JPS61163309A - Refractive index distributed single lens - Google Patents

Abstract

PURPOSE:To obtain a lens which has aberrations improved and has a high NA and use it as an object lens of a video disc or the like by constituting the lens so that it has a specific refractive index distribution and has one spheri cal refracting surface at least and satisfies specific conditions. CONSTITUTION:This lens has a refractive index (n) given by an equation n<2>= n<2>0{1-(gr)<2>+h4(gr)<4>+h6 (gr)<6>} where n0 is the refractive index of the center and (r) is the distance from the optical axis in the diametric direction and has one spherical refracting surface at least and satisfies conditions of 1.54f<D and -4<(g-0.5)D. In said equation and formulas, (g) is the parameter indicating the degree of the refractive index gradient, and h4 and h6 are coefficients of terms of fourth and sixth orders of the refractive index distribution respectively, and D is the lens length, and (f) is the focal length of the lens. Since the refrac tive index distribution is symmetrical with respect to the optical axis and said conditions are satisfied, the distributed index lens is obtained which has various aberrations compensated well and has not only >=0.5 NA but also >=0.7 NA which is difficult normally with respect to design.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a lens using a non-homogeneous medium, and particularly to a gradient index single lens used as an objective lens for optical video discs and the like.

[Prior art]

The objective lens that is conveniently used for recording information that is recorded at high density on a recording medium such as an optical video disk is small in order to directly focus on this objective lens.
quantity is required.

In addition, a high HA is required in order to further reduce the spot diameter to be focused on the recording medium.

Conventionally, as such a pair of object lenses, a combination of several homogeneous spherical lenses or a homogeneous aspherical single lens, which is intended to be particularly compact and lightweight, have been used.

Furthermore, recently, in addition to these homogeneous lenses, gradient index single lenses using a non-homogeneous medium have become known for economical processing, miniaturization, and weight reduction.
There are those described in JP-A No. 354, JP-A-58-122512, JP-A-59-62815, and the like.

In all of these conventional examples of gradient index single lenses, spherical aberration is corrected by making at least one refractive surface spherical.
22512 and JP-A-59-62815 attempt to correct not only spherical aberration but also off-axis aberration, especially coma aberration.

However, all of these conventional examples only take into consideration the NA of 0.45, and it cannot be said that they fully satisfy the requirements in that respect.

〔the purpose〕

An object of the present invention is to provide a gradient index single lens whose various aberrations are well corrected and whose NA is 0.5 or more.

〔overview〕

The gradient index single lens of the present invention has a refractive index distribution that is rotationally symmetrical with respect to the optical axis and is expressed by the following equation.

nz=no2(1-(gr)2+h4(gr)'+
h6(gr)6 madashi n. is the refractive index at the center of the lens, r
is the radial distance from the optical axis, g is a parameter indicating the degree of refractive index gradient, and h4. h6 is a coefficient of the fourth-order term and the sixth-order term of the refractive index distribution.

The lens of the present invention has the above refractive index distribution, at least one refractive surface is a spherical surface, and the length of the lens is D.
, the following condition (ih, (2)) is satisfied when the focal length is f.

<1) 1.54f < D (2) -4< (g-0°5) A recording/reproducing lens for a D-optical video disk, etc. has a diameter of 0.1 to 0.2 mm centered on the optical axis on the image plane. Aberrations must be sufficiently corrected within this range. Therefore, it is necessary to properly correct at least spherical aberration, coma aberration, and astigmatism.

When the NA is about 0.45, if the spherical aberration and coma aberration (sine condition) are corrected for a gradient index single lens, astigmatism will also be within the allowable Wm range, so off-axis aberrations will be Correcting the conditions is almost sufficient. However, as HA becomes larger, the permissible range of astigmatism becomes narrower, and correction must be made by considering both astigmatism and the sine condition. Also, spherical aberration must be corrected more severely as the NA increases.

Among the above conditions, condition (1) relates to the lens length and is for correcting astigmatism. This condition (1
) If not satisfied, astigmatism will not be fully corrected when the dog is made to have an NA of around 10.7.

Condition (2) relates to the refractive power of the lens medium. In a gradient index lens, the refractive power of the entire lens can be divided into the refractive power at the refractive surface and the refractive power due to the medium, and the balance between these is important when correcting multiple aberrations. If this condition (2) is not satisfied, spherical aberration worsens and cannot be completely corrected.

〔Example〕

Next, examples of the gradient index single lens of the present invention will be shown. The lens of the present invention is made of a non-homogeneous medium, and its refractive index distribution is expressed by the above formula. In each embodiment, the surfaces R1 and R2 are made spherical as shown in FIG. 1 or 2. be. The data of each example is as follows.

In the data of each example above, R□ and R2 are the radius of curvature of the incident side and exit side surfaces, D is the lens length, and n
(1 is the refractive index at the center of the lens, g is a parameter indicating the degree of refractive index gradient, h4.h6 are coefficients of the fourth and sixth terms of the refractive index distribution, respectively, f is the focal length of the lens, and NA is the numerical aperture on the exit side. It is.

Among these Examples, Examples 1 to 5 and Example 7
Examples 9 to 11 are biconvex lenses as shown in FIG. 1, and Example 6. Example 10, Example 12.
All of Example 13 are positive meniscus lenses as shown in FIG.

Each of these examples is designed to minimize the aberrations caused by a disk with a thickness of 0.288 m and a refractive index of 1.55, and the refractive index of the disk is λ = 780 ns. This is the value at , and the lens is designed so that aberrations are minimized at this wavelength. The aberration curves of these examples are shown in Figures 3 to 1.
This is the passage shown in Figure 5.

〔Effect of the invention〕

As explained in detail above, and as is clear from the examples, according to the present invention, the NA can be reduced to 0.5 while maintaining good aberrations.
Not only can the above be achieved, but it is also possible to obtain a gradient index lens whose NA exceeds 0.7, which is extremely difficult with normal designs.

[Brief explanation of drawings]

FIG. 1 shows examples 1 to 5 of the present invention. Examples 7 to 9. FIG. 2 is a sectional view of Example 11, and FIG. 2 is a sectional view of Example 11. Example 10. Example 12. A cross-sectional view of Example 13,
3 to 15 are aberration curve diagrams of Examples 1 to 13 of the present invention, respectively. Applicant: Olympus Optical Industry, a closed company agent Hiroshi Mukai - Fig. 1 Fig. 3 Fig. 4 Spherical aberration o s c' Astigmatism Fig. 5 Fig. 6 Spherical aberration OS σ Astigmatism Fig. 7 Fig. 8 Spherical aberration o s c' Astigmatism Fig. 9 Spherical aberration o s c'' Astigmatism Fig. 10 Fig. 11 Fig. 12 Fig. 13 Spherical aberration o s c' Astigmatism Fig. 14 Fig. 15 Spherical aberration Aberration o s c' Astigmatism procedure correction document June 10, 1985

Claims (1)

[Claims] A gradient index single lens whose refractive index n is expressed by the following formula when the central refractive index is n_0 and the radial distance from the optical axis is r, and at least one refractive surface is A gradient index single lens having a spherical surface and further satisfying the following conditions (1) and (2). n_2=n_0^2{1-(gr)^2+h_4(gr
)^4+h_6(gr)^6}(1)1.54f<D (2)-4<(g-0.5)D where g is a parameter indicating the degree of refractive index gradient, h_
4, h_6 are the coefficients of the fourth-order term and the sixth-order term of the refractive index cloth, D
is the lens length, and f is the focal length of the lens.