JP2574915B2 - Optical device for reproducing magneto-optical recording media - Google Patents

Description

The present invention relates to an optical device for reproducing a magneto-optical recording medium,
In particular, the present invention relates to a reproducing optical device for differentially detecting a magneto-optical signal by utilizing a circular dichroism effect of a magnetic material.

[Conventional technology]

An optical disk using a rare earth transition metal alloy thin film as a recording medium has entered a practical stage as a digital memory. In order to reproduce information recorded on an optical disk, a method of irradiating a recording medium with linearly polarized light emitted by a semiconductor laser and converting the rotation of the plane of polarization of the reflected light into light intensity by an analyzer is usually employed.

To explain the principle of the reproducing method utilizing the so-called Kerr effect described above, as shown in FIG. 5, the reflected light R ₁₁ in the recording medium of the linearly polarized light by the semiconductor laser is emitted, the analyzer 31
It is led to. In the analyzer 31, the detection light D ₁₁ separated by the difference of the inclination of the polarization plane corresponding to the difference of the vertical magnetization direction of the recording medium and detecting light D ₁₂ is 32 - each photodetector
33 is converted into an electrical signal by the reproduced signal S ₁₁ and the reproduction signal S ₁₂ is obtained.

The specific direction of the perpendicular magnetization direction of the recording medium is (+), the opposite direction is (-), the reflected light vector of the recording bit magnetized in the (+) direction is α, and the recording bit magnetized in the (−) direction. Is a reflected light vector and γ is an incident light vector. As shown in FIG. 6, the reflected light vector α
Is the Kerr rotation angle + θ _{K with} respect to the incident light vector γ.
Is only spinning. The polarization plane of the reflected light vector β is rotated by the Kerr rotation angle −θ _{K with} respect to the incident light vector γ.

The reflected light vectors α and β are detected in two orthogonal polarization directions X and Y of the analyzer 31. For the polarization direction X, a signal corresponding to the component beta _X of the reflected light vector beta is output at a high level by the light detector 32, a signal corresponding to the component alpha _X of the reflected light vector alpha is the optical detector 32 Output at low level. Thus, high level (-) reproduced signal S ₁₁ corresponding to the recording, which is magnetized in the direction is output from the photodetector 32.

On the other hand, for the polarization direction Y, the component α _X
When it is outputted as a low level from the signal corresponding to the component alpha _Y of the reflected light vector alpha is output at a high level by the light detector 33. Further, when the component beta _X is output from the photodetector 32 as a high level, a signal corresponding to the component beta _Y of the reflected light vector beta is output at a low level by the light detector 33. Thereby, the output from the photodetector 32, a high level (-) and the reproduced signal S ₁₁ corresponding to the recording, which is magnetized in the direction, is output from the photodetector 33, a high level (+) is magnetized in a direction Playback signal S corresponding to the recorded
₁₂ is a signal whose phase is shifted by half a cycle from each other and whose polarities are inverted from each other.

The thus obtained reproduced signal S ₁₁ and the reproduced signal S
Numeral ₁₂ is a signal obtained based on the rotation of the plane of polarization of the reflected light, so that it is less affected by dust or the like attached to the optical disk and hardly contains disk noise. Further, in order to improve the S / N ratio, these are input to a differential amplifier, and information is reproduced based on the output signal.

However, the reproducing method utilizing the Kerr effect which is usually performed for magneto-optical recording as described above requires high accuracy in setting the analyzer 31, and has the problems of increasing the price of the reproducing apparatus. doing.

Therefore, as a method of simplifying the reproducing optical device without using an analyzer and reducing the cost of the reproducing device, a method of irradiating the recording medium with circularly polarized light and responding to the difference in the perpendicular magnetization direction of the recording medium. A reproduction method utilizing the so-called circular dichroism effect, which utilizes the occurrence of anisotropy in the intensity and phase of the reflected light, is also theoretically considered.

As shown in FIG. 7, the refractive index of the medium on the incident light side of the recording medium 34 is n ₀ , and the recording bit in the upward magnetization direction of the recording medium 34 is
The refractive index of 34a is n ₊ , the complex reflectivity is r ₊ , the refractive index of the recording bit 34b in the downward magnetization direction is n ₋ , and the complex reflectivity is r ₋ .
Such recording medium 34, for example, when irradiated with circularly polarized light L ₁₁ clockwise as you face the traveling direction of the light, reflected light is circularly polarized light L ₁₂ next to the left direction in the recording bit 34a, the recording bit 3
Light reflection at 4b is a circularly polarized light L ₁₃ small counterclockwise ^{_{strength, (r +) 2 - (}} r -) 2 reflected light intensity difference represented by the formula ... (1) can be obtained (the recording When the medium 34 is irradiated with left-handed circularly polarized light, the reflected light becomes clockwise, and the relationship between the reflected light intensities at the recording bits 34a and 34b is opposite to the above. Note that r ₊ and r _- are
Using n ₀ , n ₊ , and n ₋ , r ₊ = (n ₀ −n ₊ ) / (n ₀ + n ₊ ) (2) r ₋ = (n ₀ −n ₋ ) / (n ₀ + n ₋₎ ) …… (3)

[Problems to be solved by the invention]

However, in the reproducing method using the circular dichroism effect, since the magnetization direction of the recording medium is detected based on the difference in reflected light intensity, foreign matter such as dust attached to the optical disk affects the reflected light intensity, and the reproduced signal is Will contain noise. As a result, there is a problem that the signal quality is more likely to be degraded than in the conventional reproducing method using the Kerr effect.

It is an object of the present invention to simplify the optical system by utilizing the circular dichroism effect without using the Kerr effect for reproducing information on a magneto-optical recording medium, and to enable differential amplification of a reproduction signal as in the related art. To provide a reproducing optical device for a magneto-optical recording medium.

[Means for solving the problem]

In order to solve the above-mentioned problems, an optical device for reproducing a magneto-optical recording medium according to the present invention irradiates a magneto-optical recording medium made of, for example, a rare-earth transition metal alloy thin film, and reflects the reflected light to a light detecting means. In the optical device for reproducing the magneto-optical recording medium, which reads the information recorded on the magneto-optical recording medium by the difference in the magnetic direction,
A first light source that emits s-polarized light and a second light source that emits p-polarized light
And converts the S-polarized light and P-polarized light into circularly polarized light.
A quarter-wave plate, and simultaneously guides the circularly-polarized S-polarized light and the circularly-polarized P-polarized light converted by the quarter-wave plate to the magneto-optical recording medium, and sets the magnetic direction of each circularly polarized light in the magneto-optical recording medium. Each reproduction signal output from the light detecting means is differentially amplified in response to a change in the intensity of the reflected light due to the difference between the two.

(Operation)

According to the above configuration, the S-polarized light emitted by the first light source is used as a quarter-wave plate (if the optical path difference between linearly polarized light vibrating in directions perpendicular to each other is within a quarter wavelength ± 20%). it can)
Then, the light is converted into, for example, right-handed circularly polarized light and irradiated on the magneto-optical recording medium. At the same time, the P-polarized light emitted by the second light source is
The light is converted into, for example, left-handed circularly polarized light by a quarter-wave plate, and is applied to the magneto-optical recording medium. The circularly polarized light of the S-polarized light and the P-polarized light is simultaneously guided to the magneto-optical recording medium.

For example, a magneto-optical recording medium made of a rare-earth transition metal alloy thin film has circular dichroism, so that right-handed circularly polarized light applied to a magnetic domain in which the magnetization direction of the magneto-optical recording medium is vertically downward has intensity. And is reflected as a largely attenuated left circularly polarized light. On the other hand, the irradiated left circularly polarized light is reflected as right circularly polarized light having a small attenuation in intensity.

Further, the left circularly polarized light applied to the magnetic domain whose magnetization direction is vertically upward is reflected as right circularly polarized light having a large attenuation. On the other hand, the irradiated right circularly polarized light is reflected as left circularly polarized light having a small attenuation in intensity. As a result,
The change in the reflected light intensity of the right circularly polarized light applied to the magneto-optical recording medium and the change in the reflected light intensity of the irradiated left circularly polarized light are shifted by a half cycle, and the polarities are reversed.

The light detecting means detects each circularly polarized light reflected on the magneto-optical recording medium and outputs the detected light as a reproduction signal. Therefore, the polarity of each reproduction signal is opposite to each other in accordance with the magnetization direction of the recording medium, so that differential amplification can be performed. Even if each reproduction signal based on the intensity of the reflected light includes noise due to the influence of dust or the like, the noise is offset by differential amplification, so that a practical level of S / N ratio can be obtained.

Embodiment 1 An embodiment of the present invention is described below with reference to FIGS. 1 to 3.

As schematically shown in FIG. 1, a main part of the reproducing optical device according to the present invention includes a first light source and a second light source which emit laser beams having different wavelengths and having polarization directions orthogonal to each other. Semiconductor lasers 1, 2, polarizing beam splitter 3, half mirror 4, collimating lens 5 for forming a parallel light beam, quarter-wave plate 6 for converting linearly polarized light to circularly polarized light, objective lens 7, transmitting only light of a specific wavelength. Wavelength filter
10 and photodetectors 11 and 12 as photodetection means.

The polarization beam splitter 3 has a function of totally reflecting s-polarized light whose electric field vector is perpendicular to the incident surface and totally transmitting p-polarized light whose electric field vector is parallel to the incident surface. The linearly polarized light L _1S emitted from the semiconductor laser 1 becomes S-polarized with respect to the polarization beam splitter 3, and the linearly polarized light L _2P emitted from the semiconductor laser 2 becomes
(Therefore, the polarization directions of the linearly polarized light _L1S and the linearly polarized light _L2P are orthogonal to each other).

The quarter-wave plate 6 has a function of delaying the phase of the component of the electric field vector of the incident light in the direction of the main axis M by a quarter wavelength. A method of arranging the spindle M, as shown in FIG. 2, the polarization direction N _S of the linearly polarized light L _1S the incident has theta = 45 ° inclination with respect to the main axis M, The polarization of the linearly polarized light L _2P direction N _P is adapted to have a theta = 45 ° inclination. Such linearly polarized light L
_1S is a quarter-wave plate 6 in which the phase of the component in the main axis M direction is 1/4.
If the wavelength is delayed, it changes to right circularly polarized light. On the other hand, when the phase of the component in the direction of the principal axis M is delayed by 1/4 wavelength, the linearly polarized light _L2P as described above changes to left circularly polarized light.

The recording medium 9 of the optical disk 8 is provided perpendicular to the optical axis of the objective lens 7 and is made of a rare earth transition metal alloy thin film. Information is recorded by the difference in the perpendicular magnetization direction of the recording medium 9.

In the above configuration, since the linearly polarized light L _1S emitted from the semiconductor laser 1 is S-polarized, it is totally reflected by the polarization beam splitter 3 in the optical axis direction of the objective lens 7. Then, the linearly polarized light L _1S passes through the half mirror 4 and is converted into a parallel light beam by the collimating lens 5, and then, as described above, is changed to right circularly polarized light by the 既 に wavelength plate 6. The right circularly polarized light is condensed on a beam spot by the objective lens 7 and irradiates the recording medium 9 of the optical disk 8. The right circularly polarized light is reflected by the recording medium 9 as left circularly polarized light (as viewed from the recording medium 9) as described with reference to FIG. /
The light returns to the four-wave plate 6 as right circularly polarized light (as viewed from the quarter-wave plate 6). In the quarter-wave plate 6, when the phase of the component of the component in the direction of the main axis M is delayed by a quarter wavelength, the transmitted light of the right circularly polarized light is linearly polarized L _1P of the P-polarized light.
Becomes The linearly polarized light L _1P is reflected by the half mirror 4 via the collimator lens 5 toward the wavelength filter 10, selectively transmitted by the wavelength filter 10, and detected by the photodetector 11.

On the other hand, since the linearly polarized light L _2P emitted from the semiconductor laser 2 (having a different wavelength from the linearly polarized light L _1S emitted from the semiconductor laser 1) is P-polarized, it is totally transmitted through the polarization beam splitter 3. I do. And linearly polarized light L
Like the linearly polarized light L _1S , _2P is changed to left circularly polarized light by the 波長 wavelength plate 6 via the half mirror 4 and the collimating lens 5. The left circularly polarized light applied to the recording medium 9 via the objective lens 7 is reflected as right circularly polarized light (as viewed from the recording medium 9) on the recording medium 9, passes through the objective lens 7, and returns to 1 / The light returns to the four-wavelength plate 6 as left-handed circularly polarized light (as viewed from the quarter-wavelength plate 6). In the quarter-wave plate 6, the transmitted light of the left circularly polarized light becomes the linearly polarized light _L2S of the S polarized light. The linearly polarized light L _2S is reflected by the half mirror 4 toward the wavelength filter 10 via the collimating lens 5 and totally reflected by the wavelength filter 10,
The light is detected by the photodetector 12.

Signal strength change in the reproduced signals S ₁ to the optical detector 11 is outputted,
This corresponds to a change in the light intensity of the linearly polarized light _L1P that has been reflected by the recording medium 9. As shown in FIG. 3, in the magnetic domain 9b magnetized vertically downward of the recording medium 9, the reflected light intensity of the right circularly polarized light radiated by the circular dichroism effect as described with reference to FIG. is because attenuated, low level of the reproduced signals S ₁ corresponds to the domain 9b. Therefore, the high level of the reproduced signals S ₁ corresponds to the domain 9a.

The signal intensity variation of the reproduced signal S ₂ the photodetector 12 is output corresponds to the light intensity variation of the linearly polarized light L _2S passed through the reflection at the recording medium 9. In the magnetic domain 9b magnetized vertically downward of the recording medium 9, the reflected light intensity of the irradiated left circularly polarized light is attenuated in the magnetic domain 9a, contrary to the right circularly polarized light.
Low level of the reproduced signal S ₂ corresponds to the magnetic domain 9a. Therefore,
High level of the reproduced signal S ₂ corresponds to the magnetic domain 9a.

Thus, the signal strength of the reproduction signal S ₁ and the reproduced signal S ₂ corresponding to the magnetic domain 9a and the magnetic domain 9b are polarity is opposite to each other. Therefore, such a reproduced signal S ₁ and the reproduced signal
By entering the S ₂ to the differential amplifier, the reproduction signal having a practical S / N ratio can be obtained. Further, the reflected light intensity by foreign matter such as dust adhering on the substrate of the optical disc 8 is influenced, for example, signal intensity of the reproduced signal S ₁ is from the original value Δ
S even smaller, since the semiconductor laser 1 and the semiconductor laser 2 is performed simultaneously irradiated onto the recording medium 9, the influence of the same foreign object, the signal strength of the reproduction signal S ₂ also ΔS smaller. Therefore, the reproduction signal S ₁ and the reproduced signal S ₂ is once input to the differential amplifier, [Delta] S is canceled. As a result, disc noise other than the reproduced signal of the recorded information is reduced.

Embodiment 2 Another embodiment of the present invention will be described with reference to FIG.
It is as follows. In addition, for convenience of explanation, the first embodiment is used.
The members having the same functions as the members shown in the drawings are denoted by the same reference numerals, and description thereof will be omitted.

In the first embodiment, two reflected lights on the recording medium 9 are:
The case where the light is separated by the wavelength filter 10 using the difference in wavelength is shown. In this embodiment, a case is shown in which the reflected light is split by the polarization beam splitter 3 using the difference in the polarization direction.

As shown in FIG. 4, the reproducing optical device of the present embodiment
A half mirror 13 is provided between the semiconductor laser 1 and the polarization beam splitter 3, and a half mirror 14 is provided between the semiconductor laser 2 and the polarization beam splitter 3.
On the optical axis of the objective lens 7, a collimator lens 5, a quarter-wave plate 6, an optical disk 8 and its recording medium 9 are provided as in FIG. Further, as described later, each reflected light on the recording medium 9 is transmitted to the polarization beam splitter 3.
After that, one is guided to the photodetector 12 by the half mirror 13 and the other is guided to the photodetector 11 by the half mirror 14 at the same time.

In the above configuration, since the linearly polarized light L _1S emitted from the semiconductor laser 1 is S-polarized light, it passes through the half mirror 13 and is totally reflected by the polarization beam splitter 3, and the collimating lens 5, the / 4 wavelength plate 6, Then, through the objective lens 7, the light is radiated to the recording medium 8 of the optical disk 8 as right circularly polarized light. The reflected light is 1 /
The light is converted into P-polarized linearly polarized light L _{1P by the} four-wavelength plate 6, so that it is completely transmitted through the polarization beam splitter 3, totally reflected by the half mirror 14, and then guided to the photodetector 11.

The linearly polarized light _{L2P of} P-polarized light emitted from the semiconductor laser 2 and having a different wavelength from the linearly-polarized light _L1S is also 1
The light is returned as S-polarized linearly polarized light L ₂ S by the action of the / 4 wavelength plate 6, and is totally reflected by the polarization beam splitter 3, totally reflected by the half mirror 13, and then guided to the photodetector 12.

Hereinafter, the reproduction signal S ₁ output from the photodetector 11, the photodetector 12
Are the same as those in the first embodiment, and a description of the reproduced signal S ₂ output by the first embodiment and the reproduced signal obtained by differential amplification will be omitted.

〔The invention's effect〕

As described above, the reproducing optical device for a magneto-optical recording medium according to the present invention includes the first light source that emits S-polarized light, the second light source that emits P-polarized light, and the S-polarized light and the P-polarized light. A quarter-wave plate for converting the light into polarized light, and simultaneously guiding the circularly-polarized light of S-polarized light and the circularly-polarized light of P-polarized light converted by the quarter-wave plate to the magneto-optical recording medium, In this configuration, each reproduction signal output from the light detection means is differentially amplified in response to a change in the intensity of reflected light due to a difference in the magnetization direction in the medium.

Therefore, each circularly polarized light simultaneously guided to the recording medium changes due to the circular dichroism effect so that the polarities of the reflected light intensities are opposite to each other in accordance with the magnetization direction of the recording medium. It becomes possible to differentially amplify each reproduction signal output from the detection means. As a result, the disk noise is canceled out by the differential amplification, and the S / N ratio at a practical level can be obtained with a simple optical device without using an expensive analyzer as in the past. Play.

[Brief description of the drawings]

1 to 3 show an embodiment of the present invention. FIG. 1 is a structural view of a main part of an optical device for reproducing a magneto-optical recording medium. FIG. 2 is an explanatory diagram relating to the operation of the quarter-wave plate. FIG. 3 is a waveform diagram of a reproduction signal corresponding to the magnetization direction of the recording medium. FIG. 4 shows another embodiment of the present invention and is a configuration diagram of a main part of an optical device for reproducing a magneto-optical recording medium. 5 to 7 show a conventional example. FIG. 5 is a configuration diagram of a main part of an optical device for reproducing a magneto-optical recording medium. FIG. 6 is an explanatory diagram showing the relationship between the Kerr effect and the polarity of the reproduced signal. FIG. 7 is an explanatory diagram relating to the circular dichroism effect in the magneto-optical recording medium. 1 is a semiconductor laser (first light source), 2 is a semiconductor laser (second light source), 6 is a quarter-wave plate, 9 is a recording medium (magneto-optical recording medium), and 11 and 12 are photo detectors (light detection Means), L _1S is linearly polarized light (S polarized light), L _2P is linearly polarized light (P polarized light), S ₁ · S
₂ is a reproduction signal.

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Kenji Ota 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (56) References JP-A-1-59656 (JP, A) Shokai Sho 61- 118122 (JP, U)

Claims (1)

(57) [Claims] 1. A magneto-optical recording medium for reading information recorded on the magneto-optical recording medium by irradiating the magneto-optical recording medium with a light beam and detecting reflected light thereof by a light detecting means. In the reproducing optical device, a first light source that emits S-polarized light and a second light source that emits P-polarized light
And converts the S-polarized light and P-polarized light into circularly polarized light.
A quarter-wave plate, and simultaneously guides the circularly-polarized S-polarized light and the circularly-polarized P-polarized light converted by the quarter-wave plate to the magneto-optical recording medium, and the magnetization direction of each circularly polarized light in the magneto-optical recording medium. An optical device for reproducing a magneto-optical recording medium, which differentially amplifies each reproduction signal output from the light detecting means in response to a change in the intensity of reflected light due to the difference between the two.