JPS6218502A - Optical device - Google Patents

Abstract

PURPOSE:To obtain a parallel flux having a uniform intensity distribution by dropping the diffraction efficiency in the center of a grading lens rather than in its vicinity. CONSTITUTION:The diffraction efficiency of a strong incident light part is dropped. To realize said dropping, the depth of a diffraction grating groove is made shallow. Namely, the depth is made gradually shallower toward center part 24 from periphery part 23. Thus Gauss type light beam having a light intensity distribution which is largest in the center of a light emitting source and gradually smaller toward the periphery can be shaped to a light beam with a uniform intensity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an optical device having a light emitting source with non-uniform light intensity distribution and a grating lens.

[Background Art and Problems Therein] In optical information processing devices such as optical pickups, various gas lasers and semiconductor lasers are used as light emission sources. The light waves from these lasers are generally called Gaussian beams and have a specific phase distribution and width distribution. Its phase distribution differs from a plane wave and a spherical wave near the light source, but approaches a spherical wave as it moves away from the light source.

However, the spread width distribution in a plane perpendicular to the direction of travel is a Gaussian distribution even at a location away from the light source. Therefore, although it is possible to convert the diverging light from a laser into a parallel beam with a uniform wavefront by using a suitable lens system, the intensity distribution is not uniform and always becomes a Gaussian distribution.

Incidentally, the optical pickup has a structure in which the light from the laser light source is collimated into a parallel beam using a collimating lens, and then focused onto a disk using an objective lens.

In this case, a so-called tracking mechanism is required to prevent the focus of the objective lens from deviating from the fine pit rows carved on the information recording surface of the disk.

To do this, the objective lens must be moved several mm in a plane perpendicular to the optical axis. Therefore, the diameter of the parallel beam from the collimating lens is larger than the aperture of the objective lens.

However, since the parallel light from the collimating lens does not have a uniform intensity but has a crow-shaped distribution, if the objective lens moves from side to side, the light incident on the lens will have an asymmetric intensity distribution. Therefore, the convergence spot diameter at the focal point becomes large and asymmetrical. The situation is shown in FIG. (a) is when the objective lens OL is shifted to the left with respect to the beam center Y, (b) is when the optical axis of the objective lens OL is aligned with the beam center Y, and (C) is with respect to the beam center Y. A case in which the optical axis of the objective lens OL is shifted to the right is schematically shown,
Each spot pattern S in that state is shown. This is the information recording surface of the disc. This asymmetry is particularly noticeable in writeable optical disk systems (DRAW).
This is a serious defect. Furthermore, if the convergence spot is asymmetrical, the reflected and diffracted light from the disk will also be asymmetrical, resulting in a large offset in the focus error signal that is calculated by 1q by the astigmatism method or the critical angle method.

In order to solve the above-mentioned problems, conventional optical pickups employ a method of reducing the numerical aperture (NA>) of the collimating lens. This method uses only light near the center of the Gaussian distribution.

Therefore, the asymmetry caused by the movement of the objective lens is certainly reduced, but on the other hand, it has the disadvantage that only a small amount of light from the light source can be used.

[Object of the Invention] The object of the present invention is to provide an optical device capable of obtaining a parallel light beam having a substantially uniform intensity distribution.

[Summary of the Invention] That is, the present invention provides a light emitting source having a light intensity distribution in which the light intensity is high in the center and low in the peripheral area, and a light emitting source installed at a predetermined distance from the light emitting source and having at least one surface. The present invention relates to an optical device including a grating lens in which a concentric diffraction grating is formed, and in which the diffraction efficiency at the center of the grating lens is lower than that at the periphery. The focal length of the grating lens is determined by the pitch of the concentric non-uniformly spaced diffraction gratings.

On the other hand, the diffraction efficiency varies greatly depending on the depth and shape of the grooves in the diffraction grating. Therefore, in a grating lens with a predetermined focal length, by changing the depth and shape of the grooves of each diffraction grating according to the intensity of the incident light, it is possible to obtain parallel light with a uniform intensity distribution. [Embodiment of the Invention] Fig. 1 shows an embodiment in which the present invention is applied to optical pickup, in which light 12 emitted from a semiconductor laser diode light source 11 is received by a grating lens 13 serving as a collimating lens. After the light is made into a parallel beam, it passes through a beam splitter 14 and reaches the position of an objective lens 15 . The effective surface of the objective lens 15 is made smaller than the parallel beam diameter, and the objective lens 1
The range of movement of 5 is provided with some leeway. The light beam is focused by the objective lens 15 on the information recording surface of the disk 17, and illuminates the pit array that carries information. The reflected light from the pit passes through the same objective lens 15, becomes a parallel beam again, is reflected by the beam splitter 14, enters the photodetector 20 through the detection optical system 19, and is read as information.

The grating lens 13 is made of glass, as shown in FIG. 2, and has a diffraction grating surface 22 formed on the side of the laser diode light source 11. As shown in FIG. It has a grid. From FIG. 2, if the wavelength of the incident light from the light source 11 is λ and the focal length is f, then the radius of the m-th ring zone rm is rm 2 = 2rrrtλ + (mλ) 2・−・・−(1
)become.

Now, in the case of the rectangular diffraction grating in this example, the first-order diffraction efficiency η2 is 772 = (4/ y'r2) -5inDr-Δn, where d is the groove depth.
-d/λ)...(2) It is given by: However, Δn is the refractive index difference between air and the diffraction grating material. From the above equation, it can be seen that the maximum diffraction efficiency of 40.5% can be obtained at Δnd-λ/2. Therefore, when a spherical wave centered at the focal point is incident on a grating lens in which each annular zone has the same depth, a plane wave having a uniform intensity value 5 is obtained. However, such a lens has a non-uniform intensity valve like a Gaussian beam.
When a diverging light beam having . In order to correct this non-uniformity, this embodiment lowers the diffraction efficiency of the strong portion of the incident light. For that purpose (2)
According to the formula, the depth of the diffraction grating groove is made shallow. That is, the depth gradually becomes shallower from the peripheral portion (23) to the center portion (24). As a result, we created a Gaussian light beam with a light intensity distribution where the light intensity is high at the center of the light source and gradually decreases toward the periphery, as shown in Figure 4(a).
It is possible to shape the light beam into a light beam with uniform intensity as shown in b).

A quantitative explanation is as follows. Now normal union u
The force ``intensity of parallel light after passing through a lath lens or grating lens with the same depth, i [is f(r) = Ae
Assume that it is a Gaussian type xp(-αr") ...-<3>. Here, r is the distance from the optical axis. Also,
If the effective diameter of the lens is r max, then the parallel beam has an intensity of A at the center and Aexp (-λr max") at the outermost periphery. Therefore, the groove depth dm of the m-th annular zone is π2 λ - exp (-αr max2)...
...If it is determined to satisfy (4), 1q of parallel light beams with a uniform intensity distribution can be obtained. In the example applied to the sawtooth diffraction grating shown in FIG. 5, in addition to the structure in which the groove depth is changed as in the previous embodiment, the groove bottom of the diffraction grating 31 of the grating lens 30 is made flat. Side 3
2 is wide at the center and gradually narrows or disappears toward the periphery, thereby adjusting and lowering the diffraction efficiency at the center of the lens to 100% or less.

In the embodiment of the sawtooth diffraction grating shown in FIG. 6, the grating 34 of the grating lens 33 has a triangular cross section, and the angle of the slope 35 forming the grating groove is gentle at the center with respect to the lens optical axis. Efforts are being made to reduce diffraction efficiency.

In this way, the light intensity can be made uniform.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing one embodiment of the present invention, and FIG.
A schematic diagram showing a part of the figure; Figure 3 is a cross-sectional view of the grating lens in Figure 1; Figure 4 is a curve diagram explaining the embodiment of Figure 1; (a) is the intensity of light from the light emitting source. distribution, (b
) is a diagram showing the light intensity distribution after passing through the grating lens, FIG. 5 is a partial cross-sectional view of another embodiment of the present invention, and FIG. 6 is a partial cross-sectional view of a further embodiment of the present invention. Figure 7 (
a), (b), and (C) are diagrams illustrating a conventional device. 11... Light source, 12... Light, 13.
... Grating lens, 22 ... Diffraction grating surface 1.23 ... Peripheral part, 24 ...
·Central part. Figure 1 Figure 2 Figure 3 ■ Figure 5 Figure 6 (a) (b)
(C) Figure 7

Claims (4)

[Claims] (1) A light emitting source with a light intensity distribution where the light intensity is high at the center and low at the periphery, and a concentric diffraction grating installed at a predetermined distance from this light source on at least one surface. What is claimed is: 1. An optical device comprising a grating lens, characterized in that diffraction efficiency at a central portion of the grating lens is lower than at a peripheral portion. (2) The optical device according to claim 1, wherein the grooves of the diffraction grating are shallow in depth at the center. (3) The optical device according to claim 2, wherein the diffraction grating has a sawtooth cross section, the bottom of the groove is flat, and the area is wide at the center. (4) The optical device according to claim 1, wherein the grating of the diffraction grating has a triangular cross section and the angle of the slope is gentle at the center.