JPS6254212A - Aspheric surface single lens - Google Patents

Abstract

PURPOSE:To secure the action distance and the image-forming performance and to execute the aberration correction over the visual field with the image forming-scale factor of about -1/4--1/6 and the focus distance of about 4-5mm by using the aspheric surface single lens to satisfy the special conditions. CONSTITUTION:The lens is the double aspheric surface lens used by the reduction scale factor, has the shape where the radius of curvature comes to be gradually large from the center to the circumference for both surfaces, the light source side is the first surface, the disk side is the second surface, the curvature radius near the optical axis is gamma1 at the first surface, the radius is gamma2 at the second surface, the wall thickness of the equivalent lens is (d), the focus distance is (f) and the image-forming scale factor is beta, and then, the lens is the aspheric surface single lens to satisfy the conditions of the formula. Thus, while the action distance and the image-forming performance are held, the correction of the sphere aberration, the frame aberration and the astigmatism can be executed over the wide visual field.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field The present invention relates to an imaging optical system used for video and audio discs, optical memory devices, etc., and particularly relates to an imaging optical system with an imaging magnification of 11/4 to -176 and an NA of 0. This relates to a double-sided aspherical single lens for finite imaging that corrects aberrations over a relatively wide field of view with a diameter of about 45.

(2) Prior art Conventionally, as an optical system aimed at reducing the cost of an imaging optical system for optical memory, a finite imaging optical system that integrates a collimator lens and an objective lens and uses an imaging magnification of -1/4.33 is disclosed in Japanese Patent Application Laid-Open No. 60-26917. The optical system disclosed here is a lens system with 4 elements in 4 groups, and usually 1
This has the advantage that the number of lens barrels can be reduced to one compared to a lens configuration consisting of two groups of collimator lenses and three groups of three objective lenses. However, compared to a normal lens configuration, the number of lenses was reduced by only one, and the cost could not be reduced sufficiently.

On the other hand, a single lens with aspherical surfaces on both sides as an objective lens for optical memory was disclosed in Japanese Patent Application Laid-open No. 57-64714 and Japanese Patent Laid-open No. 57-765.
12. JP-A-57-201210, JP-A-58-687
11. JP-A-59-26714, JP-A-60=1203
10 etc. These double-sided aspherical single lenses have only been disclosed with an imaging magnification of -1717.7 at the maximum, and this type of lens has an imaging magnification of -1/4 to -1.
/6, and when a numerical aperture NA of about 0.45 is applied, there is a significant deterioration in imaging performance. Furthermore, JP-A-57-7651
2. Japanese Patent Publication No. 57-201210, Japanese Patent Application Publication No. 58-6871
1. JP-A-59-26712, JP-A-60-12031
0 mainly focused on spherical aberration and coma aberration, and aimed at correcting near-axial aberrations of about 0.1 to 0.2 mmφ on the disk surface. That is, in all of these, it is assumed that a collimator lens is used in combination, and the area in which aberration correction is necessary is sufficient to be a minute area near the axis. However, the imaging magnification -1
In a finite imaging system of about /4 to -1/6, the parallel movement of the lens during tracking results in a large off-axis image to the lens, so compared to a conventional infinity imaging system, the lens itself is It becomes necessary to design a wide aberration correction range. That is, imaging performance close to the diffraction limit is required over a range of approximately 0.4 to 0.5 mmφ on the disk surface, and in addition to the conventional correction of spherical aberration and coma, correction of astigmatism is essential. In the above-mentioned Japanese Patent Application Laid-Open No. 57-64714, correction conditions are described in which astigmatism and field curvature are each made equal to zero, and as examples, the imaging magnification is -1/17.7 to -1.
Two examples of meniscus shaped lenses used in /20.2 are disclosed. However, these lenses are designed with a focal length of about 7.5 mm, and if the focal length is set to about 4 to 5 mm for miniaturization, the second surface in particular becomes concave with respect to the disk surface. This has the disadvantage that a sufficient working distance cannot be obtained because of the

(3) Summary of the invention In view of the above-mentioned problems, an object of the present invention is to increase the imaging magnification to 11.
The object of the present invention is to provide an aspherical single lens having a focal length of about /4 to -1/6 and a focal length of about 4 to 5 mm, which can ensure operating distance and imaging performance, and can correct aberrations over a wide field of view.

In order to achieve the above object, the present invention has an aspherical shape, the light source side is the first surface, the disk side is the second surface, and the radius of curvature near the optical axis is γ1 on the first surface, and γ1 on the second surface. When γ2 is the thickness of the lens, d is the focal length, and β is the imaging magnification, (1) −0,25<β<−0,15 (2) −0,45<γ1/γ2 <-0,35(3)1.
5<d/γl<-1,3 (4) 1.2<d/f<1.3.

In addition, in the present invention, as essential components of a device such as an optical memory, a prism material of approximately 5 to 10 mm for optical path branching is provided on the light source side, and a prism material of approximately 1 to 1.5 mm is provided on the disk side for protecting the information surface.
It is assumed that an optical medium made of glass, plastic, etc. of approximately 1.0 mm is arranged.

(4) Examples Below, before showing examples, the above-mentioned conditional expressions will be explained.

First, equation (1) represents the imaging magnification according to the present invention. This means that the NA required to read the information density of an optical disc is 0.4.
N A of 0.075 to 0.12 is assumed to be about 5 to 0.50, and a semiconductor laser is used as a light source to cover the emission angle.
5 shows the specifications of the imaging system according to the present invention, which is aimed at That is, when the lower limit of formula (1) is exceeded, (i) the pupil diameter relative to the focal length becomes large, and therefore, especially high-order spherical aberration,
The occurrence of astigmatism increases, making it difficult to correct the aberration with a single lens. (11) NA on the light source side is affected by the astigmatism of the semiconductor laser (LD) itself (i i i)
As the angle becomes larger, the effect that the emission angle of the semiconductor laser (LD) is not isotropic increases, and as a result, the spot shape on the disk surface becomes elliptical. Moreover, if the upper limit of equation (1) is exceeded, (i) the coupling efficiency of the semiconductor laser decreases, the energy on the disk surface and the light receiving surface decreases, and the S/N
This results in a decrease in (ii) The distance between the light source and the disk increases, resulting in an increase in the size of the system. This results in disadvantages such as: Next, the above equations (2), (3), and (4) are conditional equations to remove spherical aberration, coma aberration, and astigmatism by satisfying these simultaneously, and to obtain the operating distance required for the system. . Each conditional expression will be explained below assuming that each other expression is satisfied. Equation (2) shows the curvature ratio of the first surface and the second surface near the optical axis, and when it is below the lower limit, astigmatism mainly occurs, and when it is below the upper limit, it is particularly difficult to reduce the size. When the focal length of the lens is shortened, it becomes difficult to obtain a desired operating distance. Also, equation (3) is the radius of curvature γ1 near the optical axis of the first surface.
This shows the relationship between the lens thickness d and the lens thickness d, and shows the power arrangement of the first and second surfaces after determining the refractive index and focal length of the lens. That is, when the lower limit value in equation (3) is less than the astigmatism value, the astigmatism will be under-produced, and when it exceeds the -limit value, the astigmatism will be over-produced. Furthermore, equation (4) has thickness d and focal length 111! Regarding f, if it is below the lower limit value, astigmatism will occur too much, and if it is above the - limit value, it will be difficult to obtain the desired operating distance. Note that spherical aberration and comatic aberration can be removed by optimally selecting an aspherical shape after satisfying equations (1) to (4).

The aspherical shape can be designed to have a radius of curvature that gradually increases from the center to the periphery in order to correct in a well-balanced manner the spherical aberration that occurs significantly under the competition.

Examples of the present invention will be shown below.

FIG. 1 is a diagram showing a first embodiment of the aspherical single lens according to the present invention, and shows an optical path diagram when it is arranged in an optical system of an optical disc. Further, FIG. 2 shows the wavefront aberration on the axis of the paraxial image plane and at an image height of 0.2 mm in the first embodiment. In Figure 1,
L is the aspherical lens according to the present invention, C is the medium for protecting the information surface, P is the optical path branching prism, γ1 is the radius of curvature near the optical axis on the first surface of the lens L, and A2 is the radius of the lens L. The radius of curvature near the optical axis on the second surface, d is the thickness of the lens L, n is the refractive index of the lens L at wavelength input = 780 nm, WD is the working distance, n and p are the optical path branching prism P refractive index, tp
is the thickness of the optical path branching prism P, nc is the refractive index of the information surface protection medium C, tc is the thickness of the information surface protection medium C, and L
D indicates the light beam emission position of the semiconductor laser.

Note that the focal length of the wooden non-ball lens is expressed as f, the numerical aperture as NA, and the imaging magnification as β. Also, in Figure 2, h is the image height,
S indicates wavefront aberration in the sagittal direction, and M indicates wavefront aberration in the meridional direction.

The shape of the aspherical surface of the lens L is defined by the terms up to the 10th power of H based on a rotationally symmetric quadratic curved surface, when the origin is the vertex of each surface of the lens L, the optical axis is the optical axis, and the incident height is H. Contains the following (5)
Expressed by the formula.

In equation (5), K is P [constant, B, C, D, E, A',
B'.

C' and D' are all constants of each order. (Hereinafter, K, B
.

C, D, E, A', B', C', and D' are written as aspheric coefficients. )+BH4+CH6+DH8+EH10 +A'lH13+B'lH15 〇'lHI 7+D'
lHI 9--- (5) Note that the order restriction in the above equation is for convenience and is not limited to this.

Table IA and Table IB below show the values of each parameter (including lens data) shown in FIG. 1 and the value of the aspherical aberration coefficient in equation (5), respectively. Incidentally, the subscripts 1 and 2 in the numbers in 1B correspond to the first and second surfaces of the lens L, respectively.

Table IA Table IB As can be seen from Table IA and the wavefront aberration diagram in FIG. 2, the aspheric single lens according to this example has a focal pressure @ f = 4. Imaging magnification β=- at 2mm
Even when used at 1/4.7, it has good imaging performance and operating distance.

That is, it can be said that there is almost no axial wavefront aberration on the image plane, and even off-axis aberrations sufficiently meet the required performance, and good imaging performance is maintained over a wide field of view.

Subsequently, Tables 2A, 2B to 5A, and 5B below show the values of each parameter and the value of the aspheric coefficient, including lens data from the second example to the fifth example of the aspheric single lens according to the present invention. . Also, FIGS. 3 to 6 are similar to the first embodiment, respectively.
It is a figure which shows the wavefront aberration in 5th Example from an Example.

Therefore, the symbols in the figures and tables have the same meanings as in the embodiments described above, and the description formats are also the same.

Table 2 A Table 2B Table 3 A Table 3 B Table 4 A Table 4 B Table 5 A Table 5B (5) Effects of the Invention As explained, the aspheric single lens according to the present invention has the following advantages: A single lens that can correct spherical aberration, coma aberration, and astigmatism over a wide field of view while maintaining performance, and is extremely useful in reducing the cost of optical head optical systems such as optical discs. It is.

[Brief explanation of drawings]

FIG. 1 is an optical path diagram of a first embodiment in which an aspherical single lens according to the present invention is arranged in an optical system of an optical disk, and FIGS. 2 to 6 are paraxial diagrams of the first to fifth embodiments, respectively. A diagram showing wavefront aberration at an image plane "axis" and an image height of 0.2 mm. P: Optical path branching prism, S: Wavefront aberration in the sagittal direction, L: Double-sided aspherical single lens, M: Wavefront aberration in the meridional direction, C...
Information surface protection medium, LD... Light beam emission position of semiconductor laser.

Claims (1)

[Claims] (1) A double-sided aspherical lens used at a reduction magnification,
Both sides have a shape in which the radius of curvature gradually increases from the center to the periphery, with the light source side being the first surface and the disk side being the second surface, and the radius of curvature near the optical axis being γ_1 on the first surface and γ_2 on the second surface. , when the lens thickness is d, the focal length is f, and the imaging magnification is β, (1) −0.25<β<−0.15 (2) −0.45<γ_1/γ_2<− 0.35 (3)
1.5<d/γ_1<-1.3 (4) An aspheric single lens that satisfies 1.2<d/f<1.3.