JPS63220210A - Objective lens - Google Patents

Abstract

PURPOSE:To satisfactorily correct aberrations and to shorten the overall length of a lens system to make it miniaturized and light-weight by satisfying a specific condition between radiuses of curvature of the object-side surface and the image- side surface of a first lens group consisting of a positive meniscus lens whose convex is directed to the object side. CONSTITUTION:In the case of the use for reduction power, a first lens group I consisting of a positive meniscus lens whose convex is directed to the object side, a second negative lens group II, a third positive lens group III, and a fourth lens group IV consisting of a positive meniscus lens whose convex is directed to the object side are provided in order from the object side. This lens system satisfies condition 0.17<r1/r2<0.40 where r1 is the radius of curvature of the object-side surface of the first lens group I and r2 is that of the image-side surface of this group I. Thus, the miniaturized and light-weight lens system where aberrations are satisfactorily corrected is obtained.

Description

[Detailed Description of the Invention] [Technical Field] The present invention is used when recording information on an information recording carrier or reproducing information in an optical recording and/or reproducing device. This relates to objective lenses.

[Prior art]

In recent years, optical discs such as video discs and compact discs have been widely used as large storage capacity record carriers.

Furthermore, optical cards, which have advantages such as large storage capacity and good portability, are also attracting attention as optical record carriers similar to optical disks.

In order to record information with high density on this type of record carrier and to accurately reproduce recorded information, an objective lens used in an information recording/reproducing apparatus is required to have a resolution of several μm.

It is also necessary to provide a sufficient distance between the surface of a carrier such as an optical disk or an optical card and the objective lens to prevent contact between the two to avoid damage to the record carrier or the objective lens.

Furthermore, in the above-mentioned information recording/reproducing apparatus, the mainstream method is to move the objective lens in the optical axis direction or in a direction orthogonal to the optical axis direction in order to perform autofocus or autotracking. Therefore, in order to improve response characteristics, this type of objective lens is required to be smaller and lighter.

Conventionally, as this type of objective lens, Japanese Patent Application Laid-Open No. 58-420
Lens systems having a configuration of four groups and four branches are disclosed in Japanese Patent Application Publication No. 21, Japanese Patent Application Laid-open No. 58-208719, Japanese Patent Application Laid-open No. 60-122915, and the like.

However, the objective lenses disclosed in these publications have a large overall length of the lens system, and cannot be made smaller and lighter as described above.

In addition, the objective lens shown in the above publication is designed in accordance with the specifications of the optical disk, and when used for a record carrier with a thin protective layer covering the recording surface, such as an optical card, it may cause aberrations. It is difficult to make corrections.

The thickness of the optical card is about 0.8 mm, which is about the same as the thickness of commonly distributed magnetic cards, so when considering the strength of the optical card, the thickness of the transparent protective layer in the optical card t
is approximately 0.4 mm.

In the objective lens of four groups and four elements shown in the above publication, if the focal length of the objective lens is r, the thickness t of the transparent protective layer of the record carrier to which these objective lenses can be applied is 0.26.
f to about 0.34f. Therefore, when the thickness t of the transparent protective layer in the optical card is 0.4 mm, the focal length f of these objective lenses must be set to about 1.17 to 1.54.

However, as this type of objective lens, the focal length f=1
．． When designing a lens system of about 17 to 1.54 mm,
The radius of curvature of each lens has become too small, making manufacturing extremely difficult.

In order to overcome the above-mentioned drawbacks of the conventional objective lens, as shown in FIG.
A parallel plate 106 for thickness correction may be interposed between the two.

According to the specifications of conventional objective lenses, the thickness t of the transparent protective layer 103 is optimized to be approximately 1.04 to 1.36 mm, so the thickness t of the transparent protective layer 103 of the optical card 101 is 0.
．． For the thickness exceeding 4 mm, t' = 0.64 to 0
．． Parallel flat plate 1 for correction having a thickness t' of about 96 mm
By correcting according to 06, aberrations can be corrected using a lens system that can be manufactured.

However, although the axial thickness TD of the conventional objective lens 105 with 4 elements in 4 groups is usually about TD>1.67f, the parallel plate for correction has a thickness t' of about 0.16f to 0.24f. Adding 106 to the lens system is clearly contrary to the desire for miniaturization of the lens system and is not preferred.

[Summary of the invention]

The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to provide an objective lens in which aberrations are well corrected and the overall length of the lens system can be shortened to reduce the size and weight. There is.

In order to achieve the above object, the objective lens according to the present invention includes:
When used at reduced magnification, a first group lens consisting of a positive meniscus lens with a convex surface facing the object side in order from the object side, a negative second group lens, a positive third group lens, and a convex surface facing the object side. and a fourth group lens consisting of a positive meniscus lens, and the radius of curvature of the object side surface of the first group lens is r1.
, when the radius of curvature of the image side surface is r2, it is characterized by satisfying the following condition: 0.17<r I/r 2 <0.40- (1).

Further, the present invention has the above characteristics, and when the radius of curvature of the object side surface of the second group lens is r3, -7,0<r
By satisfying the condition 2/r 3 <-2,7 (2), good aberration correction is performed while keeping the overall length short.

Further, the present invention has all of the above characteristics, and the radius of curvature of the object side surface and image side surface of the fourth group lens is '? + r
8, furthermore, +0.44 < r 7 / r 8 < 0.73...
- Strict aberration correction is achieved by satisfying the condition (3).

According to the present invention, by designing the lens so as to satisfy each of the above-mentioned conditions, it is possible to obtain various small and lightweight lens systems in which aberrations are well corrected.

Further, the objective lens of the present invention is a lens system that can sufficiently correct aberrations even if the protective layer covering the recording surface of an optical card or the like is as thin as about 0.4 mm. Further features of the present invention will become clear from the examples given below.

〔Example〕

FIG. 1 is a sectional view schematically showing an objective lens according to the present invention.

In the figure, L is the objective lens, ! is the first group lens,
It consists of a meniscus lens with positive refractive power. IT is a second lens group, which is composed of a lens having negative refractive power.

[1, [ is the third group lens, which is composed of a lens having positive refractive power. IV is the fourth lens group and is composed of a meniscus lens having positive refractive power.

Further, 101 is an optical card which is an information recording carrier, +02 is a support substrate of the optical card +01, 103 is a transparent protective layer on the support substrate +02, and 104 is an information recording layer formed on the upper surface of the support substrate +02, which is transparent. It is covered with a protective layer 103.

Other symbols in FIG. 1 are parameters indicating the shape and arrangement of the objective lens L and the optical card 101.

Among these codes, ri (i=1, 2, 3...)
is the radius of curvature of each surface, di (i=1.2, 3...)
is the axial distance between each surface, WD is the working distance, and t is the optical card 1
The thickness of the transparent protective layer 103 of No. 01 is shown.

In FIG. 1, a light beam from a light source such as a semiconductor laser (not shown) enters the objective lens L from the left direction in the figure. This objective lens L focuses this light flux (for example, parallel light flux) onto the information recording layer 104 via the transparent protective layer 103, forms a bit pattern on the information recording layer 104, and collects reflected light from the information recording layer. It receives the signal and directs it to the detection system.

The first lens group (■) of the objective lens L is a meniscus lens with a convex surface facing the object side, that is, the light source side, and the conditional expression (
1) is satisfied.

Conditional expression (1) is a condition for correcting both spherical aberration and coma aberration while shortening the overall length of the lens system. rl/
If the value of '2 is less than 0.17, spherical aberration will be insufficiently corrected, making it difficult to correct aberrations in the rear group lenses r1, III, and IV. Conversely, if the value of r H/ r 2 is 0.
If the value exceeds 40, a large amount of inward comatic aberration will occur, and similarly, it will be difficult to correct the aberration with the lenses rI, uI, and IV in the rear group.

Therefore, in the objective lens of the present invention, the value of r + / r 2 is set to satisfy the conditional expression (1), and the first group lens I and the rear group lens 11. We are optimizing the shape and placement of III and IV.

Furthermore, according to the present invention, the conditional expression (2)? : By setting the lens system so that 4 is added, aberrations can be corrected well. Conditional expression (2) is the ratio r of the radius of curvature r of the first lens group I and the radius of curvature r3 of the second lens group ■ shown in FIG.
2/r3, and in order to better correct off-axis aberrations and improve imaging characteristics, the conditional expression (11 ) to l+S! It is desirable to add i.

The above conditional expression (4) indicates the conditions for good correction of spherical aberration and astigmatism, and the refractive power (1/f) of the objective lens L and the third group lens ■ in FIG. optical card 10
The ratio to the radius of curvature r6 of the surface on the first side (image side) is defined. When 1/r6・f becomes -0.22 or less, spherical aberration is undercorrected, and conversely, when 1/r6・f becomes 0.02 or more, astigmatism increases, and in both cases, off-axis The imaging characteristics of the image will deteriorate.

By having the above-mentioned features, the objective lens of the present invention can record information on the optical card 101 shown in FIG.
It becomes an objective lens suitable for reproducing information from the optical card 101.

This condition is a condition for satisfactorily correcting spherical aberration.

When the value of r2/r3 becomes -7.0 or less, spherical aberration becomes overcorrected. Conversely, when the value of r2/r3 becomes -2.7 or more, spherical aberration becomes insufficiently corrected.

The above conditional expression (3) is a condition for correcting spherical aberration, coma aberration, and astigmatism in a well-balanced manner, and the ratio of the radii of curvature of both sides of the fourth lens group v shown in FIG. 1 r 7 / r
It stipulates B. In conditional expression (3), r7/r
If the value of s becomes +0.44 or less, even if coma aberration and astigmatism can be corrected, spherical aberration will be insufficiently corrected. On the contrary, r7/
When the value of rg exceeds 0.73, high-order coma aberration increases and astigmatism also increases, making it difficult to correct.

Therefore, in order to properly correct various aberrations, one conditional expression (1) must be used.
In addition, it is desirable to design the lens so that conditional expression (2) and conditional expression (3) are satisfied.

Further, according to the objective lens of the present invention, the optical card 1 can be used without disposing the correction parallel plate 106 shown in FIG. 2 in the optical path.
Even for record carriers such as 01 and the like having a thin transparent protective layer 103, it is possible to design a compact and lightweight objective lens with good aberration correction.

In particular, when the focal length of the objective lens is 0.04
The present invention is suitable for providing an objective lens used for recording and reproducing information on a record carrier having a transparent protective layer having a thickness t such that 4F≦t≦0,133f.

Hereinafter, specific numerical examples of the present invention will be shown.

Implementation 4! A11 to Example 5 show lens data of the objective lens according to the present invention and parameters in the conditional expressions (1) to (4) above.

In Examples 1 to 5, the focal length of the objective lens, which is the basis of the design, is standardized to f=1.o, the numerical aperture NA is NA=0.46, and the design wavelength λ is λ= 830(n
m).

ri (λ= 1.2, 3...) is the radius of curvature of the i-th surface counting from the light source side, di (i= 1.2,
3. ) is the axial wall thickness or axial air gap between the i-th and i+1-th surfaces counting from the light source side, ni, v
i (i==1, 2, 3°4) represents the refractive index and Abbe number of the i-th lens counting from the light source side, respectively.

Further, TD is the total length of the objective lens, WD is the working distance, t is the thickness of the transparent protective layer of the record carrier, nl is the refractive index of the transparent protective layer, and SI is the object distance.

Also, r+ /r2 + r2/r3 + l/r6 +
r7/rB is each parameter related to the conditional expressions (1) to (4) above, and the objective lenses shown in Examples 1 to 5 below are lens systems that satisfy all of the conditional expressions (1) to (4). This is a design example.

Figures 3 (A), (B) to Figure 7 (A), ([
3) is an aberration diagram showing various aberrations of the objective lenses shown in Examples 1 to 5.

Here, Fig. 3(A) to Fig. 7(A) show the longitudinal aberrations of spherical aberration, astigmatism, and distortion, and Fig. 3(B) to Fig. 7(B)- show the image height Y. '=0.0.0.02.0.0
It shows the lateral aberration at 3°0.04.0.05.

Further, ΔS and ΔM represent aberration curves at the sagittal image plane and the meridional image plane, respectively.

1"L = 1.7399 d, = 0
．． 193I n, = 1.81720V
l=37.2r2=4.7203d
2= 0.0943rs =-1,5826(1+
= 0.1123 nz = 1.49199
V2 =65.0r4=-32,30456a
= 0.0112rs = 1.3619
ds = 0.2200 ns = 1.8
1720 u3=37.2ra=84.082
9 d6=Q,0112ry=0.
7637 dy = 0.2088 n4
= 1.81720 ν4 = 37.2ra
= 1.1506 f=1.0, TD=0.851
, WD=0.616, NA=0.4fit
”0.085, nt=1.50974
, λ = 830 nm, S, = −C
X) r+/r2=0.37, r2/rs=-2
,98,1/ra=0.012,rt/ra
=o,a6ken'1'L presentationri = 1.7029 d+ = 0.
1937 11 = 1.81720 17+
=37.2r2 = 5.5859 d2
= 0.0856rs = -1,77[11
ds = 0.1126 n2 = 1.4919
9112=65. Or4 = 8.1070
d4 = 0.0653rg = IJ800
d5 = 0.1869 ns =
1.81720us=37.2re” 26
．． 1442 d6= 0.0135r7=
0.7661 dt = 0.1960 1
4 = 1.81720 'Lla = 37.2ra
= 1.2615 f=1.0, TD=0.854,
WD=0.632, NA=0.46t=0
．． 085, nt = 1.50974
, λ = 830 nm, S, = −(x
) rl/r2 = 0.30, r2/rs = -3
,21,1/ra=0.0:1B, r,/ra
= 0.61 1" L yuri = 1.7264 d+ =
0.1933 1+ = 1.81720 u
+ =37.2r2=6.1939d
2 = 0.0854rs = -1,7662d
s = 0.1169 nz = 1.4
9199 1j3 = [i5.0r4= 6.0
829 d4= 0.1011rs”
1.4853 ds = 0.1865
n, = 1.81720 'l/s = 37
．． 2r6” 206.9153 da =
0.0135ry” 0.7644
dv = 0.2023 n4- 1.111
720 +4 = 37.2ra ” 1.3
089 f=1. o, TD=0.899
, WD=0. l134, NA=0.46t
=0.085, nt =1.50974
, λ =830na+, S+ =-
ωrl/r2=0.28, r2/r3=-3,
51, l/r,,=0.0048, r7/r6
=0.58 doctor jl (+14 ri = 1.7745 d+ = 0
．． 1821 n+ = 1.81720 1J
r = 37.2 rx = 6.5385
d2= 0.0878r3= -1,7997d
s = 0.114I n2- 1.52331
+2 =65.0r4” 13.1306
d4 = 0.1602rs = 1.4699
ds = 0.1821 ns = 1
．． 81720 shi3=37.2ra ”1247.3
178 da -0,0132ry”
0.7465 dt = 0.1821
14 = 1.77227 174 =44.2r6
= 1.2926 f=1.0, TD=0.922
, WD=0.651, NA=0.46t=0
．． 083, nt = 1.50974
, λ =830na+ , S, = −
(X) rl/r2 = 0.27, r2/r3 =
-3,63, 1/ra=0.00080, r7/r
6 = 0.58 prongs 1" L2 rl -1,8457d+ -0,1830J = 1
．． 81720 v+ =37.2r2 = 9.
5236 d, -0,09413r3 =
-1,4767d3 = 0.1521 nz =
1.52331 +2 =65.0r4=5
．． 5846 d4=0.1323rs”
1.9487 ds = 0.1918
n3 = 1.81720 +3 = 37
．． 2r6 = −5,0779d6− 0.0110
rt=0.8120dy=
0.11130 n4 = 1.81720
νa = 37.2ra = 1.6535 f = 1.0, TD = 0.948,
WD=0.891, NA=0.46t=
0.084, n t =1.50974
, λ = t130nm, S, = -o.

rl/r2 =0.19, r2/r+ =-6,
45, 1/ra=0.20, r,/ra=0.4
9 [Effects of the Invention] As described above, the objective lens according to the present invention is a lens system composed of four groups with a positive, negative, positive and positive refractive power arrangement, and when the first group lens is used at a reduction magnification, the object side (light source side) By constructing the objective lens with a convex meniscus lens and designing the ratio rl/r2 of the radius of curvature of both surfaces to a predetermined value, the overall length of the lens system can be shortened and the objective lens can be made smaller and lighter. Needless to say, aberrations are well corrected and an objective lens with high imaging performance can be provided.

Further, according to the objective lens according to the present invention, it is possible to design a lens system that is small, lightweight, and has high performance, regardless of the thickness of the transparent protective layer of the information recording carrier.

In particular, the thickness t of the transparent protective layer of an optical card etc. is 0.044f≦t≦0.133f with respect to the focal length of the objective lens.
It is suitable as an objective lens for recording or reproducing information on a medium-sized information recording carrier.

[Brief explanation of the drawing]

FIG. 1 is a lens sectional view schematically showing an objective lens according to the present invention. FIG. 2 is an explanatory diagram showing a thickness correction method when a conventional objective lens is used for recording and reproducing an optical card. 3(A) and 3(B) are aberration diagrams of the objective lens shown in Example 1. FIGS. 4(A) and 4(B) are aberration diagrams of the objective lens shown in Example 2. FIGS. 5A and 5B are aberration diagrams of the objective lens shown in Example 3. FIGS. 6(A) and 6(B) are aberration diagrams of the objective lens shown in Example 4. FIGS. 7(A) and 7(B) are aberration diagrams of the objective lens shown in Example 5. L, 105・・・・・・・・・・・・・・・・・・
Objective lens 1, r1, 111. IV・・・・・・・・・
...First group lens (i=1,2,3.4) 101...
・・・・・・・・・・・・・・・・・・・・・ Optical card 103・・・・・・・・・・・・・・・・・・・・・
...Transparent protective layer 106...
・・・・・・・・・・・・Parallel plate ΔS for thickness correction
・・・・・・・・・・・・・・・・・・・・・・・・
Aberration curve ΔM of sagittal section...
・・・・・・・・・・・・Aberration curve of meridional cross section Fig. 1 Fig. 2 μυ /〃 /q Fig. 4 (A) A/A Y'Y' peripheral plane W
Difference 1 number order Distortion prisoner ζ%) Figure 4 (B
) Kinuta 6 (￥] CA) NA Y'Y' stock ゛ music time L 弗 1 times, convergence L λ side 1 deviation stand) Fig. 6 CB) Fig. 7 (A) Ahead of the run...route, convergence destination hIII]us+t<
%] Figure 7 CB)

Claims (3)

[Claims] (1) When used at reduced magnification, from the object side, the first group lens consists of a positive meniscus lens with a convex surface facing the object side, the negative second group lens, the positive third group lens, and the and a fourth group lens consisting of a positive meniscus lens with a convex surface facing, and when the radius of curvature of the object side surface of the first group lens is r_1 and the radius of curvature of the image side surface is r_2, 0.17 An objective lens that satisfies the condition <r_1/r_2<0.40. (2) The radius of curvature of the object-side surface of the second group lens is r_
3, the objective lens according to claim (1) satisfies the following condition: -7.0<r_2/r_3<-2.7. (3) When the radius of curvature of the object-side surface and image-side surface of the fourth group lens is r_7 and r_8, respectively, the scope of claim 1 satisfies the following condition: +0.44<r_7/r_8<0.73 Objective lens described in section 2).