JPWO2010004858A1 - Objective lens and optical pickup device - Google Patents

Abstract

In order to provide an optical pickup device capable of appropriately recording / reproducing information with respect to an optical disc having a plurality of information recording layers, the maximum value of the sine condition violation amount when the incident light beam is parallel light in the objective lens OL1. Is SCmax and the focal length is f, the sine condition violation amount is set so as to satisfy the conditional expression (1). The following spherical aberration is sufficiently small, and recording / reproducing can be performed with respect to the multilayer optical disk MLD. 0.015 <SCmax / f <0.045 (1)

Description

  The present invention relates to an objective lens for an optical pickup device capable of recording / reproducing information on / from an optical disc having a plurality of information recording layers.

  In a DVD (Digital Versatile Disc), a BD (Blu-ray Disc), or the like, one having two information recording layers is known as means for increasing the recording capacity. In response to the recent demand for larger capacity, optical discs (multilayer optical discs) and optical pickup devices having a plurality of information recording layers of three or more layers have been developed.

  As an example, Patent Document 1 discloses information on an optical disc including a guide track layer having a concavo-convex portion and a plurality of information recording layers made of an optical recording material provided above and below the guide track layer. An optical pickup device capable of recording / reproducing data is disclosed.

JP 2008-21348 A

  In the optical pickup device, spherical aberration occurs when the thickness of the protective layer up to the information recording layer changes. In the optical pickup device of Patent Document 1, when condensing a light beam on any of the plurality of information recording layers, by displacing a coupling lens disposed between the light source and the objective lens in the optical axis direction, By changing the divergence angle of the light beam incident on the objective lens, that is, by changing the magnification of the objective lens, the spherical aberration caused by the change in the thickness of the protective layer is corrected. However, in an optical pickup device for an optical disc having a plurality of information recording layers of 3 layers or more, particularly 8 layers or more, when the numerical aperture NA is as large as about 0.85 as in the BD specification, it is incident on the objective lens. The third-order spherical aberration can be corrected by changing the divergence angle of the luminous flux, that is, by changing the magnification of the objective lens, but there is a problem that a large amount of higher-order spherical aberration remains.

  The present invention has been made in view of the problems of the prior art, and provides an objective lens and an optical pickup device capable of appropriately recording / reproducing information with respect to an optical disc having a plurality of information recording layers. Objective.

The objective lens according to claim 1 is for an optical pickup device for recording / reproducing information on / from an optical disc having a plurality of information recording layers stacked in the thickness direction and a guide layer on which position information is recorded. In the objective lens, the optical pickup device includes a first light source that emits a light beam having a wavelength λ1, a second light source that emits a light beam having a wavelength λ2 (λ1 ≠ λ2), and an objective lens. The lens reads the positional information by condensing the luminous flux having the wavelength λ2 emitted from the second light source onto the guide layer of the optical disc via the objective lens, and at the same time, emits the light from the first light source. Information is recorded / reproduced by condensing the light beam having the wavelength λ1 to any one of the information recording layers via the objective lens, and the objective lens has an NA of 0.7. In the above single lens, the change in spherical aberration due to the change in magnification of the objective lens of the light beam having the wavelength λ1 is almost the same as the change in spherical aberration due to the change in the position of the information recording layer for recording / reproducing information. When the light beam having the wavelength λ1 is incident on the objective lens at a design magnification, the sine condition violation amount has a positive maximum value between 50% and 90% of the effective aperture radius h. The coma aberration correction state is set so that the sine condition violation amount is reduced in the peripheral portion, and the following conditional expression (1), where SC _max is the maximum value of the sine condition violation amount and f is the focal length:
0.015 <SC _max /f<0.045 (1)
It is characterized by satisfying.

According to the present invention, the change in spherical aberration due to the change in magnification of the objective lens of the light beam having the wavelength λ1 is substantially similar to the change in spherical aberration due to the change in position of the information recording layer for recording / reproducing information. It is. More specifically, when the light beam having the wavelength λ1 is incident on the objective lens at a design magnification, the sine condition violation amount has a positive maximum value between 50% to 90% of the effective aperture radius h, and The coma aberration correction state is set so that the sine condition violation amount is further reduced in the peripheral portion, and the conditional expression (1) is satisfied, where SC _max is the maximum value of the sine condition violation amount and f is the focal length. That is. As a result, it is possible to appropriately correct the spherical aberration caused by the change in the position of the information recording layer for recording / reproducing information without changing the higher-order aberration, by changing the divergence convergence degree of the incident light. Become. Here, “substantially similar” means that when spherical aberration caused by a change in the position of the information recording layer for recording / reproducing information is corrected by a change in the magnification of the objective lens of the light beam having the wavelength λ1, It means that the next spherical aberration is below the Marechal limit (0.07λrms). “High-order spherical aberration” refers to fifth-order or higher spherical aberration. When the light beam having the wavelength λ1 is incident on the objective lens at the design magnification, it is preferable that the sine condition violation amount monotonously decreases in the peripheral portion from the position where the sine condition violation amount has a positive maximum value. “Monotonically decreasing” means that it continues to decrease, at least means that it decreases once and never increases, and preferably, the reduction rate does not change greatly at a certain position. This also means. Preferably, when the light beam having the wavelength λ1 is incident on the objective lens at the design magnification, the sine condition violation amount has a positive maximum value between 60% and 90% of the effective aperture radius h.

  The objective lens described in claim 2 is characterized in that, in the invention described in claim 1, the design magnification of the objective lens with respect to the light beam having the wavelength λ1 is substantially zero. Note that “substantially zero” means −0.02 or more and +0.02 or less, preferably −0.01 or more and +0.01 or less.

  The objective lens according to claim 3 is the objective lens according to claim 1 or 2, wherein the objective lens is a single lens having an NA of 0.70 or more, and the magnification of the objective lens with respect to the light beam having the wavelength λ1. The correction state of the spherical aberration of the objective lens is set so that the change of the spherical aberration due to the change of the position of the information recording layer for recording / reproducing information can be corrected by the change of the spherical aberration due to the change of Features.

  According to a fourth aspect of the present invention, there is provided the objective lens according to the first or second aspect, wherein the optical disc has eight or more layers of the information recording layer, and the information recording layer excluding a protective layer on the surface thereof. The total thickness is 0.1 mm or more. Preferably, it is 10 layers or more, and the total thickness is 0.2 mm or more.

  The optical pickup device according to claim 5 is an optical pickup device for recording / reproducing information on / from an optical disc having a plurality of information recording layers stacked in the thickness direction and a guide layer on which position information is recorded. The optical pickup device includes a first light source that emits a light beam having a wavelength λ1, a second light source that emits a light beam having a wavelength λ2 (λ1 ≠ λ2), and the objective according to any one of claims 1 to 4. The optical pickup device reads the position information by condensing the light beam having the wavelength λ2 emitted from the second light source onto the guide layer of the optical disc via the objective lens. At the same time, information is recorded / reproduced by condensing the light beam having the wavelength λ1 emitted from the first light source onto one of the information recording layers via the objective lens. , And having a first magnification converting means for changing the magnification of the objective lens of the light flux of the wavelength λ1 incident on the objective lens.

  According to a sixth aspect of the present invention, in the optical pickup device according to the fifth aspect, the optical pickup device has first focusing means for focusing the light flux having the wavelength λ1 on one of the recording layers. It is characterized by.

  According to a seventh aspect of the present invention, there is provided the optical pickup device according to the sixth aspect of the invention, wherein the optical pickup device has a second focusing means for focusing the light flux having the wavelength λ2 on the guide layer. To do.

  According to an eighth aspect of the present invention, in the invention according to the seventh aspect, the second focusing means includes an actuator that moves and adjusts the position of the objective lens in the optical axis direction.

  An optical pickup device according to a ninth aspect of the present invention is the optical pickup device according to any one of the fifth to eighth aspects, wherein the objective lens has a negative magnification of the light beam having the wavelength λ2. As described above, the light beam having the wavelength λ2 is incident.

  An optical pickup device according to a tenth aspect of the present invention is the optical pickup device according to any one of the fifth to ninth aspects, wherein the optical pickup device changes a magnification of the objective lens of the light flux having the wavelength λ2. It has a magnification conversion means.

  With this configuration, it is possible to cope with an optical disc having a plurality of guide layers.

  The optical pickup device according to the present invention includes at least two light sources, ie, a first light source and a second light source, but may include a third light source or a fourth light source. Furthermore, the optical pickup device of the present invention condenses the light beam having the wavelength λ2 emitted from the second light source on the guide layer of the optical disk, and simultaneously converts the light beam having the wavelength λ1 emitted from the first light source to the optical disk. A condensing optical system for condensing light on any one of the information recording layers is provided. The optical pickup device of the present invention includes a light receiving element that receives a reflected light beam from the information recording surface and the guide layer of the optical disc.

  The wavelength λ1 of the first light source is preferably 0.39 μm or more and 0.42 μm or less. The wavelength λ2 of the second light source is preferably 0.48 μm or more and 0.85 μm or less.

  An optical disk used in the optical pickup device of the present invention has a guide layer on which position information is recorded and a plurality of information recording layers stacked in the thickness direction. There may be only one guide layer or a plurality of guide layers, but it is preferably less than the number of information recording layers, and it is less than half the number of information recording layers. This facilitates the design and is preferable. The optical disk preferably has a protective layer on the surface, and the thickness of the protective layer is not particularly limited, but is 0.6 mm or less when the numerical aperture of the objective lens is 0.7 or more. It is preferable that it is 0.2 mm or less. Further, when the information recording layer is 8 layers or more, or when the thickness of the plurality of information recording layers excluding the protective layer is 0.1 mm or more, the effect of the present invention becomes more remarkable, which is preferable. .

  The numerical aperture of the objective lens in recording / reproducing of an optical disc having a guide layer in which position information is recorded and a plurality of information recording layers is 0.7 or more, and preferably 0.8 or more and 0.9 or less.

  However, the optical pickup device of the present invention may be an optical pickup device that is compatible with at least one or all of BD, DVD, and CD in addition to the optical disc.

  In the BD, information is recorded / reproduced by an objective lens having an NA of 0.85, and the thickness of the protective substrate is about 0.1 mm. DVD is a general term for DVD-series optical discs in which information is recorded / reproduced by an objective lens having an NA of about 0.60 to 0.67, and the thickness of the protective substrate is about 0.6 mm. DVD-ROM, DVD -Video, DVD-Audio, DVD-RAM, DVD-R, DVD-RW, DVD + R, DVD + RW, etc. CD is a general term for CD optical discs in which information is recorded / reproduced by an objective lens having an NA of about 0.45 to 0.53, and the thickness of the protective substrate is about 1.2 mm. 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 the present specification, the light sources such as the first light source, the second light source, and the third 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 wavelength λ2 of the light beam emitted from the second light source is preferably longer than the wavelength λ1 with respect to the wavelength λ1 of the light beam emitted from the first light source, and the wavelength λ3 of the light beam emitted from the third light source is the wavelength λ2. Longer is preferred.

  Further, in the case where the optical pickup apparatus is also compatible with BD, DVD, or CD, the wavelength λB of the BD light source is preferably 0.35 μm or more and 0.44 μm or less, more preferably 0.39 μm or more, 0 The wavelength λD of the DVD 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, and the wavelength λC of the CD light source is preferably Is not less than 0.75 μm and not more than 0.88 μm, more preferably not less than 0.76 μm and not more than 0.82 μm.

  The first light source may also serve as a BD light source, and the second light source may serve as a DVD light source or a CD light source. As the second light source, a green laser may be used separately from the DVD light source and the CD light source.

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

  The optical pickup device may include a light receiving element that receives a light beam reflected from the optical disk. 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 light amount due to the spot shape change and position change on the light receiving element, performs focus detection and track detection, and based on this detection, the objective lens can be moved for focusing and tracking 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 used for the optical pickup device has an objective lens. The condensing optical system may include only the objective lens, but may include a coupling lens such as a collimator in addition to the objective lens. The coupling lens is a single lens or a lens group that is disposed between the objective lens and the light source and changes the divergence angle of the light beam. The collimator is a type of coupling lens, and is a lens that emits light incident on the collimator 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 lens 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 the light beam emitted from the light source onto the information recording surface of the optical disk. Preferably, the objective lens is an optical system which is disposed at a position facing the optical disk in the optical pickup device and has a function of condensing the light beam emitted from the light source on the information recording surface of the optical disk, and further includes an actuator An optical system that can be integrally displaced at least in the optical axis direction.

  The objective lens is preferably a single objective lens, but may be formed of a plurality of optical elements. The objective lens preferably has a refractive surface that is aspheric.

  The objective lens may be a glass lens, a plastic lens, or a hybrid lens in which an optical path difference providing structure or the like is provided on a glass lens with a photocurable resin or the like.

  When the objective lens is a glass lens, the influence of temperature change in the optical pickup device can be reduced.

When the objective lens 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 ) is −20 × 10 ^{−5 to} −5 × 10 ^{− with} respect to the wavelength of 405 nm in accordance with the temperature change within the temperature range of −5 ° C. to 70 ° C. ⁵ (more preferably, −10 × 10 ^{−5 to} −8 × 10 ⁻⁵ ) is used. When the objective lens 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 lens is 50 or more.

  The objective lens may have an optical path difference providing structure for correcting a spherical aberration caused by a change in refractive index with respect to a temperature change or a spherical aberration caused by a change in wavelength of the light source. The optical path difference providing structure referred to in this specification is a general term for structures that add an optical path difference to an incident light beam. The optical path difference providing structure also includes a phase difference providing structure for providing a phase difference. The phase difference providing structure includes a diffractive structure. The optical path difference providing structure has a step, preferably a plurality of steps. This step adds an optical path difference / phase difference to the incident light flux. The optical path difference added by the optical path difference providing structure may be an integer multiple of the wavelength of the incident light beam or a non-integer multiple of the wavelength of the incident light beam. 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. Preferably, the optical path difference providing structure is a diffractive structure.

  It should be noted that information recording / reproduction of an information recording layer (referred to as layer A) of an optical disc having a plurality of information recording layers is performed, and then information recording / reproduction of another information recording layer (referred to as layer B) is performed. When performing the above, spherical aberration due to the difference between the thickness from the optical disk surface to the layer A and the thickness from the optical disk surface to the surface layer B occurs. Accordingly, there is a need for spherical aberration correction means for correcting the spherical aberration that occurs due to the difference in thickness between the optical disk surface and the information recording layer in different information recording layers.

  Examples of such spherical aberration correction means include a liquid crystal device that corrects spherical aberration of a light beam having a wavelength λ1, and at least one lens that is movable in the optical axis direction. Examples include first magnification conversion means for changing. Examples of the first magnification conversion unit include the above-described single ball coupling lens movable in the optical axis direction, a single ball collimator lens, at least one positive lens and at least one negative lens. Examples thereof include a two-group coupling lens that can move these in the optical axis direction, a beam expander that has at least one positive lens and at least one negative lens, and one of them can move in the optical axis direction.

  Further, the optical pickup device may include a second magnification conversion unit that changes the magnification of the objective lens of the light beam having the wavelength λ2. Examples of the second magnification conversion means include the above-described single coupling lens movable in the optical axis direction, a single collimator lens, at least one positive lens and at least one negative lens, Examples thereof include a two-group coupling lens that can move these in the optical axis direction, a beam expander that has at least one positive lens and at least one negative lens, and one of them can move in the optical axis direction. By including the second magnification conversion means, it is possible to easily cope with an optical disc having a plurality of guide layers.

  Furthermore, it is preferable that the optical pickup device has first focusing means for focusing a light beam having a wavelength λ1 on any one of the information recording layers.

  The optical pickup device preferably has second focusing means for focusing the light beam having the wavelength λ2 on the guide layer. For example, it is preferable to have an actuator for moving and adjusting the position of the objective lens in the optical axis direction for this purpose.

  The optical information recording / reproducing apparatus preferably has 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.

  ADVANTAGE OF THE INVENTION According to this invention, the objective lens and optical pick-up apparatus which can record / reproduce information appropriately with respect to the optical disk which has several information recording layers can be provided.

It is a figure which shows schematically the structure of optical pick-up apparatus PU1. 6 is a graph showing spherical aberration (SA) and a sine condition amount (SC) in a state where light is condensed at a recording position of a multilayer optical disc using the objective lens in Example 1. 3 is a cross-sectional view of an objective lens in Example 1. FIG. It is a graph which shows the spherical aberration (SA) in the state condensed to the recording position of the multilayer optical disk using the objective lens in Example 2, and a sine condition amount (SC). 5 is a cross-sectional view of an objective lens in Example 2. FIG. It is a graph which shows the spherical aberration (SA) in the state condensed to the recording position of the multilayer optical disk using the objective lens in Example 3, and a sine condition amount (SC). 10 is a cross-sectional view of an objective lens in Example 3. FIG. It is the figure which plotted the value of Examples 1-3 by taking the residual wavefront aberration after correction | amendment on a vertical axis | shaft, and taking the protective layer thickness change from a design reference | standard state on a horizontal axis | shaft. It is the figure which plotted the value of Examples 1-3 by taking the residual wavefront aberration after correction | amendment on the vertical axis | shaft, and taking an angle of view on the horizontal axis | shaft. It is a graph which shows the spherical aberration (SA) in the state condensed to the recording position of the multilayer optical disk using the objective lens in Example 4, and a sine condition amount (SC). 10 is a cross-sectional view of an objective lens in Example 4. FIG. It is a graph which shows the spherical aberration (SA) in the state condensed to the recording position of the multilayer optical disk using the objective lens in Example 5, and a sine condition amount (SC). FIG. 6 is a cross-sectional view of an objective lens in Example 5. It is a figure which shows schematically the structure of optical pick-up apparatus PU2.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to FIG. Note that the optical pickup device according to the first embodiment can be incorporated into an optical disk drive device. FIG. 1 is a diagram illustrating a schematic configuration of the optical pickup device according to the first embodiment.

  The optical pickup apparatus shown in FIG. 1 is capable of recording / reproducing information in a manner compatible with three types of optical disks, namely, a multilayer optical disk (MLD), a Blu-ray disk (BD), and a digital versatile disk (DVD). Here, the multilayer optical disk MLD is a plurality of (eight or more layers) information recording layers RL capable of recording information stacked in the thickness direction, and a guide track layer in which pits are formed as position information for tracking servo control. GL.

  1 includes a blue-violet semiconductor laser LD1, a red semiconductor laser LD2, a first polarizing beam splitter PBS1, a second polarizing beam splitter PBS2, a first lens BE1 and a second lens of a beam expander. BE2, dichroic prism DBS1, λ / 4 wavelength plate QWP, rising mirror ML, objective lens OL1, first sensor lens SL1, second sensor lens SL2, pinhole PH, and first light detection A detector PD1, a second photodetector PD2, a biaxial actuator AC1, a first actuator AC2, and a second actuator AC3.

  The red semiconductor laser LD2 emits red laser light having a wavelength of λ2 = 650 nm to 680 nm. The red laser beam is used as servo light that is applied to the guide track layer GL to read tracking information when information is recorded / reproduced with respect to the multilayer optical disk MLD, and information is recorded on the information recording layer of the DVD. It is also used as recording / reproducing light for recording / reproducing, and its numerical aperture is NA 0.65. On the other hand, the blue-violet semiconductor laser LD1 emits blue-violet laser light having a wavelength of λ1 = 390 nm to 420 nm. The blue-violet laser light is focused on each information recording layer of the multilayer optical disk MLD, used as recording / reproducing light for recording / reproducing information, and recording / reproducing information on the BD information recording layer. Is used as recording / reproducing light for recording, and its numerical aperture is NA 0.85.

  As a laser light source having the above-mentioned wavelength, a semiconductor laser used in a general optical pickup device that records / reproduces information in a manner compatible with BD and DVD is suitable. Small and inexpensive. The relationship between the wavelength of the servo light and the recording / reproducing light and the numerical aperture is not limited to the above, and can be appropriately changed according to the material characteristics of the information recording layer.

  The multilayer optical disk MLD may have a structure in which the guide track layer GL is disposed only on one side of the information recording layer as disclosed in Japanese Patent Laid-Open No. 2002-31958, or Japanese Patent Laid-Open No. 2008-21348. A structure in which the guide track layer GL is disposed in the middle of the information recording layer as disclosed in Japanese Patent Laid-Open No. 2005-18852, or a guide track layer as disclosed in Japanese Patent Application Laid-Open No. 2005-18852. A structure in which the GL is disposed on both sides of the information recording layer may be employed. Further, when the same light source is used for the servo light that is applied to the guide track layer GL to read tracking information and the light flux used for DVD recording / reproduction as in the present embodiment, the guide track layer GL is used as the DVD information. A structure in which the recording layer is disposed at a position equal to the protective layer thickness may be employed.

  In the present embodiment, a beam expander including a negative first lens BE1 and a positive second lens BE2 is provided, and the negative first lens BE1 is displaced in the optical axis direction by the second actuator AC3. The information recording layer RL of the multilayer optical disk MLD can be selected, and the spherical aberration of the BD recording / reproducing light can be corrected by displacing the positive second lens BE2 in the optical axis direction by the first actuator AC2. ing. That is, in the present embodiment, the beam expander is the first magnification conversion unit. However, a liquid crystal element may be used instead of the beam expander. As a liquid crystal element capable of correcting spherical aberration, an element disclosed in Japanese Patent No. 3795998 can be used.

The objective lens OL1 is an objective lens that can be applied to three types of optical disks, that is, a multilayer optical disk MLD, BD, and DVD. In the objective lens OL1, when the light beam of λ1 is incident on the objective lens as a parallel light beam, the sine condition violation amount has a positive maximum value between 50% and 90% of the effective aperture radius h. The coma aberration correction state is set so that the sine condition violation amount monotonously decreases. When the maximum value of the sine condition violation amount is SC _max and the focal length is f, conditional expression (1),
0.015 <SC _max /f<0.045 (1)
Since the sine condition violation amount is set so as to satisfy the above, even when the information recording layer RL of the multilayer optical disc MLD is changed to any one, the high-order spherical aberration is sufficiently small, and the multilayer optical disc MLD is recorded well. / Playback can be performed.

  In addition, since the specifications for recording / reproduction of the multilayer optical disk MLD and the specifications for recording / reproduction of the BD are the same, the objective lens OL1 can perform good recording / reproduction with respect to the BD. Further, the red laser beam emitted from the red semiconductor laser LD2 is incident on the objective lens in a divergent light beam state (the objective lens has a negative value), thereby performing good recording / reproduction on the DVD. be able to. This will be specifically described below.

  First, information recording / reproduction in the multilayer optical disk MLD will be described. In the optical pickup device shown in FIG. 1, the divergent light beam emitted from the red semiconductor laser LD2 passes through the second polarization beam splitter PBS2, is reflected by the dichroic prism DBS1, passes through the λ / 4 wavelength plate QWP, and rises. After being reflected by the mirror ML and incident on the objective lens OL1, it becomes a spot formed on the guide layer GL via the protective substrate of the multilayer optical disk MLD.

  The reflected light beam modulated by the pits in the guide layer GL again passes through the objective lens OL1, is reflected by the rising mirror ML, passes through the λ / 4 wavelength plate QWP, is reflected by the dichroic prism DBS1, and is further reflected by the second polarization. The light is reflected by the beam splitter PBS2 and enters the second photodetector PD2 via the second sensor lens SL2. The tracking position of the multilayer optical disk MLD can be read from the output signal of the second photodetector PD2. Based on such tracking information, the biaxial actuator AC1 can be driven to perform tracking servo control on the track of the guide layer GL, whereby the tracking information is recorded on the information recording layer RL of the multilayer optical disk MLD. Even without this, information can be recorded / reproduced appropriately. Therefore, the irradiation with the red semiconductor laser LD2 is continuously performed while information is recorded / reproduced with respect to the information recording layer RL.

  Here, when recording / reproducing information with respect to any of the information recording layers RL, the second actuator AC3 is driven to displace the first lens BE1 of the beam expander in the direction of the optical axis, and the objective lens The divergence angle of the light beam incident on OL1 is changed, and the magnification of the objective lens is changed. Thereby, any one of the information recording layers RL desired to be recorded / reproduced can be arbitrarily selected.

  Furthermore, the blue-violet semiconductor laser LD1 is caused to emit light when information is recorded on or reproduced from the multilayer optical disk MLD. The divergent light beam emitted from the blue-violet semiconductor laser LD1 passes through the first polarizing beam splitter PBS1, the first and second lenses BE1 and BE2 of the beam expander, the dichroic prism DBS1, and the λ / 4 wavelength plate QWP, and rises. After being reflected by the mirror ML and incident on the objective lens OL1, it becomes a spot formed on the selected information recording layer RL via the protective substrate of the multilayer optical disk MLD. The objective lens OL1 is driven by a biaxial actuator AC1, and focusing and tracking are performed.

  The reflected light flux modulated by the information pits in the selected information recording layer RL again passes through the objective lens OL1, is reflected by the rising mirror ML, and is reflected by the λ / 4 wavelength plate QWP, the dichroic prism DBS1, and the beam expander. Another information recording layer that is not a desired layer by passing through the second lens BE2 and the first lens BE1, reflected by the first polarizing beam splitter PBS1, passing through the first sensor lens SL1, and passing through the pinhole PH The stray light reflected by is removed and enters the first photodetector PD1. Information recorded on the selected information recording layer RL can be read by the output signal of the first photodetector PD1.

  Next, recording or reproduction of information on the BD will be described. When the blue-violet semiconductor laser LD1 emits light, the divergent light beam emitted from the blue-violet semiconductor laser LD1 is a first polarizing beam splitter PBS1, a first lens BE1 and a second lens BE2 of a beam expander, a dichroic prism DBS1, a λ / 4 wavelength plate QWP. , Reflected by the rising mirror ML, and incident on the objective lens OL1, then becomes a spot formed on the information recording layer RL via the protective substrate of the BD. The objective lens OL1 is driven by a biaxial actuator AC1, and focusing and tracking are performed.

  The reflected light beam modulated by the information pits in the information recording layer RL again passes through the objective lens OL1, is reflected by the rising mirror ML, and is a λ / 4 wavelength plate QWP, a dichroic prism DBS1, and a second lens BE2 of the beam expander. And the first lens BE1, and further reflected by the first polarization beam splitter PBS1, passes through the first sensor lens SL1, passes through the pinhole PH, and the stray light is removed, and enters the first photodetector PD1. To do. Information recorded on the selected information recording layer RL can be read by the output signal of the first photodetector PD1.

  Although not shown, the detection of the focusing state when the light beam of the blue-violet semiconductor laser LD1 forms a spot on the information recording layer RL of the multilayer optical disk MLD or BD, for example, the first polarization beam splitter PBS1 and It is possible to detect by arranging a beam splitter between the first sensor lenses SL1 and arranging a focus detection sensor lens and a photodetector for the light beams separated by the beam splitter.

  Next, recording or reproduction of information on the DVD will be described. When the red semiconductor laser LD2 emits light, the divergent light beam emitted from the red semiconductor laser LD2 passes through the second polarization beam splitter PBS2, is reflected by the dichroic prism DBS1, passes through the λ / 4 wavelength plate QWP, and is raised by the rising mirror ML. After being reflected and incident on the objective lens OL1, it becomes a spot formed on the information recording layer RL via the DVD protective substrate.

  The reflected light beam modulated by the pits in the information recording layer RL again passes through the objective lens OL1, is reflected by the rising mirror ML, passes through the λ / 4 wavelength plate QWP, is reflected by the dichroic prism DBS1, and is further reflected by the second. The light is reflected by the polarization beam splitter PBS and enters the second photodetector PD2 via the second sensor lens SL2. The DVD tracking position can be read from the output signal of the second photodetector PD2.

  The objective lens OL1 of the present embodiment is designed so that the spherical aberration is optimized for the light flux of the blue-violet semiconductor laser LD1 that records / reproduces information on / from the information recording layer RL of the multilayer optical disk MLD. Therefore, for the luminous flux of the red semiconductor laser LD2 that performs servo at the position of the guide layer GL, the magnification of the objective lens OL1 is arbitrarily selected depending on the arrangement of the guide layer GL, and the information recording layer RL and the guide layer GL are protected. The spherical aberration due to the difference in layer thickness and the difference in wavelength is corrected. However, an optical path difference providing structure (preferably a diffractive structure) for correcting the spherical aberration is provided on the optical surface of the objective lens OL1. It is good also as a type formed in. When recording / reproducing information on / from a DVD, the red laser light emitted from the red semiconductor laser LD2 is incident on the objective lens OL1 in the form of a divergent light beam, so that the multilayer optical disk MLD and the DVD are recorded. Is a type that corrects the spherical aberration due to the difference in the thickness of the protective layer and the wavelength difference between the multilayer optical disc MLD and the DVD, but the phase structure for correcting the spherical aberration is on the optical surface of the objective lens OL1. It is good also as a type formed in. By doing so, the spherical aberration can be corrected in the state where the red laser light is incident as parallel light on the objective lens OL1 by the action of the phase structure. Therefore, during tracking of the objective lens OL1. The generated coma aberration can be suppressed. Further, the phase structure is not necessarily formed on the optical surface of the objective lens OL1, and an optical element on a flat plate on which the phase structure is formed is prepared separately, and the optical element on the flat plate and the objective lens are provided in a lens frame. An integrated objective unit may be used. An example of such a phase structure is a diffractive structure as disclosed in JP-A-2005-38575. Of course, the phase structure is not limited to the diffractive structure. In addition, at least one surface of the objective lens is divided into two regions, which are used only for the light flux of the blue-violet semiconductor laser LD1 and the light flux of the red semiconductor laser LD2, and only for the light flux of the blue-violet semiconductor laser LD1. The surface shape of each region to be optimized may be optimized.

  In the first embodiment, for the BD and DVD, the biaxial actuator AC1 and the objective lens OL1 correspond to the first focusing means.

  On the other hand, for the multilayer optical disk MLD, at least one of the first actuator AC2 and the second lens BE2 of the beam expander, the second actuator AC3 and the first lens BE of the beam expander 1, the biaxial actuator AC1 and the objective lens OL1. One set corresponds to the first focusing means.

  The second focusing means corresponds to the biaxial actuator AC1 and the objective lens OL1.

  The first magnification conversion means corresponds to the first actuator AC2 and the second lens BE2 of the beam expander, and the second actuator AC3 and the first lens BE1 of the beam expander.

(Second Embodiment)
Hereinafter, a second embodiment of the present invention will be described with reference to FIG. Note that the optical pickup device PU2 according to the second embodiment can be incorporated into an optical disk drive device. FIG. 14 is a diagram showing a schematic configuration of the optical pickup device PU2. The optical pickup device PU2 is capable of recording / reproducing information so as to be compatible with four types of optical discs such as a multilayer optical disc (MLD), a Blu-ray disc (BD), a digital versatile disc (DVD), and a compact disc (CD). .

  The biggest difference from the first embodiment is that the present embodiment has two objective lenses, a first objective lens OL1 and a second objective lens OL2. The first objective lens OL1 is used for recording / reproduction of a multilayer optical disc, a BD, and a DVD, and the second objective lens OL2 is used for recording / reproduction of a CD. In the following description, description of points that are common to the first embodiment will be omitted.

  The optical pickup device PU2 includes a blue-violet semiconductor laser LD1, an LD2 in which a red semiconductor laser and an infrared semiconductor laser are integrated in one package, a first polarization beam splitter PBS1, a first lens BE1 of a beam expander, and a first lens BE1. A two-lens BE2, a first dichroic prism DBS1, a second dichroic prism DBS2, a λ / 4 wavelength plate QWP, a raising mirror ML, a first objective lens OL1, a second objective lens OL2, and a first sensor. A lens SL1, a second sensor lens SL2, a pinhole PH, a first photodetector PD1, a second photodetector PD2, a coupling lens CP1, a coupling lens CP2, a biaxial actuator AC1, 1st actuator AC2, 2nd actuator AC3, 3rd actuator A Having four.

  The red semiconductor laser in the LD 2 emits red laser light having a wavelength of λ2 = 650 nm to 680 nm. The red laser beam is used as servo light that is applied to the guide track layer GL to read tracking information when information is recorded / reproduced with respect to the multilayer optical disk MLD, and information is recorded on the information recording layer of the DVD. It is also used as recording / reproducing light for recording / reproducing, and its numerical aperture is NA 0.65. On the other hand, the blue-violet semiconductor laser LD1 emits blue-violet laser light having a wavelength of λ1 = 390 nm to 420 nm. The blue-violet laser light is focused on each information recording layer of the multilayer optical disk MLD, used as recording / reproducing light for recording / reproducing information, and recording / reproducing information on the BD information recording layer. Is used as recording / reproducing light for recording, and its numerical aperture is NA 0.85. The infrared semiconductor laser in the LD 2 emits infrared laser light having a wavelength of λ3 = 760 nm to 820 nm. Infrared laser light is used as recording / reproducing light for recording / reproducing information on / from a CD.

  As in the first embodiment, the first objective lens OL1 is an objective lens that can be used for three types of optical disks, namely, multilayer optical disks MLD, BD, and DVD. On the other hand, the second objective lens OL2 is an objective lens that is compatible with CD.

  Information recording / reproduction in the multilayer optical disk MLD will be described. In the optical pickup device PU2 shown in FIG. 14, the divergent light beam emitted from the red semiconductor laser LD2 passes through the second polarization beam splitter PBS2 and the coupling lens CP2, is reflected by the first dichroic prism DBS1, and has a λ / 4 wavelength. After passing through the plate QWP, reflected by the second dichroic prism DBS2, and incident on the first objective lens OL1, it becomes a spot formed on the guide layer GL via the protective substrate of the multilayer optical disk MLD.

  In the present embodiment, the coupling lens CP2 can be moved in the optical axis direction by the third actuator AC4, and the magnification of the first objective lens OL1 at the wavelength λ2 can be converted. Even if it has a guide layer, it is possible to cope with the change in temperature when the first objective lens is a plastic lens, and even a minute change in the protective layer thickness up to the guide layer It becomes.

  The reflected light beam modulated by the pits in the guide layer GL again passes through the first objective lens OL1, is reflected by the second dichroic prism DBS2, passes through the λ / 4 wavelength plate QWP, and is reflected by the first dichroic prism DBS1. Then, the light passes through the coupling lens CP2, is further reflected by the second polarization beam splitter PBS2, and enters the second photodetector PD2 via the second sensor lens SL2. The tracking position of the multilayer optical disk MLD can be read from the output signal of the second photodetector PD2. Based on such tracking information, the biaxial actuator AC1 and / or the third actuator AC4 can be driven to perform tracking servo control on the track of the guide layer GL, and thereby the information recording layer RL of the multilayer optical disk MLD. Even if tracking information is not recorded on the disc, information can be recorded / reproduced appropriately. Therefore, the irradiation with the red semiconductor laser LD2 is continuously performed while information is recorded / reproduced with respect to the information recording layer RL.

  Here, when information is recorded / reproduced with respect to any one of the information recording layers RL, the second actuator AC3 is driven to displace the first lens BE1 of the beam expander in the optical axis direction, so that the first The divergence angle of the light beam incident on the objective lens OL1 is changed, and the magnification of the first objective lens is changed. Thereby, any of the information recording layers desired to be recorded / reproduced can be arbitrarily selected.

  Furthermore, the blue-violet semiconductor laser LD1 is caused to emit light when information is recorded on or reproduced from the multilayer optical disk MLD. The divergent light beam emitted from the blue-violet semiconductor laser LD1 is a first polarizing beam splitter PBS1, a coupling lens CP1, a first lens BE1 and a second lens BE2 of a beam expander, a first dichroic prism DBS1, and a λ / 4 wavelength plate. After passing through the QWP, reflected by the second dichroic prism DBS2, and incident on the first objective lens OL1, it becomes a spot formed on the selected information recording layer RL via the protective substrate of the multilayer optical disk MLD. The first objective lens OL1 is driven by a biaxial actuator AC1, and focusing and tracking are performed.

  The reflected light beam modulated by the information pits in the selected information recording layer RL again passes through the first objective lens OL1, is reflected by the second dichroic prism DBS2, and is λ / 4 wavelength plate QWP, first dichroic prism DBS1, By passing through the second lens BE2, the first lens BE1, and the coupling lens CP1 of the beam expander, reflected by the first polarization beam splitter PBS1, passing through the first sensor lens SL1, and passing through the pinhole PH. The stray light reflected by another information recording layer that is not a desired layer is removed, and is incident on the first photodetector PD1. Information recorded on the selected information recording layer RL can be read by the output signal of the first photodetector PD1.

  Next, recording or reproduction of information on the BD will be described. When the blue-violet semiconductor laser LD1 emits light, the divergent light beam emitted from it emits the first polarizing beam splitter PBS1, the coupling lens CP1, the first and second lenses BE1 and BE2 of the beam expander, and the first dichroic prism DBS1. After passing through the λ / 4 wavelength plate QWP, reflected by the second dichroic prism DBS2, and incident on the first objective lens OL1, it becomes a spot formed on the information recording layer RL via the protective substrate of the BD. The first objective lens OL1 is driven by a biaxial actuator AC1, and focusing and tracking are performed.

  The reflected light beam modulated by the information pits in the information recording layer RL again passes through the first objective lens OL1, is reflected by the second dichroic prism DBS2, and is a λ / 4 wavelength plate QWP, the first dichroic prism DBS1, and a beam expander. The stray light is removed by passing through the second lens BE2, the first lens BE1, and the coupling lens CP1, and reflected by the first polarizing beam splitter PBS1, passing through the first sensor lens SL1, and passing through the pinhole PH. And enters the first photodetector PD1. Information recorded on the selected information recording layer RL can be read by the output signal of the first photodetector PD1.

  Although not shown, the detection of the focusing state when the light beam of the blue-violet semiconductor laser LD1 forms a spot on the information recording layer RL of the multilayer optical disk MLD or BD, for example, the first polarization beam splitter PBS1 and It is possible to detect by arranging a beam splitter between the first sensor lenses SL1 and arranging a focus detection sensor lens and a photodetector for the light beams separated by the beam splitter.

  Next, recording or reproduction of information on the DVD will be described. When the red semiconductor laser LD2 emits light, the divergent light beam emitted therefrom passes through the second polarization beam splitter PBS2 and passes through the coupling lens CP2. In addition, it is also possible to correct the spherical aberration generated in the DVD by moving the coupling lens CP2 to a desired position in the optical axis direction by the third actuator AC4. The light beam that has passed through the coupling lens CP2 is reflected by the dichroic prism DBS1, passes through the λ / 4 wavelength plate QWP, is reflected by the second dichroic prism DBS2, enters the first objective lens OL1, and then is a protective substrate for the DVD. It becomes a spot formed on the information recording layer RL via the.

  The reflected light beam modulated by the pits in the information recording layer RL again passes through the first objective lens OL1, is reflected by the second dichroic prism DBS2, passes through the λ / 4 wavelength plate QWP, and is reflected by the first dichroic prism DBS1. Then, the light passes through the coupling lens CP2, is further reflected by the second polarization beam splitter PBS2, and enters the second photodetector PD2 through the second sensor lens SL2. DVD information can be read by the output signal of the second photodetector PD2.

  Next, recording or reproduction of information with respect to a CD will be described. When the infrared semiconductor laser LD2 emits light, the divergent light beam emitted therefrom passes through the second polarization beam splitter PBS2 and passes through the coupling lens CP2. Note that the spherical aberration occurring in the CD can be corrected by moving the coupling lens CP2 to a desired position in the optical axis direction by the third actuator AC4. The light beam that has passed through the coupling lens CP2 is reflected by the first dichroic prism DBS1, passes through the λ / 4 wavelength plate QWP, passes through the second dichroic prism DBS2, is reflected by the rising mirror ML, and is reflected by the second objective lens. After entering the OL2, the spot is formed on the information recording layer of the CD through the CD protective substrate.

  The reflected light beam modulated by the pits on the information recording layer of the CD again passes through the second objective lens OL2, is reflected by the rising mirror ML, passes through the second dichroic prism DBS2, and passes through the λ / 4 wavelength plate QWP. Then, it is reflected by the first dichroic prism DBS1, passes through the coupling lens CP2, is further reflected by the second polarization beam splitter PBS2, and enters the second photodetector PD2 via the second sensor lens SL2. CD information can be read by the output signal of the second photodetector PD2.

  In the second embodiment, for the BD, DVD and CD, the biaxial actuator AC1 and the objective lens OL1 correspond to the first focusing means.

  On the other hand, for the multilayer optical disk MLD, at least one set of the first actuator AC2 and the second lens BE2 of the beam expander, the second actuator AC3 and the first lens BE of the beam expander 1, the 2-axis actuator AC1 and the objective lens OL1. Corresponds to the first focusing means.

  The second focusing means corresponds to the biaxial actuator AC1 and the objective lens OL1.

  The first magnification conversion means corresponds to the first actuator AC2 and the second lens BE2 of the beam expander, and the second actuator AC3 and the first lens BE1 of the beam expander.

  The second magnification converting means corresponds to the third actuator AC4 and the coupling lens CP2.

Next, examples that can be used in the above-described embodiment will be described. The objective lens of this example can be applied to either the objective lens OL1 of the first embodiment described above or the first objective lens OL1 of the second embodiment. In the following table, ri represents the radius of curvature, di represents the position in the optical axis direction from the i-th surface to the (i + 1) -th surface, and ni represents the refractive index of each surface. In the following (including the lens data in the table), a power of 10 (for example, 2.5 × 10 ⁻³ ) is expressed using E (for example, 2.5 × E−3). The optical surface of the objective optical element is formed as an aspherical surface that is axisymmetric about the optical axis, each of which is defined by a mathematical formula obtained by substituting the coefficient shown in Table 1 into Formula 1. The design magnification is represented by m in the table.

Here, X (h) is an axis in the optical axis direction (the light traveling direction is positive), κ is a conical coefficient, A _i is an aspherical coefficient, h is a height from the optical axis, and r is a paraxial curvature. Radius.

Example 1
Table 1 shows lens data of Example 1. FIG. 2 is a graph showing the spherical aberration (SA) and the sine condition amount (SC) in a state where light is condensed at the recording position of the multilayer optical disk using the objective lens in Example 1, and the vertical axis is NA. The horizontal axis represents the defocus amount, (a) shows a state where light is condensed at a recording position (reference position) with a protective layer thickness of 0.20 mm, and (b) shows a recording with a protective layer thickness of 0.35 mm. FIG. 4C shows a state where light is condensed at a position, and FIG. 5C shows a state where light is condensed at a recording position having a protective layer thickness of 0.05 mm. FIG. 3 is a cross-sectional view of the objective lens in the first embodiment. As shown in FIG. 2A, at the reference position which is the central information recording layer, the sine condition violation amount has a positive maximum value at 80% of the effective opening radius h, and the sine condition violation amount at the peripheral portion. The spherical aberration correction state is set so as to decrease monotonically. In this embodiment, SC _max /f=0.022.

(Example 2)
Table 2 shows lens data of Example 2. FIG. 4 is a graph showing the spherical aberration (SA) and the sine condition amount (SC) in a state where light is condensed at the recording position of the multilayer optical disk using the objective lens in Example 2, and the vertical axis is NA. The horizontal axis represents the defocus amount, (a) shows a state where light is condensed at a recording position (reference position) with a protective layer thickness of 0.30 mm, and (b) shows a recording with a protective layer thickness of 0.55 mm. FIG. 4C shows a state where light is condensed at a position, and FIG. 5C shows a state where light is condensed at a recording position having a protective layer thickness of 0.05 mm. FIG. 5 is a cross-sectional view of the objective lens according to the second embodiment. As shown in FIG. 4A, at the reference position, which is the central information recording layer, the sine condition violation amount has a positive maximum value at 63% of the effective opening radius h, and the sine condition violation amount at the peripheral portion. The spherical aberration correction state is set so as to decrease monotonically. In this embodiment, SC _max /f=0.043.

(Example 3)
Table 3 shows lens data of Example 3. FIG. 6 is a graph showing the spherical aberration (SA) and the sine condition amount (SC) in a state where light is condensed at the recording position of the multilayer optical disk using the objective lens in Example 3, and the vertical axis is NA. The horizontal axis represents the defocus amount, (a) shows a state where light is condensed at the recording position (reference position) with a protective layer thickness of 0.15 mm, and (b) shows the recording with a protective layer thickness of 0.25 mm. FIG. 4C shows a state where light is condensed at a position, and FIG. 5C shows a state where light is condensed at a recording position having a protective layer thickness of 0.05 mm. FIG. 7 is a cross-sectional view of the objective lens in the third embodiment. As shown in FIG. 6A, at the reference position which is the central information recording layer, the sine condition violation amount has a positive maximum value at 80% of the effective opening radius h, and the sine condition violation amount at the peripheral portion. The spherical aberration correction state is set so as to decrease monotonically. In this embodiment, SC _max /f=0.016.

  FIG. 8 shows the corrected residual wavefront aberration on the vertical axis and the protective layer thickness change from the design reference state on the horizontal axis, for Examples 1 to 3. FIG. 9 is a graph showing the corrected residual wavefront aberration on the vertical axis and the angle of view on the horizontal axis for Examples 1 to 3.

As shown in FIG. 8, in Example 2 in which SC _max / f is set to SC _max /f=0.043, which is a value larger than 0.03, the value of the residual wavefront aberration after correction is less than the Marechal limit. The protective layer thickness range is larger than that in Example 1 in which SC _max / f is set to 0.022. In other words, even when the protective layer thickness changes by ± 0.2 mm or more from the reference state, it is possible to cope with it, and even when the information recording layer is formed in a wide range in the thickness direction of the optical disc, a good spot is formed. it can. Further, as shown in FIG. 9, in Example 2 in which SC _max /f=0.043 is set, off-axis aberrations at an angle of view of 0.25 ° can be suppressed below the Marechal limit, but SC _max / When f exceeds 0.045, the off-axis aberration becomes excessively greater than the Marechal limit, and the tolerance for decentration becomes small, which makes it difficult to align the optical axis when assembling the optical pickup device. . Therefore, the objective lens of the present invention, like the conditional expression _(1), it is desirable _{SC max} / f is smaller than 0.045.

In order to increase the storage capacity, in a multilayer optical disc, it is preferable that the residual aberration after correction is less than the Marechal limit for a protective layer thickness change of 0.20 mm or more from the design reference state. As shown in FIG. 8, in Example 3 where SC _max /f=0.016 was set, the corrected residual aberration was below the Marshallal limit for a substrate thickness change of 0.20 mm or more from the total reference state. However, when SC _max / f is 0.015 or less, there is a problem that the residual aberration after correction exceeds the marshal limit for a substrate thickness change of 0.20 mm or more from the design reference state. Therefore, the objective lens of the present invention, like the conditional expression _{(1), SC max} / f is preferably larger than 0.015.

Example 4
Table 4 shows lens data of Example 4. In the present embodiment, the material of the protective layer of the optical disk is changed from that of the first embodiment. FIG. 10 is a graph showing the spherical aberration (SA) and the sine condition amount (SC) in a state where the light is condensed at the recording position of the multilayer optical disk using the objective lens in Example 4, and the vertical axis is NA. The horizontal axis represents the defocus amount, (a) shows a state where light is condensed at a recording position (reference position) with a protective layer thickness of 0.20 mm, and (b) shows a recording with a protective layer thickness of 0.35 mm. FIG. 4C shows a state where light is condensed at a position, and FIG. 5C shows a state where light is condensed at a recording position having a protective layer thickness of 0.05 mm. FIG. 11 is a cross-sectional view of the objective lens according to the fourth embodiment. As shown in FIG. 10A, at the reference position which is the central information recording layer, the sine condition violation amount has a positive maximum value at 81% of the effective opening radius h, and the sine condition violation amount at the peripheral portion. The spherical aberration correction state is set so as to decrease monotonically. In this embodiment, SC _max /f=0.024.

(Example 5)
Table 5 shows lens data of Example 5. In this embodiment, the wavelength of the light beam for recording / reproducing on the multilayer optical disk is changed from 405 nm to 530 nm as compared with the first embodiment. FIG. 12 is a graph showing the spherical aberration (SA) and the sine condition amount (SC) in a state where light is condensed at the recording position of the multilayer optical disk using the objective lens in Example 5, and the vertical axis is NA. The horizontal axis represents the defocus amount, (a) shows a state where light is condensed at a recording position (reference position) with a protective layer thickness of 0.2 mm, and (b) shows a recording with a protective layer thickness of 0.35 mm. FIG. 4C shows a state where light is condensed at a position, and FIG. 5C shows a state where light is condensed at a recording position having a protective layer thickness of 0.05 mm. FIG. 13 is a cross-sectional view of the objective lens in the fifth embodiment. As shown in FIG. 12A, at the reference position, which is the central information recording layer, the sine condition violation amount has a positive maximum value at 80% of the effective aperture radius h, and the sine condition violation amount at the peripheral portion. The spherical aberration correction state is set so as to decrease monotonically. In this embodiment, SC _max /f=0.024.

LD1 Blue-violet semiconductor laser (Multi-layer optical disc and BD recording / reproducing light)
LD2 Red semiconductor laser (servo light for multilayer optical disk, DVD recording / reproducing light)
PBS1 first polarizing beam splitter PBS2 second polarizing beam splitter BE1 first lens of beam expander (focusing lens for recording / reproducing light of multilayer optical disk)
BE2 Beam expander second lens (multilayer optical disc and spherical aberration correction lens for BD recording / reproducing light)
DBS1 dichroic prism (transmits blue-violet laser light, reflects red laser light)
QWP λ / 4 wave plate ML Standing mirror OL1 Objective lens SL1 First sensor lens (for multilayer optical disc and BD recording / reproducing light)
SL2 Second sensor lens (for servo light of multi-layer optical disk, DVD recording / reproducing light)
PH pinhole (for removing stray light from other layers of multilayer optical disks)
PD1 first photodetector (for recording / reproducing light reading of multilayer optical disk and BD)
PD2 2nd photo detector (for servo light of multilayer optical disk, DVD recording / reproducing light reading)
AC1 biaxial actuator (for focusing / tracking of servo light of multilayer optical disk and recording / reproducing light of BD / DVD)
AC2 first actuator (for correcting spherical aberration of multilayer optical disc and BD)
AC3 2nd actuator (for multi-layer optical disc focusing)

Claims (10)

In an objective lens for an optical pickup device for recording / reproducing information with respect to an optical disc having a plurality of information recording layers stacked in the thickness direction and a guide layer on which position information is recorded,
The optical pickup device includes a first light source that emits a light beam having a wavelength λ1, a second light source that emits a light beam having a wavelength λ2 (λ1 ≠ λ2), and an objective lens.
The objective lens reads the positional information by condensing the light beam having the wavelength λ2 emitted from the second light source onto the guide layer of the optical disc via the objective lens, and at the same time, the first light source. Information is recorded / reproduced by condensing the light beam having the wavelength λ1 emitted from the light beam onto one of the information recording layers via the objective lens,
The objective lens is a single lens having an NA of 0.70 or more, and a change in spherical aberration due to a change in magnification of the objective lens of the light beam having the wavelength λ1 is a position of the information recording layer where information is recorded / reproduced. It is almost similar to the change of spherical aberration due to change,
When the light beam having the wavelength λ1 is incident on the objective lens at the design magnification, the sine condition violation amount has a positive maximum value between 50% and 90% of the effective aperture radius h, and the sine condition is more in the periphery. The coma correction state is set so that the amount of violation is reduced,
An objective lens satisfying the following conditional expression (1), where SC _max is the maximum value of the sine condition violation amount, and f is the focal length.
0.015 <SC _max /f<0.045 (1)   The objective lens according to claim 1, wherein a design magnification of the objective lens with respect to the light beam having the wavelength λ <b> 1 is substantially zero.   The objective lens is a single lens having an NA of 0.70 or more, and the position of the information recording layer where information is recorded / reproduced by a change in spherical aberration due to a change in magnification of the objective lens of a light beam having the wavelength λ1. The objective lens according to claim 1, wherein a correction state of the spherical aberration of the objective lens is set so that a change in the spherical aberration due to the change can be corrected.   3. The optical disc has eight or more information recording layers, and a total thickness of the information recording layers excluding a protective layer on the surface is 0.1 mm or more. Objective lens described in 1. In an optical pickup device for recording / reproducing information with respect to an optical disc having a plurality of information recording layers stacked in the thickness direction and a guide layer in which position information is recorded,
The optical pickup device includes: a first light source that emits a light beam having a wavelength λ1, a second light source that emits a light beam having a wavelength λ2 (λ1 ≠ λ2), and any one of claims 1 to 4. And an objective lens described in
The optical pickup device reads the position information by condensing the light beam having the wavelength λ2 emitted from the second light source onto the guide layer of the optical disc via the objective lens, and simultaneously reading the first information. Information is recorded / reproduced by condensing the light beam having the wavelength λ1 emitted from the light source onto one of the information recording layers via the objective lens,
An optical pickup device comprising first magnification conversion means for changing the magnification of the objective lens of the light beam having the wavelength λ1 incident on the objective lens.   6. The optical pickup device according to claim 5, further comprising first focusing means for focusing the light beam having the wavelength [lambda] 1 on any one of the information recording layers.   The optical pickup device according to claim 6, further comprising second focusing means for focusing the light beam having the wavelength λ2 on the guide layer.   8. The optical pickup device according to claim 7, wherein the second focusing means includes an actuator that moves and adjusts the position of the objective lens in the optical axis direction.   9. The optical pickup device causes the light beam having the wavelength λ <b> 2 to be incident so that the magnification of the objective lens of the light beam having the wavelength λ <b> 2 becomes a negative value. The optical pickup device according to one item.   10. The optical pickup according to claim 5, wherein the optical pickup device includes a second magnification conversion unit that changes a magnification of the objective lens of the light flux having the wavelength λ <b> 2. apparatus.