JPS59125A - Optical system - Google Patents

Abstract

PURPOSE:To read out correctly information, by arranging a coupling lens between a semiconductor laser which emits an ellipsoidal beam and an objective lens and forming a minute circular spot on a disk. CONSTITUTION:A beam from a semiconductor laser light source 1 which has a luminous area of a different light-width ratio and, therefore, whose emissive light distribution forms an ellipsoidal beam pattern is irradiated upon the whole surface of an objective lens 3 by means of a coupling lens 2 and a spot 4 stopped down on the surface of a disk 5 by the lenses 2 and 3 is adjusted to a half- value width of about 1mumphi. The optical system becomes more effective when a lens of an N.A of about 0.4-0.5 having 20-30-fold magnification is used for the lens 3 and a lens having seven-fold or less magnification is used for the lens 2.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical system, and particularly to an optical system suitable for reproducing dot information recorded on a predetermined disk using a semiconductor laser.

The dot information recorded on the disk usually has a size of about 1 .mu.m to 2 .mu.m, and is recorded in a spiral or concentric pattern on the disk.

Therefore, in order to accurately reproduce the dot information and signal, it is necessary to focus the laser beam into a circular spot with a diameter of about 1 μm (half width).

Conventionally, in a video disc playback device that uses a semiconductor laser light source as a laser light source, a buried Peter structure is used in which the semiconductor laser light emitting area has a square shape of 1 μm x 1 μm. A spot of about 1 μmφ was obtained on the top. Here, 1 is a semiconductor laser light source, 2 is a first lens (coupling lens), 3 is a second lens (objective lens), 4 is a focused light spot, 5 is a disk,
It is. In such an optical system, for example, both the coupling lens and the objective lens have a magnification of 25
A lens with a multiplication factor, N, and A (numerical aperture) of 0.4 has been used.

However, if a laser with a 1 μm x 1 μm light emitting area is used, the narrowed spot 4 has the advantage of being circular, but it also has the disadvantage that the laser light output is small because the light emitting area is small. If the light emitting area is small, a lens with high magnification and high NA is required, which also causes the disadvantage that it is difficult to adjust the optical system, especially the position of the coupling lens. Therefore, when reproducing dot signals from dot information, a stable signal having a high signal-to-noise ratio cannot be obtained. Considering this point, conventionally, in order to obtain a large laser output, the light emitting area was reduced to 1 μm in the vertical direction.
In video disc devices that use semiconductor lasers with a width of 3 μm to 5 μm in the lateral direction, a cylindrical lens is used to make the narrowed spot on the disc close to a circular shape. It has the disadvantage that it is difficult and expensive, and the arrangement of the optical system becomes complicated.Also, converging the spot into a circular spot of 1 μm has a large astigmatism due to the use of a cylindrical lens. It is difficult because of the influence.

In order to eliminate such drawbacks, the present invention uses a semiconductor laser as a light source, which has light emitting areas with different aspect ratios [7, and therefore has an elliptical beam pattern with an emitted light distribution]. By having more light distribution patterns in the vertical direction than in the horizontal direction, it is possible to form a minute circular spot on the disk, thereby accurately relying on the dot information on the disk.

Hereinafter, the present invention will be explained by examples.

In Fig. 2, a spot 4 narrowed down by a lens 2.3 on the surface of the disk 5 has a half-width of about 1 μmφ (if the zero point of the diffracted light distribution in the case of a circular aperture is taken as the spot diameter, the diameter is about 2.5 μmφ). ), the following conditions must be met.

1.22λ/n' ≦ 2.5μm (1) Here, λ
is the wavelength of light, which is 0.83 μm in the case of a semiconductor laser, υ, and n/ is N and A of the lens 3, so according to equation (1), n/≧0.4.

If a lens 3 with N and A of 0.4 or more is used, a microscope objective lens with a magnification of 20 times or more is usually used. However, in general, the magnification and N
, A, the depth of focus will be shallow, and as shown in FIG. 2, the focus will be lost due to vibration during rotation of the disk 5, making it difficult to obtain a good reproduced signal.

Therefore, as the lens 3, a lens having a magnification of 20 to 30 times, N, A, 0.4 to 0.5 is suitable.

In the second step, if the magnification of the lens 3 is m' and N and A are n', the angles u4 and n' that define N and A shown in FIG.
The relationship expressed by the following equation holds true in the general equation.

sin (u4) = n' f2J In order for the focused spot of the lens 3 to be sufficiently small, the entire lens 3 must be irradiated with the light from the semiconductor laser 1. In order to satisfy this condition, it is necessary to satisfy the following equation (3).

Here, since m' is 20 to 301n' is 0.4 to 0.5, the minimum value on the right side of equation (3) is 0.0133.

Therefore, the following relationship exists between the emission angles u2 and u3 of the semiconductor laser from the light source 1 shown in FIG.

u2 = mu3 (41 where m
is the magnification of lens 2.

From equations (3) and (4), the magnification m is expressed by the following equation (5).

U.

In order to determine u2 in equation (5), an example of the characteristics of the semiconductor laser light from the light source 1 in FIG. 2 is shown in FIG. In Figure 3, the horizontal axis shows the emission angle n2 (degrees) and the vertical axis shows the light intensity (arbitrary unit). For example, the light emitting area is 1 μm long x 3.5 μm wide.
A far field image of a semiconductor laser from a degree
d pattern).

In Figure 3, the half-width in the vertical and horizontal directions is 3
4.5°, 10', and in the case of a light-emitting region of about 1 μm in length x 5 μm in width, the far-field pattern is about 34.5°, 7°.

The angle u2 in equation (5) is set to give the half width of the lateral far-field pattern shown in FIG. This is because when u2 is set as above, the output light distribution of the semiconductor laser is treated as a nearly circular light distribution within an angle of 02 in both the vertical and horizontal directions, and the light distribution incident on the lens 3 is also circular. The narrowed spot 4 also has a circular shape. Therefore, when u2 is expressed in radians, it becomes. However, the lateral light emitting area of the semiconductor laser light source 1 is set to 7 from 3.5/1 m to 5 μm.

Taking the larger angle 5 as u2, from equation (5)
m≦6.6 Therefore, it is effective to use a lens with a magnification of 7 times or less as lens 2.

Also, N and A of lens 2 are the smaller angle 35 of u2.
It is necessary that the angle be larger than 5in3.5°=006.

If a lens 2 with a magnification higher than this is used, the spot 4 narrowed down on the disk by J' will have an elliptical shape instead of a circular shape.

As described above, according to the present invention, it is possible to focus the light into a circular light spot using a relatively small semiconductor laser light source, and a better reproduction signal than that from a disc can be obtained.

[Brief explanation of the drawing]

FIGS. 1 and 2 are diagrams for explaining the configuration of an optical system according to the present invention, and FIG. 3 is a diagram showing a far-field image from a semiconductor laser.・、-／′ Beauty l Sonodake 3 Kuni

Claims (1)

[Claims] A semiconductor laser that emits an elliptical beam, an objective lens whose entire surface is irradiated with the laser beam, and a coupling lens disposed between the objective lens and the laser. An optical system consisting of