JPH0619838B2 - Optical playback device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical reproducing apparatus using a semiconductor laser light source as a laser light source, and a 3-beam type tracking error detection and astigmatism focusing with a simple optical system. The present invention relates to an optical reproducing device capable of detecting an error.

[Background of the Invention]

Conventionally, in tracking error detection by the three-beam method, as shown in, for example, Japanese Patent Application Laid-Open No. 57-205833, it is known to arrange a phase type diffraction grating in the optical path between a semiconductor laser light source and a beam splitter. There is. This method is widely used for optical audio disc players because it is possible to detect the tracking error of the three-beam system with a simple optical system. In order to do so, there is a problem that an expensive special optical component called a diffraction grating is separately required. Further, in the focus error detection by the astigmatism method, there is a problem that an expensive and special optical component called a cylindrical lens is required to generate astigmatism.

[Object of the Invention]

An object of the present invention is to provide an optical reproducing device which has a function of a beam splitter and a function of generating three beams with one optical component. Another object of the present invention is to provide an optical reproducing device capable of detecting a focus error by the astigmatism method with a simple structure that does not use a cylindrical lens.

[Outline of Invention]

In order to achieve the above object, a divergent beam from a semiconductor laser light source is made into a parallel beam by a collimator lens, the parallel beam is focused by an objective lens and irradiated onto an optical recording medium, and a reflected beam from the optical recording medium is obtained. In the optical reproducing apparatus for irradiating the photodetector with a photodetector to obtain a reproduction signal, in the optical path between the collimator lens and the photodetector, a reflection type diffraction grating internally provided with an uneven beam splitter surface. Are tilted with respect to the optical axis, and a divergent beam from the semiconductor laser light source is reflected by the beam splitter surface to be separated into 0th order and ± 1st order diffracted beams, which are guided to the collimator lens. The first, second and third photodetectors are irradiated with 0th and ± 1st order diffracted reflected beams from the recording medium, respectively, and reproduction signals are produced from the first photodetector. And a tracking error signal is obtained from each of the second and third photodetectors. Further, the reflection type diffraction grating is arranged so as to be tilted in the recording track direction of the optical recording medium with respect to the optical axis and rotated by about 45 ° around the optical axis, Astigmatism is caused in the reflected beam, and the focusing shape of the reflected beam is detected by the first photodetector to obtain a focus error signal.

Example of Invention

The first embodiment of the present invention will be described below with reference to FIG.
A divergent beam 2 from a semiconductor laser light source (laser diode) 1 is incident on a reflection type diffraction grating 3 which is arranged to be inclined with respect to the optical axis. In the figure, it is inclined at about 45 °. The reflection type diffraction grating 3 is a so-called phase type diffraction grating having an uneven beam splitter surface 3a therein. The diverging beam 2 has an uneven beam splitter surface 3a.
Are reflected by the 0th-order diffracted beam 2a and the + 1st-order diffracted beam 2
The beams b, -1st order diffracted beams 2c are separated, and the separated beams 2a, 2b, 2c are incident on the collimator lens 4 to be parallel beams, respectively, and then are incident on the objective lens 5 to be a focused beam. The recording medium 6 is aligned in a line and focused.

It should be noted that each of the beams 2a, 2b, 2 focused on the recording medium 6 is
c are arranged in the direction y orthogonal to the recording track direction x, and the + 1st-order diffracted beam 2 is arranged before and after the 0th-order diffracted beam 2a.
The b, -1st order diffracted beam 2c is located. Recording medium 6
The respective reflected beams 2a, 2b, 2c of the backward direction travel backward, this time after passing through the reflection type diffraction grating 3, the concave lens 7 and the cylindrical lens 8
It is incident on the photodetectors 9, 10, and 11 via. The photodetectors 10 and 11 are provided separately from the photodetector 9 for obtaining the reproduction signal, and the + 1st-order and -1st-order diffracted beams 2b,
Receive 2c. The 0th-order diffracted beam 2a is detected by the photodetector 9
Incident on. Note that the present invention mainly relates to tracking error detection by the three-beam method, and the concave lens 7 and the cylindrical lens 8 are not directly related to the essence of the present invention.

FIG. 2 shows the detection surfaces of the photodetectors 10, 9 and 11. The photodetector 9 is a four-quadrant photodetector 9a, 9b, 9c,
It is divided into 9d. The reproduction signal is obtained by the sum of the detection outputs of the detection units 9a, 9b, 9c, 9d of the photodetector 9.

Due to the astigmatism generated by the cylindrical lens 8, the focus error signal is detected by the detectors 9 in the first and third quadrants of the photodetector 9.
It is obtained by the difference between the sum of the detection outputs of a and 9c and the sum of the detection outputs of the detection units 9b and 9d in the second and fourth quadrants. The spot 2a 'on the photodetector 9 has a substantially circular shape in the focused state and an elliptical shape in the defocused state. The focus error signal moves the objective lens 5 in the axial direction.

The tracking error signal is obtained by the difference between the detection outputs of the photodetectors 10 and 11. By this tracking error signal, the objective lens 5 is moved in the direction y orthogonal to the recording track direction x of the recording medium 6.

According to the first embodiment, since the reflection type diffraction grating 3 is used, the function of the beam splitter and the function of the beam splitter can be realized by this one optical component.
There is an effect that it is possible to combine the function of beam generation.

Next, a second embodiment of the present invention will be described with reference to FIG.
The divergent beam 2 from the semiconductor laser light source 1 is incident on the reflection type diffraction grating 13. The reflection type diffraction grating 13 is arranged so as to be inclined with respect to the optical axis in the recording track direction x of the recording medium 6 and rotated by about 45 ° around the optical axis.
The diffraction grating 3 is basically the same as the reflection type diffraction grating 3 described in the first embodiment, and is a so-called phase type diffraction grating having an uneven Heme splitter surface 13a therein. Since this diffraction grating 13 is the same as a parallel plate that is inclined and arranged, astigmatism occurs in transmitted refracted light. However, since the distance between the surface 13b of the reflection type diffraction grating 13 on the collimator lens 4 side and the beam splitter surface 13a is sufficiently small, the 0th-order diffracted light 2 reflected and separated by the beam splitter surface 13a is separated.
Astigmatism does not occur in the a, + 1st-order diffracted light 2b and the -1st-order diffracted light 2c. This separated beams 2a, 2b, 2
After being incident on the collimator lens 4 and being made into a parallel beam, the beam c is made incident on the objective lens 5 to be a focused beam, which is aligned in a line on the recording medium 6 and focused. In addition,
The beams 2a, 2b, 2c focused on the recording medium 6 are arranged in a direction y orthogonal to the recording track direction x, and a + 1st-order diffracted beam 2b and a -1st-order diffracted beam 2c are arranged before and after the 0th-order diffracted beam 2a. positioned. Each reflected beam 2a, 2b, 2c from the recording medium 6 goes backward, and from now on, after being reflected and refracted by the reflection type diffraction grating 13, it enters the photodetectors 9, 10, 11 through the concave lens 7. The photodetectors 10 and 11 are provided separately from the photodetector 9 for obtaining the reproduction signal, and receive the + 1st-order and -1st-order diffracted beams 2b and 2c, respectively. The 0th-order diffracted beam 2a is incident on the photodetector 9.

FIG. 2 shows the detection surfaces of the photodetectors 10, 9 and 11. The photodetector 9 is a four-quadrant photodetector 9a, 9b, 9c,
It is divided into 9d. The reproduction signal is obtained by the sum of the detection outputs of the detectors 9a, 9b, 9c, 9d of the photodetector 9.

The focus error signal is the sum of the detection outputs of the detection units 9a and 9c in the first and third quadrants of the photodetector 9 due to the astigmatism generated when transmitting and refracting the reflection type diffraction grating 13 arranged in an inclined manner, It is obtained by the difference with the sum of the detection outputs of the detection units 9b and 9d in the second and fourth quadrants. The spot 2a 'on the photodetector 9 has a substantially circular shape in the focused state and an elliptical shape in the defocused state. The focus error signal moves the objective lens 5 in the axial direction.

The tracking error signal is obtained by the difference between the detection outputs of the photodetectors 10 and 11. By this tracking error signal, the objective lens 5 is moved in the direction y orthogonal to the recording track direction x of the recording medium 6.

According to the second embodiment, since the reflection type diffraction grating 13 is used as in the first embodiment, it is possible to combine the function of the beam splitter and the function of generating three beams with this one optical component. There is an effect that it becomes possible. In addition, since the astigmatism generated by the reflection type diffraction grating 13 arranged at an inclination can be used for focus error detection, the cylindrical lens which is an expensive and special optical component conventionally used for generating astigmatism is omitted. can do.

An example of the schematic grating pattern of the reflection type diffraction gratings 3 and 13 used in the first and second embodiments is shown in FIG. 4, respectively.
It is shown in FIG. The reason why the grating pattern is asymmetric is to correct the shift of the focal position of the + 1st-order diffracted light 2b and the -1st-order diffracted light 2c which is generated by tilting the diffraction grating.

FIG. 6 shows an example of a schematic sectional structure of the reflection type diffraction gratings 3 and 13 used in the first and second embodiments. In this example, the reflection enhancing film 23 is formed on the uneven surface 22 of the uneven diffraction grating 21 formed by glass vapor deposition or glass etching.
Was formed, and an optical plastic layer 24 having a refractive index close to that of glass was formed thereon by molding. A metal thin film (not shown) may be used instead of the enhanced reflection film 23.

〔The invention's effect〕

According to the present invention, since the reflection type diffraction grating arranged obliquely is used, it is possible to combine the function of the beam splitter and the function of generating three beams in this one optical component. Further, there is an effect that it is possible to detect a focus error due to astigmatism with a simple configuration that does not use a cylindrical lens.

[Brief description of drawings]

FIG. 1 is a side view of the first embodiment of the present invention, and FIG.
The front view of the photodetector, FIG. 3 is the side view of the second embodiment,
4 and 5 are schematic diagrams of the diffraction grating patterns of the first and second embodiments, respectively, and FIG. 6 is a schematic diagram of the sectional structure of the diffraction grating. 1 ... Semiconductor laser light source, 2 ... Beam, 3 ... Diffraction grating, 3a ... Beam splitter surface, 4 ... Collimating lens, 5 ... Objective lens, 6 ... Recording medium, 7 ... Concave lens, 8 ... ... Cylindrical lens, 9, 10, 11 ... Photodetector, 13 ... Diffraction grating, 13a ... Beam splitter surface, 21 ... Convex-concave diffraction grating, 22 ... Convex / concave surface, 23 ...
… Reflecting film, 24 …… Optical plastic layer.

Claims (2)

[Claims] 1. A beam from a laser light source is separated into 0th order and ± 1st order diffracted beams by a diffraction grating, and the 0th order and ± 1st order diffracted beams are focused by an objective lens and irradiated onto an optical recording medium, The 0th and ± 1st order diffracted reflected beams from the optical recording medium are guided to the light detection means by a beam splitter,
The first, second and third photodetectors constituting the photodetection means are respectively irradiated with 0th and ± 1st order diffracted reflected beams from the optical recording medium, and the first photodetector In an optical reproducing device for obtaining a reproduction signal and obtaining a tracking error signal from the second and third photodetectors, the diffraction grating and the beam splitter are provided, and an uneven beam splitter surface is provided inside thereof. An optical reproducing device characterized in that the optical reproducing device is composed of a single reflection type diffraction grating and is arranged so as to be inclined with respect to the optical axis. 2. The optical reproducing apparatus according to claim 1, wherein the reflection type diffraction grating is tilted with respect to an optical axis in a recording track direction of the optical recording medium, and the optical axis. By arranging them by rotating them by about 45 °, astigmatism is caused in the reflected beam from the optical recording medium, and the focusing shape of the reflected beam is detected by the first photodetector. An optical reproducing device characterized in that a signal is obtained.