JPH0589553A - Magneto-optical disk reproducing device - Google Patents

Abstract

PURPOSE:To obtain a differential signal without being buried by the noise and a magneto-optical signal having high C/N without generating the heat noise even in an area wherein the output is low. CONSTITUTION:When a magneto-optical disk 7 is irradiated with a laser beam flux from a laser beam source 3 at the prescribed period by a pulse generator 1 and an obtained returning light is received by two photodetectors 12 and 13, the regenerative signal is obtained after taking a differential by making other optical flux than the returning light incident on the photodetectors 12 and 13. By such a constitution, the average incident quantity of light to the magneto-optical disk 7 is very little and the photoelectric conversion is made by the photodetectors 12 and 13 in the range wherein the shot noise of PIN diode is dominant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical disk reproducing apparatus for reproducing a signal recorded on a magneto-optical recording medium.

[0002]

2. Description of the Related Art In a signal reproducing method for a magneto-optical recording medium, for example, a magneto-optical disk, information recorded as a magnetization direction on the magneto-optical disk is subjected to Kerr effect or Farade effect which is an interaction between light and magnetization. It is a method of reproducing by changing the minute rotation of the polarization plane. However, p that is responsible for photoelectric conversion
Since the in-photodiode or the like has no selectivity for polarization, a method of converting the rotation of the polarization plane into a change in the intensity of light and reproducing the light is used by using a polarization beam splitter or an analyzer.

A conventional signal reproducing device for a magneto-optical disk will be described with reference to FIG. In FIG. 4, the laser light of the laser light source 31 is the laser light of the P-polarized component (P wave). The light flux emitted from the laser light source 31 is made into a parallel light flux by the collimator lens 32, passes through the first polarization beam splitter (PBS) 33, and the objective lens 34.
Is incident on the signal recording layer 7a of the magneto-optical disk 7. The reflected light flux reflected by the signal recording layer 7a of the magneto-optical disk 7 is passed through the objective lens 34 and then passed through the first PBS 3
3 reaches the reflecting surface 33a. The reflection surface 33a of the first PBS 33 changes the direction of the reflected light flux by 90 degrees. This 9
The reflected light beam whose direction has been changed by 0 degrees is rotated by the half-wave plate 36 by 45 degrees, is converged by the condenser lens 37, and is incident on the second polarization beam splitter (PBS) 38. .. The reflected light flux incident on the second PBS 38 is separated by the reflection surface 38a of the second PBS 38 into polarization components having polarization planes orthogonal to each other. Second above
The first polarized light component (P polarized light component) that passes through the reflection surface 38a of the PBS 38 is radiated to the photodetector 39 configured by, for example, a pin photodiode. On the other hand, the second P
The second polarization component (S-polarization component) reflected by the reflecting surface 38a of the BS 38 is applied to the photodetector 40 which is, for example, a pin photodiode. At this time, the output signal O ₁ 'of the _first photodetector 39 is the differential amplifier 41
Is supplied to one input side of the second photodetector 4 and
The output signal O ₂ 'of 0 is supplied to the other input side of the differential amplifier 42.

Here, the signal recording layer 7 of the magneto-optical disk 7
The polarization plane of the reflected light flux reflected from a is rotated according to the direction of magnetization of the perpendicular magnetization film by the Kerr effect. FIG. 6 shows the polarization component of the reflected light from the magneto-optical disk 7. In this figure, P of the laser light incident on the magneto-optical disk is
The polarization direction is the vertical axis (P wave axis). In this case, the reflected light reflected by the magneto-optical disk 7 is represented by I ₀ ⁺ and I _O ⁻ , respectively, depending on the direction of magnetization of the signal recording layer 7a of the magneto-optical disk 7. The angle between the P-wave axis and the reflected light I ₀ ⁺ is θ _K , and the angle between the P-wave axis and the reflected light I _O ⁻ is −θ _K, and the S-polarized component (S-wave component) and the P-polarized light of the reflected light I _O are defined. _{Let the} components (P wave components) be I _S and I _P , respectively. This I
_S is the magneto-optical signal and I _P is the in-phase component. Further, the θ _K is the rotation angle (Kerr rotation angle) of the polarization plane due to the Kerr effect described above.

For example, when the direction of magnetization is one direction, the plane of polarization rotates by θ _K degrees, and when the direction of magnetization is the other direction, it rotates by -θ _K degrees. for that reason,
When the magnetization direction of the perpendicular magnetization film is one direction,
While the polarization component of the first polarization plane that is transmitted through the second PBS 38 and is irradiated on the photodetector 39 is small or large,
The polarization component of the second polarization plane reflected by the second PBS 38 and irradiated on the photodetector 40 becomes large or small. On the other hand, when the magnetization direction of the perpendicular magnetization film is the other direction, the above-mentioned reverse state is obtained. Therefore, the differential amplifier 41
The signal output from the output terminal 42 is the differential amplifier 4
The in-phase component I _P is removed by 2 and the magneto-optical disk 3
5, a magneto-optical signal (MO) recorded in the signal recording layer 35a
Only I _S.

A means for evaluating the performance of the magneto-optical reproducing apparatus shown in FIG. 4 is C / N (carrier to noise ratio). This C / N characteristic will be described. The cause of the noise here is, for example, noise of the laser diode when the laser light source is a laser diode, minute irregularities on the disk surface when the laser light is reflected by the magneto-optical disk, and fluctuations in the characteristics of the recording medium. There are generated medium noise, thermal noise of the pin photodiode, and shot noise of the pin photodiode. The total of these noises becomes the noise (N) in C / N. The noise of the laser diode is the S / N of the laser diode.
(Signal to noise ratio) is 70 to 80 dB or more, which is a level that does not pose a problem for the above C / N. There is always a certain level of thermal noise in pin photodiodes, and it is difficult to remove it at room temperature. The medium noise is in-phase component noise with respect to the above-mentioned optical differential method, and can be removed to some extent. These relationships are shown in FIG.

In FIG. 5, the horizontal axis represents the input light quantity of the pin photodiode, and the vertical axis represents the output power of the pin photodiode. The thermal noise N ₁ shows a substantially constant value regardless of the amount of incident light. Shot noise N ₂ is noise generated when light is converted into electricity and is proportional to the square root of the incident light quantity. The medium noise N ₃ and the laser noise N ₄ increase as the amount of incident light increases, but the level of the laser noise N ₄ is smaller.
The carrier signal C also increases as the incident light amount increases, but decreases when the input light amount exceeds a certain level. This is because the amount of light applied to the magneto-optical disk 35 becomes too large, the temperature of the part irradiated with the laser light rises, and the Kerr rotation angle θ _K decreases. Therefore, the values of C / N are CNR ₁ , CNR ₂ and CN shown in the figure.
It increases with an increase in R ₃ and the amount of incident light, and becomes a constant value CNR _{4 in} the range of the amount of incident light in which shot noise is dominant.

The above C / N is approximately proportional to the product of the square root of the incident light amount I on the signal recording layer 7a of the magneto-optical disk 7 and the Kerr rotation angle θ _K. Therefore, in order to increase the C / N, it is possible to increase the power of the laser light source 31 and increase the incident light amount I. However, if the incident light amount I is simply increased, the temperature of the perpendicularly magnetized film of the signal recording layer 35a of the magneto-optical disk 35 locally rises, the Kerr rotation angle θ _K is decreased, and the increase of C / N is suppressed. It This is similar to the suppression of the increase of the carrier signal C in FIG. 5 described above. This tendency becomes more remarkable as the linear velocity of the magneto-optical disk during reproduction decreases.

The magneto-optical disk 7 can be considered there.
The magneto-optical disk reproducing apparatus aims to improve the C / N by increasing the incident light amount I without increasing the temperature of the perpendicularly magnetized film of the signal recording layer 7a. This has been proposed by the applicant in Japanese Patent Application No. 62-3398. That is, even if the incident light amount I is increased by not irradiating the magneto-optical disk 7 continuously with the laser beam, but by irradiating it intermittently at the sampling cycle, the temperature rise of the perpendicular magnetization film is not caused. It is the one. But,
As described above, since the shot noise N ₂ is proportional to the square root of the incident light amount I, the C / N when the incident light amount I is increased even when the magneto-optical disk 7 is intermittently irradiated with the laser light flux. Rises only in proportion to the square root of the incident light quantity I. This shot noise is random noise, which cannot be removed by the differential amplifier as it is. On the other hand, the applicant of the present invention has disclosed that
In -298734, a magneto-optical disk reproducing apparatus was proposed in which an integrating means was provided after the photodetector to reduce the influence of random noise such as shot noise so as to improve C / N.

The applicant does not use the reflected light flux of the magneto-optical disk as the laser light flux added to increase the incident light quantity of the photodetector to the region where the shot noise of the pin photodiode is dominant. A signal detecting device for a magneto-optical recording medium characterized by using a laser beam which is not affected by medium noise is also disclosed in Japanese Patent Application Laid-Open No. 2-19200.
Proposed in 8044.

[0011]

By the way, as described above, the magneto-optical disk is intermittently irradiated with the laser light flux, and the intermittent reflected light flux from the magneto-optical disk is received by the photodetector, and its photodetection is performed. Even in a magneto-optical reproducing apparatus which is intended to reduce the influence of random noise such as shot noise by an integrating means provided in the latter stage of the device, thermal noise is conspicuous as shown in FIG. 2D in a low output region. The resulting differential signal may be buried in noise.

Therefore, the present invention has been made in view of the above circumstances, and an object thereof is to provide a magneto-optical disk reproducing apparatus having a high C / N.

[0013]

According to the present invention, a laser light source is pulsed at a predetermined period to irradiate a magneto-optical disk with a laser beam, and at the same time, return light from the magneto-optical disk is received by two photodetectors. Then, in the magneto-optical disk reproducing apparatus which obtains the reproduction signal by taking the differential from the photodetector, after the light flux other than the return light from the magnetooptical disk is incident on the photodetector, , Is characterized by taking a differential.

[0014]

According to the present invention, the laser light source is intermittently powered on at a predetermined cycle to intermittently irradiate the optical flux from the laser light source to the magneto-optical disk, and the average incident light amount on the magneto-optical disk is made extremely high. By making it small, the temperature of the perpendicular magnetization film will not rise even if the amount of incident light increases, and
By adding a light flux other than the return light from the magneto-optical disk to the photodetector, photoelectric conversion is performed by the photodetector in a region where the shot noise of the pin photodiode is dominant. Improve.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a magneto-optical disk reproducing apparatus according to the present invention will be described below with reference to FIGS. In FIG. 1, the laser light source 3 is a single mode laser diode, and the emitted light flux emitted is a P wave. The laser light source 11 is a multi-mode laser diode, and the emitted light flux emitted is, for example, P-wave laser light having a wavelength of 780 nm.

First, a pulse signal generated from the pulse generator 1 in a sampling cycle, for example, passes through an amplifier 2
For example, it is supplied to the laser light source 3 such as a laser diode. The laser light source 3 emits an intermittent emitted light beam corresponding to the pulse signal with an output having a peak value of about 6 mW. The emitted light flux is made into a parallel light flux by the collimator lens 4, and passes through the beam splitter 5 having polarization selectivity,
It reaches the objective lens 6 and is focused by the objective lens 6 on the signal recording layer 7 a of the magneto-optical disk 7 and irradiated.
Here, for example, the transmittance T _P for the P wave of the beam splitter 5 is 98%, the reflectance R _P is 2%, the transmittance T _S for the S wave is 0%, and the reflectance R _S is 100%.
That is, the amount of light applied to the magneto-optical disk 7 is 98% of the total amount of light of the laser light source 3. The reflected light beam reflected by the magneto-optical disk 7 is incident on the beam splitter 5 again and is reflected by the reflecting surface 5a. The plane of polarization of this reflected light flux is rotated by 45 ° by the half-wave plate 8 and is incident on the polarization beam splitter 10 via the condenser lens 9. The polarization beam splitter 10 separates the polarization components having mutually orthogonal polarization planes, and the laser light from the laser light source 11 is added to each of the separated polarization components. It can be said that the laser light from the laser light source 11 is a getter in-phase component that does not include so-called medium noise. Interface 10 of the polarization beam splitter 10
The laser light obtained by adding the first polarization component of the intermittently reflected light flux from the magneto-optical disk 7 that has passed through a and the getter in-phase component is incident on the photodetector 12. Further, the laser light in which the second polarization component of the intermittently reflected light flux from the magneto-optical disk 7 reflected by the interface 10a of the polarization beam splitter 10 and further reflected by the reflection surface 10b and the getter in-phase component are added, It is incident on the photodetector 13.

The operation of the first embodiment will be described with reference to FIG. In the first embodiment of FIG. 1, the magneto-optical disk 7 is intermittently irradiated with laser light at sampling intervals. In this case, the generation time of the laser light is set to 1/5 to 1/20 of the sampling cycle. At this time, the output signals O ₁ and O ₂ of the photodetectors 12 and 13 are, for example, as shown in FIG.
As shown in FIG. 2B. These output signals O ₁ and O ₂ contain getter in-phase components such that shot noise becomes a dominant region. Therefore, the differential amplifier 1
The 4 output terminal 15 derived from the magneto-optical signal O _O that Getah-phase component as shown in Figure C has been removed is output. This output signal MO is one in which the thermal noise noticeable at a low level of the output signal O _O 'obtained by the conventional pulse read method shown in FIG. 3D is removed, and is not buried in the thermal noise.

That is, while the average incident light quantity is reduced by intermittently irradiating the magneto-optical disk 7 with laser light, the incident light quantity I is increased and the temperature of the perpendicularly magnetized film of the magneto-optical disk 7 is not increased. While doing so, the photodetector is irradiated with the laser light in which the in-phase component of the gedda is added so that the shot noise of the pin photodiode becomes a dominant region. As a result, the thermal noise shown in FIG.
In order to improve / N, as shown in FIG. 6, C / of the photodetector
N can be increased to the theoretical limit value CNR ₅ .

Next, a second embodiment of the magneto-optical disk reproducing apparatus according to the present invention will be described with reference to FIG. In FIG. 3, a pulse signal generated from the pulse generator 21 in a sampling cycle, for example, is supplied to a laser light source 23 such as a laser diode via an amplifier 22. The laser light source 23 emits an intermittent emitted light beam according to the pulse signal. Here, this emitted light flux is
It is a P-polarized component. The emitted light flux is made into a parallel light flux by the collimator lens 24, and is radiated to the signal recording layer 7a of the magneto-optical disk 7 via the beam splitter 25 having the mirror surface 25a and having polarization selectivity and the objective lens 26.
Here, for example, the transmittance T _P for the P wave of the beam splitter 25 is 98%, the reflectance R _P is 2%, the transmittance T _S for the S wave is 0%, and the reflectance R _S is 100%.
In this case, the amount of light applied to the magneto-optical disk 7 is 98% of the total amount of light of the laser light source 31. On the other hand, 2% (getter in-phase component) of the laser light is reflected by the interface 25b of the beam splitter 25 and again by the mirror surface 25a. Next, the laser light reflected by the magneto-optical disc is incident on the beam splitter 25 again, and 100% of the magneto-optical signal I _S is input at the interface 25b of the beam splitter 25.
Is 2% of the P wave component I _P of the reflected light of the magneto-optical disk 7.
(In-phase component) is reflected and the mirror surface 25
It is added to the getter in-phase component reflected at a. The plane of polarization of the added light flux is rotated by 45 degrees by the half-wave plate 27, is incident on the polarization beam splitter (PBS) 29 via the condenser lens 28, and the planes of polarization orthogonal to each other are made by the PBS 29. It is separated into its polarized components. This PB
The polarization component having the first polarization plane that has passed through S29 is incident on the first photodetector 30a configured by, for example, a pin photodiode. On the other hand, the polarization component having the second plane of polarization reflected by the PBS 29 is incident on the second photodetector 30b configured by, for example, a pin photodiode. The output signals of the photodetectors 30a and 30b are input to the input terminals of a differential amplifier (not shown), in-phase components are removed, and only the MO signal is detected.

That is, the amount of light incident on the photodetector is defined as pin
In order to increase the area where the shot noise of the photodiode is dominant, the reflected light flux of the magneto-optical disk 7 is not used as the added light flux, but a part of the light flux emitted from the laser light source 23 is used.

The operation of the second embodiment can also be described with reference to FIG. 2, as in the first embodiment described above.

As described above, in the second embodiment of the magneto-optical disk reproducing apparatus according to the present invention, the average incident light quantity is made small by irradiating the magneto-optical disk 7 with laser light intermittently, but the incident light quantity I To prevent the temperature rise of the perpendicular magnetization film of the magneto-optical disk 7,
The photodetector is irradiated with laser light to which the in-phase component of the gedda is added so that the shot noise of the n-photodiode becomes the dominant region. As a result, the thermal noise shown in FIG. 2D can be removed, the C / N can be improved, and the C / N of the photodetector can be increased to the theoretical limit value CNR _{5 as} shown in FIG.

Needless to say, the magneto-optical disk reproducing apparatus according to the present invention is not limited to the above-described embodiments, and for example, a pin photo of a photodetector without using a differential optical system. Since the medium noise contained in the laser beam incident on the diode is small, a self-coupling type capable of detecting a magneto-optical signal with a single photodetector may be used.

[0024]

In the magneto-optical disk reproducing apparatus according to the present invention, the laser light source is pulsed at a predetermined cycle to intermittently generate a luminous flux irradiating the magneto-optical disk, and the photodetector is subjected to medium noise. By adding light fluxes that do not include the photodetector, photoelectric conversion is performed by the photodetector in a region where the shot noise of the pin photodiode is dominant, and a differential that can be obtained without generating thermal noise even in a low output region It is possible to obtain a magneto-optical signal having a high C / N without the signal being buried in noise.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a first embodiment of a magneto-optical signal reproducing device according to the present invention.

FIG. 2 is a first and second magneto-optical signal reproducing apparatus according to the present invention.
FIG. 6 is a diagram for explaining the operation of the embodiment of FIG.

FIG. 3 is a configuration diagram of a second embodiment of a magneto-optical signal reproducing device according to the present invention.

FIG. 4 is a block diagram of a conventional magneto-optical signal reproducing device.

FIG. 5 is a diagram showing C / N characteristics.

FIG. 6 is a diagram showing a polarization component of reflected light from a magneto-optical disk.

[Explanation of symbols]

 1 pulse generator 3 laser source 4 collimator lens 5 polarization beam splitter 6 objective lens 7 magneto-optical Disk 8 ... 1/2 wave plate 9 ... Condensing lens 11 ... LED 12, 13 ... Photodetector 14 ... Differential amplifier

Claims (1)

[Claims] 1. A laser light source generates pulses in a predetermined cycle to irradiate a magneto-optical disk with a laser beam, and the return light from the magneto-optical disk is received by two photodetectors.
In a magneto-optical disk reproducing apparatus for obtaining a reproduction signal by taking a differential from the photodetector, a light beam other than the return light from the magnetooptical disk is made incident on the photodetector, and A magneto-optical disk reproducing device designed to move.