JP5152524B2 - Optical pickup device and objective lens unit - Google Patents

Description

  The present invention relates to an optical pickup device and an objective lens unit capable of recording / reproducing information with respect to 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 discs) and optical pickup devices having a plurality of information recording layers of three or more layers have been developed, particularly having eight or more information recording layers. The optical disk is called a super multi-layer disk.

  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.

  By the way, the information recording layer for recording / reproducing information is selected by moving the objective lens in the optical axis direction and correcting the spherical aberration caused by the difference in the thickness of the information recording layer. This can be done by moving the coupling lens or the like arranged between and in the optical axis direction and changing the magnification of the objective lens. However, when the magnification of the optical system is changed, the divergence angle or convergence angle of the light beam incident on the objective lens changes, and the numerical aperture (NA) of the objective lens, which should be essentially constant, may change. The change in NA causes a change in spot diameter on the surface of the information recording layer, which may cause a reading error. In the conventional optical pickup device, the number of information recording layers is as small as about 1 to 2 layers, and the change in the magnification for accommodating a plurality of layers is 0 or small, so the problem of change in NA does not become obvious. However, when the number of information recording layers is increased, the thickness change is as large as 0.2 mm to 0.6 mm, and the NA is high NA such as 0.7 or more and 0.9 or less, the information A change in magnification when selecting a recording layer is increased. In such an optical pickup device, the present inventor has found that the problem of NA change accompanying change in magnification becomes particularly significant.

JP 2008-21348 A JP 2002-170274 A

  On the other hand, according to Patent Document 2, in the next-generation optical disc system, when an objective lens having a high numerical aperture of NA = 0.85 is used, the spherical aberration is corrected according to the variation in the thickness of the cover layer of the optical disc. It is described that when the magnification is changed by the beam expander, a change in NA occurs. This corrects the spherical aberration caused by the thickness variation of one cover layer, not a plurality of information recording layers, but is the same phenomenon as the magnification change at the time of selecting the information recording layer in the super multi-layer disc. It can be said. However, as a countermeasure against the change in NA, in the prior art of Patent Document 2, the aperture stop is separated from the objective lens by a substantial focal length to the light source side. In this case, the change in NA increases due to the change in magnification. Regardless of the description in Patent Document 2, it can be said that the change in NA cannot be suppressed.

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

The optical pickup device according to claim 1 includes a first light source, a spherical aberration correction element, an aperture limiting element, and an objective lens, and a light beam having a wavelength λ1 emitted from the first light source. It has a plurality of information recording layers that are incident on the objective lens through the spherical aberration correction element and the aperture limiting element and are stacked in the optical axis direction, and the thickness from the surface to the deepest information recording layer is 0. Information is recorded / reproduced by focusing on one of the information recording layers of the first optical disc having a diameter of 2 mm or more and 0.6 mm or less, and the spherical aberration correction element attempts to record / reproduce information. An optical pickup device adapted to correct spherical aberration caused by a difference in information recording layer by changing the magnification of the objective lens,
The objective lens is a single lens, and the aperture limiting element is disposed closer to the optical disc side than the light source side surface apex of the objective lens, and further satisfies the following expression.
0.7 ≦ NA ≦ 0.9 (1)
0.19 ≦ D / f ≦ 0.44 (2)
However, NA is the numerical aperture D [mm] of the objective lens, and the distance f [mm] in the optical axis direction from the vertex of the light source side surface of the objective lens to the aperture limiting position of the aperture limiting element is the objective at the wavelength λ1. Lens focal length

  The principle of the present invention will be described with reference to FIG. FIG. 1A is an optical path diagram of the objective lens OL and the aperture limiting element AP shown as a comparative example, and FIG. 1B is an optical path diagram of the objective lens OL and the aperture limiting element AP shown as an example of the present invention. Yes, the left side is the light source side, and the right side is the optical disc side. In the comparative example of FIG. 1A, the aperture limiting element AP is disposed closer to the light source than the surface vertex P of the objective lens OL. Therefore, when the numerical aperture when the parallel light beam indicated by the solid line is incident on the objective lens OL is NA0, the magnification of the objective lens is changed, and the numerical aperture when the divergent light indicated by the dotted line is incident is NA1 or It can be seen that the numerical aperture NA2 is NA1> NA0> NA2 when the magnification of the objective lens is changed and the convergent light indicated by the alternate long and short dash line is incident, and the numerical aperture is changed.

  On the other hand, according to the present invention, as shown in FIG. 1B, the aperture limiting element AP is arranged on the optical disc side with respect to the surface vertex P of the objective lens OL. When the numerical aperture when the parallel light beam indicated by the solid line is incident on the objective lens OL is NA0, the magnification of the objective lens is changed, and the numerical aperture when the divergent light indicated by the dotted line is incident is NA1 or the objective lens. The numerical aperture NA2 when the convergent light indicated by the alternate long and short dash line is incident is NA1≈NA0≈NA2, and the change in the numerical aperture can be suppressed.

  From the above, it can be seen that a favorable result can be obtained by the technical idea of disposing the stop closer to the optical disc side than the surface vertex of the objective lens in order to suppress the change of the numerical aperture (NA). At this time, the present inventor has examined how much the aperture should be arranged on the optical disc side with respect to the surface vertex of the objective lens.

  As a result, when the NA is small, such as 0.6, it has been found that the problem of the present invention that the NA changes with a change in magnification does not become large. The above is the scope of the present invention. When NA is 0.7 or more, in an optical disc having a plurality of information recording layers, the magnification fluctuation necessary for correcting spherical aberration that occurs when focusing on an information recording layer different from an information recording layer is large. Therefore, the subject of the present invention that the NA change generated in association therewith increases. On the other hand, when NA is larger than 0.9, the spherical aberration generated when the light is condensed on an information recording layer different from a certain information recording layer becomes excessive, and it is not at a level that can be corrected by a variation in magnification. Therefore, NA of 0.9 or less was set as the scope of the present invention.

  Therefore, in the range of NA 0.7 or more and NA 0.9 or less, when the spherical aberration is corrected by changing the magnification of the objective lens, the range of D / f that can suppress the change of NA most was examined. (D [mm] is the distance in the optical axis direction from the vertex of the light source side surface of the objective lens to the aperture limiting position of the aperture limiting element, and f [mm] is the focal length of the objective lens at the wavelength λ1.) Specifically, when the magnification is changed in the information recording layer shallow from the surface, the value of D / f that can most suppress the change in NA, and the change in NA when the magnification is changed in the information recording layer deep from the surface. The value of D / f that can minimize the above was examined. From the result, it was found that the D / f range is preferably 0.24 or more and 0.41 or less.

  Further, when the study was continued, it was found that if the change in NA is about 2%, there is a possibility that it can be accepted as an error. As a result of the examination, the formula (2) was obtained.

  Therefore, in order to correct the spherical aberration so as to focus on the information recording layer of the multiple-layer optical disk that satisfies the NA of the formula (1) and the change in the substrate thickness is in the range of 0.2 mm to 0.6 mm, Even if a large magnification change is made, the change in NA can be suppressed by satisfying the equation (2), and stable recording / reproducing of the optical disc can be performed.

Further, as can be seen from the above, it is more preferable to satisfy the following conditional expression (2 ′).
0.24 ≦ D / f ≦ 0.41 (2 ′)

Further, when the following conditional expression (1 ′) is satisfied, the effects obtained by the conditional expressions (2) and (2 ′) are particularly prominent.
0.8 ≦ NA ≦ 0.9 (1 ′)

According to a second aspect of the present invention, there is provided the optical pickup device according to the first aspect, wherein the following expression is satisfied.
0.18 ≦ | H1-D | /f≦0.42 (3)
However, H1 is the distance from the light source side surface vertex of the objective lens to the image side principal point position.

  The distance D from the light source side surface vertex of the objective lens to the aperture limiting element and the distance H1 from the light source side surface vertex of the objective lens to the image side principal point position are actually used when they are in a range satisfying the expression (3). It can be said that it is preferable.

  Conditional expression (3) is an expression showing a preferable range when conditional expression (2) is expressed by using H, which is the distance from the light source side surface vertex to the image side principal point position. The effect obtained by satisfying is the same as the conditional expression (2).

More preferably, the following conditional expression (3 ′) is satisfied.
0.23 ≦ | H1-D | /f≦0.37 (3 ′)

According to a third aspect of the present invention, there is provided an optical pickup device according to the first or second aspect of the present invention, wherein when the light flux having the wavelength λ1 is incident at a design magnification, the sine is between 50% and 90% of the effective aperture radius h. The coma aberration correction state is set so that the condition violating amount has a positive maximum value, and the sine condition violating amount is reduced in the peripheral portion, and the following equation is satisfied.
0.015 <SC _max /f<0.045 (4)
However, SC _max is the maximum value of the sine condition violation amount.

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 of 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 amount of violation of the sine condition is reduced at the periphery, and the conditional expression (4) is satisfied, where SC _max is the maximum value of the sine condition violation amount and f is the focal length. It 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, the “substantially similar shape” means a higher order 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 of λ1. It means that the 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.

  According to a fourth aspect of the present invention, in the invention according to any one of the first to third aspects, the wavelength λ1 is 390 nm ≦ λ1 ≦ 420 nm.

  According to a fifth aspect of the present invention, in the invention according to any one of the first to third aspects, the wavelength λ1 is 520 nm ≦ λ1 ≦ 540 nm.

  According to a sixth aspect of the present invention, there is provided the optical pickup device according to any one of the first to fifth aspects, wherein a light beam having a wavelength λ1 emitted from the first light source is recorded on a Blu-ray Disc by the objective lens. Since the information is recorded / reproduced by focusing on the surface, the information can be recorded / reproduced to be compatible with the super multi-layer disc and the Blu-ray Disc.

  According to a seventh aspect of the present invention, in the optical pickup device according to any of the first to sixth aspects, in addition to the first light source, a second light source that emits a light beam having a wavelength λ2 (λ1 <λ2). And recording / reproducing information by making a light beam of wavelength λ2 emitted from the second light source incident on the objective lens and condensing it on an information recording layer of a second optical disc. To do.

  An optical pickup device according to an eighth aspect of the present invention is the invention according to any one of the first to sixth aspects, further comprising a second objective lens in addition to the objective lens, and a wavelength in addition to the first light source. an information recording layer of a second optical disc having a second light source that emits a light beam having a wavelength of λ2 (λ1 <λ2), the light beam having a wavelength of λ2 emitted from the second light source being incident on the second objective lens; It is characterized in that information is recorded / reproduced by condensing light on the light.

The invention according to claim 9 is characterized in that, in the invention according to any one of claims 1 to 8, the following conditional expression (2 ′) is satisfied.
0.24 ≦ D / f ≦ 0.41 (2 ′)

The objective lens unit according to claim 10 includes an objective lens unit having an aperture limiting element and an objective lens, a first light source, and a spherical aberration correction element, and a wavelength emitted from the first light source. A light beam of λ1 is made incident on the objective lens through the spherical aberration correction element and the aperture limiting element, and has a plurality of information recording layers stacked in the optical axis direction, from the surface to the deepest information recording layer. Information is recorded / reproduced by focusing on any information recording layer of the first optical disc having a thickness of 0.2 mm or more and 0.6 mm or less, and the spherical aberration correction element is configured to record / reproduce information. This objective lens unit is used in an optical pickup device adapted to correct spherical aberration caused by a difference in the information recording layer to be reproduced by changing the magnification of the objective lens. , The objective lens is single lens, the aperture limiting element, said being arranged on the optical disk side than the light source side apex of the objective lens, characterized by further satisfying the following expression.
0.7 ≦ NA ≦ 0.9 (1)
0.19 ≦ D / f ≦ 0.44 (2)
However, NA is the numerical aperture D [mm] of the objective lens, and the distance f [mm] in the optical axis direction from the vertex of the light source side surface of the objective lens to the aperture limiting position of the aperture limiting element is the objective at the wavelength λ1. Lens focal length

The invention described in claim 11 is characterized in that, in the invention described in claim 10, the following conditional expression (2 ′) is satisfied.
0.24 ≦ D / f ≦ 0.41 (2 ′)

  The optical pickup device according to the present invention includes a first light source that emits a light beam having a wavelength λ1, but a second light source that emits a light beam having a wavelength λ2 (λ1 <λ2) and / or a wavelength λ3 (λ2 <λ3). You may have the 3rd light source which radiate | emits this light beam. 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. You may have the condensing optical system for condensing in any layer of an information recording layer. Moreover, the optical pickup device of the present invention may have a light receiving element that receives a reflected light beam from the information recording surface and the guide layer of the optical disc.

  The first optical disk used in the optical pickup device of the present invention has a plurality of information recording layers stacked in the thickness direction (optical axis direction). The optical pickup device of the present invention can be more suitably used in an optical disc having eight or more information recording layers. More preferably, it has 8 or more and 20 or less information recording layers. The thickness from the optical disk surface to the information recording surface of the deepest information recording layer is 0.2 mm or more and 0.6 mm or less. Further, the optical disc may have a guide layer in which position information is recorded. 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. In addition, when the numerical aperture NA necessary for recording / reproducing information on the optical disc is 0.7 or more and 0.9 or less, the effect of the present invention becomes more remarkable, and the NA is 0.8 or more. When the ratio is 0.9 or less, it becomes more prominent. Further, when NA is 0.8 or more and 0.9 or less, compatibility with BD is facilitated, which is also preferable in this respect. 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. .

  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 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 expression.

1.5 × λ1 <λ2 <1.7 × λ1
1.9 × λ1 <λ3 <2.1 × λ1

  The first wavelength λ1 of the first light source is preferably 350 nm or more and 440 nm or less, more preferably 390 nm or more and 420 nm or less, and the second wavelength λ2 of the second light source is preferably 570 nm or more and 680 nm or less, More preferably, the third wavelength λ3 of the third light source is preferably 750 nm or more and 880 nm or less, and more preferably 760 nm or more and 820 nm or less when the third light source is provided. As the first light source, one having a first wavelength λ1 of 520 nm or more and 540 nm or less may be used.

  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.

  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 lens 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 collimating lens is a kind of coupling lens, and is a lens that emits light incident on the collimating 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 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 a single objective lens. 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 accompanying the temperature change within the range of −5 ° C. to 70 ° C. ^It is more preferable to use a resin material within a range of ⁵ (more preferably, −10 × 10 ^{−5 to} −8 × 10 ⁻⁵ ). 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.

  In addition, information recording / reproduction is performed on an information recording layer (referred to as layer A) of an optical disc having a plurality of information recording layers, and then information recording / reproduction on other information recording layers (referred to as layer B) 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. Therefore, a spherical aberration correction element that corrects the spherical aberration caused by the difference in thickness from the optical disk surface to the information recording layer in different information recording layers is required.

  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, and the magnification of the objective lens of the light beam having a wavelength λ1 is increased. 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 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.

  Moreover, 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 when the light beam having the wavelength λ2 can be used. 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 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 the light beam having the wavelength λ1 on any one of the information recording layers.

  In addition, the optical pickup device preferably includes 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 pickup device of the present invention has an aperture limiting element. Examples of the aperture limiting element include a generally used donut-shaped aperture, and the case where the aperture of the pickup bobbin also serves as the aperture.

  As described in FIGS. 1 and 2, the objective lens and the aperture limiting element may be separated. At that time, the relative distance between the objective lens and the aperture limiting element is preferably fixed. Further, as shown in FIGS. 9 and 10, an objective lens unit in which the objective lens OL and the aperture limiting element AP are integrated may be used. FIG. 9 shows an example in which the objective lens OL and the aperture AP which is an aperture limiting element are integrated by a lens barrel LB. FIG. 10 shows an example in which the flange portion FL of the objective lens OL is extended to the light source side in the optical axis direction, and the aperture AP, which is an aperture limiting element, is fixed to the light source side tip of the flange portion and integrated.

As shown in FIG. 1B, the aperture limiting element is disposed closer to the optical disc than the vertex of the light source side surface of the objective lens. Moreover, the following formula | equation (2) is satisfy | filled.
0.19 ≦ D / f ≦ 0.44 (2)
Here, D [mm] represents the distance in the optical axis direction from the vertex of the light source side surface of the objective lens to the aperture limiting position of the aperture limiting element, and f [mm] represents the focal length of the objective lens at the wavelength λ1.

More preferably, the following expression (2 ′) is satisfied.
0.24 ≦ D / f ≦ 0.41 (2 ′)

  The distance in the optical axis direction to the aperture limiting position of the aperture limiting element will be described below. When a very thin diaphragm as shown in FIG. 11 is used as the aperture limiting element, the aperture limiting position is AL, and D is the length shown in FIG. 11 (from the light source side lens surface vertex to the surface of the aperture limiting element). Distance). On the other hand, when an edge-shaped stop as shown in FIG. 12 is used as the aperture limiting element, the aperture limiting position is AL at the edge tip, and D is the length shown in FIG. Distance to the edge tip of the limiting element). Further, when a thick flat diaphragm as shown in FIG. 13 is used as the aperture limiting element, the aperture limiting position is set as an intermediate point AL of the thickness in the optical axis direction of the aperture, and D is a length ( Distance from the vertex of the light source side lens surface to the intermediate point of the aperture limiting element).

  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.

  According to the present invention, when information is recorded / reproduced on an optical disc having a plurality of information recording layers and a large required numerical aperture, the numerical aperture is changed even if the spherical aberration is corrected by changing the magnification. It is possible to provide an optical pickup device and an objective lens unit that can suppress and appropriately record / reproduce information.

  Hereinafter, an embodiment of the present invention will be described with reference to FIG. The optical pickup device PU1 according to the present embodiment can be incorporated in an optical disk drive device. FIG. 1 is a diagram showing a schematic configuration of the optical pickup device PU1. The optical pickup device PU1 is capable of recording / reproducing information so as to be compatible with four types of optical discs, that is, a multilayer optical disc (MLD), a Blu-ray disc (BD), a digital versatile disc (DVD), and a compact disc (CD). is there. Here, the multilayer optical disk MLD is a guide track in which a plurality of (eight or more layers) information recording layers RL capable of recording information stacked in the thickness direction and pits as position information for tracking servo control are formed. Layer GL.

The optical pickup device PU1 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. Lens SL1, second sensor lens SL2, pinhole PH, first photodetector PD1, second photodetector PD2, first coupling lens CP1, second coupling lens CP2, biaxial Actuator AC1, first actuator AC2, second actuator AC3, third actuator Having over data AC4. The single objective lens OL1 is used for recording / reproduction of a multilayer optical disc, BD, and DVD, and the second objective lens OL2 is used for recording / reproduction of a CD. As shown in FIG. 1B, the first aperture limiting element AP1 arranged on the light source side of the first objective lens OL1 is provided on the optical disc side from the surface vertex of the first objective lens OL1, and the following equation is used. Look.
0.7 ≦ NA (1)
0.24 ≦ D / f ≦ 0.41 (2)
0.24 ≦ | H1-D | /f≦0.37 (3)
0.015 <SC _max /f<0.045 (4)
However, NA is the numerical aperture D [mm] of the objective lens OL1, the distance f [mm] from the vertex of the light source side surface of the objective lens OL1 to the aperture limiting element AP1 is the focal length of the objective lens OL1, and H1 is the light source of the objective lens OL1. Distance from side vertex to image side principal point position SC _max is the maximum value of sine condition violation amount

  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 light 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. Is used as recording / reproducing light for performing 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 beam is focused on each information recording layer of the multilayer optical disc MLD, used as recording / reproducing light for recording / reproducing information, and also for recording / reproducing information on the BD information recording layer. It is also used as recording / reproducing light for reproducing, 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.

  The first objective lens OL1 is an objective lens that can be applied to three types of 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 PU1 shown in FIG. 2, the divergent light beam emitted from the red semiconductor laser of LD2 passes through the second polarization beam splitter PBS2 and the second coupling lens CP2, is reflected by the first dichroic prism DBS1, and λ / 4 wave plate QWP, reflected by the second dichroic prism DBS2, passes through the first aperture limiting element AP1, enters the first objective lens OL1, and then passes through the protective substrate of the multilayer optical disk MLD. It becomes a spot formed on GL.

  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 plurality of guide layers, it is possible to cope with the change in temperature when the first objective lens is a plastic lens, and also for the minute change in the protective layer thickness up to the guide layer. It becomes possible.

  The reflected light beam modulated by the pits in the guide layer GL again passes through the first objective lens OL1 and the first aperture limiting element AP1, is reflected by the second dichroic prism DBS2, passes through the λ / 4 wavelength plate QWP, The light is reflected by the 1 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. 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 of the multilayer optical disc MLD. Even if tracking information is not recorded in the RL, information can be recorded / reproduced appropriately. Accordingly, the irradiation of the red semiconductor laser of the LD 2 is continuously performed while information recording / reproduction is being performed on the information recording layer RL.

  Here, when recording / reproducing information with respect to any information recording layer RL of the multilayer optical disc MLD, the second actuator AC3 is driven to move the first lens BE1 of the beam expander in the optical axis direction. Displacement is performed to change the divergence angle of the incident light beam to the first objective lens OL1, and to change the magnification of the first objective lens. Thereby, any of the information recording layers desired to be recorded / reproduced can be arbitrarily selected.

  Further, 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 lens BE1 and second lens BE2 of the beam expander, the first dichroic prism DBS1, and the λ / 4 wavelength plate QWP. After being reflected by the second dichroic prism DBS2 and entering the first objective lens OL1 via the first aperture limiting element AP1, it is formed on the selected information recording layer RL via the protective substrate of the multilayer optical disk MLD. It becomes a spot. The first 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 first objective lens OL1 and the first aperture limiting element AP1, is reflected by the second dichroic prism DBS2, and is reflected by the λ / 4 wavelength plate QWP. , Passes through the second dichroic prism DBS1, the second lens BE2 and the first lens BE1 of the beam expander, is further reflected by the first polarizing beam splitter PBS1, passes through the first sensor lens SL1, and passes through the pinhole PH. Thus, 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 is caused to emit light, the divergent light beam emitted from the laser beam is a first polarizing beam splitter PBS1, a first lens BE1 and a second lens BE2 of a beam expander, a first dichroic prism DBS1, and a λ / 4 wavelength. The light passes through the plate QWP, is reflected by the second dichroic prism DBS2, passes through the first aperture limiting element AP1, enters the first objective lens OL1, and then is formed on the information recording layer RL via the BD protective substrate. It becomes a spot. 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 and the first aperture limiting element AP1, is reflected by the second dichroic prism DBS2, and is reflected by the λ / 4 wavelength plate QWP and the dichroic prism. DBS1 passes through the second lens BE2 and the first lens BE1 of the beam expander, is further reflected by the first polarizing beam splitter PBS1, passes through the first sensor lens SL1, passes through the pinhole PH, and detects the first light. Is incident on the device PD1. Information recorded on the information recording layer RL of the BD can be read by the output signal of the first photodetector PD1.

  Next, recording or reproduction of information on the DVD will be described. When the red semiconductor laser of LD2 emits light, the divergent light beam having the wavelength λ2 emitted therefrom passes through the second polarizing 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, passes through the first aperture limiting element AP1, and passes through the first objective lens. After entering the 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 first objective lens OL1 and the first aperture limiting element AP1, is reflected by the second dichroic prism DBS2, passes through the λ / 4 wavelength plate QWP, The light is reflected by the first dichroic prism DBS1, further reflected by the second polarization beam splitter PBS2, and enters the second photodetector PD2 via 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 of LD2 is caused to emit light, a divergent light beam having a wavelength λ3 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 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 aperture limiting element AP2. After passing through and entering the second objective lens OL2, it becomes a spot formed on the CD information recording layer via the CD protective substrate.

  The reflected light beam modulated by the pits in the information recording layer of the CD again passes through the second objective lens OL2 and the second aperture limiting element AP2, is reflected by the rising mirror ML2, passes through the second dichroic prism DBS2, and λ / 4 wave plate QWP, reflected by the first dichroic prism DBS1, further reflected by the second polarization beam splitter PBS2, and incident on the second photodetector PD2 via the second sensor lens SL2. CD information can be read by the output signal of the second photodetector PD2.

Next, examples that can be used in the above-described embodiment will be described. The objective lens of the present embodiment can be applied to the objective lens OL1. 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 represented 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 aspheric 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. 3 shows a case where the thickness of the protective layer of the optical disk is changed by taking the position of the aperture limiting element (aperture) in the objective lens in Example 1 on the horizontal axis and the position of the marginal ray on the vertical axis. Each is shown. In FIG. 3 and FIGS. 4 to 8 to be described later, when the aperture position is zero, the aperture limiting element is positioned on the surface vertex of the objective lens, and the amount of movement to the optical disk side is expressed as positive. Further, if the marginal ray is set to zero as a reference and then increased or decreased, the numerical aperture also increases or decreases accordingly.

In the case of the first embodiment, from the relationship of FIG. 3, the position of the aperture limiting element has a small change in the numerical aperture at a position displaced by 0.43 mm to 0.46 mm from the surface vertex of the objective lens to the optical disk side. It turns out that it is preferable. In Example 1, D / f = 0.244 to 0.261, | H1-D | /f=0.349 to 0.366, and SC _max /f=0.022.

(Example 2)
Table 2 shows lens data of Example 2. FIG. 4 shows a case where the horizontal axis indicates the position of the aperture limiting element (aperture) in the objective lens in Example 2 and the vertical axis indicates the marginal ray position, and the thickness of the protective layer of the optical disk is changed in three types. Each is shown. In the case of the second embodiment, from the relationship of FIG. 4, the position of the aperture limiting element has a small change in the numerical aperture at a position displaced from the surface vertex of the objective lens to the optical disk side by 0.56 mm to 0.65 mm. It turns out that it is preferable. In Example 2, D / f = 0.317 to 0.368, | H1-D | /f=0.249 to 0.300, and SC _max /f=0.043. In this embodiment, if the change in NA is allowed to be 2% as an error, D / f = 0.19 to 0.44, and | H1-D | /f=0.18 to 0.42. It becomes.

(Example 3)
Table 3 shows lens data of Example 3. FIG. 5 shows a case where the horizontal axis indicates the position of the aperture limiting element (aperture) in the objective lens in Example 3 and the vertical axis indicates the position of the marginal ray, and the thickness of the protective layer of the optical disk is changed in three types. Each is shown. In the case of the third embodiment, from the relationship of FIG. 5, the position of the aperture limiting element is slightly changed in numerical aperture at a position displaced from the surface vertex of the objective lens to the optical disc side by 0.535 mm to 0.600 mm. It turns out that it is preferable. In Example 3, D / f = 0.303 to 0.34, | H1-D | /f=0.26 to 0.297, and SC _max /f=0.016.

Example 4
Table 4 shows lens data of Example 4. FIG. 6 shows a case where the horizontal axis indicates the position of the aperture limiting element (aperture) in the objective lens in Example 4 and the vertical axis indicates the marginal ray position, and the thickness of the protective layer of the optical disk is changed in three types. Each is shown. In the case of the fourth embodiment, from the relationship of FIG. 6, the position of the aperture limiting element is slightly changed in numerical aperture at a position displaced from the surface vertex of the objective lens to the optical disc side by 0.45 mm to 0.54 mm. It turns out that it is preferable. In Example 4, D / f = 0.255 to 0.306, | H1-D | /f=0.305 to 0.356, and SC _max /f=0.024.

(Example 5)
Table 5 shows lens data of Example 5. FIG. 7 shows a case where the horizontal axis indicates the position of the aperture limiting element (aperture) in the objective lens in Example 5 and the vertical axis indicates the marginal ray position, and the thickness of the protective layer of the optical disk is changed in three types. Each is shown. In the case of the fifth embodiment, from the relationship of FIG. 7, the position of the aperture limiting element is slightly changed by a numerical aperture at a position displaced by 0.475 to 0.54 mm from the surface vertex of the objective lens to the optical disc side. It turns out that it is preferable. In Example 5, D / f = 0.269 to 0.306, | H1-D | /f=0.306 to 0.342, and SC _max /f=0.024.

(Example 6)
Table 6 shows lens data of Example 6. FIG. 8 shows a case where the horizontal axis indicates the position of the aperture limiting element (aperture) in the objective lens in Example 6 and the vertical axis indicates the marginal ray position, and the thickness of the protective layer of the optical disk is changed in three types. Each is shown. In the case of the sixth embodiment, from the relationship of FIG. 8, the position of the aperture limiting element has a small change in the numerical aperture at a position displaced 0.41 mm to 0.57 mm from the surface vertex of the objective lens to the optical disk side. It turns out that it is preferable. In Example 6, D / f = 0.290 to 0.404, | H1-D | /f=0.235 to 0.348, and SC _max /f=0.022.

  Table 7 summarizes the values for each example. The symbols in Table 7 correspond to the symbols in the claims, but D1 indicates a short end of a preferred distance from the light source side surface apex of the first objective lens to the first aperture limiting element, and D2 indicates the first objective lens. The long end of the preferable distance from a light source side surface vertex to a 1st aperture limiting element is shown.

It is a figure which shows the principle of this invention. It is a figure which shows schematically the structure of optical pick-up apparatus PU1. It is a figure which shows the relationship between the position of the aperture limiting element (diaphragm) in Example 1, and the position of a marginal ray. It is a figure which shows the relationship between the position of the aperture limiting element (stop) in Example 2, and the position of a marginal ray. It is a figure which shows the relationship between the position of the aperture limiting element (stop) in Example 3, and the position of a marginal ray. It is a figure which shows the relationship between the position of the aperture limiting element (stop) in Example 4, and the position of a marginal ray. It is a figure which shows the relationship between the position of the aperture limiting element (stop) in Example 5, and the position of a marginal ray. It is a figure which shows the relationship between the position of the aperture limiting element (stop) in Example 6, and the position of a marginal ray. It is a figure which shows the relationship between an objective lens and the type with an aperture limiting element. It is a figure which shows the relationship between another type of an objective lens and an aperture limiting element. It is a figure which shows the relationship between another type of an objective lens and an aperture limiting element. It is a figure which shows the relationship between another type of an objective lens and an aperture limiting element. It is a figure which shows the relationship between another type of an objective lens and an aperture limiting element.

LD1: Blue-violet semiconductor laser (Multi-layer type 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: the second lens of the beam expander (spherical aberration correction lens for multilayer optical disk and BD recording / reproducing light)
DBS1: Dichroic prism (transmits blue-violet laser light and reflects red laser light)
QWP: λ / 4 wavelength plate ML: Raising mirror OL1: First objective lens OL2: Second objective lens SL1: First sensor lens (for recording / reproducing light of multilayer type optical disc and BD)
SL2: Second sensor lens (for servo light of multilayer optical disk, DVD recording / reproducing light)
PH: Pinhole (for removing stray light from other layers of multilayer optical disks)
PD1: First optical detector (for recording / reproducing light reading of a multilayer optical disk and a BD)
PD2: Second photodetector (for servo light of multilayer optical disk, for reading / recording light of DVD)
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 disk and BD)
AC3: Second actuator (for focusing servo light on multilayer optical disks)
AP1: first aperture limiting element AP2: second aperture limiting element

Claims (11)

A first light source, a spherical aberration correction element, an aperture limiting element, and an objective lens. A light beam having a wavelength λ1 emitted from the first light source is converted into the spherical aberration correction element and the aperture limiting element. A plurality of information recording layers stacked in the optical axis direction and having a thickness from the surface to the deepest information recording layer of 0.2 mm or more and 0.6 mm or less. Information is recorded / reproduced by focusing on one of the information recording layers of the optical disc, and the spherical aberration correcting element is adapted to reduce spherical aberration caused by the difference in the information recording layer for recording / reproducing information. , An optical pickup device adapted to correct by changing the magnification of the objective lens,
The objective lens is a single lens, and the aperture limiting element is disposed closer to the optical disc than the vertex of the light source side surface of the objective lens, and further satisfies the following expression.
0.7 ≦ NA ≦ 0.9 (1)
0.19 ≦ D / f ≦ 0.44 (2)
However, NA is the numerical aperture D [mm] of the objective lens, and the distance f [mm] in the optical axis direction from the vertex of the light source side surface of the objective lens to the aperture limiting position of the aperture limiting element is the objective at the wavelength λ1. Lens focal length The optical pickup device according to claim 1, wherein the following expression is satisfied.
0.18 ≦ | H1-D | /f≦0.42 (3)
However, H1 is the distance from the light source side surface vertex of the objective lens to the image side principal point position. When the light beam having the wavelength λ1 is incident 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 violation amount decreases in the peripheral portion. The optical pickup device according to claim 1, wherein the coma aberration correction state is set so as to satisfy the following expression.
0.015 <SC _max /f<0.045 (4)
However, SC _max is the maximum value of the sine condition violation amount.   The optical pickup device according to claim 1, wherein the wavelength λ1 is 390 nm ≦ λ1 ≦ 420 nm.   The optical pickup device according to claim 1, wherein the wavelength λ1 satisfies 520 nm ≦ λ1 ≦ 540 nm.   6. Information recording / reproduction is performed by condensing a light beam having a wavelength λ1 emitted from the first light source onto an information recording surface of a Blu-ray Disc by the objective lens. The optical pickup device according to any one of the above.   In addition to the first light source, a second light source that emits a light beam having a wavelength λ2 (λ1 <λ2) is provided, and the light beam having a wavelength λ2 emitted from the second light source is incident on the objective lens. The optical pickup device according to claim 1, wherein information is recorded / reproduced by focusing on an information recording layer of the second optical disc. A second objective lens in addition to the objective lens;
In addition to the first light source, a second light source that emits a light beam having a wavelength λ2 (λ1 <λ2) is provided, and the light beam having a wavelength λ2 emitted from the second light source is supplied to the second objective lens. 7. The optical pickup device according to claim 1, wherein the information is recorded / reproduced by being incident and condensed on an information recording layer of the second optical disc. The optical pickup device according to claim 1, wherein the following conditional expression (2 ′) is satisfied.
0.24 ≦ D / f ≦ 0.41 (2 ′) An objective lens unit having an aperture limiting element and an objective lens, a first light source, and a spherical aberration correction element, and a light beam having a wavelength λ1 emitted from the first light source is converted into the spherical aberration correction element, A plurality of information recording layers that are incident on the objective lens through the aperture limiting element and are stacked in the optical axis direction have a thickness from the surface to the deepest information recording layer of 0.2 mm or more and 0.6 mm. Information is recorded / reproduced by focusing on one of the following information recording layers of the first optical disc, and the spherical aberration correction element is caused by the difference in the information recording layer to record / reproduce information. An objective lens unit used in an optical pickup device adapted to correct the generated spherical aberration by changing the magnification of the objective lens,
The objective lens unit is a single lens, and the aperture limiting element is disposed closer to the optical disc than the vertex of the light source side surface of the objective lens, and further satisfies the following expression.
0.7 ≦ NA ≦ 0.9 (1)
0.19 ≦ D / f ≦ 0.44 (2)
However, NA is the numerical aperture D [mm] of the objective lens, and the distance f [mm] in the optical axis direction from the vertex of the light source side surface of the objective lens to the aperture limiting position of the aperture limiting element is the objective at the wavelength λ1. Lens focal length The objective lens unit according to claim 10, wherein the following conditional expression (2 ′) is satisfied.
0.24 ≦ D / f ≦ 0.41 (2 ′)