JPH01285046A - Magneto-optical head for magneto-optical recording and reproducing device - Google Patents

Abstract

PURPOSE:To facilitate the assembly and adjustment of an optical system by returning a linearly polarized light transmitted through or reflected at a plate polarizing beam splitter to the plate polarizing beam splitter again in rotating a polarizing surface and reflecting or transmitting the light at or through the plate polarizing beam splitter. CONSTITUTION:P polarized components transmitted through two-sheet laminating plate polarizing beam splitter 12 are converted to S polarized components by a second optical system 21 composed of a 1/4 wavelength plate 13 and a reflecting mirror 14, the components are returned to the two-sheet laminating plate polarizing beam splitter 12 again, and they are reflected at the two-sheet plate polarizing beam splitter 12. Consequently, the same result as the case where two sheets of parallel flat plates with the same thickness as the two-sheet laminating plate polarizing beam splitter 12 are inclined at an inclination angle equal in positive and negative reverse directions to an optical axis is obtained, and a coma can be canceled. Thus, since the parallel flat plates as a conventional case can be made unnecessary, the adjustments of the inclinations, thicknesses, etc., of the parallel flat plates can be made unnecessary, and the assembly and adjustment of the optical system can be facilitated.

Description

[Detailed Description of the Invention] Technical Field> The present invention is directed to a recording medium having a magnetized film that is uniformly magnetized in a specific direction during recording by transmitting light from a light source such as a semiconductor laser through various optical components. At the same time, by applying an auxiliary magnetic field to reverse the direction of magnetization, information is recorded on the recording medium.At the time of playback, the recording medium is irradiated with polarized light to generate information using the magneto-optical effect (Kerr effect). The present invention relates to a magneto-optical head of a magneto-optical recording/reproducing device configured to detect polarized light with a polarization direction corresponding to the magnetization direction of the magnetized film and reproduce information.

<Prior art> Figure 3 shows a magneto-optical head 5 of a conventional magneto-optical recording/reproducing device.
1 is a diagram showing the configuration of FIG.

First, the operation when recording an information signal on the magneto-optical recording medium 8 will be explained. The laser beam emitted from the semiconductor laser light source 2 becomes a parallel beam by the collimating lens 3, is converted from an elliptical cross-section beam to a circular cross-section beam by the heem shaping prism 4, passes through the beam splitter 5, and is collimated by the objective lens 6 to the substrate 7. A condensed light beam is irradiated onto the magneto-optical recording medium 8 via the magneto-optical recording medium 8.

The magneto-optical recording medium 8, as shown in FIG. 4(a),
The magneto-optical recording medium 8 is magnetized in the opposite direction to the direction in which it has already been magnetized by the bias magnetic field coil 9 or the like in the area including the part where the entire surface is previously magnetized in one direction and where the laser beam is focused and irradiated. By applying an auxiliary magnetic field,
Information is recorded as shown in FIG. 4(b).

The reflected light from the magneto-optical recording medium 8 passes through the objective lens 6 again, is reflected by the beam splitter 5, passes through the I5/2 wavelength plate 10, and is converged by the condenser lens 11. This convergent light passes through a plane-parallel plate 52 and a two-plate polarizing heem splitter 53 formed by integrating two flat plates, and enters the four-split photodetector 15 .

Here, as shown in FIG. 5, if a parallel plane plate 52 is inserted at an angle with respect to the optical axis of the convergent light, the imaged point by the meridional light beam will be Astigmatism occurs due to the mismatch with the imaging point due to the spherical beam. Therefore, the cross-sectional shape of the light beam with astigmatism has a straight part (a in Figure 5), a circular part (b in Figure 5), and a straight part (Fig. 5). C) in the figure is produced.

The astigmatism in this case, IIWa (distance between a and C in Figure 5), is Wa=d(n'-1)sin''U/[(n''-s r
n! u) 3) l/', and the thickness d of the plane-parallel plate 52, the refractive index n, and the inclination between the optical axis of the convergent light and the normal line of the plane-parallel plate (f4) can be arbitrarily determined by appropriately selecting u. It is possible to obtain an astigmatism mWa of .

However, as shown in FIG. 6(a), when only - parallel plane plates 52 are used, not only astigmatism but also other aberrations, especially coma, occur, making it difficult to use for focus control. This makes it difficult to detect the cross-sectional shape of the light beam.

The coma aberration fiWk in this case is Wk=d(n'-1) sinu -cosu ・s
i n 3α/2(n ″−s i n″u) 6/″, and α is the convergence angle of the convergent light. Therefore, the sixth
If another plane-parallel plate 62 is inserted with the same angle of inclination as the plane-parallel plate 52 and tilted in the opposite direction as shown in Figure (b), comatic aberration will be canceled, while astigmatism will be reduced. The effects of the two parallel planes 52 and 62 are added.

In the conventional magneto-optical head 51 shown in FIG. 3, a two-plate polarizing beam splitter 53 plays the role of the parallel plane plate 62.

Therefore, when the convergent light enters the plane parallel plate 52 and the two-ply polarizing beam splitter 53, which are set at an angle with respect to the optical axis, an imaging point is formed by the meridional beam and an image is formed by the spherical beam. Since astigmatism occurs due to the mismatch with the point, the cross-sectional shape of the light beam is approximately circular immediately after the two-ply flat plate polarizing beam splitter 53;
As it moves away, it becomes elliptical, and finally becomes linear (Fig. 7a), then becomes elliptical again, and then returns to a circular shape (
Figure 7b) It then becomes elliptical again, becomes straighter (Figure 7C), returns to an ellipse, and then returns to a circle.

As shown in FIG. 2, four light-receiving elements 15a, 15b, I5 are placed at the position where the cross-sectional shape is circular (FIG. 7b).
A 4-split photodetector 15 composed of 5d and 5d is arranged to detect the cross-sectional shape of the astigmatic light beam.

That is, when the distance between the objective lens 6 and the magneto-optical recording medium 8 is the focus distance, the cross-sectional shape of the light beam with astigmatism becomes circular as shown in FIG. The output of will be equal. Next, if the distance between the objective lens 6 and the magneto-optical recording medium 8 is smaller than the focus distance, the position where the cross-sectional shape of the astigmatic light beam becomes circular moves away from the two-ply flat plate polarizing beam splitter 53. 4, the cross-sectional shape of the light beam on the light-receiving surface of the 4-split photodetector 15 becomes an ellipse long in the X-axis direction (b2 in FIG. 2), and the output of the light-receiving elements 15a and 15c is small, The output will be larger.

Next, when the distance between the objective lens 6 and the information recording medium 8 becomes larger than the focus distance, the position where the cross-sectional shape of the light beam with astigmatism becomes circular approaches the two-ply flat plate polarizing beam splitter 53. The cross-sectional shape of the light-receiving surface of the 4-split light detector 515 is an ellipse (second
Figure b3), and the light receiving element! The outputs of 5a and 5C become large, and the outputs of light receiving elements 15b and 15d become small.

Therefore, based on the output of the 4-split photodetector I5, it is detected whether the distance between the objective lens 6 and the information recording medium 8 is at the focus distance, closer to the focus distance, or farther than the focus distance. be able to.

Recording is performed while performing focus control based on the output of this 4-split photodetector 15.

Next, when reproducing information, the laser beam emitted from the semiconductor laser 2 as P-polarized light is converted into a parallel beam by the collimating lens 3, and the beam shaping prism 4, beam splitter 5,
The light passes through the objective lens 6 and the substrate 7 and is focused onto the magneto-optical recording medium 8 .

The plane of polarization of the reflected light from the magneto-optical recording medium 8 is rotated by an angle θ in one direction depending on the direction of magnetization of the magneto-optical recording medium 8 due to the magneto-optic effect (Kerr effect). The reflected light whose polarization plane has been rotated enters the objective lens 6 again.

The laser beam that has passed through the objective lens 6 is reflected by the beam splitter 5 and enters the I/2 wavelength plate 10.

The laser light incident on the 1/2 wavelength plate 10 has a polarization plane of 45
The light is rotated by .degree., converged by the condenser lens 11, transmitted through a parallel plane plate 52, and directed to a two-plate polarizing beam splitter 53 which functions as an analyzer. Of the light directed toward the two-ply flat plate polarizing beam splitter 53, the S-polarized component is reflected by the reflective surface 54 of the two-ply flat plate polarizing beam splitter 53 and enters the photodetector I6.

The 10,000P polarized light component passes through the two-ply polarized beam splitter 53 and enters the four-split photodetector 15.

Therefore, of the laser beam modulated by the magneto-optical recording medium 8, the P-polarized light component enters the four-split photodetector 15, and the S-polarized light component enters the photodetector 16. The P polarized light component incident on the 4-split photodetector 15 is @element 1 of the 4-split photodetector 15.
It can be obtained by taking the sum of 5a to 15d. And 4
By taking the differential signal of the output photoelectrically converted by the split photodetector 15 and the photodetector 16, a recorded information signal can be obtained.

The laser light incident on the four-split photodetector 15 is also used as a signal for focus control, and information is reproduced while performing focus control in the same way as during recording.

However, in such a conventional magneto-optical head 51, the inclination angle, thickness, etc. of the two parallel plane plates, the parallel plane plate 52 and the two-plate bonded flat plate polarizing beam splitter 53, must be adjusted respectively. The problem was that it became difficult to assemble and adjust the system.

<Objective of the Invention> The present invention has been made in view of the above-mentioned points, and an object of the present invention is to provide a magneto-optical head whose optical system is easy to assemble and adjust.

<Structure of the Invention> In the present invention, in order to achieve the objects mentioned in L, the magneto-optical recording medium is irradiated with light from a laser light source.1. In a magneto-optical head of a magneto-optical recording/reproducing device that records or reproduces information using a magneto-optical recording medium, a magneto-optical head separates reflected light from the magneto-optical recording medium from an optical path from the laser light source to the magneto-optical recording medium and converts it into convergent light. 1 optical system, a flat plate polarizing beam splitter configured by integrating at least two flat plates and arranged to be inclined with respect to the optical axis of the convergent light so as to produce astigmatism; A second optical system converts the polarization plane of the transmitted light or reflected light from the flat plate polarizing beam splitter into linearly polarized light by rotating it by 90 degrees and returns it to the flat plate polarizing beam splitter. and a photodetector for detecting the linearly polarized light that is rotated and returned to the flat polarizing beam splitter and reflected or transmitted by the flat polarizing beam splitter, and focus control is performed based on the output of this photodetector. ing.

According to the above configuration, the linearly polarized light transmitted or reflected by the flat plate polarizing beam splitter is rotated by the second optical system, returns to the flat plate polarizing beam splitter again, and is reflected or transmitted by the flat plate polarizing beam splitter. Therefore, it is equivalent to installing two parallel plane plates of the same thickness as the flat plate polarizing beam splitter with equal inclination angles in opposite directions, and coma aberration can be avoided even if the parallel plane plate 52 is not provided as in the conventional example. will be cancelled.

<Examples> The present invention will be described in more detail below based on examples shown in the drawings.

FIG. 1 is a block diagram of a magneto-optical head l according to an embodiment of the present invention, and parts corresponding to the conventional example in FIG. 3 are given the same reference numerals.

In FIG. 1, 2 is a semiconductor laser as a laser light source, 3 is a collimating lens that converts the light from the semiconductor laser 2 into a parallel beam, 4 is a beam shaping prism that converts a beam with an elliptical cross section into a beam with a circular cross section, and 5 is a beam 3 is a splitter, 6 is an objective lens, 7 is a substrate, 8 is a magneto-optical recording medium, 9 is a bias magnetic field coil, IO is a 1/2 wavelength plate, and 11 is a condensing lens. This is the same as the conventional example.

Beam splitter 5. The 1/2 wavelength plate 10 and the condenser lens 11 separate the reflected light from the magneto-optical recording medium 8 from the optical path from the laser light source 2 to the magneto-optical recording medium 8, and convert it into convergent light. 1 optical system 20 is configured.

In the magneto-optical head (2) of this embodiment, a two-plate polarizing beam splitter is constructed by integrating two flat plates! 2 is arranged to be inclined with respect to the optical axis of the convergent light from the first optical system 20 so as to cause astigmatism,
Furthermore, the polarization plane of the transmitted light (P-polarized light) from the two-ply flat plate polarizing beam splitter 12 is rotated by 90 degrees, converting it into linearly polarized light (S-polarized light), and then sending it back to the two-ply flat plate polarizing beam splitter 12. a second optical system 21 for returning;
The plane of polarization is rotated by this second optical system 21' and returned to the two-ply polarized beam splitter 12, and the linearly polarized light (S polarized light) reflected by the flat plate polarized beam splitter 12 is detected. A four-split photodetector 15 is provided as a detector. Further, as in the conventional example, a photodetector 16 is provided to detect the light (S-polarized light) reflected by the two-ply flat plate polarizing beam splitter 12 from among the convergent destinations from the condensing lens 11.

The configuration of the two-ply bonded flat plate polarizing beam splitter 12 is the same as the conventional two-ply bonded flat plate polarizing beam splitter 53 shown in FIG. The structure is similar to that of the conventional example.

The second optical system 21 is! It is composed of a /4 wavelength plate I3 and a total reflection mirror 14, and reflects the transmitted light (P-polarized light) from the optical beam splitter 12 as a two-ply bonded flat plate to the two-ply bonded flat plate polarizing beam splitter 12. At the same time, the plane of polarization is rotated 900 times to convert it into S-polarized light and returned.

Therefore, the linearly polarized light that is returned to the two-ply flat plate polarizing beam splitter 12, reflected, and given to the four-split photodetector 15 is provided at equal inclination angles in positive and negative directions with respect to the optical axis of the convergent light. This is equivalent to passing through two parallel plane plates, and coma aberration is canceled, while astigmatism is doubled.

Next, a recording and reproducing operation by the magneto-optical head l having the above configuration will be explained.

First, recording of information signals will be explained. Laser light emitted from the laser light source 2 in the P polarization direction becomes a parallel light beam by the collimating lens 3, is converted from an elliptical cross-section light beam to a circular cross-section light beam by the beam shaping prism 4, passes through the beam splitter 5, and passes through the objective lens 6. Accordingly, the board 7
The condensed light is irradiated onto the magneto-optical recording medium 8 via.

The magneto-optical recording medium 8 is magnetized in one direction over the entire surface in advance, and the magneto-optical recording medium 8 is already magnetized by a bias magnetic field coil 9 or the like in the area including the part where the laser beam is focused and irradiated. Recording is performed by applying an auxiliary magnetic field in the direction opposite to the direction of magnetization.

The irradiated laser beam is reflected by the magneto-optical recording medium 8,
The reflected light enters the objective lens 6 again.

The laser beam that has passed through the objective lens 6 is reflected by the beam splitter 5, passes through the 1/2 wavelength plate 10, and passes through the condenser lens 11.
The light is made into a convergent light and directed to the two-plate polarizing beam splitter I2. The laser beam that has passed through the two-ply flat plate polarizing beam splitter 12 passes through the 1/4 wavelength plate 13, is reflected by the reflection mirror 14, passes through the 1/4 wavelength plate 13 again, and becomes the two-ply flat plate polarized beam. The light is reflected by the reflecting surface 17 of the beam splitter 12 and enters the four-split photodetector 15 .

As explained in the related art of Kamiren, astigmatism occurs when convergent light is incident on a plane parallel plate set at an angle with respect to the optical axis. The amount of astigmatism Wa due to - parallel plane plates is Wa=d(n'-1)sin"u/((n'-sin
'u) 3) '/'', and the amount of comatic aberration Wk is Wk=d(n''-1) sinu Icosu asin
3α/2(n′−s i n ”u) 5ρ.

Here, d is the thickness of the parallel plane plate, n is the refractive index, U is the inclination angle, and α is the convergence angle of the convergent light.

In the magneto-optical head I of the present invention, two bonded flat plate polarizing beam splitters 12, ! Using the configuration of the /4 wavelength plate I3 and the reflection mirror 14, the P polarized light component transmitted through the two-ply flat plate polarizing beam splitter 12 is divided into 1/4
A second optical system 21 consisting of a wavelength plate 13 and a reflection mirror 14 converts the light into an S-polarized light component, returns it to the two-ply flat plate polarizing beam splitter 12, and reflects it at the two-ply flat plate polarizing beam splitter 12. Because
This is the same as tilting two parallel plane plates of the same thickness as the two-ply flat plate polarizing beam splitter I2 at equal inclination angles in opposite directions with respect to the optical axis, and coma aberration is canceled while The point aberration becomes twice as large.

This eliminates the need for the parallel plane plate 52 as in the conventional example, and therefore eliminates the need to adjust the inclination, thickness, etc. of the parallel plane plate 52, making it easy to assemble and adjust the optical system.

The four-split photodetector 15 is placed at a position where the cross-sectional shape of the astigmatic light beam becomes circular when the distance between the objective lens 6 and the magneto-optical recording medium 8 is the focus distance.

Then, as shown in FIG. 2, four light receiving elements 15a-
15d in combination, so that the cross-sectional shape of the light beam can be detected.

If the objective lens 6 and the magneto-optical recording medium 8 are closer than the focus distance, the cross-sectional shape of the light beam will be an ellipse long in the X-axis direction in FIG. 2, and if it is far away, it will be an ellipse long in the Y-axis direction.

Therefore, from the output of the 4-split photodetector 15, it is determined whether the distance between the objective lens 6 and the magneto-optical recording medium 8 is at the focus distance, closer than the focus distance, or farther than the focus distance. can be detected.

Information is recorded while performing focusing using the signal obtained from the four-split photodetector 15.

Next, in information reproduction, the laser light emitted from the laser light source 2 in the P polarization direction becomes a parallel light beam by the collimating lens 3, is converted from an elliptical cross-section light beam to a circular cross-section light beam by the beam shaping prism 4, and is sent to the beam splitter. 5 and is condensed and irradiated by an objective lens 6 onto a magneto-optical recording medium 8 via a substrate 7 .

The plane of polarization of the reflected light from the magneto-optical recording medium 8 is rotated by an angle θ in one of ten directions depending on the direction of magnetization of the magneto-optical recording medium 8 due to the magneto-optic effect (Kerr effect). The reflected light whose polarization plane has been rotated enters the objective lens 6 again.

The laser beam that has passed through the objective lens 6 is reflected by the beam splitter 5 and enters the 1/2 wavelength plate 10.

The laser light incident on the 1/2 wavelength plate IO has a polarization plane of 45
The light is rotated by 0.degree., is turned into a convergent beam by a condenser lens 11, and is directed toward a two-plate polarizing beam splitter 12, which functions as an analyzer. Of the laser light directed to the two-ply flat plate polarizing beam splitter 12, the S-polarized component is
It is reflected by the two-ply flat plate polarizing beam splitter I2 and enters the photodetector 16.

On the other hand, the P-polarized light component is a two-ply polarized beam splitter 12,! /4 wavelength plate 13 and reflects mirror 1.
4, passes through the 1/4 wavelength plate 13 again, is converted into an S-polarized laser beam, is reflected by the two-ply flat plate polarizing beam splitter 12, and enters the 4-split photodetector 15. Therefore, of the laser beam modulated by the magneto-optical recording medium 8, the P-polarized component converted to the S-polarized component enters the four-split photodetector 15, and the S-polarized component enters the photodetector 16. P incident on the 4-split photodetector i5
The polarized light component is transmitted to each element 15a-+5 of the 4-split photodetector 15.
It can be obtained by taking the sum of d.

Then, by taking the differential signal of the output photoelectrically converted by the 4-split photodetector 315 and the photodetector 16, the recorded information signal can be obtained. Here, the laser light incident on the 4-split photodetector 15 is also used as a signal for focus control, and information is reproduced without performing focus control in the same way as during recording.

In the above-mentioned embodiment, the light transmitted through the two-ply flat plate polarizing beam splitter 12 is configured to be returned by rotating the plane of polarization by the second optical system 21. Of course, the configuration may be such that the plane of polarization of the light reflected from the two-layered flat plate polarizing beam splitter 12 is rotated and returned to the two-layered flat plate polarizing beam splitter 12.

<Effects of the Invention> As described above, according to the present invention, the polarization plane of the transmitted light or reflected light from the flat plate polarizing beam splitter is rotated by 90 degrees, converting the polarized light into linearly polarized light, and returning the polarized light to the flat plate polarizing beam splitter again. Since the configuration is configured to detect the linearly polarized light reflected or transmitted by this flat plate polarizing beam splitter, there is no need to provide a parallel plane plate separately from the flat plate polarizing beam splitter as in the conventional example. There is no need to adjust the thickness, etc., making it easier to assemble and adjust the optical system compared to the conventional example, and reducing costs.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is a plan view of a four-split photodetector, Fig. 3 is a block diagram of a conventional example, and Fig. 4 is a diagram of magnetization of a magneto-optical recording medium. Figure 5 is a diagram to explain the occurrence of astigmatism due to a parallel plane plate, Figure 6 is a diagram showing the occurrence of coma aberration due to a parallel plane plate and its correction, and Figure 7 is a diagram to explain the occurrence of astigmatism due to a parallel plane plate. FIG. 3 is a diagram for explaining focus error detection in a conventional example using a face plate. l... magneto-optical head, 8... magneto-optical recording medium, 12
...Two-ply bonded flat plate polarizing beam splitter! 3.
・l/4 wavelength plate, 14... Total reflection mirror, 15...
4-split photodetector, 20...first optical system, 2I...second optical system.

Claims (1)

[Claims] (1) In a magneto-optical head of a magneto-optical recording/reproducing device that records or reproduces information by irradiating light from a laser light source onto a magneto-optical recording medium, the light reflected from the magneto-optical recording medium is emitted from the laser light source. It includes a first optical system that separates from the optical path leading to the magneto-optical recording medium and converts the convergent light into convergent light, and is configured by integrating at least two flat plates, and is arranged on the optical axis of the convergent light so as to produce astigmatism. a flat plate polarizing beam splitter disposed at an angle with respect to the flat plate polarizing beam splitter; a second optical system, and a photodetector for detecting linearly polarized light whose polarization plane is rotated by the second optical system and returned to the flat polarizing beam splitter and reflected or transmitted by the flat polarizing beam splitter, A magneto-optical head for a magneto-optical recording/reproducing device, which performs focus control based on the output of a photodetector.