JPS61259214A - Lens for optical disc - Google Patents

Abstract

PURPOSE:To obtain a lens system which has a small number of constituting lenses and is made lightweight, by satisfying prescribed conditions in the lens system consisting of four lenses of three groups where the first - the third lens groups are arranged in order from the light source side. CONSTITUTION:The first lens group I, the second lens group II, and the third lens group III arranged in order from the light source-side constitute the lens system. This land system is constituted of the first lens group I consisting of the first positive double convex lens L1, the second lens group II consisting of a cemented lens of the second positive lens L2 and the third negative lens L3, and the third lens group III consisting of the fourth positive meniscus lens L4 whose convex is directed to the light source-side. This lens system is constituted to satisfy conditions of formulas I and II where (f), f23, and f4 are the focal length of the whole of the system, that of the second lens group II, and that of the third lens group respectively and r6 is the radius of curvature of the light source-side surface of the fourth lens L4.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is primarily concerned with large-diameter lenses whose aberrations are corrected to below the diffraction limit so as to be able to focus laser light down to a minute spot, particularly for digital audio discs and video discs. The present invention relates to lenses for optical discs such as discs.

[Prior art]

An optical disc lens used as a pickup lens for a digital audio disc or a video disc is attached to a small actuator such as a pickup unit, and is driven by control such as focusing, so it is first required to be small and lightweight. Furthermore, the above driving is required to have a sufficient working distance so that the lens does not collide with the disk surface during driving. In addition, since the laser beam must be focused as a minute spot on the disk surface, aberrations must be well corrected to below the diffraction limit while maintaining a large NA (numerical aperture). is necessary. The pond is required to be easy to make, low cost, stable performance, and durable. Thermal Theory It goes without saying that providing a lens system that satisfies these conditions requires precise technology in both design and manufacturing.

As is well known, various types of lens systems have been reported as lenses for optical discs that satisfy these conditions. Most of these lenses are condensing lenses that are used under the condition that the incident laser beam is parallel to the optical axis.

(Problem to be solved by the invention) When a condenser lens is used under the condition that the incident light beam is a parallel light beam, the optical system that constitutes the pickup converts the laser beam into the desired beam diameter. An optical system is required. Particularly when a semiconductor laser is used as a light source, a collimating lens is required to convert the diverging light beam from the semiconductor laser into a parallel light beam. In general, it is sufficient for the collimating lens used for pickups to cover a small NA, but at least two lenses are required, and condensing lenses often have three lenses, so common pickups It can be said that approximately five lenses are used. In a pickup optical system that has a part that converts a light beam into a parallel light beam, it is common to drive only the condenser lens, while the focusing and tracking parts are driven. Optical lenses and the like are not integrated and controlled and driven. The reason for this is that when integrated, undesirable phenomena such as an increase in the weight of the driving part and the need for a large driving force, and a decrease in the cutoff frequency occur.

(Means for Solving the Problems) An object of the present invention is to integrate the functions of a collimating lens and a condensing lens into one set of lenses, and to connect a light source and a condensed spot with a finite magnification. It is an object of the present invention to provide an optical disk lens that is light in weight by reducing the number of lenses and is suitable for the optical system that can be integrated and controlled.

(Structure of the Invention) The lens system according to the present invention is structured as follows, as shown in the Makisei diagram in FIG.

That is, from the light source side to the condensing side, the 18th lens, the 2nd lens, etc.
It is composed of three lens groups: a group lens and a third group lens. In order from the light source side, the first group lens is composed of a first lens that is a biconvex positive lens, and the second group lens is a positive lens. The lens is composed of a cemented lens consisting of a second lens and a third lens which is a negative lens, and the third lens group consists of a fourth lens which is a positive meniscus lens with its convex surface facing the light source side.It is composed of 4 elements in 3 groups. This lens system is characterized by satisfying the following conditions.

(1) 4.0<I23/f<7.0(2>
0.5 <rs / f4 < 0.7 However, n is the focal length of the entire system, fts is the focal length of the second group lens, I4 is the focal length of the third group lens, and rs is the light source of the fourth lens. is the radius of curvature of the side.

The above conditional expression (1) is a condition for maintaining a good working distance. If the upper limit is exceeded, the power borne by the fourth lens of the third lens group becomes too large, making it difficult to satisfactorily correct each aberration.

Moreover, if the lower limit is exceeded, the working distance becomes too small, causing a problem in the focusing flill@ drive.

Conditional expression (2) is a condition for properly correcting the sine condition. If the upper limit is exceeded, the sine condition will be undercorrected, and if the lower limit is exceeded, the sine condition will be overcorrected, and off-axis aberrations will worsen, making it difficult to ensure good light collection performance off-axis.

By satisfying the above configuration and conditions, it is possible to realize an optical disc lens suitable for controlling drive by integrating the light source section and optical system, etc., and since it is a finite system, the number of lenses can be reduced, resulting in low cost. , it is also possible to realize compactness.

〔Example〕

Examples based on the present invention will be shown below.

If FIG. 1 does not show the structure of an embodiment of the optical disc lens of the present invention, the first lens group I, the second lens group ■, and the third lens group ■ are shown in order from the light source side toward the condensing side. It is composed of three lens groups, and in order from the light source side, the first group lens ■ consists of the first lens L1, which is a biconvex positive lens, and the second group lens ■ consists of the second lens L2, which is a positive lens. It is composed of a cemented lens with a third lens L3 which is a negative lens, and the third lens group (3) is composed of a fourth lens L4 which is a positive meniscus lens with its convex surface facing the light source side.
This is a lens system with a group of four elements. Note that the reference numeral 1 indicates a light source side cover glass, and the reference numeral 2 indicates a light condensing side cover glass. Second
The figure is an aberration diagram.

However, rj, r2...rj: radius of curvature d of each component lens,
, d2...d6: Axial thickness and axial air spacing nl of each constituent lens, n2...n4 = wavelength 780nR of each constituent lens
+ refractive index ν1, ν2...C4: Number f of each constituent lens
: Focal length of the entire system ft3 : Focal length of the second group lens f4 : Focal length of the third group lens NA : Numerical aperture M : Magnification tdl : Thickness of the 7J bar glass on the light source side of the lens t(
12: Thickness tnl of 7J bar glass on condensing side of lens
: The wavelength of the cover glass on the light source side of the lens is 780. The refractive index at rv is tnz. The wavelength of the cover glass on the condensing side of the lens is 780.
Refractive index Y at nIIl High Example 1 f=1 NA=0.45 M=-0,342r
1=30.7346 61=0.3202n 1
= 1,81961 ν, =37,3r2 =-
2,6124d 2 = 0.0820r 3 = 2
．． 0490 d a = 0.4610n 2
= 1.55749 ν2 =60.8r 4
= −1,1525d 4 = 0.2049n 3
= 1.78571 3 = 25.5r 5
= -8,9284d 5 = 0.2049r 6
=0.8785 dB =0.3586n
4 = 1,81961 ci, = 37,3r 7 =
3.3501 td1=1゜1013 tn 1= 1.51
118td2 = 0.3073t
n2=1.51frs/f=5.8
8 r s / f 4 =0.64 Example 2 f −I N A = 0.45 M = −
0,342r + =32.8731 d t
=0.2739n, =1.81961ν,
=37.3r2=-2,7942d2=0.
0877r 3 =2.1915 d 3 =
0.4794n 2 = 1.55749 C2=
60.8r 4 = -1,2327d 4 = 0.
1507n 3 = 1,78571 ν3 = 2
5.5rs = -9,549665 = 0.027
4r, =0.8903 d, =0.3
698n 4 = 1.81961 ν4 = 37
．． 3r 7 =2.5118 td1=1.1780 tn1=1.5111
8td2 = 0.3287 tn2 =
1.51j n/f = 6.29 r s
/ f 4 =0.58 Example 3 f=1 N△=0.45 M=-0,36
4r 1 =68.4856 d l
=0.2739n 1 = 1.81961
ν1 = 37.3r 2 = -2,4929d 2
=0.0274r3=2.7668
d a = 0.4931n 2 = 1.5
5749 shi2=eo, ar4=1
．． 1232 d 4 =0.1507n 3
= 1,78571 ν3 ==25.
5rs = -3,8626d 5 = 0.02
74r 6 =0,8766 d b'
=0.3698r14 = 1,81961
C4=37,3r7=2.3230 td t=1.1780 tn 1=
1.51118td2 = 0.3287
tn 2 = 1.51fn/f = 4.96
r5 /f4 =0.57 Example 4 f=1 NA=0.45 M=-0,342r
1 =73.4819 d l =0.293
9n 1 = 1.8196+ Si1 = 37.3r
2 = -2,7923d 2 = 0.0294rs
=2,9687 da =0.52
91n 2 = 1.55749 C2 = 6
0.8r4=-1,205164=0.16
17n 3 = 1,78571 ν3
=25.5rs = -4,144465='0.
0294r 6 =0,9112d
a = 0.3968n 4 = 1.81961
C4=37.3r7=2.3103 tdl=L2639 ti,=1.51
118td2 shi0.3527 jn 2
= 1.55jn/j=5.32rs
/f<=0.56 [Effects of the Invention] With the above configuration, the optical disc lens of the present invention is suitable for optical discs where the light source unit, optical system, etc. are integrated to perform focusing and tracking control. In addition, since it is a finite system, the number of lenses can be reduced, making it possible to achieve low cost and compactness.

[Brief explanation of the drawing]

FIG. 1 is an aberration diagram of the first embodiment, showing (a) spherical aberration, (0) sine condition, and (c) astigmatism. (The aberration diagrams of Examples 2 to 4 are omitted because they are similar to Example 1.) rl...rl: radius of curvature of each constituent lens, dl...
d8: Axial thickness and axial air spacing of each component lens, ■ = 1st group lens, ■: 2nd group lens, ■: 3rd group lens, 1: 1st lens, L2: 2nd lens, L3 : 3rd lens, L4: 4th lens.

Claims (1)

[Claims] It is composed of three lens groups, a first group lens, a second group lens, and a third group lens, in order from the light source side toward the condensing side.In order from the light source side, the first group lens is composed of a first lens which is a biconvex positive lens, a second lens group is composed of a cemented lens of a second positive lens and a third lens which is a negative lens, and the third lens group is a light source. A lens system for an optical disk, which is a lens system having four lenses in three groups, including a fourth lens that is a positive meniscus lens with a convex surface facing the side, and which satisfies the following conditions. (1) 4.0<f_2_3/f<7.0 (2) 0.5<r_6/f_4<0.7 However, f is the focal length of the entire system, f_2_3 is the focal length of the second group lens, and f_4 is Focal length of the third group lens, r_6
is the radius of curvature of the fourth lens on the light source side.