JPH0642016B2 - Gradient index type single lens - Google Patents

Description

TECHNICAL FIELD The present invention relates to a gradient index single lens.

[Prior Art] In recent years, devices such as optical video discs and digital audio discs for reading information recorded at high density on a recording medium by converging laser light on a minute spot have become widespread. As an optical system that focuses laser light on a minute spot, a lens system in which several homogeneous spherical lenses are combined has been used so far. Recently, a lens using an aspherical single lens has been developed for the purpose of downsizing and weight reduction. On the other hand, a gradient index single lens has been attracting attention as an alternative to the aspherical lens. The earliest of these types of lenses only considers spherical aberration correction, but as is well known, focusing lenses used for optical video discs etc. have a diameter of 0.
Aberrations must be sufficiently corrected in the range of 1 to 0.2 mm, and it is necessary to correct not only spherical aberration but also coma. As a gradient index single lens considering comatic aberration, Japanese Patent Laid-Open Nos. 58-122512 and 59-62815 are available.
There is one disclosed in the issue. Both of these are intended to favorably correct both spherical aberration and coma by combining the refractive surface of the gradient index lens as a spherical surface and combining the curvature of this spherical surface with the higher-order terms of the refractive index distribution.

[Problems to be Solved by the Invention] However, it is hard to say that the former is always sufficiently corrected for aberrations. In the latter case, aberrations are relatively well corrected, but the meniscus lens has a strong lens shape, so that there is a problem that manufacture is not easy.

The present invention provides a gradient index single lens in which at least one refracting surface has a gentle curvature and is well corrected for aberrations.

[Means for Solving Problems] The gradient index single lens of the present invention has a refractive index n of n ² = n ² where no is the central refractive index and r is the radial distance from the optical axis. o {1- (gr) ² + h ₄ (gr) ⁴ + h ₆ (gr) ⁶ }, and (1) gZ <0.43 (2) 0.3 <gφ <0.7 (3) 0.28f <Z Each of the conditions is satisfied, and at least one refracting surface is a spherical surface.

However, g is a parameter indicating the degree of the refractive index gradient, h ₄ ,
h ₆ is the coefficient of the 4th and 6th terms of the refractive index distribution,
Z is the length of the lens, φ is the outer diameter of the lens, and f is the focal length of the lens.

[Operation] In the gradient index single lens, the refractive power of the entire lens can be divided into the refractive power of the refractive surface and the refractive power of the medium. Of these, the refractive power of the medium is JO
SA, vo161, no7, pp879-885 "-I nhomogeneous Lense
sIII Paraxial Optics ”, it can be obtained more exactly, but in the case where gZ is smaller than π / 2, it is possible to know the magnitude of the refractive power of the medium from the value of gZ. The degree of the gradient of the refractive index distribution is indicated by the parameter g, which is a value that depends on the shape of the lens outer diameter, etc. That is, in a normal homogeneous lens, the focal length, the radius of curvature of the refracting surface, the lens, and the like. Even if the length and outer diameter are constant multiples of the original lens, it becomes an optically equivalent lens, but in the gradient index lens, it is
(1 / constant) is multiplied to obtain an equivalent lens. Therefore, by taking the value of gφ, it is possible to know the degree of the gradient of the refractive index distribution regardless of the shape.

Conditions (1) and (2) are conditions for the refractive power of the medium of the gradient index lens and the degree of gradient of the gradient index,
It is necessary to loosen the curvature of at least one refracting surface when the aberration is satisfactorily corrected.

As the value of gZ increases, spherical aberration and coma that occur when light rays propagate through the medium increase, and in order to correct this, it is necessary to make the refracting surface on the opposite side of the far conjugate point a strong concave surface. come. If the condition (1) is exceeded, the curvature of this surface becomes too strong, which makes lens manufacturing difficult.

Further, even if gZ is within the range of the condition (1), when gφ exceeds the upper limit of the condition (2), the gradient of the refractive index becomes strong and the light beam is largely bent in the medium. In order to satisfactorily correct the aberration in this state, the refracting surface on the side opposite to the far conjugate point must be made strongly concave, which is contrary to the object of the present invention.

The lower limit of the condition (2) and the condition (3) are provided to satisfactorily correct various aberrations. If the lower limit of the condition (2) is exceeded or the condition (3) is not satisfied, It becomes impossible to correct both spherical aberration and coma.

[Examples] The attached Table 1 shows data of the examples of the present invention. Each of the embodiments is designed as a condenser lens for an optical video disc, etc., where R1 and R2 are the radius of curvature of the lens surface, Z is the length of the lens, no is the central refractive index, and g is the central refractive index.
Is a parameter indicating the refractive index gradient, h4 and h6 are coefficients of the fourth and sixth terms of the refractive index distribution, φ is the outer diameter of the lens,
f is the focal length of the lens, NA is the numerical aperture on the disk side, W
D is the distance from the lens to the disc. The coefficient of the refractive index distribution, g and the value of the refractive index are all wavelength λ = 780 nm
Against.

1 to 3 are views showing the lens construction of each embodiment, in which D is a disk. Example 1 is the first
As shown in the figure, it is a plano-convex lens convex on the light source side (far conjugate side) not shown. Further, Examples 2, 12 and 13 are biconvex lenses having a strong convex surface on the light source side as shown in FIG. Furthermore, the remaining Examples 3 to 11 are positive meniscus lenses convex toward the light source side as shown in FIG.

In each of the examples, the surface on the disk side is a flat surface or a loose spherical surface, and the shape is easy to manufacture.

4 to 16 are aberration curve diagrams of Examples 1 to 13, respectively. In each figure, SA is spherical aberration, OS
C'is the amount of unsatisfaction of the sine condition, and AS is astigmatism.

In each of the examples of the present invention, aberrations are corrected including a disk having a thickness of 1.2 and a refractive index of 1.55.

It is clear from the aberration diagrams that the aberration correction is sufficiently satisfactorily performed as the condensing disc for the optical disc in each of the examples.

[Effect of the Invention] According to the present invention, it is possible to obtain a gradient index single lens that is satisfactorily corrected for aberrations and that has at least one refracting surface having a gentle curvature and is easy to manufacture.

[Brief description of drawings]

1 to 3 are lens configuration diagrams of an embodiment of the present invention,
4 to 16 are aberration curve diagrams of the present invention.

Claims (1)

[Claims] 1. The refractive index n is n ² = n ₀ ² {1- (gr) ² + h ₄ (gr) ⁴ where n _{0 is the} central refractive index and r is the radial distance from the optical axis. In the gradient index single lens represented by + h ₆ (gr) ⁶ }, at least one of the following conditions is satisfied: (1) gZ < 0.43 (2) 0.3 <gΦ <0.7 (3) 0.28 <Z A graded-index single lens having a spherical refracting surface. However, g is a parameter indicating the degree of the refractive index gradient, h ₄ ,
h ₆ is the coefficient of the 4th and 6th terms of the refractive index distribution,
Z is the length of the lens, Φ is the outer diameter of the lens, and f is the focal length of the lens.