JPH03196011A - Aspherical face single lens - Google Patents

Abstract

PURPOSE:To obtain the aspherical face single lens which is as bright as about 0.55 N.A. and is well corrected in various aberrations at a short focal length by determining the focal length, lens thickness, refractive index, etc., so as to satisfy specific conditions. CONSTITUTION:A transparent parallel plane plate 2 satisfying the conditions 0.4 <=t/f <= 0.5 when the thickness on the plane side of the aspherical face single lens 1 is designated as (t) is disposed in the aspherical face single lens 1 having the 1st lens face consisting of the aspherical face and the 2nd face consisting of an approximately a plane and having a focal length (f). The conditions of equation I to equation III are satisfied when the refractive index at the use wavelength of the material of the aspherical face single lens 1 is designated an (n), the axial thickness as (d), the difference in the direction parallel with the optical axes of the aspherical face at 10% within the effective diameter of the lens determined by the numerical aperture of the 1st lens face and the reference spherical face determined by the paraxial radius of curvature as (10) and the direction of the difference where the curvature of the aspherical face weakens as positive. The lens has the brightness of about 0.55 in numerical aperture and the aberrations are well corrected within the range of about 1 angle of view.

Description

Detailed Description of the Invention (g. Field of Industrial Application) The present invention relates to an aspherical single lens, particularly an optical lens with high optical performance used for recording and reproducing in video and audio discs, optical memory devices, etc. N for pickup,
This relates to an aspheric single lens in which various aberrations with A of about 0.55 are well corrected.

(Prior Art) In recent years, optical disks such as magneto-optical disks and phase change disks have been widely used as large storage capacity recording media.

In this type of optical memory device, the optical pickup objective lens used to optically read the minute signals recorded on the disk and erase the recorded signals has good aberrations. It is required to have high corrected optical performance.

For example, the pitch of the signal recorded on the disk is 1~
In order to optically read very fine objects of about 2 μm, a high resolution close to the limit resolution is required.

In addition, in order to perform high-speed automatic focusing in order to read signals on a disk that is rotating at high speed, and to move Tracking III etc. in various directions with high responsiveness,
It is required to be as small and light as possible, and to have a predetermined working distance to the extent that external vibrations and shocks do not hit the disk.

Such an objective lens for optical pickup is disclosed in, for example, Japanese Patent Application Laid-open Nos. 60-181714 and 61-88213.
It is proposed in the publication No. 132915 of JP-A-6-132915. In these valley publications, in order to lower the manufacturing angle of 8174 degrees for the objective lens,
The first surface is an aspherical lens, and the second surface is a substantially flat single lens, which corrects various aberrations in a well-balanced manner.

(Membrane point to be solved by the invention) The objective lens proposed in each of the above-mentioned publications has a t/r<0.317, where f is the focal length and L is the thickness of the optical disc used. used in the range. That is, the focal length is relatively long compared to the thickness of the optical disc, and the objective lens as a whole cannot necessarily be said to be small and lightweight.

For example, if the thickness L of the optical disc is the thickness t of the compact disc = 1.2 mm, the focal length of the objective lens is 3.
It was longer than 79 mm.

In general, if you try to maintain N and A (numerical aperture) at around 0.55 and make the entire lens system smaller and lighter, various aberrations such as spherical aberration will occur, making it difficult to obtain high optical performance. It's coming.

In the present invention, by appropriately setting the aspherical surface, N and A can be reduced to 0.
．． The object of the present invention is to provide an aspheric objective lens for light pickup, which has a brightness of about 55 degrees and has high optical performance with good aberration correction within a field angle of about 1 degree.

Another object of the present invention is to create a short focal length aspherical monolith whose ratio of the focal length f to the thickness t of the optical disc is O4≦t/f≦0.5, in order to achieve miniaturization and weight reduction. Our goal is to provide lenses.

Still another object of the present invention is to provide an aspherical single lens in which the first surface is an aspherical surface and the second surface is a flat surface, in order to reduce manufacturing difficulty and reduce costs. It is in.

The above objects of the present invention are achieved by the aspheric lens of the present invention described below.

(Means for Solving the Problems) The aspherical objective lens of the present invention is an aspherical lens having a focal length f in which the first lens surface is an aspherical surface and the second lens surface is a substantially flat surface, and the aspherical single lens is It is used by arranging a transparent parallel plane plate on the plane side that satisfies the following conditions: 04≦t, /f≦05, where the thickness is t; The refractive index is n, and the axial thickness is d.

t51 10 within the lens effective diameter determined by N and A of the lens surface
When the difference in the direction parallel to the optical axis between the aspheric surface at 71 and the reference sphere determined by the paraxial radius of curvature is Δ(10), and the direction in which the curvature of the aspheric surface becomes weaker among the differences is defined as positive, 1.72≦ n≦1. aa・----(1)・ ・ ・
・ ・ ・ ・ ・ (2) 0.8≦d/(ft)
≦1.3 ・ ・ It is characterized by satisfying the following condition (3).

(Embodiment 9#) FIG. 1 is a schematic diagram showing D!JgI of the aspherical single lens of the present invention and a transparent substrate (optical disc) for protecting a recording medium.

2 to 7 are lateral aberration diagrams of numerical examples 1 to 6 of the present invention, which will be described later.

In the m1 diagram, l is an aspherical single lens, 2 is a transparent substrate (optical disk) made of a parallel plane plate, and W and D are working distances.

The aspherical single lens in this example has N and A of about 0.55, a focal length of f, and a thickness of the transparent substrate 2 of t, which is 0.
．． It is suitable for use within the range of 4≦t/f≦0.5, and in this case, it has high optical performance close to the diffraction limit within a field angle of about 1 degree, and is suitable for high-density recording and playback. It is designed to perform well.

In this example, as described above, the 1121st surface is formed with an aspherical surface of a predetermined shape, and the second lens surface is made substantially flat, thereby simplifying the lens shape, facilitating manufacturing with mounting accuracy, etc. The aim is to obtain high optical performance within this range.

Next, the technical meaning of each of the above conditional expressions will be explained.

Conditional expression (1) relates to the refraction -Kn of the material of the aspherical single lens at the wavelength used, and is mainly intended to correct spherical aberration and coma aberration in a well-balanced manner when an aspherical surface is used.

Since the aspherical single lens according to the present invention has a flat second lens surface, when the focal length f is constant, the radius of curvature R1 of the PJJl lens surface becomes R1=f (n-1). Therefore, conditional expression (1) If the lower limit of is exceeded, the curvature half 1'lR1 becomes too small and a lot of under-spherical aberration occurs.If an aspherical shape is set to correct this, the axial imaging performance can be maintained well. However, it is not good because the deterioration of off-axis imaging performance, especially the deterioration of imaging performance due to coma aberration, increases.

The upper limit of conditional expression (1) is a value determined based on the refractive index of materials that can currently be used, including cost.

Conditional expression (2) relates to the aspherical shape of the 1121st surface, and is intended to appropriately set the amount of aspherical surface at the outermost periphery of the effective diameter, mainly to correct spherical aberration and comatic aberration in a well-balanced manner.

If the lower limit of conditional expression (2) is exceeded, spherical aberration will be under-corrected, and if the upper limit is exceeded, spherical aberration will be over-corrected.
In either case, the best way to correct coma aberration is to correct it in a well-balanced manner.
It's getting louder.

Since the second lens surface of the aspherical single lens according to the present invention is made of a flat surface, the axial thickness d is irrelevant to the focal length f, and the working distances W and D and the thickness t of the optical disk are taken into account. Conditional expression (3) can be set freely by setting the thicknesses d and t appropriately at this time,
This is to satisfactorily correct various aberrations.

If the lower limit of conditional expression (3) is exceeded, the edge thickness as a lens with a predetermined curvature becomes too thin, which is not good as it becomes difficult to handle during manufacturing. This is difficult, and the working distances W and D (lens back) are too short, making it unsuitable for use as a single lens for optical discs.

Further, in the present invention, in addition to the conditions (1) to (3) above, when the difference in the amount of aspherical surface at 70% of the effective diameter of the 1121st surface is Δ(7)... (4) If the upper limit or lower limit of condition (4), which is preferably satisfied, is exceeded, it becomes difficult to correct axial aberrations and off-axis aberrations in a well-balanced manner and obtain high optical performance.

In the present invention, the second lens surface of the aspherical single lens is not completely flat, but may be substantially flat in terms of aberrations.

Next, numerical examples of the present invention will be shown.
1, R2 are the radii of curvature of the first and second lens surfaces of the aspherical single lens, d and t are the thicknesses of the aspherical objective lens and the disk,
n and nt are the refractive indexes of the aspherical objective lens and disc materials at the wavelength used; When the distance H1 is the paraxial radius of curvature R1, the conic constant is Ai (ix3.4, 5,...), and the #spherical coefficient is Ai (ix3.4, 5,...) + A ff H3 + A 4 H' + A S
It is expressed by the formula H' + .

Furthermore, Table 1 shows the relationship between each of the above-mentioned conditional expressions and the numerical values in the numerical examples.

Numerical Example 1 f=1. ONA=0.55 R,=0.79155 d=0.5536R*
” Co W, D, = 0.4182t = 0
．． 4286 n = 1.79155 nt = 1.57095 -4,58714xlO-" A,, -1,09021Xl0- Aa --5,21529X In-"A,, -3,6
4995x 10-' ^1゜, 4.050+4 XIO"A1□ -1,
35919xlO6 A,. 8.47045 xlO-'A16□ 2.
27748 xlO61,51074x 7.73387X 2.89891X -3,32066X 1.72484X 1.47110x -5,26755X Numerical Example 2 r=1. ONA=0.55 R1=0.7915S d=0.5893R,
= OOW, D, = 0.3983 t = 0.4286 n = 1.79155 nt = 1.57095 K -5,85505 A,, -1,07778 A,, -5,59007 As --3,62741 A1゜, 4.011482 A++-1,35061 A, 4. 8.60255 A, @, 2.19213 XIO-koX In-' Xl01 XIO-'"3.08495X As = -3,29230x A,, = 1.79047X A+s" 1.41491x A+s= -5,17771X Numerical Example 3 f=1. o NA=0.55 R1=0.79+55 d=0.6250Rx
= ooW, D, = 0.3783t = 0.428
6 n = 1.79155 nt = 1.57095 K -1,01385 A4 yo -1.0789O A, l , -5,63341 A,, -3,65017 A,, 4.11137 A 1 g w publication, 35555 A,,, 8.67529 8,6□ 2.05711 X 10-" X 10-'Xl0-" X In-' X 10-' × 100 XIO"' 03562X A? "3.1I240x A, = -3,36869X A++" 1.88774x A+s" 1.41665x A+s" -s, +7476X Numerical Example 4 f=1. ONA=O R= 0.80204 Rt= ω -1,136+4 A,, -1,03674 A6 O -5.27570 A81 -3.32695 A1゜+ 3.64293 A1□ -1,17244 A,,, 7.32123 A+a-1,68939 5 d= 0.6333 W, D, = 0.3818 t = 0.4430 n = 1.80204 nt" 1.51462 As = -1,20608x As : -6,68066X Ay = 2.87405X As = -3,03305x A,, = 1.64953X A+s = 1.2Q963x A+i" -4,30096x Numerical Example 5 f=1. ONA=0.55 R=0.79155 d=0.6607R*”
00 W, D, =0.3584t =0.
4286 to -1,9547s xlO-"A4, -1.
07938 Xl0-'As --6,35727X
10-”As --3,55780X 10-'A
1°, 3.40882 Xl0-'A++--1
,35811X 10゜A+4- 9.27736X 10-'A+s-1
, 78290X In.

Numerical Example 6 f=1. ONA=0.55 R+” 0.79323 d=0.6263t1w
oo W, D, = 0.37021t = 0.4
402 K -1,243+4 A4 O -1.07163 A,, -5,51925 As, -3,60811 A1゜, 4.08505 A1□ -1,31851 A,,, 8.73274 A+s-1,99909 XIO -'Xl0-' Xl01 X 10-'Xl0-' ×! Mouth 0 XIO-' 57251 1. 14472x -6,83g34X 3.01020X -3,33787X 1.86170x 1.38106X -5,09266x Table 1 (Effects of the Invention) According to the present invention, by setting each element as described above, To achieve an aspherical single lens that is bright with N and A of about 0.55, has a short focal length, and satisfactorily corrects various aberrations, is compact and easy to manufacture, has high optical performance, and is suitable for optical memory devices, etc. be able to.

In addition, since one of the aspherical lenses of the present invention is a flat surface, 5
Another feature is that the difficulty level of ``8i construction'' is low.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a lens according to an embodiment of the present invention, and FIG.
The figures are transverse aberration diagrams of numerical examples 1 to 6 of the present invention, respectively. This is shown in (A) in the aberration diagram when normalized to f=1.
is on-axis, (B) is an angle of view of 05°, and (C) is an angle of view of 1°. The horizontal axis is the pupil radius coordinate, and the vertical axis is the lateral aberration value. In the figure, l is an aspheric objective lens, 2 is a transparent substrate (optical disk), W and D are working distances, ΔS is a sagittal image plane, and ΔM is a meridional image plane.

Claims (2)

[Claims] (1) In an aspherical single lens with a focal length f where the first lens surface is an aspherical surface and the second lens surface is a substantially flat surface, the thickness of the aspherical single lens on the plane side is 0.4≦ It is used by arranging a transparent parallel plane plate that satisfies the condition t/f≦0.5, where the refractive index of the material of the aspherical single lens at the wavelength used is n, and the axial thickness is d. , N. at the first lens surface. Let Δ(10) be the difference in the direction parallel to the optical axis between the aspherical surface at 100% of the lens effective diameter determined by A and the reference spherical surface determined by the paraxial radius of curvature, and within this difference, the direction in which the curvature of the aspherical surface becomes weaker When is positive, 1.72≦n≦1.88 2.4≦100・(n-1)/(NA)・Δ(10)
An aspheric single lens that satisfies the following condition: /f≦3.00.8≦d/(f−t)≦1.3.