JPH0981953A - Recording and reproducing optical system of optical recording information medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an information pickup device and an optical disk device capable of recording/reproducing optical disks different in thickness of substrate one from another by one optical pickup. SOLUTION: The recording/reproducing optical system comprises a laser light source and a finite conjugate type objective lens having positive refractive power and converging a projected luminous flux from the light source on an information recording surface through the transparent substrate of an optical information recording medium. The laser light source is moved along the optical axis according to the thickness of a transparent substrate, when mti is a lateral magnification of the whole system from the laser light source to the recording surface of an optical information medium and the thickness of the transparent substrate has a relation of t1<t2, mt1>mt2 is satisfied. Both surfaces on the side of the light source and on the side of the information recording surface of the objective lens are aspherical surfaces and the surface on the side of the light source is a convex surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system for recording and reproducing optical information by condensing a light beam such as a laser beam on an optical information recording medium.

[0002]

2. Description of the Related Art An optical system used in a recording / reproducing apparatus for an information recording medium such as an optical disk is one of the most popular optical systems in recent years. There is one that focuses the light directly on the information recording surface. Since this method does not require a collimator lens, the cost can be kept low as compared with the method using a collimator lens, so in recent years, this method has been adopted more often.

Further, in recent years, in a recording / reproducing apparatus for an information recording medium such as an optical disk, it has been necessary to reduce the light spot condensed by an objective lens in order to cope with higher density. Therefore, an objective lens having a large numerical aperture (NA) (for example, NA 0.6) is required. Also, a high-density disc with a substrate thickness of 0.6 mm has been put into practical use,
Conventional disks (CD, CD-RO) with a substrate thickness of 1.2 mm
There is a demand for an optical system that can handle both M. When the NA is large as described above, a large spherical aberration occurs when the thickness of the substrate placed in the condensed light flux deviates from a predetermined thickness. For example, in an objective lens having an NA of 0.60 and a magnification of 1/12, when the laser light emitted from a laser light source is optimized under the conditions of a wavelength of 635 nm, a substrate thickness of 0.6 mm, and a substrate refractive index of 1.58, If the thickness of the substrate is changed, 0.01 λrm will occur for every 0.01 mm deviation.
The aberration increases by about s. Therefore, the substrate thickness is ± 0.0
A deviation of 7 mm results in an aberration of 0.07 λrms, which reaches the Marechal limit value that is a standard for normal reading.

For this reason, when a substrate having a thickness of 1.2 mm, for example, is to be reproduced instead of a substrate having a thickness of 0.6 mm,
It is necessary to prepare two objective lenses corresponding to the respective thicknesses so that the reproduction is performed by switching to the objective lenses corresponding to the thickness of 1.2 mm. Alternatively, a method in which two information pickups for a substrate having a thickness of 0.6 mm and a substrate having a thickness of 1.2 mm are attached to the device can be considered. Further, there is also a method in which a hologram is provided in the information pickup and each of the 0th-order light and the 1st-order light transmitted therethrough is condensed on the information recording surface as a light spot corresponding to a 0.6 mm thick substrate and a 1.2 mm thick substrate. Conceivable.

[0005]

As described above, in order to provide an apparatus capable of reproducing optical disks having different substrate thicknesses by one optical disk apparatus, for example, the substrate thickness of the disk is 0.6 mm and 1.2 mm. Two objective lenses corresponding to each use are attached, and the substrate thickness of the disc is 0.6m
With the method of attaching two optical pickups for m and 1.2 mm to the device, the information pickup device, the optical disc device, and the optical disc device cannot be made compact and low cost. A hologram is provided during information pickup,
The method of converging each of the 0th-order light and the 1st-order light passing therethrough on the information recording surface as light spots corresponding to the 0.6 mm thick substrate and the 1.2 mm thick substrate is always directed toward the information recording surface.
Since one light flux is emitted, when information is read from a light spot by one light flux, the other light flux becomes unnecessary light that does not contribute to the reading. Further, since the light is separated into two spots by diffraction, diffracted light other than the two spots actually used is generated, resulting in a large light amount loss. Therefore, the S / N ratio is reduced due to the decrease in the light amount,
If the amount of light is increased, the laser life will be shortened. SUMMARY OF THE INVENTION It is an object of the present invention to provide a compact and inexpensive information pickup device, which enables recording and reproduction of optical disks having different substrate thicknesses with one optical pickup.

[0006]

A recording / reproducing optical system for an optical information recording medium according to the present invention comprises at least a laser light source and a light beam emitted from the light source on an information recording surface via a transparent substrate of the optical information recording medium. Characterized in that it has a finite conjugate type objective lens having a positive refracting power for condensing to, the laser light source moves along the optical axis according to the thickness of the transparent substrate, and satisfies the following conditions: . mt1> mt2 (1) where mti is the lateral magnification of the entire system from the laser light source to the recording surface of the optical information medium, and the thickness t i of the transparent substrate has a relationship of t1 <t2.

Both the light source side and the information recording surface side of the objective lens for focusing on the information recording surface are aspherical surfaces, and the surface on the light source side is a convex surface, and −2.1 ≦ G ≦ −0. .5 [2] where G = Δt · (F−m1 · Δd) / (m1 ² · Δd · F)
^{· (Nt 2 -1) / nt} 3 where △ d = d2-d1 △ t = t2-t1 ti: thickness of the transparent substrate (t1 <t2) di: distance from the light source relative to the transparent substrate to the objective lens m1: Transparent It is characterized in that the lateral magnification F of the objective lens which gives the best wavefront aberration at the thickness t1 of the substrate is F: the focal length of the objective lens nt is the refractive index of the transparent substrate.

When the numerical aperture of the objective lens is NA1 when the transparent substrate has a first thickness and NA2 when the transparent substrate has a second thickness larger than the first thickness, NA2 <NA1 ... [3] 0 .30 ≤ NA1 ≤ 0.65 ... [4] 0.30 ≤ NA2 ≤ 0.65 ... [5]. In the case of 0.50 ≤ NA2 ≤ 0.65 ... [5 '], it is characterized by satisfying -1.5 ≤ G ≤ -0.80 ... [2'].

The objective lens has the following properties: 0.035 ≦ NA1 · | m1 | ≦ 0.15 ... [6] 0.035 ≦ NA2 · | m2 | ≦ 0.15 ... [7] m1 <0, m2 <0 ... [8] where NA1: numerical aperture when the thickness t1 (<t2) of the transparent substrate NA2: numerical aperture when the thickness t2 of the transparent substrate m1: objective lens when the thickness t1 of the transparent substrate Lateral magnification m2: It is characterized by satisfying the condition of the lateral magnification of the objective lens when the thickness t2 of the transparent substrate.

The objective lens has the following formula: | m1 | · F · NA1 ⁴ ≦ 0.061

[9] | m2 | · F · NA2 ⁴ ≦ 0.061 ... [10] is satisfied. Further, the objective lens must satisfy the condition of | m1 | · F · NA1 ⁴ ≦ 0.045 ・ ・ ・ [9 ′] | m2 | · F · NA2 ⁴ ≦ 0.045 ・ ・ ・ [10 ′] Is desirable.

[0011]

In the optical system of the present invention, the spherical aberration generated by the change Δt in the thickness of the substrate is canceled out by the spherical aberration generated by moving the light source along the optical axis, thereby correcting the aberration well. The purpose is to do. Actually, the amount of change in spherical aberration relative to the change in substrate thickness Δt ΔSAt
Are proportional to each other at the same NA and can be expressed as follows. Δt · (nt ² −1) / nt ³ · α = ΔSAt (α: proportional constant) (1) where nt is the refractive index of the transparent substrate. It can be considered that the spherical aberration change amount ΔSAm due to the single lens objective lens magnification change Δm is in a substantially proportional relationship. F · Δm · β = ΔSAm (β · proportional constant) (2) where F is the focal length of the objective lens. Therefore, in order to correct spherical aberration as a whole, ΔSAt + ΔSAm = 0 (3) may be set. That is, Δt · (nt ² −1) / (nt ³ · F · Δm) = − β / α (constant) (4)

At this time, in the equation (1), nt is constant and Δ
When t is positive (> 0), spherical aberration moves in the over direction.
That is, ΔSAt> 0. As a result, since nt> 1, the constant α becomes positive. In the equation (2), if the lateral magnification change is positive (Δm> 0) in the lateral magnification change Δm (in the case of a real image system that is not a reflection system, if the absolute value of the lateral magnification is small), spherical aberration Moves over. Therefore ΔS
Am> 0. As a result, since F> 0, the constant β is positive. As a result, if .DELTA.t (= t2-t1> 0) is positive, .DELTA.m is negative (<0) from the equation (4).

The lateral magnification at which the spherical aberration on the transparent substrate t1 is best corrected is m1, and the transparent substrate t2 (t2> t1).
, .DELTA.t = t2-t1), the lateral magnification at which the spherical aberration is best corrected is m2, which can be expressed by .DELTA.m = m2-m1 (5). As a result, m1> m2 (6) holds. By making the surface of the objective lens on the light source side convex and making both surfaces aspheric, spherical aberration can be corrected well.

Next, in order to satisfactorily correct the spherical aberration ΔSAt that occurs when the thickness of the transparent substrate changes (Δt), the movement amount Δd for moving the light source and the lateral amount of the objective lens at that time are corrected. Find the relationship of magnification change Δm. The movement amount Δd of the light source is Δd = (1-1 / m2) · F− (1-1 / m1) · F = {(1 / m1) − (1 / m2)} · F ... (7 ) Can be represented. Here, F is the focal length of the objective lens. By transforming this, m2 = m1 · F / (F−m1 · Δd) (8) Substituting this into equation (5), Δm = m1 ² · Δd / (F−m1 · Δd) ... (9)

Substituting the above into equation (4), the relationship between Δd and Δt and the objective lens is Δt · (F−m1 · Δd) / (F · m1 ² · Δd) · (nt ² − ) / Nt ³ = −β / α = constant (10) Here, the left side of the equation (12) is set to G for simplification. That is, G = Δt · (F−m1 · Δd) / (F · m1 ² · Δd) · (nt ² −1) / nt ³ (11) Here, in order to suppress the spherical aberration generated with respect to the thickness change Δt of the transparent substrate, the allowable error is considered to be a margin such as an arrangement error with respect to the Marechal limit (wavefront aberration 0.07λ). And the spherical aberration 0.045λ in the wavefront aberration RMS value
It is necessary to change Δd and set G so as to keep G. If Δd is set so that G exceeds the upper limit (-0.5) of the conditional expression, the spherical aberration becomes under, and even if NA2 is 0.3 level, the spherical aberration exceeds 0.045λ in wavefront aberration RMS value. I will end up. G is the lower limit (-2.
If Δd is set so as to be smaller than 1), the spherical aberration becomes over, and the spherical aberration exceeds 0.045λ in terms of wavefront aberration RMS value even when NA2 is at the 0.3 level. If NA2 becomes 0.50 or more (conditional expression [5 ']), the spherical aberration will maintain the wavefront aberration RMS value within 0.045λ unless the value of G is -1.5 or more and -0.8 or less. It is difficult (conditional expression [2 ']). If NA1 and NA2 exceed 0.65, good performance cannot be obtained. In particular, by setting G to -1.3 or more and -1.0 or less, the spherical aberration approaches 0 and better performance can be obtained.

Let NAo1 be the numerical aperture on the light source side in the first arrangement (arrangement at t1) and NAo2 be the numerical aperture on the light source side in the second arrangement (arrangement at t2). N
Each of Ao2 must satisfy 0.035 ≤ NAo1 ≤ 0.15 0.035 ≤ NAo2 ≤ 0.15. If the numerical aperture on the light source side is smaller than 0.035, the light amount of the laser decreases due to the divergence angle of the laser, which causes troubles in recording and reproducing on the optical information recording medium. On the other hand, if it is larger than 0.15, the laser divergence angle causes a problem due to the astigmatic difference of the laser and the uneven light amount.

The numerical aperture on the light source side can be represented by the product of the numerical aperture on the image side and the absolute value of the lateral magnification of the optical system. That is, NAo1 = | m1 | .NA1 NAo2 = | m2 | .NA1 As a result, the conditional expressions [6] and [7] must be satisfied. Here, since the objective lens converts the divergent light from the light source into the convergent light by the objective lens, m1 <0, m2 <0 (conditional expression [8]).

In the objective lens, the distance from the light source to the image point (inter-image distance) changes due to the blurring of the transparent substrate of the optical information recording medium, and spherical aberration occurs when the objective lens follows the auto focus. If the first arrangement is | m1 |
If so F · NA1 ⁴ exceeds 0.061, the amount of the spherical aberration due to the change in the distance between the object and the image may exceed the allowable value (condition

[9]). The same applies to the second arrangement, and if | m2 | · F · NA2 ⁴ exceeds 0.061, the amount of spherical aberration generated due to the change in the object-image distance exceeds the allowable value (conditional expression). [10]). In addition,
In the first and second arrangements | m1 | ・ F ・ NA1 ⁴
And | m2 | · F · NA2 ⁴ is 0.045 or less, more favorable performance can be maintained (conditional expression [9 ′],
[10 ']).

[0019]

EXAMPLES Examples of the optical system of the present invention will be shown below. In each of the embodiments, the numerical example is such that the laser light source is the 0th surface, the radius of curvature of the i-th surface (including the diaphragm surface) is r ₁ , the i-th surface and the i + 1-th surface in this order. Is represented by di, the thickness on the optical axis, the distance is represented by ni, and the refractive index at the wavelength of the laser light source of the medium between the i-th and (i + 1) -th medium is represented by ni. Also,
The refractive index of air is 1. Further, when an aspherical surface is used for the lens surface, the aspherical shape has an apex of the surface as an origin and an orthogonal coordinate system in which the optical axis direction is the X axis,
Let κ be the conic coefficient, Ai be the aspherical surface coefficient, and Pi be the power of the aspherical surface.

[Equation 1] It is represented by

In these embodiments, the wavelength of the light source is set to λ = 635 nm, the first thickness of the transparent substrate corresponding to the first arrangement is t1 = 0.6 mm, and the first thickness corresponds to the second arrangement. The second thickness is t2 = 1.2 mm. The refractive index at this time is set to nt = 1.58.
Further, the numerical aperture NA1 in the first arrangement is set to 0.6. The numerical aperture NA2 in the second arrangement is assumed to be the same as that in the first arrangement (the same arrangement with respect to the objective lens and the same diaphragm diameter, and the diaphragm diameter at this time is φ1). Then, although the aberration diagram and the wavefront aberration change are evaluated and simulated, the diaphragm diameter may be made variable and smaller than φ1. By making the aperture diameter smaller than φ1, the value of NA2 becomes smaller, and it is clear that the amount of aberration and the amount of wavefront aberration become smaller than that at φ1. F in the table is the focal length of the objective lens,
U1 and U2 are the distances from the light source to the information recording medium (inter-object distance) in the first arrangement and the second arrangement, respectively, m1,
m2 represents the lateral magnification of the objective lens in the first arrangement and the second arrangement, respectively. Also, T1 and T2 are respectively in the first arrangement,
The distance from the objective lens to the light source in the second arrangement (the direction from the light source to the information recording medium is positive) is shown.

Example 1 F = 3.2410959 First arrangement T1 = −34.713 U1 = 40.000 m1 = −0.1000 Second arrangement T2 = −22.383 U2 = 28.089 m2 = − 0.1614 Ri d1i d2i Ni 1 2.080 3.10 3.10 1.49446 2 -3.539 1.587 1.406 3 ∞ 0.60 1.2 1.58000 4 ∞ Aspheric surface coefficient 1st surface κ = −6.08630 × 10 ⁻¹ A1 = −4.207090 × 10 ⁻⁴ P1 = 4.0000 A2 = −1.49720 × 10 ⁻⁴ P2 = 6.0000 A3 = −1.04560 × 10 ⁻⁶ P3 = 8.00000 A4 = −4. 67950 × 10 ⁻⁷ P4 = 1.0000 2nd surface κ = −1.36490 × 10 A1 = 3.72820 × 10 ⁻³ P1 = 4.0000 A2 = −2.85100 × 10 ⁻⁴ P2 = 6.00000 A3 = 1.49930 × 10 ⁻⁵ P3 = 8.00000 A4 = 1.89980 × 10 ⁻⁹ P4 = 10.00000

The distance di from the light source to the objective lens for each transparent substrate is d1 = -T1 d2 = -T2, respectively. Therefore, the amount of movement of the light source is .DELTA.d = d2-d1 = T1-T2 = -12.33 mm. Further, since Δt = t2-t1 = 0.6 mm, G = Δt · (F−m1 · Δd) / (F · m1 ² · Δd) · (nt ² −1) / nt ³ = -1.1438.

When the same aperture is used for the first arrangement and the objective lens (a diaphragm having the same arrangement and the same aperture diameter as the objective lens), the numerical aperture NA2max in the second arrangement is used.
Is NA2max = 0.595. When NA2 = NA2max = 0.595 NA1. | M1 | = 0.0600 NA2. | M2 | = 0.0958 Also | m1 | .F.NA1 ⁴ = 0.0420 | m2 | .F.NA2 ⁴ = It becomes 0.0654. If the aperture diameter is variable and the numerical aperture NA2 in the second arrangement is 0.53, NA2 · | m2 | = 0.0853 | m2 | · F · NA2 ⁴ = 0.0412 and the aperture diameter is variable. Assuming that the numerical aperture NA2 in the second arrangement is 0.45, NA2. | M2 | = 0.0725 | m2 | .F.NA2 ⁴ = 0.0214. The NA2 · When NA2 = 0.38 | m2 | = 0.0612 | a ^{· F · NA2 4 = 0.0109 |} m2.

FIG. 1 shows the first arrangement and the second arrangement of the first embodiment.
Is shown. FIG. 2 shows an aberration diagram of spherical aberration when NA2-NA2max. FIG. 2A is a spherical aberration diagram in the first arrangement when t1 = 0.6 mm. FIG. 2B is a spherical aberration diagram in the case where the thickness t2 of the transparent substrate is 1.2 mm in the first arrangement. At this time, spherical aberration moves in the over direction. FIG. 2C shows a spherical aberration diagram when the lens is brought to the second arrangement in the state of t2 = 1.2 mm. At this time, the spherical aberration is almost corrected. Further, FIG. 3 shows the wavefront when the light source is moved so that the spherical aberration is minimized at the thickness of each transparent substrate when the thickness of the transparent substrate is changed from 0.6 mm to 1.2 mm in Example 1. It is a change in the aberration RMS value. At this time, when the thickness of the transparent substrate is 0.6 mm, the numerical aperture is NA1 = 0.60, and the thickness of the transparent substrate is 1.2.
The numerical aperture for mm is NA2 = NA2 max = 0.595, which are all values around NA0.6. t = 1.2m
In m, the wavefront aberration RMS value is 0.015λ, which is an influence of high-order spherical aberration. However, this is a problem-free level.

Example 2 F = 3.33763632 First arrangement T1 = -29.470 U1 = 35.000 m1 = -0.1250 Second arrangement T2 = -20.819 U2 = 26.768 m2 =- 0.1839 Ri d1i d2i Ni 1 2.180 3.10 3.10 1.49446 2 -3.775 1.830 1.649 3 ∞ 0.60 1.2 1.58000 4 ∞ aspherical coefficient 1st surface κ = -5.56960 x 10 ^-1 A1 = −1.38700 × 10 ⁻³ P1 = 4.0000 A2 = −2.05900 × 10 ⁻⁴ P2 = 6.0000 A3 = −5.04330 × 10 ⁻⁶ P3 = 8.00000 A4 = −7. 19700 × 10 ⁻⁷ P4 = 1.0000 2nd surface κ = −1.27410 × 10 A1 = 2.53780 × 10 ⁻³ P1 = 4.0000 A2 = −1.91030 × 10 ⁻⁴ P2 = 6.00000 A3 = 1.24000 × 10 ⁻⁵ P3 = 8.00000 A4 = −2.74030 × 10 ⁻⁷ P4 = 10.00000

When Δd = T1−T2 = −8.651 G = −1.14646 NA1 = 0.6 NA2max = 0.596 NA2 = NA2max = 0.596 NA1 · | m1 | = 0.0750 NA2 · | M2 | = 0.1097 and | m1 | · F · NA1 ⁴ = 0.0547 | m2 | · F · NA2 ⁴ = 0.0784. When the numerical aperture NA2 is 0.53, NA2. | M2 | = 0.0975 | m2 | .F.NA2 ⁴ = 0.0490, and the numerical aperture NA2 is 0.45 in the second arrangement by changing the diaphragm diameter. Then, NA2 · | m2 | = 0.0828 | m2 | · F · NA2 ⁴ = 0.0255. When NA2 = 0.38, NA2 · | m2 | = 0.0699 | m2 | · F · NA2 ⁴ = 0.0129.

Example 3 F = 3.0107542 First Arrangement T1 = -45.217 U1 = 50.000 m1 = -0.07000 Second Arrangement T2 = -24.567 U2 = 29.764 m2 =- 0.13465 Ri d1i d2i Ni 1 1.911 2.800 2.800 1.49446 2 -3.469 1.383 1.197 3 ∞ 0.600 1.200 1.58000 4 ∞ Aspherical coefficient 1st surface κ = -6.04930 x 10 ^-1 A1 = 1.24640 × 10 ⁻⁴ P1 = 4.0000 A2 = −1.49370 × 10 ⁻⁴ P2 = 6.0000 A3 = 1.36210 × 10 ⁻⁶ P3 = 8.00000 A4 = −2.995790 × 10 ⁻⁷ P4 = 1.0000 2nd surface κ = −1.57360 × 10 A1 = 5.52940 × 10 ⁻³ P1 = 4.0000 A2 = −5.447660 × 10 ⁻⁴ P2 = 6.0000 A3 = 2.58450 × 10 ^-5 P3 = 8.00000 A4 = 1.12380 × 10 ^-6 P4 = 10.00000

Δd = T1−T2 = −20.650 G = −1.16969 NA1 = 0.6 NA2max = 0.596 NA2 = NA2max = 0.596 NA1 · | m1 | = 0.0420 NA2 · | M2 | = 0.0805 In addition, | m1 | .F.NA1 ⁴ = 0.0273 | m2 | .F.NA2 ⁴ = 0.0513. When the numerical aperture NA2 is 0.53, NA2. | M2 | = 0.0718 | m2 | .F.NA2 ⁴ = 0.0321, and the numerical aperture NA2 in the second arrangement is variable by changing the diaphragm diameter. Then, NA2 · | m2 | = 0.0608 | m2 | · F · NA2 ⁴ = 0.0167. The NA2 · When NA2 = 0.38 | m2 | = 0.0513 | a ^{· F · NA2 4 = 0.0085 |} m2.

Example 4 F = 3.10523331 First arrangement T1 = -24.956 U1 = 30.000 m1 = -0.14000 Second arrangement T2 = -17.998 U2 = 23.460 m2 =- 0.20400 Ri d1i d2i Ni 1 2.580 2.600 2.600 1.72623 2 -10.323 1.844 1.662 3 ∞ 0.600 1.200 1.58000 4 ∞ Aspheric surface coefficient 1st surface κ = -7.75620 x 10 ^-1 A1 = 9.49130 × 10 ⁻⁴ P1 = 4.0000 A2 = 3.31790 × 10 ⁻⁵ P2 = 6.0000 A3 = 4.71180 × 10 ⁻⁶ P3 = 8.0000 A4 = −3.448490 × 10 ^-6 P4 = 10.0000 2nd surface κ = -1.63440 x 10 A1 = 7.67570 x 10 ^-3 P1 = 4.0000 A2 = -1.52800 x 10 ^-3 P2 = 6.0000 A3 = 8 0.96720 × 10 ^-5 P3 = 8.00000 A4 = 1.76040 × 10 ^-6 P4 = 10.00000

Δd = T1 −T2 = −6.958 G = −1.1455 NA1 = 0.6 NA2max = 0.593 NA2 = NA2max = 0.593 NA1 · | m1 | = 0.0840 NA2 · | M2 | = 0.1210 and | m1 | .F.NA1 ⁴ = 0.0563 | m2 | .F.NA2 ⁴ = 0.0783. When the numerical aperture NA2 = 0.53, NA2 · | m2 | = 0.1081 | m2 | · F · NA2 ⁴ = 0.0500, and the numerical aperture NA2 in the second arrangement is variable with the diaphragm diameter variable. Then, NA2 · | m2 | = 0.0918 | m2 | · F · NA2 ⁴ = 0.0260. When NA2 = 0.38, NA2 · | m2 | = 0.0775 | m2 | · F · NA2 ⁴ = 0.0132.

[0031]

As described above, according to the present invention, an optical information recording medium having a different substrate thickness can be obtained by simply moving the light source.
Thus, an information pickup device and an optical disc device having a simple structure, a compact structure, and a low cost, capable of recording and reproducing with one information pickup device and having compatibility with a plurality of substrate thicknesses, were obtained. Furthermore, support for any thickness of the substrate,
It is possible to easily deal with the variation in the thickness of each substrate. It should be noted that although the working distance slightly changes as the divergence angle of the incident light to the objective lens changes, it is within the working range of the focusing actuator, and it is not necessary to consider this.

[Brief description of drawings]

FIG. 1 is an optical path diagram showing first and second arrangements of a first embodiment of the present invention.

FIG. 2 is a spherical aberration diagram of Example 1 of the present invention.

FIG. 3 is a change diagram of wavefront aberration when the substrate thickness changes from 0.6 mm to 1.2 mm in Example 1.

FIG. 4 is an optical path diagram showing the first and second arrangements of the second embodiment of the present invention.

FIG. 5 is a spherical aberration diagram of Example 2 of the present invention.

FIG. 6 is a diagram showing a change in wavefront aberration when the substrate thickness changes from 0.6 mm to 1.2 mm in Example 2.

FIG. 7 is an optical path diagram showing the first and second arrangements of the third embodiment of the present invention.

FIG. 8 is a spherical aberration diagram of Example 3 of the present invention.

FIG. 9 is a change diagram of the wavefront aberration when the substrate thickness changes from 0.6 mm to 1.2 mm in Example 3.

FIG. 10 is an optical path diagram showing the first and second arrangements of Example 4 of the present invention.

FIG. 11 is a spherical aberration diagram of Example 4 of the present invention.

FIG. 12 is a change diagram of the wavefront aberration when the substrate thickness changes from 0.6 mm to 1.2 mm in Example 4.

Claims (8)

[Claims] 1. A recording on an optical information recording medium having at least a laser light source and an objective lens having a positive refractive power for condensing a light beam emitted from the light source on an information recording surface through a transparent substrate of the optical information recording medium. In the reproducing optical system, the recording / reproducing optical system for an optical recording information medium, wherein the laser light source moves along the optical axis according to the thickness of the transparent substrate, and the following condition is satisfied. mt1> mt2 where mti: lateral magnification of the entire system from the laser light source to the recording surface of the optical information medium, and the thickness ti of the transparent substrate has a relationship of t1 <t2. 2. The optical recording / reproducing optics for an optical information medium according to claim 1, wherein both the light source side and the information recording surface side of the objective lens are aspherical surfaces, and the light source side surface is a convex surface. system. 3. -2.1 ≦ G ≦ −0.5 where G = Δt · (F−m1 · Δd) / (m1 ² · Δd · F)
^{· (Nt 2 -1) / nt} 3 where △ d = d2-d1 △ t = t2-t1 ti: thickness of the transparent substrate (t1 <t2) di: distance from the light source relative to the transparent substrate to the objective lens m1: Transparent 3. The optical information recording medium according to claim 2, wherein the following conditions are satisfied: lateral magnification of objective lens F: focal length of objective lens nt: refractive index of transparent substrate, where wavefront aberration is best when substrate thickness t1. Playback optical system. 4. When the numerical aperture of the objective lens focused on the information recording surface is NA1 when the transparent substrate has a first thickness and NA2 when the second thickness is thicker than the first thickness, The optical system for recording / reproducing an optical information medium according to claim 2, wherein NA2 <NA1 0.30 ≤ NA1 ≤ 0.65 0.30 ≤ NA2 ≤ 0.65 is satisfied. 5. The light according to claim 2, wherein NA2 <NA1 0.30 ≦ NA1 ≦ 0.65 0.50 ≦ NA2 ≦ 0.65 −1.5 ≦ G ≦ −0.80 is satisfied. Optical system for recording and reproducing information media. 6. The objective lens comprises: 0.035 ≦ NA1 · | m1 | ≦ 0.15 0.035 ≦ NA2 · | m2 | ≦ 0.15 m1 <0 m2 <0 where NA1: transparent substrate thickness t1 ( Numerical aperture at <t2) NA2: Numerical aperture at transparent substrate thickness t2 m1: Lateral magnification of objective lens at transparent substrate thickness t1 m2: Lateral magnification of objective lens at transparent substrate thickness t2 3. The recording / reproducing optical system for an optical information medium according to claim 2, wherein the condition (1) is satisfied. 7. The objective lens is: | m1 | · F · NA1 ⁴ ≦ 0.061 | m2 | · F · NA2 ⁴ ≦ 0.061 (unit:
mm) is satisfied, the recording / reproducing optical system of the optical information medium according to claim 2. 8. The objective lens is: | m1 | · F · NA1 ⁴ ≦ 0.045 | m2 | · F · NA2 ⁴ ≦ 0.045 (unit:
mm) is satisfied, the recording / reproducing optical system of the optical information medium according to claim 6.