JPS59168952A - Reproducing system of optical magnetic memory - Google Patents

Abstract

PURPOSE:To reproduce information with a high quality and a high S/N ratio simplify the adjustment by making a beam, whose polarization direction is rotated with a frequency twice or more as high as a maximum frequency of a reproducing signal, incident to an optical magnetic memory and detecting the phase of a change in the polarization direction of a reflected light or a transmitted light on a basis of the rotation frequency of the incident light to reproduce information. CONSTITUTION:A frequency twice or more as high as a maximum frequency of the reproducing signal is selected as the frequency, with which the polarization direction is rotated, in accordance with the sampling theorem. The parallel luminous flux from a laser light source 26 becomes a linearly polarized light by a polarizer 28 and is divided to diffracted lights 30 and 31 in an acoustooptic modulator 29, and they are made incident to lambda/4 plates 33A and 33B to become clockwise and counterclockwise circularly polarized lights. Two circularly polarized lights become rotating linear polarized lights by mirrors 34 and 35 and are divided to a phase detection reference signal, which is made incident to a photodetector 38, and a light, which is condensed on an optical magnetic memory medium 40 and passes the mirror 36 again and passes an analyzer 41 and a photodetector 42, by a half mirror 36, and phase comparison is performed in a phase detector 43B to reproduce information recorded on the memory 40.

Description

DETAILED DESCRIPTION OF THE INVENTION Industrial Application Field 4-: The present invention relates to a reproduction method for a magneto-optical memory 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. Especially among them,
MnB1,7 has an easy axis of magnetization perpendicular to the film surface.
A magneto-optical memory using a magnetic thin film such as TbDyFe or TbGdFa as a memory medium is attracting attention as a high-density recording carrier from which information can be easily erased. A method for recording information in a magneto-optical memory will be explained using FIG. 1. Before recording information on the magnetic thin film 1Fi, the magnetization 2 was in the direction perpendicular to the film surface, as shown in Figure 1 (a).
The entire surface of the medium is magnetized so as to face in one direction.

A laser beam 3 is irradiated onto this film 1 to heat a part of the film 1 to locally raise the temperature to around the Curie point, and an external magnetic field 4 is applied to the area including this part to record the film. When a magnetic field is applied in the opposite direction to the previous magnetization direction, the magnetization direction of the portion irradiated with the laser beam 3 is reversed, and information is recorded. Therefore, the information U page is accumulated on the magnetic thin film 1 as a difference in the direction of magnetization. On the other hand, reading out the stored information is also carried 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 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 film can be reflected light from the film or transmitted light, but both reflected light and transmitted light are magnetized in the same direction. In contrast, the change in polarization direction occurs in the same direction.

A method of detecting information will be described using transmitted light as an example, with reference to FIG. When the magnetic thin film 5 on which information is recorded is irradiated with a laser beam 7 polarized in the direction of the arrow 6 as shown in FIG. The light is rotated by 9 degrees and becomes polarized in 9 directions. On the other hand, Figure 2 (
As shown in I)), on the magnetic thin film, the magnetization direction is in the opposite direction.

Similarly, when the laser beam 7 polarized in the direction of arrow 6 is irradiated, the transmitted light 1o is rotated by # in -θ with respect to the polarization direction 6, and becomes polarized light in the direction of 11. 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 passing through the analyzer, and the information will be reproduced by the ink that performs 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 converted into a parallel beam by a collimating lens 13, then linearly polarized by a polarizer 14 such as a Glan-Thompson prism, and then sent to a half mirror 1.
5 and magneto-optical memory medium 1 by a condensing lens 16.
The light is focused on 7. The reflected light whose polarization direction has been rotated by the magneto-optical memory medium 17 passes through the half mirror 16 again, and is further divided into two parts by the half mirror 18, each of which is sent to an analyzer 19.
Man.

The light passes through 19B and is received by photodetectors 2OA and 20B. The analyzers 19A and 19B are constructed of Glan-Thompson prisms, etc., and extinguish light whose plane of polarization has been rotated by +θX and light whose plane of polarization has been rotated by −θ, respectively.

It is set in such a position. Photodetector 2OA.

The output of 20B is a differential output obtained by a differential amplifier 21, and is configured such that only a rotational component of the polarization direction due to a change in the direction of magnetization is detected.

In FIG. 3, if the intensity of reflected light from the magneto-optical memory medium 17 is I, the amount of light entering the detection system after the half mirror 18 is reduced by the half mirror 15. As mentioned above, the angles of the analyzers 19A and 19B are
Only light whose polarization direction of reflected light has been rotated by 10 degrees or -θX# is allowed to pass through. In other words, the angle of the analyzer is 24.25 with respect to the polarization direction of the reflected light shown in Fig. 4, 22. The amount of light emitted is −
IBm2θx/2, and the amount of light received by the other photodetector becomes zero. Therefore, the amplitude of the reproduced signal obtained as the output of the differential amplifier 21 has a value proportional to IR8 2θK.

However, in general, the rotation angle θ of the polarization direction has a value of 1° or less, so in the conventional configuration, the amount of light received by the photodetector lR5
There is a drawback that /2 becomes very small in 1n2θ, and the 8/N ratio of the signal obtained through the photoelectric conversion process is very poor. Also, the reproduced signal amplitude lR5 obtained as a differential output
1n2θX is also a very small value, further degrading the signal quality. Furthermore, in the configuration shown in FIG.
The crystal axis directions of 9A and 19B need to be precisely aligned so that they form an angle of 2θ with each other, and the crystal axes of
Considering this, it also has the disadvantage that it is difficult to adjust the optical system.

Purpose of the Invention The present invention has been made in view of the various drawbacks mentioned above, and provides a reproduction method that can reproduce information recorded on a magneto-optical memory medium with high quality and a good S/N ratio, and can also easily adjust the optical system. The purpose is to provide a method.

Structure of the Invention The present invention - irradiates a laser beam onto a magneto-optical memory;
When receiving the reflected light or transmitted light and optically reproducing information recorded as changes in magnetization on a photomagnetic memory, the beam is combined with 77. −□.

at least two of the highest frequencies that the information signal to be reproduced has;
Using linearly polarized light whose polarization direction rotates at a frequency that is more than double This method is characterized in that information is reproduced by performing phase detection based on the rotation frequency of the polarization direction of the beam.

Further, the present invention is characterized in that an acousto-optic light modulation element is used as a means for creating rotation of the polarization direction of the incident light beam.

DESCRIPTION OF THE EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 6 is a diagram showing the configuration of a first embodiment of a magneto-optical memory reproducing apparatus using the present invention.

The light emitted from the laser light source 26 passes through the collimating lens 2
The light beam is converted into a parallel light beam by a polarizer 7, and is converted into a linearly polarized light perpendicular to the plane of the paper by a polarizer 28, and enters an acousto-optic light modulation element 29. Inside the acousto-optic light modulator 29, the laser beam incident on the acousto-optic light modulator 29 is diffracted by the ultrasonic wave of the frequency fA, and is divided into the 0th-order diffracted light 3o and the 1st-order diffracted light 31, each having the same intensity. There is a frequency difference by the ultrasonic frequency fm. These two light beams remain linearly polarized perpendicular to the plane shown in Figure 6.
Reflected by mirror 32B, λ/4 plate 33A, 33
incident on B. The direction of the crystal axes of the two λ/4 plates is the sixth
As shown in the figure, 46 people, 4
It is aimed at 6B. When passing through the λ/4 plate placed in this manner, the incident linearly polarized light becomes clockwise and counterclockwise circularly polarized light, respectively. The two circularly polarized lights pass through the mirror 34 and the half-
When the optical paths are matched by the mirror 36, the light becomes linearly polarized light that rotates at the ultrasonic frequency fm, and enters the half mirror 36. One of the two lights divided by the half mirror 36 passes through the analyzer 37 and enters the photodetector 38, where it becomes a reference signal for detecting one phase.・・－fu・
The other light separated by the mirror 36 is sent to e, by a condensing lens 39.

The light is focused onto the magneto-optical memory medium 4o. The light reflected from the magneto-optical memory medium 40 is again...
The light passes through the analyzer 41 and enters the photodetector 42 . The output of the photodetector 38 is shifted by a suitable amount by a phase shifter 43, and using the resulting signal as a reference, the output of the photodetector 42 is transferred to a phase detector 43B using a balanced modulator or the like.
The information recorded on the magneto-optical memory medium 40 is reproduced by performing a phase comparison and removing frequency components of f or more using a low-pass filter 43OK.

Next, the process of changing the polarization direction of the light beam and the process of signal processing will be explained in detail. For simplicity, within the plane of FIG. 6, the X-axis is taken in a direction perpendicular to the traveling direction of the light beam, and the y-axis is taken in a direction perpendicular to the plane of FIG. 6.

The X component and X component of the electric field of each circularly polarized light after passing through the λ/4 plates 33A and 33B are - Since the intensities of the two lights are equal,
1o7 for right-handed circularly polarized light. , for left-handed circularly polarized light.

becomes. Here, φ indicates the phase difference between the two light beams. Also, between the frequencies ωR1ωb of each light beam,
There is a frequency difference f created by the acousto-optic light modulator. In other words, ωL-ωR=2πfm...
...Used by...Tagawa...(5). This 2
When the two circularly polarized lights are superimposed by the half mirror 36,
The polarization state is the X component.

This indicates light with a y component. If the analyzer 37 is set at an angle that allows only the X component to pass through, the intensity of the light that passes through the half mirror 36 and the analyzer 37 and is detected by the photodetector 38 is determined by )/2 because it does not respond.

=”O(”I +cts (2rrflt, +φ))・
・・・・・・・・・・・・(8) Engineering here. indicates the incident light intensity.

On the other hand, the polarization direction of the light reflected by the magneto-optical memory medium 40 is rotated to +θ or -θ depending on the direction of magnetization on the memory, and each component of the electric field is as follows. Here, R indicates the reflectance of the magneto-optical memory medium 4o. Analyzer 41
Similarly to 37, if the angle is set to allow only the component in the X direction to pass through, the intensity of the light that passes through the half mirror 36 and the analyzer 41 and is detected by the photodetector 42 is calculated as in equation (8). It becomes -13, -ji. Here, IR is the intensity of reflected light from the magneto-optical memory. The AC component of the reference signal for phase detection given by equation (8) is shifted by 9θ0 phase using a phase shifter 43, and this output and the AC component of the information signal shown in equation 09 are transferred to a balanced modulator, etc. When the phase detection circuit 43B used is multiplied by
+φ±2491)・・・・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・D. This inner cylinder term is a harmonic component, and the filter circuit a
30, and as a result, the signal S±= + -IR2θX reflecting the information of the magneto-optical memory is... 13 is obtained. Therefore, the signal amplitude is S = 18+ -S-l = In5In2θx
-----It becomes f141.

The signal amplitude In5in2θI obtained using the present invention.

Signal amplitude In5In20 obtained with the conventional configuration! Comparing 1 to θ is usually ~1°.

20 S stones! /S stone 2θ~6o・・・・・・・・・
It can be seen that the signal amplitude increases by about 60 times in the reproduction using the present invention.Also, the light received by the photodetector 42 The amount of light is also l in the case of the conventional example.
R51n2θ, /2 was extremely small, whereas in the case of the single-wire system, the average intensity IR was approximately 106 times the light intensity, as expressed by the equation Ql), and the signal quality in the photoelectric conversion process was 16 2 , Beto ramie]! No more deterioration. Furthermore, in the above description, analyzer 37. The set angles of 41 and 41 were both in the X direction, but in reality, the set angles can be corrected by adjusting the amount of phase shift of the phase shifter 43, so the set angle of the analyzer can be set in any direction. Therefore, when this method is used, adjustment of the analyzer is completely unnecessary.The present invention allows light whose polarization direction changes with time to enter the magneto-optical memory, and allows the magneto-optical memory to adjust the polarization direction slightly. This method detects changes in the phase of the signal being measured as changes in the phase of the signal being measured.According to the sampling theorem, the frequency at which the polarization direction is rotated is at least twice the highest frequency of the information signal to be reproduced. You need to choose. When an acousto-optic light modulator is used as a means for producing light whose polarization direction is rotated as in the above embodiment, the frequency f of the ultrasonic wave that provides the rotational frequency can be set to approximately 80M)[z. , it is possible to use the reproduction method of the present invention for information signals up to 40 MHz.

The configuration of a second embodiment using the present invention is shown in FIG. This improves the utilization efficiency of laser light compared to the first embodiment, and at the same time simplifies the design of the optical system. The 0th order diffracted light 30 emitted from the acousto-optic light modulator enters the polarizing beam splitter 46 as an S-polarized wave perpendicular to the plane of FIG. 7, and is totally reflected.

On the other hand, -order diffracted light 31 becomes P-polarized by the λ/2 plate 47, enters the polarizing beam splitter 46 by the -mirror 48, and is completely transmitted. The two lights whose polarization planes are orthogonal to each other emitted from the polarizing beam splitter 46 are split to the right by a λ/4 plate 49 whose crystal axis direction is set at 46° with the polarization direction of the two incident lights. At the same time, linearly polarized light that is superimposed and rotated is formed as circularly polarized light that rotates around and to the left. According to the second embodiment, the loss of light amount that occurred in the half mirror 35 of the first embodiment is eliminated, resulting in a partially simpler optical system.

The configuration of a third embodiment using the present invention is shown in FIG. This allows the reference signal to be obtained more stably than in the first embodiment. Half-mirror 17, some of the light emitted from G-36 is mirror 60m.

The light passes through 50B and is incident on the mirror 51, which moves along with the condenser lens 39 to follow the irregularities on the surface of the magneto-optical memory medium 4o. The reflected light from the mirror 61 passes through the analyzer 52 and is received by the photodetector 63, and the signal obtained by photoelectric conversion is used as a reference signal for phase detection. With this configuration, there is no change in the amount of time delay between the reference signal and the information signal due to unevenness on the surface of the magneto-optical memory medium, and the reference signal and the information signal remain constant except for phase changes due to Kerr rotation. It becomes possible to use a phase difference, making single-phase detection more stable.

As described in detail about the invention, when the present invention is used to reproduce information from a magneto-optical memory, the signal can be reproduced with high quality, for example, about 10 times the amount of signal light and about 60 times the amplitude of the reproduced signal. At the same time, an optical system that does not require any fine adjustment of the analyzer can also be realized.

[Brief explanation of drawings]

Figure 1 shows the principle of the recording process of magneto-optical memory.
Fig. 2 is a side view showing the principle of the reproducing process of magneto-optical memory, Fig. 3 is a block diagram showing the configuration of a conventional magneto-optical memory reproducing method, and Fig. 4 is a diagram of the conventional reproducing method. 6 is a block diagram showing the first embodiment of the magneto-optical memory reproducing method of the present invention. FIG. 6 is a diagram explaining the state of the polarization direction in the same embodiment of the present invention. A diagram explaining the state,
FIG. 7 is a block diagram showing a second embodiment of the present invention;
The figure is a block diagram showing a third embodiment of the present invention. 26...L/-f-i source'-27...
Collimating lens, 28...Polarizer, 29...
...Acousto-optic light modulation element-30...0th order diffracted light, 31...1st order diffracted light. 32mm, 32B... Mirror + 33mm, 33B.
・Group・λ/4 plate, 34...Mirror-35, 36
...Half mirror-537...Analyzer,
38...Photodetector, 39...Condensing lens-40...Magneto-optical memory medium, 41...
...Analyzer, 42...Photodetector, 43m...
... Phase shifter, 43B ... Phase detector, 43C
......Low pass filter, 44...Polarization direction, 46M, 46B...Crystal axis direction of λ/4 plate, 46...Polarized beams''-Pezo 7' IJ footer 147...λ/2 root -4g・
...Mirror-49----...λ/4 plate-5OA
, 50B------Bella--s 1...Bella-
152...Analyzer, 53...Photodetector. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure (A) (Logic Figure 2 (A) (B) Figure 4?3 Figure 5 Figure 6 Figure 8 Guide β 43Q

Claims (2)

[Claims] (1) When reading information recorded as changes in magnetization perpendicular to the film surface of a magnetic thin film using a light beam. A linearly polarized beam whose polarization direction is rotated at a frequency that is at least twice the highest frequency of an information signal reproduced as a light beam is used, and the light beam is incident on the magnetic thin film. By detecting the phase of changes in the polarization direction of reflected light or transmitted light based on the rotation frequency of the polarization direction of the incident light beam. A magneto-optical memory reproducing method characterized by reproducing the information. (2) Claim 1, which is characterized by black ink using an acousto-optic light modulation element as a means for producing rotating linearly polarized light.
Magneto-optical memory reproducing method described in .