JPH0542741B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a magneto-optical memory reproducing device that reads information using the magneto-optical effect.

2. Description of the Related Art Structures of Conventional Examples and Their Problems In recent years, optical memory devices have been developed that use laser light to record and reproduce information at high density. Among them, MnBi, which has an easy axis of magnetization perpendicular to the film surface, amorphous TbDyFe,
Magneto-optical memories that use magnetic thin films such as TbCdFe as memory media are attracting attention as high-density recording carriers that allow information to be easily erased. A method for recording information in a magneto-optical memory will be explained using FIG. 1. Before recording information, the magnetic thin film 1 is magnetized so that the magnetization 2 is perpendicular to the film surface and oriented in one direction over the entire medium, as shown in FIG. 1A. This film 1 is irradiated with a laser beam 3 as shown in FIG. When a magnetic field is applied in the opposite direction to the magnetization direction before recording, the direction of magnetization of the part irradiated with the laser beam 3 is reflected,
Information is recorded. Therefore, information is stored on the magnetic thin film 1 as a difference in the direction of magnetization. on the other hand,
The stored information is also read out by irradiating the magnetic thin film with laser light and optically detecting the difference in the direction of magnetization. This detection method using laser light utilizes the Kerr effect or the Faraday effect, and uses a phenomenon in which the polarization direction of light irradiated onto a magnetic material changes depending on the direction of magnetization. The reproduction light that returns from the magnetic thin film with its polarization direction changed by the information stored in the magnetic thin film can include reflected light and transmitted light from the film.
If both reflected light and transmitted light are magnetized in the same direction, the polarization direction changes in the same direction. A method of detecting information will be described using transmitted light as an example, with reference to FIG. As shown in FIG.
When irradiated with , the transmitted light 8 is rotated by +θ _k degrees with respect to the polarization direction 6 before irradiation, and becomes polarized light in the direction of an arrow 9. On the other hand, as shown in Figure 2B, on the magnetic thin film,
When laser light 7, which is also polarized in the direction of arrow 6, is irradiated onto a part whose magnetization direction is opposite, the transmitted light 10 is rotated by −θ _k degrees with respect to the polarization direction 6, in the direction of arrow 11. It becomes polarized light. Therefore, if you use an analyzer to distinguish these two lights with different polarization directions, the difference in polarization direction will result in a difference in the intensity of the light that passes through the analyzer, and information will be reproduced by performing photoelectric conversion. That's why.

A conventional example of reproducing information from a magneto-optical memory using the above-described reproducing method will be described with reference to FIG. The light from the laser light source 12 is turned into a parallel beam by a collimating lens 13, linearly polarized by a polarizer 14 such as a Glan-Thompson prism, passed through a half mirror 15, and then sent to a condenser lens 1.
6, the light is focused onto a magneto-optical memory medium 17. The reflected light whose polarization direction has been rotated by the magneto-optical memory medium 17 passes through the half mirror 15 again.
It is further divided into two parts by a half mirror 18, and passes through analyzers 19A and 19B respectively to photodetectors 20A and 20B.
The light is received by the Analyzers 19A and 19B are also composed of Glan-Thompson prisms, etc., and their planes of polarization each form an angle of 45 degrees with respect to the polarization direction of the light incident on the magnetic memory medium 17, and are set at positions that are orthogonal to each other. . A differential amplifier 21 obtains a differential output from the photodetectors 20A and 20B, and the structure is such that only the rotational component of the polarization direction due to the change in the direction of magnetization is detected.

However, in general, the rotation angle θ _k of the polarization direction is less than 1°, so in the conventional configuration, the amplitude of the signal obtained through the photoelectric conversion process is very small, and the S/N ratio of the reproduced signal is insufficient. It has drawbacks.

Purpose of the Invention The present invention has been made in view of the above-mentioned drawbacks.
The purpose is to reproduce information recorded on a magneto-optical memory medium with higher quality and a good S/N ratio.

Structure of the Invention The present invention irradiates a magneto-optical memory with a laser beam, receives the reflected light or transmitted light, and
When optically reproducing information recorded as changes in magnetization on a magneto-optical memory, two linearly polarized beams having a frequency difference of at least twice the highest frequency of the information signal to be reproduced are used as the beams. A first photodetector detects the reflected light from the magnetic thin film obtained by making one of the linearly polarized beams incident on the magnetic thin film, or the transmitted light and the other one of the linearly polarized beams. The information on the magnetic thin film is reproduced by demodulating the signal obtained by making the two linearly polarized beams incident on the second photodetector, based on the signal obtained by making the two linearly polarized beams incident on the second photodetector. It is characterized by:

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 4 is a diagram showing the configuration of an embodiment of a magneto-optical memory reproducing apparatus using the present invention. The light emitted from the laser light source 22 passes through the collimating lens 2
3 into a parallel beam of light, and a polarizer 24 into linearly polarized light perpendicular to the plane of the paper, which enters an acousto-optic light modulation element 25 . Inside the acousto-optic light modulator 25, the incident laser beam is diffracted by the ultrasonic wave of frequency _fA and separated into 0th-order diffracted light 26 and 1st-order diffracted light 27, but at this time, there is a gap between these two light beams. There is a frequency difference equal to the ultrasonic frequency f _A. These two light beams remain linearly polarized perpendicular to the plane of the paper in FIG.
It is reflected at 9. Of the two beams, the 0th order diffracted light 26 is sent to the half mirror 3 as a beam for signal reproduction.
0 and 31, and further enters the magneto-optical memory medium 36 through the condenser lens 32. On the other hand, the first-order diffracted light 27 serves as a reference beam for detecting the optical beat signal, is reflected by the half mirror 33, and is tracked to the half mirror 30 and through the half mirror 33.
The light enters the half mirror 31 through the λ/2 plate 35 and the λ/2 plate 35, respectively. In the half mirror 30, the signal reproduction beam and the reference beam incident on the magneto-optical memory medium are superimposed, and the photodetector 37 detects a beat signal that matches the frequency difference f _A between the two beams. On the other hand, in the half mirror 31, the light reflected from the magneto-optical memory medium 36 and the reference beam whose polarization direction has been rotated by the λ/2 plate 35 are superimposed, and the photodetector 38 detects the frequency difference between the two beams. A beat signal depending on the polarization direction is detected. The output of the photodetector 38 containing the information signal stored on the magneto-optical memory medium is subjected to AM demodulation by the AM demodulator 39 using the output of the photodetector 37 as a reference signal, thereby reproducing the information signal.

Next, the relationship between the change in the polarization direction of the reflected light from the magneto-optical memory medium and the beat signal detected by the photodetector 38 will be explained. In FIG. 5, assuming that the polarization direction of the light incident on the magneto-optical memory medium is 40, the polarization directions of the reflected light are 41A and 41A, depending on the direction of magnetization on the medium.
The light is in the direction of 1B. With respect to this reflected light, the polarization direction of the reference beam is changed by the λ/2 plate 35 to a direction perpendicular to one of the two polarization directions of the reflected light, for example, the direction 42 in FIG. I can do it. This can be easily achieved by rotating the crystal axis direction of the λ/2 plate 35. Therefore, two beams having polarization directions as shown in FIG. 5 are superimposed by the half mirror 31 and can be made incident on the photodetector 38. Here, a reference beam with 42 polarization directions and 41A or 41B
There is still a frequency difference of f _A between the reflected light from the magneto-optical memory medium with the polarization direction of . On the other hand, the optical beat signal is not observed when the polarization directions of the two lights are perpendicular to each other. The optical beat signal will be observed only when facing the direction 41A in the figure. That is, the output signal of the photodetector 38 becomes as shown in FIG. 6c. In FIG. 6, a indicates the direction of magnetization on the magneto-optical memory medium, and b indicates a change in the rotation angle of the polarization direction. The signal shown in FIG. 6c is
The information signal recorded on the magneto-optical memory medium is intensity modulated (AM) at the beat frequency _fA , and is demodulated by the AM demodulator 39 in Fig. 4 to reproduce the original information signal. becomes possible.

The present invention detects minute changes in the polarization direction caused by magneto-optical memory, which were conventionally detected as weak signals.
By causing an optical beat, the signal is converted into an intensity modulation signal whose carrier is a frequency twice or more higher than the maximum frequency component of the information signal band, and the signal is reproduced. The method of detecting such a signal by modulating it at a high frequency has been widely used for detecting small signals. (i) The signal can be amplified by an amplifier with a sufficiently narrow band compared to the carrier frequency; Noise is reduced.

(ii) The signal is smoothed by synchronously detecting the intensity modulated signal at the carrier frequency.
Noise is reduced.

As a result, the S/N ratio of the reproduced signal can be easily improved compared to the conventional case where changes in the polarization direction are directly measured.

In the present invention, according to the sampling theorem, the beat frequency must be selected to be at least twice the highest frequency of the information signal to be reproduced.
When using an acousto-optic light modulator as in the above embodiment, the frequency f _A of the ultrasonic wave that provides the beat frequency
can be set to about 80 MHz, for example, and the reproduction method of the present invention can be used for information signals up to 40 MHz.

Effects of the Invention As explained above, by using the present invention, when reproducing information from a magneto-optical memory, it is possible to reduce the noise during signal amplification, and also to remove noise by smoothing the signal, resulting in a high quality signal. It becomes possible to perform playback.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing the principle of the recording process of a magneto-optical memory, Fig. 2 is a perspective view showing the principle of the reproducing process of a magneto-optical memory, and Fig. 3 is a block diagram showing the configuration of a conventional magneto-optical memory reproducing method. FIG. 4 is a block diagram showing one embodiment of the magneto-optical memory reproducing method of the present invention.
FIG. 5 is a diagram for explaining the state of the polarization direction in the same embodiment of the present invention, and FIG. 6 is a diagram showing an example of a detection signal in the same embodiment of the present invention. 22... Laser light source, 23... Collimating lens, 24... Polarizer, 25... Acousto-optic light modulator, 26... 0th order diffracted light, 27... 1st order diffracted light, 28, 29, 34... Mirror, 30, 31,
33...Half mirror, 32...Condensing lens, 3
5...λ/2 plate, 36... Magneto-optical memory medium, 3
7, 38...Photodetector, 39...AM demodulator, 4
0, 41A, 41B, 42...Polarization direction.

Claims (1)

[Claims] 1. When reading information recorded as changes in magnetization perpendicular to the film surface of a magnetic thin film using a light beam,
Magnetism obtained by using two linearly polarized beams having a frequency difference of at least twice the highest frequency of the information signal to be reproduced as light beams, and making one of the linearly polarized beams incident on the magnetic thin film. A signal obtained by inputting the reflected light from the thin film or the transmitted light and the other one of the linearly polarized beams into a first photodetector, and the two linearly polarized beams are detected by a second photodetector. 1. A magneto-optical memory reproducing device characterized in that the information on the magnetic thin film is reproduced by demodulating the signal obtained by inputting the signal into the device. 2. The magneto-optical memory reproducing device according to claim 1, characterized in that an acousto-optic light modulation element is used as means for producing two linearly polarized beams having a frequency difference.