JP3057903B2 - Magneto-optical disk drive - Google Patents

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 drive and, more particularly, to an optical system suitable for detecting minute signals.

[0002]

2. Description of the Related Art At present, a signal S /
N is worse than other optical disk media. The main cause of the noise N is a component proportional to the DC light amount (laser noise, laser noise, etc.) generated because a DC light amount not related to a signal is used to detect a signal from a domain recorded on the disk surface. Disk noise, shot noise).

FIG. 1 shows an optical system of a conventional magneto-optical disk. Light emitted from the semiconductor laser 1 passes through the coupling lens 2 and the beam conversion optical element 3 and passes through the first beam splitter 4. The deflection direction of the laser light is set to, for example, p deflection, and the deflection characteristic of the first beam splitter is p deflection.
The deflection transmittance is set to 70% and the s deflection reflectance is set to 100%. The light beam that has passed through the first beam splitter 4 is converged on the surface of the magneto-optical disk 6 by the objective lens 5 to form a minute spot. The minute spot is positioned along a guide groove formed on the disk surface 6, and a domain corresponding to a recording signal is recorded on the disk surface. In the reproduction, the spot is positioned in the recorded domain based on the guide groove, the reflected light beam is condensed by the objective lens 5, and returns to the original optical path again. The s-deflection component of the reflected light beam is reflected by the first beam splitter 4 in the direction of 90 °, is separated from the original optical path, and 30% of the p-deflection component is similarly reflected to bend the optical path. This split light beam enters the second beam splitter 15 and is split into a magneto-optical signal detection optical system and a control signal detection optical system. The deflection characteristics of the second beam splitter 15 are s deflection 100% reflection and p deflection 60% transmission. After passing through the second beam splitter 15, the p-polarized component of the separated light beam passes through the convex lens 14, and is p-polarized, s
Beam splitter 1 with 50% transmittance / reflectance for both deflection
3, the light is separated into two convergent lights, and is made to enter the first photodetector 11 and the second photodetector 12, respectively.
The light beam having the s and p deflection components reflected by the beam splitter 15 is 4
The beam splitter 7 is set so as to have an analyzer function of 5 °, is split into two light beams corresponding to s-deflection and p-deflection, and signals are detected by separate photodetectors 9 and 10. Signals from these detectors are input to a differential amplifier, and the output is used as a detection signal for a magneto-optical disk.

Now, the deflection rotation angle due to the Kerr effect of the magneto-optical disk is θk, the set angle of the analyzer is θa, the electric charge is e, the incident light power to the analyzer is Po, the photoelectric conversion efficiency of the photodetector is η, Assuming that the current-voltage conversion resistance of the preamplifier is Rg and the modulation degree of the signal is m, the signal C is

[0005]

(Equation 1)

Here,

[0007]

(Equation 2)

[0008] Rp ₁ and Rp ₂ are reflectances of the first and second beam splitters for p-polarization, respectively.

The shot noise Nshot is

[0010]

(Equation 3)

The disk noise Ndisk is

[0012]

(Equation 4)

The laser noise Nlaser is

[0014]

(Equation 5)

Where c ″ is the extinction ratio, Δf is the signal band,
ηdisk and ηlaser are the relative noise ratios of the laser and the disk per signal band.

The above is the analysis of the signal from one photodetector of the differential detection system. In the current detection system, θa is + 45 ° and −45 ° for each deflection. In this case, when the difference between the signals from the respective photodetectors is obtained, the signal has twice the magnitude since the signals have different polarities. Further, in principle, disk noise and laser noise are canceled, but shot noise Nshot is not canceled because there is no correlation between both detectors due to randomness. Actually, disk noise and laser noise are ηdisk and ηla in s deflection and p deflection.
The ser is not completely canceled due to different or various other reasons and remains. In this detection system, S / N
By increasing Po, P other than the above-mentioned noise
The influence of components that do not depend on o, for example, the amplifier noise of the preamplifier is reduced, but converges to a certain value even if Po is increased. The reason is that the component in which both the S component and the N component are proportional to Po becomes dominant. This value is determined by the characteristics of the medium of the magneto-optical disk and the detection optical system, and the characteristics cannot be further improved. In addition, if Po is to be increased, the output power from the semiconductor laser must be increased, and the temperature on the recording film surface during reproduction rises, causing data destruction.
At present, the above values cannot be achieved and are used in situations where the S / N is poor.

[0017]

SUMMARY OF THE INVENTION A novel signal detection method for detecting a signal from a magneto-optical disk by reducing the influence of the characteristics of a medium and an optical system and detecting a signal with a good S / N ratio without destroying a recording domain, and a novel signal detection method. The idea is to propose a device for realization.

[0018]

SUMMARY OF THE INVENTION An optical system for guiding a light beam emitted from a light source to a magneto-optical disk, and a light quantity related to a recording domain signal from reflected light or transmitted light from the magneto-optical disk using deflection characteristics. And a means for generating a spatially and temporally coherent light beam with the light beam emitted from the light source, and an optical system for guiding the light beam and the separated signal light to the same photodetector.

[0019]

When a signal light and a light beam having a coherent relationship with a light beam emitted from a light source (now called local light) are square-detected on a photodetector surface, a signal is generated between the signal light and the local light. Product, the shot noise of the local light is added to the noise,
As the magnitude of the local light increases, the magnitude of the signal becomes relatively larger than the noise component. Therefore, the S / N of the entire detection system can be improved as compared with the related art by controlling the amount of local light.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to FIG. The optical system for condensing the light beam emitted from the laser 1 on the surface of the magneto-optical disk 6 by the objective lens 5 is the same as that of the conventional apparatus. In the present invention, a fourth beam splitter 20 for separating light emitted from a light source is inserted in the optical path to the objective lens 5. As the deflection characteristics of the beam splitter 20, if the deflection direction of the light beam emitted from the laser 1 is p, it is preferable that the deflection characteristics reflect about 20% of the p deflection component. This reflected light beam is passed through a half-wave plate 21 to form a triangular prism 22.
And reflected by the triangular prism 23 and the triangular prism 24. The adjustment of the optical path length is performed by moving the triangular prism 23. The light beam split by the beam splitter 20 is incident on the beam splitter 28 via the gray plate 25. The reflected light from the disk 6 is separated from the light beam incident on the objective lens 5 by the beam splitter 4 as in the related art, and guided to the beam splitter 29. The deflection characteristics of the beam splitter 29 are such that the s deflection component is totally reflected and the p component is almost transmitted. In this way, the p-polarization component that does not contribute to the signal can be used for the control signal detection system without entering the information signal detection system, and the S / N ratio of the control signal can be improved. The light reflected by the beam splitter 29 enters the beam splitter 28. The beam splitter 28 has a transmission and reflection characteristic of 50% for each of the s and p deflections in consideration of ease of production. In this way, half the light passes through the beam splitter 28 with respect to the s deflection. A half-wave plate 21 for transmitting the reflected light of the beam splitter 20 described above.
Is rotated so that the deflection after passing through the half-wave plate 21 is set to be the s component, and is incident on the beam splitter 28. Then, half of the s-polarized light is reflected, and the photodetector 26
The signal light is superimposed on the surface and squared detection is performed. The S / N with this configuration is shown below.

Due to the Kerr effect of the magneto-optical disk, the deflection of the reflected light from the disk surface is ±.
Only θk is rotated depending on the presence or absence of a domain. This reflected light is almost completely separated by the beam splitter 4 and the beam splitter 29 into only the s-polarized component. The signal is -θ
The difference between the reflected light at the time of k rotation and the reflected light at the time of + θk rotation is a signal. The respective s-deflection components have different phases as light, but have the same intensity. Signal cannot be detected. Therefore, the triangular prism 23 is moved so that the phase of the light becomes equal to the phase of the light from the reflected light during rotation of either -θk or + θk, and is superimposed on the signal light as local light. Now, the amplitude of the signal from the domain on the photodetector surface is Es, and the amplitude of the local light is E
l, the signal is

[0022]

(Equation 6)

Is proportional to

Further, a DC component is generated,

[0025]

(Equation 7)

Is proportional to

The noise is disk noise, shot noise, and laser noise generated from the DC component. Among them, the disk noise is proportional to Es ² , the laser noise is proportional to the DC component, and the shot noise is proportional to the route of the DC component.

Therefore, since S / N is approximately the ratio of the above signal and noise,

[0029]

(Equation 8)

[0030]

(Equation 9)

[0031]

(Equation 10)

[0032]

[Equation 11]

[0033]

(Equation 12)

If El is larger than Es,
For example, if the value is increased by one digit, the disk noise can be ignored. When El is increased, laser noise becomes more dominant than shot noise.
/ N is

[0035]

(Equation 13)

## EQU1 ##

Looking at the relationship between El and S / N, as shown in FIG.
There is a value at which the S / N is maximized as El is increased. For this purpose, the size of El is controlled by the density plate 25 and set to a value that maximizes S / N. This value is not affected by the characteristics of the disk and the characteristics of the detection optical system from the above-described examination results.

FIG. 3 shows another embodiment of the present invention. In the above-described embodiment, the light emitted from the laser is used as the local light. However, in this method, the phase of the signal light and the local light is changed due to a temporal change due to many optical components through which the signal light passes. The components may move and the phase relationship may change. Therefore, local light is generated from the reflected light of the disk, and the local light is adjusted in a locally fixed optical system to adjust the phase and the light amount, and is superimposed on the signal light on the photodetector surface. This embodiment is different from the above-described embodiment in that an amount proportional to the local light El ² is added to the disk noise. However, the behavior of C / N is similar to Equation 13, and can be improved as compared with the conventional case. The ninth beam splitter 31 has a deflection characteristic of p-deflection total reflection and s-deflection total transmission. Also, the eighth beam splitter 30
Has 50% transmission and reflection characteristics for p and s deflection. The operations of the half-wave plate 21 and the density plate 25 are the same as the operations described with reference to FIG.

[0039]

According to the present invention, high S / N signal detection of a signal of a magneto-optical disk can be performed without increasing the amount of reproduction light to destroy data, without being affected by characteristics of an optical system and characteristics of a medium. Can be.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a conventional magneto-optical disk signal detection system.

FIG. 2 is a configuration diagram of a magneto-optical disk signal detection optical system according to the present invention.

FIG. 3 is a configuration diagram of another magneto-optical disk signal detection optical system of the present invention.

FIG. 4 is a diagram showing a relationship between local light and C / N.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G11B 11/105 G11B 7/135

Claims (3)

(57) [Claims] 1. A light source, a first optical system for guiding a laser beam emitted from the light source to a recording medium, and a second optical system for guiding a reflected light beam or a transmitted light beam from the recording medium to a photodetector. System and the separated light beam separated from the laser light beam
A third optical system for leading to the photodetector, wherein the light of the separated light beam is
And a third optical system having a means for controlling the amount . 2. An optical system for guiding a light beam emitted from a light source to a magneto-optical disk, and an optical system for separating a light amount related to a recording domain signal from reflected light or transmitted light from the magneto-optical disk using polarization characteristics. Means and a spatially and temporally coherent light beam with the light beam emitted from the light source, while controlling the light amount
An optical system for guiding the coherent light beam and the separated signal light to the same photodetector. An optical system for guiding the optical disk a light beam emitted from wherein the light source, optical means separated using polarization characteristics amount related to the recorded domain signal from the reflected light from the magneto-optical disc, the upper Symbol Light that guides the separated signal light to a photodetector
And the optical signal is guided to a photodetector.
The optical system generates a local light that further separates the separated signal light.
Create and control the amount of local light and integrate with the original signal light
A magneto-optical disk device having means for guiding the light to the photodetector .