JPS63165811A - Condenser lens for optical memory - Google Patents

Abstract

PURPOSE:To form a miniaturized and lightweight condenser lens for an optical memory with efficient image formation by displaying an aspheric surface based on a specific formula and satisfying prescribed conditional equation. CONSTITUTION:When it is defined in a single lens 1 having a face opposed to a light source and a face on the opposite side respectively constituted of aspherical surfaces that a distance between a point on the spherical surface having high H from an optical axis and a contact plane on the vertex of the aspherical surface is X, the radius of curvature on the base curve of the nu-th face (nu=1, 2) is Rnu, the conical constant of the aspherical surface of the nu-th face (nu=1, 2) is Knu, and the aspherical factor of the aspherical surface on the nu-th face (nu=1, 2) is Anui (i=1-m), the aspherical surface can be expressed by an equation I and the conditions of inequalities II are satisfied. Provided that F is the focal distance of the lens, WD is a working distance, (t) is the thickness of protection layer on a memory using the lens, and (n) is a refractive index in used wavelength.

Description

[Detailed Description of the Invention] [Technical Field] The present invention relates to recording and/or
The present invention also relates to a condensing lens suitable for a pickup used for reproduction.

<Prior art> Conventionally, this type of condensing lens was
Like the lens systems shown in Japanese Patent Publication No. 52-148143, the lens system was composed of three to five spherical lenses. However, as a condensing lens for optical memory, it is important to not only perform imaging performance but also to perform high-speed automatic focusing.
Since it is necessary to perform tracking control, etc., it is desirable to make the lens as small and lightweight as possible, and in this respect, conventional lens systems consisting of three to five spherical lenses have limited response performance.

Furthermore, the above-mentioned combination lens requires troublesome optical axis alignment and assembly adjustment of each surface, which also causes an increase in cost.

Examples of using double-sided or single-sided aspherical single lenses to solve such problems are JP-A-57-76512, JP-A-57-201210, and JP-A-58-17.
This method has been proposed in Japanese Patent Application Laid-open No. 409, Japanese Unexamined Patent Publication No. 59-26714, and the like.

However, the condensing lenses proposed in these conventional proposals still cannot be said to be sufficient in terms of lens size, weight, and imaging performance.

<Summary of the Invention> The purpose of the present invention is to solve the above-mentioned drawbacks of the conventional condensing lens.
It is an object of the present invention to provide a condensing lens for an optical memory, which is made of aspherical surfaces on both sides, is small and lightweight, and has good imaging performance.

In order to achieve the above object, the condensing lens according to the present invention is a single lens whose first and second surfaces are both aspherical in order from the light source side, and the height is l from the optical axis on the aspherical surface. The distance from the tangential plane of the aspherical vertex of the point -I is 1.2) Let the conic constant of the aspherical surface be ν,
Apt
(i=1.2, 3...), the aspherical surface is expressed by and satisfies the following conditions.

(1) -0,54<R, /R2<-0,42(2
) 0.57 < (WD 10t/n)2 < 0.67
Here, F is the focal length of the lens, WD is the working distance, t,
n indicates the thickness of the protective layer on the memory and the refractive index at the wavelength used when this condensing lens is used.

In addition, in order to obtain even better imaging performance, some forms of this condensing lens meet the following conditions (1) and (2) in addition to the above conditions (1) and (2).
It is characterized by satisfying 3) to (6).

(3) −0,7<ψ, −F'< −0,4(4)−
4,0<ψ2・F"<-2,0(5) -0,7<A
, 6・F'<-0,3(6) -2,0< A2[i-
F”< −0,9 Here, ψv ” bu (Nv' N,) γ22Rv / (1+2A v 2 Rν) However, N
,,N,' are the refractive indices of the medium on the incident side and the exit side at the νth surface (ν=1.2).

<Example> The condensing lens of the present invention is one in which aberrations mainly near the axis are favorably corrected. It is particularly suitable for condensing lenses for optical disk recording and reproducing devices that perform focusing and tracking control by moving the lens itself.

Specifically, this type of condenser lens has an NA of about 0.5 and is usually 1.1 = 1 because it is necessary to perform high-density recording and playback and take into account the effects of surface runout, eccentricity, etc. of the disk.
．． Transparent substrate (transparent protective layer) equivalent to a 3 mm thick disk
It has optical performance close to the analytical limit in the range of o, i to 0.3 mmφ through the lens. In principle, these condensing lenses can correct aberrations at only one point on the optical axis, but in practice, errors during processing and setting, focus, movement of the lens itself due to tracking servo, and rotation of the disk can be corrected. In order to tolerate to some extent the effects of surface wobbling and eccentricity caused by the lens, it is necessary to correct aberrations in the range of about 0.1 mm to 0.3 mmφ.

Furthermore, this condensing lens takes into consideration performance deterioration due to the relative eccentricity of the first and second surfaces, and has less sensitivity to eccentricity in performance, making it easy to manufacture and advantageous in terms of cost.

The configuration of the present condenser lens will be explained in detail below.

In addition, before showing specific examples, the above-mentioned conditions (1) to (
6) will be explained in detail. For convenience of explanation, the above conditions (1) ~
(6) is listed again.

(1) -0,54<R, /R2<-0,42(2)
0.57 < (WD+t/n) -< 0.67 (3
)-0,7<ψ1・R3<-0,4(4)-4,0<
ψ2・R3< −2,0(5) −0,7<A,6・
F'< -0,3(6) -2,0<A26・F'
<-0,9 Condition (1) is a condition regarding the ratio of the radius of curvature of the base curved surfaces of the first surface and the second surface constituting the single lens.

In condition (1), if the value is outside this range, it becomes difficult to satisfy the sine condition. Furthermore, if it is below the lower limit, the working distance will not be sufficient and practical problems will occur.On the other hand, if it exceeds the upper limit, R2 will become too small compared to R1, and the spherical aberration generated at the second surface will become large and under-performing. This occurs and is difficult to correct.

Condition (2) is mainly a condition for reducing the amount of unsatisfactory sine condition.

In order to achieve the purpose of the condensing lens according to the present invention, it is necessary to correct not only the on-axis aberration but also the off-axis aberration. As for off-axis aberrations, it is sufficient to mainly correct comatic aberrations. To this end, it is sufficient to realize an isoplanatic lens in which the amount of unsatisfied with the sine condition is 0.

If the range of condition (2) is exceeded, the amount of unsatisfactory sine condition becomes so large that off-axis performance deteriorates. Further, if the value is below the lower limit, there will be practical problems such as an increase in the weight of the lens due to the thick wall thickness and an inability to measure the working distance sufficiently. On the other hand, when the upper limit is exceeded, comatic aberration and other sinus aberrations also increase and off-axis performance deteriorates significantly.

According to the present invention, good imaging performance can be obtained by satisfying the above conditions (1) and (2) and appropriately setting the aspherical shapes of the first and second surfaces.

Conditions (3) and (4) are conditions for properly correcting spherical aberration and coma aberration in the third-order region.

According to "Lens Design Method" by Yoshiya Matsui (Kyoritsu Shuppan), the third-order spherical aberration coefficient of the first and second surfaces is 1. , I2 and the third-order coma aberration coefficients n, , m2 of the first surface and the second surface are expressed as follows when the entrance pupil coincides with the first surface and the object distance is infinite.

Here ψ2. Each ψ2 is a third-order aspherical term.

Therefore, the spherical aberration coefficient and coma aberration coefficient I of the entire lens
, II are determined by the sum of the aberration coefficients on each surface: I=I, +I2 2 1 + 2.

As is clear from Equation 1, when the shape of the lens is determined according to values such as focal length and working distance, R1, R2, and D are determined.

The value of N is almost fixed. Therefore, the aberration coefficient is 1゜■
There are only two degrees of freedom left in order to set the value to an appropriate value: ψ1°ψ2. Therefore, it is best to first set ψ2 so that the comatic aberration coefficient ■ of the entire system becomes an appropriate value, and then select ψ1 so that the spherical aberration coefficient I of the entire system becomes an appropriate value. ψ2 is each condition (3).

It is desirable that it be within the range shown in (4).

Conditions (5) and (6) are conditions for correcting axial and off-axis aberrations in a well-balanced manner in the fifth-order region. When the aspherical coefficients A 161 A 3 of H6 of the first and second surfaces are outside the ranges indicated in the respective conditions, it becomes difficult to correct aberrations of higher-order terms, and the amount of wavefront aberration becomes significantly large especially around the effective diameter of the lens. Both the above and off-axis performance deteriorate.

Examples 1 to 3 of the condensing lens according to the present invention are described in Tables 1 to 3 below.

Table 3 below shows the lens data of this condensing lens, the aspherical coefficients and conic constants of the first and second surfaces, and the values of conditions (1) to (6) for each lens. .

In the table, F is the focal length of the lens, NA is the numerical aperture, β is the paraxial lateral magnification, R1 and R2 are the radius of curvature of the base curved surfaces of the first and second surfaces, and D is the axial thickness of the lens. , WD is the working distance, t is the thickness of the transparent protective layer facing the lens, and N and n are the refractive indices of the lens and the transparent protective layer at a wavelength of 780 nm, respectively.

Note that the aspherical shape is expressed by the following equation as described above, with the vertex of the lens 6 surface as the origin, the optical axis direction as the X axis, and the incident height as II.

(ν=1.2) FIG. 1 is a sectional view of a condensing lens according to the present invention.

In the figure, 1 is a main condensing lens, 2 is a transparent protective layer of an optical disk, etc., and other symbols indicate each parameter in the above-mentioned lens data.

Further, FIGS. 2 to 4 are aberration diagrams of the condensing lenses of Examples 1 to 3 described above.

Here, spherical aberration, astigmatism, and distortion aberration are shown, where SA is the spherical aberration, SC is the amount of unsatisfactory sine condition, M is the meridional surface aberration, and S is the sagittal surface aberration.

<Effects of the Invention> As described above, according to the present invention, it is possible to provide a condensing lens in which aberrations are well corrected both on-axis and off-axis.

Moreover, since the present condenser lens is a single lens, it is possible to reduce the load on an actuator etc. when used as a pickup for an optical disc or the like.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a condensing lens according to the present invention. FIG. 2 is an aberration diagram of the condenser lens of Example 1. FIG. 3 is an aberration diagram of the condensing lens of Example 2. FIG. 4 is an aberration diagram of the condensing lens of Example 3.

Claims (5)

[Claims] (1) It is a single lens whose first and second surfaces are both aspherical in order from the light source, and the aspherical surface is such that Distance from the plane, H
is the height from the optical axis, and R_ν is the νth when ν=1, 2
The radius of curvature of the base curved surface of the surface, K_ν is the conic constant of the νth surface aspherical surface, and A_ν_i is each aspherical coefficient of the νth surface aspherical surface, X=[H^2/R_ν]/[1+√{1−( 1+K_ν
)(H/R_ν)^2}]+A_ν_2H^2+A_ν
A condensing lens for optical memory, which is represented by _3H^3+A_ν_4H^4+... and satisfies the following conditional expression. (1) -0.54<R_1/R_2<-0.42(2)
0.57<(WD+t/n)1/F<0.67 where,
F is the focal length of the lens, WD is the working distance, and n is the thickness of the protective layer on the memory and the refractive index at the wavelength used when this condensing lens is used. (2) Let the refractive index of the medium on the incident side and exit side of the ν-th surface of the single lens be N_ν, N_ν', and ψ_ν=b_
ν(N_ν'-N_ν) b_ν=8A_ν_4+(K_ν/R_ν^3)-2A
＿ν_2 (4A_ν_2^2+[3/R_νγ_ν])
When γ_ν=R_ν/1+2A_ν_2R_ν, (3) −0.7<ψ_1F^3<−0.4 (4) −4.0<ψ_2F^3<−2.0 (5)-0.7<A_16F^5<-0.3(6)-2
．． A condensing lens for an optical memory according to claim (1), which satisfies 0<A_26F^5<-0.9.