JPS61179409A - Objective of optical information reader - Google Patents

Abstract

PURPOSE:To improve aberration compensation and size and weight reduction by providing an objective with two refracting aspherical surfaces which have a common optical axis and expressed by specific aspherical surface expansion equations, and satisfying specific condition. CONSTITUTION:The objective 3 has the two refracting aspherical surfaces which have the common optical axis and its 1st refracting surface S1 on which a laser beam is incident is an elliptic surface expressed by an aspherical expansion equation shown by I when A10 is an aspherical surface coefficient. The 2nd refracting surface S2 facing a recording disk 2 is an aspherical surface expressed by an aspherical surface expansion equation II. Then, the condition shown by an inequality III is satisfied. The inequality III shows the condition for suppressing the generation of coma aberration and the condition for simplify the form of the 2nd refracting surface S2 by decreasing the aspherical surface coefficient of high degree of the 2nd refracting surface S2. The condition shown by the inequality III is satisfied to suppress the generation of a spherical aberration of high degree. In said equations and inequal ity, K1, A4, A6 and A8 are coefficients of aspherical surfaces and X is the distance of the peak point of the (n)th refracting surface Sn at height Y from the optical axis on the (n)th refracting surface Sn from a contact plane; and Y is the height from the optical axis and Cn is the curvature at the peak point of the (n)th refracting surface Sn.

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, passes through a substrate 2α of a recording disk 2, and is then focused onto a recording surface 2b. It becomes a spot light for information 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 from the tangent plane at the apex of the first refracting surface S0 of the point whose height from the optical axis is Y on the first refractive surface S0,
C is the curvature at the apex of the first refractive surface S0, Y is the height from the optical axis, K, , 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, C8 is the curvature at the apex of the second refractive surface S2, and K
2 is the aspheric coefficient of the second refractive surface.

Now, for the aspherical coefficients, ,A, ,A6. A8. for,
For example, the values are shown in the following equations.

K, = −2,41688 (3) A
,=0.62875X10'...
...(4) A6=-0,21838X10-"
・・・・・・・・・(5) A8= 0.67
164X10-' ・・・・・・・・・(6)
K, = -149,999 (7) Also, the radius of curvature r, (=1/C,) at each vertex of the first and second refractive surfaces S and S, r21 (= t/Cs
).

The distance between the vertices of the first and second refractive surfaces S0 and 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 WD, and the refraction of the objective lens 1 For example, the ratio rL1 is calculated using the following formula: % Since a resolving power of about 1000 lines/wn is required, the numerical apertures N and A are about 0.45 to 0.5, and the residual aberrations are set to the diffraction limit. Aberrations have been corrected to keep it within the range.

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 increase the focal length f while satisfying the condition shown in the following equation.

If the condition of the α3 formula is not satisfied, astigmatism cannot be corrected. With this type 13, in order to increase the working distance WD while achieving good aberration correction, the thickness d of the lens must be made small, which makes it difficult to reduce the size and weight. Further, in the conventional objective lens, the first and second refractive surfaces S and S are both aspherical, making it difficult to fabricate them.

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. It is.

The objective lens of the optical information reading device according to the present invention has two refractive aspherical surfaces having a common optical axis, and these two refractive aspherical surfaces have a diameter of 80°. When is an aspherical coefficient, it is expressed by two aspherical expansion formulas, and is formed to satisfy the following.

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS. 2 to 5.

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 first refractive surface S0 of the objective lens 3 on which the laser beam is incident is an elliptical surface expressed by an aspherical expansion formula as shown in equation I. Further, the second refractive surface S facing the recording disk 2 is an aspherical surface expressed by an aspherical surface expansion formula as shown in equation (2). Then, the condition shown in equation (g) is satisfied.

Equation (G) expresses the conditions for suppressing the occurrence of coma aberration and the conditions for simplifying the form of the second refractive surface S by reducing the high-order aspherical coefficients of the second refractive surface S. It shows.

Furthermore, by satisfying the condition shown in equation (A), the occurrence of higher-order spherical aberration is suppressed.

Next, specific numerical examples are shown in the table below. In the same table below, ru is the refractive index of the substrate 2cL, and t is the thickness of the substrate 2α.

The spherical aberration, coma aberration, sine condition, and astigmatism in Numerical Example I are as shown in FIGS. 3(3) to 0, respectively. Similarly, the characteristics in each of the numerical examples ■ to ■ are as follows:
Figure 4 (5) to Figure 4 (0). Figure 5 (5) to figure 0. 6(N to 0) and FIG. 7(8) to 7(8), respectively.

Numerical example 1 from Fig. 3 (5) to Fig. 7). It can be seen that in all cases of V, sufficient aberrations including the substrate of the recording disk are corrected, and off-axis aberrations are also corrected to a practically necessary degree.

Effects of the Invention As detailed above, the objective lens of the optical information reading device according to the present invention has two refractive aspherical surfaces represented by the formulas (141 and (G)), and satisfies the conditions shown by the QQ formula. Therefore, there are no restrictions on the selection of the lens thickness d when achieving good aberration correction, and it is possible to achieve good aberration correction while reducing the size and weight, and at the same time increasing the working distance. In addition, one refractive aspherical surface expressed by equation I is an ellipsoid and has a simple shape, and the curvature of the reference sphere of the other refractive aspherical surface is a dog from equation (G), so it is easy to create. Become.

[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 7, showing one embodiment of the present invention are as follows:
It is a graph which shows the characteristic in each of numerical example 1-V.

Claims (1)

[Scope of Claims] An objective lens for an optical information reading device having two refractive aspherical surfaces having a common optical axis, one of the two refractive aspherical surfaces: X: on the refractive aspherical surface. Distance Y from the tangent plane at the vertex of the refractive aspherical surface of a point whose height from the optical axis is Y: Height from the optical axis C_1: Curvature at the one vertex K_1: When set as a constant, X = (C_1Y ^2)/(1+√[(1-K_1C^2
_1Y^2)], and the other of the two refractive aspherical surfaces is: C_2: Curvature at the other vertex A_4, A_6, A_8, A_1_0: When set as a constant, X= (C_2Y^2)/ I +√[(1-C^2_2Y
^2)]+A_4Y^4+A_6Y^6+A_8Y^8
+A_1_0Y^1^0, where n: refractive index f: focal length r_1: curvature at one vertex r_2: curvature at the other vertex, r_1/r_2<(n-1 ) f/r_2<n-2K_1
An objective lens for an optical information reading device, characterized in that it satisfies -100(A_4+A_6).