JPH02192053A - Magneto-optical disk device - Google Patents

Abstract

PURPOSE:To obtain a high recording density and an excellent reproducing S/N ratio, and to reduce an bit error rate by making light fluxes from a light source into the two light fluxes having fine open angles by mans of a plural- image element, and condensedly irradiating an optical disk by the light fluxes. CONSTITUTION:The light flux from a semiconductor laser 7 is polarized and divided into two directions by a plural-image element 4, an objective lens 2 condenses two light fluxes 17 and 18 from a beam splitter 3, and a recording medium surface is irradiated by the two fine spots. The reflected light from a recording medium 1 is polarized and separated through the beam splitter 3 and a 1/4 wavelength plate 8 and a lens 9 by a polarized beam splitter 10. The respective light receiving surface of photo detectors 11 and 12 are divided into front and rear parts in a track direction, receives the reflected light emitted from the polarized beam splitter 10, and a current in proportion to each incident light intensity is outputted. A differential amplifier 15 differential amplifies two intensity differential signals 101 and 102, adds an information signal, and outputs it as a read signal 103. Consequently only the information of a recording bit is correctly reproduced at the read signal 103.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a magneto-optical disk device that reproduces magnetization information by optical differentiation.

[Conventional technology]

A method for recording information in a magneto-optical disk device is performed by thermomagnetic recording in which a recording medium made of a magnetic thin film is irradiated with focused laser light and information is recorded as changes in magnetization on the medium.

That is, in advance, an external magnetic field is applied perpendicularly to the entire film surface of the recording layer to magnetize the recording layer so that it is magnetized upward to write a "0", and then a laser beam is applied to the part where a "1" is to be written. irradiate and heat. The coercive force Hc of the heated minute part becomes small, and if a weak external bias magnetic field is applied in the direction of downward magnetization during laser beam irradiation, the magnetization is reversed and "1" is recorded. A method is used to form a magnetic recording pattern depending on whether or not to irradiate the recording layer, that is, whether or not to raise the temperature of the minute spot irradiated onto the recording layer.

On the other hand, as a method of reading information, for example, when a magnetic recording pattern is irradiated with a linearly polarized laser beam, the effect of rotating the polarization plane of the reflected or transmitted light (
For example, when using the magnetic Kerr effect, it is important to note that the rotation angle θ1 of the polarization plane of reflected light differs depending on the direction of recording magnetization. Using this method, the reflected light passes through an analyzer before entering the photodetector, and information corresponding to the direction of magnetization is read out as changes in the amount of light. This type of magneto-optical disk device is attracting attention as a highly useful file device that can rewrite information, unlike a write-once optical disk.

Conventionally, reading information in such a magneto-optical disk device has been carried out by a method in which a plane of polarization rotated by magnetization information recorded on the surface of a recording medium is converted into a change in light intensity using an analyzer.

In this signal reading method, the portion of the recording medium surface where the magnetization has changed becomes bright or dark, and the playback signal is read by detecting the change in the amount of reflected light as a whole, similar to a reflectance-changing optical disk device. It will be read out. Such detection methods include those that use a single photodetector, and those that use a polarization beam splitter to split the beam into two, which are each received by two photodetectors, and the difference between the outputs is calculated. There are motion detection methods, etc. These methods detect changes in the total amount of light that has passed through the analyzer, and are essentially the same as methods for detecting changes in brightness and darkness.

[Problem to be solved by the invention]

However, in the reading method in the conventional magneto-optical disk device described above, the intensity distribution of the reading light spot focused on the recording medium surface is wide (Gaussian), so the intensity change of the reflected light is steep. It is not. Therefore, the timing information of the reproduced signal is inaccurate,
Moreover, it has the disadvantage that it is easily affected by variations in the intensity of the irradiated light spot, variations in the reflectance of the recording medium, variations in the characteristics of the reproducing circuit, etc., and read errors are likely to occur during signal reproduction.

Furthermore, when attempting to record (pulse width modulation) and reproduce information using changes in the length of the area where magnetization information is recorded (hereinafter referred to as recording peer), a DC component or a low frequency close to it is generated. If the components were not amplified accurately, signal disturbances would become large and accurate information reproduction would not be possible.

Therefore, as the recording density is increased, stable reproduction becomes impossible, the bit error rate increases, and the recording capacity of the disc is limited.

In order to solve these drawbacks of the conventional reading method of magneto-optical disk devices, Japanese Patent Application No. 61-30058 (Japanese Patent Application Laid-Open No. 61-30058)
2-188047), a method is proposed in which a change in the magnetization of a recording medium is converted into a phase delay (or advance) of light, and a signal is read out by capturing a change in the far field pattern caused by the phase difference. has been done. Although this method solves many of the problems mentioned above, it also has the disadvantage that the readout signal tends to contain a lot of noise because the phase change of the light is also caused by irregularities on the surface of the recording medium and non-uniformity of the pre-group. have

Furthermore, since signal detection is performed in the far field, the influence of diffraction due to focusing remains, and the reproduction S/N ratio is degraded compared to a condensing optical system.

The purpose of the present invention is to improve the above-mentioned drawbacks, to achieve high recording density, good stability of the reproduced signal, and good quality of the reproduced signal, and therefore to enable readout with a high reproduction signal-to-noise ratio, resulting in a low error rate. The purpose of the present invention is to provide a magneto-optical disk device.

[Means to solve the problem]

The magneto-optical disk device of the present invention comprises: a light source; a double image element that polarizes and splits the light beam from the light source into two directions with a small aperture angle; , an optical system that condenses and irradiates light beams divided into two directions so that they overlap partially along the recording trunk direction of a magneto-optical recording medium; a beam splitter that reflects part of the reflected light or transmitted light, and a quarter placed in the optical path from this beam splitter.
A photodetector consisting of a wavelength plate, an analyzer that separates the two light beams that have passed through the quarter-wave plate into two polarized components, and at least two light receiving sections that receive light from the analyzer. and circuit means for extracting a difference signal or a sum signal of the output signals from each light receiving section, and the difference signal or sum signal outputted by the circuit means is used as a reproduction signal of magneto-optical information. There is.

[Effect]

According to the present invention, signals are read out by capturing changes in the polarization of each reflected light beam by two focused beams, so compared to a method that simply captures intensity changes due to polarization,
A read signal with high resolution and little signal level fluctuation can be obtained. In addition, noise caused by minute irregularities on the surface of the recording medium cancels out and becomes extremely small by taking the difference or sum of the outputs of the two photodetectors.
A read signal with less noise can be obtained, and therefore information can be recorded and reproduced with high density and high reliability.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing a first embodiment of the present invention. This magneto-optical disk device consists of a semiconductor laser 6 which is a light source, and a magneto-optical disc device which is arranged in an optical path from the semiconductor laser 6 to an objective lens 2 which is a condensing lens, and which records a magneto-optical focus row which is information. A Wollaston prism 4, which constitutes a double image element that polarizes and splits the light beam from a semiconductor laser 6 into two directions with a small aperture angle, is applied to the magneto-optical disk that is the recording medium 1. For example, two optical systems constituting an optical system that converges and irradiates the magnetic recording medium 1 so as to partially overlap each other along the recording track direction of the magnetic recording medium 1.
An objective lens 2 mounted on an axial actuator, a beam splitter 3 that reflects a part of the reflected light from the same bit of each focused light spot, and a quarter beam disposed in the optical path from the beam splitter 3. a wavelength plate 8; a lens 9 that converges the two light beams that have passed through the 1/4 wavelength plate 8;
A light detection device consisting of a polarizing beam splitter 10 that constitutes an analyzer that separates two light beams that have passed through a wavelength plate 8 and a lens 9 into two polarized components, and two light receiving sections that each receive the separated polarized components. Vessel IL 12 and
An arithmetic circuit 2 comprising a differential amplifier 13, 14, 15, which extracts a difference signal between output signals from the light receiving portion of each photodetector.
1, and a signal processing circuit 16 that outputs the difference signal output from the arithmetic circuit 21 as a reproduction signal of magneto-optical information.

Next, the operation of this embodiment will be explained.

The semiconductor laser 6 is supplied with current from the laser drive circuit 7 and emits a laser beam for reproduction. Collimator lens 5
converts the diverging laser beam emitted from the semiconductor laser 6 into a parallel laser beam.

The light beam from the semiconductor laser 6 is polarized into two directions by the Wollaston prism 4, which is a double image element. The separation direction of the light beam divided into two directions is set in the direction in which the light beam is focused and irradiated by the objective lens 2 along the traveling direction 25 (recording track direction) of the recording medium 1. The beam brick 3 serves to align the polarizations of the two separated beams in one direction.

That is, if the polarization of the light beam emitted from the semiconductor laser 6 is P-polarized, the beam splitter 3 has a characteristic of, for example, transmitting 60% of the P-polarized light and reflecting all the S-polarized light. As described above, the objective lens 2 is mounted on the two-axis actuator, and focuses the two light beams 1, 7, and 18 from the beam splitter 3 and irradiates them onto the surface of the recording medium as two minute spots. The two light beams 17 and 18 condensed onto the recording medium are shown in FIG.
), for example, the light beams are condensed and irradiated so that they are half-shaped. FIG. 2(b) shows the light intensity distribution of the luminous flux 17.18.

Each light spot irradiated onto the recording medium 1 is reflected with its polarization direction slightly Kerr-rotated depending on the magnetization state (recording state) of the recording medium. Here, the recording medium 1 is magnetized in one direction (for example, "I") perpendicular to the surface of the recording medium in the information recording part (focus part), and in the opposite direction in the non-recording part (erased part). There is.

The two light beams 17 and 18 reflected from the recording medium are returned to parallel light beams by the objective lens 2. A portion of the reflected light from the recording medium 1 is reflected by the beam splitter 3 and its optical path is bent at right angles to the quarter-wave plate 8 . The light exiting the quarter-wave plate 8 is converged by a lens 9 and polarized by a polarizing beam splitter 10. At this time, the light flux exiting the quarter-wave plate 8 does not necessarily have to be circularly polarized light, and may have any inclination. That is, the light beam separated by the polarizing beam splitter 10 is
The main axes of polarized light may be arranged in polarization directions perpendicular to each other and have characteristics of equal intensity.

The two light beams separated by the polarizing beam splitter 10 enter a photodetector 11 and a photodetector 12, respectively.

The photodetectors 11 and 12 each have a light receiving surface in the track direction (
It is divided into two parts, front and rear, with respect to the recording medium traveling direction).
Polarized beam subric 10eJ: receives the reflected light coming out from the recording medium 1 in the front and rear directions, and outputs a current proportional to the intensity of each incident light.

As mentioned above, two light beams 1 are focused on the recording medium.
As shown in FIG. 2, 7.18 is condensed and irradiated so that, for example, the light beams are half-shaped. Of course, since the photodetector IL 12 for reproducing the signal is separate, there is no effect due to the overlap. Furthermore, although the same light source is separated and focused at almost the same position, no interference occurs because there is no phase difference in the optical path difference.

In order to condense light under these conditions, the separation angle θ is important for the Wallas I. prism 4. When condensing light onto the recording medium surface with the objective lens 2, for example, the numerical aperture NA of the objective lens 2
0.55, focal length 3.9mm, beam diameter 1.4
When it is assumed to be μm, the separation angle θ is approximately 0.01°.

FIG. 3 is a diagram for explaining the principle of reproduction according to the present invention. A readout beam 17 is provided at the leading edge of the recording bit 30.
When one recording bit is irradiated, diffraction occurs due to the bit, and the amount of reflected light received differs between the front and rear of the recording bit. this is,
This is basically the same as the diffraction phenomenon caused by conventionally known phase-type diffraction gratings, and since the magnetization direction differs in phase by 180° between the recording focus and the non-recording area, the reflected light beam does not exhibit the diffraction phenomenon. I will receive it. In this sense, the discussion may be focused only on the amount of light received by the photodetector IL 12.

Similarly, when the reading beam 18 is irradiated to the leading edge of the recording focus, diffracted light is generated spatially shifted by half the width of the beam 17 and is reflected. In the photodetectors 11 and 12, the reflected light fluxes of the beams 17 and 18 are converged on the front and rear divided portions, respectively, and guided to the differential amplifier 1314.

The differential amplifier 13 receives the output currents from the divided front and rear parts of the photodetector 12, amplifies the difference, and outputs an intensity difference signal 101. In the above example, this intensity difference signal 101 has a positive peak at the leading edge of the recorded bit 30 and a negative peak at the trailing edge of the recorded bit.

On the other hand, the differential amplifier 14 receives output currents from the divided front and rear parts of the photodetector 11, amplifies the difference, and outputs an intensity difference signal 102. In the above example, the intensity difference signal 102 has a negative peak at the leading edge of the recording focus 30 and a positive peak at the trailing edge of the recording focus, having a polarity opposite to that of the intensity difference signal 101.

The differential amplifier 15 receives these two intensity difference signals 101°10
2 differential amplification is performed, the information signals are added, and the readout signal 1 is obtained.
Output as 03. Of course, here, the differential amplifier 13
If the connections are reversed, it becomes possible to configure an adder circuit using the differential amplifier 15. The read signal 103 outputted from each differential amplifier 15 is input to a signal processing circuit 16, and edge information of the recorded bit can be detected in the signal processing circuit 16 from the timing relationship with the reproduction clock, for example.

Since optical differential detection as described above is possible, the intensity difference signal has opposite polarity at the leading and trailing edges of the recording focus, so basically no DC component is included, and therefore accurate amplification down to low frequencies is possible. There is no need to do this. In addition, changes in the intensity of the reflected light in the front and back direction due to minute irregularities on the surface of the recording medium, changes in reflectance, changes in the intensity distribution of the laser beam, etc. other than those caused by the recording focus affect both the light beams incident on the photodetectors 11 and 12. Since it appears in the same way, it appears as in-phase noise for the intensity difference signals 101 and 102. Since the readout signals 103 perform differential amplification of the intensity difference signals 101 and 102, they cancel each other out and noise is removed. Therefore, only the information of the recorded bits is accurately reproduced in the read signal 103.
The influence of other factors (noise) is kept extremely small.

FIG. 4 shows a second embodiment of the invention. The difference from the first embodiment is that the analyzer is not a polarizing beam splitter (for example, a double image element 19 such as a Savart plate or a Wollaston prism is used, and the photodetector 20 is a double-image element 19, and two sets of two-split prisms are used as the photodetector 20. It has a multi-division detector configuration.

In the above embodiments, an example of a basic optical system has been shown, but the influence of retardation occurring in optical elements and magneto-optical recording media can also be compensated for by inserting a phase compensation plate having a phase compensation effect into the reproduction optical system. good. Furthermore, although the servo information detection optical system is omitted in the above embodiment, for example, an optical system that can implement the astigmatism method may be added after the quarter-wave plate 8, or the beam splitter 3 and Wollaston prism 4
A servo optical system may be constructed by additionally inserting another beam splitter between the two. Further, in the above embodiments, the information signal is detected using reflected light from the magneto-optical recording medium, but a structure may be adopted in which light transmitted through the magneto-optical recording medium is used. Further, similar effects can be obtained by using a configuration using a two-beam laser and a Wollaston prism.

〔Effect of the invention〕

As explained above, the optical disk device of the present invention converts the light beam from the light source into two light beams with a small aperture angle using the double image element, and condenses and irradiates these light beams onto the optical disk. Since the two beams are focused on the photodetector, the reproduction characteristics are significantly improved, and information can be reproduced with a high recording density, a good reproduction S/N ratio, and a small bit error rate.

In other words, with the above configuration, in-phase noise such as changes in the intensity of reflected light in the front and rear directions due to minute irregularities on the surface of the recording medium other than those caused by recording bits, changes in reflectance, and changes in the intensity distribution of laser light, etc. It has the effect of being canceled out. Furthermore, only the information of the recorded bits is accurately reproduced in the read signal, and the influence (noise) caused by other factors is suppressed to an extremely low level. Furthermore, since the intensity difference signal has opposite polarities at the leading and trailing edges of the recording bit, it basically does not contain a direct current component, and therefore has the effect that signal amplification for accurate amplification up to low frequencies is not required.

Therefore, since the edge information of the recording focus can be detected, there is an effect that the recording density can be set more than twice as high as that of the conventional method.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a first embodiment of the magneto-optical disk device of the present invention, FIG. The figure is a diagram for explaining the principle of photodetection according to the present invention, and Fig. 4 is a diagram showing a second embodiment of the magneto-optical disk device of the present invention. ...Objective lens 3 ...Beam splitter 4 ...Wollaston prism 5 ...Collimating lens 6 ...Semiconductor laser 7 ...Laser drive circuit 8 ... Quarter wavelength plate 9 ... Lens 10 ... Polarizing beam splitter 11, 12゜13, 14゜16 ... 17.18. 19. ... 21 ... 25・ ・ ・ 30 ・ ・ 20...Photodetector 15...Differential amplifier...Signal processing circuit...Focused beam...Double image element...Arithmetic circuit...Recording medium traveling direction...Focus

Claims (1)

[Claims] (1) A light source and a double image element that polarizes and splits the light flux from the light source into two directions with a small aperture angle on a magneto-optical disk on which a magneto-optical bit string that is information is recorded. an optical system that condenses and irradiates the light beams adjacent to each other so that they partially overlap along the recording track direction of a magneto-optical recording medium; and a part of reflected light or transmitted light from the same bit of each focused light spot. a beam splitter that reflects the
A photodetector consisting of a wavelength plate, an analyzer that separates the two light beams that have passed through the quarter-wave plate into two polarized components, and at least two light receiving sections that receive light from the analyzer. and circuit means for extracting a difference signal or a sum signal of the output signals from each light receiving section, and the difference signal or sum signal outputted by the circuit means is used as a reproduction signal of magneto-optical information. magneto-optical disk device.