JPS61200518A - Objective for recording and reproduction of optical information recording medium - Google Patents

Abstract

PURPOSE:To compensate sine condition, a spherical aberration, and astigmatism excellently and to obtain a large-diameter objective for recording and reproduction which has a wide visual field by making both surfaces of a single lens aspherical. CONSTITUTION:The objective is a biconvex lens which has a large-curvature surface on an object side and both of the object-side and image-side surfaces are aspherical and satisfy conditions shown by expressions. In the expressions, (f) is the focal length of the lens, (n) is the refractive index of the lens, and r1 and r2 are the top radii of curvature of the object side and the image side; and (d) is the on-axis thickness of the lens, NA is the numerical aperture, and DELTA1 is the difference between the aspherical surface at the effective diameter outermost periphery (at position on the light-source side where a peripheral light beam is incident) of the surface at the light source side and a reference spherical surface which has the top radius r1 of curvature of the spherical surface in the direction of the optical axis and positive when the aspherical surface is displaced to the light source side according to the distance from the optical axis. Then, DELTA2 is the difference between the aspherical surface at the effective diameter outermost periphery of the image-side surface and a reference spherical surface having the radius r2 and is positive when the aspherical surface is displaced to the light source side according to the distance from the optical axis.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention uses an optical system KIN that serves as a recording optical system, a reproducing optical system, or both of them for an optical information recording medium to image a light source light into a minute spot. The present invention relates to objective lenses for recording and reproducing optical information recording media that are optimally used for this purpose.

[Conventional technology]

When using a semiconductor laser as the light source, the objective lens used in the optical information recording medium's reproduction optical system has a numerical aperture (
NA) is 0.45 to 0.0 for compact discs.
47. For video discs, it is necessary to have a large diameter of 0.5 to 0.53 and also to have diffraction-limited performance. On the other hand, a recording optical system, a DRAW optical system, and a magneto-optical recording optical system require NAo of 5 to O26.

In recent years, a number of single lenses have been proposed in which the refractive surface of the lens is made aspherical and the on-axis performance is corrected to within the diffraction limit performance.
45 with a sufficiently wide field of view has been put into practical use.

(MICRO0PTIC8NEWSVol 3 tJn
(IP20) However, when the aperture becomes large with an NA of about 0.6, sine conditions,
It is essential to properly correct astigmatism. In order to satisfactorily correct the spherical aberration and sine conditions of a large-diameter single lens, it is necessary to make both surfaces aspherical.

JP-A-50-156945, JP-A-57-765
12, JP-A No. 57-201210, JP-A No. 58-68711-W, JP-A No. 59-26714, and JP-A No. 59-23313. There is a description of a double-sided aspherical lens that was proposed as a lens.

Many of these have NAs of 0.45 to 0.5, and although the spherical aberration and sine conditions are good, the astigmatism is insufficient, and if the aperture is increased as is, a practically sufficient field of view will be secured. I can't.

Japanese Unexamined Patent Publication No. 156945/1983 describes a means for securing the field of view, and the first embodiment has a NA of 0.
Although it has a large aperture such as 6, the overall length of the lens is long (and the back focus is short (because the curvature of the object side surface is large and positive) according to the means of securing the field of view described here. Therefore, in order to use it as an optical system for recording and reproducing optical information recording media, it is necessary to increase the focal length in order to secure the necessary working distance, and it is not possible to make the optical system lightweight and compact.

Furthermore, the lens described in JP-A-57-76512 has good astigmatism, but high-order aberrations are generated under spherical aberration and sine conditions. Ta.

[Problem that the invention seeks to solve]

The present invention not only satisfies sinusoidal conditions and spherical aberrations by forming aspherical surfaces on both sides of the oval lens, but also satisfactorily corrects astigmatism, resulting in a wide field of view despite its large aperture.・It is intended to obtain an objective lens for single-use recording and reproduction of optical information recording media.

[Means for solving problems]

The present invention aims to achieve the above object, and the present invention is a biconvex lens with a surface having a strong curvature facing the object side, and both the object side surface and the image 1 brightness 1 surface are non-convex. It is a spherical surface and the following conditions (110, 35<-< 0.7 f +2) 0.50(Tπ, (1・0+31 0
．． 05 <△1-~<0.25 where f: focal length of the lens n: refractive index of the lens r, - vertex radius of curvature of the object side surface r2: vertex radius of curvature of the image side surface d: on the axis of the lens Thickness NA: Numerical aperture Δ,: Reference spherical surface having an aspheric surface at the most peripheral effective diameter of the surface on the light source side (the position on the surface on the light source side where the peripheral rays of the above NA are incident) and a radius of curvature at the μ point r. A case where the aspherical surface is displaced toward the light source as the distance from the optical axis increases due to the difference in the optical axis direction from the optical axis is defined as positive.

Δ,: The difference in the optical axis direction between the aspheric surface at the most peripheral effective diameter of the image side surface and the reference spherical surface having the radius of apex curvature r2,
A case where the aspherical surface is displaced toward the light source as it moves away from the optical axis is considered positive.

The object of the present invention is to provide an objective lens for recording and reproducing optical information recording media, which satisfies the following.

[For production]

Condition (1) is related to the radius of curvature rl of the apex on the light source side. Since both surfaces of the lens of the present invention are aspherical, it can be said that the necessary conditions for correcting spherical aberration and sine conditions are satisfied. However, unless r is selected optimally, high-order comatic aberration will occur, and it is difficult to properly correct this with an aspheric surface, especially in the case of a large-diameter lens.

When the upper limit of condition (1) is exceeded, a large amount of outward coma aberration occurs, and when the lower limit is exceeded, a large amount of inward coma aberration occurs. Even if an attempt is made to correct these coma aberrations by making the surface aspheric, the unevenness of the sine condition increases, making it impossible to obtain a wide field of view.

Condition (2) is a condition for correcting astigmatism. If the value exceeds the upper limit and becomes too large, the astigmatism becomes excessive, and if it exceeds the lower limit and becomes small, the astigmatism becomes insufficient, and in either case, it is not possible to widen the field of view.

Condition (3) is a condition regarding the amount of aspherical surface in order to satisfactorily correct spherical aberration. As is clear from aberration theory, third-order spherical aberration is proportional to the fourth power of the aperture when considered in terms of wavefront aberration. Therefore, the aspherical amount needs to be normalized by the fourth power of the numerical aperture. Furthermore, the higher the refractive index of the lens, the smaller the amount of aspherical surface needed to correct spherical aberration. Actually, the aspherical amounts △5 and △2 for the peripheral rays of the light source side surface and the image side surface are set to 1.
/(n-1 house, (NA)4, loss normalized by f is Δ2
, Δ2, the greater the positive value of Δ1, and the smaller the negative value of Δ, the greater the effect of overdoing the spherical aberration. Therefore, in order to correct spherical aberration, Δ and −Δ must be within a certain range. It is necessary to be within.

Condition (3) defines this range; if the upper limit is exceeded, the spherical aberration will be over-corrected, and if the lower limit is exceeded, the spherical aberration will be under-corrected.

Furthermore, it is desirable to satisfy condition (4) regarding correction of the sine condition.

If this range is not satisfied, the unevenness of the sine condition will become large and it will be impossible to widen the field of view.

(Example) Examples of the objective lens of this invention will be shown below.

In addition to the above, the symbols in the table are r, 2 W from the light source side, vertex curvature radius d of the 1st-IIIth lens surface: distance n of the 1st lens surface from the light source side,:
The refractive index of the first lens material from the light source side is ν, 2. The Abbe number for the d-line and the aspherical shape of the first lens material from the light source side have the apex of the surface as the origin and the optical axis direction as the X axis. In the orthogonal coordinate thread, the apex curvature is C1, the conic constant, and the aspherical coefficient Ai*Pt is the aspherical power number (Pi>
2.0), then φ=5'';F (C=l/r
).

In addition, the value of cover glass G is also shown in the table.

Hl and H7 are the heights of the peripheral rays at 1' on the light source side surface and the image side surface, respectively.

When the aspherical shape is expressed as above, the aspherical quantities Δ2 and Δ2 are as follows: Δj = Xsp, j - XAg, j (j = 1
+ 2) where Kj: conic constant of the j-plane Ail': aspheric coefficient of the j-plane p, fj): power of the aspheric surface of the j-plane.

Margin Example 1 f = 1. ONA O,45m =04
o.

Aspheric coefficient/power number H, = 0.450 H, = 0.324Δ
, -Δ, = 0.075 Δ1/Δ, = -3
, 53 Example 2 f ” 1.0 NA 0.45 m
=04 o.

Aspheric coefficient/power number H, = 0.450 8. = 0.278Δ,
−tq = 0.070 ΔI/Δ, = −3
, 56 Example 3 f=Lo NAo, 45 m=
0 Aspheric coefficient/Beginner number Ht = 0.450 Hz = 0.2
38Δ1−Δt = 0.069 Δ1/Δ,
= -5,48 Example 4 f=1. ONA O, 6m=0 4 o.

Aspheric coefficient/power number H, = 0.602 Ht = 0.47
1Δ, -Δ, = 0.179 Wealth/Δ, = -6,3
2 Example 5 f=1. Q NAo, 6 m=0
4 o.

Aspheric coefficient/power number Hi = 0.602 Ht = 0.391 Buichitomi = 0.092 ∆, /∆ = -6.35 Example 6 f = 1. ONA O,ei m=
0rt d, nth + 1.15621 1.3476 1.911
80 27.92 -1.91568 0.2721 3 heads 0.2667 1.55000
30.04 o.

Aspheric coefficient/power value A6 = -3,06251D-01P6 = 14.00
00Δ1−Δ, = 0.094 Δ, /Δ,
=-9,41 [Effect of the Invention] Although the lens of the present invention has an extremely simple structure as shown in Fig. 1, the aberration diagrams of each embodiment are shown in Figs. 2 to 7. As shown, all aberrations are well corrected.

FIG. 8 and FIG. 9 show examples 1 to 3 of the present invention, respectively.
, shows the relationship between the image height and the effective value of the wavefront aberration when Examples 4 to 6 are implemented at a focal length of 4.5 m, and ■ to ■ are the relationship curves for Examples to 6, respectively.

An optical information medium in which the effective value of the wavefront aberration is within 0.07λ (Marechal's tolerance) as the wavelength λ, and the maximum image height is 0.19 or more when NAo is 45, and 0.11 or more when NAo is 6. It can be seen that this lens has a much wider field of view as an objective lens for recording and reproducing.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of one embodiment of the objective lens of the present invention, @2
7 to 7 are diagrams showing various aberrations of the first to sixth embodiments of the present invention, respectively. 'W, Figures 8 and 9 are 1# when each embodiment is implemented with a focal length of 4.5! The relationship between height and the effective value of wavefront aberration is shown. Applicant: Konishi Roku Photo Industry Co., Ltd. (Cfh 11.11) 1 瀘i6b t, L 9 ku, (and 44←
``To, 昂,, Collect (L (裳1!/・32) 工艈面 Ｇｕｒｕｎｄｅｒ
xr, a, destination Figure 3 (Abutokuchiku, 13) (Jinno-Kai 14) Figure 5 (Motou example 5) Figure 6 (Shuddering t Ushii now, 16) Figure 7

Claims (1)

[Claims] A biconvex lens with a surface with strong curvature facing the object side, where both the object side surface and the image side surface are aspheric surfaces, and the following condition (1) 0.35<r_1 /nf<0.7 (2) 0.50<d/|r_2|<1.0 (3) 0.05<■-■<0.25 ■=(n-1)^3Δ_1/(NA)^ 4f■=(n-
1) ^3Δ_2/(NA)^4f where f: Lens focal length n: Lens refractive index r_1: Vertex radius of curvature of object side surface r_2: Vertex radius of curvature of image side surface d: Axial thickness of lens NA: Numerical aperture Δ_1: Effective diameter of the surface on the light source side The distance in the optical axis direction between the aspheric surface at the outermost periphery (the position on the light source side surface where the peripheral rays of the above NA are incident) and the reference spherical surface having the apex radius of curvature r_1 The difference is defined as positive if the aspherical surface is displaced toward the light source as the distance from the optical axis increases. Δ_2: The difference in the optical axis direction between the aspheric surface at the most peripheral effective diameter of the image side surface and the reference spherical surface having the apex radius of curvature r_2,
A case where the aspherical surface is displaced toward the light source side as the distance from the optical axis increases is considered positive. An objective lens for recording and reproducing optical information recording media, which satisfies the following.