JPS61198458A - Method for reproducing magnetooptic information - Google Patents

Abstract

PURPOSE:To eliminate the effect due to fluctuation of the distance between a recording and the detection system by splitting a luminous flux into the 1st luminous flux comprising a polarized component in a prescribed direction and the 2nd luminous flux comporising the polarized component in a direction vertical to the prescribed direction and interferring the 1st and 2nd luminous fluxes to detect information. CONSTITUTION:Light irradiated from a light source 61 is collimated by a collimator lens 62, becomes a luminous flux 20 subject to linear polarization into P polized light through a polizer 3, condensed on an optomagnetic disc 24 and reflected. In this case, the polarized azimuth of the reflected light is rotated to produce an S polarized component, which is reflected by a polarized beam splitter 22, bent by a mirror 27 and becomes a luminous flux 25 without almost any loss of luminous amount. On the other hand, the P polarized light component component transmits through the polarized beam splitter 22, is reflected by the beam splitter 2 and becomes a luminous flux 26. The luminous fluxes 25, 26 are interferred by a beam splitter 30 and propagate in two directions and are condensed on photodetectors 33, 34 by condenser lenses 31, 32 respectively and detected. Thus, the adverse effect due to the change in the distance between the recording medium and the detection system is eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a magneto-optical information reproducing method for reproducing high-density magnetically recorded information using the magneto-optic effect.

[Prior art]

In recent years, rewritable magneto-optical systems using magneto-optic effects have been viewed as promising in the field of high-density recording of information.

Conventionally, a reading optical system as shown in FIG. 6 has been used to reproduce the above-mentioned magneto-optical recording. Here, the light emitted from a light source 1 such as a semiconductor laser is passed through a collimator lens 2.
The light becomes a parallel light beam 3, which is linearly polarized in a certain direction through a polarizer 4, passes through a beam splitter 5, and an objective lens 6, and is incident on a magneto-optical disk 7 in the form of a bright spot. This incident light corresponds to the difference in the upward or downward direction of the perpendicular magnetization of the magneto-optical disk, and the polarization plane of the light flux is rotated θ in opposite directions due to the magneto-optic Kerr effect.
It receives and reflects k. For example, if the incident light flux is P polarized light,
Assuming that the plane of polarization changes +θk for upward magnetization and 10 for downward magnetization so that it has an S-polarized component, the reflected light will become as shown in A and H in FIG.

The reflected light passes through the objective lens 6 and the beam splitter again and is bent in the direction of a parallel beam 8, and as shown in FIG. The light passes through an analyzer 9 arranged at an angle ψ and reaches a photodetector 10. The intensity of the light passing through the analyzer at this time corresponds to the diagonal component shown in C in FIG. 7, and is detected as a strength or weakness depending on whether the magnetization is upward or downward.

However, the Kerr rotation angle θ due to the magneto-optical Kerr effect is a minute angle on the order of 0.1 degrees, and the absolute amount of light reaching the photodetector is small, so noise components from lasers, magneto-optical disk photodetectors, etc. had a large effect on the signal, and the deterioration of the 8N ratio had become a problem.

On the other hand, the so-called optical mixing method has been proposed as a method that eliminates the above-mentioned drawbacks and obtains a larger S/N ratio than the conventional reproducing method, which can overcome the 8N ratio deterioration in minute signal reading that is a problem in magneto-optical reproduction. A magneto-optical information reproducing method using the method has been proposed (Publication No. 135646/1982).
.

FIG. 8 is an explanatory diagram of a magneto-optical information reproducing method using such an optical mixing method.

A light beam 11Fi linearly polarized in a predetermined direction passes through a beam splitter 12 and an objective lens 13 to a magneto-optical disk 14.
It is incident on the top in the form of a spot.

The reflected light flux, which has been modulated according to the upward or downward magnetization of the magneto-optical disk as described above, passes through the objective lens 13 and is bent by the beam splitter 12 to become a light flux 15, which is sent to the light source by the beam splitter 17 in an appropriate manner. The light is mixed with the divided reference light beam 16 and sent to the analyzer 1.
8 and is detected by a photodetector 19. At this time, the luminous flux 1
A method in which the frequencies of the light flux 5 and the light flux 16 are the same is called a homodyne method, and a method in which the frequencies are different is called a heterodyne method. Both are capable of photodetector shot noise limit detection,
8N ratio can be improved.

However, in such a magneto-optical disk reproducing apparatus, the disk surface usually moves up and down with respect to the detection system due to the disk being tilted or the like. Therefore, in the conventional method using the light mixing method as described above, the beam splitter 12
As the distance between the magneto-optical disk 14 changes, the optical path lengths of the light beams 15 and 16 change, and as a result, there are cases in which signals from the magneto-optical disk cannot be read.

[Summary of the invention]

An object of the present invention is to eliminate the drawbacks of the prior art described above, improve the S/N ratio of signal reading, and eliminate the effects of fluctuations in the distance between the recording medium and the detection system. Our goal is to provide the following.

The above-mentioned object of the present invention is to irradiate a recording medium on which information is magnetically recorded with a light beam polarized in a predetermined direction, and to convert the light beam whose polarization direction has changed in accordance with the information into a light beam consisting of a polarization component in the predetermined direction. This is achieved by dividing the light beam into 10 light beams and a second light beam consisting of a polarization component in a direction perpendicular to a predetermined direction, and then interfering with the first light beam and the second light beam to detect the information.

〔Example〕

The present invention will be described in detail below with reference to one drawing.

FIG. 1 is a schematic diagram showing an example of the configuration of a detection system using the method of the present invention. Here, 61 is a light source such as a semiconductor laser,
62 collimator lenses, 63 a polarizer that transmits only P-polarized light, and 20 a linearly polarized light beam of P-polarized light. Also, 21
is a beam splitter that splits the luminous flux at a predetermined ratio, 22
23 is an objective lens, 24 is a magneto-optical disk, 25
is S of the reflected light beam split by the polarizing beam splitter
26 is a polarizing beam splitter 22.
The P-polarized light beam of the reflected light beam split by the beam splitter 21 is further reflected by the beam splitter 21, the reflecting mirror 27,
28 changes the polarization direction from the P polarization direction to the S polarization direction 7
A wavelength plate 29 is a phase plate that adjusts the phase of the light flux that has passed through the seven-wavelength plate 28, 30 is a beam splitter that causes the light fluxes 25 and 26 to interfere, 31 and 32 are condensing lenses, 33
and 34 are photodetectors.

The light emitted from the light source 61 becomes parallel light by the collimator lens 62, passes through the polarizer 63, and becomes a light beam 20 linearly polarized into P-polarized light.

This luminous flux of 20, beam splitter 21. [The magneto-optical disk 2 passes through the optical beam splitter 22 and the objective lens 23.
, 4 in the form of a spot and is reflected. On this occasion,
As mentioned above, depending on the direction of perpendicular magnetization of the magnetic film, the polarization direction of the reflected light is rotated by +θk or only partially.
This produces an S-polarized component. The 8 polarized light components of the reflected light pass through the objective lens 23, are reflected by the polarizing beam splitter 22, are further bent by the mirror 27, and reach the luminous flux 25 with almost no loss in light quantity.

On the other hand, the P-polarized component of the reflected light is transmitted through the objective lens 23. The light passes through the polarizing beam splitter 22, is reflected by the beam splitter 21, and becomes a light beam 26.

The light beam 26 is passed through a 100-wave plate 28 to change the polarization direction from the P polarization direction to the S polarization direction, and then the phase is adjusted by the phase plate 29. The phase plate 29 is set to maximize the interference effect when interfering with the light beam 25. Luminous flux 2
5.26 is interfered by beam splitter 30. The interference light travels in two directions, and is focused onto photodetectors 33 and 34 by condenser lenses 31 and 32, respectively, and detected.

Next, the principle of reading information on a magneto-optical disk by the interference of the light beams 25 and 26 will be explained with reference to FIG. Figure 2 (A)
Assuming that is a model of perpendicular magnetization on the magneto-optical disk 24,
(B) is the intensity of the light beam 25, and (C) is the phase of the light beam 250. (D) is the intensity of the light beam 26, and (E) is the phase of the light beam 25.

Here, the horizontal axis corresponds to the position on the magneto-optical disk (A). Also, O<Is<<1. IP>1. As is clear from the figure, the phases of the light beams 25 and 26 are in phase or opposite (or opposite or in phase) due to the difference in the upward and downward directions of perpendicular magnetization. The present invention focuses on this point and reads signals by interference.

The luminous flux 25.26 is expressed by the formula (1) and (
2) It becomes the formula.

, //TF3e 1 (ωt + If)
(1) 〆Shame e' (a+t・δ) (2) However, δ' is δ when the vertical magnetization is upward (or downward), δ - π when the vertical magnetization is downward (or upward), and ω The frequency of dimming, t, is a time variable.

Therefore, the intensity of the interference light is expressed by equation (3).

l, 'r';el(ωt-δ')+6eI(ωt-JT
= Is+Ip+24 cos (δ-δ')
(3) In equation (3), the first term and the second term are DC components.

The third term is an alternating current component, which corresponds to the signal of the magneto-optical disk. In other words, the third term is -2/Cn when the perpendicular magnetization is upward (or downward), and -24 from δ-J'=π when the perpendicular magnetization is downward (or upward).
becomes.

In this way, the signal component of the interference light is almost unaffected by noise, and by increasing the overall absolute light amount, the S/N can be improved.
It can be detected as a signal with improved ratio. In addition, in the configuration shown in FIG. 1, if the signals detected by the photodetectors 33 and 34 are further differentially detected by a differential amplifier (not shown), the result will be 8/8.
N ratio improves.

According to the method of the present invention, since a part of the reflected light from the disk is used as diagnostic illumination light, even if the distance between the recording medium and the detection system changes, the optical path length of the two light beams to be interfered with will not change in the same way. Therefore, stable signal detection is possible without creating an optical path difference.

FIG. 3 is a partial schematic diagram showing another example of the configuration of a detection system using the method of the present invention. In this example, the polarizer 6 of the example in FIG.
3 to the polarizing beam splitter 22 and in the optical path up to the beam splitter 301C, the other configurations are the same as those in FIG. 1 and are not shown.

In the figure, 35 is an incident light beam that is linearly polarized to P polarization, and 36 is S
Polarizing beam splitter that reflects polarized light and transmits P-polarized light (for example, S-polarized light reflectance 99%, P-polarized light reflectance 98%), 37Fi7 Faraday rotator using an element with Alladay effect, 38 is a single wavelength plate , 39 is the reflected light flux from the magneto-optical disk. The polarization state of each optical element in FIG. 3 is shown in FIG. 4, and its characteristics will be explained.

The incident light beam 35 is a linearly polarized P-polarized light as shown in FIG. 4(A). Most of the light beam 35 passes through the polarizing beam splitter 36, and the polarization direction is rotated by 45 degrees by the Faraday rotator 37 as shown in (B). The light is then converted back into linearly polarized P-polarized light by the one-wavelength plate 38 as shown in FIG.

On the other hand, the P-polarized component of the light flux reflected by the magneto-optical disk is
The light passes through the seven-wavelength plate 38 from bottom to top in FIG. 3, and its polarization direction becomes as shown in (E). The light then passes through the 7 Alladey rotator 37 and is changed into S-polarized linearly polarized light as shown in (F), and is almost reflected by the polarizing beam splitter 36 to become a light beam 39.

In this way, if the configuration shown in FIG. 3 is adopted, it is Kyoto, that is, in the detection system shown in FIG. By arranging the 7Alade rotator 37 and the 7 wavelength plate between the beam splitter 22, the light from the light source can be used with almost no loss, and at the same time it functions as an optical isolator to prevent light from returning to the light source. Detection with a high 87N ratio becomes possible.

FIG. 5 is a schematic diagram showing still another configuration example of a detection system using the method of the present invention. In the figure, 40 semiconductor lasers, 4
1 is a collimator lens, 42 is a polarizer, 43 is a beam splitter surface, 44 is a polarizing beam splitter surface, 45 objective lenses, 46 is a magneto-optical disk, 47 is a single wavelength plate, 4
8 is a mirror surface, 49 is a beam splitter surface, 50° 51
52 and 53 are photodetectors. Also, 5
4.55 indicates the light flux reflected by the magneto-optical disk.

The light emitted from the semiconductor laser 40 passes through the collimator lens 4
1, the light is made into a parallel light beam, and the polarizer 42 turns the light into a linearly polarized P-polarized light, which is transmitted through a beam splitter surface 43 that also serves as beam shaping. In addition, 1 polarized beam splitter surface 4
4. The light passes through the objective lens 45 and enters the magneto-optical disk 46 .

As mentioned above, the 8 polarized light components and the P polarized light component of the reflected light modulated by the signal from the magneto-optical disk are separated by the polarizing beam splitter 44, and the S polarized light component becomes the light beam 54.
The P polarized light component is reflected by the beam splitter 43,
The luminous flux becomes 55. The light flux 54.55 has P polarization direction and S
The one-wavelength plate 47 has its crystal axis arranged at an angle of 45° with respect to the polarization direction, so that each light becomes circularly polarized.

The light beam 54 passes through the mirror surface 47 and interferes with the light beam 55 on the beam splitter 49 .

Interference light travels in two directions, each with a condensing lens 50°51
After the light is collected, it is detected by the light detectors 52 and 53.

In this embodiment, during manufacturing, the surfaces 44 and 49 are placed on the same plane, and the distances between this surface and the surfaces 43 and 48 are the same, so that the luminous flux divided by the polarizing beam splitter 44 is The optical path difference between the two beams is made zero before they are interfered by the beam splitter 49 again. In this way, it is possible to eliminate the adverse effects of vibrations in the optical system and fluctuations in the frequency of the laser used as a light source, and a stable interference effect can be obtained.

The present invention is not limited to the above embodiments, but can be applied in various ways. For example, the above embodiment uses the so-called Hosodine method, in which laser beams of two different frequencies are incident on the magneto-optical disk.

After dividing the reflected light from the magneto-optical disk into two beams, selecting only the beams of different frequencies in each optical path and interfering with each other, it is also possible to perform reproduction by the so-called heterodyne method. Furthermore, although the embodiments show signal reading using reflected light from a recording medium, the present invention can also be applied to optical signal reading using transmitted light from a recording medium using the Faraday effect.

〔Effect of the invention〕

As explained above, the present invention splits light whose polarization direction has changed according to information into light with polarized components in two directions, and detects these lights by interfering with each other, thereby achieving a high 87N ratio for information reading. It is possible to obtain the effect of eliminating the adverse effects caused by changes in the distance between the recording medium and the detection system while maintaining the same.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an example of the configuration of a detection system using the present invention;
@Figure 2 is a diagram explaining the reproduction principle of the present invention, Figure 3 is a partial schematic diagram showing another configuration example of a detection system using the present invention, and Figure 4 is a diagram explaining the reproduction principle of the present invention.
The figure shows the polarization state of each optical element in Fig. 3, Fig. 5 is a schematic diagram showing another example of the configuration of the detection system using the present invention, and Fig. 6 shows the configuration of the conventional detection system. Schematic,! 7
This figure is a diagram explaining the principle of reproduction according to the conventional method, and FIG. 8 is a schematic diagram showing the configuration of another conventional detection system. 21.30...Beam splitter, 22-Polarizing beam splitter, 23...Objective 7X', 24...Magneto-optical disk, 27...Mirror, 28-1 wavelength plate, 29...・Phase plate% 31.32...Condensing lens, 3
3.34...Photodetector, 61...Light source, 62...
Collimator lens %63...Polarizer. (A)

Claims (1)

[Claims] (1) A light flux polarized in a predetermined direction is irradiated onto a recording medium on which information is magnetically recorded, and the light flux whose polarization direction has been changed according to the information is converted into a first light flux consisting of a polarization component in the predetermined direction. A magneto-optical information reproducing method that detects the information by dividing the first light beam into a second light beam consisting of a polarization component in a direction perpendicular to a predetermined direction and then interfering with the first light beam and the second light beam.