JPS61141415A - Objective lens of optical information reader - Google Patents

Abstract

PURPOSE:To make possible good aberration correction, the reduction in size and weight and the extension of a working distance by constituting an objective lens of a convex meniscus lens having a single refractive spherical face and single refractive aspherical face in common use of the optical axis. CONSTITUTION:An objective lens 3 consists of the convex meniscus lens lens and the 1st refractive face S1 to which a laser beam is made incident is a spherical face. The 2nd refractive face S2 facing a recording disk 2 is an aspherical face. Since the one refractive face of the objective lens is the convex meniscus lens of the aspherical face, the thickness of the lens including the substrate to the recording disk is decreased and the reduction in the size and weight of the lens is made possible while the satisfactory aberration correction is executed. The increase in the working distance is also made possible.

Description

[Detailed description of the invention] Technical field The present invention relates to an objective lens for an optical information reading device.

BACKGROUND ART A large diameter aspherical lens as shown in FIG. 1, which is suitable for use as an objective lens of an optical information reading device such as an optical video disc player, is disclosed in Japanese Patent Application Laid-Open No. 57-76512. In FIG. 1, a laser beam emitted from a light source such as a laser tube (not shown) is refracted by an objective lens 1 and transmitted through a substrate 2α of a recording disk 2, and then converged onto a recording surface 2b to provide information. It becomes a spot light for detection. Then, the recorded information is read by detecting a change in the amount of transmitted light or reflected light from the recording surface 2b. In the objective lens 1, the first refractive surface S0 onto which the laser beam is incident is an aspherical surface expressed by an aspherical expansion formula as shown in the following equation.

・・・・・・・・・(1) Here, X is the distance s C1 from the tangential plane at the apex of the first refracting surface S1 of the point whose height is Y from the optical axis on the first refracting surface S0. is the curvature at the apex of the first refractive surface S, Y is the height from the optical axis, K is A, , A6. A is an aspheric coefficient.

Further, the second refractive surface S of the objective lens 1 facing the recording disk 2 is a hyperbolic surface expressed by an aspheric expansion formula as shown in the following equation.

Here, C2 is the curvature at the apex of the second refractive surface S, and K
, is the aspheric coefficient of the second refractive surface.

Now, the aspheric coefficient is + A4 + A6 + A@,
Km has, for example, a value shown in the following equation.

K engineering=-2,41688...(3)^=0.6
2875xlO・・・・・・(4)＾=-0,2183
8xlO...(5) A, = 0.67164xl
O...(6)K, =-149,999...
...(7) Also, the radius of curvature R (= 1/C) and the radius of curvature (=
1/CZ).

The distance between the vertices of the first and second refractive surfaces S, that is, the thickness d of the objective lens, the distance between the apex of the second refractive surface S, and the recording disk 2, that is, the working distance, and the distance of the objective lens 1. The refractive index L has, for example, the values shown in the following equations.

R, = 3.3710 C word] ...
・(8)R,=-14,768C") °°°°
°−(9), l□d=4.0 (
[Gift] ...α0WD= 1.400
[136) ・-・・-C1l) le,
=1.70214 (2
) Since a resolving power of approximately H1iooo lines/s+s is required, the numerical apertures N and A are approximately 0.45 to 0.5, and aberration correction is performed so that residual aberrations are kept within the diffraction limit.

In order to increase the working distance WD while performing good aberration correction in the conventional objective lens as described above, it is necessary to set the focal length f to a value that satisfies the condition shown in the following equation.

1,45 < □ < 1.65 ・・・・・・C
3 (Le, -i>f If the condition of formula 03 is not satisfied, it will not be possible to correct astigmatism. According to formula 03, υ makes the working distance WD a dog while achieving good aberration correction. To avoid this, the thickness d of the lens must be increased, making it difficult to reduce the size and weight.Also, in the conventional objective lens, both the first and second refractive surfaces S and S are non-uniform. The drawback was that it was difficult to create because it was spherical.

SUMMARY OF THE INVENTION An object of the present invention is to provide an objective lens for an optical information reading device that can perform good aberration correction, be small and lightweight, and at the same time can have a long working distance, and is easy to manufacture. .

The objective lens of the optical information reading device according to the present invention is composed of a convex meniscus lens having a single refracting spherical surface and a single refractive aspherical surface having a common optical axis. This will be explained in detail with reference to FIGS. 2 to 8.

In FIG. 2, an objective lens 3 and a recording disk 2 according to the invention are arranged in the same way as the objective lens 1 and the recording disk 2 in FIG. The objective lens 3 is a convex meniscus lens.
The refractive surface S1 is a spherical surface. Also, recording disk 2
The second refractive surface S2, which faces S2, is an aspherical surface expressed by an aspherical expansion formula as shown in the following equation.

....alpha.4 Here, s AIO is an aspheric coefficient similar to Km + A4 to A8.

Then, the conditions shown in the following equation are satisfied.

0.02<C5(FLl-1)f<0.2 -...
-031-0,001<Cs/ ((4+K)<0.
15...Equation (2)0119 shows the conditions for suppressing the occurrence of coma aberration. If the condition shown in equation (2) is not satisfied, the performance of the lens installation deteriorates because comatic aberration becomes a problem with respect to the inclination of the lens optical axis caused by an error or the like. Further, equation (to) indicates a sloppy condition for satisfactorily correcting higher-order spherical aberration. If the conditions shown in equation (a) are not met, it becomes necessary to set the higher-order aspherical coefficients to 100%, making it difficult to form the second refractive surface S2.Next, specific numerical examples are shown in the table below. show. In the table below, L is the refractive index of the substrate 2α, and t is the thickness of the substrate 2α.

In each of numerical examples 1 to 2, the resolution is on-axis and off-axis 0.750, as shown in FIGS. 3 to 5, respectively. 1.0° off-axis
In both cases, solid lines and dashed lines, respectively.

Good results are obtained as shown by the dashed line. Furthermore, in all of these numerical examples [--2], the spherical aberration is well corrected as shown in FIGS. 6 and 7, respectively.

Effects of the Invention As detailed above, the objective lens of the optical information reading device according to the present invention is a convex meniscus lens with one refractive surface being an aspherical surface, so that it can sufficiently correct aberrations including the substrate of the recording disk. This makes it possible to make the lens smaller and lighter, while also increasing the working distance. In addition, as is clear from Figures 3 to 5, a resolution of about 1000 lines per stroke can be obtained, and there is no decrease in resolution even for obliquely incident light, that is, off-axis light, which occurs due to mounting errors other than on-axis. It can be within the permissible range. In addition, in the objective lens according to the present invention, since the second refractive surface is an aspherical surface to correct spherical aberration, the curvature at the apex of the aspherical surface is thickened to about 20 to 30 degrees when the focal length is about 4.5 blinds. be able to. Therefore, it becomes easy to process an aspherical surface, and for example, the processing performance of a press die can be improved.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing a conventional objective lens, and FIG. 2 is a cross-sectional view showing a conventional objective lens.
The sectional views, FIGS. 3 to 5, showing one embodiment of the present invention are as follows:
Graph showing the resolution in each of numerical examples I to ■, No. 6
Figures 8 through 8 are graphs showing spherical aberrations in each of numerical examples 1 to (2).

Claims (1)

Claims: (1) An objective lens for an optical information reading device, comprising a convex meniscus lens having a single refractive spherical surface and a single refractive aspherical surface having a common optical axis. (2) The refractive aspherical surface has the following formula: : Curvature K at the apex of the refractive aspherical surface, A_4
, A_6, A_8, A_1_0: When constants ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ It is expressed by the aspheric expansion formula, where n: refractive index f: focal length C_1: curvature of the refracting sphere 0 .02<C_2(n-1)f<0.2 -0.001<C_2/(C_1+K)<0.15, the optical information reading device according to claim 1 objective lens.