JP2690943B2 - Optical disk device - Google Patents

Description

The present invention relates to an optical disc device, and more particularly to a device suitable for focus detection.

[Conventional technology]

Conventionally, focus detection in an optical disk device has been performed by the method shown in FIG. That is, the laser light emitted from the semiconductor laser 1 becomes parallel light by the coupling lens 2 and is narrowed down by the objective lens 4 to a minute light spot on the optical disk 6. The reflected light of the optical spot focused on the optical disk 6 is detected by the beam splitter 3 in the photodetector 9
It is led to.

The focus error between the objective lens 4 and the optical disc position is detected by the optical system shown in FIG. When the reflected light from the optical disc is narrowed down by the convex lens 7 and the cylindrical lens 8, a light intensity distribution as shown in FIGS. If a four-division photodetector is arranged near this focal point, the optical disc 6 and the objective lens 4 are arranged.
According to the distance between them, the light intensity distribution 10,1 as shown in FIG.
1,12 is obtained. Therefore, the light intensity incident on the four-division photodetector is I = (I (9a) + I (9b))-(I (9c) +
When calculated according to I (9d)), a focus error signal is obtained as shown in FIG.

[Problems to be Solved by the Invention] The above-described conventional technique has a problem in that the optical system is not downsized and adjustment is easy.

That is, as shown in FIG. 2c, it is necessary to accurately align the light intensity distribution and the four-division light detecting element
When the focal lengths of the lenses 7 and 8 are shortened in order to downsize the optical system shown in FIG. 2, the light distribution becomes minute and the position adjustment with the 4-division sensing element becomes difficult.

An object of the present invention is to provide an optical disk optical system that is small and easy to adjust.

[Means for solving the problem]

The above purpose can be achieved by detecting the intensity of the returning light when reflected, instead of detecting the change in the shape of the fine pattern of the returning light that is reflected from the optical disk to detect the focus error. And

Therefore, the present invention can be achieved by performing focus error detection using a diffraction grating that forms at least three or more light spots at different positions in the depth direction of focus.

〔Example〕

 Hereinafter, an embodiment of the present invention will be described with reference to FIG.

The light from the semiconductor laser 1 becomes parallel light by the coupling lens 2 and is split into three lights by the focus detection diffraction grating 16. When the three branched lights are narrowed down by the objective lens 4, they are narrowed down to three positions in different focal depth directions (17a to 17c). These return lights are divided into three light detection elements 13 through a beam splitter β and a lens 7.
It is narrowed down to a, 13b, 13c. Two of the side spots 17a and 17c of the light spots 17a to 17c for the three branched lights are used for focus detection.

FIGS. 1 (b) and 1 (c) show the light detection signal by the side spots. FIG. 1 (b) shows a side spot 17a.
Is closer to the position of the optical disc 6 than 17c, and when the signals recorded on the optical disc 6 are detected by the respective light detecting elements 13a to 13c, the side spot 17a near the optical disc position is located.
As a result, the modulation degree of the signal detected by the detection element 13a becomes larger than that of 17c.

In this case, the position of the main spot in the middle of the side spots is also displaced from the optical disc position. Therefore, as shown in FIG. 1 (c), if the modulation degree of the detection signal by the side spots is made equal, the main spots are
The position of 17b is in focus. In order to perform this control automatically, the photodetector elements 13a, 13 that detect the side spots are used.
The output of c is detected by the envelope detection circuits 14a and 14b,
The signal is differentially amplified by the differential amplifier 15 to drive the lens driving coil 5.

The focus detection diffraction grating used in the present invention will be described below. FIG. 3A is a diagram used for designing a diffraction grating for focus detection. The focal length of the objective lens 4 is f, and the image formation position of the side spot is a from the lens.
The distance c and the position c are away from the optical axis. For the sake of simplicity, it is still common to think of the diffraction grating as the surface of an objective lens. When there is a point light source at point X, the phase distribution A (c, δ) for creating the diffraction grating is obtained.

The phase P to be given to the diffraction grating is A (c, δ) and A (-
The following equation is obtained from c, −δ).

P = | A (c, δ) + A (−c, −δ) | ² When creating a diffraction grating with a binarized pattern, m is an integer.

Given by That is, And a curve group as shown in FIG.

[Advantages of the Invention] According to the present invention as described above, since the formation of the side spots by the diffraction grating and the focusing control of the objective lens by the comparison of the light intensity reflected from the side spots are performed, the focusing as in the prior art is performed. An optical system such as a combination of a lens and a cylindrical lens is not necessary, and the difficulty in downsizing the alignment of the optical system and downsizing is eliminated.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an optical system of an optical disc device of the present invention, and a waveform diagram of detected light intensity, and FIG. 2 is a schematic diagram of an optical system of a conventional device and an explanatory diagram of focus state detection, FIG. FIG. 3 is an explanatory diagram for designing a diffraction grating for focus detection of the present invention.

Claims (1)

(57) [Claims] 1. At least 3 at different positions in the depth of focus direction.
An optical disk device that detects a focus error using a diffraction grating that forms optical spots of more than one point.