JPH032812A - Objective lens for optical disk - Google Patents

Abstract

PURPOSE:To sufficiently correct the chromatic aberration even if a sudden fluctuation occurs in the laser oscillation wavelength by constituting this lens of two groups and four pieces of a 2-piece cemented lens formed by joining a negative lens and a positive lens from a light source side and a 2-piece cemented lens formed by a positive lens and a negative lens, and allowing them to satisfy specific conditions. CONSTITUTION:A first group is a 2-piece cemented lens formed by joining a negative lens 10 and a positive lens 12 successively from a light source side, and a second group is a 2-piece cemented lens formed by joining a positive lens 20 and a negative lens 22, and this lens is constituted of two groups and four pieces as a whole. In this state, when a composite focal distance of the whole system, a focal distance of a first group, a radius of curvature of an i-th lens surface from the light source side, and a refractive index and an Abbe number of a j-th lens from the light source side are denoted as f, f1, ri = (i = 1 - 6), and nj, nuj (j = 1 - 4), respectively, this lens is allowed to satisfy the conditions of the expression I. In such a way, a satisfactory spot can be obtained, and even if a sudden wavelength variation is generated in the light source, it does not occur that the spot becomes blurred due to defocus.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an objective lens used in an optical pickup for an optical disc.

[Prior Art] Since high-density information is handled when recording, reproducing, and erasing information on an optical disk, coherent monochromatic light such as laser light is generally used.

For this reason, conventional objective lenses for optical discs are only corrected for aberrations at a single wavelength, and sufficient consideration has not been given to chromatic aberrations.

[Invention tries to solve! I! i] Semiconductor lasers have recently come to be used as light sources for optical pickups, but semiconductor lasers have a rapid wavelength change due to so-called mode hopping, and when such a wavelength change occurs, the objective lens If chromatic aberration is not taken into consideration sufficiently, there is a problem that defocus will occur.

Wavelength changes due to mode hopping occur not only due to changes in environmental temperature, but also when switching the output of a semiconductor laser in conjunction with mode switching between recording or erasing mode and reproduction mode.

In optical pickups, focusing control is performed using voice coil actuators, etc., but when wavelength changes occur due to mode hopping, the control speed of focusing control cannot follow the defocus caused by wavelength changes.
This makes high-speed data recording and playback difficult.

Furthermore, a sufficient operating distance is required to avoid contact between the objective lens and the optical disk when the focusing control is pulled in or when a focus malfunction occurs.

The present invention has been made in view of the above-mentioned circumstances, and includes sufficient correction of chromatic aberration to enable good recording, erasing, and reproduction even when there is a sudden change in the laser oscillation wavelength.
Another object of the present invention is to provide a novel objective lens for optical discs that can realize a sufficiently long operating ratio n.

[Means to solve the problem] The present invention will be explained below.

As shown in FIG. 1, the objective lens for optical disks of the present invention has the first and second groups arranged in this order from the light source side (left side in FIG. 1) to the optical disk side (right side in the same figure). It will be arranged.

The first group includes, in order from the light source side, a negative lens 10, a positive lens I
The second group is a two-piece cemented lens formed by cementing a positive lens 20 and a negative lens 22 in order from the light source side. Therefore, the overall configuration is four elements in two groups.

The composite focal length of the entire system is f, the focal length of the first group is f1, and the radius of curvature of the i-th lens surface from the light source side is rl (i=
= i ~ 6), and the refractive index and Atsube number of the i-th lens from the light source side are respectively nJ + νj (j''1 ~ 4), then the objective lens of the present invention is (I) 0.9 (( nz/n+)”(f/
r*)<1-7, rx>0(IT) 1
5 < (,, -ν,) (III) 8
< (v, -94) (IV) 2. Of
< f L < 9.0f is satisfied.

Note that the refractive index n is for light with a wavelength of 830 nm, and the Abbe number ν is a value at the d-line. Further, in FIG. 1, reference numeral 30 denotes an optical disk, and its recording surface is formed on the right side flange surface of the optical disk 30 in the figure.

[Production use] The above conditions (I) to (IV) will be explained.

Condition (1) is a condition for correcting spherical aberration that occurs in large amounts on the third and sixth surfaces.If the lower limit of condition (I) is exceeded, both spherical aberration and chromatic aberration will be insufficiently corrected. If the upper limit is exceeded, spherical aberration will be overcorrected.

Conditions (II) and (III) are conditions for satisfactorily correcting monochromatic aberration, in order to offset the negative chromatic aberration occurring in the positive lens 12.20 with the positive chromatic aberration occurring in the negative lens 10.22. This is the condition. These conditions (II
) and (III), chromatic aberration correction becomes insufficient.

Condition (IV) is a condition for ensuring a sufficiently long operating distance by appropriately distributing the focal length of the first group with respect to the composite focal length of the entire system.

When the upper limit of condition (IV) is exceeded, spherical aberration and coma aberration become large, and when the lower limit is exceeded, the operating distance becomes small and there is a risk of contact between the optical disk and the objective lens.

[Example] Six specific examples are listed below.

As shown in FIG. 1, in each embodiment, rl(i=1
~6) is the radius of curvature of the i-th lens surface from the light source side,
di (i: 1 to 5) represents the i-th interplanar spacing from the light source side *njyνJ (j''1 to 4) is the refractive index (for wavelength 830 nm) of the i-th lens from the light source side It represents the Abbe number at the d-line, and V and D represent the distance from the final lens surface, that is, the sixth surface, to the optical disc.

The specifications of the optical disc are as follows.

Disk thickness dc: 0.2687 Refractive index nc = 1.57035 (for wavelength 830 nm) Abbe number ν. =30.8 (value at d-line) Furthermore, it is assumed that the light source exists at infinity.

Example 1 f=1.0. N, A, 0.50. W, D=0.4)05
i rl di jnn 'V
jl 1.8149 0.111! 1
1.82156 23.782 0.7351 0
．． 3333 2 1.76028 49.653 4
1.4431 0.0444 4 0.6380 (L2889 3 1.7
0247 51.935 -4.8180 0.11
11 4 1.82156 23.786 1.5
138 (nz/n+)/(f/r2)=1.41, L/f
=2.82 Example 2 f=1.0. N, A=0.50. V, D=0.3956
rs di j njvJ1, 882
7 0.1111 1 1.82158 23.7
80.6644 0.3778 2 1.7820
2 49.6520.5453 0.0222 0.6282 0.2889 3 1.70399
53.9310.1360 0.1111 4
1.82156 23.788 1.3140 (nz/nl)/(f/r2)=1.56, L/f
"2.73 Example 3 f=1.0.N-A=0.50.W, D=0.2992
i ridt J nJ vjl
1.7038 0.1333 1 1.82
156 23.782 0.9478 0.488
9 2 1.63143 60.153-14.93
29 0.0364 4 0.6453 0.3773 3 1.76
028 49.655 -1.6649 0.149
3 4 1.82156 23.786 1.18
53 (nt/n+)/(f/r2)1.18, f+/f
=3.07 Example 4 f=1. o, N, A=0.50. V, D=0.3954
r5 di j nj'S' s2.2
398 0.1111 1 1.8215B 2
3.780.7313 0.3267 2 1.7
3313 44.90-9.7304 0.0316 0.6402 0.3333 3 1.70399
53.93-5.2722 0.1333 4
1.82156 23.786 1.4889 (nz/n+)/(f/r2)=1.44, L/f
"3.18 Example 5 f=1.0.N, A=0.50.W, D=0.5338
i rldt j nj'S' jl
2.9104 0.1111 1 1.82
172 23.832 0.7324 0.488
9 2 1.56080 56.043 -2.75
89 0.0222 4 0.7627 0.3333 3 1.76
028 49.655 -9.1271 0.13:
13 4 1.821?2 23.836 4.4
860 (nt/n+)/(f/r2)=1.59, L/f
"7.34 Example 6 f=1.o, N, A=0.50.W, D=0.4705
i rs di j nJ
' Jl 1.6289 0.1111 1
1.82156 23.782 0.7207
0.3333 2 1.68672 55.483-
16.6867 0.0258 4 0.5987 0.2480 3 1.63
135 60.215 -46.3529 0.07
96 4 1.82158 23.786 1.
7196 (nz/ns)/(f/r2)=1.50, L/f
=2.78 Figures 2 to 7 show aberration diagrams corresponding to Examples 1 to 6 above. In each of the examples, chromatic aberration is sufficiently taken into consideration, so that the sharp changes in wavelength can be adequately dealt with. I can handle it.

[Effects of the Invention] As described above, according to the present invention, a novel objective lens for optical discs can be provided.

With this objective lens, it is possible to obtain a good spot narrowed down to approximately 1 μm or less, and the spot does not become blurred due to defocusing even if a sudden wavelength change occurs in the light source. High-speed playback processing is possible.

Furthermore, since a long operating distance can be obtained, there is little risk of contact with the optical disc even when focusing is pulled in or when a focusing malfunction occurs.

Furthermore, despite being super achromatic, the lens structure is simple, and assembly is easy and can be realized at low cost.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining the lens structure of the objective lens for an optical disk of the present invention, and FIGS. 2 to 7 are aberration diagrams regarding the side mold of the JIIJt embodiment, respectively.

Claims (1)

[Claims] A first group and a second group are arranged in this order from the light source side toward the optical disk side, and the first group is made up of a negative lens and a positive lens cemented in order from the light source side. The second group is a two-piece cemented lens consisting of a positive lens and a negative lens sequentially cemented from the light source side, and has a composition of four elements in two groups, with a composite focal length of the entire system. is f, the focal length of the first group is f_1,
The radius of curvature of the i-th lens surface from the light source side is r_i(i
= 1 to 6), and when the refractive index and Abbe number of the j-th lens from the light source are n_j and ν_j (j=1 to 4), respectively, (I)0.9<(n_2/n_1)・(f /r_2)
<1.7, r_2>0(II) 15<(ν_2-ν_1) (III) 8<(ν_3-ν_4) (IV) 2.0f<f_1<9.0f. , objective lens for optical discs.