JPS62116915A - Condenser lens for optical memory - Google Patents

Abstract

PURPOSE:To compensate aberrations excellently and to compact a condenser lens by compositing the lens of the 1st positive lens group, the 2nd negative lens group, and the 3rd positive lens group successively from a light source side and satisfying specific conditional inequalities. CONSTITUTION:The lens system has the 1st positive lens group L1, the 2nd negative lens group L2, and the 3rd positive lens groups L3 successively from the light source side, and the conditional inequalities I-V hold, where R1 is the curvature of the surface of the lens L1 on the light source side, R3 and R4 the curvature values of the lens L2 on the light source side and image side, R6 the curvature of the image-side surface of the lens L3, D4 the air gap between the lenses L2 and L3, F the focal length of the whole system, F1.2 the composite focal length of the lenses L1 and L2, and F1 and F2 the focal lengths of the lenses L1 and L2 respectively. Consequently, the aberrations are compensated excellently and the condenser lens for optical memory which consists of a small number of lens elements is realized.

Description

Detailed Description of the Invention (1) Technical Field The present invention relates to optically reproducing information recorded on a disk-shaped or card-shaped recording medium, or optically transmitting information to this type of recording medium. The present invention relates to a condensing lens for optical memory used for recording.

(2) Prior Art Conventionally, various proposals have been made regarding condenser lenses (or objective lenses) used in pickups in optical memories such as optical disks and optical cards.

There are several lens systems that have been put into practical use.

This type of condensing lens for optical memory is required to have high resolution in order to reproduce or record minute recording patterns, and its aperture number (N).

A) also Q,'? It is necessary to make it sufficiently usable.
be. Furthermore, since the depth of focus becomes shallow, it becomes necessary to move the lens in the optical axis direction using automatic focusing means (autofocus) to respond sensitively to out-of-focus during scanning. Therefore, the lens system must be as lightweight as possible, and it has been conventionally required to reduce the number of lenses in the lens system. Furthermore, from a cost standpoint, it was necessary to have a simple structure and easy manufacture. In addition, since optical discs rotate at high speed and must follow tracks on the disc that have some fluctuation, a sufficient working distance is required between the disc and the lens, and the image must be flat. It was also required that

In addition, as a noise countermeasure, it is necessary to make the transmittance of light at the wavelength used as high as possible, so it is necessary to apply a multilayer anti-reflection coating to the lens surface, and from this point of view, it is necessary to reduce the price. It is advantageous to have fewer lenses.

However, although conventional condensing lenses for optical memory meet some of the above requirements, they cannot be said to be fully satisfactory lens systems, and are not suitable for the current situation where higher performance and lower cost lens systems are required. It was not possible to provide a condensing lens for optical memory.

(3) Summary of the Invention An object of the present invention is to provide a compact condensing lens for optical memory that satisfies the above requirements, can satisfactorily correct aberrations, and has a small number of constituent lenses.

The present invention includes, in order from the light source side, a positive first group lens L1, a negative second group lens L2, and a positive third group lens L3,
The curvature of the light source side surface of the first group lens Ll is R1, and the curvature of the light source side surface and image side surface of the second group lens L2 is R3.
, R4, the curvature of the image side surface of the third group lens L3 is R6, the axial air distance between the second group lens L2 and the third group lens L3 is D4, the focal length of the entire system is F, the first group lens When the focal lengths are F1 and F2, respectively, (1) O<R1/|R3|<1.5. R3<0(2)
0<l/IR4+<0.035/F (3) 2.9F<
R6<9F (4) -0,1<D4/F1.2<0.1(5)
The objective is to achieve the above object with a condensing lens for optical memory that satisfies -1,05<Fl/F2<-0,05.

(4) Examples Hereinafter, each of the conditions (1) to (5) will be explained before showing examples of the condensing lens for optical memory according to the present invention.

Condition (1) is for good correction of spherical aberration; if this range is exceeded, the positive spherical aberration generated on the surface of the second lens group L2A on the light source side becomes too large, and the spherical aberration is corrected. It becomes additive.

The ma condition (2) is a term related to spherical aberration and coma aberration correction. That is, when the curvature R4 of the second group lens L2 is positive, if the curvature R4 becomes larger than 0.035/F, the amount of positive spherical aberration generated becomes too large, and the occurrence of outward coma becomes large, making it difficult to correct. Become. Further, in the case where the curvature R4 is negative, if one curvature R4 becomes smaller than -0,0357F, the amount of negative spherical aberration generated becomes too large, and the outward coma is generated too thick, making it difficult to correct.

Condition (3) is for properly correcting coma aberration; if the curvature R6 of the third lens group L3 is smaller than 2.9F, an inward coma will occur, and if it is larger than 9F, an outward coma will occur. becomes large and correction becomes difficult.

Condition (4) is a condition for obtaining a sufficiently large working distance so that, for example, when the optical disk is displaced in the optical axis direction, it does not come into contact with the lens. If D4/F 1.2 is larger than 0.1, it will not be possible to obtain a sufficiently large working distance, and if it is smaller than -〇, l, the negative composite refractive power of the first group lens L1 and second group lens L2 will be strong. Too much, 3rd
The positive refractive power of the group lens L3 must be made stronger than necessary, which is undesirable in terms of aberration correction.

Condition (5) is for properly correcting spherical aberration by determining the focal length of the first lens group LL and controlling the amount of negative spherical aberration generated in the first lens group Ll. Fl/F2 is -0
, 05, the negative spherical aberration of the first lens group L1 becomes too large, and it becomes difficult to correct it with the positive spherical aberration generated in the second lens group L2. On the other hand, -1
.

If the configuration satisfies the above conditions (1) to (5), it is possible to realize a condensing lens for optical memory according to the present invention, but by adding the following conditions, even higher performance condensing lens for optical memory can be realized. lenses can be realized.

(El) -1,05<F3/F2<-0,05(7
)0.3<F3/Fl<1.0 (8) -15F(Fa<-2F (9) 0.4F(TD<2F However, here, R1 is the curvature of the light source side of the first lens group Ll, R3 is the curvature of the light source side surface of the second lens group L2, and R4
is the curvature of the image side surface of the second group lens L2, R6 is the curvature of the image side surface of the third group lens L3, D4 is the axial air distance between the second group lens L2 and the third group lens L3, F is the focal length of the entire system, F1, F2 . F3 is each first group lens L1
, second group lens L2. The focal length of the third group lens L3 is
F1 and 2 are the combined focal lengths of the first group lens Ll and second group lens L2, and Fa is composed of the image side surface of the first group lens L1, the light source side surface of the second group lens L2, and the air gap therebetween. TD is the focal length of the air lens, and TD is the first group lens LL.
The axial spacing between the light source side surface and the image side surface of the third lens group L3 is shown.

The above conditions (6) to (9) will be explained below.

Condition (6) is a condition for determining the focal length of the second group lens L2 and controlling the amount of positive spherical aberration generated in the second group lens L2 to satisfactorily correct the spherical aberration. That is, F3/
When F2 is larger than -0.05, the amount of positive spherical aberration generated in the second group lens L2 becomes too small, and the negative spherical aberration generated in the positive lenses of the first group lens L1 and the :1IJ third group lens L3 is corrected. It becomes difficult to do so. On the other hand, -1
.

Condition (7) is the first group lens L1 and the third group lens L3.
This is for determining the focal length distribution of the positive lens and reducing spherical aberration.1 In the optical system according to the present invention, the third
The height of the light beam incident on the group lens L3 is the height of the first group lens Ll.
The height of the third lens group L is lower than the height of the incident light ray.
Spherical aberration is reduced by setting the focal length of lens L1 to be shorter than that of the first group lens L1. F3/Fl
becomes larger than 1.0, the focal pressure 1 of the first group lens L1
If iIIFI becomes too small and the negative spherical aberration becomes large, and if F 3 /F l becomes smaller than 0 or 3, Fl becomes too large and the positive spherical aberration becomes large, making it difficult to correct.

Condition (8) defines the focal length of an air lens consisting of the image side surface of the first group lens Ll, the light source side surface of the second group lens L2, and the air gap therebetween, and the positive spherical aberration that occurs in this air lens. This is to control the spherical aberration to satisfactorily correct spherical aberration. Here, if the focal length Fa of this air lens becomes larger than -2F, the amount of positive spherical aberration generated by the air lens becomes too large, resulting in excessive correction.

On the other hand, when Fa is smaller than -15F, the amount of positive spherical aberration generated by the air lens becomes too small, making it difficult to correct the negative spherical aberration generated on the side surface of the light source of the first lens group Ll.

Condition (9) is a condition for reducing the size of the optical system according to the present invention, and if TD becomes shorter than 0.4F, sufficient lens thickness and edge thickness cannot be obtained, making it difficult to manufacture. . Moreover, if TD is longer than 2F, the lens becomes large and the weight increases, which is not preferable.

By satisfying each of the conditions explained above, especially conditions (1) to (5), the condensing lens for optical memory according to the present invention satisfies various requirements for this type of lens system, and is a high-performance, compact, and inexpensive lens system. An example of the present invention will be shown below with zero order.

Figures 1 and 2, Figures 3 and 4, Figures 5 and 6
The figures show cross-sectional views of optical systems and aberration diagrams of the optical systems of the first to third embodiments of the present invention, respectively. In the cross-sectional view, Ll is the first group lens, L2 is the second group lens, L3 is the third group lens, P is the protective film on the recording surface, and WD is the image side surface of the third group lens L3 and the protective film P. distance between (operating pressure till), Ri
(i = 1, 2, 3 ----) is the first surface counting from the light source side, Di (i = 1, 2,
3 ----1) is the 1st and f+1th counting from the light source side
In addition, the aberration diagram shows the spherical aberration, astigmatism, and distortion of each example.
, A is spherical aberration, S, C sine condition 1M is meridional direction, S is sagittal direction. As shown in each cross-sectional view, the condensing lens for optical memory according to the present invention has a positive first group in order from the light source side. It is composed of a lens LL, a negative second lens group L2, and a positive third lens group L3, and is designed to satisfy the conditions (1) to (5) above.

Also, as you can see from each aberration diagram, not only one spherical aberration,
Since the sine condition is satisfied, a lens system that can satisfactorily correct comatic aberration near the axis and also correct astigmatism and distortion can be achieved.

Tables IA and IB to Tables 3A and 3B below show lens data and various design values for the lens systems shown in the first to third embodiments. ---
-) is the curvature of the first surface counting from the light source side, Di(
i=1,2.3---> is the axial air gap or axial wall thickness between the first surface and the i+1th surface counting from the light source side, and N1 (i=1.2. 3) and Vi (i=1.2.
3. ) are the refractive index and Abbe number for the D line of the first lens counting from the light source side for wavelength input = 0.78 pm, NA is the numerical aperture, WD is the working distance, F is the focal length of the entire system, and Fi (+=1.2.3) is the first one counting from the light source side.
The focal length of the th lens, F1, 2 is the first group lens LL
and the second group lens L2, and Fa is the focal length of the air lens composed of the image side surface of the first group lens Ll, the light source side surface of the second group lens L2, and the air distance between the respective surfaces. , TD indicates the axial distance from the light source side of the first lens group Ll to the image side of the third lens group L3. All values in the table are normalized by the focal length of the entire system, except for the aperture L&NA. It is something that (F=1.0) Table IA Table IB Table 2 A Table 2 B Table 3 A Table 3 B Each example shown above is applicable not only to conditions (1) to (5) but also to conditions (6) to (9). However, in the present invention, it is possible to obtain a lens system with sufficient performance even if conditions (6) to (9) are not necessarily satisfied. Further, if a lens system is designed based on the idea of the present invention, substantially the same effects as those of the above embodiments can be obtained, and a wide variety of configurations can be considered.

Furthermore, as mentioned above, the condensing lens for pilotage memory can be applied to various optical memories such as optical disks, magneto-optical disks, and optical cards that optically record and/or reproduce information, and the recording method, medium shape, etc. are not limited. . That is, the device to which the single optical memory condensing lens is applied is designed in accordance with its use.

(5) As described in detail of the invention, the condenser lens for optical memory according to the present invention has spherical aberration, coma aberration, astigmatism, and distortion well corrected, and has a sufficient working distance and numerical aperture. It is a compact and high-performance lens system with

[Brief explanation of drawings]

ttg and FIG. 2 are a sectional view of an optical system and its aberration diagram showing a first embodiment of the condensing lens for optical memory. 3 and 4 are a sectional view of an optical system and its aberration diagram showing a second embodiment of the condensing lens for optical memory. 5 and 6 are a sectional view of an optical system and its aberration diagram showing a third embodiment of a condensing lens for pilot memory. L L ------1st group lens. 2nd-----1 2nd group lens, 3rd---1--3rd group lens, p----1 Recording surface protective film, S, A
--- One spherical aberration, S, C --- One right side condition, M --- Meridional direction, S ---
--1--Sagittal direction. 20th LA map

Claims (1)

[Claims] (1) An optical memory having, in order from the light source side, a positive first group lens L1, a negative second group lens L2, and a positive third group lens L3, and which satisfies the following conditions. Condenser lens. (1) 0<R1/|R3|<1.5, R3<0(2)0
<1/|R4|<0.035/F (3) 2.9F<R6<9F (4) -0.1<D4/F1.2<0.1 (5) -1.05<F1/F2 <-0.05 However, R
1 is the curvature R3 of the light source side of L1, R4 is the curvature of the light source side and image side of L2, R6 is the curvature of the image side of L3, D4 is the axial air distance F between L2 and L3, and F1 and 2 are the entire system and L1, respectively. , L2's composite focal lengths F1 and F2 indicate the focal lengths of L1 and L2, respectively. (2) An air lens configured by the focal length of the third lens group L3 being F3, the image side surface of the first group lens L1, the light source side surface of the second group lens L2, and the air gap between the respective surfaces. When the focal length of is Fa and the axial distance from the light source side surface of the first group lens L1 to the image side surface of the third group lens L3 is TD, (6) -1.05<F3/F2<-0.05. (7) 0.
3<F3/F1<1.0 (8) -15F<Fa<-2F (9) 0.4F<TD<2F as described in claim (1). condensing lens for optical memory.