JPS6235310A - Lens for optical disk - Google Patents

Abstract

PURPOSE:To especially correct the chromatic aberration satisfactorily by being a three-group four-sheet constituting lens, and specializing the relation of the focal length of all systems and the first group lens, the Abbe's number of the positive lens and the negative lens in the first lens and the curvature radius of the sticking surface of the first group lens. CONSTITUTION:The lens is a three-group four-pieces constituting lens and satisfies formulas 1-3. Here, f is the synthesized focal length of total lens system, f1 is the focal length of the first group lens, nu+ and nu- are Abbe's num ber of the positive lens and the negative lens in the first group lens and r2 is the curvature radius of the sticking surface of the first group lens. When f1 is smaller than the lower limit, the negative spherical aberration quantity to occur at the first group lens comes to be too large, and reversely, when it is larger than the upper limit, the burden of the third group lens and the correction of the spherical aberration comes to be hard. When ¦nu+-nu-¦ is larger than the upper limit, the action to the correction of the chromic aberration is too small, and when ¦r2¦ is large than the upper limit, the achromatic action by the positive lens and the negative lens in the first group lens comes to be small and the correction of the chromatic aberration is short.

Description

Detailed Description of the Invention a. Technical Field The present invention relates to a lens for optical discs (high-density information recording media).

b. Prior art and its problems A lens used to read high-density information recorded on an optical disk must read high-density signals, so its resolution is required to be about 1 μm.

In the past, many inventions have been made for lenses for optical discs and optical systems to meet this requirement, but most of them have only corrected aberrations at a single wavelength. In cases where the light source (in most cases laser light) is stable, or in optical systems (products) where changes in lens imaging performance due to slight wavelength fluctuations can be ignored, aberration correction at a single wavelength is sufficient. However, in optical systems where the light source is unstable and the wavelength fluctuates, and the resulting fluctuations in aberrations cannot be ignored, the above requirements cannot be met by aberration correction alone at a single wavelength.

C0 Purpose The present invention aims to provide an optical disc lens that corrects aberrations not only at a single wavelength but also at multiple wavelengths and minimizes changes in aberrations due to fluctuations in the wavelength of a light source.

Solution to Problem d6 The optical disc lens of the present invention includes, from the light source side, a first group lens of a cemented lens of a negative lens and a positive lens or a cemented lens of a positive lens and a negative lens.

The second lens group is a negative meniscus lens with its concave surface facing the light source, and the third lens group is a positive meniscus lens with its convex surface facing the light source.
The lens is composed of four elements in three groups, and is characterized by satisfying the following conditions.

(1) 1.5f < f! <3.5f (2) 1 Y/1 ν-|>20 (3) I r21<f Here, f is the composite focal length of the entire lens system + fl is the focal length of the first group lens, and sha and ν- are Atsube numbers of the positive and negative lenses in the first lens group.

r2 is the radius of curvature of the bonding surface of the first lens group.

80 effects Next, each of the above conditions will be explained.

Condition (1) is a condition for properly correcting spherical aberration, and f! When is smaller than the lower limit, the amount of negative spherical aberration generated in the first lens group becomes too large and it becomes difficult to correct the spherical aberration.

On the other hand, when it is larger than the upper limit, the amount of negative spherical aberration generated in the first group lens becomes small, but the burden on the third group lens increases, and as a result, it becomes difficult to correct the spherical aberration, which is not preferable.

Condition (2) is related to condition (3) and is a condition for improving the correction of aberrations at multiple wavelengths, that is, chromatic aberration, which is a feature of the present invention.

1ν. When -v-1 is larger than the upper limit, the effect on correction of chromatic aberration is too small, making it difficult to satisfactorily correct aberrations at a plurality of target wavelengths.

Condition (3), like condition (2), is a condition for good correction of monochromatic aberration, and when 1r21 is larger than the upper limit, achromatization by the positive lens and negative lens in the first group lens The effect of this decreases, and correction of monochromatic aberration becomes insufficient, making it difficult to correct aberrations at a plurality of target wavelengths.

f. Examples Below, numerical values of Examples 1 to 4 in the present invention will be described. Here, f is the focal length.

NA is the numerical aperture, ω is the half angle of view yfB is the back focus,
r is the radius of curvature of each lens surface, d is the lens thickness or distance between lenses, N is the refractive index of each lens, and ν is the Abbe number of each lens.

[Example 1] f=1. ONA=0.47 ω=±1″f B
=0.569=0.57 f plane Na r d N ν
1 2.2051 0.2211 1.84666
23.92 0.6567 0.7175 1.69
680 55.53 -2.4234 0.0880 4 -0.9486 0.6264 1.80518
25.45 -1.1241 0.5510 6 0.7781 0.2457 1.88300
40.87 2.5748 f t =2.349=2.35 f 1 shear ν-1 = 155.5-23.91 =31
．． 61 r 21 = I O, 65671=t), 6
6 f [Example 2] f = 1. ONA=0.47 ω=゛±l@f
B=0.569=0.57 f shame r d N wage 1
2.4978 0.2211 1.84666 23.
92 0.570B 0.6153 1.7725
0 49.63 -2.2940 0.0734 4 -0.9825 0.7765 1.80518
25.45 -1.1492 0.3226 6 0.7037 0.2609 1.77250
49.67 1.5076 f r =1.932=1.93f | ν+-ν-1= l 49.6-23.9 l =2
5.71 r 2 I = l O, 57081=0.
57 f [Example 3] f = i, o NA = 0.47 ω = ±1
″f B=0.569=0.57 f plane Na r d N ν1
2.061! 0.5915 1.72916 5
4.72 -0.7061 0.2211 1.846
66 23.93 -3.7650 0.1183 4 -0.9397 0.3155 1.84666
23.95 0.9740 0.2670 6 0.7125 0.2447 1.88300
40.87 1.4981 f , =2.462=2.46f 1ν. −ν−1 = +54.7−23.91 =3
0.81 r21 = 1-0.70611 =0.
71 f [Example 4] f = 1. ONA=0.47 ω=±1@f B
:0.547=0.551 Surface Har d N ν1
1.9446 0.5920 1.77250 4
9.62 -0.6983 0.2211 1.846
66 23.93 -4.1729 0.1123 4 -0.9083 0.3588 1.80518
25.45 -0.9733 0.1932 6 0.6824 0.2467 1.77250
49.67 1.5260 f! =2.092=2.09f 1v Yer ν-1= 149.6-23.91 =25.
71 r 21 = 1-0.6983% = 0.7
Ofg, Effects As explained above, the present invention is a lens composed of 4 elements in 3 groups, and by satisfying each of the conditions (1) to (3) above, various aberrations (especially chromatic aberration) can be well corrected. It is possible to provide an optical disk lens that has a resolution of about 1 μm and exhibits very little change in performance due to wavelength fluctuations.

[Brief explanation of the drawing]

Figure 1 is a lens configuration diagram of Example 1 of the present invention, Figure 2 is a diagram of various aberration curves of Example 1, Figure 3 is a lens configuration diagram of Example 2 of the invention, and Figure 4 is Example 2. 5 is a lens configuration diagram of Example 3 of the present invention, and FIG. 6 is a diagram of Example 3 of the present invention.
FIG. 7 is a lens configuration diagram of Example 4 of the present invention, and FIG. 8 is a diagram of various aberration curves of Example 4. Patent applicant: Asahi Kogaku Kogyo Co., Ltd. Figure 1 Figure 2 Sine condition Figure 3 Figure 4 Sine condition Figure 5 Figure 6 Sine condition Figure 7 Figure 8 Sine condition procedural amendment November 27, 1985 Dear Commissioner of the Patent Office, □Required. /, Indication of the case Japanese Patent Application No. 175426/1983 Name of the invention Optical disk lens 3, Person making the correction Relationship to the case Patent applicant address 2-36-9 Maeno-cho, Itabashi-ku, Tokyo Name (0
52) M Kogaku Kogyo Co., Ltd. Representative: Tetsuda Matsumoto 0 Agent Address: 2-36-9 Maeno-cho, Itabashi-ku, Tokyo Address: 174 Telephone: 03-960-5162 6. Subject of amendment (1) Clarification of "" in the actual details Detailed explanation of 1. Contents of amendment (1) In the "Detailed description of the invention" section of the specification, change "greater than the upper limit" to "less than the lower limit" on page 4, line 15. and correct it.

Claims (1)

[Claims] From the light source side, a first group lens of a cemented lens of a negative lens and a positive lens or a cemented lens of a positive lens and a negative lens;
The second lens group is a negative meniscus lens with its concave surface facing the light source, and the third lens group is a positive meniscus lens with its convex surface facing the light source.
1. A lens for an optical disc, which is a lens consisting of four elements in three groups, and is characterized by satisfying the following conditions. (1) 1.5f<f_I<3.5f (2) |ν_+−ν_−|>20 (3) |r_2|<f Here, f is the composite focal length of the entire lens system, and f_I is the first
The focal lengths of the group lenses, ν_+ and ν_-, are the Abbe numbers of the positive and negative lenses in the first group lens, and r_2 is the radius of curvature of the bonding surface of the first group lens.