JP2551129B2 - Magneto-optical disk device - Google Patents

Description

The present invention relates to a magneto-optical disk device for reproducing magneto-optical information based on optical differentiation.

(Prior Art) A method of 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 focused and irradiated with laser light to record information as a change in magnetization on the medium. That is, an external magnetic field is applied vertically to the entire surface of the recording layer in advance to magnetize the recording layer so as to have an upward magnetization, write “0”, and then apply a laser beam to the portion where “1” is written. Irradiate and heat. The coercive force Hc becomes small in the heated minute portion, and when 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. Depending on whether or not to irradiate the laser beam, that is, whether or not to raise the temperature of the minute spot irradiated on the recording layer,
A method of forming a magnetic recording pattern is adopted. On the other hand, as a method of reading information, for example, when a linearly polarized laser beam is applied to the magnetic recording pattern, the effect of rotating the polarization plane of the reflected light or the transmitted light (called the Kerr effect and the magnetic Faraday effect, respectively) ) Is included in the recording layer, the reflected light is reflected by utilizing the fact that the rotation angle θ _k of the polarization plane of the reflected light differs depending on the recording magnetization direction when the magnetic Kerr effect is used. Before entering the photodetector, the light is passed through an analyzer and information corresponding to the direction of magnetization is read out as a light amount change. This type of magneto-optical disk device is attracting attention as a highly useful file device capable of rewriting information, unlike a write-once optical disk.

Conventionally, the reading of information in such a magneto-optical disk device has been performed by a method in which a polarization plane rotated by magnetization information recorded on the medium surface is converted into a light intensity change by an analyzer. In such a signal reading method, a portion where the magnetization is changed on the medium surface becomes bright or dark, and the reproduction signal is read by detecting the change in the amount of the entire reflected light as in the case of the reflectance changing type optical disk device. It will be. Such a detection method includes an operation using a single photodetector or an operation in which two photodetectors receive the light beams split into two polarization beams by a polarization beam splitter and the difference between their outputs is taken. There are detection methods. These methods are
It detects changes in the total amount of light flux that has passed through the analyzer, and is essentially the same as the method for detecting changes in brightness.

(Problem to be Solved by the Invention) However, in the above-described reading method in the conventional magneto-optical disk device, since the intensity distribution of the reading light spot focused on the medium surface has a broadness (Gaussian distribution), The intensity change of the reflected light does not become steep. Therefore, there are drawbacks in that the timing information of the reproduction signal is inaccurate, and it is easily affected by fluctuations in the intensity of the radiated light spot, fluctuations in the reflectance of the medium, fluctuations in the characteristics of the reproduction circuit, etc., and read errors during signal reproduction are likely to occur. Had. Further, when the information is recorded (pulse width modulation) by utilizing the change in the length of the area where the magnetization information is recorded (hereinafter referred to as a recording pit) and is reproduced, even a DC component or a low frequency component close to it is detected. If amplification is not performed accurately, the signal will be disturbed significantly, and accurate information reproduction will not be possible. Therefore, if the recording density is increased, stable reproduction cannot be performed, the bit error rate increases, and the recording capacity of the disk is limited.

In order to solve these drawbacks of the reading method of the conventional magneto-optical disk device, in Japanese Patent Application No. 62-151809, the change in the magnetization of the medium is changed by a quarter-wave plate as shown in FIG. A method has been proposed in which a signal is read out by converting into a phase delay (or a lead) and catching a change in a far field pattern caused by the phase difference. Although this method solves many of the problems described above, since signal detection is performed in the far field, it is weaker in tolerance such as inclination of the optical head with respect to the disk surface than in a condensing optical system, and the reproduction SN ratio is deteriorated. It has the drawback.

The object of the present invention is to improve the above-mentioned drawbacks and to improve the stability of the reproduced signal even if the recording density is increased, and the quality of the reproduced signal is good. The object is to provide a magneto-optical disk device with less power consumption.

(Means for Solving the Problem) In order to achieve the object, a magneto-optical disk device of the present invention is a magneto-optical disk device which uses a laser as a light source and records and reproduces information by using a magneto-optical pit row. A beam splitter that reflects a part of the reflected light or transmitted light from the pit row, a wave plate disposed in the optical path of the light beam reflected from the beam splitter, and a polarized light beam that has passed through the wave plate. A condensing lens, a wedge prism for amplitude-dividing the reflected light beam from the recording pit in the front-back direction with respect to the jitter direction in the optical path condensed by the condensing lens, and the wedge prism An analyzer composed of a compound image element that divides each of the light fluxes into two light fluxes having different polarizations with a small divergence angle or a parallel divergence in the direction perpendicular to the jitter direction is arranged. Each of the four-division photodetector and the four-division photodetector, which includes four light-receiving portions that receive the four light beams separated by the wedge prism and the compound image element at the condensing position of the condensing lens. Is a magneto-optical disk device having circuit means for extracting a difference between output signals from the light receiving section and a sum signal. 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 as the wave plate. A Wollaston prism, a Savart plate, a lotion prism, or the like can be used as the analyzer including the compound image element.

(Function) According to the present invention, a change in the magnetization (recording pit row) of the medium is converted into a phase delay (or advance) of light by the wave plate, and a change in the intensity of the far field pattern caused by the phase difference is changed on the far field. Signal-efficient detection is achieved because it can be captured in a focused form rather than at.

Further, noise due to minute irregularities on the medium surface is canceled out by the difference (or sum) of the outputs of the two photodetectors and becomes extremely small, so that a read signal with less noise can be obtained, and therefore high noise can be obtained. It is possible to record and reproduce information with high density and high reliability.

(Example) Below, this invention is demonstrated in detail with reference to drawings. FIG. 1 is a schematic view showing an embodiment of the present invention. The semiconductor laser 1 is supplied with a current from a laser drive circuit (not shown) and emits a laser beam for recording and reproduction.
The collimator lens 2 and the beam shaping prism 3 convert the divergent laser light emitted from the semiconductor laser 1 into a circular and parallel laser light flux. The light flux is focused and irradiated by the objective lens 6 mounted on the actuator along the traveling direction (jitter direction) of the magneto-optical medium 7. At this time, the beam splitters 4 and 5 have a function of aligning the polarization of the light flux irradiating the medium in one direction (linear polarization). That is, if the polarized light of the light beam emitted from the semiconductor laser 1 is P-polarized light, the characteristic of the beam splitter is such that 60% of P-polarized light is transmitted and all S-polarized light is reflected.

The objective lens 6 is mounted on, for example, a biaxial actuator, focuses the light beam from the beam splitter 5 and irradiates a minute light spot on the surface of the recording medium. The light spot applied to the recording medium is inverted (or transmitted) with its polarization direction slightly rotated by Kerr according to the magnetization state (recording state) of the medium. The light beam reflected from the medium is returned to the parallel light beam by the objective lens 6. The beam splitter 5 reflects a part of the P-polarized component and all the S-polarized component of the reflected light from the medium, bends the optical path at a right angle, and travels to the wave plate 8. Here, for example, a quarter-wave plate is used as the wave plate, and the quarter-wave plate 8 is used.
The light exiting is converged by the condenser lens 9 and wedge wedge 10
Is divided into two directions. At this time, the light flux emitted from the quarter-wave plate is substantially circularly polarized light having a phase difference according to the magnetization information of the recording pit row. The axis line divided by the wedge prism 10 is a direction perpendicular to the track direction (jitter direction) in which the light beam scans, and is installed at the center position of the light beam. Next, the light beams that are divided into these two directions and are directed toward the condensing light are polarized and separated into two directions with an opening angle by the Wollaston prism 11, for example. The light beam separated by the Wollaston prism 11 has a characteristic that the main axes of polarization are orthogonal to each other and the intensity is equal to each other, and the light beams are separated in a direction perpendicular to the direction of division in the wedge prism. Will be placed. At this time, the luminous flux after polarization division is 4
It will be a book. The four light beams separated by the Wollaston prism 10 receive the respective light beams.
Incident on.

FIG. 4 shows an example of a four-division photodetector according to the present invention.
The respective light receiving portions A, B, C and D show the average light amount distribution divided by the wedge prism and the warlast prism. Axis lines 1 and 2 of the four-division photodetector 12 are installed in a direction (axis line 2) perpendicular to the medium traveling direction (jitter direction) and the track direction, and output currents proportional to the respective incident light intensities. It is a composition.

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 portion P (recording pit), and in the non-recording portion N.
Is magnetized in the opposite direction. When the reading beam 15 is applied to the leading edge of the recording pit, the spatial light amount distribution after passing through the analyzer on the recording pit P type and the non-recording portion N side is different. That is, since the recording pit P and the non-recording N have different magnetization directions by 180 degrees and have Kerr rotations of ± θ _K , the reflected light passes through the quarter-wave plate and then the phase difference is Kerr rotation angle. The resulting circularly polarized light is different. Mathematically speaking, the amplitude Φ _n of the reflected light at the non-recording portion N is It is represented by. Where ω is the angular frequency of light and α is a quarter
It is a relative azimuth with respect to the ordinary polarization direction of the wave plate. on the other hand,
The amplitude Φ _p of the reflected light at the recording pit P is Φ _p = exp (jα)) [{cos (α-θ) -sin (α-
θ)} exp (jωt) + {cos (α−θ) −sin (α−θ)} exp (−jω
t)] / 2 (2). Where θ is the Kerr rotation angle θ _k .

It can be seen that both formulas (1) and (2) result in counterclockwise circularly polarized light. Therefore, when the reproducing beam is applied to the boundary (pit edge) between the recording pit P and the non-recording portion N, the polarization state of the reflected light is spatially divided into two with respect to the medium traveling direction (jitter direction). Becomes Of course, at this time, the recording pit row is apparently considered to be the same as that of the phase diffraction grating, so that a diffraction phenomenon occurs but it is negligible because it is minute.

When the circularly polarized lights with different phase differences pass through the Wollaston prism, the principal axes of the polarization components differ by 90 degrees.
It is polarized and separated in the direction. At this time, the polarization separation direction is set to be a direction perpendicular to the jitter direction.

Among the amplitude components polarized and separated in the two directions, the amplitude component for the S-polarized component is It is expressed as On the other hand, in the recording pit P Similarly, for the P polarized component, Becomes On the other hand, in the recording pit P Becomes

Due to these, the recording pit P for S-polarized light
The reflected light component E _ps from the non-recording portion N is the reflected light component E _ns
It can be seen that the phase advances by θ. On the other hand, for P-polarized light, the reflected light component E _pp from the recording pit P is
It can be seen that the phase is delayed by θ from the reflected light component E _np from.

In this way, the reflected light at the recording pit P and the non-recorded portion N has a phase difference by the Kerr rotation angle θ _k , and at the pit edge, the light intensity in the forward and backward directions of the jitter direction in the reflected light far field is increased. It will change. Therefore, as described above, the recorded information can be detected by dividing the light intensity into the jitter direction on the far field by the wedge prism and capturing the change in the light intensity for each of the S polarization component and the P polarization component. Becomes

FIG. 5 shows a configuration example of the signal processing system 13 according to the present invention. In FIG. 7A, a differential amplifier 17 is provided so that the light receiving portions A and B of the four-division photodetector detect, for example, a change in the light intensity of the P polarization component in the jitter direction, and the light intensity of the S polarization component in the jitter direction is provided. A differential amplifier 18 is provided to detect the change. Therefore, from the outputs A-B and C-D of each differential amplifier,
The edge information of the recording pit can be output using 19. Since this is equivalent to (A + D)-(B + C) as a result, the differential amplifier 20 performs the summation of A and D and B and C at the stage of the photodetector output as shown in FIG. With this, it is possible to realize the configuration in which the edge information of the recording pit is output.
Further, an adding circuit 21 may be added to detect the preformatted signal.

On the other hand, it is easily understood that the calculation of (A + B)-(C + D) is sufficient to obtain a signal similar to the conventional differential type detection.

FIG. 6 shows a diagram for explaining the operation according to the present invention. The intensity difference signal (A-B) described above has a positive peak when the reproducing 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)
Is 180 degrees out of phase with the intensity difference signal (A-B),
A negative peak occurs at the leading edge of the recording pit and a positive peak occurs at the trailing edge of the recording pit. The differential amplification unit 19 differentially amplifies these two intensity difference signals to calculate an information signal and outputs it as a read signal (A + D)-(B + C).

As described above, since the recording pits can be optically differentiated and detected, the intensity difference signals at the leading edge and the trailing edge of the recording pits have basically opposite polarities, and thus do not basically include a DC component.
Therefore, it is not necessary to perform accurate amplification up to low frequencies.
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 the reflectance, changes in the intensity distribution of the laser light, etc., of the light flux incident on the photodetectors A, B, C, and D. The same difference appears in both, so the intensity difference signal (AB)
And (C-D) appear as in-phase noise. Since the read 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 recording pit is accurately reproduced in the read signal, and the influence (noise) due to other factors can be suppressed to an extremely small level. As described above, the read signal (A + D)-(B + C) output from the differential amplifier 20 makes it possible to accurately and easily detect the edge information of the recording pit from the timing relationship with the reproduction clock, for example.

FIG. 2 shows a second embodiment of the present invention. In the first embodiment, the luminous flux is divided into four by the combination of the wedge prism and the Wollaston prism. Therefore, for example, the wedge prism itself has an effect of a compound image element such as a Wollaston prism, and the number of parts is reduced.

As described above, in the present invention, the polarization division is performed after the amplitude division. However, the same effect can be obtained even if the amplitude division is performed after the polarization division. Further, although the present invention shows a basic optical system, a structure in which a phase compensating plate having a phase compensating effect for retardation caused by an optical element or a magneto-optical recording medium is inserted in front of a quarter-wave plate. You may A half-wave plate or the like is considered as an example of the phase compensation plate. Further, in the present invention, the push-pull differential detection is performed by making the circular polarization having the phase difference of the Kerr rotation angle by the quarter wavelength plate and performing the push-pull differential detection. Pull differential detection may be performed. In this case as well, as the phase compensation plate, for example, a quarter-wave plate may be inserted in front of the half-wave plate. Further, in the present invention, the information signal is detected using the reflected light from the magneto-optical storage medium, but the transmitted light of the magneto-optical recording medium may be used. Further, although a Wollaston prism is used as the analyzer in the present invention, the same effect can be obtained by using other compound image elements such as a Savart plate or a lotion prism.

(Effects of the Invention) As described above, the magneto-optical disk device of the present invention is
Minute irregularities on the surface of the medium other than the recording pit row, reflectance fluctuation,
In-phase noises such as intensity changes in the front-back direction of reflected light due to variations in the intensity distribution of the laser light have the effect of canceling each other out. Further, only the information of the recording pit is accurately reproduced in the read signal, and the influence (noise) due to other factors can be suppressed to a very small level. In addition, since the intensity difference signals have opposite polarities at the leading edge and the trailing edge of the recording pit, basically no DC component is included, so that there is also an effect that signal amplification for accurately amplifying to a low frequency is unnecessary.

Therefore, since the edge information of the recording pit can be detected, there is an effect that the recording density can be set to twice as high as the conventional one.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of an optical disk device of the present invention, FIG. 2 is a diagram showing a second embodiment of a magneto-optical disk device of the present invention, and FIG. 3 is an optical device of the present invention. FIG. 4 is a diagram for explaining the principle of magnetic 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. , FIG. 6 is a diagram 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 ...
Beam shaping prism, 4, 5 ... Beam splitter, 6 ...
… Objective lens, 7… Magneto-optical disk, 8… Quarter
Wave plate, 9 ... Condensing lens, 10 ... Wedge prism,
11 …… Wollaston prism, 12 …… Quarantine photodetector, 13 …… Signal processing system, 16 …… Complex element.

Claims (2)

(57) [Claims] 1. A magneto-optical disk device for recording / reproducing information using a magneto-optical pit train using a laser as a light source, the beam splitter reflecting a part of reflected light or transmitted light from the pit train. A wavelength plate disposed in the optical path of the light beam reflected from the beam splitter, a condenser lens that collects the polarized light beam that has passed through the wavelength plate, and the recording in the optical path that is condensed by the condenser lens. A wedge prism that amplitude-divides the reflected light beam from the pits in the front-back direction with respect to the jitter direction, and each of the two light beams divided by the wedge prism in a direction perpendicular to the jitter direction has a minute opening angle or a parallel opening. And an analyzer composed of a compound image element that divides the light beam into two light beams having different polarizations, and each of the four light beams separated by the wedge prism and the compound image element. A four-division photodetector consisting of four light-receiving portions that receive light at the condensing position of the condenser lens, and a circuit means for extracting the difference and the sum signal of the output signals from the respective light-receiving portions of the four-division photodetector. A magneto-optical disk device having. 2. A magneto-optical disk device for recording / reproducing information using a magneto-optical pit train using a laser as a light source, the beam splitter reflecting a part of reflected light or transmitted light from the pit train. A wavelength plate disposed in the optical path of the light beam reflected from the beam splitter, a condenser lens that collects the polarized light beam that has passed through the wavelength plate, and the recording in the optical path that is condensed by the condenser lens. From a composite element that splits the reflected light beam from the pit into amplitudes in the front-back direction with respect to the jitter direction, and splits into two light beams with different polarizations with a small opening angle or a parallel opening in the direction perpendicular to the jitter direction. And a four-division light detector comprising four light-receiving portions for receiving each of the four light fluxes separated by the composite element at the condensing position of the condensing lens. Magneto-optical disk apparatus characterized by comprising a circuit means for taking out a difference and sum signal of output devices each output signal from the light receiving portion of the.