JP2000019389A - Optical system for optical disk - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical system for an optical disk in a proximate field having excellent image-formation performance and easily making a numerical aperture large by making an objective lens include at least a spherical lens and making the lens have the composition where a spherically symmetric refractive index distribution is in the spherical lens. SOLUTION: This optical system is constituted of a lens L1 and a spherical lens L2 and the lens L1 is a plano-concave lens whose object side is plane and whose image side is concave and where the refractive index is homogeneous. Further, the lens L2 is a refractive index distribution type spherical lens where the refractive index on the spherical surface is equal to that of the lens L1. Further, an optical disk recording surface 4 is arranged at an interval equal to or under wavelength order on the image side of the lens L2, and a diaphragm 5 is provided on the object side of the lens L1. Then, a light beam 6 is a parallel beam most separate from an optical axis, passes through the maximum aperture of a diaphragm 5 and the lens L1, passes through the lens L2 having the refractive index distribution and is formed into an image at a point (p) on the spherical surface on the image side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system for an optical disk, and more particularly, to an optical system for recording or reproducing information on or from an optical disk, wherein a distance between an objective lens and a recording surface of the optical disk is less than a wavelength order. The present invention relates to an optical system for an optical disk in a near field.

[0002]

2. Description of the Related Art For example, US Pat. No. 5,125,750 discloses an optical system in a so-called near field in which the distance between an objective lens and the recording surface of an optical disk is less than the wavelength order. This technical content uses an optical system having a large numerical aperture NA by using two separated lenses having a uniform refractive index, particularly using a hemispherical lens as a lens on the optical disk side.

Conventionally, as an optical system using a spherical lens having a refractive index distribution, for example, Marcha
GRADENT INDEX OPTIC
S "(ACADEMCC PRESS 1978) p1
It is known as a Luneburg lens from 2 to p14. FIG. 4 shows an optical axis sectional view of this optical system. As shown in the figure, this optical system comprises only a spherical lens L3,
Reference numeral 3 denotes a gradient index lens. The refractive index on the spherical surface is n ₀ , and when n ₀ = 1, that is, the refractive index on the surface of the optical disk spherical surface is equal to the refractive index of air.
The ray 6 is the farthest parallel ray incident on the spherical lens. The ray 6 passes through the maximum aperture of the stop 5, passes through the spherical lens L3, and forms an image on the spherical surface (point P) opposite to the incident side with no aberration. . The refractive index distribution in the radial direction of the spherical lens L3 is shown in FIG.
Shown in

The numerical aperture NA is a quantity representing performance related to the brightness and the resolving power of the optical system. The angle at which the radius of the entrance pupil is large at the image point P on the optical axis is θ, and the medium (sphere) on the object side is When the refractive index of the lens is n, it is represented by n · sin θ. In this optical system, the numerical aperture NA is 1 or less.

[0005]

However, as a problem of the technique disclosed in the prior art US Pat. No. 5,125,750, it is costly to manufacture a minute lens by polishing or the like, and it is necessary to assemble and adjust the lens centering. There is a problem that it takes time.

In the case of the conventional Luneburg lens, a large numerical aperture NA is required in the optical disc optical system. However, since the refractive index at the image forming position is 1.0, the numerical aperture NA is 1.0 or more. There is a problem that does not become.

The present invention has been made in view of the above problems, and an object of the present invention is to provide good imaging performance and a numerical aperture N.
An object of the present invention is to provide an optical system for an optical disk in a near field, in which A can be easily increased.

[0008]

The above object can be achieved by any of the following means. (1) An optical system for an optical disk, which is used in an apparatus for recording or reproducing information on an optical disk, wherein an interval between an objective lens and a recording surface of the optical disk is set to be equal to or shorter than a used light source wavelength. Is an optical system for an optical disk, including at least a spherical lens, wherein the spherical lens has a spherically symmetric refractive index distribution.

According to the present invention, there is provided an optical system for an optical disk in a near field, which has good imaging performance and is easy to increase the numerical aperture NA.

(2) The optical system according to claim 1, wherein the spherical lens has a spherical shape.

According to the second aspect of the present invention, since the lens is a spherical lens, it can be easily processed. For example, when the refractive index at the spherical surface is n ₀ = 1, which is unique to the spherical lens, the spherical lens on the opposite side to the parallelly incident light beam. Rays form an image with no aberration.

(Claim 3) The refractive index distribution of the spherical lens is approximately represented by the following equation:
Or the optical system for an optical disk according to 2.

[0013]

(Equation 2)

Here, n ₀ : refractive index on the spherical surface r ₀ : radius of curvature of the spherical surface of the spherical lens r: polar coordinates with the center of the sphere as the origin In claim 3, the function n (r) of the refractive index distribution is According to the equation (1), good imaging performance in the near-field optical system is obtained.

(4) The optical system for an optical disk according to any one of (1) to (3), wherein a lens having a uniform refractive index is bonded to the object side of the spherical lens.

According to the fourth aspect of the present invention, the processing is facilitated by the bonding, the assembling adjustment can be simplified, and the numerical aperture NA can be increased by increasing the refractive index of the lens having a uniform refractive index.
Can be set to a large value exceeding 1.0.

(5) The refractive index of the spherical lens on the spherical surface is substantially equal to the refractive index of the lens having the uniform refractive index.
An optical system for an optical disc according to the above item.

According to the fifth aspect, by making the refractive indices substantially equal, it is possible to obtain good imaging performance with no aberration at the image forming point.

(Claim 6) The optical system according to claim 4 or 5, wherein the lens having a uniform refractive index is a plano-concave lens having a flat surface on the object side and a concave surface on the image side.

According to the sixth aspect, the plane wave state without aberration is maintained in the plano-concave lens with respect to the incident parallel light.

(7) The optical system according to (1), wherein the refractive index distribution of the spherical lens satisfies the following condition.

1.2 <n (0) / n (r ₀ ) <1.5 Equation (2) where n (0): refractive index at the center of the sphere n (r ₀ ): at the surface of the sphere Refractive index In claim 7, the refractive index distribution of the gradient index lens is ideally in accordance with the above-mentioned equation (1), but is preferably in accordance with equation (2) in consideration of factors such as errors in manufacturing.

If the upper and lower limits of the equation (2) are exceeded, the spherical aberration is too large. Preferably, 1.3 <n (0) / n (r 0) <1.45 (3) is a formula.

The refractive index distribution lens here is a lens whose refractive index changes continuously, and has a spherical distribution in the radial direction.

(8) The optical system for an optical disk according to item 1, wherein the refractive index distribution of the spherical lens satisfies the following condition.

1.1 <n (0) / n ( ²⁻¹ / ² · r ₀ ) <1.3 Equation (4) where n (0): refractive index at the center of the sphere n (2 ^{−1 / 2} · r ₀ ): Radius 2 ^-1/2 , where r ₀ is the radius of the sphere
In the refractive index claims 8 at the position of the-r _0, (4) characteristics of the spherical aberration becomes worse Above the upper limit and lower limit of the expression, imaging state is deteriorated. Preferably, 1.15 <n (0) / n ( ²⁻¹ / ² r ₀ ) <1.25 (5)

(Claim 9) The numerical aperture of the objective lens is N
9. The optical system for an optical disk according to claim 1, wherein when A is satisfied, the following condition is satisfied.

0.5 <NA <3.0 (6) In the ninth aspect, when the value exceeds the upper limit of the expression (6), the cost increases, and when the value exceeds the lower limit, the numerical aperture becomes too small. The spot system becomes large. Preferably, 1.0 <NA <2.0 (7).

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One example of an optical system for an optical disk according to an embodiment of the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiment.

The reference numerals used in the embodiments are as follows.

R: radius of curvature of lens d: surface interval n: refractive index NA: numerical aperture f: focal length The refractive index distribution in the radial direction of the spherical lens of the embodiment is shown in FIG.
Shown in

Example 1 FIG. 1 shows an optical axis sectional view of the optical system for an optical disk of Example 1, and Table 1 shows lens data.

[0033]

[Table 1]

The optical system comprises a lens L1 and a spherical lens L2. The lens L1 is a plano-concave lens having a flat surface on the object side and a concave surface on the image side, and has a uniform refractive index. The spherical lens L2 is a gradient index type spherical lens. The image side of the lens L1 and the object side of the spherical lens L2 are attached to each other. The refractive index of the spherical lens L2 on the surface of the spherical surface is equal to the refractive index of the lens L1. The optical disk recording surface 4 is provided on the image side of the spherical lens L2 at an interval of a wavelength order or less. An aperture 5 is provided on the object side of the lens L1.

The ray 6 is a parallel ray farthest from the optical axis. The ray 6 passes through the maximum aperture of the stop 5, passes through the lens L1, passes through the spherical lens L2 having a refractive index distribution, and passes through the spherical surface on the image side. Is formed at the point P. FIG. 5 shows a change in the refractive index in the radial direction of the first embodiment.

As described above, the numerical aperture NA can be increased.
A = 1.2. At point P on the spherical surface, the light beam forms an image with no aberration.

Example 2 FIG. 2 is a cross-sectional view of the optical system for an optical disk of Example 2 along the optical axis, and Table 2 shows lens data. The same components as those in FIG. 1 are denoted by the same reference numerals and description thereof is omitted.

[0038]

[Table 2]

The numerical aperture NA can be increased, and NA = 1.6. At the point P on the spherical surface, the light beam forms an image with no aberration.

(Embodiment 3) FIG. 3 shows an optical axis sectional view of the optical system for an optical disk of Embodiment 3, and Table 3 shows lens data. The same components as those in FIG. 1 are denoted by the same reference numerals and description thereof is omitted.

[0041]

[Table 3]

The numerical aperture NA can be increased, and NA = 1.3. At point P on the spherical surface, the light beam forms an image with no aberration.

[0043]

According to the structure described above, the following effects can be obtained. According to the optical system for an optical disk of the present invention,
An optical system for an optical disc in a near field, which has good imaging performance and is easy to increase the numerical aperture NA.

[Brief description of the drawings]

FIG. 1 is an optical axis sectional view of an optical disc optical system according to a first embodiment of the present invention.

FIG. 2 is an optical axis sectional view of an optical disc optical system according to a second embodiment of the present invention.

FIG. 3 is an optical axis sectional view of an optical disc optical system according to a third embodiment of the present invention.

FIG. 4 is an optical axis sectional view of a conventional optical system.

FIG. 5 is a refractive index distribution diagram of a spherical lens of an example and a conventional example, showing a change in a refractive index in a radial direction.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st surface 2 2nd surface 3 3rd surface 4 Recording surface 6 Light ray L1 lens (plano-concave lens) L2 spherical lens (spherical lens) P Imaging point

Claims (9)

[Claims] 1. An optical disc optical system used for an apparatus for recording or reproducing information on an optical disc, wherein an interval between an objective lens and a recording surface of the optical disc is set to be equal to or less than a used light source wavelength. An optical system for an optical disk, comprising at least a spherical lens, wherein the spherical lens has a spherically symmetric refractive index distribution. 2. The optical system according to claim 1, wherein the spherical lens has a spherical shape. 3. The optical system for an optical disk according to claim 1, wherein the refractive index distribution of the spherical lens is represented by the following equation. (Equation 1) Where n ₀ : refractive index on the spherical surface r ₀ : radius of curvature of the spherical surface of the spherical lens r: polar coordinates with the origin at the center of the sphere 4. The optical system for an optical disk according to claim 1, wherein a lens having a uniform refractive index is bonded to the object side of the spherical lens. 5. The optical disk according to claim 1, wherein a refractive index of the spherical lens on a spherical surface is substantially equal to a refractive index of the lens having a uniform refractive index. Optical system. 6. The optical system according to claim 4, wherein the lens having a uniform refractive index is a plano-concave lens having a flat surface on the object side and a concave surface on the image side. 7. The optical system according to claim 1, wherein a refractive index distribution of the spherical lens satisfies the following condition. 1.2 <n (0) / n (r ₀ ) <1.5 where n (0): refractive index at the center of the sphere n (r ₀ ): refractive index at the surface of the sphere 8. The optical system according to claim 1, wherein the refractive index distribution of the spherical lens satisfies the following condition. 1.1 <n (0) / n ( ²⁻¹ / ² · r ₀ ) <1.3 where n (0): refractive index at the center of the sphere n ( ²⁻¹ / ² · r ₀ ): sphere The radius of r- ₀ and the radius 2 ^-1/2
The refractive index at the position r ₀ 9. The optical system for an optical disk according to claim 1, wherein the following condition is satisfied when the numerical aperture of the objective lens is NA. 0.5 <NA <3.0