JPH02183452A - Magneto-optical disk device - Google Patents

Abstract

PURPOSE:To stabilize a reproducing signal even if recording density is made high by receiving four luminous fluxes separated by a wedge prism and a multiple image element by means of a four-division detector and fetching the difference signal and the sum signal of the output signal of the detector by means of a differential amplifier. CONSTITUTION:A laser light beam for recording and reproducing is emitted from a semiconductor laser 1 and made incident on a magneto-optical medium 7 after arranging the polarized light of a luminous flux in one direction by beam splitters 4 and 5. Then, the luminous flux from the medium 7, which is rotated with kerr is divided in two directions perpendicular to a jitter direction by the wedge prism 10 through the splitter 5, a wavelength plate 8 such as a 1/4 wavelength plate and a condensing lens 9. Then, the luminous fluxes are polarized and separated to two directions by a Wollaston prism 11, etc. Next, the separated four luminous fluxes are received by the four-division photodetector 12 and the change of the intensity of light in the jitter direction of P and S polarized component, for example, is detected by the differential amplifiers 17 and 18 of a signal processing system 13 and the edge information of a recording bit obtained from the output of the amplifiers 17 and 18 is outputted by the differential amplifier 19.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a magneto-optical disk device that reproduces magneto-optical information using optical differentiation.

(Prior Art) 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, an external magnetic field is applied perpendicularly to the entire film surface of the recording layer in advance to magnetize the recording layer so as to cause upward magnetization to write +1011, and then a laser beam is irradiated spot-on to the part where 61'' is to be written. The coercive force He of the heated minute part becomes small, so when irradiating the laser beam, apply a weak external bias magnetic field in the direction of downward magnetization, and the magnetization will be reversed and "1" will be recorded. A method is used to form a magnetic recording pattern depending on whether or not to irradiate a laser beam, that is, whether to raise the temperature of a minute spot irradiated on the recording layer.On the other hand, as a method for reading information, For example, when the magnetic recording pattern is irradiated with a linearly polarized laser beam, the recording layer has the effect of rotating the plane of polarization of the reflected or transmitted light (called the magnetic Kerr effect and magnetic Faraday effect, respectively). , for example, when using the magnetic Kerr effect, the rotation angle θ of the polarization plane of the reflected light,
Taking advantage of the fact that the amount of light differs depending on the direction of recorded magnetization, the reflected light is passed through an analyzer before entering the photodetector, and information corresponding to the direction of magnetization is read out as a change 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 write-once optical disks.

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 medium surface is converted into a change in light intensity using an analyzer. In this signal reading method, the part of the medium surface where the magnetization has changed becomes bright or dark.
Similar to the reflectance change type optical disk device, a reproduced signal is read out by detecting a change in the amount of reflected light as a whole. Such detection methods include those that use a single photodetector, and those that use two photodetectors to receive a beam split into two polarized beams by a polarizing beam splitter, and then calculate the difference between the outputs. There are motion detection methods, etc.

These methods detect changes in the total amount of light transmitted 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 medium surface is broadened (Gaussian distribution).
Therefore, the intensity change of the reflected light will not be steep. Therefore, the timing information of the reproduced signal is inaccurate, and the intensity fluctuation of the irradiated light spot and the reflectance fluctuation of the medium,
This method has disadvantages in that it is easily affected by variations in the characteristics of the reproducing circuit, and reading 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 a recording pit), even direct current components or low frequency components close to them If it is not amplified accurately,
This had the disadvantage that the signal became more disturbed and accurate information could not be reproduced. Therefore, if the recording density is increased, stable reproduction is not possible, the pit error rate increases, and the recording capacity of the disc is limited.

In order to solve these shortcomings of the conventional reading methods of magneto-optical disk devices, Japanese Patent Application No. 151809/1983 proposes that changes in the magnetization of the medium can be detected using a quarter-wave plate using a quarter-wave plate, as shown in FIG. A method has been proposed in which the signal is read out by converting the signal into a phase lag (or phase lead) and capturing a change in the far field pattern caused by the phase difference. This method solves many of the problems mentioned above, but since signal detection is performed in the far field, it is less sensitive to tolerances such as the tilt of the optical head with respect to the disk surface than a condensing optical system, and the reproduction S/N ratio deteriorates. It has drawbacks such as:

The purpose of the present invention is to improve the above-mentioned drawbacks, and to improve the stability of the reproduced signal even when the recording density is increased, and the quality of the reproduced signal is also good. Therefore, it is possible to perform readout with a high reproduction S/N ratio, thereby reducing the error rate. The object of the present invention is to provide a magneto-optical disk device with a small amount of noise.

(Means for Solving the Problems) In order to achieve the object, the magneto-optical disk device of the present invention is a magneto-optical disk device that uses a laser as a light source and uses a magneto-optical pit train to record and reproduce information. a beam splitter that reflects part of the reflected light or transmitted light from the pit row; a wavelength plate disposed in the optical path of the light beam reflected from the beam splitter; and a wavelength plate that condenses the polarized light beam that has passed through the wavelength plate. a wedge prism that divides the amplitude into two directions corresponding to the jitter direction of the recording pit in the optical path condensed by the condenser lens; an analyzer consisting of a double image element that splits polarized light with a minute aperture angle or a parallel aperture in a direction perpendicular to the jitter direction;
a 4-split photodetector comprising four light-receiving sections that receive each of the four light beams separated by the wedge prism and the double-image element at a condensing position of the condensing lens; This is a magneto-optical disk device characterized by having circuit means for extracting a difference and a sum signal of output signals from each light receiving section. In particular, it is effective to use a quarter-wave plate, a half-wave plate, or a combination of a half-wave plate and a quarter-wave plate.

(Function) According to the present invention, a change in the magnetization (recording pit array) of the medium is converted into a phase delay (or advance) of light by a wavelength plate,
Since the intensity change of the far field pattern caused by the phase difference can be captured in a condensed form rather than being detected on the far field, detection with high signal efficiency is achieved.

In addition, noise caused by minute irregularities on the medium surface cancels out and becomes extremely small by taking the difference (or sum) of the outputs of the two photodetectors, so a readout signal with less noise can be obtained, resulting in a high It is possible to record and reproduce information with high density and high reliability.

(Example) The present invention will be described in detail below with reference to the drawings. FIG. 1 is a schematic diagram showing an embodiment of the present invention. The semiconductor laser 1 is supplied with current from a laser drive circuit (not shown) and emits laser light for recording and reproduction. The collimating lens 2 and the beam shaping prism 3 convert the diverging laser beam emitted from the semiconductor laser 1 into a circular, parallel laser beam. The light beam is focused and irradiated along the traveling direction (jitter direction) of the magneto-optical medium 7 by an objective lens 6 mounted on the actuator. At this time, the beam splitter 4.5 polarizes the light beam irradiating the medium in one direction (
It works to align the light to linearly polarized light. That is, if the polarization of the light beam emitted from the semiconductor laser 1 is P-polarized, the beam splitter has a characteristic of transmitting, for example, 60% of P-polarized light and reflecting all S-polarized light.

The objective lens 6 is mounted, for example, on a two-axis actuator, and focuses the light beam from the beam splitter 5 to irradiate a minute light spot onto the surface of the recording medium. The polarization direction of the light spot irradiated onto the recording medium is reversed (or transmitted) with a slight Kerr rotation depending on the magnetization state (recording state) of the medium. The light beam reflected from the medium is returned to a parallel light beam by the objective lens 6. The light reflected from this medium is reflected by the beam splitter 5 as a part of the P-polarized light component and all of the S-polarized light component, and its optical path is bent at right angles to the wavelength plate 8 . Here, a quarter-wave plate, for example, is used as the wavelength plate, and the light exiting the quarter-wave plate 8 is converged by a condenser lens 9 and split into two directions by a wedge prism 10. At this time, the light beam exiting the quarter-wave plate becomes approximately circularly polarized light having a phase difference according to the magnetization information of the recording pit array. The axis of division by this wedge prism 10 is perpendicular to the track direction (jitter direction) scanned by the light beam, and is located at the center of the light beam. Next, the light beams that are divided into two directions and directed to convergence are each polarized and separated into two directions with an aperture angle by a Wollaston prism 11, for example. Note that the light beams separated by the Wallaston prism 11 have a characteristic that the main axes of polarization are in polarization directions perpendicular to each other and have equal intensities, and are separated in a direction spatially perpendicular to the dividing direction by the wedge prism. will be placed in At this time, the number of light beams after polarization division becomes four. The four beams separated by the Wallaston prism 10 enter a four-part detector 12 that receives each beam.

FIG. 4 shows an example of a four-split photodetector according to the present invention. In each of the light receiving portions A, B, C, and D, the average light amount distribution divided by the wedge prism and the Wollaston prism is shown. Axis 1 of 4-split photodetector 12
．． 2 is installed in a direction (axis 2) perpendicular to the medium traveling direction (jitter direction) and the track direction, and is configured to output a current proportional to the intensity of each incident light.

FIG. 3 is a diagram for explaining the principle of reproduction according to the present invention. Here, the medium is magnetized in one direction perpendicular to the medium surface in the information recording part P (recording pit), and the non-recording part N is magnetized in the opposite direction. When the leading edge of the recording pit is irradiated with the readout beam 15, the spatial light intensity distribution after passing through the analyzer will be different between the irradiated part (the recording pit) P side and the non-recorded part N side. . In other words, the magnetization directions of the recording pit P and the non-recording pit N are different by 180 degrees, and the Kerr rotation is ±θ, so after the reflected light passes through the quarter-wave plate, the phase difference is caused by the Kerr rotation. The light becomes roughly circularly polarized. To explain it mathematically, the amplitude Φ of the reflected light at the non-recording area N is φ=exp[-j(ωt-a)]/v'r, , , ,
,,, It is expressed as (1). Here, ω is the angular frequency of light, and α is the relative orientation of the quarter-wave plate with respect to the ordinary light polarization direction.

On the other hand, the amplitude φ of the reflected light at recording pitch P is φ=exp
(ja) [(cos(a-θ)-sin(a-θ))e
xp(jωt)+(cos(a-θ)-5in(a-
θ)) exp(-jω to month 72, ,,, (2)
becomes. Here, θ is the Kerr rotation angle θ.

It can be seen that both equations (1) and (2) result in counterclockwise circularly polarized light. Therefore, the boundary (
When a reproduction beam is irradiated onto the pit edge, the polarization state of the reflected light is spatially divided into two with respect to the medium traveling direction (jitter direction). Of course, at this time, since the recording pit array is apparently considered similar to a phase diffraction grating, a diffraction phenomenon occurs, but it is minute and can be ignored.

When circularly polarized light with different phase differences passes through the Wollaston prism, the principal axes of the polarized light components differ by 90 degrees2.
The polarization is separated in the direction. At this time, the direction of polarization separation is set perpendicular to the jitter direction.

Among the amplitude components polarized in these two directions, the amplitude component for the S-polarized component is E = exp[j(
a-n/4) l cos(ωt-a)/v'r ,,,
, , , (3) It is expressed as S. On the other hand, in the recording pit P, E =exp[j(a-n/4)lcos(ωt-a+
(4) Similarly, for the P-polarized component, E = expi (a+n/4) in the non-recording area N.
lcos(ωt+a)/V'2' , , , , , ,
(5) It becomes p. On the other hand, in the recording pit P, E=exp[j(a+n/4)lcos(ωt+a−θ
)/v';' ,,,, (6) p.

From these facts, it can be seen that for S-polarized light, the reflected light component E from the recording pit P leads the phase of the reflected light component E from the non-recorded portion N by θ by θ. On the other hand, for P-polarized light, it can be seen that the reflected light component Epp from the recording area P is delayed in phase by θ from the reflected light component E from the non-recording area N.

In this way, a phase difference occurs between the reflected light at the recorded pit P and the non-recorded area N by the Kerr rotation angle 02, and at the pit edge, the change in light intensity in the front and back direction of the jitter direction is caused in the far field of the reflected light. will occur. Therefore, as mentioned above, recorded information can be detected by dividing the light in the jitter direction in the far field using a wedge prism and capturing changes in the light intensity for each of the S-polarized light component and the P-polarized light component. becomes.

FIG. 5 shows an example of the configuration of the signal processing system 13 according to the present invention. In the same figure (a), a differential amplifier 17 is installed so that the light receiving parts A and B of the 4-split photodetector detect a change in light intensity in the jitter direction of the P polarization component, and the light intensity in the jitter direction of the S polarization component is A differential amplifier 18 is provided to detect the change. Therefore, the edge information of the recording pit can be output using the differential amplifier 19 from the outputs A-B and CD of each differential amplifier. This results in (A+D)-(B+C)
Since it is equivalent to
This can also be realized with a configuration that outputs edge information of recording pits. Furthermore, an adder circuit 21 may be added to further detect the preformat signal.

On the other hand, it is easy to see that in order to obtain a signal similar to that of conventional differential detection, it is sufficient to perform the calculation of (A+B)-(C+D).

FIG. 6 shows a diagram for explaining the operation according to the present invention. The above-mentioned intensity difference signal (A-B) has a positive peak when the reproduction beam 15 scans the leading edge of the recording pit, and a negative peak at the trailing edge of the recording pit. On the other hand, the intensity difference signal (C-D
) has a phase difference of 180 degrees from the intensity difference signal (A-B), and has a negative peak at the leading edge of the recorded pit and a positive peak at the trailing edge of the recorded pit. The differential amplifier 19 differentially amplifies these two intensity difference signals, calculates an information signal, and outputs it as a read signal (A+D)-(B10).

As mentioned above, optical differential detection of recorded pits is possible, and since the intensity difference signal has opposite polarity at the front and trailing edges of recorded pits, it basically does not contain a DC component, and therefore detects up to low frequencies. There is no need to perform accurate amplification. In addition, changes in the intensity of the reflected light in the jitter direction due to minute irregularities on the medium surface other than the recording pits, changes in reflectance, changes in the intensity distribution of the laser beam, etc., cause changes in the light flux incident on the photodetectors A, B, C, and D. Since it appears similarly in both, the intensity difference signal (A-B)
and (CD) appear as in-mode noise. Since the readout signals (A+D)-(B+C) perform differential amplification of the intensity difference signal, they cancel each other out and noise is removed. Therefore, only the information of the recorded pits is accurately reproduced in the read signal, and the influence (noise) caused by other factors is suppressed to an extremely low level. In this way, by using the read signal (A + D) - (B + C) outputted by the differential amplifier 20, it is possible to accurately and easily detect the edge information of the recorded pit from the timing relationship with the reproduction clock, for example. be.

FIG. 2 shows a second embodiment of the invention. In the first embodiment, the light beam is divided into four parts by a combination of a wedge prism and a Wollaston prism. Therefore, for example, the number of parts is reduced by giving the wedge prism itself the effect of a double-image element such as a Wallaston prism.

In the above example of the present invention, the polarization division is performed after the amplitude division, but the same effect can be obtained even if the amplitude division is performed after the polarization division. In addition, in the example of the present invention, a basic optical system example is shown, but a phase compensating plate that has the effect of phase compensating for the effects of retardation occurring in optical elements and magneto-optical recording media is inserted in front of the quarter-wave plate. It may be configured to do so. As an example of the phase compensation plate, a 1/2 wavelength plate can be considered. In addition, in the present invention, push-pull differential detection is performed by converting the light into approximately circularly polarized light with a phase difference equivalent to the Kerr rotation angle using the quarter-wave plate, but with the half-wave plate, the polarized light is pushed while remaining approximately linearly polarized. Pull differential detection may also be performed. In this case as well, a configuration may be adopted in which, for example, a quarter-wave plate or the like is inserted in front of the half-wave plate as a phase compensation plate. Further, in the present invention, the information signal is detected using the reflected light from the magneto-optical recording medium, but it may be configured to use the transmitted light of the magneto-optical recording medium. Further, in the present invention, a Wollaston prism is used as an analyzer, but similar effects can be obtained by using other double-image elements such as a Savart plate or a Rochon prism.

(Effects of the Invention) As explained above, the magneto-optical disk device of the present invention has an advantage in that the strength of the reflected light in the front and back direction is caused by minute irregularities on the medium surface other than the recording pit array, reflectance fluctuations, laser beam intensity distribution fluctuations, etc. Common mode noises such as changes have the effect of canceling each other out. Furthermore, only the information of the recorded pits is accurately reproduced in the readout signal, and there is no influence (noise) caused by other factors.
can be kept extremely small. Furthermore, since the intensity difference signal has opposite polarity at the leading edge and trailing edge of the recording pit, basically no direct current component is included, so there is an effect that signal amplification for accurate amplification up to low frequencies is not required.

Therefore, since edge information of recording pits 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 optical disk device of the present invention, FIG. 2 is a diagram showing a second embodiment of the magneto-optical disk device of the present invention, and FIG. 3 is a diagram showing a second embodiment of the optical disk device of the present invention. FIG. 4 is a diagram for explaining the principle of magneto-optical detection, FIG. 4 is a diagram for explaining a configuration example of a photodetector according to the present invention, and FIG. 5 is a diagram for explaining a configuration example of a signal processing system according to the present invention. 6 are diagrams for explaining the operation according to the present invention, and FIG. 7 is a diagram for explaining an example of a conventional device. 1... Semiconductor laser, 2... Collimator lens, 3
...Gun-shaping prism, 4, 5... Beam splitter, 6... Objective lens, 7... Magneto-optical disk,
8... Quarter wavelength plate, 9... Condensing lens, 10...
...Wedge prism, 11...Double image element, 12...
・4-split photodetector, 13...signal processing system, 16...
Composite element.

Claims (1)

[Claims] A magneto-optical disk device that uses a laser as a light source to record and reproduce information using a magneto-optical pit string, the device comprising: a beam splitter that reflects part of the reflected light or transmitted light from the pit string; and the beam splitter. a wavelength plate disposed in the optical path of the light beam reflected from the wave plate; a condenser lens that condenses the polarized light beam that has passed through the wave plate; It consists of a wedge prism that splits the amplitude in two directions correspondingly, and a double image element that splits the polarization of each of the light beams split into two by the wedge prism at a minute aperture angle or a parallel aperture in a direction perpendicular to the jitter direction. a 4-split detector comprising four light receiving sections each receiving the four light beams separated by the wedge prism and the double image element at the focusing position of the focusing lens; 1. A magneto-optical disk device comprising circuit means for extracting a difference and a sum signal of output signals from each light receiving section of a four-part photodetector.