JP2011119026A - Optical pickup device and objective lens used for the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To focus laser light suitable for each of a plurality of information recording surfaces provided in an information recording medium. <P>SOLUTION: An optical pickup device 1 includes a collimator lens 13 displaced on an optical axis AX from a reference position and changing a shape of the laser light. An objective lens 15 is so constituted that the size of a spot formed by the objective lens 15 is minimized in a position separated from the surface on a light source 10 side of the information recording medium 20 by a designed center thickness regulated by expression (1):L<SB>c</SB>=(L<SB>0</SB>+L<SB>n</SB>)/2 when the collimator lens 13 is positioned in the reference position, wherein L<SB>c</SB>: the designed center thickness, L<SB>0</SB>: a distance between the surface on the light source side of the information recording medium and an information recording surface positioned closest to the surface, L<SB>n</SB>: a distance between the surface on the light source side of the information recording medium and an information recording surface positioned furthest from the surface. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical pickup device and an objective lens used therefor. Specifically, the present invention relates to an optical pickup device for focusing laser light on each information recording surface of an information recording medium having a plurality of information recording surfaces, and an objective lens used therefor.

  In recent years, research and development of an information recording medium having a plurality of information recording surfaces (for example, an optical disc such as a BD (Blu-ray Disc (registered trademark)) has been advanced. However, when a plurality of information recording surfaces are provided, since the distance from the light source side surface of the information recording medium to each information recording surface is different from each other, usually, one objective lens is used for each of the plurality of information recording surfaces. When trying to focus the laser beam, spherical aberration occurs due to the difference in distance from the light source side surface of the information recording medium to each information recording surface. It is difficult to focus the light. In view of such a problem, for example, Patent Document 1 discloses a light source for an information recording medium by changing a divergence angle of a beam incident on an objective lens by driving a beam expander lens or a collimator lens in the optical axis direction. A technique for correcting spherical aberration due to a difference in distance from the side surface to each information recording surface is disclosed.

JP 2006-31901 A

  However, there is a possibility that the aberration cannot be corrected sufficiently by simply driving the collimator lens or the like in the optical axis direction. In particular, when there are three or more information recording surfaces, for example, four surfaces, it is difficult to sufficiently correct aberration for each information recording surface.

  The present invention has been made in view of such a point, and an object of the present invention is to suitably focus laser light on each of a plurality of information recording surfaces provided on an information recording medium. Is to provide a simple optical pickup device.

In order to achieve the above object, an optical pickup device according to the present invention is an optical pickup device that focuses laser light on each of a plurality of information recording surfaces of an information recording medium having a plurality of information recording surfaces parallel to each other. It is located at the reference position between the light source that emits laser light, the objective lens that focuses the laser light on each information recording surface, and the light source and the objective lens, and is displaced on the optical axis from the reference position It is configured so that it is displaced on the optical axis from the reference position so that the laser beam is focused on the information recording surface to be focused on among the plurality of information recording surfaces and is incident on the objective lens. And an aberration correction element that changes the shape of the laser beam. When the aberration correction element is located at the reference position, the objective lens has a spot size formed by the objective lens from the light source side surface of the information recording medium. under Characterized in that it is configured so as to minimize the position separated by the design center thickness defined by Equation (1).
L _c = (L ₀ + L _n ) / 2 (1)
However,
L _c : design center thickness,
L ₀ : distance between the light source side surface of the information recording medium and the information recording surface located closest to the surface,
L _n : the distance between the light source side surface of the information recording medium and the information recording surface located farthest from the surface,
It is.

  The objective lens according to the present invention focuses laser light on each of a plurality of information recording surfaces of an information recording medium having a plurality of information recording surfaces parallel to each other, and a light source for emitting the laser light; The optical pickup device is provided with an aberration correction element that is located at a reference position closer to the information recording medium than the light source on the optical axis and changes the shape of the laser light by displacing the optical axis from the reference position. An objective lens for focusing light from the aberration correction element on each information recording surface, and when the aberration correction element is located at the reference position, is defined by the above equation (1) from the light source side surface of the information recording medium. The minimum spot is formed at a position separated by the design center thickness.

  ADVANTAGE OF THE INVENTION According to this invention, the optical pick-up apparatus which can focus a laser beam suitably with respect to all the information recording surfaces provided in the information recording medium is realizable.

2 is a diagram illustrating a configuration of a main part of the optical pickup device 1. FIG. It is sectional drawing to which the structure of the objective optical system 14 periphery was expanded. FIG. 6 is a diagram illustrating an optical path around the objective lens 15 of the optical pickup device according to Numerical Example 1. 5 is a diagram illustrating spherical aberration when parallel light is incident on an objective lens 15 in Numerical Example 1. FIG. 5 is a diagram illustrating a sine condition when parallel light is incident on an objective lens 15 in Numerical Example 1. FIG. It is a figure showing the optical path in the periphery of the objective lens 15 of the optical pick-up apparatus based on Numerical Example 2. FIG. In Numerical Example 2, it is a diagram showing spherical aberration when parallel light is incident on the objective lens 15 (object point distance: ∞, magnification: ∞). In Numerical Example 2, it is a diagram showing a sine condition when parallel light is incident on the objective lens 15 (object point distance: ∞, magnification: ∞). It is a figure showing the optical path in the periphery of the objective lens 15 of the optical pick-up apparatus which concerns on the numerical Example 3. FIG. In Numerical Example 3, it is a diagram illustrating spherical aberration when parallel light is incident on the objective lens 15 (object point distance: ∞, magnification: ∞). In Numerical Example 3, it is a diagram showing a sine condition when parallel light is incident on the objective lens 15 (object point distance: ∞, magnification: ∞).

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

  FIG. 1 is a diagram illustrating a configuration of a main part of an optical pickup device 1 according to the present embodiment.

  FIG. 2 is an enlarged cross-sectional view of the configuration around the objective optical system 14.

  The optical pickup device 1 according to the present embodiment is for focusing laser light on the information recording surface 21 of the information recording medium 20. In detail, the information recording medium 20 includes a plurality of information recording surfaces 21 arranged in parallel to each other and spaced apart from each other, and the optical pickup device 1 includes each of the plurality of information recording surfaces 21. Is for focusing the laser beam on. Specifically, the information recording medium 20 includes a first information recording surface 21a, a second information recording surface 21b, a third information recording surface 21c, and a fourth information disposed in this order from the light source 10 side. The information recording surface 21 has four information recording surfaces 21 and 21d. A first protective layer 22a is provided on the light source 10 side of the first information recording surface 21a. A second protective layer 22b is provided between the first information recording surface 21a and the second information recording surface 21b. A third protective layer 22c is provided between the second information recording surface 21b and the third information recording surface 21c. A fourth protective layer 22d is provided between the third information recording surface 21c and the fourth information recording surface 21d. The first protective layer 22a, the second protective layer 22b, the third protective layer 22c, and the fourth protective layer 22d may have substantially the same layer thickness or different layer thicknesses. It may be. Here, the case where the four information recording surfaces 21 are provided will be described as an example, but the present invention is not limited to this. The pickup device according to the present invention focuses laser light on each information recording surface of an information recording medium having two or three information recording surfaces or five or more information recording surfaces, for example. May be.

  Here, the type of the information recording medium 20 is not particularly limited. The information recording medium 20 may be, for example, a CD (Compact Disk Rerecordable), a CD-RW (Compact Disk Read Only Memory), a CD-ROM (Compact Disk Read Only Memory), a DVD (Digit Discrete Memory), or a DVD (Digit Digital Memory). DVD-R (Digital Versatile Disc Recordable), DVD-RW (Digital Versatile Disc Rewriteable), DVD-ROM (Digital Versatile Disc Read Only Memory), DVD-ROM (Digital Versatile Disc Read Only Memory), DVD-ROM (Digital Versatile Disc Rewriteable Memory) EVD (Enhanced Versatile Disc), EVD-R (Enhanced Versatile Disc Recordable), EVD-RW (Enhanced Versatile Disc ReWritable), EVD-ROM (Enhanced Versatile Disc Read Only Memory), EVD-RAM (Enhanced Versatile Disk Random Access Memory) BD (Blu-ray Disc Read Only Memory), BD-R (Blu-ray Disc Recordable), BD-RW (Blu-ray Disc Rewritable), BD-ROM (Blu-ray Disc Read Only Memory), BD-RAM lu-ray Disc Random Access Memory), HD-DVD (High Definition Digital Versatile Disc), HD-DVD-R (High Definition Digital Versatile Disc Recordable), HD-DVD-RW (High Definition Digital Versatile Disc ReWritable), HD- DVD-ROM (High Definition Digital Versatile Disc Read Only Memory), HD-DVD-RAM (High Definition Digital Versatile Disc Access Memory, etc.) Disk (or more, all registered trademark) may be.

  The optical pickup device 1 includes a light source 10, a beam shaping lens 11, a beam splitter 12, a collimator lens 13, an objective optical system 14, and a detection system 16. The light source 10 emits laser light (diverged light) having a wavelength corresponding to the type of the information recording medium 20. For example, when the information recording medium 20 is BD (registered trademark), a laser beam having a wavelength of 378 to 438 nm (may be less than 420 nm) can be emitted. On the other hand, when the information recording medium 20 is a DVD (registered trademark), a laser beam having a wavelength of 630 to 690 nm can be emitted. When the information recording medium 20 is a CD (registered trademark), a laser beam having a wavelength of 750 to 810 nm can be emitted. Further, for example, when the optical pickup device 1 is for focusing laser light on three types of optical discs of CD, DVD, and BD, the type of optical disc in which the light source 10 is disposed with respect to the optical pickup device 1 It is possible to selectively emit light having a wavelength according to the above.

  A beam shaping lens 11 is disposed in front of the light source 10. The laser beam emitted from the light source 10 by this beam shaping lens 11 is shaped into a desired shape. The laser beam shaped by the beam shaping lens 11 is reflected toward the information recording medium 20 by the reflecting surface 12 a of the beam splitter 12. Between the beam splitter 12 and the information recording medium 20, a collimator lens (which may be constituted by a single lens or may be constituted by a plurality of lenses) 13 and each of the information recording medium 20. An objective optical system 14 for focusing the laser beam on the information recording surface 21 is arranged. In the present embodiment, the objective optical system 14 is composed of only the objective lens 15, but the objective optical system 14 may be replaced with the objective lens 15 and another one such as a phase correction element or a beam expander lens as necessary. You may comprise with a some optical element.

  The NA of the objective lens 15 is not particularly limited. In particular, when the optical pickup device 1 focuses the laser beam on BD (registered trademark) or the like, the NA is, for example, 0.8 or more. Is preferred.

  The laser beam focused on the information recording surface 21 of the information recording medium 20 by the objective optical system 14 is reflected by the information recording surface 21. The reflected light from the information recording surface 21 is again transmitted through the objective optical system 14, the collimator lens 13, and the beam splitter 12 and is incident on the detection system 16. The detection system 16 includes a detector 18 and a detection lens 17 for focusing on the detector 18, and is configured so that the reflected light focused by the detection lens 17 is detected by the detector 18. Has been.

  In the present embodiment, the collimator lens 13 has a function as an aberration correction element, and is located at a reference position between the beam splitter 12 and the objective lens 15 (objective optical system 14), and the optical axis from the reference position. It is configured to be displaceable on AX. Then, the objective lens 15 (objective optical system 14) is displaced from the reference position on the optical axis AX so that the laser beam is focused on the information recording surface 21 to be focused on the laser beam among the plurality of information recording surfaces 21. The laser beam is suitably focused on the information recording surface 21. In other words, the position on the optical axis AX of the collimator lens 13 as an aberration correction element is adjusted according to the position on the optical axis AX of the information recording surface 21 to focus the laser beam on each information recording surface 21. The laser beam is suitably focused. Specifically, for example, when the collimator lens 13 is positioned at the reference position, the objective lens 15 is optically designed so that the laser light is suitably focused on the first information recording surface 21a by the objective optical system 14. In the case where the laser light is focused on the information recording surface 21 other than the first information recording surface 21a, for example, the second information recording surface 21b, the objective optical system 14 causes the second information recording surface to be focused. The position of the collimator lens 13 on the optical axis AX is adjusted so that the laser beam is suitably focused on 21b, and the shape (divergence angle, etc.) of the laser beam is adjusted. Therefore, in the optical pickup device 1 according to the present embodiment, the laser beam can be suitably focused on each information recording surface 21 of the information recording medium 20 having the plurality of information recording surfaces 21.

Further, in the present embodiment, the objective lens 15 constituting the objective optical system 14 is such that the size of the spot formed by the objective lens 15 is the information recording medium when the collimator lens 13 as the aberration correction element is located at the reference position. 20 is configured to be minimized at a position away from the surface 20a on the light source 10 side by a design center thickness defined by the following formula (1).
L _c = (L ₀ + L _n ) / 2 (1)
However,
L _c : design center thickness,
L ₀ : distance between the surface of the information recording medium 20 on the light source 10 side and the information recording surface 21a located closest to the surface,
L _n : the distance between the surface of the information recording medium 20 on the light source 10 side and the information recording surface 21d located farthest from the surface,
It is.

  With this configuration, aberration deterioration (spherical aberration, various aberrations caused when off-axis laser light is incident, the objective lens 15 is tilted with respect to the optical axis AX) on all of the plurality of information recording surfaces 21. In the optical recording medium 20 and deterioration due to various aberrations that occur when the information recording medium 20 is tilted with respect to the optical axis AX can be effectively reduced. Further, with this configuration, the amount of displacement on the optical axis AX of the collimator lens 13 as an aberration correction element can be made relatively small, and the amount of displacement from the reference position toward the light source 10 and the information recording medium The difference from the displacement amount toward the 20 side can be reduced. Therefore, stable reproduction of the information recording medium 20 can be performed. These effects can also be obtained when there are two information recording surfaces 21. However, when there are three or more information recording surfaces 21, there are four or more information recording surfaces 21. Especially great. Further, it is particularly effective in an optical pickup device used for DVD (registered trademark), BD (registered trademark), etc., in which the wavelength of the laser beam used is relatively short.

  The distance between the surface 20a of the information recording medium 20 and the information recording surface 21a located closest to the surface 20a is preferably less than 100 μm. More preferably, it is less than 75 micrometers, More preferably, it is less than 60 micrometers. According to this configuration, even when the information recording medium 20 is bent, surface-blurred, tilted, or the like, the aberration generated on the information recording surface 21a can be suppressed to a small value.

The objective lens 15 is preferably configured such that the distance from the surface 20a at the position on the optical axis AX where the aberration is minimized when parallel light enters the objective lens 15 is less than 80 μm. According to this configuration, it is possible to maintain a good balance of high-order aberrations that occur when the objective lens 15 is used while changing the magnification, and to make the entire thickness of the information recording medium 20 relatively thin. . Further, in this configuration, the objective lens 15 is preferably configured to satisfy the following conditional expression (2), and further configured to satisfy the following conditional expression (2-1). preferable. According to this configuration, it is possible to perform good recording / reproduction particularly when parallel light is incident. In this configuration as well, high-order aberrations occur when non-parallel light is incident. Therefore, it is preferable to set aberrations that occur when parallel light is incident as small as possible. In particular, it is preferable to set it to 50 mλ or less.
λ ₁ <70 mλ (2)
However,
λ ₁ : aberration at a position on the optical axis AX where the aberration is minimized when parallel light is incident on the objective lens 15;
λ: wavelength of laser light,
It is.
λ ₁ <50 mλ (2-1)
However,
λ ₁ : aberration at a position on the optical axis AX where the aberration is minimized when parallel light is incident on the objective lens 15;
λ: wavelength of laser light,
It is.

  Note that, here, an example of an optical pickup device configured to focus laser light only on one type of information recording medium has been described, but laser light is applied to each of a plurality of types of information recording media. A configuration capable of focusing may be used. In this case, the objective optical system 14 may be configured by the objective lens 15 and a phase correction element that corrects the phase of light incident on the objective lens 15, a beam expander lens, or the like. In addition, each information recording medium may have a dedicated objective lens.

  Although the case where the collimator lens 13 is used as an aberration correction element has been described as an example here, the aberration correction element is a single beam expander lens disposed between the collimator lens and the objective lens, or a beam expander. You may comprise with a lens and a collimator lens. Further, a liquid crystal lens, a liquid lens, or the like may be used as the aberration correction element.

  In the present embodiment, the optical element such as a lens may be constituted by only a refractive surface having substantially only a refractive action. For example, other optical elements such as a diffraction surface and a phase step surface may be used. It may have a functional aspect. Furthermore, the material of the optical element such as a lens is not particularly limited, and may be made of, for example, glass or resin.

  The optical pickup device 1 may further include an element between the light source 10 and the information recording medium 20 that does not substantially affect the transmitted wavefront aberration.

  Hereinafter, the optical pickup device 1 embodying the present invention will be described more specifically with reference to construction data, aberration diagrams, and the like. In each numerical example, it is shown that the surface to which the aspheric coefficient is given is an aspherical refractive optical surface or a surface having a refractive action that is transparent to the aspherical surface (for example, a diffractive surface). It is defined by the following conditional expression (*) representing the surface shape.

However,
X: distance from the tangent plane of the aspherical vertex of the aspherical point whose height from the optical axis is h,
h: height from the optical axis,
C _j : curvature of the aspherical vertex of the lens j-th surface (C _j = 1 / R _j ),
K _j : Conic constant of the lens j-th surface,
A _{j, n} : nth-order aspherical constant of lens j-th surface,
And
j = 1, 2, 3, 4,
It is.

-Numerical Example 1-
The construction data in Numerical Example 1 are shown in Tables 1 to 3 below.

In this numerical example 1,
Distance from the surface 20a of the information recording medium 20 to the first information recording surface 21a: 40.0 μm,
Distance from the surface 20a of the information recording medium 20 to the second information recording surface 21b: 67 μm,
Distance from the surface 20a of the information recording medium 20 to the third information recording surface 21c: 86 μm,
Distance from the surface 20a of the information recording medium 20 to the fourth information recording surface 21d: 100 μm,
Design center thickness: 70 μm
Focal length of the objective lens 15: 1.34 mm
Effective diameter of the objective lens 15: 2.3 mm,
It is.

  Table 4 below shows the aspherical coefficient of the first surface (# 1: the side surface of the light source 10 of the objective lens 15). Table 5 below shows the aspheric coefficient of the second surface (# 2: side surface of the information recording medium 20 of the objective lens 15).

  FIG. 3 is a diagram illustrating an optical path around the objective lens 15 of the optical pickup device according to the first numerical embodiment.

  FIG. 4 is a diagram showing spherical aberration when parallel light is incident on the objective lens 15 (object distance: ∞, magnification: ∞) in the present numerical example 1.

  FIG. 5 is a diagram showing a sine condition when parallel light is incident on the objective lens 15 in this numerical example 1 (object distance: ∞, magnification: ∞).

  Table 6 below shows the total aberration and the like in the present numerical example 1.

-Numerical Example 2-
Construction data in the present numerical example 2 are shown in Tables 7 to 9 below.

In this numerical example 2,
Distance from the surface 20a of the information recording medium 20 to the first information recording surface 21a: 10.0 μm,
Distance from the surface 20a of the information recording medium 20 to the second information recording surface 21b: 32.5 μm
Distance from the surface 20a of the information recording medium 20 to the third information recording surface 21c: 77.5 μm,
Distance from the surface 20a of the information recording medium 20 to the fourth information recording surface 21d: 100 μm,
Design center thickness: 55 μm,
Focal length of the objective lens 15: 1.34 mm
Effective diameter of the objective lens 15: 2.3 mm,
It is.

  Table 10 below shows the aspherical coefficient of the first surface (# 1: the light source 10 side surface of the objective lens 15). Table 11 below shows the aspheric coefficients of the second surface (# 2: side surface of the information recording medium 20 of the objective lens 15).

  FIG. 6 is a diagram illustrating an optical path around the objective lens 15 of the optical pickup device according to the second numerical embodiment.

  FIG. 7 is a diagram showing spherical aberration when parallel light is incident on the objective lens 15 in this numerical example 2 (object point distance: ∞, magnification: ∞).

  FIG. 8 is a diagram illustrating a sine condition when parallel light is incident on the objective lens 15 in this numerical example 2 (object distance: ∞, magnification: ∞).

  Table 12 below shows the total aberration and the like in the present numerical example 1.

-Numerical Example 3-
Construction data in the present numerical example 3 are shown in Tables 13 to 15 below.

In this numerical example 3,
Distance from the surface 20a of the information recording medium 20 to the first information recording surface 21a: 59.5 μm,
Distance from the surface 20a of the information recording medium 20 to the second information recording surface 21b: 67 μm,
Distance from the surface 20a of the information recording medium 20 to the third information recording surface 21c: 86 μm,
Distance from the surface 20a of the information recording medium 20 to the fourth information recording surface 21d: 100 μm,
Design center thickness: 79.75 μm,
Focal length of the objective lens 15: 1.4 mm
Effective diameter of the objective lens 15: 2.4 mm,
It is.

  Table 16 below shows the aspheric coefficients of the first surface (# 1: the side surface of the light source 10 of the objective lens 15). Table 17 shows the aspheric coefficients of the second surface (# 2: side surface of the information recording medium 20 of the objective lens 15).

  FIG. 9 is a diagram illustrating an optical path around the objective lens 15 of the optical pickup device according to the third numerical embodiment.

  FIG. 10 is a diagram illustrating spherical aberration when parallel light is incident on the objective lens 15 in this numerical example 3 (object distance: ∞, magnification: ∞).

  FIG. 11 is a diagram illustrating a sine condition when parallel light is incident on the objective lens 15 in this numerical example 3 (object distance: ∞, magnification: ∞).

  Table 18 below shows the total aberration and the like in the present numerical example 1.

  Since the optical pickup device according to the present invention can suitably focus laser light on each of a plurality of information recording surfaces, a CD (Compact Disc), a DVD (Digital Versatile Disc), and a BD (Blu). -Ray Disc), EVD (Enhanced Versatile Disc), and HD-DVD (High Definition Digital Versatile Disc) for recording and reproduction, for example, information equipment such as computers, video equipment, and sound Useful for equipment.

1 Optical pickup device
10 Light source
11 Beam shaping lens
12 Beam splitter
13 Collimator lens
14 Objective optical system
15 Objective lens
16 Detection system
17 Detection lens
18 Detector
20 Information recording media
21 Information recording surface
22 Protective layer

Claims (10)

An optical pickup device that focuses laser light on each of the plurality of information recording surfaces of an information recording medium having a plurality of information recording surfaces parallel to each other,
A light source that emits laser light;
An objective lens for focusing the laser beam on each information recording surface;
It is located at a reference position between the light source and the objective lens, and is configured to be displaceable on the optical axis from the reference position, and attempts to focus the laser light among the plurality of information recording surfaces. An aberration correction element that changes the shape of the laser beam incident on the objective lens by displacing the optical axis from the reference position so that the laser beam is focused on the information recording surface;
With
In the objective lens, when the aberration correction element is located at the reference position, the size of the spot formed by the objective lens is defined by the following formula (1) from the surface on the light source side of the information recording medium. An optical pickup device configured to be minimized at a position separated by a design center thickness;
L _c = (L ₀ + L _n ) / 2 (1)
However,
L _c : design center thickness,
L ₀ : distance between the light source side surface of the information recording medium and the information recording surface located closest to the surface,
L _n : the distance between the light source side surface of the information recording medium and the information recording surface located farthest from the surface,
It is. The optical pickup device according to claim 1,
The aberration correction element is an optical pickup device that is a collimator lens, a beam expander lens, or a liquid crystal element. The optical pickup device according to claim 1,
An optical pickup apparatus, wherein a distance between the light source side surface of the information recording medium and an information recording surface located closest to the surface is less than 100 μm. The optical pickup device according to claim 1,
An optical pickup device in which the wavelength of the laser beam is less than 420 nm. The optical pickup device according to claim 1,
An optical pickup device having an NA of 0.8 or more in an area where the laser beam is incident on the objective lens. The optical pickup device according to claim 1,
The objective lens is an optical pickup configured such that the distance from the light source side surface of the information recording medium at a position on the optical axis at which the aberration is minimized when parallel light is incident is less than 80 μm. apparatus. The optical pickup device according to claim 6,
An optical pickup device in which the objective lens is configured to satisfy the following conditional expression (2);
λ ₁ <70 mλ (2)
However,
λ ₁ : aberration at a position on the optical axis where the aberration is minimized when parallel light enters the objective lens,
λ: wavelength of laser light,
It is. The optical pickup device according to claim 1,
The objective lens is an optical pickup device constituted by a single lens. The optical pickup device according to claim 1,
An optical pickup device further comprising a detector for detecting reflected light of the laser beam on each information recording surface. A laser beam is focused on each of the plurality of information recording surfaces of an information recording medium having a plurality of information recording surfaces parallel to each other. The light source that emits the laser light and the light source on the optical axis Is also used in an optical pickup device provided with an aberration correction element that is positioned at a reference position near the information recording medium and that changes the shape of the laser beam by displacing the optical axis from the reference position. An objective lens for focusing the light from each of the information recording surfaces,
When the aberration correction element is located at the reference position, a minimum spot is formed at a position away from the light source side surface of the information recording medium by a design center thickness defined by the following formula (1). Constructed objective lens;
L _c = (L ₀ + L _n ) / 2 (1)
However,
L _c : design center thickness,
L ₀ : distance between the light source side surface of the information recording medium and the information recording surface located closest to the surface,
L _n : the distance between the light source side surface of the information recording medium and the information recording surface located farthest from the surface,
It is.

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