JPS59140414A - Objective lens for optical disk - Google Patents

Abstract

PURPOSE:To make a numerical aperture of an objective lens brighter, and to correct satisfactorily various aberrations, especially, a spherical aberration by constituting a titled lens of a two group and two piece constituting lens for satisfied conditions, and constituting both the first group and the second group of a positive meniscus lens whose convex surface is faced to a light source side. CONSTITUTION:A titled lens consists of a two group and two piece constituting lens, the first group R1 is constituted of a positive meniscus lens whose convex surface is faced to a light source side, the second group R2 is also constituted of a positive meniscus lens whose convex surface is faced to the light source side, and when a composite focal distance of the whole lens system, a focal distance of the second group, a radius of curvature of the i-th lens, the i-th lens thickness or lens interval, and a refractive index of the j-th lens are denoted as F, F2, ri, di and Nj, respectively, they are constituted so as to satisfy each condition of the expressions. The conditions of the expressions (2), (3) correct a comatic aberration by putting a sine condition in order, and the conditions of the inequalities (1), (4) correct satisfactorily a spherical aberration.

Description

[Detailed description of the invention]

The present invention relates to an objective lens for optical discs (high-density information recording media). As is already known, an objective lens used to read high-density information recorded on an optical disk must have a resolution of about 1F because it must read high-density signals. Therefore, the numerical aperture (NA) of the objective lens
requires a brightness of about 0.48. Moreover, this objective lens is often movable for tracking and focusing, and is required to be small and lightweight. In addition, since this objective lens is often movable, the distance between the objective lens and the disk surface, that is, the working distance, must be larger than a certain level, otherwise the objective lens will hit the disk surface. This may cause practical inconvenience. The lens according to the present invention takes into consideration the above points, has a bright NA of 0.48, and has a working distance of 0.39F to
Although it is sufficiently large at 0.40F, and is also small and lightweight, it successfully corrects various aberrations, especially spherical aberration. The present invention consists of two lenses in two groups.The first group is a positive meniscus lens with its convex surface facing the light source, and the second group is also a positive meniscus lens with its convex surface facing the light source. , F is the composite focal length of the entire lens system, F2 is the second
focal length of the group, ri is the first radius of curvature, di is the first
This objective lens for an optical disk is characterized in that it satisfies the following conditions, where the thickness of the th lens or the lens interval, Nj, is the refractive index of the first lens group. (1) 2. OF < F2 < 2.4F (2) I
OF < r 2 < 20F (3) 0.5F <
ra<0.7F(4)Ns-N2>
1.83 Each condition will be explained below. The condition (1) is for good correction of spherical aberration, and since the lens of the present invention is composed of two lenses in two groups, if the focal length of the second group lens is determined, the focal length of the first group lens is The focal length of the lens is also almost fixed. 6 Now that the power distribution of each lens is fixed, F2
When is larger than the upper limit, the focal length of the first group lens becomes too small, the amount of negative spherical aberration generated in the first group lens increases, and it becomes difficult to correct the spherical aberration. or,
When F2 is small beyond the lower limit, the focal length of the second group lens becomes too small, and the amount of negative spherical aberration generated in the second group lens increases, making it difficult to correct the spherical aberration. Conditions (2) and (3) are conditions for correcting coma aberration by adjusting the sine condition, and correction of aberration of an objective lens for optical disks basically involves improving the image performance on the optical axis of the lens. Although spherical aberration can be corrected to improve the image quality, optical axis misalignment due to lens setting is unavoidable, and off-axis image performance must be corrected to some extent. To achieve this, it is necessary to improve the sine conditions and correct coma aberration. Now, when F2 in condition (2) is large beyond the upper limit, the amount of unsatisfactory sine condition becomes positive and too large, which leads to an increase in coma aberration, which is not preferable. or,
When F2 is small beyond the lower limit, the amount of unsatisfactory sine condition becomes negative and too thick, which leads to an increase in coma aberration, which is undesirable. When F4 in condition (3) is large beyond the upper limit, the amount of unsatisfactory sine condition becomes positive and too large, which leads to an increase in coma aberration, which is undesirable. Moreover, when F4 is small beyond the lower limit, the amount of unsatisfactory sine condition becomes negative and too large, leading to an increase in coma aberration, which is not preferable. Condition (4) is a condition for good correction of spherical aberration, and when N1*N2 is small beyond the lower limit, the radius of curvature of each lens becomes small in order to obtain the same focal length, and each The amount of spherical aberration generated on the surface increases, making it difficult to correct the amount of spherical aberration to the desired amount. The lens according to the present invention satisfies the conditions (1) to (4) above, and provides an objective lens for an optical disk in which n and aberration are well corrected. Next, Example 1 of the lens of the present invention. Example 2. The numerical values of Example 3 are shown. However, Wt is the air distance (working distance) between the objective lens and the cover glass of the disk, t is the thickness of the cover glass of the disk, and Nt is the refractive index of the cover glass of the disk. Further, all refractive indices are values at a wavelength of 800 nm.

[Example 1] r t = 1.543 d s = 0.572
N r =1.84850r2 =10.823
d2=0.007r:]=0.593
d : l = 0.327 N 2 = 1.8
4850r 4 = 0.675 W t = 0.352 t = 0.289
Nt=1.51F2=2.029

[Example 2] r t = 1.558 d s = 0.368
N1=1.86796r2=13.099
d2=0.014r3'' 0.617
d3. =0.325 N2 = 1.867
96r 4 = 0.681 F=1.000 NA=0.48 Wt=0.
387t = 0.291 N t = 1.51
F2=2.256

[Example 3] r + = 1.626 d t = 0.309
N1=1.87955r2=12.129
d2=0.014r:]=0.627
d3=0.335 N2=1.87
955r a = 0.681 W t = 0.398 t = 0.291 N
t=1.51F2=2.307

[Brief explanation of drawings]

Fig. 1 is a lens configuration diagram of Example 1, Fig. 2 is a coma aberration and wavefront aberration diagram of Example 1, Fig. 3 is a lens configuration diagram of Example 2, and Fig. 4 is a coma aberration and wavefront aberration diagram of Example 2. Wavefront aberration diagram. FIG. 5 is a lens configuration diagram of Example 3, and FIG. 6 is a diagram of coma aberration and wavefront aberration of Example 3. Figure 1 Z country? qYnu i ---Noriev f+
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Claims (1)

[Claims] Consisting of two lenses in two groups, the first group is a positive meniscus lens with a convex surface facing the light source side, and the second group is also a positive meniscus lens with a convex surface facing the light source side, An objective lens for optical discs, characterized by satisfying the following conditions. (1) 2. OF < F2 < 2.4F (2) I
OF < r 2 < 20F (3) 0.5F <
r a < 0.7F (4) N 1. N2>
1. '83 Here, F is the composite focal length of the entire lens system.
F2 is the focal length of the second group lens, F2 is the radius of curvature of the side surface of the disk of the first group lens, F4 is the radius of curvature of the side surface of the disk of the second group lens, Nl is the refractive index of the first group lens, N2
is the refractive index of the second group lens.