JP2010055732A - Objective optical element and optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective optical element and optical pickup using the same, which increase transmissivity, facilitate processing, and properly record/reproduce information to/from different optical disks. <P>SOLUTION: A first two-stage diffractive structure is set in a central area, the transmissivity of light when a light beam of wavelength λ1 passes through the central area nearest to a peripheral area is lower by 20% or more than the transmissivity of the light when the light beam of wavelength λ1 passes through the peripheral area nearest to the central area, and the first diffractive structure satisfies a conditional equation (1): 2<L<3 μm when the length in the light axis direction in the step surface is L. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical pickup apparatus capable of recording and / or reproducing information interchangeably with different types of optical disks, and an objective optical element used therefor.

  In recent years, research and development of high-density optical disc systems that can record and / or reproduce information (hereinafter, “recording and / or reproduction” is referred to as “recording / reproduction”) using a blue-violet semiconductor laser having a wavelength of about 400 nm. Development is progressing rapidly. As an example, in an optical disc for recording / reproducing information with specifications of NA 0.85 and light source wavelength 405 nm, so-called Blu-ray Disc (hereinafter referred to as BD), DVD (NA 0.6, light source wavelength 650 nm, storage capacity 4.7 GB) It is possible to record 25 GB of information per layer on an optical disc having a diameter of 12 cm which is the same size as the above.

  By the way, it can not be said that the value of an optical disc player / recorder (optical information recording / reproducing device) as a product is sufficient only by appropriately recording / reproducing information on such a high-density optical disc. In light of the reality that DVDs and CDs (compact discs) on which a wide variety of information is recorded are currently being sold, it is not possible to record / reproduce information on high-density optical discs. Similarly, making it possible to appropriately record / reproduce information on DVDs and CDs leads to an increase in commercial value as an optical disc player / recorder for high-density optical discs. From such a background, an optical pickup device mounted on an optical disc player / recorder for high density optical discs can appropriately receive information while maintaining compatibility with both high density optical discs, DVDs, and even CDs. It is desired to have a performance capable of recording / reproducing.

  As a method for recording / reproducing information appropriately while maintaining compatibility with both high-density optical discs and DVDs, and even CDs, optical systems for high-density optical discs and optical systems for DVDs and CDs are used. A method of selectively switching the system to and from the recording density of an optical disk for recording / reproducing information is conceivable, but a plurality of optical systems are required, which is disadvantageous for miniaturization and increases the cost.

  Therefore, in order to simplify the configuration of the optical pickup device and reduce the cost, the optical system for high-density optical discs and the optical system for DVDs and CDs must be shared in compatible optical pickup devices. It is preferable to reduce the number of optical components constituting the optical pickup device as much as possible. In addition, it is most advantageous to simplify the configuration of the optical pickup device and to reduce the cost by using a common objective optical element disposed opposite to the optical disk. In order to obtain a common objective optical element for a plurality of types of optical disks having different recording / reproducing wavelengths, an optical path difference providing structure having a wavelength dependency of spherical aberration is formed in the objective optical system. It is necessary to reduce the spherical aberration that occurs due to the difference in thickness and the thickness of the protective layer.

Patent Document 1 describes an optical element that has an optical path difference providing structure and is used as an objective optical element that can be used in common with high-density optical discs and conventional DVDs and CDs.
JP 2005-259332 A

  In the optical element of Patent Document 1, an infrared laser beam for CD and a laser beam having a wavelength for BD or DVD are made incident within the numerical aperture of the CD on the optical surface of the objective optical element to record each information. A diffractive structure is provided in a common region used for condensing light onto the surface to distribute the light flux. In this case, if the infrared laser beam is condensed using diffracted light other than the 0th order, and the laser beam having a wavelength for BD or DVD is condensed using the 0th order diffracted light, the objective optical element This is preferable because the degree of freedom in design increases. As one type of such a diffractive structure, a binary type diffraction having a two-level optical function surface shifted in the optical axis direction with a step of about 3.6 μm (a light beam with a wavelength for BD has a phase difference of about 5λ). The structure is known.

  By the way, the diffractive structure that generates such diffracted light has the following problems. First, when light is bent by diffraction, a region that becomes a shadow with respect to incident light is generated due to a minute step, and the light transmittance is reduced (shadow effect). In addition, the step is processed into a shape different from the design, that is, a shape having a gradient and a sag due to mold processing accuracy, molding error, and the like, so that an area that becomes a shadow with respect to incident light increases. When the transmittance of light passing through the diffractive structure is reduced, more laser power is required when using the optical pickup device. In addition, when a fine shape corresponding to such a diffractive structure is engraved in a mold for forming an objective optical element, a long cutting tool must be used because the step is relatively large at about 3.6 μm. In addition, there is a problem that it is difficult to process the mold. Furthermore, the transmissivity becomes discontinuous due to the shadow effect being different due to the difference in diffraction structure between the common region and the adjacent region, and there is a possibility that the focused spot size becomes inappropriate due to the so-called super-resolution phenomenon. There are various factors in the difference in the diffraction structure. In the case of a DVD / CD 3 compatible objective optical element, the annular zone pitch in the diffractive structure in the common region often becomes narrower as it moves away from the optical axis, and accordingly, transmission is performed near the boundary between the same region and the peripheral region. The drop in rate becomes large and the super-resolution phenomenon tends to occur.

  The present invention has been made in consideration of the above-mentioned problems, and has an objective optical element that can increase the transmittance, can be easily processed, and can appropriately record / reproduce information with respect to different optical disks, and the same. An object is to provide an optical pickup device.

The objective optical element according to claim 1, a first light source that emits a first light flux having a wavelength λ1, a third light source that emits a third light flux having a wavelength λ3 (1.8λ1 <λ3 <2.0λ1), An objective optical element, and the objective optical element condenses the first light flux on an information recording surface of a first optical disc having a protective layer having a thickness t1, and the third light flux has a thickness t3 (t1). In an objective optical element for an optical pickup device that records and / or reproduces information by focusing on an information recording surface of a third optical disc having a protective layer <t3),
The objective optical element has at least a central region including an optical axis and a peripheral region provided around the central region, and a first diffractive structure is formed in the central region,
The first diffractive structure includes a plurality of concentric annular zones centered on an optical axis, and a cross-sectional shape of the plurality of annular zones including the optical axis of the objective optical element extends in parallel with the optical axis. A plurality of step surfaces, a light source side optical functional surface that connects light source side ends of the adjacent step surfaces, and an optical disc side optical functional surface that connects optical disc side ends of the adjacent step surfaces, The light source side optical functional surface and the optical disc side optical functional surface are alternately arranged along the direction intersecting the optical axis,
The light transmittance when the light beam having the wavelength λ1 passes through the central region closest to the peripheral region is the light transmittance when the light beam having the wavelength λ1 passes through the peripheral region closest to the central region. 20% or more lower than
The first diffractive structure satisfies the following conditional expression, where L is the length of the step surface in the optical axis direction.
2 μm <L <3 μm (1)

  According to the present invention, by satisfying the conditional expression (1), the length of the step surface can be suppressed, and the transmittance can be improved by reducing the shadow area with respect to the incident light. The processing of the mold for molding the objective optical element of the present invention can be facilitated. Furthermore, by reducing the shadow effect, the reduction width of the transmittance due to the shadow effect increasing as the distance from the center region approaches the peripheral region can be reduced. By bringing the transmittance at the boundary between the central region and the peripheral region close to each other, it is possible to suppress the super-resolution phenomenon and form an appropriate condensing spot.

  The objective optical element according to claim 2 is the center of the invention according to claim 1, wherein the light source side optical functional surface of the first diffractive structure has a distance from the optical axis equal to the light source side optical functional surface. Inclined with respect to the mother aspherical surface of the region.

  According to the present invention, by tilting the light source side optical functional surface, it becomes possible to adjust the diffraction efficiency without changing the diffraction order of the maximum amount of diffracted light generated by passing through the first diffractive structure. . Therefore, it is possible to appropriately adjust the diffraction efficiencies of the first light flux and the third light flux in accordance with which of the first optical disk and the third optical disk is important.

  According to a third aspect of the present invention, there is provided the objective optical element according to the first or second aspect, wherein one of the two step surfaces sandwiching the one optical function surface has a length in the optical axis direction of one of the step surfaces. The length of the other step surface in the optical axis direction is different within a range of 0.44 μm or less.

  According to the present invention, it is possible to tilt the light source side optical functional surface, and it is possible to adjust the diffraction efficiency without changing the diffraction order of the maximum amount of diffracted light generated by passing through the first diffractive structure. It becomes. Therefore, it is possible to appropriately adjust the diffraction efficiencies of the first light flux and the third light flux in accordance with which of the first optical disk and the third optical disk is important.

  When the objective optical element according to claim 4 takes the transmittance distribution of the light in the direction perpendicular to the optical axis in the light flux with wavelength λ1 in the invention according to any one of claims 1 to 3, the transmittance is In the central region, the distance gradually decreases as the distance from the optical axis increases, and increases discontinuously from the central region to the peripheral region.

  According to the present invention, the effect of shadows can be reduced in an objective optical element in which the light transmittance distribution gradually decreases as the distance from the optical axis increases in the central region and increases discontinuously from the central region to the peripheral region. Since the width of the discontinuous increase can be reduced, the effect of the present invention becomes more remarkable.

  The objective optical element according to claim 5 is the objective optical element according to any one of claims 1 to 4, wherein the zero-order diffracted light has another order when the light beam having the wavelength λ1 passes through the first diffractive structure. Most generated as compared to diffracted light, and when a light beam having the wavelength λ3 passes through the first diffractive structure, m (m is an integer other than 0) order diffracted light is compared with diffracted light of other orders. It is characterized by occurring most frequently.

  The objective optical element according to a sixth aspect is the invention according to any one of the first to fifth aspects, wherein a blaze type or a step type second diffraction structure of three or more levels is formed in the peripheral region. It is characterized by that.

  As described above, in the objective optical element in which the structure of the central region and the peripheral region is different, the light transmittance tends to increase discontinuously from the central region to the peripheral region, thus reducing the shadow effect. According to the present invention, the width of the discontinuous increase in transmittance can be reduced, and the effect of the present invention becomes more remarkable.

An objective optical element according to a seventh aspect is the invention according to any one of the first to fifth aspects, wherein a second diffractive structure is formed in the peripheral region, and the second diffractive structure has an optical axis. A plurality of concentric annular zones around the center, and the cross-sectional shape of the plurality of annular zones including the optical axis of the objective optical element includes a cylindrical step surface extending in the optical axis direction and the step And an optical functional surface extending in a direction intersecting the surface alternately, and the length in the optical axis direction of the step surface of the first diffractive structure closest to the second diffractive structure is A, and the first diffraction The following conditional expression is satisfied, where B is the length in the optical axis direction of the step surface of the second diffractive structure closest to the structure.
0.65 ≦ B / A ≦ 1 (2)

  As described above, in the objective optical element in which the optical axis direction length of the step surface changes abruptly between the central region and the peripheral region, the light transmittance increases discontinuously from the central region to the peripheral region. Therefore, according to the present invention that can reduce the shadow effect, the width of the discontinuous increase in light transmittance can be reduced, and the effect of the present invention becomes more remarkable.

The objective optical element according to claim 8 is the invention according to any one of claims 1 to 5, wherein a second diffractive structure is formed in the peripheral region, and the second diffractive structure has an optical axis. A plurality of concentric annular zones around the center, and the cross-sectional shape of the plurality of annular zones including the optical axis of the objective optical element includes a cylindrical step surface extending in the optical axis direction and the step And an optical functional surface extending in a direction intersecting the plane alternately, the ring zone pitch of the first diffractive structure closest to the second diffractive structure is α, and the first diffractive structure closest to the first diffractive structure is When the annular zone pitch of the two-diffractive structure is β, the following conditional expression is satisfied.
5 ≦ β / α ≦ 10 (3)

  As described above, in an objective optical element in which the annular zone pitch changes abruptly between the central region and the peripheral region, the light transmittance tends to increase discontinuously from the central region to the peripheral region. Therefore, according to the present invention that can reduce the shadow effect, the width of the discontinuous increase in light transmittance can be reduced, and the effect of the present invention becomes more remarkable.

The objective optical element according to a ninth aspect is the invention according to the eighth aspect, wherein the annular zone pitch of the first diffractive structure is gradually decreased as the distance from the optical axis is increased, and the ring closest to the optical axis is provided. When the belt pitch is X and the ring zone pitch closest to the peripheral region is Y, the following conditional expression is satisfied.
Y / X ≦ 0.05 (4)

  As described above, in the objective optical element in which the annular zone pitch of the first diffractive structure gradually decreases in the central region, the pitch changes suddenly in the second diffractive structure, and the light transmittance from the central region to the peripheral region. Therefore, the present invention that can reduce the shadow effect can reduce the width of the discontinuous increase in light transmittance, and the effect of the present invention becomes more remarkable. .

  According to a tenth aspect of the present invention, there is provided the objective optical element according to any one of the first to ninth aspects, further comprising a third light source that emits a second light flux having a wavelength λ2 (1.5λ1 <λ2 <1.7λ1). The objective optical element records and / or reproduces information by condensing the second light flux on the information recording surface of the second optical disc having a protective layer having a thickness t2 (t1 <t2 <t3). It is characterized by performing.

  An objective optical element according to an eleventh aspect is the optical element according to the tenth aspect, wherein the light transmittance when the light beam having the wavelength λ2 passes through the central region closest to the peripheral region is the light transmittance of the wavelength λ2. It is characterized in that the luminous flux is 10% or more lower than the light transmittance when passing through the peripheral area closest to the central area.

  In the objective optical element in which the light transmittance distribution discontinuously increases from the center region to the peripheral region not only in the first light beam but also in the second light beam, the effect of shadow can be reduced. Since the width of the discontinuous increase can be reduced, the effect of the present invention becomes more remarkable.

  The objective optical element according to claim 12 is the objective optical element according to claim 10 or 11, wherein the zero-order diffracted light is changed to other orders of diffracted light when the light beam having the wavelength λ2 passes through the first diffractive structure. It is characterized by the most frequent occurrence.

  The objective optical element according to a thirteenth aspect is the invention according to any one of the first to twelfth aspects, wherein the first diffractive structure in the vicinity of the optical axis does not satisfy the conditional expression (1), and is in the peripheral structure. The close first diffraction structure satisfies the conditional expression (1).

  According to the present invention, the first diffractive structure near the optical axis maintains a high transmittance, while the first diffractive structure near the peripheral region also reduces the transmittance gap at the boundary between the central region and the peripheral region. This is preferable because it is possible.

  The objective optical element according to claim 14 is the optical disk according to any one of claims 1 to 13, wherein the optical function surface of the first diffractive structure has a distance from the optical axis that is the optical function of the optical disk. It is inclined with respect to the mother aspherical surface of the central region equal to the surface.

  An objective optical element according to a fifteenth aspect of the present invention is the optical disc according to any one of the first to thirteenth aspects, wherein the optical function surface of the first diffractive structure has a distance from the optical axis that is the optical function of the optical disc. It is not inclined with respect to the mother aspherical surface of the central region equal to the surface.

  An optical pickup device according to a sixteenth aspect includes the objective optical element according to any one of the first to fifteenth aspects.

  The optical pickup device according to the present invention has at least two light sources, a first light source and a third light source, but may have a second light source. Furthermore, the optical pickup device of the present invention includes a condensing optical system for condensing the first light flux on the information recording surface of the first optical disc and condensing the third light flux on the information recording surface of the third optical disc. However, the second light beam may be condensed on the information recording surface of the second optical disk by the condensing optical system. The optical pickup device of the present invention has a light receiving element that receives a reflected light beam from the information recording surface of the first optical disk or the third optical disk, and further receives a reflected light beam from the information recording surface of the second optical disk. You may have a light receiving element. That is, the present invention has only two light sources, and is applied to an optical pickup device corresponding to two disks of the first optical disk and the third optical disk and an objective lens used therefor, and has three light sources, In addition to the first optical disk and the third optical disk, the present invention is also applied to an optical pickup device corresponding to the second optical disk and an objective lens used therefor. Of course, the present invention is also applied to an optical pickup device corresponding to four or more types of optical disks and an objective lens used therefor.

  The first optical disc has a protective substrate having a thickness t1 and an information recording surface. The second optical disc has a protective substrate having a thickness of 2 (t1 <t2) and an information recording surface. The third optical disc has a protective substrate having a thickness t3 (t2 <t3) and an information recording surface. The first optical disc is preferably a BD (Blu-ray Disc), the second optical disc is preferably a DVD, and the third optical disc is preferably a CD, but is not limited thereto. The first optical disc, the second optical disc, or the third optical disc may be a multi-layer optical disc having a plurality of information recording surfaces. Note that the thickness of the protective substrate includes the case of 0, or when the protective film having a thickness of several to several tens of μm is applied to the optical disk, the thickness thereof is also included.

  In the BD, information is recorded / reproduced by an objective optical element having an NA of 0.85, and the thickness of the protective substrate is about 0.1 mm. Furthermore, DVD is a general term for DVD series optical discs in which information is recorded / reproduced by an objective optical element having an NA of about 0.60 to 0.67 and the thickness of the protective substrate is about 0.6 mm. ROM, DVD-Video, DVD-Audio, DVD-RAM, DVD-R, DVD-RW, DVD + R, DVD + RW, and the like are included. Further, in this specification, the CD is a CD series optical disc in which information is recorded / reproduced by an objective optical element having a NA of about 0.45 to 0.53 and the protective substrate has a thickness of about 1.2 mm. It is a generic name and includes CD-ROM, CD-Audio, CD-Video, CD-R, CD-RW, and the like. As for the recording density, the recording density of BD is the highest, followed by HD, DVD, and CD in that order.

  In addition, regarding the thicknesses t1, t2, and t3 of the protective substrate, it is preferable to satisfy the following conditional expressions (5), (6), and (7), but is not limited thereto. The thickness of the protective substrate referred to here is the thickness of the protective substrate provided on the surface of the optical disk. That is, the thickness of the protective substrate from the optical disc surface to the information recording surface closest to the surface.

0.0750 mm ≦ t1 ≦ 0.1125 mm (5)
0.5mm ≦ t2 ≦ 0.7mm (6)
1.0mm ≦ t3 ≦ 1.3mm (7)

  In the present specification, the first light source and the third light source (further, the second light source) are preferably laser light sources. As the laser light source, a semiconductor laser, a silicon laser, or the like can be preferably used. The first wavelength λ1 of the first light beam emitted from the first light source, the second wavelength λ2 (λ2> λ1) of the second light beam emitted from the second light source, and the third of the third light beam emitted from the third light source. The wavelength λ3 (λ3> λ2) preferably satisfies the following conditional expressions (8) and (9).

1.5 × λ1 <λ2 <1.7 × λ1 (8)
1.8 × λ1 <λ3 <2.0 × λ1 (9)

  When BD, DVD, and CD are used as the first optical disc, the second optical disc, and the third optical disc, respectively, the first wavelength λ1 of the first light source is preferably 0.35 μm or more and 0.44 μm or less. Preferably, it is 0.39 μm or more and 0.42 μm or less, and the second wavelength λ2 of the second light source is preferably 0.57 μm or more and 0.68 μm or less, more preferably 0.63 μm or more and 0.67 μm or less. The third wavelength λ3 of the third light source is preferably 0.75 μm or more and 0.85 μm or less, more preferably 0.76 μm or more and 0.82 μm or less.

  Further, at least two of the first light source, the second light source, and the third light source may be unitized. The unitization means that the first light source and the second light source are fixedly housed in one package, for example. However, the unitization is not limited to this, and the two light sources are fixed so that the aberration cannot be corrected. Is widely included. In addition to the light source, a light receiving element to be described later may be packaged.

  As the light receiving element, a photodetector such as a photodiode is preferably used. Light reflected on the information recording surface of the optical disc enters the light receiving element, and a read signal of information recorded on each optical disc is obtained using the output signal. Furthermore, it detects the change in the amount of light due to the change in the shape and position of the spot on the light receiving element, performs focus detection and track detection, and moves the objective optical element for focusing and tracking based on this detection I can do it. The light receiving element may comprise a plurality of photodetectors. The light receiving element may have a main photodetector and a sub photodetector. For example, two sub photodetectors are provided on both sides of a photodetector that receives main light used for recording and reproducing information, and the sub light for tracking adjustment is received by the two sub photodetectors. It is good also as a simple light receiving element. The light receiving element may have a plurality of light receiving elements corresponding to the respective light sources.

  The condensing optical system has an objective optical element. The condensing optical system may include only the objective optical element, but the condensing optical system may include a coupling lens such as a collimator lens in addition to the objective optical element. The coupling lens is a single lens or a lens group that is disposed between the objective optical element and the light source and changes the divergence angle of the light beam. The collimator lens is a kind of coupling lens, and is a lens that emits light incident on the collimator lens as parallel light. Further, the condensing optical system has an optical element such as a diffractive optical element that divides the light beam emitted from the light source into a main light beam used for recording and reproducing information and two sub light beams used for tracking and the like. May be. In this specification, the objective optical element refers to an optical system that is disposed at a position facing the optical disk in the optical pickup device and has a function of condensing a light beam emitted from a light source on an information recording surface of the optical disk. Preferably, the objective optical element is an optical system that is disposed at a position facing the optical disc in the optical pickup device and has a function of condensing a light beam emitted from the light source on the information recording surface of the optical disc, An optical system that can be integrally displaced at least in the optical axis direction by an actuator. The objective optical element is preferably a single objective optical element, but may be formed from a plurality of optical elements. The objective optical element may be a glass lens, a plastic lens, or a hybrid lens in which a diffractive structure or the like is provided on a glass lens with a photocurable resin or the like. The objective optical element preferably has a refractive surface that is aspheric. In the objective optical element, it is preferable that a base surface (also referred to as a mother aspheric surface) on which a diffractive structure is provided is an aspheric surface. Note that the effect of the present invention is remarkable particularly in the case of a single objective optical element from the viewpoint of easy manufacture of the diffractive structure.

  Moreover, when using an objective optical element as a glass lens, it is preferable to use the glass material whose glass transition point Tg is 500 degrees C or less, and it is more preferable that it is 480 degrees C or less. By using a glass material having a glass transition point Tg of 500 ° C. or lower, molding at a relatively low temperature is possible, so that the life of the mold can be extended.

  Furthermore, since the specific gravity of the glass lens is generally larger than that of the resin lens, if the objective optical element is a glass lens, the weight is increased and a load is imposed on the actuator that drives the objective optical element. Therefore, when the objective optical element is a glass lens, it is preferable to use a glass material having a small specific gravity. Specifically, the specific gravity is preferably 3.0 or less, and more preferably 2.75 or less.

  Specific examples of such a glass material include Examples 1 to 12 of JP-A-2005-306627. For example, in Example 1 of JP-A-2005-306627, the glass transition point Tg is 460 ° C., the specific gravity is 2.58, the refractive index nd is 1.594, and the Abbe number is 59.8.

When the objective optical element is a plastic lens, it is preferable to use a cyclic olefin-based resin material. Among the cyclic olefin-based materials, the refractive index at a temperature of 25 ° C. with respect to a wavelength of 405 nm is 1.52 to 1.60. The refractive index change rate dN / dT (° C. ⁻¹ ) with respect to the wavelength of 405 nm accompanying the temperature change within the temperature range of −5 ° C. to 70 ° C. is −20 × 10 ^{−5 to} −5 × 10 ^It is more preferable to use a resin material in the range of ⁻⁵ (more preferably −10 × 10 ^{−5 to} −8 × 10 ⁻⁵ ). When the objective optical element is a plastic lens, the coupling lens is preferably a plastic lens.

  Moreover, it is preferable that the Abbe number of the material which comprises an objective optical element is 50 or more.

  The objective optical element is described below. On the optical surface of the objective optical element, at least a first diffractive structure which is a diffractive structure is formed. In addition, at least one optical surface of the objective optical element has a central region and a peripheral region around the central region. In the case where the objective lens is applied to an optical pickup device having a second light source in addition to the first light source and the third light source, at least one optical surface of the objective optical element is the outermost periphery around the peripheral region. It may have a region. The central region is preferably a region including the optical axis of the objective optical element. However, a minute region including the optical axis may be an unused region or a special purpose region, and the periphery thereof may be the central region. It is preferable that the central region, the peripheral region, and the most peripheral region are provided on the same optical surface. As shown in FIG. 1, it is preferable that the center region CN, the peripheral region MD, and the most peripheral region OT are provided concentrically around the optical axis on the same optical surface.

  The central region of the objective optical element has a central region diffractive structure. The central region diffractive structure may be composed of only the first diffractive structure, or the first diffractive structure and the fourth diffractive structure (for example, a blazed diffractive structure to be described later, where the first light beam is incident). Among the diffracted lights generated in the second diffracted light, the second-order diffracted light has the largest amount of diffracted light, and among the diffracted lights generated when the second light flux is incident, the first-order diffracted light has the largest amount of diffracted light, and the third light flux Of the diffracted light generated when the light enters, the first-order diffracted light having the maximum amount of diffracted light (hereinafter referred to as a 2/1/1 diffraction structure) may be superimposed.

  When the objective lens is applied to an optical pickup device having only the first light source and the third light source, the peripheral region may be a refractive surface or a peripheral region diffractive structure may be provided. When the objective lens is applied to an optical pickup device having a second light source in addition to the first light source and the third light source, the peripheral region of the objective optical element preferably has a peripheral region diffractive structure. The peripheral region diffractive structure may be configured by only the second diffractive structure, or may be configured by superimposing the second diffractive structure and the fifth diffractive structure.

  The outermost peripheral region of the objective optical element may have the outermost peripheral region diffractive structure, or may be composed only of a refractive surface. The central region, the peripheral region, and the most peripheral region are preferably adjacent to each other, but there may be a slight gap between them.

  The central diffractive structure is preferably provided in a region of 70% or more of the area of the central region of the objective optical element, and more preferably 90% or more. More preferably, the central diffractive structure is provided on the entire surface of the central region. The peripheral diffractive structure is preferably provided in a region of 70% or more of the area of the peripheral region of the objective optical element, and more preferably 90% or more. More preferably, the peripheral diffractive structure is provided on the entire surface of the peripheral region. The most peripheral diffraction structure is preferably provided in a region of 70% or more of the area of the most peripheral region of the objective optical element, and more preferably 90% or more. More preferably, the outermost peripheral diffractive structure is provided on the entire surface of the outermost peripheral region.

  Note that the diffractive structure referred to in this specification is a general term for structures that have a step and have a function of condensing or diverging a light beam by diffraction. The diffractive structure preferably has a plurality of steps. The steps may be arranged with a periodic interval in the direction perpendicular to the optical axis, or may be arranged with a non-periodic interval in the direction perpendicular to the optical axis.

  The diffractive structure preferably has a plurality of concentric annular zones centered on the optical axis. The diffractive structure can take various cross-sectional shapes (cross-sectional shapes on the plane including the optical axis), and the cross-sectional shapes including the optical axis are roughly classified into a blazed structure and a staircase structure.

  As shown in FIGS. 2A and 2B, the blazed structure is a sawtooth shape in cross section including the optical axis of an optical element having a diffractive structure. It has an oblique surface that is neither perpendicular nor parallel to the spherical surface. In the example of FIG. 2, it is assumed that the upper side is the light source side and the lower side is the optical disk side, and a diffractive structure is formed on a plane as a mother aspherical surface.

  In addition, as shown in FIGS. 2C and 2D, the staircase structure is a structure in which the cross-sectional shape including the optical axis of an optical element having a diffractive structure is a small staircase (referred to as a staircase unit). That is to have more than one. In the present specification, the “X level” means an annular surface corresponding to (or facing) the optical axis vertical direction in one step unit of the staircase structure (hereinafter sometimes referred to as an optical function surface). Is divided by X steps, and is divided into X ring zones. Particularly, a three-level or higher staircase structure has a small step and a large step. In the staircase unit, the smallest step in the optical axis direction is meant, and the “large step” means the largest step in the optical axis direction in one staircase unit.

  The diffractive structure shown in FIG. 2C is called a five-level step structure, and the diffractive structure shown in FIG. 2D is called a two-level step structure. The first diffractive structure is a two-level staircase structure, which includes a plurality of concentric annular zones centered on the optical axis, and the cross-sectional shape of the plurality of annular zones including the optical axis of the objective optical element is optical A plurality of step surfaces Pa and Pb extending parallel to the axis, a light source side optical functional surface Pc connecting the light source side ends of the adjacent step surfaces Pa and Pb, and an optical disc side end of the adjacent step surfaces Pa and Pb The light source side optical functional surface Pc and the optical disk side optical functional surface Pd are alternately arranged along the direction intersecting the optical axis.

    In the staircase structure, the length in the direction perpendicular to the optical axis of one step unit is referred to as a pitch P. The step surface is preferably parallel or substantially parallel to the optical axis, but the optical functional surface may be inclined with respect to the mother aspheric surface as well as when it is parallel to the mother aspheric surface.

  The diffractive structure is preferably a structure in which a certain unit shape is periodically repeated. As used herein, “unit shape is periodically repeated” naturally includes shapes in which the same shape is repeated in the same cycle. In addition, the unit shape that is one unit of the cycle has regularity, and the shape in which the cycle gradually increases or decreases gradually is also included in the “unit shape is periodically repeated”. Suppose that

  When the diffractive structure has a blazed structure, a sawtooth shape as a unit shape is repeated. As shown in FIG. 2 (a), the same sawtooth shape may be repeated, and as shown in FIG. 2 (b), the shape of the sawtooth shape gradually increases as it proceeds in the direction of the mother aspheric surface. It may be a shape that increases in size or a shape that decreases. Moreover, it is good also as a shape which combined the shape in which the magnitude | size of the serrated shape became large gradually, and the shape in which the magnitude | size of a serrated shape became small gradually. However, even when the size of the serrated shape changes gradually, it is preferable that the size of the step amount in the optical axis direction (or the direction of the passing light beam) hardly changes in the serrated shape. In addition, in some areas, the blazed structure has a step opposite to the optical axis (center) side, and in other areas, the blazed structure has a step toward the optical axis (center). It is good also as a shape by which the transition area | region required in order to switch the direction of the level | step difference of a blaze | braze type | mold structure is provided in the meantime. This transition region is a region corresponding to a point that becomes an extreme value of the optical path difference function when the optical path difference added by the diffractive structure is expressed by the optical path difference function. Note that if the optical path difference function has an extreme point, the inclination of the optical path difference function becomes small, so that the annular zone pitch can be widened, and the decrease in transmittance due to the shape error of the diffractive structure can be suppressed.

  When the diffractive structure has a stepped structure, there may be a shape or the like in which five-level step units as shown in FIG. 2C are repeated. Furthermore, the shape of the staircase gradually increases as it advances in the direction of the mother aspheric surface, or the shape of the staircase gradually decreases. It is preferable that the level difference in the direction of the light beam to be changed hardly changes.

  Hereinafter, the first diffraction structure will be described in detail. The first diffractive structure is a two-level step structure as described above. The first diffractive structure is a structure that enables at least the compatibility between the first optical disc and the third optical disc. Therefore, the first diffractive structure is different from the thickness t1 of the protective substrate of the first optical disc and the thickness t3 of the protective substrate of the third optical disc with respect to the first light flux and the third light flux that pass through the first diffractive structure. It is preferable to correct spherical aberration that occurs and / or spherical aberration that occurs due to a difference in wavelength between the first light beam and the third light beam.

The first diffractive structure preferably satisfies the following conditional expression, where L is the length of the step surface in the optical axis direction.
2 μm <L <3 μm (1)

  If the length L in the optical axis direction of the step surface of the first diffractive structure is, for example, 3.6 μm, the transmittance can be maximized when a light beam having the wavelength λ1 is incident. However, as the length L increases, the area that becomes a shadow with respect to incident light increases and the transmittance decreases, and it is difficult to process a mold for molding the objective optical element having the first diffractive structure. It becomes. Therefore, by shortening the length L so as to satisfy the conditional expression (1), it is possible to reduce the area that becomes a shadow with respect to the incident light and improve the transmittance, and to achieve the objective optical element of the present invention. The mold for molding can be easily processed. Furthermore, since the transmittance at the boundary between the central region and the peripheral region can be made closer, an appropriate condensing spot can be formed while suppressing the super-resolution phenomenon.

  The light transmittance when the light beam having the wavelength λ1 passes through the central region closest to the peripheral region is 20% or more with respect to the light transmittance when the light beam having the wavelength λ1 passes through the peripheral region closest to the central region. When it is low, the effect of the present invention can be enjoyed, which is preferable. That is, when the first diffractive structure does not satisfy the conditional expression (1), the light transmittance when the light beam having the wavelength λ1 passes through the central region closest to the peripheral region is the light beam having the wavelength λ1 most in the central region. This indicates that the transmittance of light when passing through a nearby peripheral region is very low, and the difference in transmittance can be reduced to about 20% or more by the present invention, which is preferable. If it is lower by 25% or more, the effect of the present invention becomes more remarkable, which is further preferable. The difference between the light transmittance when the light beam having the wavelength λ1 passes through the central region closest to the peripheral region and the light transmittance when the light beam having the wavelength λ1 passes through the peripheral region closest to the central region is as follows. , 40% or less, more preferably 30% or less. Further, the light transmittance when the light beam having the wavelength λ2 passes through the central region closest to the peripheral region is 10% of the light transmittance when the light beam having the wavelength λ2 passes through the peripheral region closest to the central region. % Is preferably lower, and more preferably 15% or lower. In addition, it is preferable that it is 30% or less, and it is still more preferable that it is 20% or less. Note that the transmittance in the present specification is the ratio of the amount of light that contributes to spot formation on the information recording surface of the optical disc to the amount of light of a predetermined wavelength incident on a predetermined region of the objective optical element.

  It is preferable that the light source side optical functional surface of the first diffractive structure is inclined with respect to the mother aspherical surface in the central region whose distance from the optical axis is equal to the light source side optical functional surface. In FIG. 7 schematically showing a cross section in the optical axis direction of the first diffractive structure according to an example, a mother aspheric surface BAP indicated by a dotted line defines a mother aspheric surface in the central region. For example, the light source side optical functional surface Pc Can be obtained by connecting the midpoints (not limited to the midpoint but may be the corresponding corners). “The distance from the optical axis is inclined with respect to the mother aspherical surface in the central region equal to the light source side optical functional surface” means that when such a mother aspherical surface BAP is overlapped with the light source side optical functional surface Pc, In the overlapped position Px, it means that the mother aspheric surface BAP and the light source side optical functional surface Pc intersect at an inclination angle θc (≠ 0). By inclining the light source side optical functional surface Pc in this way, the balance between the diffraction efficiency of the first light beam having the wavelength λ1 and the diffraction efficiency of the light beams having other wavelengths can be changed. If L is made small so as to satisfy the conditional expression (1), the transmittance of the first light flux having the wavelength λ1 and the transmittance of the second light flux having the wavelength λ2 are reduced, but by tilting the light source side optical function surface Pc. The transmittance of the second light flux having the wavelength λ2 can be increased, and the transmittance of the first light flux having the wavelength λ1 is also coupled with the gentle efficiency distribution with respect to the distance from the optical axis (FIG. 6 described later). As a result, it is possible to secure a high total transmittance and to suppress a change in transmittance of the light flux having the wavelength λ1 between the central region and the peripheral region.

  It should be noted that the light source side optical functional surface Pc is inclined so that the normal line of the light source side optical functional surface Pc faces outward in the direction perpendicular to the optical axis with respect to the normal line of the mother aspheric surface BAP (FIG. 7). There is a case where the light source side optical functional surface Pc is inclined so that the normal line of the side optical functional surface Pc is directed inward in the direction perpendicular to the optical axis with respect to the normal line of the mother aspheric surface BAP (FIG. 8). As shown in FIG. 8, it is preferable that the light source side optical functional surface Pc is inclined because the transmittance of the light beam having the wavelength λ2 and the transmittance of the light beam having the wavelength λ3 can be improved. The optical function surface Pd on the optical disc side may or may not be inclined with respect to the mother aspheric surface BAP. By tilting the optical disc side optical functional surface Pd with respect to the mother aspheric surface, the diffraction efficiency of the second light beam can be mainly changed.

Of the two step surfaces sandwiching one optical function surface, the length in the optical axis direction of one step surface and the length in the optical axis direction of the other step surface are preferably different within a range of 0.44 μm or less. With such a configuration, the light source side optical functional surface of the first diffractive structure can be tilted with respect to the mother aspherical surface of the central region whose distance from the optical axis is equal to the light source side optical functional surface. In the example of FIG. 7, of two step surfaces sandwiching one light source side optical functional surface Pc, the length in the optical axis direction of one step surface Pa is L1, and the length in the optical axis direction of the other step surface Pb. When L2 is L2, the following formula is established. Note that L = (L1 + L2) / 2 can be approximated.
0 μm <| L1-L2 | ≦ 0.44 μm

  When the light transmittance distribution in the direction orthogonal to the optical axis of the light flux with wavelength λ1 is taken, the transmittance gradually decreases in the central region as it moves away from the optical axis, and is discontinuous from the central region to the peripheral region. In the case where it increases, the effect of satisfying conditional expression (1) of the present invention becomes remarkable, which is preferable. In FIG. 6, when the transmittance of the light beam having the wavelength λ1 is taken on the vertical axis and the height from the optical axis is taken on the horizontal axis, the solid line satisfies the conditional expression (1) as an example of the present invention, and the light source side The present invention relates to an objective optical element in which a diffractive structure whose optical function surface is tilted with respect to the mother aspheric surface is formed in the central region. The dot-and-dash line is L = 3.6 μm as a comparative example, and the light source side optical function surface is The present invention relates to an objective optical element in which a diffractive structure equal to a mother aspheric surface is formed in a central region. In the present invention, the light transmittance when the light beam having the wavelength λ1 passes through the central region closest to the peripheral region, and the light transmittance when the light beam having the wavelength λ1 passes through the peripheral region closest to the central region. The difference δ is 20% or more. The effect of this invention becomes more remarkable when it is 25% or more. It is preferably 40% or less, and more preferably 30% or less. That is, also in the solid line portion in FIG. 6, the difference in light transmittance δ between the central region and the peripheral region is 20% or more.

  Referring to FIG. 6, in both the comparative example and the example, the transmittance at the wavelength λ1 gradually decreases as the distance from the optical axis increases in the central region, and increases discontinuously from the central region to the peripheral region. However, in the case of the comparative example, the transmittance decreases in a quadratic curve due to the shadow of the step surface as it goes to the peripheral region, whereas in the present invention, the light flux having the wavelength λ1 is near the optical axis. However, since the degree of decrease in transmittance is low even when approaching the peripheral area, the total amount of light can be secured and the change in the transmittance between the central area and the peripheral area can be kept small. Super-resolution phenomenon can be suppressed.

  When the light beam having the wavelength λ1 passes through the first diffractive structure, the 0th-order diffracted light is generated more than the diffracted light of other orders, and when the light beam having the wavelength λ3 passes through the first diffractive structure, m (M is an integer other than 0) It is preferable that the most diffracted light is generated as compared with diffracted light of other orders. In addition, it is preferable that when the light beam having the wavelength λ2 passes through the first diffractive structure, the 0th-order diffracted light is generated most as compared with other orders of diffracted light. Here, zero-order diffracted light means transmitted light. Preferably, when the light beam having the wavelength λ1 passes through the first diffractive structure, the 0th-order diffracted light is generated more than the diffracted light of other orders, and the light beam having the wavelength λ2 passes through the first diffractive structure. In addition, when the 0th-order diffracted light is generated most in comparison with other orders of diffracted light and the light beam having the wavelength λ3 passes through the first diffractive structure, the + 1st order diffracted light and the −1st order diffracted light are diffracted at other orders It is the most frequently generated structure compared to light.

  Hereinafter, the second diffractive structure will be described. The second diffractive structure is preferably a blaze type or a step type structure of three or more levels. By adopting such a structure, the central diffractive structure in the central region and the peripheral diffractive structure in the peripheral region are different from each other, so that the transmittance easily changes discontinuously at the boundary between the central region and the peripheral region. Therefore, the effect of the present invention by satisfying conditional expression (1) becomes more remarkable. More preferably, the second diffractive structure is a blazed 2/1/1 diffractive structure, or the second diffractive structure is a five-level step-type structure, A preferred example is a structure that generates the most 0th-order diffracted light, generates the most 1st-order diffracted light with respect to the second light flux, and generates the most 0th-order diffracted light with respect to the third light flux.

The second diffractive structure includes a plurality of concentric annular zones centered on the optical axis, and a cross-sectional shape of the plurality of annular zones including the optical axis of the objective optical element is a cylindrical shape extending in the optical axis direction. The step surface and the optical functional surface extending in the direction intersecting the step surface are alternately combined, and the length in the optical axis direction of the step surface of the first diffractive structure closest to the second diffractive structure is A, first. When the length in the optical axis direction of the step surface of the second diffractive structure closest to the diffractive structure is B, it is preferable that the following conditional expression is satisfied.
0.65 ≦ B / A ≦ 1 (2)

In an objective optical element in which the length in the optical axis direction of the step surface changes abruptly between the central region and the peripheral region that satisfies the conditional expression (2), the light transmittance is from the central region to the peripheral region. Since there is a tendency to increase discontinuously, the present invention that can reduce the shadow effect can reduce the width of the discontinuous increase in light transmittance, and the effect of the present invention becomes more remarkable. Where A
Is the length L in the optical axis direction of the step surface of the first diffractive structure closest to the second diffractive structure, and B is the step shown in FIG. 2A when the second diffractive structure is a blazed diffractive structure. When the second diffractive structure is a staircase type diffractive structure, the step amount B shown in FIG. In particular, the second diffractive structure is a 2/1/1 diffractive structure, or the second diffractive structure is a five-level stepped structure, and generates the most 0th-order diffracted light with respect to the first light flux, The present invention is effective when the first-order diffracted light is generated most for the second light beam and the 0-order diffracted light is generated most for the third light beam.

The second diffractive structure includes a plurality of concentric annular zones centered on the optical axis, and the cross-sectional shape of the plurality of annular zones including the optical axis of the objective optical element is a cylindrical shape extending in parallel to the optical axis Of the first diffractive structure closest to the second diffractive structure and α (see FIG. 2 (d)) by alternately combining the step surface and the optical functional surface extending in the direction intersecting the step surface. P) When the annular zone pitch of the second diffractive structure closest to the first diffractive structure is β (P in FIG. 2A or 2C), it is preferable that the following conditional expression is satisfied.
5 ≦ β / α ≦ 10 (3)

  In an objective optical element in which the annular zone pitch changes suddenly between the central region and the peripheral region that satisfies the conditional expression (3), the light transmittance increases discontinuously from the central region to the peripheral region. Therefore, according to the present invention that can reduce the shadow effect, the width of the discontinuous increase in light transmittance can be reduced, and the effect of the present invention becomes more remarkable. Here, the zone pitch refers to the value P shown in FIG. In particular, the second diffractive structure is a 2/1/1 diffractive structure, or the second diffractive structure is a five-level stepped structure, and generates the most 0th-order diffracted light with respect to the first light flux, The present invention is effective when the first-order diffracted light is generated most for the second light beam and the 0-order diffracted light is generated most for the third light beam.

The annular zone pitch of the first diffractive structure gradually decreases as the distance from the optical axis increases, and the annular zone pitch closest to the optical axis is X (P in FIG. 2 (d)), and the annular zone closest to the peripheral region. When the pitch is Y (P in FIG. 2D), it is preferable that the following conditional expression is satisfied.
Y / X ≦ 0.05 (4)

  In an objective optical element in which the annular pitch of the first diffractive structure gradually decreases in the central region that satisfies the conditional expression (4), the pitch changes suddenly in the second diffractive structure, and the central region to the peripheral region. Since the light transmittance tends to increase discontinuously over time, the present invention that can reduce the shadow effect can reduce the width of the light transmittance discontinuous increase, and the effect of the present invention can be further improved. It will be remarkable.

  The first diffractive structure near the optical axis may not satisfy the conditional expression (1), and the first diffractive structure close to the peripheral structure may satisfy the conditional expression (1). Specifically, in the graph shown in FIG. 6, H is the height in the optical axis direction at which the transmittance (solid line) with the tilted optical function surface and the transmittance without tilting (the one-dot chain line) intersect in the central region. Then, “near the optical axis” corresponds to a height of 0 to H, and “close to the peripheral structure” can correspond to a height equal to or higher than H. By adopting such a structure, the first diffractive structure near the optical axis maintains a high transmittance with respect to the first light beam, while the first diffractive structure close to the peripheral area has a central region and a peripheral region. This is preferable because the gap of the transmittance of the first light flux at the boundary can be reduced.

  The optical disc side optical functional surface of the first diffractive structure may be inclined with respect to the mother aspherical surface in the central region whose distance from the optical axis is equal to the optical disc side optical functional surface. In FIG. 9 schematically showing a cross section in the optical axis direction of the first diffractive structure, a mother aspheric surface BAP indicated by a dotted line defines a mother aspheric surface in the central region. For example, the middle point of the optical function surface Pd on the optical disc side (It is not limited to the midpoint, and a corresponding corner may be used). “The distance from the optical axis is inclined with respect to the mother aspheric surface in the central region equal to the optical function surface on the optical disk side” means that such a mother aspheric surface BAP is superimposed on the optical function surface Pd on the optical disk side. In the superimposed position Py, the mother aspheric surface BAP and the optical disc side optical functional surface Pd intersect at an inclination angle θd (≠ 0). Thus, by tilting the optical function surface Pd on the optical disc side, the balance between the diffraction efficiency of the first light beam having the wavelength λ1 and the diffraction efficiency of the light beams having other wavelengths can be changed. This is particularly effective for adjusting the diffraction efficiency of the second light beam. In addition, you may incline with the light source side optical function surface Pc.

  Note that the optical disc side optical functional surface Pd is inclined so that the normal line of the optical disc side optical functional surface Pd faces outward in the direction perpendicular to the optical axis with respect to the normal line of the base aspheric surface BAP (FIG. 9). In some cases, the optical disc side optical functional surface Pd is tilted so that the normal line of the side optical functional surface Pd is directed inward in the direction perpendicular to the optical axis with respect to the normal line of the base aspheric surface BAP (FIG. 10). As shown in FIG. 10, it is preferable that the optical disc-side optical functional surface Pd is inclined because the transmittance of the light beam having the wavelength λ2 and the transmittance of the light beam having the wavelength λ3 can be improved.

  In addition to the central diffractive structure provided in the central region of the objective optical element, when the peripheral diffractive structure is provided in the peripheral region of the objective optical element, it may be provided on different optical surfaces of the objective optical element, but the same optical surface It is preferable to provide in. Providing them on the same optical surface is preferable because it makes it possible to reduce eccentricity errors during manufacturing. Further, it is preferable that the central diffractive structure and the peripheral diffractive structure are provided on the surface on the light source side of the objective optical element rather than the surface on the optical disc side of the objective optical element.

  The objective optical element condenses the first light beam, the second light beam, and the third light beam that pass through the central region in which the central diffractive structure is provided so as to form a condensing spot. Preferably, the objective optical element condenses the first light beam passing through the central region provided with the central diffraction structure so that information can be recorded and / or reproduced on the information recording surface of the first optical disc. The objective optical element condenses the second light flux that passes through the central region provided with the central diffractive structure so that information can be recorded and / or reproduced on the information recording surface of the second optical disc. Further, the objective optical element condenses the third light flux that passes through the central region provided with the central diffraction structure so that information can be recorded and / or reproduced on the information recording surface of the third optical disc. The objective optical element corresponds to the second optical disc in addition to the first optical disc and the third optical disc, and the thickness t1 of the protective substrate of the first optical disc and the protective substrate of the second optical disc. When the thickness t2 of the first optical disc is different, the central diffractive structure has a thickness t1 of the protective substrate of the first optical disc and a thickness of the protective substrate of the second optical disc with respect to the first light flux and the second light flux passing through the central diffractive structure. It is preferable to correct spherical aberration generated due to a difference in t2 and / or spherical aberration generated due to a difference in wavelength between the first light flux and the second light flux. Further, the central diffractive structure is generated due to the difference between the thickness t1 of the protective substrate of the first optical disc and the thickness t3 of the protective substrate of the third optical disc with respect to the first light flux and the third light flux that have passed through the central diffractive structure. It is preferable to correct spherical aberration that occurs and / or spherical aberration that occurs due to a difference in wavelength between the first light flux and the third light flux.

  Further, when the objective optical element is provided with a peripheral diffraction structure, the objective optical element condenses the first light beam and the second light beam that pass through the peripheral region so as to form a condensing spot, respectively. . Preferably, the objective optical element condenses the first light flux that passes through the peripheral region provided with the peripheral diffraction structure so that information can be recorded and / or reproduced on the information recording surface of the first optical disc. In addition, when the objective optical element is provided with a peripheral diffraction structure, the second optical flux passing through the peripheral region can be recorded and / or reproduced on the information recording surface of the second optical disk using this. Condensate. In addition, it is preferable that the peripheral diffractive structure corrects chromatic spherical aberration caused by the difference in wavelength between the first light beam and the second light beam that pass through the peripheral diffractive structure.

  In addition, as a preferable aspect, there is an aspect in which the third light flux that has passed through the peripheral region is not used for recording and / or reproduction of the third optical disk. It is preferable that the third light flux that has passed through the peripheral region does not contribute to the formation of a focused spot on the information recording surface of the third optical disc. That is, when the objective optical element is provided with a peripheral diffraction structure, it is preferable that the third light beam passing through the peripheral region thereby forms a flare on the information recording surface of the third optical disk. As shown in FIG. 4, in the spot formed on the information recording surface of the third optical disc by the third light beam that has passed through the objective optical element, the light amount density is increased in the order from the optical axis side (or the spot center) to the outside. It has a high spot center portion SCN, a spot intermediate portion SMD having a light amount density lower than that of the spot center portion, and a spot peripheral portion SOT having a light amount density higher than that of the spot intermediate portion and lower than that of the spot center portion. The center portion of the spot is used for recording and / or reproducing information on the optical disc, and the spot intermediate portion and the spot peripheral portion are not used for recording and / or reproducing information on the optical disc. In the above, this spot peripheral part is called flare. However, the spot peripheral portion is also referred to as flare even when a spot has a spot peripheral portion around the center portion of the spot and there is a spot peripheral portion, that is, when a light spot with a large light is formed around the condensed spot. That is, the third light flux that has passed through the peripheral diffraction structure provided in the peripheral region of the objective optical element forms a spot peripheral portion on the information recording surface of the third optical disc.

  Moreover, as a preferable aspect in the case of having the outermost peripheral area, the first light flux that has passed through the outermost peripheral area is used for recording and / or reproduction of the first optical disc, and the second and third light fluxes that have passed through the outermost peripheral area. Includes an aspect that is not used for recording and / or reproduction of the second optical disc and the third optical disc. It is preferable that the second light flux and the third light flux that have passed through the outermost peripheral region do not contribute to the formation of a condensed spot on the information recording surfaces of the second optical disc and the third optical disc, respectively. That is, when the objective optical element has the outermost peripheral region, the second light flux and the third light flux that pass through the outermost peripheral area of the objective optical element form a flare on the information recording surfaces of the second optical disc and the third optical disc. Is preferred. In other words, the second light flux and the third light flux that have passed through the most peripheral area of the objective optical element preferably form a spot peripheral portion on the information recording surfaces of the second optical disc and the third optical disc.

  When the most peripheral region has the most peripheral diffractive structure, the most peripheral diffractive structure causes spherochromatism (color) generated by a slight variation in the wavelength of the first light source with respect to the first light flux that has passed through the most peripheral diffractive structure. (Spherical aberration) may be corrected. A slight change in wavelength refers to a change within ± 10 nm. For example, when the first light beam changes by ± 5 nm from the wavelength λ1, the most peripheral region diffraction structure compensates for the variation in spherical aberration of the first light beam that has passed through the most peripheral region, and on the information recording surface of the first optical disc. It is preferable that the change amount of the wavefront aberration is 0.001λ2 rms or more and 0.070λ2 rms or less.

  In addition, by superimposing the first diffractive structure and the fourth diffractive structure to form the central diffractive structure, the directions of the emitted light of all of the first light flux, the second light flux, and the third light flux that have passed through the central diffractive structure are changed. Since the first light beam, the second light beam, and the third light beam all enter the objective optical element with the same imaging magnification (for example, all parallel light beams), different types of optical disks can be used. It is possible to correct the aberration caused by using the lens and to make it compatible. A preferred fourth diffractive structure is a blazed 2/1/1 diffractive structure.

  When the objective optical element is a plastic lens, the seventh diffractive structure may be used as the temperature characteristic correcting structure, and the first diffractive structure or the first diffractive structure and the fourth diffractive structure may be further overlapped as the central diffractive structure. Good. Specifically, the step amount in the optical axis direction of the seventh diffractive structure gives an optical path difference of about 10 wavelengths of the first wavelength to the first light flux, and about 6 of the second wavelength to the second light flux. A step amount that gives an optical path difference corresponding to the wavelength and gives an optical path difference equivalent to approximately 5 wavelengths of the third wavelength to the third light flux, or an optical path corresponding to approximately 2 wavelengths of the first wavelength relative to the first light flux. The difference in level is such that a difference is given, an optical path difference of about one wavelength of the second wavelength is given to the second light flux, and an optical path difference of about one wavelength of the third wavelength is given to the third light flux. Is preferred.

  As described above, it is preferable that the level difference is not too large. When the level difference of the annular zone with the diffraction structure obtained by superimposing a plurality of diffraction structures is higher than the reference value, by reducing the level difference of the annular zone by 10 · λ1 / (n−1) (μm), It is possible to reduce an excessively large step amount without affecting the optical performance. An arbitrary value can be set as the reference value, but 10 · λ1 / (n−1) (μm) is preferably used as the reference value.

  Further, from the viewpoint that it is preferable from the viewpoint of manufacturing that the number of elongated ring zones is small, it is preferable that the value of (step amount / level width) is 1 or less in all ring zones of the central diffractive structure. It is 8 or less. More preferably, the value of (step amount / level width) is preferably 1 or less, and more preferably 0.8 or less, in all annular zones of all diffractive structures.

  The objective optical element necessary for reproducing and / or recording information on the first optical disk is NA1, and the objective optical necessary for reproducing and / or recording information on the second optical disk is NA1. The image side numerical aperture of the element is NA2 (NA1 ≧ NA2), and the image side numerical aperture of the objective optical element necessary for reproducing and / or recording information on the third optical disk is NA3 (NA2> NA3). . NA1 is preferably 0.6 or more and 0.9 or less, and more preferably 0.7 or more and 0.9 or less. In particular, NA1 is preferably 0.85. NA2 is preferably 0.55 or more and 0.7 or less. In particular, NA2 is preferably 0.60 or 0.65. NA3 is preferably 0.4 or more and 0.55 or less. In particular, NA3 is preferably 0.45 or 0.53.

  The boundary between the central region and the peripheral region of the objective optical element is 0.9 · NA 3 or more and 1.2 · NA 3 or less (more preferably 0.95 · NA 3 or more, 1.15 · It is preferably formed in a portion corresponding to the range of NA3 or less. More preferably, the boundary between the central region and the peripheral region of the objective optical element is formed in a portion corresponding to NA3. The boundary between the peripheral area and the most peripheral area of the objective optical element is 0.9 · NA 2 or more and 1.2 · NA 2 or less (more preferably 0.95 · NA 2 or more, 1) when the second light beam is used. .15 · NA2 or less) is preferable. More preferably, the boundary between the peripheral region and the most peripheral region of the objective optical element is formed in a portion corresponding to NA2.

  When the third light beam that has passed through the objective optical element is condensed on the information recording surface of the third optical disk, it is preferable that the spherical aberration has at least one discontinuous portion. In that case, the discontinuous portion has a range of 0.9 · NA 3 or more and 1.2 · NA 3 or less (more preferably 0.95 · NA 3 or more and 1.15 · NA 3 or less) when the third light flux is used. It is preferable that it exists in.

  Further, when spherical aberration is continuous and does not have a discontinuous portion, and the third light flux that has passed through the objective optical element is condensed on the information recording surface of the third optical disk, NA2 It is preferable that the absolute value of the longitudinal spherical aberration is 0.03 μm or more, and in NA3, the absolute value of the longitudinal spherical aberration is 0.02 μm or less. More preferably, in NA2, the absolute value of longitudinal spherical aberration is 0.08 μm or more, and in NA3, the absolute value of longitudinal spherical aberration is 0.01 μm or less.

  Also, the diffraction efficiency for each wavelength in the central region can be set as appropriate according to the use of the optical pickup device. For example, in the case of an optical pickup device that performs recording and reproduction on the first optical disc and only reproduction on the second and third optical discs, the diffraction efficiency of the central region and / or the peripheral region is expressed as the first luminous flux. It is preferable to set with emphasis. On the other hand, in the case of an optical pickup device that performs only reproduction with respect to the first optical disc and performs recording and reproduction with respect to the second and third optical discs, the second and third light fluxes are emphasized with respect to the diffraction efficiency of the central region. It is preferable to set the diffraction efficiency of the peripheral region with the second light flux as important.

  In any case, by satisfying the following conditional expression (10), it is possible to ensure high diffraction efficiency of the first light flux calculated by the area weighted average of each region.

η11 ≦ η21 (10)
However, η11 represents the diffraction efficiency of the first light flux in the central region, and η21 represents the diffraction efficiency of the first light flux in the peripheral region. When the diffraction efficiency of the central region is focused on the light fluxes of the second and third wavelengths, the diffraction efficiency of the first light flux in the central region is low, but the numerical aperture of the first optical disc is the numerical aperture of the third optical disc. If it is larger than, the lowering of the diffraction efficiency in the central region does not have a significant effect when considering the entire effective diameter of the first light flux.

In addition, the diffraction efficiency in this specification can be defined as follows.
[1] Measure the transmittance of an objective optical element that has the same focal length, lens thickness, and numerical aperture, is formed of the same material, and has no central and peripheral diffractive structures, divided into a central region and a peripheral region. To do. At this time, the transmittance of the central region is measured by blocking the light beam incident on the peripheral region, and the transmittance of the peripheral region is measured by blocking the light beam incident on the central region.
[2] The transmittance of the objective optical element having the central and peripheral diffraction structures is measured separately for the central region and the peripheral region.
[3] The value obtained by dividing the result of [2] by the result of [1] is the diffraction efficiency of each region.

  The light utilization efficiency of any two of the first to third light fluxes is 70% or more, and the light utilization efficiency of the remaining one light flux is 30% or more and 70% or less. Good. The light utilization efficiency of the remaining one light beam may be 40% or more and 60% or less. In this case, it is preferable that the light beam having the light use efficiency of 30% or more and 70% or less (or 40% or more and 60% or less) is the third light beam.

  The light utilization efficiency here is formed on the information recording surface of the optical disc by an objective optical element having a central region diffractive structure (the peripheral region diffractive structure and the most peripheral region diffractive structure may be formed). The light amount of the focused light spot in the Airy disk is LA, which is formed from the same material and has the same focal length, axial thickness, numerical aperture, and wavefront aberration. When the amount of light in the Airy disk of the focused spot formed on the information recording surface of the optical information recording medium is LB by the objective optical element in which the diffractive structure and the most peripheral region diffractive structure are not formed, it is calculated by LA / LB. Shall. Here, the Airy disk refers to a circle having a radius r ′ centered on the optical axis of the focused spot. r ′ = 0.61 · λ / NA.

When the focal length of the first optical flux of the objective optical element is f1 (mm) and the center thickness of the objective optical element is d (mm), it is preferable that the following expression (11) is satisfied.
0.7 ≦ d / f1 ≦ 1.5 (11)

In addition, it is more preferable to satisfy | fill following formula (11) '.
1.0 ≦ d / f1 ≦ 1.3 (11) ′

  With the above configuration, the working distance of the CD as the third optical disk can be secured without reducing the pitch of the diffractive structure, the objective optical element can be easily manufactured, and the light utilization efficiency is maintained high. It becomes possible to do.

Moreover, it is preferable to satisfy the following conditional expressions.
2.1mm ≦ φ ≦ 4.2mm
Φ represents the effective diameter of the objective optical element when the second optical disk is used. By satisfying the above range, the working distance of the CD as the third optical disk is secured at a distance that does not cause a problem in practical use, and even if the objective optical element is a plastic lens, the aberration changes when the temperature changes Can be maintained at a problem-free level.

The first light beam, the second light beam, and the third light beam may be incident on the objective optical element as parallel light, or may be incident on the objective optical element as divergent light or convergent light. Preferably, the imaging magnification m1 of the objective optical element when the first light beam enters the objective optical element satisfies the following expression (12).
−0.02 <m1 <0.02 (12)

On the other hand, when the first light beam enters the objective optical element as diverging light, the imaging magnification m1 of the objective optical element when the first light beam enters the objective optical element satisfies the following expression (12 ′). It is preferable.
−0.10 <m1 <0.00 (12 ′)

When the second light beam is incident on the objective optical element as parallel light or substantially parallel light, the imaging magnification m2 of the objective optical element when the second light beam enters the objective optical element is expressed by the following equation (13). It is preferable to satisfy.
−0.02 <m2 <0.02 (13)

  On the other hand, when the second light flux is incident on the objective optical element as diverging light, the imaging magnification m2 of the objective optical element when the second light flux is incident on the objective optical element satisfies the following expression (13 ′). It is preferable.

  −0.10 <m2 <0.00 (13 ′)

When the third light beam is incident on the objective optical element as parallel light or substantially parallel light, the imaging magnification m3 of the objective optical element when the third light beam is incident on the objective optical element is expressed by the following equation (14). It is preferable to satisfy. When the third light flux is parallel light, a problem easily occurs in tracking. However, even if the third light flux is parallel light, the present invention can obtain good tracking characteristics, and can be used for three different optical disks. On the other hand, recording and / or reproduction can be appropriately performed.
−0.02 <m3 <0.02 (14)

On the other hand, when the third light beam is incident on the objective optical element as diverging light, the imaging magnification m3 of the objective optical element when the third light beam enters the objective optical element satisfies the following formula (14 ′). It is preferable.
−0.10 <m3 <0.00 (14 ′)

  In addition, the working distance (WD) of the objective optical element when using the third optical disk is preferably 0.20 mm or more and 1.5 mm or less. Preferably, it is 0.3 mm or more and 1.20 mm or less. Next, the WD of the objective optical element when using the second optical disk is preferably 0.4 mm or more and 1.3 mm or less. Further, the WD of the objective optical element when using the first optical disk is preferably 0.4 mm or more and 1.2 mm or less.

  An optical information recording / reproducing apparatus according to the present invention includes an optical disc drive apparatus having the optical pickup device described above.

  Here, the optical disk drive apparatus provided in the optical information recording / reproducing apparatus will be described. The optical disk drive apparatus can hold an optical disk mounted from the optical information recording / reproducing apparatus main body containing the optical pickup apparatus or the like. There are a system in which only the tray is taken out, and a system in which the optical disc drive apparatus main body in which the optical pickup device is stored is taken out to the outside.

  An optical information recording / reproducing apparatus using each of the above-described methods is generally equipped with the following components, but is not limited thereto. An optical pickup device housed in a housing or the like, a drive source of an optical pickup device such as a seek motor that moves the optical pickup device together with the housing toward the inner periphery or outer periphery of the optical disc, and the optical pickup device housing the inner periphery or outer periphery of the optical disc These include a transfer means of an optical pickup device having a guide rail or the like that guides toward the head, a spindle motor that rotates the optical disk, and the like.

  In addition to these components, the former method is provided with a tray that can be held in a state in which an optical disk is mounted and a loading mechanism for sliding the tray, and the latter method has no tray and loading mechanism. It is preferable that each component is provided in a drawer corresponding to a chassis that can be pulled out to the outside.

  According to the present invention, it is possible to provide an objective optical element that can increase the transmittance, can be easily processed, and can appropriately record / reproduce information with respect to different optical disks, and an optical pickup device using the objective optical element. It becomes possible.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 5 is a diagram schematically showing a configuration of the optical pickup device PU1 of the present embodiment that can appropriately record and / or reproduce information on BD, DVD, and CD, which are different optical disks. Such an optical pickup device PU1 can be mounted on an optical information recording / reproducing device. Here, the first optical disc is a BD, the second optical disc is a DVD, and the third optical disc is a CD. The present invention is not limited to the present embodiment.

  The optical pickup device PU1 emits a laser beam (first beam) having a wavelength of λ1 = 405 nm that is emitted when information is recorded / reproduced with respect to the objective optical element OBJ, aperture stop ST, collimator lens CL polarization dichroic prism PPS, BD. The unit includes a unit MD1, a laser module LM, and the like in which a first semiconductor laser LD1 (first light source) to be emitted and a first light receiving element PD1 that receives a reflected light beam from the information recording surface RL1 of the BD are integrated.

  The laser module LM includes a second semiconductor laser EP1 (second light source) that emits a laser beam (second beam) having a wavelength λ2 = 658 nm and is emitted when information is recorded / reproduced with respect to a DVD, and a CD. A third semiconductor laser EP2 (third light source) that emits a laser beam (third beam) having a wavelength λ3 = 785 nm and is reflected from the information recording surface RL2 of the DVD. It has a second light receiving element DS1 that receives the light beam, a third light receiving element DS2 that receives the reflected light beam from the information recording surface RL3 of the CD, and a prism PS.

As shown in FIGS. 1A and 1B, in the objective optical element OBJ of the present embodiment, a central region CN including the optical axis on the aspherical optical surface on the light source side, and a periphery disposed around the central region CN The region MD and the outermost peripheral region OT disposed around the region MD are formed concentrically around the optical axis. Although not shown, a central diffractive structure in which the first diffractive structure and the fourth diffractive structure are superimposed is formed in the central region CN, and a peripheral diffractive structure is formed in the peripheral region MD. The outermost peripheral region OT includes a diffractive structure formed and a diffractive structure formed without a diffractive structure. The first diffractive structure includes a plurality of concentric ring zones centered on the optical axis, and a plurality of rings including the optical axis of the objective optical element OBJ, as shown in any of FIGS. The shape of the cross section of the belt is such that a plurality of step surfaces extending in parallel to the optical axis, a light source side optical functional surface connecting the light source side ends of the adjacent step surfaces, and an optical disc side end of the adjacent step surface The light source side optical functional surface and the optical disc side optical functional surface are alternately arranged along the direction intersecting the optical axis, and the light flux having the wavelength λ1. When the light beam passes through the central region closest to the peripheral region, the light transmittance is 20% or more lower than the light transmittance when the light beam having the wavelength λ1 passes through the peripheral region closest to the central region. When the length of the step surface in the optical axis direction is L, In addition, the following conditional expression is satisfied.
2 μm <L <3 μm (1)

  The divergent light beam of the first light beam (λ1 = 405 nm) emitted from the blue-violet semiconductor laser LD1 is transmitted through the polarization dichroic prism PPS, converted into a parallel light beam by the collimator lens CL, and then straightened by a quarter wavelength plate (not shown). The polarized light is converted into circularly polarized light, the diameter of the light beam is regulated by the stop ST, and is incident on the objective optical element OBJ. Here, the light beam condensed by the central region, the peripheral region, and the most peripheral region of the objective optical element OBJ is a spot formed on the information recording surface RL1 of the BD via the protective substrate PL1 having a thickness of 0.1 mm. It becomes.

  The reflected light beam modulated by the information pits on the information recording surface RL1 is again transmitted through the objective optical element OBJ and the aperture stop ST, and then converted from circularly polarized light to linearly polarized light by a quarter wavelength plate (not shown), and by the collimating lens CL. A converged light beam is transmitted through the polarization dichroic prism PPS, and then converges on the light receiving surface of the first light receiving element PD1. Then, by using the output signal of the first light receiving element PD1 to focus or track the objective optical element OBJ by the biaxial actuator AC, information recorded on the BD can be read.

  The divergent light beam of the second light beam (λ2 = 658 nm) emitted from the red semiconductor laser EP1 is reflected by the prism PS, then reflected by the polarization dichroic prism PPS, converted into a parallel light beam by the collimator lens CL, and is not shown in the figure. The light is converted from linearly polarized light to circularly polarized light by the quarter wavelength plate and enters the objective optical element OBJ. Here, the light beam condensed by the central region and the peripheral region of the objective optical element OBJ (the light beam that has passed through the most peripheral region is flared and forms a spot peripheral part) is a protective substrate PL2 having a thickness of 0.6 mm. And the spot formed on the information recording surface RL2 of the DVD, forming the center of the spot.

  The reflected light beam modulated by the information pits on the information recording surface RL2 is again transmitted through the objective optical element OBJ and the aperture stop ST, and then converted from circularly polarized light to linearly polarized light by a quarter wavelength plate (not shown), and by the collimating lens CL. After being reflected by the polarization dichroic prism PPS, after being reflected by the polarization dichroic prism PPS, it is reflected twice in the prism and then converges on the second light receiving element DS1. The information recorded on the DVD can be read using the output signal of the second light receiving element DS1.

  The divergent light beam of the third light beam (λ3 = 785 nm) emitted from the infrared semiconductor laser EP2 is reflected by the prism PS, then reflected by the polarization dichroic prism PPS, converted into a parallel light beam by the collimator lens CL, and then shown in the figure. It is converted from linearly polarized light to circularly polarized light by the quarter wave plate that is not, and enters the objective optical element OJT. Here, the light beam condensed by the central region of the objective optical element OBJ (the light beam that has passed through the peripheral region and the most peripheral region is flared and forms a spot peripheral part) is a protective substrate PL3 having a thickness of 1.2 mm. And the spot formed on the information recording surface RL3 of the CD.

  The reflected light beam modulated by the information pits on the information recording surface RL3 is transmitted again through the objective optical element OBJ and the aperture stop ST, and then converted from circularly polarized light to linearly polarized light by a quarter wavelength plate (not shown), and by the collimating lens CL. After being reflected by the polarization dichroic prism PPS, after being reflected by the polarization dichroic prism PPS, it is reflected twice in the prism and then converges on the third light receiving element DS2. The information recorded on the CD can be read using the output signal of the third light receiving element DS2.

(Example)
Examples that can be used in the above-described embodiment will be described below. In the following (including lens data in the table), a power of 10 (for example, 2.5 × 10 ⁻³ ) may be expressed using E (for example, 2.5 × E−3). In addition, the optical surface of the objective optical element is formed as an aspherical surface that is symmetric about the optical axis and is defined by mathematical formulas obtained by substituting the coefficients shown in Table 1 into Formula 1.

  Here, X (h) is an axis in the optical axis direction (with the light traveling direction being positive), κ is a conical coefficient, Ai is an aspherical coefficient, h is a height from the optical axis, and r is a paraxial radius of curvature. It is.

  Further, in the case of the embodiment using the diffractive structure, the optical path difference given to the light flux of each wavelength by the diffractive structure is defined by an equation in which the coefficient shown in the table is substituted into the optical path difference function of Formula 2. .

λ is the wavelength of the incident light beam, λB is the manufacturing wavelength (blazed wavelength), dor is the diffraction order, and C _i is the coefficient of the optical path difference function.

  Table 1 shows lens data of Example 1. In the first embodiment, the central diffractive structure in the central region overlaps the two-level staircase-type first diffractive structure and the fourth diffractive structure that is a blaze-type 2/1/1 diffractive structure. Of the diffracted light generated when the first light beam is incident on the first diffractive structure, the 0th-order diffracted light has the maximum amount of diffracted light, and the diffracted light generated when the second light beam is incident on the second diffractive structure Of these, the 0th-order diffracted light has the maximum amount of diffracted light, and the ± 1st-order diffracted light has the maximum amount of diffracted light among the diffracted light generated when the third light beam enters the second diffractive structure. Of the diffracted light generated when the first light beam enters the fourth diffractive structure, the second-order diffracted light has the maximum amount of diffracted light, and the diffracted light generated when the second light beam enters the first diffractive structure. Of these, the first-order diffracted light has the maximum amount of diffracted light, and the first-order diffracted light has the maximum amount of diffracted light among the diffracted light generated when the third light beam enters the first diffractive structure. On the other hand, the peripheral diffraction structure in the peripheral region has a five-step step-type second diffraction structure. Of the diffracted light generated when the first light beam is incident on the second diffractive structure, the 0th-order diffracted light has the largest amount of diffracted light, and the diffraction is generated when the second light beam is incident on the second peripheral region diffractive structure. Of the light, the first-order diffracted light has the maximum amount of diffracted light. In this embodiment, the light transmittance when the light beam having the wavelength λ1 = 405 nm passes through the central region closest to the peripheral region, and the light transmittance when the light beam having the wavelength λ1 passes through the peripheral region closest to the central region. And L = 2.2 μm. In the conventional example in which L = 3.6 μm, the light transmittance when the light beam having the wavelength λ1 = 405 nm passes through the central region closest to the peripheral region is the periphery where the light beam having the wavelength λ1 is closest to the central region. It is 57% lower than the light transmittance when passing through the region, and the gap is very large. In contrast, the present invention can reduce the gap to 28%.

(A) is the figure which looked at an example of the objective optical element OBJ which concerns on this invention from the optical axis direction, (b) is sectional drawing. It is sectional drawing which shows typically some examples (a)-(d) of the diffraction structure provided in the objective optical element OBJ which concerns on this invention. It is a figure which shows the superimposition of a diffraction structure. It is the figure which showed the shape of the spot by the objective optical element which concerns on this invention. 1 is a diagram schematically showing a configuration of an optical pickup device according to the present invention. It is a graph which shows the transmittance | permeability. It is a figure which shows typically the optical axis direction cross section of the 1st diffraction structure concerning an example. It is a figure which shows typically the optical axis direction cross section of the 1st diffraction structure concerning an example. It is a figure which shows typically the optical axis direction cross section of the 1st diffraction structure concerning an example. It is a figure which shows typically the optical axis direction cross section of the 1st diffraction structure concerning an example.

Explanation of symbols

AC biaxial actuator PPS polarization dichroic prism CL collimating lens LD1 blue-violet semiconductor laser LM laser module OBJ objective optical element PL1 protective substrate PL2 protective substrate PL3 protective substrate PU1 optical pickup device RL1 information recording surface RL2 information recording surface RL3 information recording surface CN center Area MD Peripheral area OT Most peripheral area

Claims (16)

A first light source that emits a first light flux having a wavelength λ1, a third light source that emits a third light flux having a wavelength λ3 (1.8λ1 <λ3 <2.0λ1), and an objective optical element, The element condenses the first light beam on an information recording surface of a first optical disk having a protective layer having a thickness of t1, and the third optical disk has a protective layer having a thickness of t3 (t1 <t3). In an objective optical element for an optical pickup device that records and / or reproduces information by focusing on the information recording surface of
The objective optical element has at least a central region including an optical axis and a peripheral region provided around the central region, and a first diffractive structure is formed in the central region,
The first diffractive structure includes a plurality of concentric annular zones centered on an optical axis, and a cross-sectional shape of the plurality of annular zones including the optical axis of the objective optical element extends in parallel with the optical axis. A plurality of step surfaces, a light source side optical functional surface that connects light source side ends of the adjacent step surfaces, and an optical disc side optical functional surface that connects optical disc side ends of the adjacent step surfaces, The light source side optical functional surface and the optical disc side optical functional surface are alternately arranged along the direction intersecting the optical axis,
The light transmittance when the light beam having the wavelength λ1 passes through the central region closest to the peripheral region is the light transmittance when the light beam having the wavelength λ1 passes through the peripheral region closest to the central region. 20% or more lower than
The objective optical element according to claim 1, wherein the first diffractive structure satisfies the following conditional expression where L is a length of the step surface in the optical axis direction.
2 μm <L <3 μm (1)   2. The light source side optical functional surface of the first diffractive structure is inclined with respect to a mother aspherical surface of the central region whose distance from an optical axis is equal to the light source side optical functional surface. The objective optical element described.   Of the two step surfaces sandwiching one optical functional surface, the length in the optical axis direction of one of the step surfaces and the length in the optical axis direction of the other step surface are within a range of 0.44 μm or less. The objective optical element according to claim 1, wherein the objective optical elements are different.   When the transmittance distribution of light in the direction orthogonal to the optical axis in the light flux having the wavelength λ1 is taken, the transmittance gradually decreases in the central region as the distance from the optical axis increases, and from the central region to the peripheral region The objective optical element according to claim 1, wherein the objective optical element increases discontinuously.   When the light beam having the wavelength λ1 passes through the first diffractive structure, the 0th-order diffracted light is generated more than the diffracted light of other orders, and the light beam having the wavelength λ3 passes through the first diffractive structure. 5. The objective optical element according to claim 1, wherein m (m is an integer other than 0) order diffracted light is generated most in comparison with other orders of diffracted light.   The objective optical element according to any one of claims 1 to 5, wherein a blaze-type or three-level or higher step-type second diffraction structure is formed in the peripheral region. A second diffractive structure is formed in the peripheral region, and the second diffractive structure includes a plurality of concentric annular zones around the optical axis and includes the optical axis of the objective optical element. The cross-sectional shape of the annular zone is formed by alternately combining a cylindrical step surface extending in the optical axis direction and an optical functional surface extending in a direction intersecting the step surface, and the second diffractive structure. The length in the optical axis direction of the step surface of the first diffractive structure closest to the first diffraction structure is A, and the length in the optical axis direction of the step surface of the second diffractive structure closest to the first diffractive structure is B. The objective optical element according to claim 1, wherein the following conditional expression is satisfied.
0.65 ≦ B / A ≦ 1 (2) A second diffractive structure is formed in the peripheral region, and the second diffractive structure includes a plurality of concentric annular zones around the optical axis and includes the optical axis of the objective optical element. The cross-sectional shape of the annular zone is formed by alternately combining a cylindrical step surface extending in the optical axis direction and an optical functional surface extending in a direction intersecting the step surface, and the second diffractive structure. When the annular zone pitch of the first diffractive structure closest to is α and the annular pitch of the second diffractive structure closest to the first diffractive structure is β, the following conditional expression is satisfied: The objective optical element in any one of Claims 1-5.
5 ≦ β / α ≦ 10 (3) The annular pitch of the first diffractive structure gradually decreases as the distance from the optical axis increases. When the annular zone pitch closest to the optical axis is X and the annular zone pitch closest to the peripheral region is Y, The objective optical element according to claim 8, wherein the following conditional expression is satisfied.
Y / X ≦ 0.05 (4)   A third light source that emits a second light beam having a wavelength λ2 (1.5λ1 <λ2 <1.7λ1) is provided, and the objective optical element protects the second light beam with a thickness t2 (t1 <t2 <t3). The objective optical element according to claim 1, wherein information is recorded and / or reproduced by focusing on an information recording surface of a second optical disc having a layer.   The light transmittance when the light beam having the wavelength λ2 passes through the central region closest to the peripheral region is the light transmittance when the light beam having the wavelength λ2 passes through the peripheral region closest to the central region. The objective optical element according to claim 10, wherein the objective optical element is lower by 10% or more.   12. The objective optical system according to claim 10, wherein when the light beam having the wavelength λ <b> 2 passes through the first diffractive structure, the 0th-order diffracted light is generated most as compared with diffracted light of other orders. element.   The first diffractive structure near the optical axis does not satisfy the conditional expression (1), and the first diffractive structure close to the peripheral structure satisfies the conditional expression (1). The objective optical element according to any one of 12.   2. The optical disc-side optical functional surface of the first diffractive structure is inclined with respect to a mother aspherical surface of the central region whose distance from an optical axis is equal to the optical disc-side optical functional surface. The objective optical element according to any one of 13.   2. The optical disc side optical functional surface of the first diffractive structure is not inclined with respect to a mother aspherical surface of the central region whose distance from an optical axis is equal to the optical disc side optical functional surface. The objective optical element according to any one of 13.   An optical pickup device comprising the objective optical element according to claim 1.