JPH09179021A - Optical system and objective for recording and reproducing optical information recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable one optical pickup to record and reproduce optical disks which differ in substrate thickness, and compensate aberrations with respective objective lateral powers in consideration of off-axis characteristics (image height characteristics) and tilt characteristics when the power is varied. SOLUTION: In the optical system wherein the objective single body varies in power corresponding to the thickness of a transparent substrate, the objective meets a condition of 0.06>=SC(m1 :NA2 )/f>=0.002, where SC(m1 :NA2 ) is the sine unsatisfactory quantity of a light beam corresponding to the thickness t2 of the transparent substrate with the lateral power m1 corresponding to the thickness t1 to the transparent substrate, (f) the focal length of the objective, NA2 the numerical aperture of the objective with the power m2 , d2 the height of the light beam corresponding to NA2 from the optical axis at the front principal point of the objective with the power m1 , and u2 the angle of incidence of the light beam corresponding to the NA2 on the objective with the power m1 .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a recording / reproducing optical system for condensing a light beam such as a laser beam on an optical information recording medium to record or reproduce optical information, and an objective lens used in the optical system.

[0002]

2. Description of the Related Art A recording / reproducing optical system used in a recording / reproducing apparatus for an information recording medium such as an optical disk (in this specification, an optical system for performing only recording, an optical system for performing only reproducing, and both recording and reproducing are performed. The optical system is referred to as a recording / reproducing optical system.) In recent years, it is necessary to reduce the light spot condensed by the objective lens in order to cope with high density recording / reproducing on an information recording medium such as an optical disk. . Therefore, an objective lens with a large numerical aperture (NA) (for example, NA
0.6) is required. In addition, a high-density disk (DVD, etc.) having a substrate thickness of 0.6 mm has been put into practical use,
There is a demand for an optical system that can handle both conventional 2 mm discs (CD, CD-ROM, etc.). 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.
The wavelength of the laser light emitted from the laser light source is 63 in the objective lens with NA of 0.60 and magnification of -1/12.
When optimized under the conditions of 5 nm, substrate thickness of 0.6 mm, and substrate refractive index of 1.58, when the thickness of the substrate is changed,
For every 0.01 mm deviation, the aberration increases by about 0.01 λrms. Therefore, if the substrate thickness deviates by ± 0.07 mm, the aberration will be 0.07 λrms, and the Marechal limit value, which is a standard for normal reading, will be reached.

Therefore, various methods have been proposed in which the arrangement of the optical system is changed to change the magnification of the objective lens to correct the spherical aberration. These methods include changing the magnification of the objective lens by moving the lens that converts the divergence angle of the divergent light from the light source and the collimator (Fig. 25). Attaching and detaching the correction lens on the light source side of the objective lens Then, a method of changing the magnification of the objective lens alone (FIG. 26) A method of changing the magnification of the objective lens alone by placing a hologram on the light source side of the objective lens and converting the divergence angle using the first-order diffracted light beam emitted from the hologram. (FIG. 27) At least one surface of the objective lens has a hologram surface,
Method of changing the magnification of the objective lens by divergence angle conversion using the first-order diffracted light beam emitted from the hologram Method of changing the magnification of the objective lens by having two light sources with different wavelengths and switching the two light sources (Fig. 28) When this method is adopted, the occurrence of spherical aberration due to switching can be corrected by changing the optical path length from the two light sources to the objective lens. Etc.

[0004]

However, what is considered in these methods is the spherical aberration, that is, the spot on the axis, and the off-axis characteristic is not considered. If only the recording / reproducing of information is taken into consideration, it is only necessary to maintain good on-axis characteristics, but in tracking using a multi-divided light receiving element, the light receiving characteristics outside the optical axis becomes a problem, and due to manufacturing errors, etc. If the tilt occurs, the tilt characteristic becomes a problem. A change in characteristics due to image height and lens tilt is shown for an example of an objective lens in which the spherical aberration and the sine condition are best corrected at the first magnification. The objective lens is f = 3.360755 first magnification m ₁ = 0.0 NA _1: 0.60 transparent substrate thickness t ₁ = 0.6 second magnification _{m 2 = -0.055 NA 2: 0} . 38 is a design based on the specification of transparent substrate thickness t ₂ = 1.2, and an aberration diagram of its spherical aberration and sine conditions is shown in FIG. 29, an image height characteristic is shown in FIG. 30, and a lens tilt characteristic is shown in FIG. The special deterioration in the second magnification in FIG. 30 is mainly an effect because the sine condition shown in FIG. 29 is not satisfied. The present invention
Allows recording and reproduction of optical discs with different substrate thickness with one optical pickup, and when changing the magnification,
At each objective lens lateral magnification, off-axis characteristics (image height characteristics) and tilt characteristics are taken into consideration for aberration correction, and even if the lateral magnification of the objective lens is changed, the off-axis characteristics are balanced, The purpose is to maintain its performance.

[0005]

A recording / reproducing optical system according to the present invention comprises a light source, particularly a laser light source, and an objective for converging a light beam from the light source on an information recording surface via a transparent substrate of an optical information recording medium. Depending on the thickness of the lens and the transparent substrate of the optical information recording medium, the lateral magnification of the objective lens alone may be the first lateral magnification m.
_In an optical system including means for mutually changing between ₁ and the second lateral magnification m ₂ (m ₁ > m ₂ ), the ray of light corresponding to NA ₂ at the first lateral magnification m ₁ for the objective lens is Satisfaction amount of sine condition SC (m ₁ : NA ₂ ) SC (m ₁ : NA ₂ ) = d ₂ · cos (u ₂ ) / NA ₂ − (1-
It is characterized in that m ₁ ) · f satisfies the following condition. 0.06 ≧ SC (m ₁ : NA ₂ ) /f≧0.002 (1) f: focal length of the objective lens NA ₂ : numerical aperture of the objective lens at the second lateral magnification m ₂ d ₂ : Height of the ray corresponding to NA ₂ at the first lateral magnification m ₁ from the optical axis in the front principal plane of the objective lens u ₂ : Corresponding to NA ₂ at the _first lateral magnification m ₁ Incident Angle of Rays of Light to the Objective Lens The objective lens is a glass combination lens (FIG. 14), a plastic / glass hybrid lens (FIG. 15), an inhomogeneous refractive index lens (FIG. 16), a diffractive lens (FIG. 17), a hologram. A lens, a double-sided aspherical single lens, or the like can be used, but a double-sided aspherical plastic single lens is advantageous in terms of cost because it is a single lens that is particularly easy to process. The material forming this single lens aspherical single lens may be glass or resin. Further, it is desirable that 0.55 ≧ NA ₂ (2) 0.03 ≧ SC (m ₁ : NA ₂ ) / f (3) be satisfied.

On the other hand, the sine condition dissatisfaction amount SC (m ₁ : NA ₁ ) of the light beam corresponding to NA ₁ by the objective lens alone satisfies the following condition. 0.002 ≧ [SC (m ₁ : NA ₁ ) −SC (m ₁ : NA ₂ )] / f (4) SC (m ₁ : NA ₁ ) / f) ≧ −0.002 SC (m ₁₎ : NA ₁ ) = d ₁ · cos (u ₁ ) / NA ₁ − (1-
m ₁₎ · f NA _1: numerical aperture of the objective lens in the first lateral magnification m ₁ d _1: in the objective lens front principal plane of the light rays corresponding to NA ₁ in the first lateral magnification m ₁ Height from the optical axis of: u ₁ : incident angle of the ray corresponding to NA ₁ at the first lateral magnification m ₁ with respect to the objective lens. Further, it is desirable to satisfy the following conditions. [SC (m ₁ : NA ₁ ) -SC (m ₁ : NA ₂ )] / f ≧ −0.003 ... (5) −0.003 ≧ [SC (m ₁ : NA ₁ ) −SC (m ₁ : NA ₂ )] / f ≧ −0.012 (6) Further, the objective lens has a lateral magnification m ₁ in the first arrangement.
May be approximately 0.

The objective lens of the present invention preferably satisfies the following conditions. NA ₁ ≧ 0.50 (7) and / or 0.8> NA ₂ / NA ₁ (8)

[0008]

The variation ΔSAt of the spherical aberration with respect to the variation Δt of the substrate thickness is proportional to the same NA, and can be expressed as follows. Δt · (nt ² −1) / nt ³ · α = ΔSAt ... [1] where nt is the refractive index of the transparent substrate and α is a proportional constant.
On the other hand, the spherical aberration change amount ΔSAm due to the single-lens objective lens magnification change Δm can be considered to be substantially proportional. f · Δm · β = ΔSAm (2) where f is the focal length of the objective lens and β is a proportional constant. Therefore, to correct the spherical aberration as a whole, ΔSAt + ΔSAm = 0 ... [3] may be set. That is, Δt · (nt ² −1) / (nt ³ · f · Δm) = − β / α ... [4]

At this time, when nt is constant and Δt is positive in the equation [1], the spherical aberration moves in the over direction. Therefore, since 0 <ΔSAt and 1 <nt,
The constant α is positive. Further, in the equation [2], if the lateral magnification change Δm is positive (0 <Δm), the spherical aberration moves excessively. Therefore, since 0 <ΔSAm and 0 <f, the constant β is positive. As a result, Δt (= t ₂
If −t ₁ > 0) is positive, Δm is negative from the equation [4]. At this time, if the lateral magnification at which the spherical aberration on the transparent substrate t ₁ is best corrected is m ₁ and the magnification at which the spherical aberration on the transparent substrate t ₂ is best corrected is m ₂ , Δm = m ₂ -M ₁ ... [5], and as a result, m ₁ > m ₂ ... [6] holds.

In such an optical system, in order to satisfy the off-axis characteristics with one objective lens, it is necessary to satisfy the sine condition. SC at the unsatisfied amount of the sine condition of ray k at magnification m
(M: NAk) SC (m: NAk) = dk.cos (uk) / NAk- (1-m) .f [7] NAk = sin (uk ') [Matsui Yoshiya, "Lens Design Method" (Kyoritsu Shuppan: November 5, 1972, 1st edition, 1st edition), page 59 (3.47). Here, m is the lateral magnification of the lens, f is the focal length of the lens, uk is the angle of the light ray with respect to the optical axis incident on the lens, and uk 'is the angle with respect to the optical axis when the light ray exits the lens. Further, dk represents the height of the ray at the front principal point of the lens. If the light rays enter the lens as substantially parallel rays, then uk = 0, m = 0
And SC = dk / NAk-f ... [8].

To satisfy the sine condition, SC = 0 ...

[9] must be obtained. The thickness of the transparent substrate is actually t ₁
From t ₂ (t ₁ <t ₂ ) to correct the spherical aberration by changing the lateral magnification of the objective lens itself, the lateral magnification m of the objective lens at the thickness t ₁ of the transparent substrate is corrected.
₁ (first magnification), the lateral magnification m at the thickness t _2
2 (second magnification) is m ₁ > m ₂ from the equation [6]. At this time, the unsatisfied amount of the sine condition at the second magnification m ₂ is lower than the unsatisfied amount of the sine condition at the first magnification m ₁ . That is, SC (m ₁ : k)> SC (m ₂ : k) ... [10]. Therefore, if the sine condition at the first magnification is satisfied [SC (m ₁ : k) = 0], then at the second magnification, SC
(M ₂ : k) <0, and the sine condition is under.

In order to satisfy the off-axis characteristic at the second magnification m ₂ , the sine condition corresponding to the numerical aperture NA ₂ at the second magnification must be SC (m ₂ : NA ₂ ) = 0. Therefore, it is necessary to exceed the sine condition at the first magnification. Further, a light ray corresponding to the numerical aperture NA ₂ in the second magnification, since the height from the optical axis passing through the objective lens of the light rays corresponding to the numerical aperture NA ₂ in the first magnification is closer, the first sine condition at a magnification of NA ₂ unsatisfied amount SC: by setting appropriately the (m ₁ NA _2), the sine condition in the second magnification unsatisfied amount SC (m _2: NA ₂₎ the approximate be zero It is possible. Actually, the value SC obtained by normalizing the sine condition dissatisfaction amount with the focal length of the objective lens in the first arrangement
When (m ₁ : NA ₂ ) / f is smaller than 0.002, the sine condition remains under at the second magnification, and especially when NA ₂ is larger than 0.3, the performance of off-axis characteristics can be maintained. Disappear. Moreover, SC (m ₁ : NA ₂ ) / f is 0.06.
When it becomes larger, the sine condition becomes over at the second magnification as well, so overcorrection occurs and good performance cannot be maintained.

When NA ₂ is set to be smaller than 0.55, if the value is larger than the upper limit of the condition (3), the sine condition is overcorrected and overcorrected even at the second magnification, so that good off-axis characteristics cannot be maintained. Can not.

Numerical aperture NA to some extent at the first magnification
_{When 1} becomes large, the tilt angle characteristic (lens tilt characteristic) becomes severe even if the sine condition is satisfied. If the upper limit of conditional expression (4) is exceeded, the tilt characteristics will become severe. At the same time, if the lower limit of SC (m ₁ : NA ₁ ) / f) is exceeded, the image height characteristic at the first magnification cannot be maintained.

Further, in order to maintain the off-axis characteristics at the second magnification, the off-axis characteristics and the tilt angle characteristics at the first magnification become substantially the same as the off-axis characteristics and the tilt angle characteristics when the sine condition is substantially satisfied. Are conditional expressions (2) and (3)
It is only necessary to satisfy the conditional expression (5) while satisfying the above condition. Further, in order to maintain the off-axis characteristics at the second magnification and to make the effect slower than the inclination characteristics when the tilt characteristics substantially satisfy the sine condition at the first magnification, the condition (3) is set. It is sufficient to satisfy the condition (6) while satisfying.

When the numerical aperture of the first magnification is smaller than 0.5, the tolerance of each characteristic is widened, so that the off-axis characteristics may be compatible without satisfying the above conditions. But,
The above condition is particularly effective when the numerical aperture of the first magnification is 0.50 or more.

When the ratio NA ₂ / NA ₁ of the numerical aperture of the second magnification and the numerical aperture of the first magnification exceeds 0.8, the off-axis characteristics and the tilt characteristics are balanced among the respective magnifications. Then, it becomes difficult to balance at each magnification. In this way, aberration correction that takes off-axis characteristics (image height characteristics) and tilt characteristics into consideration is performed, and the performance is maintained by balancing the off-axis characteristics even when the lateral magnification of the objective lens is changed. The above method is not only feasible for objective lenses such as glass combination lenses, plastic / glass hybrid lenses, inhomogeneous refractive index lenses, diffractive lenses, hologram lenses, double-sided aspherical single lenses, etc. The present invention can also be implemented in the case of having two light sources different from each other and changing the magnification of the objective lens by switching the two light sources.

[0018]

Embodiments will be described below. In Examples 1 to 4, the transparent substrate thicknesses t ₁ = 0.6, t ₂ = 1.
2, the numerical aperture for this is NA ₁ = 0.6, NA ₂
= 0.38. As for the diaphragm position that limits the NA, the distance between the diaphragm position and the light source side surface of the objective lens is set to zero. In Example 5, the transparent substrate thickness t ₁ =
0.6, t ₂ = 1.2, the numerical aperture for this is N
A ₁ = 0.58 and NA ₂ = 0.55. In this embodiment, the diaphragm position is arranged at a position of 0.05 mm from the side of the transparent substrate of the objective lens in the direction of the transparent substrate, and the diaphragm diameter is the same in the first magnification and the second magnification. From the light source side, the radius of curvature of the i-th surface is ri, the thickness on the optical axis between the i-th surface and the (i + 1) -th surface, and the spacing is d ₁ i in the _first arrangement and the second arrangement. , D ₂ i, and the refractive index at the light source wavelength of the medium between the i-th surface and the (i + 1) -th surface is represented by ni. Further, the refractive index of air is 1. Further, regarding the aspherical shape of the lens surface, κ is a conical coefficient, Ai is an aspherical coefficient, and Pi is an exponent of an aspherical surface in an orthogonal coordinate system with the vertex of the surface as the origin and the optical axis direction as the X axis. When

[Equation 1] It is represented by

Example 1 In this example, the first magnification and the second magnification are balanced in off-axis characteristics. f = 3.360755 first magnification m ₁ = 0.0 NA _1: 0.60 stop diameter phi _1: 4.050 second magnification m ₂ = -0.0635 NA _2: 0.38 stop diameter phi ₂ : 2.684 i ri d ₁ i d ₂ i ni 1 2.065 2.60 2.60 1.49810 2 -5.140 1.57 1.408 3 ∞ 0.60 1.2 1.58000 4 ∞ Aspherical surface data First surface κ = −8.48180 × 10 ⁻¹ A ₁ = 5.09770 × 10 ⁻³ P ₁ = 4.0000 A ₂ = 4.12 210 × 10 ⁻⁴ P ₂ = 6.0000 A ₃ = 2.17950 × 10 ⁻⁵ P ₃ = 8.0000 A ₄ = −5.86930 × 10 ⁻⁶ P ₄ = 1.0000 2nd surface κ = −1.00300 × 10 A ₁ = 2.23590 × 10 ⁻² P ₁ = 4.0000 A ₂ = −8.009280 × 10 ⁻³ P ₂ = 6.0000 A ₃ = 1.56230 × 10 ⁻³ P ₃ = 8.0000 A ₄ = −1.30870 × 10 ⁻⁴ P ₄ = 1.0000 The optical layout diagram at each magnification. FIG. 1 shows the spherical aberration and the aberration diagram of the sine condition in FIG. 2, the image height characteristic in FIG. 3, and the lens tilt characteristic in FIG. NA ₁ at the first magnification,
The sine condition dissatisfaction amount for NA ₂ is SC (m ₁ = 0: NA ₁ = 0.60) /f=0.038 SC (m ₁ = 0: NA ₂ = 0.38) /f=0.0052 At the first magnification, both the off-axis characteristic and the tilt characteristic are at the same level as the conventional example in which the sine condition is satisfied at each NA at the first magnification. The off-axis characteristic at the second magnification is improved by about half in the conventional example. Regarding the tilt characteristics, the change is larger in the second embodiment at the second magnification, but the objective lens itself is the same at the first magnification and the second magnification, and therefore when the first magnification is used at the time of adjustment. If the tilt angle is suppressed, the effect of wavefront aberration due to the tilt angle at the second magnification is smaller than that at the first magnification, so that the tilt characteristic at the second magnification does not pose a problem.

Example 2 f = 3.0679968 First magnification m ₁ = 0.0 NA ₁ : 0.60 Aperture diameter φ ₁ : 3.700 Second magnification m ₂ = -0.0606 NA ₂ : 0 .38 aperture diameter φ ₂ : 2.439 i ri d ₁ i d ₂ i ni 1 1.929 2.60 2.60 1.49810 2 -4.058 1.313 1.120 3 ∞ 0.60 1. 2 1.58000 4 ∞ Aspherical surface data First surface κ = −5.58980 × 10 ⁻¹ A ₁ = 1.18930 × 10 ⁻³ P ₁ = 4.0000 A ₂ = −1.93120 × 10 ⁻⁴ P ₂ = 6.0000 A ₃ = 4.04970 × 10 ⁻⁶ P ₃ = 8.00000 A ₄ = −2.69980 × 10 ⁻⁵ P ₄ = 1.0000 2nd surface κ = −2.41900 × 10 A ₁ = 6.61370 × 10 ⁻³ P ₁ = 4.0000 A ₂ = −4.25130 × 10 ^-3 P ₂ = 6.0000 A ₃ = 7.226420 × 10 ^-4 P ₃ = 8.00000 A ₄ = -3.99040 × 10 ^-6 P ₄ = 1.0000 This example is also an example. Similar to No. 1, this is an example in which the off-axis characteristics at the first magnification and the second magnification are balanced.
FIG. 5 is an aberration diagram showing the spherical aberration and the unsatisfactory amount of the sine condition at each magnification. The sine condition dissatisfaction amount for NA ₁ and NA ₂ at the first magnification is: SC (m ₁ = 0: NA ₁ = 0.60) /f=0.0047 SC (m ₁ = 0: NA ₂ = 0 .38) /f=0.0042 The wavefront aberration change diagram showing the off-axis characteristic (image height characteristic) of this embodiment is shown in FIG. 6, and the wavefront aberration change diagram showing the tilt angle characteristic is shown in FIG.
Shown in

Example 3 f = 3.360755 First magnification m ₁ = 0.0 NA ₁ : 0.60 Aperture diameter φ ₁ : 4.034 Second magnification m ₂ = −0.0597 NA ₂ : 0 .38 stop diameter _{φ 2: 2.676 i ri d 1} i d 2 i ni 1 2.080 2.70 2.70 1.49810 2 -4.875 1.53 1.351 3 ∞ 0.60 1. 2 1.58000 4 ∞ Aspheric surface data First surface κ = −8.994520 × 10 ⁻¹ A ₁ = 6.339060 × 10 ^-3 P ₁ = 4.0000 A ₂ = 3.16090 × 10 ^-4 P ₂ = 6.00000 A ₃ = 1.76490 × 10 ^-5 P ₃ = 8.00000 A ₄ = -1.58940 x 10 ^-5 P ₄ = 1.0000 2nd surface κ = -2.03250 x 10 A ₁ = 1.98900 x 10 ^-2 P ₁ = 4.0000 A ₂ = -1.11890 x 10 ⁻² P ₂ = 6.00000 A ₃ = 2.69710 × 10 ⁻³ P ₃ = 8.00000 A ₄ = −2.64920 × 10 ⁻⁴ P ₄ = 1.0000 This example is the first This is an example in which the tilt characteristic is suppressed by the magnification and the off-axis characteristic is suppressed by the second magnification. FIG. 8 is an aberration diagram showing the spherical aberration at each magnification and the unsatisfactory amount of the sine condition. The sine condition dissatisfaction amount for NA ₁ and NA ₂ at the first magnification is SC (m ₁ = 0: NA ₁ = 0.60) /f=0.0000 SC (m ₁ = 0: NA ₂ = 0 .38) /f=0.0078 A wavefront aberration change diagram showing the off-axis characteristic of this example is shown in FIG.
FIG. 10 shows a wavefront aberration change diagram showing tilt characteristics.
At the first magnification, the tilt characteristics satisfy the sine condition at each NA at the first magnification with the same specifications (SA = 0).
Compared to the conventional example, it is suppressed to about half. Although the off-axis characteristics have changed considerably, if the objective lens is infinitely arranged, tracking does not cause any problem because it has no image height. The off-axis characteristic at the second magnification is improved to about half that of the conventional example. Regarding the tilt characteristics at the second magnification,
Although the change is larger in this example, the objective lens itself is actually the same in both the first magnification and the second magnification, so if the tilt angle at the first magnification is suppressed during adjustment, If the effect of the wavefront aberration due to the tilt angle at the second magnification is smaller than that at the first magnification, the tilt angle characteristic at the second magnification has no problem.

Example 4 f = 3.3607550 First magnification m ₁ = 0.0 NA ₁ : 0.60 Aperture diameter φ ₁ : 4.116 Second magnification m ₂ = -0.0742 NA ₂ : 0 .38 Aperture diameter φ ₂ : 2.706 i ri d ₁ i d ₂ i ni 1 2.138 2.70 2.70 1.49810 2 -4.473 1.57 1.43 3 ∞ 0.60 1. 2 1.58000 4 ∞ Aspherical surface data 1st surface κ = −4.88730 × 10 ⁻¹ A ₁ = −5.09640 × 10 ⁻⁴ P ₁ = 4.0000 A ₂ = −2.71610 × 10 ⁻⁴ P ₂ = 6.0000 A ₃ = −9.27400 × 10 ⁻⁶ P ₃ = 8. 0000 A ₄ = −2.53900 × 10 ⁻⁶ P ₄ = 1.0000 2nd surface κ = −2.24560 × 10 A ₁ = 7.73040 × 10 ⁻⁴ P ₁ = 4.0000 A ₂ = 4. 12380 × 10 ⁻⁴ P ₂ = 6.00000 A ₃ = −8.5430 ⁵ × 10 ⁻⁵ P ₃ = 8.00000 A ₄ = 8.771830 × 10 ⁻⁶ P ₄ = 1.0000 This is an example in which off-axis characteristics are suppressed by a second magnification. FIG. 11 is an aberration diagram showing the spherical aberration and the unsatisfactory amount of the sine condition at each magnification. NA ₁ and N at the first magnification
The sine condition dissatisfaction amount for A ₂ is SC (m ₁ = 0: NA ₁ = 0.60) /f=0.0212 SC (m ₁ = 0: NA ₂ = 0.38) /f=0.0065 Is. FIG. 12 is a wavefront aberration change diagram showing the off-axis characteristic of this example, and FIG. 13 is a wavefront aberration change diagram showing the tilt angle characteristic.
Shown in

Example 5 f = 3.3607544 First magnification m ₁ = 0.0 NA ₁ : 0.58 Aperture diameter φ ₁ : 2.71 Second magnification m ₂ = -0.0561 NA ₂ : 0 .55 aperture diameter φ ₂ : 2.71 i ri d ₁ i d ₂ i ni 1 2.130 2.6 2.6 1.49810 2 -4.644 0.05 0.05 aperture ∞ 1.567 1. 376 3 ∞ 0.60 1.20 1.58000 4 ∞ Aspheric surface data First surface κ = −5.97500 × 10 ⁻¹ A ₁ = 5.25390 × 10 ^-3 P ₁ = 4.0000 A ₂ = -1.95480 × 10 ^-3 P ₂ = 6.0000 A ₃ = 5.76590 × 10 ^-4 P ₃ = 8.0000 A ₄ = −6.332820 × 10 ⁻⁶ P ₄ = 1.0000 2nd surface κ = −6.35750 × 10 A ₁ = −1.31660 × 10 ⁻² P ₁ = 4.0000 A ₂ = 1.05930 18 × 10 ⁻² P ₂ = 6.0000 A ₃ = −3.46010 × 10 ⁻³ P ₃ = 8.00000 A ₄ = 3.94280 × 10 ⁻⁴ P ₄ = 1.0000 This example is shown in FIG. As described above, in both the first magnification and the second magnification, a diaphragm having the same diaphragm diameter is arranged on the transparent substrate side of the objective lens, the numerical aperture at the first magnification is 0.58, and the second magnification is This is an example when the numerical aperture in is as large as 0.55. FIG. 19 is an aberration diagram showing spherical aberration and sine condition dissatisfaction amount at each magnification. The sine condition dissatisfaction amount for NA ₁ and NA ₂ at the first magnification is SC (m ₁ = 0: NA ₁ = 0.58) /f=0.0172 SC (m ₁ = 0: NA ₂ = 0 .55) /f=0.179. FIG. 20 shows a wavefront aberration change diagram showing the off-axis characteristic (image height characteristic) of this embodiment, and FIG. 21 shows a wavefront aberration change diagram showing the tilt angle characteristic.

Here, Example 5 is compared with the conventional lens in which the sine condition is satisfied at the first magnification. As this conventional example, a lens having the same design as that described in the section “Problems to be solved by the invention” is used, the focal length is almost the same as that of the fifth embodiment, and the lateral magnification m ₁ of the first objective lens alone is used. Is 0 as in the fifth embodiment. As in the case of Example 5, the diaphragm position was changed from the transparent substrate side surface of the objective lens to 0.
The positions are separated by 05, and the aperture diameter is set so that the numerical aperture NA ₁ = 0.58 at the first magnification. Also,
The second lateral magnification m ₂ = −0.0558 and NA ₂ =
It becomes 0.55. FIG. 22 shows an aberration diagram showing the spherical aberration and the amount of unsatisfaction of the sine condition at each magnification of this conventional example, FIG. 23 shows the off-axis characteristic of this conventional example, and FIG. 24 shows the tilt angle characteristic. Regarding the off-axis characteristics, the above-mentioned conventional example and Example 5 are compared. At the first magnification, the change in wavefront aberration depending on the image height is smaller in the conventional example than in the fifth example of the present invention. However, since the first magnification is m ₁ = 0 and infinite light is incident, no image height occurs even if tracking is performed. Therefore, if the initial adjustment is sufficient, this difference in off-axis characteristics does not pose a problem. The change in wavefront aberration depending on the image height at the second magnification is smaller in Example 5 of the present invention than in the conventional example. Assuming that the tracking amount is 0.5 mm, this tracking amount corresponds to an image height of about 0.028 from the value of the magnification m ₂ . Conventionally, the wavefront aberration is larger than the Marechal limit at this image height, but in Example 5 it is almost within the Marechal limit.

[0025]

According to the present invention, it is possible to record / reproduce optical disks having different substrate thicknesses with a single optical pickup, and to change off-axis characteristics (image height characteristics) at each lateral magnification of the objective lens when the magnification is changed. ), The aberration was corrected in consideration of the tilt characteristic, and even if the lateral magnification of the objective lens was changed, the performance could be maintained by balancing the off-axis characteristic.

[Brief description of the drawings]

FIG. 1 is an optical information recording medium having a substrate thickness of 0.6 mm.
It is an optical-path figure of Example 1 of the objective lens at the time of 2 mm.

FIG. 2 is an aberration curve diagram of the objective lens of Example 1 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 3 is a wavefront aberration diagram showing the image height characteristics of Example 1 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 4 is a wavefront aberration diagram showing lens tilt characteristics of Example 2 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 5 is an aberration curve diagram of the objective lens of Example 2 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 6 is a wavefront aberration diagram showing the image height characteristics of Example 2 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 7 is a wavefront aberration diagram showing lens tilt characteristics of Example 2 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 8 is an aberration curve diagram of the objective lens of Example 3 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 9 is a wavefront aberration diagram showing image height characteristics of Example 3 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 10 is a wavefront aberration diagram showing lens tilt characteristics of Example 3 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 11 is a substrate thickness of the optical information recording medium of 0.6 mm.
FIG. 7 is an aberration curve diagram of the objective lens of Example 4 at 1.2 mm and 1.2 mm.

FIG. 12 is a wavefront aberration diagram showing the image height characteristics of Example 4 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 13 is a wavefront aberration diagram showing lens tilt characteristics of Example 4 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 14 is a lens cross-sectional view showing an example of the configuration of a glass combination objective lens.

FIG. 15 is a cross-sectional view showing an example of the configuration of a hybrid objective lens of plastic and glass.

FIG. 16 is a cross-sectional view showing an example of the configuration of a non-uniform refractive index objective lens.

FIG. 17 is a sectional view showing an example of the configuration of an objective lens in which one surface of the lens is a diffractive surface.

FIG. 18 is a layout diagram of diaphragms at a first magnification and a second magnification in the objective lens of Example 5;

FIG. 19 is a substrate thickness of the optical information recording medium of 0.6 mm.
FIG. 6 is an aberration curve diagram of the objective lens of Example 5 at 1.2 mm, and 1.2 mm.

FIG. 20 is a wavefront aberration diagram showing image height characteristics of Example 5 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 21 is a wavefront aberration diagram showing lens tilt characteristics of Example 5 when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 22 is a substrate thickness of the optical information recording medium of 0.6 mm.
FIG. 7 is an aberration curve diagram of a conventional objective lens at 1.2 mm when 1.2 mm.

FIG. 23 is a wavefront aberration diagram showing image height characteristics of a conventional example when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 24 is a wavefront aberration diagram showing a lens tilt characteristic of a conventional example when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 25 is a conceptual diagram showing an example of a method of changing the magnification of an objective lens by moving a lens that constitutes an optical system.

FIG. 26 is a conceptual diagram showing an example of a method of changing the magnification of an objective lens by attaching / detaching a correction lens.

FIG. 27 is a conceptual diagram showing an example of a method of changing the magnification of an objective lens by using a hologram.

FIG. 28 is a conceptual diagram showing an example of a method of changing the magnification of an objective lens by switching the wavelength of a light source.

FIG. 29 is a substrate thickness of the optical information recording medium of 0.6 mm.
FIG. 7 is an aberration curve diagram of a conventional objective lens at 1.2 mm when 1.2 mm.

FIG. 30 is a wavefront aberration diagram showing image height characteristics of a conventional example when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

FIG. 31 is a wavefront aberration diagram showing lens tilt characteristics of a conventional example when the substrate thickness of the optical information recording medium is 0.6 mm and 1.2 mm.

Claims (23)

[Claims] 1. A light source, an objective lens for focusing a light flux from the light source on an information recording surface via a transparent substrate of an optical information recording medium, and the objective according to the thickness of the transparent substrate of the optical information recording medium. An optical system including means for mutually changing the lateral magnification of a single lens into a first lateral magnification m ₁ and a second lateral magnification m ₂ (m ₁ > m ₂ ). The sine condition dissatisfaction amount SC (m ₁ : for the ray corresponding to NA ₂ at the _first lateral magnification m ₁
NA ₂ ) SC (m ₁ : NA ₂ ) = d ₂ · cos (u ₂ ) / NA ₂ − (1-
An optical system for recording / reproducing an optical information recording medium, characterized in that m ₁ ) · f satisfies the following conditions. _{0.06 ≧ SC (m 1: NA} 2) /f≧0.002 f: focal length of the objective lens NA _2: the second lateral magnification numerical aperture of the objective lens in m ₂ d _2: the first Height of the ray corresponding to NA ₂ at the lateral magnification m ₁ from the optical axis in the front principal plane of the objective lens: u ₂ The objective lens of the ray corresponding to NA ₂ at the first lateral magnification m ₁ Incident angle to 2. The recording / reproducing optical system for an optical information recording medium according to claim 1, wherein the objective lens is a double-sided aspherical single lens. 3. The first lateral magnification m ₁ , NA ₂ and the sine condition dissatisfaction amount SC (m ₁ : NA ₂ ) satisfy the following conditions: Optical system for recording / reproducing optical information recording medium. 0.55 ≧ NA ₂ 0.03 ≧ SC (m ₁ : NA ₂ ) / f ≧ 0.002 4. The sine condition dissatisfaction amount SC (m ₁ : NA ₁ ) of the ray corresponding to NA ₁ at the first lateral magnification m ₁ and the sine condition dissatisfaction amount SC (m ₁ : NA ₂ ) are as follows: The optical system for recording / reproducing an optical information recording medium according to claim 1 or 2, characterized in that: 0.002 ≧ [SC (m ₁ : NA ₁ ) −SC (m ₁ : N
A ₂ )] / f SC (m ₁ : NA ₁ ) / f ≧ −0.002 SC (m ₁ : NA ₁ ) = d ₁ · cos (u ₁ ) / NA ₁ − (1-
m ₁₎ · f NA _1: numerical aperture of the objective lens in the first lateral magnification m ₁ d _1: in the objective lens front principal plane of the light rays corresponding to NA ₁ in the first lateral magnification m ₁ height u ₁ from the optical axis: angle of incidence with respect to the objective lens of the light rays corresponding to NA ₁ in the first lateral magnification m ₁ 5. The recording / reproducing optical system for an optical information recording medium according to claim 4, wherein the following condition is satisfied. 0.002 ≧ [SC (m ₁ : NA ₁ ) −SC (m ₁ : N
A ₂ )] / f ≧ −0.003 6. The recording / reproducing optical system for an optical information recording medium according to claim 4, wherein the following condition is satisfied. −0.003 ≧ [SC (m ₁ : NA ₁ ) −SC (m ₁ : NA
₂ )] / f ≧ −0.012 7. The optical system for recording / reproducing an optical information recording medium according to claim 1, wherein the objective lens has a lateral magnification m _{1 of} substantially 0 in the first arrangement. 8. The recording / reproducing optical system for an optical information recording medium according to claim 1, wherein the following condition is satisfied. NA ₁ ≥ 0.50 9. The recording / reproducing optical system for an optical information recording medium according to claim 1, wherein the following condition is satisfied. NA ₂ / NA ₁ <0.8 10. The lateral magnification changing unit of the objective lens alone is a unit for moving a divergence conversion lens arranged on the light source side of the objective lens in the optical axis direction. Recording / reproducing optical system for optical information recording media. 11. The optical information recording medium according to claim 1, wherein the lateral magnification changing means of the objective lens alone is an inserting / removing means of a correction lens arranged on the light source side of the objective lens. Recording / playback optical system. 12. The lateral magnification changing means of the objective lens alone is a means using a diffracted light beam emitted from a hologram arranged on the light source side of the objective lens. Optical system for recording / reproducing optical information recording medium. 13. The one surface of the objective lens is a hologram surface, and the lateral magnification changing means of the objective lens alone is a means using diffracted light emitted from the hologram surface. Recording / reproducing optical system for optical information recording media. 14. The light source includes two light sources having different wavelengths, and the lateral magnification changing means of the objective lens alone is the above-mentioned 2
The recording / reproducing optical system for an optical information recording medium according to claim 1, which is a switching means of two light sources. 15. When the light flux from the light source is focused on the information recording surface of the first optical information recording medium via the transparent substrate having the first thickness, the first lateral magnification m ₁ and the first lateral magnification m _{1 are set} . It has an aperture NA ₁ of the case for collecting light through a transparent substrate having a second thickness of the light beam to the second optical information recording medium of the information recording surface from the light source is a second lateral magnification a objective lens having m ₂ and a second numerical aperture NA _2, a m _1> m ₂ and NA _1> NA _2, corresponding to NA ₂ in the first lateral magnification m ₁ of the objective lens Satisfaction amount of sine condition SC (m ₁ :
NA ₂ ) SC (m ₁ : NA ₂ ) = d ₂ · cos (u ₂ ) / NA ₂ − (1-
An objective lens for recording / reproducing of an optical information recording medium, characterized in that m ₁ ) · f satisfies the following conditions. _{0.06 ≧ SC (m 1: NA} 2) /f≧0.002 f: focal length of the objective lens NA _2: the second lateral magnification numerical aperture of the objective lens in m ₂ d _2: the first Height of the ray corresponding to NA ₂ at the lateral magnification m ₁ from the optical axis in the front principal plane of the objective lens: u ₂ The objective lens of the ray corresponding to NA ₂ at the first lateral magnification m ₁ Incident angle to 16. The objective lens for recording / reproducing an optical information recording medium according to claim 15, wherein the objective lens is a double-sided aspherical single lens. 17. The first lateral magnification m ₁ , NA ₂ and the sine condition dissatisfaction amount SC (m ₁ : NA ₂ ) satisfy the following conditions: Objective lens for recording and reproducing of optical information recording medium. 0.55 ≧ NA ₂ 0.03 ≧ SC (m ₁ : NA ₂ ) / f ≧ 0.002 18. A sine condition dissatisfaction amount SC (m ₁ : NA ₁ ) of a ray corresponding to NA ₁ at a _first lateral magnification m _{1 and} a sine condition dissatisfaction amount SC of a ray corresponding to said NA ₂
The objective lens for recording / reproducing of an optical information recording medium according to claim 15 or 16, wherein (m ₁ : NA ₂ ) satisfies the following condition. 0.002 ≧ [SC (m ₁ : NA ₁ ) −SC (m ₁ : N
A ₂ )] / f SC (m ₁ : NA ₁ ) / f ≧ −0.002 SC (m ₁ : NA ₁ ) = d ₁ · cos (u ₁ ) / NA ₁ − (1-
m ₁₎ · f NA _1: numerical aperture of the objective lens in the first lateral magnification m ₁ d _1: in the objective lens front principal plane of the light rays corresponding to NA ₁ in the first lateral magnification m ₁ height u ₁ from the optical axis: angle of incidence with respect to the objective lens of the light rays corresponding to NA ₁ in the first lateral magnification m ₁ 19. The objective lens for recording / reproducing of an optical information recording medium according to claim 18, which satisfies the following condition. 0.002 ≧ [SC (m ₁ : NA ₁ ) −SC (m ₁ : N
A ₂ )] / f ≧ −0.003 20. The objective lens for recording / reproducing of an optical information recording medium according to claim 18, which satisfies the following condition. −0.003 ≧ [SC (m ₁ : NA ₁ ) −SC (m ₁ : NA
₂ )] / f ≧ −0.012 21. The objective lens according to claim 1, wherein the lateral magnification m ₁ in the first arrangement is substantially zero.
The objective lens for recording / reproducing of the optical information recording medium according to claim 5 or 16. 22. The objective lens for recording / reproducing of an optical information recording medium according to claim 15 or 16, which satisfies the following condition. NA ₁ ≥ 0.50 23. The objective lens for recording / reproducing of an optical information recording medium according to claim 15 or 16, which satisfies the following condition. 0.8> NA ₂ / NA ₁