JPH0253244A - Optical head for magneto-optical recorder - Google Patents

Abstract

PURPOSE:To simplify and miniaturize the structure of the title device by arranging a photodetector having light receiving surfaces, each of which is divided into four, at the converging point of a normal light component and an abnormal light component separated on an uniaxial birefringence crystal parallel plate. CONSTITUTION:An uniaxial birefringence crystal parallel plate 50 is provided under an inclined state in the convergent luminous flux of light receiving systems 15, 30, 52, and 54 from a magneto-optical recording medium 20, and a photodetector 54 having the light receiving surfaces, each of which is divided into four, is arranged at the converging point of a normal light component (o) and an abnormal light component (e) separated by the plate 50. As the result, not only a magneto-optical signal but also a servo signal can be detected by executing addition processing and subtraction processing for each output from each divided element. Thus, the light path of a magneto-optical signal detecting system and that of a servo signal detecting system can be made common, the number of parts can be reduced, and the device can be miniaturized.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to an optical head used in a magneto-optical recording device, and more specifically, the present invention relates to an optical head used in a magneto-optical recording device.
This invention relates to an optical head in which a magneto-optical signal detection system and a servo signal detection system share a common optical path by using a divided photodetector.

[Prior art] Information is reproduced from a magneto-optical recording medium by utilizing the phenomenon that when a magnetic material is irradiated with linearly polarized light, the plane of polarization of reflected or transmitted light rotates in accordance with the direction of magnetization. be exposed.

An example of a conventional optical head in a magneto-optical recording device is shown in the fifth section.
As shown in the figure, light generated from a semiconductor laser 10 is converted into a parallel beam with a circular cross section by a collimating lens 12 and a beam shaping prism 14, passes through a beam splitter 16 and an objective lens 18, and reaches a magneto-optical disk 20. The light reflected here has a polarization angle tilted in a certain direction.

Then, it passes through the objective lens 18 and is reflected by the beam splinter 16, and is separated into a servo signal detection system and a magneto-optical signal detection system by the next beam splinter 22. In the servo signal detection system, the light passes through a condensing lens 24 and a cylindrical lens 26 and enters a four-part photodiode 28 . The astigmatism method is used to detect focus errors, and the push-pull method is used to detect tracking errors.

In the magneto-optical signal detection system, the polarization angle of the light is changed using a 1/2 wavelength plate 3°, and the polarization beam splitter 32 uses an optical differential method.
．． 36 and detected by photodiodes 38 and 40.

A magneto-optical signal is obtained by taking the differential of these outputs with a differential amplifier 42.

Note that reference numeral 44 indicates a drive coil for the objective lens 18.
6 indicates a coil for applying a magnetic field.

[Problem to be solved by the invention] In order to miniaturize the entire optical herad, there is a limit to just miniaturizing each optical component, so it is necessary to devise the optical path and reduce the number of optical components used. is important. However, in the above-described conventional technology, many optical components such as lenses and prisms must be combined, and the optical path is branched into two directions, making it extremely difficult to downsize.

Furthermore, since the optical path is branched into the servo signal detection system and the magneto-optical signal detection system, there is also the problem that the amount of light is insufficient.

The purpose of the present invention is to solve the technical problems of the prior art as described above, to simplify the structure and reduce the size, and to make the optical path of the magneto-optical signal detection system and the servo signal detection system common. An object of the present invention is to provide an optical head for a magneto-optical recording device which can obtain a large amount of light, improve a C/N ratio, and realize stable operation.

[Means for Solving the Problems] The present invention, which can achieve the above objects, provides a parallel plate of uniaxial birefringent crystal in the light receiving system into which the light from the magneto-optical recording medium is incident, and transmits and separates the transmitted light from the light receiving system. This is an optical helad configured to detect with a photodetector whose surface is divided into eight parts in total.

The basic configuration of the present invention is shown in FIG. 1. A uniaxial transversely refracting crystal parallel plate 50 is made of quartz, rutile, calcite, etc., and the optical axis of the crystal is magnetically recording so that the ordinary light component and the extraordinary light component are separated. It is tilted at an angle α with respect to the light incident surface 50a from the medium. This uniaxial transverse refraction crystal parallel flat plate 50 is placed behind the condenser lens 52 and positioned in the convergent light beam by the condenser lens 52 . And the light incidence surface 5
0a is tilted by an angle β.

The incident light is separated into an ordinary light component 0 and an extraordinary light component e by this uniaxial birefringent crystal parallel plate 50, and reaches a photodetector 54 whose light receiving surface is divided into eight parts in total. Here, the uniaxial transverse refractive crystal parallel plate 50 is tilted simply to perform the astigmatism method. When the inclination angle is 45°, the light incidence surface 50
If the optical axis is tilted by about 55 degrees with respect to a, the ordinary light component and the extraordinary light component will be largely separated, so such a configuration is preferable.

The ordinary light component O and the extraordinary light component e each form a focal point, and each of the four elements of the 8-split photodetector 54 corresponds to one focal point. In other words, as shown in Figure 2, the extraordinary light component e
For A and B.

Four elements C and D correspond, and for ordinary light component 0, E and F
, G, and H correspond to the four elements.

[Function] The reflected light from the magneto-optical recording medium is converged by the condenser lens 52, and is divided into an ordinary light component 0 and an extraordinary light component e by the uniaxial transverse refracting crystal parallel plate 50, and the light-receiving surface is 4 at each condensing point. It reaches the photodetector 54 which is divided. The beam shape at the photodetector 54 is shown in FIG. In the figure, the hatched portion in the upper three rows is the beam shape for the extraordinary light component e, and the hatched portion in the lower three rows is the beam shape for the ordinary light component 0. Further, in FIG. 2, (middle) indicates the in-focus state, (δ) indicates the case where the focal position is far away, and fcl indicates the case where the focal position approaches.

Detection of the magneto-optical signal is obtained by adding the detection outputs of a set of four elements from each element and taking the differential output.

This can be expressed mathematically as follows.

Detection of the magneto-optical signal - (A + B + C + D) - (E + F Take the differential using
It can be obtained by adding them. This can be expressed mathematically as follows.

Focus error signal -FE1+FE2- ((A+C
)-(B+D))+ ((E+G)-(F+H)) This is shown in a graph as shown in FIG.

The dashed line shows the finally obtained focus error signal.

The tracking error signal can be detected by a push-pull method. This can be expressed mathematically as follows.

Tracking error signal = TE L + TE 2- (
B-D) + CF-H) In this way, in the present invention, the necessary magneto-optical signals and focus and tracking servo signals can be obtained by adding and subtracting the signals from each light receiving element. .

[Embodiment] FIG. 4 is an explanatory diagram showing an embodiment of the optical head according to the present invention. The incident path of the read light from the semiconductor laser 10 to the magneto-optical disk 20 may be basically the same as in the prior art. The light from the semiconductor laser 10 is converted into parallel light by a collimating lens 12, passes through a beam shaping and beam splitter 15, and is focused by an objective lens 18 to illuminate a predetermined position on the optical disc 20.

The reflected light from the magneto-optical disk 20 is reflected by a beam splinter, the plane of polarization is rotated by 45 degrees by a half-wave plate 30, and the light is condensed by a condenser lens 52.

In the converging light beam, a uniaxial birefringent crystal parallel plate 50 (lI
The 8-split photodetector 54 for signal detection is located at the focal point of the light beam. The details of the condenser lens 52, the uniaxial transverse refracting crystal parallel flat plate 50.8-divided photodetector 54 are as explained in FIG. By calculating the output of each element in the eight-divided photodetector 54, not only a magneto-optical signal but also a focus error signal and a tracking error signal can be obtained.

[Effects of the Invention] As described above, the present invention tilts a uniaxial birefringent crystal parallel plate into the converging light beam of the light receiving system from the magneto-optical recording medium, and thereby focuses the separated ordinary light component and extraordinary light component. Since photodetectors each having a light-receiving surface divided into four are arranged, not only a magneto-optical signal but also a servo signal can be detected by adding and subtracting the outputs from each divided element. Therefore, the optical path of the magneto-optical signal detection system and the servo signal detection system is shared, and the number of parts can be significantly reduced and the device can be made smaller.

In addition, since the present invention uses a parallel plate of uniaxial birefringent crystal, it is 10 to 20 dB lower than a conventional polarizing beam splitter.
It has a high extinction ratio and can detect magneto-optical signals with high sensitivity. Furthermore, as mentioned above, the optical path of the magneto-optical signal detection system and the servo signal detection system is shared, and since these signals are detected by the same photodetector, it is possible to apply stable servo without insufficient light intensity. .

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the basic configuration of the present invention, and FIG. 2 (al.
, (bl, , (C1 is an explanatory diagram showing the beam shape on the light receiving surface of the photodetector, FIG. 3 is an explanatory diagram showing the amount of focus error with respect to defocus, and FIG. 4 is an explanatory diagram showing the amount of focus error with respect to defocusing. FIG. 4 is an explanatory diagram showing the beam shape on the light receiving surface of the photodetector. FIG. 5 is an explanatory diagram showing an example. FIG. 5 is an explanatory diagram showing an example of the conventional technology. 10... Semiconductor laser, 20... Magneto-optical disk,
50... Uniaxial transverse refracting crystal parallel plate, 52... Condensing lens, 54... Photodetector. Patent Applicant: Fuji Denki Kagaku Co., Ltd. Representative: Shigeru Mi Dan Diagram (b) (C)

Claims (1)

[Claims] 1. In a light-receiving system where light from a magneto-optical recording medium is incident, a parallel plate of uniaxial birefringent crystal whose optical axis is tilted with respect to the light incidence plane is used in a convergent beam so that the ordinary light component and the extraordinary light component are separated. In addition, a photodetector with a light-receiving surface divided into four parts is placed at the focal point of the ordinary light component and the extraordinary light component separated by the parallel flat plate of the uniaxial birefringent crystal, and the light incident surface is inclined with respect to the optical path. An optical head for a magneto-optical recording device, characterized in that a magnetic signal detection system and a servo signal detection system share a common optical path.