JPS62267953A - Magneto-optical recording optical head - Google Patents

Abstract

PURPOSE:To realize a photomagnetic recording optical head with few deterioration in an S/N even when a substrate with large double refraction is used, by providing a slit which passes only light in the neighborhood of a straight line extending in a direction to intersect orthogonally with each other from the periphery of the center of a light beam, and the center of the light beam, at just before and behind an analyzer. CONSTITUTION:Emitting light from a semiconductor laser 1 forms a spot on a recording medium 6 formed on a substrate 5 through a collimator lens 2, two beam splitters 3, and a condenser lens 4. After reflected light from the recording medium is separated, and only the specific part of the beam is passed through a slit 7, a signal is detected at an optical detector 9 through an analyzer 8. The reflected light separated at the beam splitter 3 on the other hand passes through a convergent lens 10, and a part of it is separated by the beam splitter 3, and introduced to a track error detector 11, and another part of it is introduced to a focus error detector 13 through a knife edge 12.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical head for magneto-optical recording that performs recording and reproduction on a magnetic medium using light.

[Conventional technology]

Magneto-optical recording uses a perpendicularly magnetized film to form recording bits by reversing magnetic domains. During reproduction, linearly polarized light is irradiated, and it is detected that the polarization axis of the reflected light rotates in different directions depending on the orientation of the magnetic domain due to the magnetic Kerr effect.

FIG. 5 shows the basic configuration of the optical head. The light emitted from the semiconductor laser l is passed through a collimating lens 2 and a beam splitter 3.
, through the condensing lens 4 and onto the recording medium 6 through the substrate 5. The reflected light beam is separated by a beam splitter 3 and guided to a photodetector 9 through an analyzer 8 set at a predetermined angle. The amount of transmitted light of the analyzer changes depending on the direction of the magnetic domain of the recording medium, and pits are detected. The reflected light beam separated by the other beam splitter is used to detect positional errors of the spot on the recording medium.

[Problem that the invention seeks to solve]

It is desirable that the recording medium be formed on a plastic substrate because the pre-group can be easily manufactured.However, unlike a glass substrate, a plastic substrate has birefringence due to residual stress during molding. In particular, polycarbonate substrates widely used in compact discs have a large refractive index difference between the in-plane and vertical directions of the substrate. In a substrate that has birefringence both in-plane and in the vertical direction, there is no effect of birefringence when a light beam passes through the substrate perpendicularly, but the effect appears when it passes obliquely.

FIG. 4 shows the relationship between the angle of incidence on a polycarbonate substrate and the amount of birefringence felt by a light beam (the amount of birefringence determined from the ellipticity of transmitted light). As the angle of incidence increases, large birefringence will occur.

In an optical head, light passes through the substrate as a convergent beam, so the beam passes obliquely through the substrate at the periphery, resulting in birefringence. When linearly polarized light passes through a birefringent material, it becomes elliptically polarized, which leads to deterioration of S/N during reproduction in magneto-optical recording.

An object of the present invention is to provide an optical head for magneto-optical recording in which the S/N ratio does not deteriorate significantly even when using a substrate having birefringence as described above.

[Means for solving problems]

The present invention has an optical system that includes an optical system that converges an emitted beam from a laser light source onto a recording medium, and an optical system that separates a reflected light beam from the recording medium and passes it through an analyzer to detect a signal with a photodetector. The optical head for magnetic recording is characterized in that a slit is provided immediately before or immediately after the analyzer through which only light near the center of the light beam and near straight lines extending from the center of the light beam in mutually orthogonal directions passes through.

[Effect]

As mentioned above, in the optical head, the light beam passes through the substrate as a convergent beam. Therefore, as the distance from the beam center increases, the angle at which the beam travels obliquely within the substrate increases, making birefringence more noticeable. However, the degree to which the polarized light becomes elliptical varies depending on the direction from the beam center. This means that the angle between the incident polarization direction and the oblique direction of the beam is Oo.
Alternatively, at 90°, no ovalization occurs, and at 45°, ovalization is greatest.

FIG. 2 shows the distribution of the ovalization rate within the beam when the incident polarization is in the X-axis direction.

The ovalization rate increases as the distance from the beam center and the X-axis and y-axis increases. From this figure, it can be seen that deterioration of S/H can be reduced if the signal is reproduced by passing only through portions with a small ovalization rate such as pinholes and slits. A slit shape as shown in FIG. 3 can be considered.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the present invention. The emitted light from the semiconductor laser 1 forms a spot on a recording medium 6 formed on a substrate 5 via a collimator lens 2, two beam splitters 3, and a condenser lens 4. The reflected light from the recording medium is separated by a beam splitter 3, and after passing only a specific portion of the beam by a pickpocket 71-7, a signal is detected by a photodetector 9 via an analyzer 8. The reflected light split by the other beam splitter 3 passes through the converging lens 10, one part is split by the beam splitter 3 and sent to the track error detector 11, and the other part goes to the focus error detector 13 via the knife 12. In this embodiment, a two-split photodetector is used to detect a tracking error in the spot position, and the Knifezi method is used to detect a disc, but other detection methods such as an astigmatism method or a three-beam method may be used.

Furthermore, a differential detection method using reflected light and transmitted light of the analyzer may be used for signal detection. Even if the slit is inserted after the analyzer, the effect is the same.

〔Effect of the invention〕

According to the present invention, it is possible to realize an optical head for magneto-optical recording with little deterioration in S/N even when a substrate with high birefringence is used.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an embodiment of the optical head of the present invention, Fig. 2 is a diagram showing the ovalization rate in the light beam, Fig. 3 is a diagram showing an example of slit formation, and Fig. 4 is a diagram showing the formation of the slit. FIG. 5 is a diagram showing the relationship between the incident angle and the amount of birefringence, and FIG. 5 is a diagram showing the configuration of a conventional optical head. 1... Semiconductor laser, 2... Collimator lens, 3
... Beam splitter, 4... Condensing lens, 5...
・Substrate, 6... Recording medium, 7... Slit, 8...
・Analyzer, 9... Photodetector, 10... Converging lens, 11... Track error detector, 12... Nifetsu, 13... Focus error detector, 14.
...Error detection optical system. The 5th toko of 1/Yu 131 △ angle of fire Φ (ko

Claims (1)

[Claims] In an optical head for magneto-optical recording, the optical head has an optical system that focuses an emitted beam from a laser light source onto a recording medium, and an optical system that separates a reflected light beam from the recording medium and passes it through an analyzer to detect a signal with a photodetector. , an optical head for magneto-optical recording, characterized in that a slit is provided immediately before or after the analyzer, through which light passes near the center of the light beam and near straight lines extending from the center of the light beam in mutually orthogonal directions. .