuit 17 supplies a drive signal to the drive circuit 18 based on this output signal, and servo-controls the optical pickup unit 25. Further, in response to an instruction from the control circuit 20, the modulation / demodulation circuit 19 outputs reproduction data to the input / output terminal 23 based on this output signal, and outputs the reproduction data to the host device.

  As described above, the information recording / reproducing apparatus of the present embodiment applies the first light beam L1 to the first optical disc 14a with the reproduction power while servo-controlling the optical pickup unit 25, thereby applying the first optical disc 14a to the first optical disc 14a. The recorded recording signal is reproduced, and reproduction data is output to the host device.

  Alternatively, when the second light beam L2 is input as the light beam switching signal, the switching circuit 22 turns on the semiconductor laser driving circuit 21b and the reproduction signal detection optical system 15b, and the third light beam L3 is input. The switching circuit 22 turns on the beam semiconductor laser drive circuit 21c and the reproduction signal detection optical system 15c. Therefore, when the second optical disk is inserted, information can be recorded or reproduced on the second optical disk by the semiconductor laser drive circuit 21b and the reproduction signal detection optical system 15b related to the second light beam L2, and the third When an optical disk is inserted, information can be recorded on or reproduced from the third optical disk by the semiconductor laser drive circuit 21c and the reproduction signal detection optical system 15c related to the third light beam L3.

  In FIG. 46, the information recording / reproducing apparatus having three light sources is shown. However, the present invention is not limited to this. For example, the optical pickup 100 as shown in FIG. 1, that is, the optical pickup having two light sources is used. Can also be applied to an information recording / reproducing apparatus.

  Hereinafter, reference examples of the present invention will be described.

  In order to solve the above problems, an optical pickup according to the present invention includes a first light source that emits a first light beam having a first wavelength λ1, and a second wavelength λ2 that is longer than the first wavelength. The second light source that emits two light beams and the first light beam is condensed on the information recording surface of the first recording medium having the first light transmission layer, and the second light transmission is thicker than the first light transmission layer. An objective lens for condensing the second light beam on an information recording surface of a second recording medium having a layer, and a diffraction grating disposed in an optical path from the first and second light sources to the objective lens; In an optical pickup including a diffractive optical element having a lens with a refractive index n, the distance between the diffraction surface of the diffraction grating and the vertex of the lens surface of the lens is a, the radius of the second light beam is R, and the second The above diffraction case through which the outermost ray of the beam passes When the pitch in the child is d, the diffractive optical element has a diffraction angle α1 of the m1-order diffracted light in the diffraction grating of the first light beam, a refractive light in the lens of the m1-order diffracted light, and the optical axis of the first light beam. The angle β1 of the second light beam, the diffraction angle α2 of the m2 order diffracted light in the diffraction grating of the second light beam, and the angle β2 between the refracted light and the optical axis in the lens of the m2 order diffracted light are

Where f (d, mx) is the following function, where X is 1 or 2;

It is characterized in that m1 and m2 are set so as to satisfy the above.

  According to the above configuration, the first light beam is used for the first recording medium, and the second light beam is used for the second recording medium. At this time, since the first light transmission layer in the first recording medium is thicker than the second light transmission layer in the second recording medium, aberration occurs due to the thickness of the second light transmission layer. However, this aberration can be corrected by satisfying the above equation. Therefore, it is possible to provide an optical pickup capable of focusing on the information recording surface of the recording medium with higher accuracy and recording and reproducing.

  That is, even when an objective lens having a large numerical aperture is used with a light source having a significantly different wavelength, a diffraction having a diffraction grating and a lens using a diffraction order satisfying the conditional expression (1) on a recording medium having a different thickness of the light transmission layer. By using optical elements, it is possible to realize optical pickups capable of recording and reproducing on recording media having different light transmission layer thicknesses and wavelengths suitable for reproduction by forming light spots that converge to the diffraction limit. .

  In the optical pickup having the above-described configuration, the first and second light beams from the first and second light sources are provided between the first light source and the diffractive optical element and between the second light source and the diffractive optical element. Even if the collimator lens is provided in which the collimated light is made incident on the diffractive optical element as parallel light, the same effect as described above can be obtained.

  In the optical pickup of the present invention, it is preferable that the diffractive optical element is formed by integrating the diffraction grating and the lens.

  According to said structure, the number of parts in the said optical pick-up can be reduced.

  The optical pickup of the present invention preferably has β1 = 0 and β2> 0.

  According to the above configuration, by setting β1 = 0, the first light beam becomes parallel light, and the shift characteristics in the objective lens can be easily ensured. At this time, spherical aberration occurs in the second light transmission layer of the second recording medium. However, as described above, β2> 0 and the second light beam is diverged to suppress the spherical aberration. it can.

  In the optical pickup of the present invention, it is preferable that the m2 order diffracted light has an order less than the m1 order diffracted light.

  According to the above configuration, the m1st-order diffracted light and the m2nd-order diffracted light do not form the same focal point on the information recording surface of the recording medium, so that the influence on reading or recording by the diffracted light is suppressed. be able to.

  In the optical pickup of the present invention, it is preferable that the diffractive optical element is set to m1 = 1 and m2 = 1. Furthermore, it is preferable that the lens is a plano-convex lens whose convex surface is a spherical surface, and the diffraction grating is provided on a plane of the plano-convex lens.

  In the optical pickup of the present invention, it is preferable that the diffractive optical element is set to m1 = 1 and m2 = 0. Furthermore, it is preferable that the lens is a plano-concave lens having a concave aspheric surface, and the diffraction grating is provided on the plane of the plano-concave lens. Furthermore, it is preferable that the diffraction grating is provided on the objective lens side.

  As described above, the optical pickup can be suitably realized by setting each condition.

  In the optical pickup of the present invention, the diffraction grating surface of the diffractive optical element has a diffraction order of m1 as the diffraction order of the first diffracted light, m2 as the diffraction order of the second diffracted light, and d as the interval between the annular grooves. When the angle inclined toward the predetermined optical axis with respect to the normal is positive,

The diffraction order m1 of the first diffracted light is preferably +1, and the diffraction order m2 of the second diffracted light is preferably 0.

  According to the above configuration, the diffraction order m1 of the first diffracted light is +1, and the diffraction order m2 of the second diffracted light is 0. Since the first diffracted light having the smallest diffraction order m1 and the second diffracted light having the smallest diffraction order m2 are used among the diffracted lights satisfying the above formula, the first and second diffracted lights The diffraction efficiency can be easily ensured.

  In the optical pickup of the present invention, it is preferable that a diffraction grating surface of the diffractive optical element has a diffraction characteristic in a direction in which the first and second light beams are brought close to the predetermined optical axis.

  According to the above configuration, the diffraction grating surface of the diffractive optical element has diffraction characteristics in a direction in which the first and second light beams are brought close to the predetermined optical axis, so that the diffraction efficiency of the first diffracted light is improved. be able to. Thereby, the utilization efficiency of the first light beam can be improved.

  In the optical pickup of the present invention, the diffractive optical element has the diffraction grating surface formed on the side on which the first and second light beams are incident, and the diffraction grating on the side on which the first and second diffracted lights are emitted. A concave surface having an optical axis common to the surface is preferably formed.

  According to said structure, a diffraction grating surface and a concave surface are formed in a diffractive optical element. The concave surface reduces the axial chromatic aberration of the objective lens. Accordingly, by forming a concave surface in addition to the diffraction grating surface, it is possible to further suppress the shift of the focal position due to the wavelength variation with respect to the first diffracted light. Further, with respect to the second diffracted light, it is possible to suppress the shift of the focal position due to the wavelength variation.

  In the optical pickup of the present invention, it is preferable that the concave surface is an aspherical surface.

  According to said structure, the concave surface formed in a diffractive optical element is an aspherical surface. By forming the concave surface into an appropriate aspherical surface, spherical aberration is further increased for each information recording surface of two recording media having different light transmission layer thicknesses compared to the case where the concave surface is a spherical surface. It is possible to reduce and form better spot lights.

  The optical pickup of the present invention condenses a first light beam having a wavelength λ1 on the information recording surface of a first recording medium having a light transmission layer having a thickness t1 on the information recording surface. An optical pickup capable of recording or reproducing information by forming a spot, wherein the first optical beam is diffracted and refracted to be emitted and a diffractive optical element having a refracting surface, and a diffraction An objective lens for focusing the diffracted light of a predetermined diffraction order of the first beam emitted from the optical element on the information recording surface of the first recording medium to form the first light spot; and the first lens A collimator lens is provided between the light source and the diffractive optical element so that the first light beam from the first light source is converted into parallel light and made incident on the diffractive optical element. Diffraction characteristics in the direction that brings the beam closer to the optical axis A, it is characterized in that the refractive surface is concave.

  According to the above configuration, even in the case of a pickup dedicated for the next-generation high-density optical disc, it is possible to further suppress the shift of the focal position due to the wavelength variation.

In the optical pickup of the present invention, when the power of the diffractive optical element is Φ, the power of the diffractive surface of the diffracted photon is ΦD, and the power of the refracting surface is ΦL,
Φ = ΦD + ΦL = 0
It is preferable that

  According to the above configuration, the first light beam emitted as parallel light by the first optical system can be emitted as parallel light even after passing through the diffractive optical element, and deterioration of aberration due to positional deviation from the objective lens can be suppressed. . In this case, the diffractive optical element can be inserted anywhere between the first light beam and the objective lens.

  The optical pickup of the present invention condenses the first light beam having the wavelength λ1 on the information recording surface of the first recording medium having the light transmission layer having the thickness t1 on the information recording surface. On the information recording surface of the second recording medium, information can be recorded or reproduced by forming one light spot, and a light transmission layer having a thickness t2 (t2> t1) is provided on the information recording surface. On the other hand, an optical pickup capable of recording or reproducing information by condensing a second light beam having a wavelength λ2 (λ2> λ1) to form a second light spot, in order to solve the above problems Further, when the first and second light beams are incident, the incident first and second light beams are diffracted and refracted to be emitted, and the first diffractive optical element is emitted from the diffractive optical element. And diffracted light of predetermined diffraction orders of the second light beam and An objective lens that focuses the information recording surfaces of the first and second recording media to form the first and second light spots, respectively, and the first light beam and the second light beam are: The light beams are incident on the diffractive optical element as light beams having different degrees of convergence or divergence.

  According to said structure, the diffracted light of the 1st and 2nd light beam from which a wavelength mutually differs is shared by the information recording surface of the 1st and 2nd recording medium provided with the light transmissive layer from which thickness differs mutually. In the case where the first and second light spots are respectively formed by condensing with the objective lens, it is incident on the objective lens in order to sufficiently reduce the wavefront aberration of each diffracted light to be collected and to obtain good condensing characteristics. It is necessary to increase the angle difference between the convergence and divergence angles of each diffraction light.

  Therefore, the first light beam and the second light beam are incident on the diffractive optical element as light beams having different degrees of convergence or divergence, so that the angle difference can be easily increased. Thereby, the restrictions on the diffraction and refraction characteristics required for the diffractive optical element can be relaxed, and the design restrictions on the diffractive optical element can be relaxed. As a result, it becomes possible to realize an optical pickup capable of sufficiently reducing the wavefront aberration of the collected diffracted light by using a diffractive optical element that is easy to manufacture.

  The first light beam and the second light beam are incident on the diffractive optical element as light beams having different degrees of convergence or divergence, for example, when one is convergent light and the other is divergent light, There may be a case where one is parallel light and the other is incident as convergent light or divergent light. Furthermore, both may be convergent light or divergent light, and the degree of convergence or divergence may be different.

  In the optical pickup of the present invention, it is preferable that the diffractive optical element includes a converging diffraction grating and a diverging lens.

  According to said structure, degradation of the wavefront aberration with respect to a wavelength fluctuation can be suppressed, and a favorable condensing characteristic can be acquired also with respect to a wavelength fluctuation.

  In the optical pickup of the present invention, preferably, the first light beam is incident as convergent light and the second light beam is incident as divergent light with respect to the diffractive optical element.

  According to the above configuration, even when the diffractive optical element and the objective lens are shifted in the optical axis direction of the first and second light beams, the deterioration of the wavefront aberration can be suppressed to a low level within a relatively wide shift range.

  Further, in the optical pickup of the present invention, the objective lens receives the second-order and first-order diffracted lights of the first and second light beams emitted from the diffractive optical element, respectively, and information on the first and second recording media, respectively. The first and second light spots are preferably formed by focusing on the recording surface.

  Alternatively, in the optical pickup of the present invention, the objective lens causes the first-order and second-order diffracted lights of the first and second light beams emitted from the diffractive optical element, respectively, and information on the first and second recording media, respectively. The first and second light spots are preferably formed by focusing on the recording surface.

  According to said structure, the diffracted light of the high diffraction efficiency by a diffractive optical element can be utilized. Thereby, the first and second light beams can be efficiently used, and the first and second recording media are irradiated with high-output light while suppressing the power consumption of the light sources of the first and second light beams. Will be able to.

  In order to solve the above-described problem, the optical pickup of the present invention emits first, second, and third light beams having wavelengths λ1, λ2, and λ3 having a relationship of λ1 <λ2 <λ3, respectively. First and second light sources (for example, semiconductor lasers) are provided, and the first, second, and third light beams are different from each other by the same condensing means (for example, an objective lens). , Second and third recording media (optical discs, for example, next-generation high-density optical discs, DVDs, CDs) in an optical pickup that records or reproduces information by focusing on each information recording surface. , Arranged in a common optical path from the second and third light sources to the objective lens, having a diffractive surface and a refracting surface, and converging / diverging each light beam according to the wavelength of the light beam, In the diffractive surface, the first light beam is a first-order diffracted light, The diffractive optical element includes a diffractive optical element that causes the second light beam and the third light beam to enter the condensing unit as diffracted light having a diffraction order equal to or lower than the diffraction order of the first light beam, The depth of the diffraction grating at the diffraction surface is set such that the diffraction efficiency of the diffracted light of each light beam incident on the light converging means is higher than the diffraction efficiency of the diffracted light of other diffraction orders, respectively. It is characterized by being.

  According to the above configuration, the optical pickup that performs recording or reproduction on the first, second, and third recording media using the first, second, and third light beams having different wavelengths, respectively, is the shortest. Condensing means in which aberration is corrected when the first light beam having the wavelength is condensed on the first recording medium is used. On the other hand, the same condensing means is used for condensing the second and third light beams on the second and third recording media having different light transmission layer thicknesses from the first recording medium. As a result, the spherical aberration of the second and third light beams increases.

  However, according to the above configuration, the diffractive optical element is arranged in a common optical path from the first, second, and third light sources to the objective lens, so that the thickness of the light transmission layer is different. The positions of the focused spots formed by the first, second, and third light beams having different wavelengths from each other on the information recording surfaces of the first, second, and third recording media are changed through the focusing means. Can be made. For this reason, according to the above configuration, the first, second, and third light beams having different wavelengths are formed on the information recording surfaces of the first, second, and third recording media having different light transmission layer thicknesses. The beam can be condensed using a single condensing means, and the degree of divergence of the second and third light beams with respect to the diffractive optical element can be suppressed. The influence of the shift of the optical lens in the radial direction can be suppressed.

  The diffraction efficiency of the diffracted light of each light beam in the diffractive optical element is determined by the depth of the diffraction grating on the diffraction surface of the diffractive optical element. According to the study by the inventors of the present application, the diffractive optical element has a diffraction surface in which the first light beam is the first-order diffracted light, and the second light beam and the third light beam are diffracted by the first light beam. Diffracted light with a diffraction order equal to or lower than the order, and the depth of the diffraction grating on the diffraction surface is such that the diffraction efficiency of the diffracted light of each light beam incident on the light converging means has a diffraction order of another diffraction order. By setting so as to be higher than the diffraction efficiency of light, the loss of the amount of light incident on each recording medium can be reduced for any diffracted light. Therefore, according to the above configuration, the diffraction efficiency of each diffracted light incident on each information recording surface of each recording medium is compatible with three recording media having different thicknesses of the light transmission layer, That is, it is possible to realize an optical pickup that can perform recording and erasing of information that requires high use efficiency of each light beam and requires a high-power light beam. Furthermore, according to the above configuration, the output of each light beam can be reduced, so that power consumption in the light source can be suppressed.

  In the optical pickup of the present invention, in order to solve the above-described problem, the diffractive optical element is configured to collect the first light beam as first-order diffracted light and the second and third light beams as zero-order diffracted light. The depth of the diffraction grating at the diffractive surface of the diffractive optical element is the highest in the diffraction efficiency of the first-order diffracted light with respect to the first light beam, and with respect to the second and third light beams. Each is characterized in that the diffraction efficiency of the 0th-order diffracted light is set to be the highest.

  According to the above configuration, the diffractive optical element causes the first light beam to be the first-order diffracted light and the second and third light beams to be incident on the condensing means as the 0th-order diffracted light. The depth of the diffraction grating at the diffraction plane is the highest for the first light beam with respect to the first-order diffracted light, and with respect to the second and third light beams, the diffraction efficiency for the zero-order diffracted light. Is set to be the highest so that the loss of the amount of light incident on each recording medium can be reduced for any diffracted light, and different light transmission layers using the same light collecting means. A good condensing spot can be formed on a thick recording medium. Further, according to the above configuration, the degree of divergence of the second and third light beams with respect to the diffractive optical element can be suppressed, and the influence of the shift in the radial direction of the condenser lens due to tracking or the like can be suppressed. Is possible. Therefore, according to the above configuration, the recording medium is compatible with three recording media having different thicknesses of the light transmission layer, and the diffraction efficiency of each diffracted light incident on each information recording surface of each recording medium is high. Both are high, and an optical pickup capable of recording and erasing information that requires a high output light beam can be realized. Furthermore, according to the above configuration, the output of each light beam can be reduced, so that power consumption in the light source can be suppressed.

  In order to solve the above problems, the optical pickup of the present invention is characterized in that the diffractive optical element includes a converging diffractive surface and a concave refracting surface as the diffractive surface and the refracting surface. .

  According to the above configuration, the usable wavelength range is wider than that of the optical pickup dedicated to the first recording medium, and the wavelength dependence characteristics are improved as compared with the case where the objective lens dedicated to the first recording medium is used alone. Can do. Therefore, in the above configuration, even when wavelength fluctuation occurs due to mode pop or the like, good light condensing characteristics can be maintained. In addition, the minimum pitch of the diffractive surface of the diffractive optical element can be increased, and the creation of the diffractive optical element can be facilitated.

  Furthermore, according to the above configuration, it is possible to make a light beam incident on the light collecting means with a predetermined degree of divergence only by entering weak divergent light into the light collecting means. For this reason, the influence of the light collecting means on the radial shift can be reduced, and the position of the light source with respect to the light collecting means can be increased, thereby facilitating the arrangement of the light sources.

  In the optical pickup of the present invention, in order to solve the above-described problems, the diffraction grating has a diffraction efficiency of 90% or more of the first-order diffracted light of the first light beam in the diffraction surface of the diffractive optical element. It is characterized by being set as follows.

  According to the above configuration, the diffraction efficiency can be improved with respect to the diffracted light of any of the first, second, and third light beams. Thereby, since the output of each light beam can be made small, the power consumption of a light source can be suppressed. Further, according to the above configuration, it is possible to realize an optical pickup with higher utilization efficiency for the first light beam for which it is difficult to create a high-power laser.

In order to solve the above problems, the optical pickup of the present invention sets the convergence / divergence degrees of the second and third light beams incident on the diffractive optical element to Φinr and ΦinIr, respectively, and enters the condensing lens. 2, If the convergence / divergence degree of the third light beam is Φoutr and ΦoutIr, respectively,
| Φoutr | >> | Φinr | and | ΦoutIr | >> | ΦinIr |
It is characterized by satisfying.

  According to the above configuration, the absolute value of the degree of convergence / divergence is larger in the second and third light beams emitted from the diffraction grating than in the second and third light beams incident on the diffractive optical element. By doing so, it is possible to converge the second and third light beams incident on the diffractive optical element while sufficiently obtaining the effect of suppressing the increase in spherical aberration caused by the different thicknesses of the light transmission layers. It is possible to reduce the absolute value of the divergence degree. That is, it becomes possible to approach parallel light. Thereby, particularly in the optical pickup of the present invention, the third light beam can be incident on the diffractive optical element with a small degree of convergence / divergence.

  In the optical pickup of the present invention, in order to solve the above-described problem, the diffractive optical element causes the first, second, and third light beams to enter the condensing unit as first-order diffracted light, respectively. The depth of the diffraction grating at the diffraction surface of the diffractive optical element is set so that the diffraction efficiency of the first-order diffracted light is the highest for any of the first, second, and third light beams. It is a feature.

  According to the above configuration, the diffractive optical element causes the first, second, and third light beams to enter the condensing unit as first-order diffracted light, respectively, and a diffraction grating on the diffractive surface of the diffractive optical element. Is set so that the diffraction efficiency of the first-order diffracted light is the highest for any of the first, second, and third light beams, and the amount of light incident on each recording medium. Loss can be reduced for any diffracted light, and a good condensing spot can be formed on recording media having different light transmission layer thicknesses using the same condensing means. Further, according to the above configuration, the degree of divergence of the second and third light beams with respect to the diffractive optical element can be suppressed, and the influence of the shift in the radial direction of the condenser lens due to tracking or the like can be suppressed. Is possible. Therefore, according to the above configuration, the recording medium is compatible with three recording media having different thicknesses of the light transmission layer, and the diffraction efficiency of each diffracted light incident on each information recording surface of each recording medium is high. Both are high, and an optical pickup capable of recording and erasing information that requires a high output light beam can be realized. Furthermore, according to the above configuration, the output of each light beam can be reduced, so that power consumption in the light source can be suppressed.

  In order to solve the above problems, the optical pickup of the present invention is characterized in that the diffractive optical element includes a diffractive diffractive surface and a convex refracting surface as the diffractive surface and the refracting surface. .

  According to said structure, a favorable condensing spot can be formed with respect to each recording medium from which the thickness of a light transmissive layer differs using one condensing means. Moreover, according to said structure, it becomes possible to inject a light beam with the predetermined divergence degree to the said condensing means only by injecting weak divergent light into the said condensing means. Therefore, according to the above configuration, the degree of divergence of the second and third light beams to the diffractive optical element can be suppressed, and the condensing characteristic is further deteriorated due to the shift of the condensing means in the radial direction. While being able to suppress effectively, it becomes possible to distance the position of the light source with respect to the said condensing means, and arrangement | positioning of a light source becomes easy.

  In order to solve the above problems, the optical pickup of the present invention is characterized in that the refracting surface of the diffractive optical element is an aspherical surface.

  According to the above configuration, it is possible to further correct the spherical aberration caused by the difference in the thickness of the light transmission layer of each recording medium, and to suppress the deterioration of the shift characteristic of the light collecting means, which is better. Light collecting characteristics can be obtained.

  In the optical pickup having each configuration described above, it is preferable that the diffractive surface of the diffractive optical element is formed on a refractive surface.

  According to the above configuration, alignment between the refracting surface and the diffractive surface of the diffractive optical element becomes unnecessary, and the creation of the diffractive optical element can be facilitated.

  In the optical pickup having each of the above-described configurations, it is preferable that a sawtooth or stepped diffraction grating is formed on the diffraction surface of the diffractive optical element.

  According to the above configuration, the diffraction efficiency of each light beam in the diffractive optical element can be improved. Thereby, since the output of the light source of each light beam can be made small, the power consumption in a light source can be suppressed. The above configuration is particularly effective for an optical pickup that records and erases information that requires a high-power beam.

The optical pickup of the present invention includes the information recording surfaces of the first, second, and third recording media each having a light transmission layer having thicknesses t1, t2, and t3 (t1 = t2 <t3) on the information recording surface. In contrast, an optical pickup capable of recording or reproducing information by condensing first, second and third light beams of wavelengths λ1, λ2 and λ3 (λ1 <λ2 <λ3), respectively. An objective lens that focuses the first, second, and third light beams on the information recording surfaces of the first, second, and third recording media, respectively, and the first, second, and second objective lenses First, second, and third light beams of a predetermined diffraction order that are provided integrally with the objective lens on the incident side of the three light beams, diffract and refract the first, second, and third light beams. A diffractive optical element that makes the objective lens incident on the objective lens, When the third light beam is incident on the object lens as diverging light, the convergence / divergence degree of the third light beam incident on the diffractive optical element is ΦinIr, and the divergence degree incident on the objective lens is ΦoutIr. ,
| ΦinIr | <| ΦoutIr |
It is characterized by satisfying.

  According to the above configuration, in the optical pickup that records or reproduces the first, second, and third recording media using the first, second, and third light beams having different wavelengths, respectively, the shortest wavelength is obtained. When the first light beam is focused on the first recording medium, an objective lens with corrected aberration is used.

  On the other hand, when the objective lens is used as it is to focus the second and third light beams on the second and third recording media having different wavelengths and light transmission layers from the first recording medium, respectively. The chromatic aberration and spherical aberration of the second and third light beams will increase. In order to suppress the increase in spherical aberration, the second and third light beams may be incident on the objective lens as divergent light so as to generate aberrations in the opposite directions for correction.

  Here, in order to sufficiently obtain the effect of suppressing the increase in spherical aberration, it is necessary to increase the degree of divergence of the third light beam incident on the objective lens. However, when the degree of divergence of the light beam incident on the objective lens is increased, the radial direction of the objective lens by tracking or the like (direction substantially orthogonal to the optical axes of the first, second, and third light beams incident on the objective lens). Due to the movement, coma added to the aperture spot on the recording medium is increased, which causes a problem that the condensing characteristic is greatly deteriorated.

Therefore, in the above configuration, a diffractive optical element provided so as to be movable integrally with the objective lens is used when the third light beam is incident on the objective lens as divergent light. And this diffractive optical element is
| ΦinIr | <| ΦoutIr |
Act to satisfy. That is, the absolute value of the degree of convergence / divergence is made larger for the third light beam emitted from the diffractive optical element than for the third light beam incident on the diffractive optical element.

  As a result, the absolute value of the degree of convergence / divergence of the third light beam incident on the unit (objective lens unit) made up of the objective lens and the diffractive optical element is made small while sufficiently increasing the spherical aberration. That is, it becomes possible to approach parallel light. As a result, in the above configuration, it is possible to suppress deterioration of light collection characteristics due to a shift in the radial direction of the objective lens unit as compared with the case where no diffractive optical element is used.

  As described above, in the above-described configuration, it is possible to record or reproduce information by using a single objective lens to form a good condensing spot on a recording medium having a used wavelength or a different light transmission layer thickness. Further, it is possible to suppress the deterioration of the light condensing characteristics due to the movement of the objective lens unit in the radial direction.

  In the optical pickup according to the aspect of the invention described above, it is preferable that the diffractive optical element causes the first light beam to enter the objective lens as parallel light.

  According to the above configuration, the first light beam having a short wavelength that requires the most stringent condensing characteristics is incident as parallel light on the objective lens, whereby the diffractive optical element and the objective lens in the use of the first light beam It is possible to suppress the occurrence of aberration due to the positional deviation.

  In the optical pickup of the present invention, in the above configuration, it is preferable that the first light beam is incident on the diffractive optical element as parallel light or convergent light.

  According to the above configuration, the absolute value of the degree of convergence / divergence of the third light beam incident on the diffractive optical element can be made relatively small. As a result, it is possible to more effectively suppress the deterioration of the light collection characteristics due to the movement of the objective lens unit in the radial direction.

  In the optical pickup of the present invention having the above-described configuration, the diffractive optical element supplies the second-order diffracted light of the first light beam, the first-order diffracted light of the second light beam, and the first-order diffracted light of the third light beam to the objective lens, respectively. In the diffractive optical element, the diffraction efficiency of the second-order diffracted light is higher than that of other diffraction orders for the first light beam, and the diffraction efficiency of the first-order diffracted light is other diffraction orders for the second light beam. It is preferable that the diffraction efficiency of the first-order diffracted light is higher than the efficiency of other diffraction orders for the third light beam.

  According to the above configuration, the diffraction efficiency of all of the first, second, and third light beams can be improved. Thereby, since the output of the light source of each light beam can be made small, the power consumption in a light source can be suppressed. The above configuration is particularly effective for an optical pickup that records and erases information that requires a high-power beam. In particular, in the diffractive optical element, the diffraction efficiency of the second-order diffracted light is preferably 90% or more for the first light beam.

  In the optical pickup of the present invention, the diffractive optical element preferably includes a converging diffractive surface and a concave refracting surface.

  According to the above configuration, the usable wavelength range is wider than that of the optical pickup dedicated to the first recording medium, and the wavelength dependence characteristics can be improved as compared with the case where the objective lens dedicated to the first recording medium is used alone. Therefore, in the above configuration, even when wavelength variation occurs due to mode hopping or the like, it is possible to maintain good light collection characteristics. In addition, the minimum pitch of the diffractive surface of the diffractive optical element can be increased, and the creation of the diffractive optical element can be facilitated.

  As described above, the optical pickup according to the present invention includes the first light source that emits the first light beam having the first wavelength λ1 and the second light beam that has the second wavelength λ2 longer than the first wavelength. The first light beam is condensed on the information recording surface of the first recording medium having the second light source to be emitted and the first light transmission layer, and the second light transmission layer is thicker than the first light transmission layer. (2) an objective lens for condensing the second light beam on the information recording surface of the recording medium, and a diffraction grating and a refractive index n arranged in an optical path from the first and second light sources to the objective lens In an optical pickup including a diffractive optical element having a lens, the distance between the diffraction surface of the diffraction grating and the vertex of the lens surface of the lens is a, the radius of the second light beam is R, and the outermost of the second beam Of the diffraction grating through which the light beam passes. When H is d, the diffractive optical element has a diffraction angle α1 of the m1-order diffracted light in the diffraction grating of the first light beam, a refractive light in the lens of the m1-order diffracted light, and an optical axis of the first light beam. The angle β1, the diffraction angle α2 of the second-order diffracted light in the diffraction grating of the second light beam, and the angle β2 between the refracted light and the optical axis in the lens of the second-order diffracted light are expressed by the following equations 28 and 29.

Where f (d, mx) is the following function, where X is 1 or 2,

In this configuration, m1 and m2 are set so as to satisfy the above.

  According to the above configuration, a diffraction grating that uses a diffraction order satisfying the conditional expression (1) on a recording medium having a different thickness of a light transmission layer even when an objective lens having a large numerical aperture is used with a light source having a significantly different wavelength. By using a diffractive optical element having a lens and a lens, a light spot that converges to the diffraction limit is formed, and recording and reproduction are possible with respect to recording media having different thicknesses and optimum wavelengths for reproduction. There is an effect that an optical pickup can be realized.

  In the optical pickup of the present invention, it is preferable that the diffractive optical element is formed by integrating the diffraction grating and the lens.

  According to said structure, there exists an effect that the number of parts in the said optical pick-up can be reduced.

  The optical pickup of the present invention preferably has β1 = 0 and β2> 0.

  According to the above configuration, the first light beam becomes parallel light by setting β1 = 0, and the shift characteristic in the objective lens can be easily ensured, but the second light transmission layer of the second recording medium can be obtained. Then, spherical aberration occurs. However, as described above, when β2> 0 and the second light beam is divergent light, this spherical aberration can be suppressed.

  In the optical pickup of the present invention, it is preferable that the m2 order diffracted light has an order less than the m1 order diffracted light.

  According to the above configuration, the m1st-order diffracted light and the m2nd-order diffracted light do not form the same focal point on the information recording surface of the recording medium, so that the influence on reading or recording by the diffracted light is suppressed. There is an effect that can be.

  In the optical pickup of the present invention, it is preferable that the diffractive optical element is set to m1 = 1 and m2 = 1. Furthermore, it is preferable that the lens is a plano-convex lens whose convex surface is a spherical surface, and the diffraction grating is provided on a plane of the plano-convex lens.

  Moreover, it is preferable that the diffractive optical element is set to m1 = 1 and m2 = 0. Furthermore, it is preferable that the lens is a plano-concave lens having a concave aspheric surface, and the diffraction grating is provided on the plane of the plano-concave lens. Furthermore, it is preferable that the diffraction grating is provided on the objective lens side.

  As described above, there is an effect that the optical pickup can be suitably realized by setting each condition.

  As described above, according to the present invention, the first and second diffracted lights are in a state in which spherical aberration is reduced on each information recording surface of two recording media having different light transmission layer thicknesses by the objective lens. Are each focused. As a result, spot light can be formed on each information recording surface of two recording media having different light transmission layer thicknesses with reduced spherical aberration. Further, regarding the first diffracted light, the change in the diffraction angle related to the diffraction by the diffraction grating surface and the change in the refraction angle related to the refraction by the objective lens, which are caused by the wavelength fluctuation of the first light beam, are canceled out. The deviation of the focal position can be suppressed.

  Furthermore, without increasing the number of optical parts compared to the conventional technology, spot light is formed by reducing spherical aberration on each information recording surface of two recording media having different light transmission layer thicknesses. In addition, since it is possible to suppress the shift of the focal position due to the wavelength variation, it is possible to prevent the cost of the optical component from being increased and to prevent the optical pickup device from being enlarged.

  The optical pickup of the present invention condenses a first light beam having a wavelength λ1 on the information recording surface of a first recording medium having a light transmission layer having a thickness t1 on the information recording surface. An optical pickup capable of recording or reproducing information by forming a spot, wherein the first optical beam is diffracted and refracted to be emitted and a diffractive optical element having a refracting surface, and a diffraction An objective lens for focusing the diffracted light of a predetermined diffraction order of the first beam emitted from the optical element on the information recording surface of the first recording medium to form the first light spot; and the first lens A collimator lens is provided between the light source and the diffractive optical element so that the first light beam from the first light source is converted into parallel light and made incident on the diffractive optical element. Diffraction characteristics in the direction that brings the beam closer to the optical axis A, wherein the refractive surface is concave.

  With the above configuration, even in the case of a pickup dedicated to the next generation high-density optical disc, it is possible to further suppress the shift of the focal position due to the wavelength variation.

In the optical pickup of the present invention, when the power of the diffractive optical element is Φ, the power of the diffractive surface of the diffracted photon is ΦD, and the power of the refracting surface is ΦL,
Φ = ΦD + ΦL = 0
It is characterized by being.

  If it is the said structure, the 1st light beam radiate | emitted by the 1st optical system 16a can be radiate | emitted with a parallel light after passing a diffractive optical element, and the deterioration of the aberration by the position shift with an objective lens is suppressed. it can. In this case, the diffractive optical element may be inserted anywhere between the first light beam 16a and the objective lens.

  Further, according to the present invention, the position of the diffractive optical element with respect to the direction parallel to the predetermined optical axis and the direction perpendicular to the predetermined optical axis are within the range irradiated with the first and second light beams. Regardless of the position of the diffractive optical element with respect to any direction, first and second diffracted light that is inclined at a desired angle toward the predetermined optical axis side is generated. Accordingly, the degree of freedom with respect to the arrangement of the diffractive optical element for allowing the first and second diffracted lights to enter the objective lens is improved.

  Further, according to the present invention, the diffraction order m1 of the first diffracted light is +1 and the diffraction order m2 of the second diffracted light is 0, so that the diffraction efficiency of the first and second diffracted lights can be easily secured. can do.

  Further, according to the present invention, the diffraction grating surface of the diffractive optical element has diffraction characteristics in a direction in which the first and second light beams are brought close to the predetermined optical axis, so that the diffraction efficiency of the first diffracted light is improved. Can be made. Thereby, the utilization efficiency of the first light beam can be improved.

  Further, according to the present invention, since the concave surface is formed in addition to the diffraction grating surface, it is possible to further suppress the shift of the focal position due to the wavelength variation with respect to the first diffracted light. Further, with respect to the second diffracted light, it is possible to suppress the shift of the focal position due to the wavelength variation.

  Further, according to the present invention, the concave surface formed in the diffractive optical element is formed into an appropriate aspheric surface, so that the two recording layers having different light transmission layer thicknesses compared to the case where the concave surface is a spherical surface. For each information recording surface of the medium, spherical aberration can be further reduced, and better spot light can be formed.

  As described above, the optical pickup according to the present invention includes a diffractive optical element that diffracts and refracts the incident first and second light beams when the first and second light beams are incident thereon, and First and second light spots are obtained by condensing the diffracted lights of predetermined diffraction orders of the first and second light beams emitted from the diffractive optical element on the information recording surfaces of the first and second recording media, respectively. The first light beam and the second light beam are incident on the diffractive optical element as light beams having different degrees of convergence or divergence.

  In the above configuration, the first light beam and the second light beam are incident on the diffractive optical element as light beams having different degrees of convergence or divergence, so that the angle difference can be easily increased. Thereby, the restrictions on the diffraction and refraction characteristics required for the diffractive optical element can be relaxed, and the design restrictions on the diffractive optical element can be relaxed. As a result, it becomes possible to realize an optical pickup capable of sufficiently reducing the wavefront aberration of the collected diffracted light by using a diffractive optical element that is easy to manufacture.

  In the optical pickup of the present invention, it is desirable that the diffractive optical element includes a converging diffraction grating and a diverging lens.

  With the above configuration, it is possible to suppress the deterioration of the wavefront aberration with respect to the wavelength variation, and it is possible to obtain a good light collection characteristic with respect to the wavelength variation.

  In the optical pickup of the present invention, it is desirable that the first light beam is incident as convergent light and the second light beam is incident as divergent light with respect to the diffractive optical element.

  In the above configuration, even when the diffractive optical element and the objective lens are shifted in the optical axis direction of the first and second light beams, the deterioration of the wavefront aberration can be suppressed in a relatively wide shift range.

  In the optical pickup of the present invention, the objective lens emits the second-order and first-order diffracted lights of the first and second light beams emitted from the diffractive optical element to the information recording surfaces of the first and second recording media, respectively. It is desirable to focus and form the first and second light spots.

  Alternatively, in the optical pickup of the present invention, the objective lens causes the third and second order diffracted lights of the first and second light beams emitted from the diffractive optical element to be recorded on the information recording surfaces of the first and second recording media, respectively. On the other hand, the first and second light spots may be formed by condensing.

  In the above configuration, diffracted light with high diffraction efficiency by the diffractive optical element can be used. Thereby, the first and second light beams can be efficiently used, and the first and second recording media are irradiated with high-output light while suppressing the power consumption of the light sources of the first and second light beams. Will be able to.

  In the optical pickup of the present invention, it is desirable that the diffractive optical element has a diffraction grating having a sawtooth or stepped cross-sectional shape.

  Since the diffraction grating having a sawtooth or stepped cross section has a high diffraction efficiency as described above, the first and second light beams can be used efficiently, and the light source of the first and second light beams can be used. It becomes possible to irradiate the first and second recording media with high output light while suppressing power consumption.

  As described above, the optical pickup according to the present invention emits the first, second, and third light beams having the wavelengths λ1, λ2, and λ3 having the relationship of λ1 <λ2 <λ3, respectively. 3, each information recording surface of the first, second and third recording media having different light transmission layer thicknesses by the same condensing means. In an optical pickup that records or reproduces information by condensing light on the optical pickup, the optical pickup is disposed in a common optical path from the first, second, and third light sources to the objective lens, and has a diffractive surface and a refracting surface. Each light beam is converged / diverged according to the wavelength of the light beam, and the first light beam is the first-order diffracted light on the diffraction surface, and the second light beam and the third light beam are the first light beam. As the diffracted light of the diffraction order equal to or lower than the diffraction order of the light beam, A diffractive optical element to be incident, and the depth of the diffraction grating at the diffraction surface of the diffractive optical element is such that the diffraction efficiency of the diffracted light of each light beam incident on the condensing means is diffracted light of other diffraction orders. It is the structure set so that it may become higher than the diffraction efficiency of this.

  According to the above configuration, the diffractive optical element is arranged in a common optical path from the first, second, and third light sources to the objective lens, so that the first light transmission layer has a different thickness. The positions of the focused spots formed by the first, second and third light beams having different wavelengths on the respective information recording surfaces of the second and third recording media through the focusing means are changed. Can do. For this reason, according to the above configuration, the first, second, and third light beams having different wavelengths are formed on the information recording surfaces of the first, second, and third recording media having different light transmission layer thicknesses. The beam can be condensed using a single condensing means, and the degree of divergence of the second and third light beams with respect to the diffractive optical element can be suppressed. There is an effect that the influence of the shift of the optical lens in the radial direction can be suppressed.

  In addition, according to the above configuration, the loss of the amount of light incident on each recording medium can be reduced for any diffracted light, so compatibility with three recording media with different thicknesses of the light transmission layer is achieved. And an optical pickup with high diffraction efficiency of each diffracted light incident on each information recording surface of each recording medium and capable of recording and erasing information that requires a high output light beam can do. Furthermore, according to said structure, since the output of each light beam can be made small, there exists an effect that the power consumption by a light source can be suppressed.

  As described above, in the optical pickup of the present invention, the diffractive optical element causes the first light beam to be the first-order diffracted light and the second and third light beams to be incident on the condensing means as the zero-order diffracted light. The depth of the diffraction grating at the diffraction surface of the diffractive optical element has the highest diffraction efficiency of the first-order diffracted light with respect to the first light beam, and 0 with respect to the second and third light beams. In this configuration, the diffraction efficiency of the next diffracted light is set to be the highest.

  According to the above configuration, the loss of the amount of light incident on each recording medium can be reduced with respect to any diffracted light, and the recording medium having different light transmission layer thicknesses using the same light collecting means. And a good condensing spot can be formed. Further, according to the above configuration, the degree of divergence of the second and third light beams with respect to the diffractive optical element can be suppressed, and the influence of the shift in the radial direction of the condenser lens due to tracking or the like can be suppressed. Is possible. Therefore, according to the above configuration, the recording medium is compatible with three recording media having different thicknesses of the light transmission layer, and the diffraction efficiency of each diffracted light incident on each information recording surface of each recording medium is high. Both of them are high, and there is an effect that it is possible to realize an optical pickup capable of recording and erasing information that requires a high output light beam. Furthermore, according to said structure, since the output of each light beam can be made small, there exists an effect that the power consumption by a light source can be suppressed.

  As described above, the optical pickup of the present invention has a configuration in which the diffractive optical element includes a converging diffractive surface and a concave refracting surface as the diffractive surface and the refracting surface.

  According to the above configuration, the usable wavelength range is wider than that of the optical pickup dedicated to the first recording medium, and the wavelength dependence characteristics are improved as compared with the case where the objective lens dedicated to the first recording medium is used alone. Therefore, even if wavelength fluctuations due to mode pop or the like occur, it is possible to maintain good light collecting characteristics. Moreover, according to said structure, the minimum pitch of the diffraction surface of a diffractive optical element can be expanded, and preparation of a diffractive optical element can be made easy. Furthermore, according to the above configuration, it is possible to make a light beam incident on the light collecting means with a predetermined degree of divergence only by entering weak divergent light into the light collecting means. Therefore, the influence on the radial shift of the light collecting means can be reduced, and the position of the light source with respect to the light collecting means can be increased, and the light source can be easily arranged. Play.

  As described above, in the optical pickup of the present invention, the diffractive optical element causes the first, second, and third light beams to enter the condensing means as first-order diffracted light, respectively. The depth of the diffraction grating on the diffractive surface is set so that the diffraction efficiency of the first-order diffracted light is the highest for any of the first, second, and third light beams.

  According to the above configuration, the loss of the amount of light incident on each recording medium can be reduced with respect to any diffracted light, and the recording medium having different light transmission layer thicknesses using the same light collecting means. And a good condensing spot can be formed. Further, according to the above configuration, the degree of divergence of the second and third light beams with respect to the diffractive optical element can be suppressed, and the influence of the shift in the radial direction of the condenser lens due to tracking or the like can be suppressed. Is possible. Therefore, according to the above configuration, the recording medium is compatible with three recording media having different thicknesses of the light transmission layer, and the diffraction efficiency of each diffracted light incident on each information recording surface of each recording medium is high. Both of them are high, and there is an effect that it is possible to realize an optical pickup capable of recording and erasing information that requires a high output light beam. Furthermore, according to said structure, since the output of each light beam can be made small, there exists an effect that the power consumption by a light source can be suppressed.

  As described above, the optical pickup of the present invention has a configuration in which the diffractive optical element includes a diffractive diffractive surface and a convex refracting surface as the diffractive surface and the refracting surface.

  According to said structure, a favorable condensing spot can be formed with respect to each recording medium from which the thickness of a light transmissive layer differs using one condensing means. Moreover, according to said structure, it becomes possible to inject a light beam with the predetermined divergence degree to the said condensing means only by injecting weak divergent light into the said condensing means. Therefore, according to the above configuration, the degree of divergence of the second and third light beams to the diffractive optical element can be suppressed, and the condensing characteristic is further deteriorated due to the shift of the condensing means in the radial direction. While being able to suppress effectively, it becomes possible to distance the position of the light source with respect to the said condensing means, and there exists an effect that arrangement | positioning of a light source becomes easy.

  In the optical pickup of the present invention, as described above, the depth of the diffraction grating at the diffraction surface of the diffractive optical element is set so that the diffraction efficiency of the first-order diffracted light of the first light beam is 90% or more. It is the composition which is.

  According to the above configuration, the diffraction efficiency can be improved with respect to the diffracted light of any of the first, second, and third light beams. Thereby, since the output of each light beam can be made small, the power consumption of a light source can be suppressed. Moreover, according to said structure, there exists an effect that an optical pick-up with higher utilization efficiency is realizable with respect to the 1st light beam for which it is difficult to produce a high output laser.

In the optical pickup of the present invention, as described above, the convergence / divergence degrees of the second and third light beams incident on the diffractive optical element are Φinr and ΦinIr, respectively, and the second and third light incident on the condenser lens. If the convergence / divergence of the light beam is Φoutr and ΦoutIr, respectively,
| Φoutr | >> | Φinr | and | ΦoutIr | >> | ΦinIr |
It is the structure which satisfies.

  According to the above configuration, the absolute value of the degree of convergence / divergence is larger in the second and third light beams emitted from the diffraction grating than in the second and third light beams incident on the diffractive optical element. By doing so, it is possible to converge the second and third light beams incident on the diffractive optical element while sufficiently obtaining the effect of suppressing the increase in spherical aberration caused by the different thicknesses of the light transmission layers. There is an effect that the absolute value of the divergence degree can be reduced.

  As described above, the optical pickup of the present invention has a configuration in which the refractive surface of the diffractive optical element is an aspherical surface.

  According to the above configuration, it is possible to further correct the spherical aberration caused by the difference in the thickness of the light transmission layer of each recording medium, and to suppress the deterioration of the shift characteristic of the light collecting means, which is better. There is an effect that a light collecting characteristic can be obtained.

  The present invention is not limited to the above-described embodiments and examples, and various modifications are possible within the scope shown in the claims, and technical means disclosed in different embodiments are appropriately combined. Examples obtained are also included in the technical scope of the present invention.

  The present invention can be applied to an optical pickup using light beams with different wavelengths.

1 is a block diagram of an optical system in an optical pickup according to an embodiment of the present invention. It is a graph which shows the relationship between the depth of the diffractive optical element of one Embodiment of this invention, and diffraction efficiency. It is a figure explaining the effect | action of the diffractive optical element in the said optical pick-up. The graph which shows the relationship between the primary diffracted light in the light beam of the 1st wavelength, and the diffracted light in the light beam of the 2nd wavelength for determining the shape of a curvature radius in the diffractive optical element of one Embodiment of this invention It is. The relationship between the primary diffracted light in the light beam of the first wavelength and the diffracted light in the light beam of the second wavelength for determining the shape of the radius of curvature in the diffractive optical element of another embodiment of the present invention is shown. It is a graph. (A) is a fragmentary sectional view of the objective lens unit for explaining the operation when the first light beam is incident on the diffractive optical element of another embodiment of the present invention, and (b) is the other in the present invention. It is a fragmentary sectional view of the objective-lens unit explaining the effect | action when a 2nd light beam injects into the diffractive optical element of the Example. (A) is a partial cross-sectional view of an objective lens unit for explaining the operation when a first light beam is incident on a diffractive optical element of still another embodiment of the present invention, and (b) is a diagram of the present invention. It is a fragmentary sectional view of the objective lens unit explaining the operation when the second light beam is incident on the diffractive optical element of still another embodiment. It is a graph which shows the wavelength dependence of wavefront aberration (lambda) rms when spot light is formed in the information recording surface of one recording medium with the optical pick-up of this embodiment. It is a schematic block diagram which shows schematic structure of the optical pick-up in one Embodiment of this invention. It is sectional drawing of the said objective lens unit which showed a mode that a light beam was condensed on the optical disk with the objective lens unit produced as an Example in one Embodiment of this invention, (a) is a 1st light beam 1st. FIG. 6B is a cross-sectional view when the second light beam is focused on the second optical disc. It is a graph which shows the relationship between the shift amount of an objective lens unit at the time of condensing a light beam with the objective lens unit of FIG. 1, and a wavefront aberration, (a) is a graph at the time of condensing a 1st light beam, b) is a graph when the second light beam is condensed. 11 is a graph showing a change in wavefront aberration with respect to a change in wavelength of the first light beam when the first light beam is condensed on the first optical disk by the objective lens unit of FIG. 10. It is sectional drawing of the said objective lens unit which showed a mode that a light beam was condensed on the optical disk with the objective lens unit produced as an Example in one Embodiment of this invention, (a) is a 1st light beam 1st. FIG. 6B is a cross-sectional view when the second light beam is focused on the second optical disc. 14 is a graph showing the relationship between the shift amount of the objective lens unit and the wavefront aberration when the light beam is collected by the objective lens unit of FIG. 13, (a) is a graph when the first light beam is collected, (b) ) Is a graph when the second light beam is condensed. It is sectional drawing of the said objective lens unit which showed a mode that a light beam was condensed on the optical disk with the objective lens unit produced as 3rd Example in one Embodiment of this invention, (a) is a 1st light beam. A sectional view in the case of condensing on the 1st optical disk, (b) is a sectional view in the case of condensing the 2nd light beam on the 2nd optical disk. FIG. 16 is a graph showing the relationship between the shift amount of the objective lens unit and the wavefront aberration when the light beam is condensed by the objective lens unit of FIG. 15, (a) is a graph when the first light beam is condensed; ) Is a graph when the second light beam is condensed. 1 is a configuration diagram showing a schematic configuration of an optical pickup according to an embodiment of the present invention. FIG. 18 is a cross-sectional view for explaining the power of the diffractive surface and the refracting surface of the diffractive optical element provided in the optical pickup of FIG. 17 and the degree of convergence / divergence of the light beam before and after the diffractive optical element. It is a graph which shows the relationship between the depth of the diffraction grating of the diffractive optical element with which the optical pick-up of FIG. 17 is provided, and the diffraction efficiency about each diffraction order. It is a graph which shows the relationship between the convergence / divergence degree of the 1st light beam which injects into the diffractive optical element with which the optical pick-up of FIG. 17 is equipped, and the convergence / divergence degree of a 2nd and 3rd light beam. It is a graph which shows the relationship between the convergence / divergence degree of the 1st light beam which injects into the diffractive optical element with which the optical pick-up of FIG. 17 is equipped, and the wavefront aberration of a 1st, 2nd and 3rd light beam. It is sectional drawing which shows one Example of the convergence / divergence degree of the light beam in the optical pick-up which concerns on one Embodiment of this invention, (a)-(b) is the case of blue light, red light, and infrared light, respectively FIG. 22 is a graph comparing the relationship between the shift amount of the objective shift and the wavefront aberration for the example and the comparative example shown in FIG. 22, and (a) to (c) are cases of blue light, red light, and infrared light, respectively. It is a graph which shows. In the Example shown in FIG. 22, it is a graph which shows the relationship between the wavelength shift of blue light, and a wavefront aberration. It is sectional drawing which shows the other Example of the convergence / divergence degree of the light beam in the optical pick-up which concerns on one Embodiment of this invention, (a)-(c) is respectively blue light, red light, and infrared light. It is sectional drawing which shows a case. FIG. 26 is a graph comparing the relationship between the shift amount of the objective shift and the wavefront aberration for the example and the comparative example shown in FIG. 25, and (a) to (c) are cases of blue light, red light, and infrared light, respectively. It is a graph which shows. In the Example shown in FIG. 25, it is a graph which shows the relationship between the wavelength shift of blue light, and a wavefront aberration. In the Example shown in FIG. 25, it is a graph which shows the relationship between the power of the refractive surface of a diffractive optical element, and the minimum pitch of a diffraction grating. In the example shown in FIG. 25, it is a graph which shows the relationship between the power of the refractive surface of a diffractive optical element, and the wavefront aberration of blue light when the shift amount of an objective shift is 200 micrometers. It is sectional drawing which shows one Example of the convergence / divergence degree of the light beam in the optical pick-up which concerns on one Embodiment of this invention, (a)-(c) is the case of blue light, red light, and infrared light, respectively FIG. 30 is a graph comparing the relationship between the shift amount of the objective shift and the wavefront aberration for the example and the comparative example shown in FIG. 30, and (a) to (c) are cases of blue light, red light, and infrared light, respectively. It is a graph which shows. In the Example shown in FIG. 30, it is a graph which shows the relationship between the wavelength shift of blue light, and a wavefront aberration. It is sectional drawing which shows the other Example of the convergence / divergence degree of the light beam in the optical pick-up which concerns on one Embodiment of this invention, (a)-(c) is respectively blue light, red light, and infrared light. It is sectional drawing which shows a case. It is a graph which compares the relationship between the shift amount of an objective shift, and a wavefront aberration about the Example shown in FIG. 33, and a comparative example, (a)-(c) is the case of blue light, red light, and infrared light, respectively. It is a graph which shows. It is a graph which shows the relationship between the pitch of the diffraction grating in a diffractive optical element, and a diffraction angle difference. It is a graph which shows the relationship between the pitch of a diffraction grating, and the curvature of the refractive surface of a diffractive optical element when the blue light which injected with the parallel light with respect to the diffractive optical element becomes parallel light after passing through a diffractive optical element. It is sectional drawing which shows the other Example of the convergence / divergence degree of the light beam in the optical pick-up which concerns on one Embodiment of this invention, (a)-(c) is respectively blue light, red light, and infrared light. It is sectional drawing which shows a case. It is a graph which compares the relationship between the shift amount of an objective shift, and a wavefront aberration about the Example shown in FIG. 33, and a comparative example, (a)-(c) is the case of blue light, red light, and infrared light, respectively. It is a graph which shows. In the Example shown in FIG. 37, it is a graph which shows the relationship between the wavelength shift of blue light, and a wavefront aberration. It is a schematic block diagram which shows the other example of the optical pick-up of this invention. (A) is explanatory drawing which shows an effect | action when a 1st light beam injects into the objective lens unit with which the concave surface of the diffractive optical element with which the optical pick-up of this invention was equipped was provided in the light source side, (b) These are explanatory drawing which shows an effect | action when a 2nd light beam injects into the objective-lens unit of the optical pick-up of the structure of (a). (A) (b) is a schematic block diagram which shows the other example of the objective lens unit of the optical pick-up of this invention. It is a graph which shows the relationship between the pitch of the diffraction grating in a diffractive optical element, and a diffraction angle difference. 6 is a graph showing the relationship between the pitch of the diffraction grating and the radius of curvature of the refractive surface of the diffractive optical element when blue light incident on the diffractive optical element as parallel light becomes parallel light after passing through the diffractive optical element. . It is a schematic block diagram which shows the other example of the optical pick-up of this invention. It is a schematic block diagram which shows an example of the recording / reproducing apparatus provided with the optical pick-up of this invention.

Explanation of symbols

1a semiconductor laser 1b semiconductor laser 1c semiconductor laser 2a collimator lens 2b collimator lens 2c correction lens 3a shaping optical system 3b shaping optical system 4a beam splitter 4b beam splitter 4c beam splitter 5 dichroic mirror 6 spherical aberration compensation system 7 dichroic mirror 8 1/4 Wave plate 9 Mirror 10 Wavelength selective aperture filter 11 Diffractive optical element 12 Objective lens 13 Objective lens unit 14a First optical disk 14b Second optical disk 14c Third optical disk 15a Reproduction signal detection optical system 15b Reproduction signal detection optical system 15c Reproduction signal detection optics System 16a first optical system 16b second optical system 16c third optical system 100 optical pickup 111 diffractive optical element 112 objective lens 113 objective lens unit 200 optical pickup 211 diffractive optical element 212 objective lens 213 objective lens unit 300 optical pickup 311 diffractive optical element 312 objective lens 313 objective lens unit 411 diffractive optical element 412 objective lens 413 objective lens unit

Claims (15)

A wavelength λ1 is applied to each of the information recording surfaces of the first, second, and third recording media having light transmission layers having thicknesses t1, t2, and t3 (t1 <t2 <t3) on the information recording surfaces, respectively. , Λ2, λ3 (λ1 <λ2 <λ3), the first, second and third light beams are condensed to record or reproduce information,
The first, second, and third light beams are movable in directions substantially perpendicular to the optical axes of the incident first, second, and third light beams, and the first, second, and third light beams are moved to the first, second, and third recording media, respectively. An objective lens for focusing on each information recording surface of
Provided integrally with the objective lens on the incident side of the first, second, and third light beams with respect to the objective lens, and diffracts the first, second, and third light beams. And a diffractive optical element that refracts and makes the first, second, and third light beams of a predetermined diffraction order incident on the objective lens,
The diffractive optical element makes the second and third light beams incident on the objective lens as divergent light,
For the second and third light beams, the convergence / divergence incident on the diffractive optical element is Φinr and ΦinIr, respectively, and the divergence incident on the objective lens is Φoutr and ΦoutIr, respectively.
| Φinr | <| Φoutr | and | ΦinIr | <| ΦoutIr |
Meet the,
Furthermore, the diffractive optical element includes a converging diffractive surface and a concave refracting surface .   The optical pickup according to claim 1, wherein the diffractive optical element causes the first light beam to enter the objective lens as parallel light. The diffractive optical element causes the second-order diffracted light of the first light beam, the first-order diffracted light of the second light beam, and the first-order diffracted light of the third light beam to enter the objective lens, respectively.
In the diffractive optical element, the first light beam has the highest diffraction efficiency of the second-order diffracted light, the second light beam has the highest diffraction efficiency of the first-order diffracted light, and the third light beam has the highest diffraction efficiency of the first-order diffracted light. The optical pickup according to claim 1, wherein is the highest.   4. The optical pickup according to claim 3, wherein in the diffractive optical element, the diffraction efficiency of the second-order diffracted light with respect to the first light beam is 90% or more.   The optical pickup according to claim 3, wherein the third light beam is incident on the diffractive optical element as divergent light.   6. The optical pickup according to claim 5, wherein the first and second light beams are incident on the diffractive optical element as parallel light and divergent light, respectively.   6. The optical pickup according to claim 5, wherein the first light beam is incident on the diffractive optical element as convergent light, and the second light beam is incident as convergent light, parallel light, or divergent light. If the effective diameter of the objective lens for the first light beam is φ,
The degree of convergence / divergence of the first, second and third light beams incident on the diffractive optical element is:
0 ≦ φ × Φinb ≦ 0.11,
−0.048 ≦ φ × Φinr ≦ 0.04, and
−0.18 ≦ φ × ΦinIr ≦ −0.1
The optical pickup according to claim 6 or 7, wherein: When the power of the diffractive optical element with respect to the first, second and third light beams is Φb, Φr and ΦIr, respectively,
−0.11 ≦ φ × Φb ≦ 0,
−0.2 ≦ φ × Φr ≦ −0.002, and
−0.16 ≦ φ × ΦIr ≦ 0.03
The optical pickup according to claim 6 or 7, wherein:   4. The optical pickup according to claim 3, wherein the refractive surface of the diffractive optical element is a spherical surface.   The optical pickup according to claim 3, wherein the power of the refractive surface of the diffractive optical element with respect to the first light beam is −0.1 or more.   A wavelength λ1 is applied to each of the information recording surfaces of the first, second, and third recording media having light transmission layers having thicknesses t1, t2, and t3 (t1 <t2 <t3) on the information recording surfaces, respectively. , Λ2, λ3 (λ1 <λ2 <λ3), the first, second and third light beams are condensed to record or reproduce information,
  The first, second, and third light beams are movable in directions substantially perpendicular to the optical axes of the incident first, second, and third light beams, and the first, second, and third light beams are moved to the first, second, and third recording media, respectively. An objective lens for focusing on each information recording surface of
  Provided integrally with the objective lens on the incident side of the first, second, and third light beams with respect to the objective lens, and diffracts the first, second, and third light beams. And a diffractive optical element that refracts and makes the first, second, and third light beams of a predetermined diffraction order incident on the objective lens,
  The diffractive optical element makes the second and third light beams incident on the objective lens as divergent light,
  For the second and third light beams, the convergence / divergence incident on the diffractive optical element is Φinr and ΦinIr, respectively, and the divergence incident on the objective lens is Φoutr and ΦoutIr, respectively.
| Φinr | <| Φoutr | and | ΦinIr | <| ΦoutIr |
The filling,
  Further, the diffractive optical element causes the second-order diffracted light of the first light beam, the first-order diffracted light of the second light beam, and the first-order diffracted light of the third light beam to enter the objective lens, respectively.
  In the diffractive optical element, the first light beam has the highest diffraction efficiency of the second-order diffracted light, the second light beam has the highest diffraction efficiency of the first-order diffracted light, and the third light beam has the highest diffraction efficiency of the first-order diffracted light. Is the highest,
  An optical pickup characterized in that a refractive surface of the diffractive optical element is a spherical surface.   A wavelength λ1 is applied to each of the information recording surfaces of the first, second, and third recording media having light transmission layers having thicknesses t1, t2, and t3 (t1 <t2 <t3) on the information recording surfaces, respectively. , Λ2, λ3 (λ1 <λ2 <λ3), the first, second and third light beams are condensed to record or reproduce information,
  The first, second, and third light beams are movable in directions substantially perpendicular to the optical axes of the incident first, second, and third light beams, and the first, second, and third light beams are moved to the first, second, and third recording media, respectively. An objective lens for focusing on each information recording surface of
  Provided integrally with the objective lens on the incident side of the first, second, and third light beams with respect to the objective lens, and diffracts the first, second, and third light beams. And a diffractive optical element that refracts and makes the first, second, and third light beams of a predetermined diffraction order incident on the objective lens,
  The diffractive optical element makes the second and third light beams incident on the objective lens as divergent light,
  For the second and third light beams, the convergence / divergence incident on the diffractive optical element is Φinr and ΦinIr, respectively, and the divergence incident on the objective lens is Φoutr and ΦoutIr, respectively.
| Φinr | <| Φoutr | and | ΦinIr | <| ΦoutIr |
The filling,
  Further, the diffractive optical element causes the second-order diffracted light of the first light beam, the first-order diffracted light of the second light beam, and the first-order diffracted light of the third light beam to enter the objective lens, respectively.
  In the diffractive optical element, the first light beam has the highest diffraction efficiency of the second-order diffracted light, the second light beam has the highest diffraction efficiency of the first-order diffracted light, and the third light beam has the highest diffraction efficiency of the first-order diffracted light. Is the highest,
  The power of the refractive surface of the diffractive optical element with respect to the first light beam is −0.1 or more. Diffraction surface of the diffractive optical element, the optical pickup according to any one of claims 1 to 13, characterized in that it is formed on the refractive surface. The optical pickup according to any one of claims 1 to 14 , wherein a diffractive surface of the diffractive optical element is formed with a sawtooth or stepped diffraction grating.

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