JPS63181125A - Optical head - Google Patents

Abstract

PURPOSE:To improve the incident/outgoing efficiency between a light waveguide and a space by providing blaze effect to a bent type diffraction grating of the optical waveguide so as to suppress the production of high-order diffraction wave. CONSTITUTION:The light from a semiconductor laser 21 is led to an optical waveguide layer 19 by the end face coupling and a minute spot is formed on a disk 23 by a coupling lens 22 and an objective lens 20. The light reflected from the disk passes through the objective lens 20, reflected in the polarized beam splitter 25 and made incident in a linear arrangement four-split photodetector 27 by a condenser lens 26. The blade type is adopted for the cross sectional shape of the objective lens 20 being the diffraction grating (on condition that the slope and the direction of light wave propagation are made perpendicular) and the production of high-order diffracted light is prevent more than the binary type, when the efficiency is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical device for performing optical recording or reproduction with higher performance in an optical information processing device such as an optical disk.

[Conventional technology]

A conventional optical head is shown in FIG. In FIG. 1, light from a semiconductor laser 1 is turned into parallel light by a coupling lens 2, reflected by a mirror 5, and a minute light spot 6 is formed by an objective lens 7.

A diffraction grating 3 is arranged between the coupling lens 2 and the objective lens, and the minute light spot 6 consists of three spots. A minute light spot is formed on an optical disk, which is a rotating recording medium, and is used to read information on the optical disk. Information on an optical disk is recorded in the form of tiny holes called bits on multiple tracks.

This information ~5. In order to read with high precision, a tracking function is required on the disc to allow a minute optical spot to follow a desired track. The light passes through the cylindrical lens 9 and reaches the photodetector 8. FIG. 2 shows the convex lens 10 shown in FIG. The operating principle of focus error signal detection using the cylindrical lens 9 will be described. The point narrowed down by the convex lens 10 is 0. If the point focused by the cylindrical lens 9 is F, then F,
The cross-sectional distribution shape of the light intensity at point M and 0 is IP and I. Now, when a focus error occurs in which the distance between the disk and the objective lens 7 is smaller than the ideal position, point 0, F in Fig.
The point moves in the direction away from the convex lens 10, so the distance is 1M.
The distribution shape of the light intensity at a point changes from IN to a shape close to IP. When the distance between the disk and the objective lens becomes larger than the ideal position, the shape at point M becomes close to zero. Therefore, if the 4-split photodetector 13 shown in FIG. 3 is placed at point M, the focus error can be detected. That is, ゛, °When light is detected, V (Da) +V (Da) - (V (
Db) +V (DC)) provides a focus error signal, which drives the autofocus mechanism.

Tracking signal detection is performed in the following manner. It is assumed that the three spots formed on the disk are arranged with respect to the tracks on the disk as shown in FIG. Main spot 15 is.

The optical system is set so that the side spot 16 is located on the track, and the side spot 16 is located slightly off the center of the track. The reflected light from the main spot 15 is detected by a photodetector 13 shown in FIG.

The photodetection voltage of photodetector 12 and 14 is set to V (DC
), V (Dl). If there is an error in tracking, v (Dl:) and v (Dt) are not equal, and the tracking signal difference (V (Dc) - V (D
The tracking mechanism operates so that l)) becomes zero.

By the way, in the conventional optical head explained in Fig. 1, bulk optical elements such as lenses, prisms, and diffraction gratings are arranged within a mechanical part with a predetermined precision. The disadvantage is that there are many adjustment points and adjustment is difficult. Furthermore, the size of the entire optical head increases due to the arrangement of individual individual components.

To solve this problem, a small optical head has been proposed that has fewer adjustment points, is inexpensive, and is suitable for mass production. In this example, we use an optical waveguide as a base material and focus on the fact that waveguide lenses, waveguide gratings, etc. can be fabricated and arranged on the surface of the optical waveguide in an integrated manner with high precision through exposure and development processes.
The aim is to provide a compact optical head using an optical waveguide, and details can be found in Japanese Patent Application Laid-Open No. 12993/1983.
8.

[Problems to be Solved by the Invention 1] Problems to be solved by the invention include poor efficiency of input and output of light waves between the two, and uneven processing accuracy of the bent diffraction grating, which is the central optical element. As an example of the input/output efficiency, the intensity ratio of the diffracted light of the bent diffraction grating to the guided light is approximately 40% (f1
Child Communication Society Journal '85/10. voQ, J68-C,
&10. There is a report that it was (p. 803).

Low efficiency is the main factor preventing the widespread use of such optical heads, and improvements have been strongly desired. In view of the above, an object of the present invention is to improve the efficiency of light wave input and output in a bent diffraction grating.

[Means for solving problems]

The bent diffraction grating of the conventional optical head in which an optical element is built into a waveguide has a binary cross-sectional shape. As a result, when the sales signal enters the area of the diffraction grating, high-order diffraction waves are generated, and these waves are emitted in a direction different from the focal point position, resulting in efficiency. Therefore, in the present invention, the diffraction grating We decided to suppress the generation of higher-order diffraction waves by devising the shape of the .

[Effect]

Generally, when a diffraction grating is used to emit a light wave from an optical waveguide, the emitting direction is mainly determined by the period of the diffraction grating, the material of the substrate, and the wavelength, and the cross-sectional shape of the diffraction grating has a relationship with its efficiency. It's good to see that there is.

As mentioned above, in the case of a binary diffraction grating, the efficiency decreases due to the generation of higher-order diffraction waves.

Therefore, the period of the diffraction grating remains the same, and the cross-sectional shape is made into a blaze type (however, the direction of propagation of the light wave is perpendicular to the slope) to prevent the generation of higher-order diffracted light as much as possible compared to the binary type. An increase in efficiency is achieved.

As an example, there is a report that an efficiency of 97% was obtained by imparting a blaze effect to a diffraction grating.

〔Example〕

8. Hereinafter, one embodiment of the present invention will be explained with reference to FIG.

・ FIG. 5 is a diagram showing an embodiment of the present invention,
(a) is a plan view, (b) is a side view, and (c) is a perspective view of the cross section of the bent diffraction grating in (a) as viewed from the front.

In the figure, numeral 18 is a ferroelectric crystal such as a LiHbOa crystal, and since the crystal surface 19 is Ti-diffused and has a refractive index slightly higher than the surrounding refractive index, it functions as an optical waveguide layer.

The light from the semiconductor laser 21 is guided to the optical waveguide layer 19 by end face coupling, and a minute spot is formed on the disk 23 by the coupling lens 22 and the objective lens 20. 24 is an electrode for exciting a surface acoustic wave (SAW), and by changing the frequency of the SAW, a light wave is diffracted by the SAW, and a minute spot formed on the disk follows a track on the disk. That's what I do. The light reflected from the disk passes through an objective lens 20 and becomes a polarized beam split. YokL, -C,y'Iyuga. □Ka, □6 are taken out.

The objective lens 2o, which is the center of the present invention, will be explained in detail below using FIG. 5(C).

First, the average refractive index of the cladding layer is ngc, the wavelength of the light wave traveling in the waveguide is λ, the propagation constant is β, and the period of one of the blazed gratings is 8. Let the angle be θ. In order to focus the light at the focal point, the pinch A of the grating needs to titrate the following equation:

ngc 2π A Then, as the focus approaches directly below the focal point, θ becomes smaller, so Δ also becomes smaller, resulting in an unequal pitch as shown in Figure 1.

In such a grating, the blazed surface of each grating is formed perpendicular to the line connecting each grating and the focal point. In this way, the output angle θ when the grating pitch is calculated using ρ, which increases the output p position by the blaze surface, differs from the actual output angle, but the thickness of the blaze is about 1 μm, and the focal point is Since it is located about 1 to 2ff1 above the waveguide surface, the error poses almost no problem.

[Effects of the Invention] As described above, according to the present invention, the generation of high-order diffracted waves can be suppressed by providing a blaze effect to the bent diffraction grating of the leading waveguide, so that the connection between the light waveguide and the space can be suppressed. It is possible to significantly improve the input and output efficiency between the two. Furthermore, by using process technology, it becomes possible to realize an optical head with high precision and low cost.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a conventional optical head, FIGS. 2, 3, and 4 are diagrams for explaining the conventional tracking principle and automatic focusing principle, and FIG. 5(a). (b) and (Q) are diagrams for explaining an embodiment of a high-efficiency optical head according to the present invention. 21... Semiconductor laser, 22... Coupling lens, 23... Recording medium, 24... Surface acoustic wave generation electrode, 25... Polarizing beam splitter 126...
・Concentration 1st figure Kaya 3rd figure? ￥4-diagram

Claims (1)

[Claims] 1. Using a bent diffraction grating formed on the waveguide, the light wave from the light source propagating within the waveguide is emitted so as to be focused on the recording medium, and the reflected light is guided again through the above-mentioned diffraction grating. In an optical head that receives light guided into a wave path and reflected by a deflection beam splitter with a photodetector, by imparting a blaze effect to the bent diffraction grating, efficiency is increased and recording information, focus error signals, and tracking signals are further improved. An optical head characterized by high performance detection.