JP2006004570A - Optical lens, optical head, optical information apparatus, computer, optical information medium recorder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical head capable of realizing a recording/reproducing operation of three kinds of optical disks having the different thicknesses by using one objective lens and to provide an apparatus applying them. <P>SOLUTION: The apparatus is equipped with: 1st, 2nd volume holograms for diffracting a red light; 3rd, 4th volume holograms for diffracting an infrared light; and a refractive lens, and a blue light is converged to a transparent layer of 0.1mm by the refractive lens through the volume hologram, and for the red light, the diffracted lights of the 1st volume hologram are partially crossed and incident to the 2nd hologram, and the diffracted lights of the 2nd volume hologram are converged to a transparent layer of 0.6mm by the refractive lens. The infrared light, is diffracted as well as the red light by the 3rd, 4th volume holograms to be converged to a transparent layer of 1.2mm by the refractive lens. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical head for recording / reproducing or erasing information stored on an optical information medium such as an optical disc, an optical lens used for the optical head, an optical information device using the optical head, and applications thereof. Related to the system.

  As high-density and large-capacity storage media, optical discs have been expanded to audio discs, video discs, document file discs, and data files. Furthermore, in recent years, with the advance of optical technology and the shortening of the wavelength of a semiconductor laser, which is a light source, development of an optical disc having a higher storage capacity than before has been progressing. As an approach for increasing the density, it has been studied to increase the numerical aperture (hereinafter referred to as NA) of an optical system that finely focuses a light beam onto an optical disk. At that time, the problem is an increase in aberration due to the inclination of the optical axis. When NA is increased, the aberration generated with respect to the inclination of the optical axis increases. In order to prevent this, the transparent layer of the optical disk may be thinned. In the present specification, the transparent layer refers to a transparent medium interposed from the surface on which the light beam enters the optical disk to the information recording surface.

  A compact disc (hereinafter referred to as CD) which can be said to be the first generation of an optical disc uses infrared light having a wavelength of 780 nm, the NA of the objective lens is 0.45, and the thickness of the transparent layer of the optical disc is 1.2 mm. The second generation digital versatile disk (hereinafter referred to as DVD) uses red light with a wavelength of 650 nm, the NA of the objective lens is 0.6, and the thickness of the transparent layer of the optical disk is 0.6 mm. The third-generation Blu-ray disc (hereinafter referred to as BD) uses blue light with a wavelength of 405 nm, the NA of the objective lens is 0.85, and the thickness of the transparent layer of the optical disc is 0.1 mm. As described above, the transparent layer of the optical disk becomes thinner as the density increases.

  In BD, which is a high-density optical disc, inheritance of assets stored on DVDs and CDs is desired. From the viewpoint of the size of the device, an optical information device capable of recording / reproducing different optical discs with one optical head is required. Yes. For this purpose, an optical head capable of focusing the light beam to the diffraction limit is required for optical disks having different transparent layer thicknesses. Further, when recording / reproducing a disc having a thick transparent layer, it is necessary to focus the light beam on a recording surface located far from the surface of the optical disc, and the focal length must be made longer.

  For this reason, a configuration for recording / reproducing different types of optical disks using light beams having a plurality of wavelengths has been disclosed. This will be described with reference to FIGS.

  FIG. 18 shows a schematic configuration of an optical head. Parallel light emitted from an optical system 60 having a light source with a wavelength of 650 nm is transmitted through a beam splitter 62 and a wavelength selection phase plate 63, and the thickness of the transparent layer is obtained by the objective lens 64. It converges on the information recording surface of the 0.6 mm optical disk 52. The light reflected by the optical disk 52 follows the reverse path and is detected by the detector of the optical system 60. The divergent light emitted from the optical system 61 having a light source having a wavelength of 780 nm is reflected by the beam splitter 62, transmitted through the wavelength selection phase plate 63, and by the objective lens 64, the information recording surface of the optical disk 53 having a transparent layer thickness of 1.2 mm. To converge. The light reflected by the optical disk 53 follows the reverse path and is detected by the detector of the optical system 61.

  The objective lens 64 is designed to converge on a DVD having a transparent layer thickness of 0.6 mm when collimated light with a wavelength of 650 nm is incident. When a CD having a transparent layer thickness of 1.2 mm is recorded and reproduced, the thickness of the transparent layer is set. Due to the difference, spherical aberration occurs. In order to correct this spherical aberration, the light beam emitted from the optical system 61 and incident on the objective lens 64 is made into divergent light, and a wavelength selection phase plate 63 is used. When divergent light is incident on the objective lens, a new spherical aberration is generated. Therefore, the spherical aberration caused by the difference in the thickness of the transparent layer is eliminated by the new spherical aberration, and the wavefront is also corrected by the wavelength selection phase plate 63. is doing.

  FIG. 19A is a plan view of the wavelength selection phase plate 63 and FIG. 19B is a side view thereof. The wavelength selection phase plate 63 includes a phase step 63a having heights h and 3h, where the refractive index at the wavelength λ2 (650 nm) is n2, and h is λ2 / (n2-1). For light having a wavelength of 650 nm, the difference in optical path length caused by the height h is the wavelength λ2, and the phase difference corresponds to 2π radians, so the phase distribution is not affected and the recording / reproduction of the optical disk 52 is affected. Not give. On the other hand, for light of wavelength λ3 (780 nm), if the refractive index is n3, an optical path length difference of h / λ3 × (n3-1) is generated. This difference in optical path length is not an integral multiple of the wavelength, and a phase difference occurs. The aberration correction described above is performed using this phase difference (for example, Patent Document 1).

  As a second conventional example, a configuration corresponding to three types of discs is disclosed by combining two wavelength selection phase plates and an objective lens. This will be described with reference to FIG. FIG. 20 shows a schematic configuration of the optical head. Blue light 71a having a wavelength of 405 nm emitted from the optical unit 70 is diffracted by the diffraction element 72 and becomes substantially parallel light by the concave lens 73 to correct chromatic aberration generated by the objective lens 77, thereby converting the transparent layer. Is converged on the information recording surface of the optical disk 51 having a thickness of 0.1 mm. The red light 71b having a wavelength of 650 nm emitted from the optical unit 70 is diverged by the concave lens 73 without being affected by the diffraction element 72, and the phase is corrected by the wavelength selection phase plate 75 so that the transparent layer has a thickness of 0.6 mm. It converges on the information recording surface of the optical disc 52. Further, the infrared light 71c having a wavelength of 780 nm emitted from the optical unit 70 becomes divergent light by the concave lens 73 without being affected by the diffraction element 72, the phase is corrected by the wavelength selection phase plate 76, and the thickness of the transparent layer 1. It converges on the information recording surface of the 2 mm optical disk 53. The light reflected by the optical discs 51, 52, 53 follows the reverse path and is detected by the detector of the optical unit 70.

  The wavelength selection phase plate 75 generates a phase step for a light beam having a wavelength of 650 nm, and a phase step 75a is provided for a light beam having a wavelength of 405 nm and a wavelength of 780 nm so that the phase difference is approximately an integer multiple of each wavelength. ing. Therefore, the phase distribution of the light beams 71a and 71c is not affected. The wavelength selection phase plate 76 has a phase step for a light beam having a wavelength of 780 nm, and a phase step 76a for the light beam having a wavelength of 405 nm and 650 nm that is substantially an integral multiple of each wavelength. For this reason, the phase distribution of the light beams 71a and 71b is not affected.

  The objective lens 77 is designed to converge on a BD having a transparent layer thickness of 0.1 mm when blue light having a wavelength of 405 nm is incident in parallel. Spherical aberration occurs due to the difference in thickness. This spherical aberration is corrected using the spherical aberration generated by making the divergent light by the concave lens 73 and the wavelength selection phase plate 75. Further, spherical aberration that occurs when recording and reproducing a CD having a transparent layer thickness of 1.2 mm is corrected by using spherical aberration that occurs when diverging light is generated by the concave lens 73 and a wavelength selection phase plate 76. (For example, Patent Document 2).

  As a third conventional example, a configuration in which a plurality of objective lenses are switched mechanically is disclosed (for example, Patent Document 3).

As a fourth conventional example, a refractive objective lens and a hologram are combined in the same manner as in the first conventional example, and the thickness of the transparent layer is increased by using chromatic aberration generated in the same order diffracted light of different wavelengths. A configuration for correcting the difference is disclosed (for example, Patent Document 4).
JP-A-10-334504 (page 7-9, FIGS. 1 to 4) JP 2003-281775 A (page 6-11, FIG. 1) Japanese Patent Laid-Open No. 11-296890 (page 4-6, FIG. 1) Japanese Unexamined Patent Publication No. 2000-81566 (page 4-6, FIGS. 1 and 2)

  The first conventional example uses a wavelength selective phase plate as an interchangeable element. When recording / reproducing an optical disk with a thick transparent layer, the recording surface is further away from the objective lens by the thickness of the transparent layer, so it is necessary to increase the focal length. The focal length can be extended by having the lens power of the compatible element, but the wavelength selective phase plate has no lens power. For this reason, even if the spherical aberration of DVD or CD is corrected with an objective lens having a short working distance such as BD, the working distance cannot be secured, and the optical disk and the objective lens collide. Furthermore, this method is a patent relating to compatibility between CD and DVD, and cannot cope with three wavelengths including BD.

  In the second conventional example, two wavelength selective phase plates are used for recording / reproducing BD, DVD, and CD. However, the diffraction element and the wavelength selective phase plate have wavelength selectivity of three wavelengths. Grooves become deeper, and light loss increases due to diffraction loss on the groove slope and errors in groove depth accuracy. Furthermore, due to the difference in NA, the aperture of DVD and CD becomes small, so that the light capture rate deteriorates. Since the emission angle of the semiconductor laser does not differ greatly between BD, DVD and CD, the light capture rate is almost proportional to the aperture. For example, in the case of a CD, since the area ratio is 26% compared to the BD, the light amount is reduced to about ¼. In addition, since the concave lens is combined with the diffraction element to correct the BD chromatic aberration, the focal length cannot be sufficiently extended for DVD and CD, and the working distance when recording / reproducing CD is 0.3 mm or less. Become. Since the CD has a 0.3 mm stack rib (ring-shaped protrusion on the inner periphery), the optical head collides with the CD in accordance with the standard. Further, this configuration requires a diffraction grating, a concave lens, two wavelength selection phase plates, and an objective lens, which makes the structure complicated and disadvantageous in terms of cost. Further, when these are integrated and focus driving or tracking driving is performed, the mass of the movable part of the actuator increases, and high-speed driving becomes difficult.

  In the third conventional example, since the objective lenses are switched, there are problems that a plurality of objective lenses are required, the number of parts is increased, and it is difficult to reduce the size of the optical head device. In addition, there is a problem that it is difficult to reduce the size of the apparatus in that a switching mechanism is required.

  In the fourth conventional example, the first-order diffraction of a hologram is used as a compatible element. This method is compatible with CD and DVD, but with compatibility including BD, the hologram groove becomes deep, so the diffraction loss on the groove slope and the light loss due to accuracy error increase, making it difficult to secure the recording light amount. . Further, when the wavelength changes, the spherical aberration increases due to the chromatic aberration of the hologram, thereby deteriorating the recording / reproducing performance. Furthermore, as described above, due to the difference in NA between BD, DVD, and CD, the light capture rate of DVD and CD deteriorates.

  From these facts, practical optical heads with three-wavelength compatibility of infrared light, red light, and blue light corresponding to the thickness of the transparent layer of the optical disk of 1.2 mm, 0.6 mm, and 0.1 mm are Not done.

  The present invention solves the conventional problems as described above, and realizes compatible reproduction and compatible recording of different types of optical disks using a single objective lens, and is sufficient for recording by increasing the light capture rate. To provide an optical head capable of ensuring stable light quantity, suppressing the change of focal length and the occurrence of spherical aberration at the time of wavelength change at the time of light quantity switching, etc., and capable of stably recording and reproducing information, and an apparatus using these optical heads With the goal.

  In order to achieve the above object, a first configuration of an optical lens according to the present invention includes first and second holograms that diffract light having a wavelength λ2 and a refractive lens, and the light having a wavelength λ1 has a thickness t1. The light having the wavelength λ2 is converged through the transparent layer having the thickness t2.

  The desirable configuration of the present invention is that the light of the wavelength λ1 enters the refractive lens through the first and second holograms and is converged through the transparent layer having the thickness t1, and the light of the wavelength λ2 is focused on the first hologram. , The at least part of the optical axis of the diffracted light of the first hologram intersects and enters the second hologram, the diffracted light of the second hologram enters the refractive lens, and the transparent layer has a thickness t2. It is characterized by being converged through. According to this configuration, it is possible to suppress an increase in spherical aberration of light that converges through the transparent layer having the thickness t2 when the wavelength of the light having the wavelength λ2 changes.

  In the first configuration of the optical lens of the present invention, it is desirable that the light of wavelength λ2 is diffracted by the first hologram and the second hologram to become divergent light and enter the refractive lens.

  In the first configuration of the optical lens of the present invention, the change in the diffraction angle of the first hologram due to the change in the wavelength of the light having the wavelength λ2 and the change in the diffraction angle of the second hologram are offset. It is desirable to be.

  In the first configuration of the optical lens according to the present invention, the light at the outermost periphery of the light having the wavelength λ2 incident on the first hologram is diffracted by the first hologram and becomes the innermost periphery of the second hologram. It is desirable to enter. According to this desirable example, there is no discontinuous region in the light intensity distribution of the light of wavelength λ2 that converges on the transparent layer, and a smooth light intensity distribution is obtained.

  In the first configuration of the optical lens of the present invention, the beam diameter when light of wavelength λ2 is incident on the first hologram is D1, and the beam diameter when diffracted by the first hologram and incident on the second hologram If D2 is D2, it is desirable that D1> D2. According to this desirable example, it is possible to prevent deterioration of the light capturing rate of the wavelength λ2 due to the difference between the NA1 and the NA2.

  In the first configuration of the optical lens of the present invention, it is preferable that the first and second holograms are volume holograms having a refractive index distribution. According to this desirable example, it is possible to realize an optical lens having good hologram wavelength selectivity, good diffraction efficiency, and little light loss.

  In the first configuration of the optical lens of the present invention, it is desirable to form the first hologram on one surface of the transparent parallel plate and the second hologram on the other surface of the transparent parallel plate. According to this desirable example, the optical lens can be made small and light, and the actuator can be driven at high speed.

  In the first configuration of the optical lens of the present invention, the numerical aperture at which the light of wavelength λ1 is converged through the transparent layer having the thickness t1 of the transparent layer is NA1, and the light of wavelength λ2 is converged through the transparent layer of the thickness t2 of the transparent layer. Assuming that the numerical aperture is NA2, it is desirable that t1 <t2 and NA1> NA2.

  The first optical head of the present invention includes the optical lens having the first configuration, a first light source that emits light having a wavelength λ1, a second light source that emits light having a wavelength λ2, and recording on an optical information medium. A light detection unit that receives light reflected by the surface and extracts an electrical signal in accordance with the amount of light, and the optical lens converts the first light beam from the first light source into a transparent layer having a transparent layer thickness t1. And converges on the recording surface of the first optical information medium through the transparent layer having the thickness t2 of the transparent layer and converges on the recording surface of the second optical information medium through the transparent layer having the thickness t2. <T2.

  In the optical lens and the first optical head of the first configuration of the present invention, it is desirable that the wavelength of the light having the wavelength λ1 is in the vicinity of 405 nm and the wavelength of the light having the wavelength λ2 is in the vicinity of 650 nm.

  In the optical lens and the first optical head of the first configuration of the present invention, it is desirable that the thickness t1 of the transparent layer is approximately 0.1 mm and the thickness t2 of the transparent layer is approximately 0.6 mm.

  A second configuration of the optical lens according to the present invention includes first and second holograms that diffract light having a wavelength λ2, third and fourth holograms that diffract light having a wavelength λ3, and a refractive lens. The light having the wavelength λ1 is converged through the transparent layer having the thickness t1, the light having the wavelength λ2 is converged through the transparent layer having the thickness t2, and the light having the wavelength λ3 is converged through the transparent layer having the thickness t3.

  A desirable configuration of the present configuration is that the light with the wavelength λ1 enters the refractive lens through the first and second holograms, is converged through the transparent layer with the thickness t1, and the light with the wavelength λ2 becomes the first hologram. Incident, at least part of the optical axis of the diffracted light of the first hologram intersects and enters the second hologram, the diffracted light of the second hologram enters the refractive lens, and passes through the transparent layer having the thickness t2. The converged light of wavelength λ3 is incident on the third hologram, at least part of the optical axis of the diffracted light of the third hologram intersects and is incident on the fourth hologram, and the diffracted light of the fourth hologram is It is incident on the refractive lens and converged through a transparent layer having a thickness t3. According to this configuration, optical disks having different transparent layer thicknesses can be recorded and reproduced with one objective lens, and an increase in spherical aberration when the wavelengths λ2 and λ3 change can be suppressed.

  In the second configuration of the optical lens of the present invention, the light of wavelength λ2 is diffracted by the first hologram and the second hologram to become divergent light, and is incident on the refractive lens, and the light of wavelength λ3 is It is desirable that the light is diffracted by the hologram 3 and the fourth hologram to become divergent light and enter the refractive lens.

  In the second configuration of the optical lens of the present invention, the change in the diffraction angle of the first hologram due to the change in the wavelength of the light having the wavelength λ2 and the change in the diffraction angle of the second hologram cancel each other. It is desirable that the change in the diffraction angle of the third hologram due to the change in the wavelength of the light having the wavelength λ3 and the change in the diffraction angle of the fourth hologram be offset.

  In the second configuration of the optical lens of the present invention, the light at the outermost periphery of the light having the wavelength λ2 incident on the first hologram is diffracted by the first hologram and is incident on the innermost periphery of the second hologram. It is desirable that the light at the outermost periphery of the light having the wavelength λ3 that enters and enters the third hologram is diffracted by the third hologram and is incident on the innermost periphery of the fourth hologram. According to this desirable example, there is no discontinuous region in the light intensity distribution of the light of wavelength λ2 and the light of wavelength λ3 that converges on the transparent layer, and a smooth light intensity distribution is obtained.

  In the second configuration of the optical lens of the present invention, the light of the wavelength λ2 is incident on the first hologram with a diameter D1, the diffracted light of the first hologram is incident on the second hologram with a diameter D2, When the light of wavelength λ3 is incident on the third hologram with a diameter D3 and the diffracted light of the third hologram is incident on the fourth hologram with a diameter D4, D1> D2 and / or D3> D4. Is desirable. According to this desirable example, it is possible to prevent deterioration of the light capturing rate of the wavelength λ2 due to the difference between the NA1, the NA2, and the NA3.

  A third configuration of the optical lens according to the present invention includes first and second holograms that diffract light having a wavelength λ1, third and fourth holograms that diffract light having a wavelength λ3, and a refractive lens. The light of wavelength λ2 is incident on the refractive lens through the first and second holograms, converged through the transparent layer of thickness t2, and the light of wavelength λ1 is incident on the first hologram. At least part of the optical axes of the diffracted light beams intersect and enter the second hologram, and the diffracted light beams of the second hologram enter the refractive lens and are converged through the transparent layer having the thickness t1, and light having wavelength λ3. Is incident on the third hologram, at least part of the optical axis of the diffracted light of the third hologram intersects and is incident on the fourth hologram, and the diffracted light of the fourth hologram is incident on the refractive lens, Through a transparent layer of thickness t3 It is characterized by convergence. According to this configuration, optical disks having different transparent layer thicknesses can be recorded and reproduced with one objective lens, and an increase in spherical aberration when the wavelengths λ1 and λ3 change can be suppressed.

  In the third configuration of the optical lens of the present invention, the light having the wavelength λ1 is diffracted by the first hologram and the second hologram to become convergent light, and is incident on the refractive lens, and the light having the wavelength λ3 is It is desirable that the light is diffracted by the hologram 3 and the fourth hologram to become divergent light and enter the refractive lens.

  In the third configuration of the optical lens of the present invention, the change in the diffraction angle of the first hologram due to the change in the wavelength of the light having the wavelength λ1 and the change in the diffraction angle of the second hologram are offset. It is desirable that the change in the diffraction angle of the third hologram due to the change in the wavelength of the light having the wavelength λ3 and the change in the diffraction angle of the fourth hologram be offset.

  In the third configuration of the optical lens of the present invention, the light at the outermost periphery of the light having the wavelength λ1 incident on the first hologram is diffracted by the first hologram and becomes the innermost periphery of the second hologram. It is desirable that the light at the outermost periphery of the light having the wavelength λ3 that enters and enters the third hologram is diffracted by the third hologram and is incident on the innermost periphery of the fourth hologram. According to this desirable example, there is no discontinuous region in the light intensity distribution of the light of wavelength λ1 and the light of wavelength λ3 that converges on the transparent layer, and a smooth light intensity distribution is obtained.

  In the third configuration of the optical lens of the present invention, the light of the wavelength λ1 is incident on the first hologram with a diameter D1, the diffracted light of the first hologram is incident on the second hologram with a diameter D2, When the light of wavelength λ3 is incident on the third hologram with a diameter D3 and the diffracted light of the third hologram is incident on the fourth hologram with a diameter D4, D1 <D2 and / or D3> D4. Is desirable. According to this desirable example, it is possible to prevent deterioration of the light capturing rate of the wavelength λ2 due to the difference between the NA1, the NA2, and the NA3.

  In the second and third configurations of the optical lens of the present invention, it is desirable that the first to fourth holograms are volume holograms having a refractive index distribution. According to this desirable example, it is possible to realize an optical lens having good hologram wavelength selectivity, good diffraction efficiency, and low light loss.

  In the second and third configurations of the optical lens of the present invention, the first and third holograms are placed on one side of the transparent parallel plate, and the second and fourth holograms are placed on the other side of the transparent parallel plate. It is desirable to form on the surface. According to this desirable example, the optical lens can be made small and light, and the actuator can be driven at high speed.

  In the second and third configurations of the optical lens of the present invention, the numerical aperture at which the light of wavelength λ1 is converged through the transparent layer of t1 is NA1, and the numerical aperture of the light at wavelength λ2 is converged through the transparent layer of thickness t2. Is NA2, and the numerical aperture at which the light with wavelength λ3 is converged through the transparent layer of thickness t3 is NA3, it is desirable that t1 <t2 <t3 and NA1> NA2> NA3.

  A second optical head of the present invention includes the optical lens having the second or third configuration, a first light source that emits light having a wavelength λ1, a second light source that emits light having a wavelength λ2, and a wavelength λ3. A third light source that emits the light and a light detection unit that receives the light reflected by the recording surface of the optical information medium and extracts an electrical signal according to the amount of the light, and the optical lens includes the first light source The first light beam from the second light beam is converged on the recording surface of the first optical information medium through the transparent layer having the thickness t1, and the second light beam from the second light source passes through the transparent layer having the thickness t2. The light beam converges on the recording surface of the information medium, and the third light beam from the third light source converges on the recording surface of the third optical information medium through the transparent layer having a thickness t3, and t1 <t2 <t3. And

  In the optical lenses of the second and third configurations of the present invention and the optical head of the second configuration, the wavelength of the light of the wavelength λ1 is about 405 nm, and the wavelength of the light of the wavelength λ2 is about 650 nm, It is desirable that the wavelength of the light having the wavelength λ3 is in the vicinity of 780 nm.

  In the optical lenses having the second and third configurations of the present invention and the optical head having the second configuration, the thickness t1 of the transparent layer is approximately 0.1 mm, and the thickness t2 of the transparent layer is approximately 0.6 mm. It is desirable that the thickness t3 of the transparent layer is approximately 1.2 mm.

  The optical information device of the present invention includes an optical head including any one of the optical lenses and a laser light source, a motor for rotating an optical information medium, a signal obtained from the optical head device, and the motor based on the signal. And an electric circuit for controlling and driving at least one of the optical lens and the laser light source.

  The computer of the present invention includes the optical information device, and performs an operation based on at least one of the input information and the information reproduced from the optical information device, the input information, the optical An output device that outputs at least one of information reproduced from the information device and a result calculated by the arithmetic device is provided.

  The optical information medium recorder of the present invention includes the optical information device and an encoder that converts image information into information recorded by the optical information device.

  The present invention realizes an optical system with good wavelength selectivity and good wavefront conversion efficiency mainly by two volume holograms having a refractive index distribution, and diffracts a hologram that diffracts light of wavelength λ2 and light of wavelength λ3. By combining holograms, spherical aberration due to the difference in transparent layer thickness between BD, DVD, and CD can be corrected, and compatibility between different types of discs can be realized. In addition, by combining two volume holograms, it is possible to prevent a decrease in light capture rate due to a difference in NA.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
An optical head according to a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a side view showing the basic configuration of the optical head and the state of light propagation in the same embodiment, FIG. 2A is a plan view of the first hologram, and FIG. 3 is a side view of the hologram and objective lens, FIG. 3 is an enlarged view showing details of the first hologram, and FIG. 4 shows a relationship between the wavelength shift of the wavelength λ2 incident on the first and second holograms and the first-order diffracted light. FIG. 5 is a diagram showing the behavior of diffracted light from the first and second holograms when wavelength variation occurs in incident light of wavelength λ2, and FIG. 6 is a diagram of when wavelength shift occurs in wavelength λ2. FIG. 7 is a diagram showing the change in the amount of aberration, FIG. 7 is a diagram showing the relationship between the refractive index change amount Δn of the first hologram and the second hologram and the first-order diffraction efficiency with respect to incident light of wavelength λ2, and FIG. 8 (a). Is the first hologram when the incident light of wavelength λ2 is aperture limited in accordance with NA Shows a plan view of the beam, FIG. 8 (b) shows a side view of the first and second hologram and the objective lens at that time.

  In FIG. 1, 1 is a first hologram and 2 is a second hologram, which are formed on the surface of 10 transparent parallel plates and diffract red light having a wavelength λ2 (approximately 650 nm). Reference numeral 11 denotes a blue laser that emits blue light having a wavelength λ1 (approximately 405 nm), and reference numeral 12 denotes a red laser that emits red light having a wavelength λ2. 14 is a collimating lens, and 15 is an objective lens. The objective lens 15 has an NA of 0.85, and is designed so that spherical aberration is reduced when a light beam having a wavelength λ1 is narrowed through a transparent layer of 0.1 mm. The first and second holograms 1 and 2, the transparent parallel plate 10 and the objective lens 15 are attached to 16 lens barrels, and the lens barrel 16 is configured to follow the track of the optical disk by an actuator.

  Reference numeral 51 denotes an optical disk having a transparent layer thickness t1 of about 0.1 mm or thinner, and is an optical information medium that is recorded and reproduced by a light beam having a wavelength λ1, for example, an optical disk for BD. Reference numeral 52 denotes an optical information medium, for example, an optical disk for DVD, which has a thickness t2 of the transparent layer of about 0.6 mm and is recorded and reproduced by a light beam having a wavelength λ2. The optical disks 51 and 52 show only the transparent layer from the surface on which the light beam is incident to the recording surface. Actually, a protective material is bonded to secure the mechanical strength and to make the outer shape 1.2 mm which is the same as that of the CD. The optical disk 52 is laminated with a protective material having a thickness of 0.6 mm, and the optical disk 51 is laminated with a protective material having a thickness of 1.1 mm. In the drawings of the present invention, the protective material is omitted for simplicity.

  When recording / reproducing the optical disk 51 having a high recording density, the light beam 41 having the wavelength λ 1 emitted from the blue laser 11 is reflected by the beam splitter 17, becomes substantially parallel light by the collimator lens 14, and is circularly polarized by the wavelength plate 19. become. The wave plate 19 is designed to act as a substantially quarter wave plate for both the wavelength λ1 and the wavelength λ2. The circularly polarized light beam 41 passes through the first hologram 1, the transparent parallel plate 10 and the second hologram 2, is narrowed down by the refractive objective lens 15, and passes through the transparent layer having a thickness of about 0.1 mm. 51 converges on the information recording surface. The light beam 41 reflected by the information recording surface of the optical disc 51 travels back along the original optical path to become linearly polarized light in a direction perpendicular to the forward path by the wave plate 19, passes through the beam splitter 17, and passes through the beam splitter 18. Reflected. The beam splitter 17 reflects linearly polarized light in one direction and transmits linearly polarized light in a direction perpendicular thereto with respect to the light beam having the wavelength λ1. As will be described later, the light beam 42 having the wavelength λ2 emitted from the red laser 12 is transmitted regardless of the polarization. Thus, the beam splitter 17 is an optical path branching element having wavelength selectivity as well as polarization characteristics, and the optical multilayer film 17a is formed so as to have such characteristics. The light beam 41 reflected by the beam splitter 18 is given astigmatism by the detection lens 20 and enters the photodetector 21. By calculating the output of the photodetector 21, servo signals and information signals used for focus control and tracking control are obtained. The focus signal can be detected by, for example, the astigmatism method, and the tracking signal can be detected by, for example, the push-pull method.

  When recording / reproducing the optical disk 52, the light beam 42 having the wavelength λ 2 emitted from the red laser 12 passes through the beam splitter 18 and the beam splitter 17, is made into substantially parallel light by the collimator lens 14, and is circularly polarized by the wavelength plate 19. become. The circularly polarized light beam 42 is diffracted by the first hologram 1, diffracted by the second hologram 2 through the transparent parallel plate 10, and diffracted in the opposite direction by the objective lens 15. It converges on the information recording surface of the optical disc 52 through a 6 mm transparent layer. The light beam reflected by the information recording surface of the optical disk 52 follows the original optical path in reverse, passes through the beam splitter 17, and is reflected by the beam splitter 18. The beam splitter 18 transmits linearly polarized light in one direction with respect to the wavelength λ2, and reflects linearly polarized light in a direction perpendicular thereto. Further, the light beam 41 having the wavelength λ1 is reflected regardless of the polarization. As described above, the beam splitter 18 is also an optical path branching element having wavelength selectivity as well as polarization characteristics, and the optical multilayer film 18a is formed so as to have such characteristics. The light beam 42 reflected by the beam splitter 18 is given astigmatism by the detection lens 20 and enters the photodetector 21.

  By calculating the output of the photodetector 21, servo signals and information signals used for focus control and tracking control are obtained. In order to obtain the servo signals of the optical disks 51 and 52 from the common photodetector 21 in this way, the emission points of the blue laser 11 and the red laser 12 are imaged with respect to the common position on the objective lens 15 side. Arrange as shown. By doing so, the number of detectors and the number of wirings can be reduced.

  Next, the function and configuration of the first hologram 1, the second hologram 2, and the objective lens 15 will be described in detail with reference to FIGS. Since the objective lens 15 is designed to reduce spherical aberration when the light beam 41 having the wavelength λ1 is focused through the optical disk having the transparent layer of 0.1 mm, the light beam having the wavelength λ2 is focused through the optical disk having the transparent layer having the thickness of 0.6 mm. When this occurs, spherical aberration occurs. The present invention uses the first and second holograms 1 and 2 to wavefront convert the light beam 42 having the wavelength λ2 into divergent light that corrects this spherical aberration.

  In the present embodiment, both the first and second holograms 1 and 2 use volume holograms. The volume hologram is a hologram having a refractive index distribution having a periodic structure or a quasi-periodic structure and having a thickness that is larger than the period. A quasi-periodic structure is a structure in which the period does not change suddenly and the period gradually changes. The first and second holograms 1 and 2 which are volume holograms have a refractive index distribution having a periodic structure or a quasi-periodic structure in the radial direction, and there is no rapid change in the radial direction except for the central portion. These volume holograms are obtained by, for example, using a photopolymer and applying a quasi-sine wave refractive index distribution by a two-beam interference method using a known Ar laser beam (for example, wavelength λ = 0.5145 μm or 0.488 μm). A periodic structure is formed. By using volume holograms for the first and second holograms 1 and 2, it is possible to realize a high diffraction efficiency of 90% or more even at a large diffraction angle such as 45 °. The material of the volume hologram is not necessarily limited to a photosensitive resin such as a fat polymer, but may be LiNbO3, BiTiO3 or the like doped with gelatin or Fe having a photorefractive effect.

  In the first hologram 1 shown in FIGS. 2A and 3, for example, the thickness of the refractive index distribution layer is L1 = 6 μm, and the refractive index change amount Δn (difference between the refractive indexes n1a and n1b in FIG. 3) is 0. .055, a quasi-periodic structure in which the refractive index distribution period Λ1 is around 0.57 μm, and the front of the fringe 1a is inclined toward the center. Further, the inclination angle φ is a half value of the diffraction angle, for example, around 22.5 °. As a result, the light beam 42 incident on the first hologram 1 is emitted as first-order diffracted light, for example, around 45 ° obliquely (θ1 is around 45 °) by Bragg diffraction.

  The second hologram 2 is different from the first hologram 1 in the period Λ2 and the fringe 2a of the refractive index distribution layer, and has an average diffraction angle smaller than that of the first hologram. The front edge of the fringe 2a is inclined in the outer peripheral direction, and the light diffracted by the first hologram 1 at an angle of about 45 ° is returned to the optical axis by the diffraction of the second hologram 2, and becomes divergent light. The light enters the lens 15. The period Λ2 and fringe 2a of the second hologram are designed so that the spherical aberration is reduced when the divergent light enters the objective lens 15 and converges through the 0.6 mm transparent layer.

  In the light beam 42 of FIG. 2B, the light incident on the point Pa of the first hologram 1 is diffracted in the central direction (left side with respect to the traveling direction of the light in the figure). Crosses the beam and enters the point Pb of the second hologram 2. The diffracted light incident on the point Pb is diffracted in the direction in which the optical axis is returned by the second hologram 2 (rightward with respect to the light traveling direction), enters the point Pc of the objective lens 15, and is refracted here. This light is further refracted at the point Pd on the exit side, passes through the optical disk 52 and converges to the point P2. The innermost light of the light beam 42 is diffracted at the point Pe of the first hologram 1 and is incident on the point Pf of the second hologram 2. The diffracted light incident on the point Pf is diffracted by the second hologram 2 and incident on the point Pg of the objective lens 15 where it is refracted. This light is further refracted at a point Ph on the exit side, passes through the optical disk 52, and converges to a point P2. Further, the light at the outermost periphery of the opening of the first hologram 1 when the light beam 42 enters the optical disk 52 with NA 0.6 is diffracted at the point Pj of the first hologram 1, and the point of the second hologram 2. Incident on Pk. Further, the diffracted light is diffracted by the second hologram 2 in the opposite direction at the same angle as the point Pj of the first hologram 1 and converges to the point P2 of the optical disk 52 through the points Pm and Pn of the objective lens 15. To do. Thus, it is desirable that the light at the outermost periphery diffracted at the point Pj of the first hologram 1 is incident on the innermost periphery of the second hologram 2.

  In order to make the light beam 42 diffracted by the second hologram 2 into divergent light for correcting spherical aberration, for example, the period Λ1 and the fringe 1a are made constant, and the period Λ2 is gradually widened from the central part toward the outer peripheral part. The inclination angle of the fringe 2a may be gradually increased from the central portion toward the outer peripheral portion, or the period Λ2 and the fringe 2a may be constant, and the period Λ1 may be gradually decreased from the central portion toward the outer peripheral portion. However, the inclination angle of the fringe 1a may be gradually increased from the central portion toward the outer peripheral portion. It can also be configured by changing both the period Λ1 and the fringe 1a and the period Λ2 and the fringe 2a. The light beam 42 diffracted by the second hologram 2 is designed not only to diverge but also to correct spherical aberration. Thus, by setting the period Λ1 of the first hologram 1 and the inclination angle of the fringe 1a and the period Λ2 of the second hologram 2 and the inclination angle of the fringe 2a, the light beam 42 can be efficiently diverged. Wavefront conversion is possible.

  On the other hand, the thickness of the transparent layer of the optical disc 52 is 0.6 mm, which is thicker than the thickness of the transparent layer of the optical disc 51, and the focal position when recording / reproducing the optical disc 52 is greater than that when recording / reproducing the optical disc 51. It is necessary to separate in the optical axis direction. By the wavefront conversion of the light beam 42 as shown in FIG. 2B, the spherical aberration can be corrected and the focal position can be separated from the objective lens.

  When the first and second holograms 1 and 2 are formed by the volume hologram as described above, the change in diffraction efficiency with respect to the wavelength change has characteristics as shown in FIG. In FIG. 4, the solid line indicates S-polarized light, and the wavy line indicates P-polarized light (the electric field direction of light is in the rz plane of FIG. 2, where r is the radial direction). The variation of the wavelength of the semiconductor laser is about ± 5 nm, and the change of the diffraction efficiency with respect to the change of the wavelength of the semiconductor laser becomes an average value of P-polarized light and S-polarized light when incident with circularly polarized light. It is a level that does not have any problem. On the other hand, the wavelength λ1 and the wavelength λ2 have a wavelength difference of about 250 nm, and the red light with the wavelength λ2 is diffracted, but the blue light with the wavelength λ1 is not diffracted, and the wavelength selectivity can be sufficiently provided.

  The wavelength of light emitted from the semiconductor laser changes by about ± 2 nm due to a change in output associated with temperature and recording / reproduction switching. In this embodiment, the amount of change in the optical axis of the diffracted light from the first hologram 1 is changed. Is in a direction that cancels out the change in the optical axis of the diffracted light from the second hologram 2. As a result, even if the wavelength changes, the divergent light emitted from the second hologram cancels out the influence of the wavelength variation, and the spherical aberration when the light is stopped through the 0.6 mm transparent layer hardly increases.

  The behavior of light when wavelength variation occurs will be specifically described with reference to FIG. When the wavelength of the light beam 42 emitted from the red laser with the wavelength λ2 does not change (Δλ = 0), the light diffracted at the point Pa of the first hologram 1 is the second hologram as shown by the solid line. 2 enters the point Pb and is diffracted as shown in the figure, and is focused by the objective lens 15 and converges to the point P2. When the wavelength fluctuates in the direction of increasing wavelength (Δλ> 0), as shown by the wavy line, the diffraction angle is increased by Δθ + in the first hologram 1, but the incident angle is increased by Δθ + in the second hologram 2. Is incident on the point Pb +. As a result, the direction of light diffracted at the point Pb + is diffracted at the point Pa + of the first hologram 1 when Δλ = 0, and coincides with the direction in which the light incident on the point Pb + of the second hologram 2 is diffracted. The fluctuation of the diffraction angle is canceled out and converges to the point P2 when the objective lens 15 is squeezed through the 0.6 mm transparent layer.

  On the other hand, when the wavelength fluctuates in the direction of shortening (Δλ <0), the diffraction angle is reduced by Δθ− in the first hologram 1 as shown by the two-dot chain line, but in the second hologram 2, Since the incident angle is decreased by Δθ−, the incident angle is incident on the point Pb−. As a result, the direction of light diffracted at the point Pb− is diffracted at the point Pa− of the first hologram 1 when Δλ = 0 and the light incident on the point Pb− of the second hologram 2 is diffracted. The diffraction angle variation is canceled out and converged to the point P2 when the objective lens 15 is squeezed through the 0.6 mm transparent layer. FIG. 6 shows changes in spherical aberration when the wavelength λ2 is shifted using the first and second holograms configured as described above. As can be seen from the figure, even when the design wavelength of the first and second holograms is 650 nm and the wavelength is shifted by ± 10 nm, the spherical aberration can be suppressed to 20 mλrms or less, and the practical performance can be sufficiently satisfied.

  In the present embodiment, volume holograms having a sinusoidal refractive index distribution are used as the first and second holograms 1 and 2. As shown in FIG. 7, the first-order diffraction efficiency has different characteristics depending on the polarization direction of incident light depending on the refractive index change amount Δn. In the figure, the solid line is for S-polarized light, the wavy line is for P-polarized light (electric field direction is in the rz plane of FIG. 2, r is the radial direction), wavelength λ = 655 nm, diffraction angle θ = 45 °, thickness L = The average refractive index n of the volume hologram was 6 μm, the fringe tilt angle φ was set to θ / 2. Black circles and triangles indicate actual measurement values. As can be seen from this, when the incident light is circularly polarized as in the present embodiment, the refractive index change amount Δn is preferably about 0.055 where the characteristics of the S-polarized light and the P-polarized light intersect. In the system, it is desirable to match the polarization.

  In the present embodiment, the case where a volume hologram having a refractive index distribution is used as the first and second holograms 1 and 2 has been described as an example. However, operation is possible even with a surface relief hologram. . However, in that case, it is difficult to achieve both a large diffraction angle and high diffraction efficiency.

  In the present embodiment, the first and second holograms 1 and 2 are formed on both surfaces of the transparent parallel plate 10, but there is no problem even if they are formed and combined on different transparent parallel plates.

  Numerical aperture NA1 when recording / reproducing the optical disk 51 with the light beam 41 (0.85 in the case of BD), and numerical aperture NA2 when recording / reproducing the optical disk 52 with the light beam 42 (in the case of DVD 0.6) However, since NA1> NA2, it is desirable to set the optimum NA by limiting the aperture of the light beam 42 with the first hologram or the second hologram. FIG. 8 shows a case in which the aperture is limited by the first hologram, and the first hologram 5 has a volume hologram formed in the range of the diameter D0, and other portions are transmitted or do not affect the convergence point P2. It is configured to diffract to a degree. By setting the diameter D0 to be NA 0.6, the aperture of the light beam 42 that converges at the point P2 can be limited.

(Embodiment 2)
A second embodiment of the present invention will be described with reference to FIG. In the present embodiment, only the first hologram and the second hologram of the first embodiment are changed. 9A is a plan view of the first hologram, and FIG. 9B is a side view of the first and second holograms and the objective lens. In the figure, the first hologram 7 and the second hologram 7 are formed on the surface of the transparent parallel plate 10 and are configured to diffract red light. Both the first and second holograms 6 and 7 have a volume. Use a hologram. The first and second holograms 6 and 7, the transparent parallel plate 10 and the objective lens 15 are attached to a lens barrel. As in the first embodiment, the objective lens 15 is designed so that spherical aberration is reduced when the light beam 41 having the wavelength λ1 is focused through the optical disk having the transparent layer of 0.1 mm. When squeezing through a 6 mm optical disk, spherical aberration occurs. In the present embodiment, the first and second holograms 6 and 7 are used to convert the light beam 42 having the wavelength λ2 into a divergent light in which the spherical aberration is corrected and to capture the light beam 42. To improve the rate.

  Since the numerical aperture NA2 when recording / reproducing the optical disk 52 with the light beam 42 is smaller than the numerical aperture NA1 when recording / reproducing the optical disk 51 with the light beam 41, for example, the diameter D0 as in the first hologram 5 of FIG. Will limit the opening. However, due to the difference between the numerical aperture NA1 and the numerical aperture NA2, the light beam 42 that converges at the point P2 is about half of the area compared to the light beam 41. The emission angle of the light emitted from the blue laser 11 and the emission angle of the light emitted from the red laser 12 are approximately the same, and the light beam 42 is deteriorated in the capture rate by the area. For this reason, in such a compatible head that records and reproduces the optical disks 51 and 52 by the single objective lens 15, the recording speed of the optical disk 52 must be reduced. Now, in order to transfer data at high speed with DVD, the speed is increased by 8 times or 16 times as compared with standard recording / reproduction. If the light uptake rate is poor, this double speed cannot be obtained and the commercial value is lowered. For this reason, in this embodiment, as shown in the figure, the beam diameter D1 of the light beam 42 when entering the first hologram 6 is larger than the beam diameter D2 of the light beam 42 when entering the second hologram 7. By configuring the first and second holograms 6 and 7 to be larger, more light is captured while maintaining the numerical aperture NA2, and the capture rate of the light beam 42 is improved.

  The front side of the fringe 6a of the first hologram 6 is inclined in the center direction. For example, the period Λ6 gradually becomes finer from the central part toward the outer peripheral part, and the inclination angle of the fringe 6a is changed from the central part to the outer peripheral part. It has a structure that gradually increases toward the front. Further, the front side of the fringe 7a of the second hologram 7 is inclined in the outer peripheral direction. The light beam 42 incident on the first hologram 6 emits first-order diffracted light at an innermost circumference of, for example, 45 ° by Bragg diffraction, and gradually increases in diffraction angle toward the outer periphery. First-order diffracted light is emitted at 55 °. Thereby, the opening of the first hologram 6 when the light beam 42 converges to P2 at NA2 is set to be approximately the same as the opening of the light beam 41. The volume hologram can secure a diffraction efficiency of 90% or more even if the angle of the first-order diffracted light is increased to 60 °, and therefore there is no problem of the diffraction efficiency at about 55 °. The diffraction angle also varies depending on the design of the objective lens 15, the innermost diffraction angle of the first hologram 6, the thickness of the transparent parallel plate 10, and the like, and it is desirable to design the optical system optimally. . More specifically, the innermost light of the light beam 42 is diffracted at the point Po of the first hologram 6 and enters the point Pp of the second hologram 7. Further, the diffracted light is diffracted by the second hologram 7 and incident on the point Pq of the objective lens 15, where it is refracted and further refracted at the point Pr and passes through the optical disk 52 and converges to the point P2. The light at the outermost periphery of the opening of the first hologram 6 when the light beam 42 enters the optical disk 52 at NA 0.6 is diffracted at a larger angle than the innermost periphery at the point Ps of the first hologram 6, Incident on the point Pt of the second hologram 7. Further, the diffracted light is diffracted by the second hologram 7 in the opposite direction at the same angle as the point Ps of the first hologram 6 and converges to the point P2 of the optical disk 52 through the points Pu and Pv of the objective lens 15. . The incidence angle and the emission angle of the central portion Pt and the outer peripheral portion Pp of the second hologram 7 are different, and the period Λ7 and the fringe 7a are designed correspondingly. As a result, the light beam 42 diffracted by the second hologram 7 is not simply diverged light, but is corrected by the spherical aberration and converged by the objective lens 15. As described here, it is desirable that the light at the outermost periphery diffracted at the point Ps of the first hologram 6 is incident on the innermost periphery of the second hologram 7.

  Further, the transparent layer of the optical disc 52 is thicker than the transparent layer of the optical disc 51, and the focal position when recording and reproducing the optical disc 52 needs to be separated in the optical axis direction than when recording and reproducing the optical disc 51. As shown in FIG. 9, spherical aberration can be corrected and the focal position can be moved away from the objective lens 15 by wavefront conversion of the light beam 42. In addition, the light emitted from the semiconductor laser changes in wavelength by about ± 2 nm due to the change in temperature and the output accompanying the switching of recording / reproduction. In this embodiment, the first hologram 6 of the first hologram 6 is changed as in the first embodiment. The amount of change in the optical axis is in a direction that cancels out the amount of change in the optical axis of the diffracted light from the second hologram 7, and the divergent light emitted from the second hologram 7 varies even when the wavelength changes. The spherical aberration is hardly increased when the light is squeezed through the 0.6 mm transparent layer.

  Thus, according to this embodiment, even if the numerical aperture NA2 is smaller than the numerical aperture NA1, the light beam 42 incident on the first hologram 6 can be captured in a large range, and the capture rate can be improved. It becomes possible. As a result, even a compatible head using one objective lens can cope with high-speed recording.

(Embodiment 3)
An optical head according to Embodiment 3 of the present invention will be described with reference to FIGS. FIG. 10 is a side view showing the basic configuration of the optical head and the state of light propagation in the same embodiment, FIG. 11A is a plan view of a third hologram, and FIG. 4 is a side view of the hologram and the objective lens, FIG. 12A is a plan view of the third hologram when the incident light of wavelength λ3 is aperture-limited in accordance with NA, and FIG. Side views of the first to fourth holograms and the objective lens are shown.

  In FIG. 10, 3 is a third hologram, 4 is a fourth hologram, and 13 is an infrared laser having a wavelength λ3 (approximately 780 nm). In this embodiment, third and fourth holograms 3 and 4, an infrared laser 13 and a beam splitter 32 are added to the first embodiment. The beam splitters 30 and 31 and the wave plate 33 are obtained by changing the beam splitters 17 and 18 and the wave plate 19 of FIG. In FIG. 1 and FIG.

  The first hologram 1 and the third hologram 3 are formed on the same surface of the transparent parallel plate 10, and the second hologram 4 and the fourth hologram 4 are formed on the other same surface of the transparent parallel plate 10. The first and second holograms 1 and 2 diffract red light having a wavelength λ2, and the third and fourth holograms 3 and 4 diffract infrared light having a wavelength λ3. The first to fourth holograms 1 to 4, the transparent parallel plate 10 and the objective lens 15 are attached to 16 lens barrels, and the lens barrel 16 is configured to follow a track provided on the optical disk by an actuator. . Reference numeral 53 denotes an optical information medium, for example, an optical disk for CD, which has a thickness t3 of the transparent layer of about 1.2 mm and is recorded and reproduced by a light beam having a wavelength λ3.

  When recording / reproducing the optical disk 51 having a high recording density, the light beam 41 having the wavelength λ 1 emitted from the blue laser 11 is reflected by the beam splitter 30, becomes substantially parallel light by the collimating lens 14, and is circularly polarized by the wavelength plate 33. become. The wave plate 33 is designed to act as a substantially quarter wave plate for the wavelengths λ1, λ2, and λ3. The circularly polarized light beam 41 passes through the first and third holograms 1, 3, the transparent parallel plate 10, and the second and fourth holograms 2, 4, and has a thickness of about 30 mm by the refractive objective lens 15. The light is converged on the information recording surface of the optical disc 51 through a transparent layer of 0.1 mm. The light beam 41 reflected by the information recording surface of the optical disc 51 travels back along the original optical path to become linearly polarized light in the direction perpendicular to the forward path by the wave plate 33, passes through the beam splitter 30, and passes through the beam splitter 31. Reflected. The beam splitter 30 reflects linearly polarized light in one direction and transmits linearly polarized light in a direction perpendicular to the light beam having the wavelength λ1. As will be described later, the light beam 42 having the wavelength λ2 and the light beam having the wavelength λ3 are transmitted regardless of the polarization. As described above, the beam splitter 30 is an optical path branching element having wavelength characteristics as well as polarization characteristics, and the optical multilayer film 30a is formed so as to have such characteristics. The light beam 41 reflected by the beam splitter 31 is given astigmatism by the detection lens 20 and enters the photodetector 21. By calculating the output of the photodetector 21, servo signals and information signals used for focus control and tracking control are obtained.

  Next, when recording / reproducing the optical disk 52, the light beam 42 having the wavelength λ2 emitted from the red laser 12 is transmitted through the beam splitters 32, 31, and 30 and becomes substantially parallel light by the collimating lens 14, and the wavelength plate 33 becomes circularly polarized light. The beam splitter 32 transmits red light having a wavelength λ2 and reflects infrared light having a wavelength λ3. The circularly polarized light beam 42 is diffracted by the first hologram 1, diffracted by the second hologram 2 in the opposite direction to the first hologram 1 through the transparent parallel plate 10, and thickened by the objective lens 15. The light is converged on the information recording surface of the optical disc 52 through a transparent layer having a thickness of about 0.6 mm. The light beam 42 reflected by the information recording surface of the optical disk 52 traces the original optical path in the opposite direction, and is linearly polarized in the direction perpendicular to the forward path by the wave plate 33, passes through the beam splitter 30, and passes through the beam splitter 31. Reflected. The beam splitter 31 transmits linearly polarized light in one direction with respect to light having wavelengths λ2 and λ3, reflects linearly polarized light in a direction perpendicular thereto, and reflects light having wavelength λ1 regardless of the polarization. Therefore, the beam splitter 31 is an optical path branching element having polarization characteristics and wavelength selectivity, and the optical multilayer film 31a is formed so as to have such characteristics. The light beam 42 reflected by the beam splitter 31 is given astigmatism by the detection lens 20 and enters the photodetector 21 to obtain a servo signal and an information signal.

  Next, when recording / reproducing the optical disk 53, the light beam 43 having the wavelength λ3 emitted from the infrared laser 13 is reflected by the beam splitter 32, passes through the beam splitters 31 and 30, and becomes substantially parallel light by the collimating lens 14. The circularly polarized light is obtained by the wave plate 33. The circularly polarized light beam 43 is diffracted by the third hologram 3, diffracted by the fourth hologram 4 in the direction opposite to the third hologram 3 through the transparent parallel plate 10, and is thickened by the objective lens 15. It converges on the information recording surface of the optical disc 53 through a transparent layer of about 1.2 mm. The light beam 43 reflected by the information recording surface of the optical disk 53 traces the original optical path in the opposite direction, becomes linearly polarized light in the direction perpendicular to the forward path by the wave plate 33, passes through the beam splitter 30, and is reflected by the beam splitter 31. Is done. The light beam 43 reflected by the beam splitter 31 is given astigmatism by the detection lens 20, enters the photodetector 21, and obtains a servo signal and an information signal.

  In the present embodiment, the first and second holograms 1 and 2 and the third and fourth holograms 3 and 4 all use volume holograms. The first and second holograms 1 and 2 have the same function as in the first embodiment, and divergence is such that spherical aberration is corrected when a light beam having a wavelength λ2 is focused through an optical disk having a transparent layer of 0.6 mm. Wavefront conversion to light. In addition, the light beam 42 having the wavelength λ2 is transmitted through the third and fourth holograms 3 and 4 without being diffracted and is not affected.

  Next, the third and fourth holograms 3 and 4 which are volume holograms have a refractive index distribution having a periodic structure or a quasi-periodic structure in the radial direction, and the thicknesses L3 and L4 of the refractive index distribution layer and the refractive index change amount. Δn and periods Λ3 and Λ4 are designed to diffract infrared light, and the fringe 3a is inclined forward in the central direction, and has a large diffraction angle such as 45 ° as in the first embodiment. However, high diffraction efficiency can be realized. Thereby, the light beam 43 incident on the third hologram 3 is emitted as first-order diffracted light by Bragg diffraction. The fourth hologram 4 is different from the third hologram 3 in the period Λ4 and the fringe 4a of the refractive index distribution layer, and has an average diffraction angle smaller than that of the third hologram. The front edge of the fringe 4a is inclined in the outer peripheral direction, and the light beam 43 diffracted obliquely by the third hologram 3 is returned to the divergent light by returning the optical axis by the diffraction of the fourth hologram 4. 15 is incident. The inclination of the period Λ4 and the fringe 4a of the fourth hologram is designed so that the spherical aberration is reduced when this diverging light enters the objective lens and converges through the 1.2 mm transparent layer.

  In the light beam 43 of FIG. 11, the light incident on the point Qa of the third hologram 3 is diffracted in the central direction, and this diffracted light is incident on the point Qb of the fourth hologram 4 crossing the central axis C. . The diffracted light incident on the point Qb is diffracted in the direction in which the optical axis is returned by the fourth hologram 4 and incident on the point Qc of the objective lens 15, and the refracted light is further refracted at the point Qd on the exit side. Then, the light passes through the optical disk 53 and converges to the point P3. The innermost light of the light beam 43 is diffracted at the point Qe of the third hologram 3 and is incident on the point Qf of the fourth hologram 4. Further, the diffracted light is diffracted by the fourth hologram 4 and incident on the point Qg of the objective lens 15 where it is refracted and further refracted at the point Qh on the exit side, and converges to the point P3 through the optical disk 53. . Further, the light at the outermost periphery of the opening of the third hologram 3 when the light beam 43 is incident on the optical disk 53 at NA 0.45 is diffracted at the point Qj of the third hologram 3, and the point of the fourth hologram 4. Incident on Qk. Further, the diffracted light is diffracted by the fourth hologram 4 in the opposite direction at the same angle as the point Qj of the third hologram 3 and converges to the point P2 of the optical disk 52 through the points Pm and Pn of the objective lens 15. To do. Thus, it is desirable that the light at the outermost periphery diffracted at the point Qj of the third hologram 3 is incident on the innermost periphery of the fourth hologram 4.

  The light beam 43 diffracted by the fourth hologram 4 is designed to be divergent light that corrects spherical aberration. For example, the period Λ3 and the fringe 3a are made constant, the period Λ4 is gradually widened from the central part toward the outer peripheral part, and the inclination angle φ of the fringe 4a is gradually increased from the central part toward the outer peripheral part. The period Λ4 and the fringe 4a are made constant, the period Λ3 is gradually made finer from the central part toward the outer peripheral part, and the inclination angle of the fringe 1a is gradually increased from the central part toward the outer peripheral part. good. It can also be configured by changing both the period Λ3 and the fringe 3a and the period Λ4 and the fringe 4a. In this way, by setting the third hologram 4 and the fourth hologram 4, the light beam 43 can be efficiently wavefront converted into divergent light.

  On the other hand, the thickness of the transparent layer of the optical disc 53 is 1.2 mm, which is thicker than the thickness of the transparent layer of the optical disc 51, and the focal position when recording / reproducing the optical disc 53 is greater than that when recording / reproducing the optical disc 51. It is necessary to separate in the optical axis direction. As shown in the figure, the spherical aberration can be corrected and the focal position can be moved away from the objective lens 15 by wavefront conversion of the light beam 43.

  When the third and fourth holograms 3 and 4 are formed by the volume hologram as described above, the change in the diffraction efficiency with respect to the wavelength change has the same characteristics as in FIG. 4 of the first embodiment, and the variation in the wavelength of the semiconductor laser. Is about ± 5 nm, the average value of P-polarized light and S-polarized light falls within a change of about 2-3% when incident with circularly polarized light. On the other hand, as shown in FIG. 4, since the wavelength λ3 and the wavelength λ2 have a wavelength difference of about 130 nm, the infrared light with the wavelength λ3 is diffracted, but the red light with the wavelength λ2 is not diffracted, and is sufficiently wavelength selective Can be given.

  The light emitted from the semiconductor laser changes in wavelength by about ± 2 nm due to a change in temperature and output accompanying switching between recording and reproduction. In this embodiment, the optical axis of the third hologram 3 is changed as in the first embodiment. The change is in a direction that cancels out the change in the optical axis of the diffracted light from the fourth hologram 4, and even if the wavelength changes, the divergent light emitted from the fourth hologram 4 is affected by the wavelength fluctuation. The spherical aberration when offset through the 1.2 mm transparent layer is hardly increased.

  Thus, using the objective lens 15 designed to reduce the spherical aberration when the blue light having the wavelength λ1 is converged through the transparent layer having the thickness of 0.1 mm, the red light having the wavelength λ2 has the thickness of 0.6 mm. The spherical aberration generated in the objective lens 15 when converging through the transparent layer is corrected by the first and second holograms 1 and 2, and the infrared light having the wavelength λ3 is objective when converging through the transparent layer having a thickness of 1.2 mm. By correcting the spherical aberration generated in the lens 15 with the third and fourth holograms 3 and 4, an optical head corresponding to three transparent layers having different thicknesses can be produced. Therefore, it becomes possible to record / reproduce the BD optical disc, the DVD optical disc, and the CD optical disc with one optical head.

  In the present embodiment, a case where a volume hologram having a refractive index distribution is used as the first to fourth holograms has been described as an example. However, operation is possible even with a surface relief hologram. However, in that case, it is difficult to achieve both a large diffraction angle and high diffraction efficiency.

  In the present embodiment, the third and fourth holograms 3 and 4 are formed on both surfaces of the transparent parallel plate 10, but there is no problem even if they are formed on separate transparent parallel plates.

  On the other hand, the numerical aperture NA1 when recording / reproducing the optical disk 51 with the light beam 41 (0.85 in the case of BD) and the numerical aperture NA2 when recording / reproducing the optical disk 52 with the light beam 42 (0.6 in the case of DVD) Since the numerical aperture NA3 (0.45 in the case of CD) when recording / reproducing the optical disk 53 with the light beam 43 is different and NA1> NA2> NA3, the light beam 43 is formed by the third hologram or the fourth hologram. It is desirable to set the optimal NA by limiting the aperture. FIG. 12 shows a case where the opening of the light beam 42 is restricted by the first hologram 5 and the opening of the light beam 43 is restricted by the third hologram 23. The first hologram 5 has a volume hologram formed in the range of the diameter D0. In the third hologram 23, a volume hologram is formed in the range of the diameter D9, and other portions are transmitted or diffracted to such an extent that the convergence is not affected. By setting the diameter D0 to be NA 0.6, the aperture of the light beam 42 that converges to the point P2 can be limited, and by setting the diameter D9 to be NA 0.45, the light that converges to the point P3. The aperture of the beam 43 can be limited.

(Embodiment 4)
A fourth embodiment of the present invention will be described with reference to FIG. FIG. 13A is a plan view of the third hologram, and FIG. 13B is a side view of the first to fourth holograms and the objective lens. In the present embodiment, only the first and second holograms and the third and fourth holograms of the third embodiment are changed. In the figure, the first and second holograms 6 and 7 and the third and fourth holograms 8 and 9 are formed on the transparent parallel plate 10 and all use volume holograms. The first and second holograms 6 and 7 correct spherical aberration when a light beam having a wavelength λ2 is narrowed through an optical disk having a transparent layer of 0.6 mm, and the third and fourth holograms 8 and 9 are lights having a wavelength λ3. Spherical aberration is corrected when the beam is focused through an optical disk having a transparent layer of 1.2 mm. The first and second holograms 6 and 7 are the same as those in the second embodiment and function in the same manner. That is, the numerical aperture NA1 (0.85 for BD) when converging the light beam 41 on the optical disc 51 and the numerical aperture NA2 (0.60 for DVD) when converging the optical beam 42 on the optical disc 52 are as follows. Although different, the aperture when the light beam 42 with the wavelength λ2 is incident on the first hologram 6 is made the same as that of the light beam 41 with the wavelength λ1 to improve the capture rate of the light beam 42.

  Next, the operation of the third hologram 8 and the fourth hologram 9 will be described with reference to FIG. Since the numerical aperture NA3 (0.45 in the case of CD) when recording / reproducing the optical disk 53 with the light beam 43 is smaller than the numerical aperture NA1, the light amount of the light beam 43 that converges at the point P3 decreases. The light beam 43 is about 1 / 3.6 in area comparison with the light beam 41. The light beam 43 has a lower light capture rate by this area. Therefore, a compatible head that records and reproduces the optical disks 51 and 53 with one objective lens has to reduce the recording speed of the optical disk 53. Since data is transferred at high speed on a CD, the speed is increased by 24 or 32 times the standard recording speed. If the light capture rate is poor, this speed cannot be obtained and the commercial value is lowered. For this reason, in this embodiment, as shown in the figure, the diameter D3 of the light beam 43 when entering the third hologram 8 is larger than the diameter D4 of the light beam 43 when entering the fourth hologram 9. As described above, the third and fourth holograms 8 and 9 are configured so as to be able to capture more light while maintaining the numerical aperture NA3.

  This will be further described. First, the fringe 8a of the third hologram 8 is inclined forward in the center direction, and the fringe 9a of the fourth hologram 9 is inclined forward in the outer peripheral direction. First, the innermost light of the light beam 43 is diffracted at the point Qo of the third hologram 8 and enters the point Qp of the fourth hologram 9. Further, the diffracted light is diffracted by the fourth hologram 9 and incident on the point Qq of the objective lens 15, where it is refracted and further refracted at the point Qr and converges to the point P3 through the optical disk 53. The light at the outermost periphery of the opening of the third hologram 8 when the light beam 43 enters the optical disk 53 at NA 0.45 is diffracted at a point larger than the innermost periphery at the point Qs of the third hologram 8, Incident on the point Qt of the fourth hologram 9. Further, the diffracted light is diffracted by the fourth hologram 9 in the opposite direction at the same angle as the point Qs of the third hologram 8 and converges to the point P3 of the optical disk 53 through the points Qu and Qv of the objective lens 15. . Here, since the emission angles of the central portion Qo and the outer peripheral portion Qs of the third hologram 8 are different, the period Λ8 is gradually finer from the central portion to the peripheral portion, and the fringe 8a is gradually inclined from the central portion to the peripheral portion. Designed to increase φ. Further, the incident angle and the outgoing angle of the central portion Qt and the outer peripheral portion Qp of the fourth hologram 9 are different, and the period Λ9 and the fringe 9a are designed correspondingly. Furthermore, the light beam 43 emitted from the fourth hologram 9 is designed not only to be divergent light but also to correct spherical aberration and converge to the point P3 through the transparent layer 1.2 mm. Further, it is desirable that the outermost light diffracted at the point Qs of the third hologram 8 is incident on the innermost periphery of the fourth hologram 9.

  Thus, by setting the period Λ3 of the third hologram 8 and the inclination angle of the fringe 8a and the period Λ9 of the fourth hologram 9 and the inclination angle of the fringe 9a, the light beam 43 can be efficiently diverged. Wavefront conversion is possible. Similarly to the third embodiment, the change in the optical axis of the diffracted light from the third hologram 8 is also different from the change in the optical axis of the diffracted light from the fourth hologram 9 with respect to the wavelength change. It cancels out and there is almost no increase in spherical aberration when squeezed through a 1.2 mm transparent layer.

  As described above, according to the present embodiment, when the light beam 41 having the numerical aperture NA1 enters the first hologram, and when the light beam 42 having the numerical aperture NA2 enters the first hologram. And the light beam diameter when the light beam 43 having the numerical aperture NA3 enters the first hologram can be made substantially equal. For this reason, it is possible to prevent the deterioration of the light capture rate due to the difference in NA.

(Embodiment 5)
A fifth embodiment of the present invention will be described with reference to FIG. FIG. 14A is a plan view of the third hologram, and FIG. 14B is a side view of the first to fourth holograms and the objective lens. In the present embodiment, the first and second holograms, the third and fourth holograms, and the objective lens of the third embodiment are changed. In the figure, the first and second holograms 24 and 25 and the third and fourth holograms 26 and 27 are formed on the transparent parallel plate 10 and all use volume holograms. The objective lens 28 has an NA of 0.7 to 0.75, for example, and is designed to reduce spherical aberration when a light beam having a wavelength λ2 is narrowed through a transparent layer of 0.6 mm.

  The first and second holograms 24 and 25 diffract blue light having a wavelength λ1, and the third and fourth holograms 26 and 27 diffract infrared light having a wavelength λ3. The light beam 42 having the wavelength λ2 is transmitted without being diffracted by the first and second holograms 24 and 25 and the third and fourth holograms 26 and 27 and is not affected by the hologram. Further, the light beam 42 having the wavelength λ2 is aperture-limited (not shown) so that the NA is 0.6, and converges on the optical disk 52 having a transparent layer of 0.6 mm. The first and second holograms 24 and 25 are wavefront-converted into convergent light so that spherical aberration is corrected when the light beam 41 having the wavelength λ1 is focused through the optical disk having a transparent layer of 0.1 mm by the objective lens 28. When the lens effect by diffraction of the first and second holograms 24 and 25 and the NA of the objective lens 28 are combined, the NA is set to 0.85. The third and fourth holograms 26 and 27 are wavefront-converted into divergent light in which spherical aberration is corrected when the light beam 43 having the wavelength λ3 is focused through the optical disk having a transparent layer of 1.2 mm by the objective lens 28.

  The first and second holograms 24 and 25 have a refractive index distribution having a periodic structure or a quasi-periodic structure in the radial direction. The period Λ24 and the fringe 24a of the first hologram 24 are set so as to diffract the blue light beam 41 in the central axis C direction, and the front of the fringe 24a is inclined in the central direction. The period Λ 25 and the fringe 25 a of the second hologram 25 are set so that the average diffraction angle is larger than that of the first hologram 24, so that the optical axis of the light beam 41 obliquely diffracted by the first hologram 24 is returned. The light is diffracted and enters the objective lens 28 as convergent light. The front of the fringe 25a is inclined in the outer peripheral direction.

  In the light beam 41 of FIG. 14, the light incident on the point Oa of the first hologram 24 is diffracted in the central direction, and this diffracted light is incident on the point Ob of the second hologram 25 while intersecting the central axis C. . The diffracted light incident on the point Ob is diffracted in the direction in which the optical axis is returned by the second hologram 25, and incident on the point Oc of the objective lens 28. The light refracted here is further refracted at the point Od on the emission side. Then, the light passes through the optical disc 51 and converges to the point P1. The innermost light of the light beam 41 is diffracted at the point Oe of the first hologram 24 and is incident on the point Of of the second hologram 25. Further, the diffracted light is diffracted by the second hologram 25 and incident on the point Og of the objective lens 28, where it is refracted and further refracted at the point Oh on the exit side, and converges to the point P1 through the optical disc 51. . Further, when the light beam 41 is incident on the optical disk 51 at NA 0.85, the light at the outermost periphery of the opening of the first hologram 24 is diffracted at the point Oj of the first hologram 24, and the point of the second hologram 25. Incident to Ok. Further, the diffracted light is diffracted by the second hologram 25 in the opposite direction at the same angle as the point Oj of the first hologram 24, and converges to the point P1 of the optical disc 51 through the point Om and the point On of the objective lens 28. To do. Thus, it is desirable that the light at the outermost periphery diffracted at the point Oj of the first hologram 24 is incident on the innermost periphery of the second hologram 25.

  The light beam 41 diffracted by the second hologram 25 is designed to be convergent light that corrects spherical aberration. For example, the period Λ24 and the fringe 24a can be made constant, the period Λ25 can be gradually made finer from the central part toward the outer peripheral part, and the inclination angle φ of the fringe 25a can be made gradually smaller from the central part toward the outer peripheral part. However, it can also be configured by changing both the period Λ24 and the fringe 24a and the period Λ25 and the fringe 25a.

  The change in diffraction efficiency with respect to the change in wavelength is the same as that in the first embodiment. If the variation of the wavelength of the semiconductor laser is about ± 5 nm, the change is within about 2-3%. Further, since the wavelength λ1 and the wavelength λ2 have a wavelength difference of about 250 nm, the first and second holograms 24 and 25 diffract the blue light having the wavelength λ1 but the red light having the wavelength λ2 is sufficiently diffracted without being diffracted. Selectivity can be given.

  Furthermore, even if there is a wavelength change, in the present embodiment, the change in the optical axis of the first hologram 24 is the same as the change in the optical axis of the diffracted light from the second hologram 25 as in the first embodiment. Even if the wavelength is changed, the convergent light emitted from the second hologram 25 is canceled out by the influence of the wavelength fluctuation, and the spherical aberration when the light is squeezed through the transparent layer of 0.1 mm hardly increases even if the wavelength is changed. .

  On the other hand, the third and fourth holograms 26 and 27 function in the same manner as the third and fourth holograms 3 and 4 of the third embodiment. Spherical aberration is corrected when squeezing through the optical disc. However, since the objective lens is different from that of the third embodiment, the spherical aberration to be corrected is different. That is, the objective lens 15 according to the first embodiment is optimized for the transparent layer of 0.1 mm, and the objective lens 28 according to the present embodiment is optimized for the transparent layer of 0.6 mm. In addition, the amount of aberration to be corrected in the fourth holograms 26 and 27 is reduced. Therefore, the period and fringe change amount of the refractive index distribution layers of the third and fourth holograms 26 and 27 are reduced, and the manufacturing error and the setting error are strong. Further, the NA of the objective lens 28 is smaller than the NA of the objective lens 15 of the third embodiment, so that the manufacturing tolerance is increased, the yield is improved, and the cost can be reduced. As described above, in the present embodiment, the optimum value of the transparent layer of the objective lens is adjusted to the middle of the three types of optical discs, so that the maximum value of spherical aberration to be corrected can be reduced, and it is easier to manufacture. Can be configured.

  As described above, in the present embodiment, the blue light having the wavelength λ1 is generated using the objective lens 28 designed to reduce the spherical aberration when the red light having the wavelength λ2 is converged through the transparent layer having a thickness of 0.6 mm. The spherical aberration generated in the objective lens 28 when converging through the transparent layer having a thickness of 0.1 mm is corrected by the first and second holograms 24 and 25, and infrared light having a wavelength λ3 passes through the transparent layer having a thickness of 1.2 mm. By correcting the spherical aberration generated in the objective lens 28 at the time of convergence by the third and fourth holograms 26 and 27, an optical head corresponding to three transparent layers having different thicknesses can be produced.

  Further, also in the present embodiment, as in the fourth embodiment, the period of the first and second holograms and the inclination angle of the fringe, and the period of the third and fourth holograms and the inclination angle of the fringe are set. Thus, the light beam diameter when the light beam 41 having the numerical aperture NA1 and the wavelength λ1 is incident on the first hologram, the light beam diameter when the light beam 42 having the numerical aperture NA2 and the wavelength λ2 is incident on the first hologram, and The diameters of the light beams 43 having the numerical aperture NA3 and the wavelength λ3 incident on the first hologram can be made substantially equal. For this reason, it is possible to prevent the deterioration of the light capture rate due to the difference in NA.

  In the present invention, with the above configuration, compatibility between different types of discs can be realized, and information can be stably reproduced and recorded even when the wavelength changes. In addition, by configuring the diameters of the light beams incident on the first hologram to be approximately equal, it is possible to prevent deterioration in the rate of capturing light due to the difference in NA. Therefore, the use of the volume hologram has a remarkable effect that the problems of compatibility between different types of disks and the light capture rate can be solved at once.

  The important point here is that a volume hologram is used to realize a hologram with wavelength selectivity that acts on one wavelength and does not act on the other two wavelengths, and is configured to increase diffraction efficiency. It has a function and is compatible. For this reason, other configurations such as a beam splitter, a detection lens, and a detection hologram are not essential, and each has an effect as a desirable configuration, but other configurations can be used as appropriate. In the first to fifth embodiments, a tracking error signal can be detected by a well-known differential push-pull (DPP) method by introducing a three-beam grating (diffraction element). In addition, by providing a configuration that converts an elliptical light distribution from a blue laser into a circular shape using an anamorphic prism or beam shaping lens, the far-field image is brought closer to a point-symmetric intensity distribution around the optical axis. Thus, the light use efficiency can be improved.

  It is also effective to change the parallelism of the light beam by moving the collimating lens 14 shown in FIG. 1 or 10 in the optical axis direction (arrow direction in the figure). Spherical aberration occurs when there is a thickness error of the transparent layer, or when the optical disk 51 is a two-layer disk, and the thickness of the transparent layer is caused by the interlayer thickness. Aberration can be corrected. As described above, the spherical aberration can be corrected by moving the collimator 14 when the NA of the convergent light with respect to the optical disk is 0.85, and the thickness of the transparent layer of ± 30 μm can be corrected. . However, there is no ability to correct spherical aberration of DVD or CD using the objective lens 15 corresponding to the transparent layer thickness of 0.1 mm, and wavefront conversion as in this embodiment is necessary.

(Embodiment 6)
An embodiment of an optical information device using the optical head of the present invention is shown in FIG. In FIG. 15, the optical disk 51 is placed on a turntable 84 and rotated by a motor 83. The optical head 81 shown in the first to fourth embodiments is transported by the optical head driving device 80 to a track on the optical disk 51 where desired information exists.

  The optical head 81 sends a focus error signal and a tracking error signal to the electric circuit 82 in accordance with the positional relationship with the optical disc 51. In response to this signal, the electric circuit 82 sends a signal for finely moving the objective lens to the optical head 81. In response to this signal, the optical head 81 performs focus control and tracking control on the optical disc 51, and the optical head 81 reads, writes, or erases information.

  The above description is the same even if the optical disk to be mounted is replaced with the optical disks 52 and 53 having a thicker transparent layer than the optical disk 51. Since the optical information apparatus of the present embodiment uses the optical head of the present invention as an optical head, a single optical head can handle a plurality of optical disks having different recording densities.

(Embodiment 7)
The present embodiment is an embodiment of a computer provided with the optical information device 86 according to the fifth embodiment. FIG. 16 is a perspective view of a computer according to the present embodiment.

  The computer 88 shown in the figure includes an optical information device 86 according to the sixth embodiment, an input device such as a keyboard 90 and a mouse 91 for inputting information, information input from the input device, and optical information. An arithmetic unit 87 such as a central processing unit (CPU) that performs an operation based on information read from the device 86, and an output of a cathode ray tube, a liquid crystal display device, a printer, or the like that displays information such as a result calculated by the arithmetic unit 87 Device 89.

  The computer according to the present embodiment includes the optical information device 86 according to the fifth embodiment, and can record or reproduce different types of optical discs stably, so that it can be used for a wide range of applications.

(Embodiment 8)
The present embodiment is an embodiment of an optical information medium recorder provided with the optical information device 86 according to the fifth embodiment. FIG. 17 is a perspective view of the optical disc recorder according to the present embodiment.

  The optical disk recorder 94 shown in this figure includes an optical information device 86 according to Embodiment 5 and an image-to-information conversion device 93 (for example, an encoder) that converts image information into information to be recorded on an optical disk by the optical information device 86. It has.

  It is also desirable to have an information-to-image conversion device 92 (decoder) that converts an information signal obtained from the optical information device 86 into an image. According to this configuration, it is possible to reproduce the already recorded portion. Further, an output device 89 such as a cathode ray tube, a liquid crystal display device, or a printer for displaying information may be provided.

  The optical disc recorder according to the present embodiment includes the optical information device 86 according to the fifth embodiment, and can record or reproduce different types of optical discs stably, so that it can be used for a wide range of applications.

  As described above, according to the present invention, compatible reproduction and recording of different types of optical disks can be realized by using one objective lens, and light transmission efficiency can be ensured even if the optical disks are different. Stable information can be reproduced or recorded by suppressing the occurrence of spherical aberration when changing. Therefore, it is useful for this application equipment, such as an optical head, an optical information device, a computer, an optical information medium recorder, and the like.

1 is a side view showing a basic configuration of an optical head and a state of light propagation in Embodiment 1 of the present invention. (A) Plan view of hologram in optical head of embodiment 1 of the present invention (b) Side view of hologram and objective lens in optical head of embodiment 1 of the present invention FIG. 4 is an enlarged view showing details of a hologram in the optical head according to the first embodiment of the present invention. The figure which shows the relationship between the wavelength shift of the hologram in the optical head of Embodiment 1 of this invention, and 1st-order diffracted light. The figure which shows the mode of the diffraction light of a hologram when incident light carries out wavelength fluctuation | variation in the optical head of Embodiment 1 of this invention. The figure which shows the change of the amount of aberrations when wavelength shift arises in the optical head of Embodiment 1 of this invention. The figure which shows the relationship between the 1st-order diffraction efficiency of a hologram in the optical head of Embodiment 1 of this invention, and refractive index variation | change_quantity. (A) A plan view of an aperture-limited hologram in the optical head according to the first embodiment of the present invention. (B) A side view of the aperture-limited hologram and objective lens in the optical head according to the first embodiment of the present invention. (A) Plan view of hologram in optical head of embodiment 2 of the present invention (b) Side view of hologram and objective lens in optical head of embodiment 2 of the present invention Side view showing a basic configuration of an optical head and a state of light propagation in Embodiment 3 of the present invention. (A) Plan view of hologram in optical head of embodiment 3 of the present invention (b) Side view of hologram and objective lens in optical head of embodiment 3 of the present invention (A) Top view of aperture-limited hologram in optical head of Embodiment 3 of the present invention (b) Side view of aperture-limited hologram and objective lens in optical head of Embodiment 3 of the present invention (A) Plan view of hologram in optical head of embodiment 4 of the present invention (b) Side view of hologram and objective lens in optical head of embodiment 4 of the present invention (A) Plan view of hologram in optical head of embodiment 5 of the present invention (b) Side view of hologram and objective lens in optical head of embodiment 5 of the present invention Schematic sectional view of an optical information device in Embodiment 5 of the present invention Schematic perspective view of a computer according to Embodiment 6 of the present invention Schematic perspective view of the optical disk recorder in Embodiment 7 of the present invention Schematic sectional view of an example of a conventional optical head (A) Plan view of wavelength selection phase plate of conventional optical head (b) Cross section of wavelength selection phase plate of conventional optical head Schematic sectional view of another example of a conventional optical head

Explanation of symbols

1, 5, 6, 24 First hologram 2, 7, 25 Second hologram 3, 8, 26 Third hologram 4, 9, 27 Fourth hologram 10 Transparent parallel plate 11 Blue laser 12 Red laser 13 Red Outer laser 14 Collimating lens 15, 28 Objective lens 16 Lens tube 17, 18, 30, 31, 32 Beam splitter 17a, 18a, 30a Optical multilayer film 19, 33 Wavelength plate 20 Detection lens 21 Photo detector 41, 42, 43 Light Beam 51, 52, 53 Optical disc 80 Optical head drive device 81 Optical head 82 Electrical circuit 83 Motor 84 Turntable 86 Optical information device 87 Arithmetic device 88 Computer 90, 91 Input device 92 Decoder 93 Encoder 89 Output device 94 Optical disc recorder

Claims (32)

First and second holograms that diffract light of wavelength λ2 and a refractive lens are provided, light of wavelength λ1 is converged through a transparent layer of thickness t1, and light of wavelength λ2 is converged through a transparent layer of thickness t2. An optical lens characterized by the above. First and second holograms that diffract light of wavelength λ2 and a refractive lens are provided, and light of wavelength λ1 enters the refractive lens through the first and second holograms, and passes through a transparent layer of thickness t1. The converged light of wavelength λ2 is incident on the first hologram, at least part of the optical axis of the diffracted light of the first hologram intersects and is incident on the second hologram, and the diffracted light of the second hologram is An optical lens that is incident on a refractive lens and converges through a transparent layer having a thickness t2. 3. The optical lens according to claim 1, wherein the light with the wavelength λ <b> 2 is diffracted by the first hologram and the second hologram to become divergent light and is incident on the refractive lens. 4. The apparatus according to claim 1, wherein a change in the diffraction angle of the first hologram due to a change in wavelength of the light having the wavelength λ <b> 2 and a change in the diffraction angle of the second hologram are offset. 5. One of the optical lenses. 5. The light according to claim 1, wherein the light at the outermost periphery of the light having the wavelength λ <b> 2 incident on the first hologram is diffracted by the first hologram and is incident on the innermost periphery of the second hologram. The optical lens described in 1. 6. The structure according to claim 1, wherein when the light having the wavelength λ <b> 2 enters the first hologram with a diameter D <b> 1 and the diffracted light of the first hologram enters the second hologram with a diameter D <b> 2, D <b> 1> D <b> 2. The optical lens according to any one of the above. The optical lens according to claim 1, wherein the first and second holograms are volume holograms having a refractive index distribution. The optical lens according to any one of claims 1 to 7, wherein the first hologram is formed on one surface of a transparent parallel plate, and the second hologram is formed on the other surface of the transparent parallel plate. If NA1 is the numerical aperture at which the light of wavelength λ1 is converged through the transparent layer having the thickness t1, and NA2 is the numerical aperture at which the light of wavelength λ2 is converged through the transparent layer having the thickness t2, then t1 <t2 and NA1> The optical lens according to claim 1, wherein the optical lens is NA2. 10. The optical lens according to claim 1, a first light source that emits light having a wavelength λ1, a second light source that emits light having a wavelength λ2, and recording of an optical information medium A light detection unit that receives light reflected by the surface and extracts an electrical signal according to the amount of light;
The optical lens focuses the first light beam from the first light source on the recording surface of the first optical information medium through the transparent layer having a thickness of t1, and the second light beam from the second light source. An optical head characterized in that it converges on the recording surface of the second optical information medium through a transparent layer having a thickness t2, and t1 <t2. 11. The optical lens according to claim 1, wherein the wavelength of the light having the wavelength λ <b> 1 is in the vicinity of 405 nm, and the wavelength of the light having the wavelength λ <b> 2 is in the vicinity of 650 nm. head. 10. The optical lens according to claim 1, wherein a thickness t <b> 1 of the transparent layer is approximately 0.1 mm, and a thickness t <b> 2 of the transparent layer is approximately 0.6 mm. 11. An optical head according to 11. The first and second holograms for diffracting the light of wavelength λ2, the third and fourth holograms for diffracting the light of wavelength λ3, and a refractive lens are provided, and the light of wavelength λ1 is converged through the transparent layer of thickness t1. The optical lens is characterized in that light having a wavelength λ2 is converged through a transparent layer having a thickness t2, and light having a wavelength λ3 is converged through a transparent layer having a thickness t3. First and second holograms for diffracting light of wavelength λ2, third and fourth holograms for diffracting light of wavelength λ3, and refractive lenses are provided, and light of wavelength λ1 passes through the first to fourth holograms. , Is incident on the refractive lens, is converged through a transparent layer having a thickness t1, light having a wavelength λ2 is incident on the first hologram, and at least a part of the optical axes of the diffracted light of the first hologram intersect with each other. 2 is incident on the second hologram, the diffracted light of the second hologram is incident on the refractive lens, is converged through the transparent layer of thickness t2, the light of wavelength λ3 is incident on the third hologram, and the diffraction of the third hologram An optical system characterized in that at least a part of the optical axes of light intersect and enter the fourth hologram, and the diffracted light of the fourth hologram enters the refractive lens and is converged through a transparent layer having a thickness t3. lens. The light of wavelength λ2 is diffracted by the first hologram and the second hologram to become divergent light and enters the refractive lens, and the light of wavelength λ3 is diffracted by the third hologram and the fourth hologram to diverge light The optical lens according to claim 13 or 14, which enters the refractive lens. The amount of change in the diffraction angle of the first hologram due to the change in wavelength of the light with wavelength λ2 and the amount of change in the diffraction angle of the second hologram cancel each other. 16. The optical lens according to claim 13, wherein the change amount of the diffraction angle of the hologram and the change amount of the diffraction angle of the fourth hologram cancel each other. The light at the outermost periphery of the light aperture having the wavelength λ2 incident on the first hologram is diffracted by the first hologram, is incident on the innermost periphery of the second hologram, and is incident on the third hologram. The optical lens according to any one of claims 13 to 16, wherein light at the outermost periphery of the aperture of the light is diffracted by the third hologram and enters the innermost periphery of the fourth hologram. The light of wavelength λ2 enters the first hologram with a diameter D1, the diffracted light of the first hologram enters the second hologram with a diameter D2, and the light of wavelength λ3 enters the third hologram with a diameter D3. The optical system according to any one of claims 13 to 17, wherein the optical system is configured such that D1> D2 and / or D3> D4 when incident and diffracted light of the third hologram is incident on the fourth hologram with a diameter D4. lens. First and second holograms for diffracting light of wavelength λ1, third and fourth holograms for diffracting light of wavelength λ3, and refractive lenses are provided, and light of wavelength λ2 passes through the first to fourth holograms. , Is incident on the refractive lens, is converged through a transparent layer having a thickness t2, light having a wavelength λ1 is incident on the first hologram, and at least a part of the optical axis of the diffracted light of the first hologram intersects with the first hologram. 2 is incident on the hologram 2, the diffracted light of the second hologram is incident on the refractive lens, is converged through the transparent layer of thickness t1, the light of wavelength λ3 is incident on the third hologram, and the diffraction of the third hologram An optical system characterized in that at least a part of the optical axes of light intersect and enter the fourth hologram, and the diffracted light of the fourth hologram enters the refractive lens and is converged through a transparent layer having a thickness t3. lens. The light of wavelength λ1 is diffracted by the first hologram and the second hologram to become convergent light, and enters the refractive lens, and the light of wavelength λ3 is diffracted by the third hologram and the fourth hologram to diverge light. The optical lens according to claim 19, which enters the refractive lens. The amount of change in the diffraction angle of the first hologram due to the change in wavelength of the light with wavelength λ1 and the amount of change in the diffraction angle of the second hologram cancel each other. 21. The optical lens according to claim 19 or 20, wherein a change in the diffraction angle of the hologram and a change in the diffraction angle of the fourth hologram are offset. The light at the outermost periphery of the light having the wavelength λ1 incident on the first hologram is diffracted by the first hologram, is incident on the innermost periphery of the second hologram, and is incident on the third hologram. The optical lens according to any one of claims 19 to 21, wherein light at the outermost periphery of the aperture of the light is diffracted by the third hologram and incident on the innermost periphery of the fourth hologram. The light of wavelength λ1 is incident on the first hologram with a diameter D1, the diffracted light of the first hologram is incident on the second hologram with a diameter D2, and the light of wavelength λ3 is incident on the third hologram with a diameter D3. The optical system according to any one of claims 19 to 22, wherein the optical system is configured so that D1 <D2 and / or D3> D4 when incident and diffracted light of the third hologram enters the fourth hologram with a diameter D4. lens. The optical lens according to any one of claims 13 to 23, wherein the first to fourth holograms are volume holograms having a refractive index distribution. The first and third holograms are formed on one surface of a transparent parallel plate, and the second and fourth holograms are formed on the other surface of the transparent parallel plate. Optical lens. NA1 is the numerical aperture at which light of wavelength λ1 is converged through a transparent layer of thickness t1, NA2 is the numerical aperture of light at which wavelength λ2 is converged through the transparent layer of thickness t2, and light of wavelength λ3 is converged through the transparent layer of thickness t3. The optical lens according to any one of claims 13 to 25, wherein t1 <t2 <t3 and NA1> NA2> NA3, where NA3 is a numerical aperture. The optical lens according to any one of claims 13 to 26, a first light source that emits light of wavelength λ1, a second light source that emits light of wavelength λ2, and light of wavelength λ3. A third light source that emits light, and a light detection unit that receives light reflected by the recording surface of the optical information medium and extracts an electric signal according to the amount of light,
The optical lens focuses the first light beam from the first light source on the recording surface of the first optical information medium through the transparent layer having a thickness of t1, and the second light beam from the second light source. The third optical beam from the third light source converges on the recording surface of the third optical information medium through the transparent layer of thickness t3, and converges on the recording surface of the second optical information medium through the transparent layer of thickness t2. An optical head, wherein t1 <t2 <t3. 27. The optical device according to claim 13, wherein the wavelength of the light of the wavelength λ1 is near 405 nm, the wavelength of the light of the wavelength λ2 is near 650 nm, and the wavelength of the light of the wavelength λ3 is near 780 nm. A lens and an optical head according to claim 27 using the lens. 27. The thickness t1 of the transparent layer is about 0.1 mm, the thickness t2 of the transparent layer is about 0.6 mm, and the thickness t3 of the transparent layer is about 1.2 mm. 29. An optical head according to claim 27 or 28, wherein the optical lens is used. An optical head according to any one of claims 10 to 12 and 27 to 29, a motor for rotating an optical information medium, and the motor, the optical lens, and the like based on a signal obtained from the optical head. And an electric circuit for controlling and driving at least one of the light sources. 32. A computing device comprising the optical information device according to claim 30, wherein the computing device performs computation based on at least one of inputted information and information reproduced from the optical information device, and the inputted information and information reproducing device A computer apparatus comprising: an output device that outputs at least one of information reproduced from the computer and a result calculated by the arithmetic device. An optical information medium recorder comprising: the optical information device according to claim 30; an encoder that converts image information into information to be recorded in the optical information device; and a decoder that converts a signal obtained from the optical information device into an image.

Images