JPH03254450A - Reproduction optical device for magneto-optical recording medium - Google Patents

Abstract

PURPOSE:To obtain the S/N of practical use with a simple device by leading S- polarized light converted with a 1/4 wavelength plate and circularly polarized light of P-polarized light to a recording medium simultaneously and applying differential amplification to each regenerative signal obtained in response to the intensity change of a reflected light resulting from each circularly polarized light in the recording medium. CONSTITUTION:This device is a reproduction optical device reading information recorded on a magneto-optical recording medium 9 depending on the difference from the magnetization direction of a reflected light and provided with a 1st light source 1 with an S-polarized light L1S outgoing therefrom, a 2nd light source 2 with a P- polarized light L2P outgoing therefrom, and a 1/4 wavelength plate 6 converting the S-polarized light L1S and the P-polarized light L2P into a circularly polarized light. Then both circularly polarized beams converted by the 1/4 wavelength plate 6 are led simultaneously to the magneto-optical recording medium 9 and regenerative signals S1, S2 outputted from photodetecting means 11, 12 are differentially amplified corre sponding to the intensity change in the reflected light from each circularly polarized light in the recording medium 9. Thus, noise is cancelled by the differential amplifica tion and the S/N of practical use is attained with the simple device.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an optical device for reproducing magneto-optical recording media, and in particular, to differential detection of magneto-optical signals using the circular dichroism effect of a magnetic material. This invention relates to a reproducing optical device.

[Conventional technology]

Optical disks using rare earth transition metal alloy thin films as recording media are entering the stage of practical use as digital memories. To reproduce information recorded on an optical disc, it is usually necessary to
A method has been adopted in which a recording medium is irradiated with linearly polarized light emitted by a semiconductor laser, and the rotation of the plane of polarization of the reflected light is converted into light intensity using an analyzer.

To explain the principle of the reproduction method using the so-called Kerr effect described above, as shown in FIG.
I am guided by 1. In the analyzer 31, the detection light Dll and the detection light DI2, which are separated by the difference in the slope of the polarization plane corresponding to the difference in the perpendicular magnetization direction of the recording medium, are converted into electric signals by the photodetectors 32 and 33, respectively. Playback signal S.

and a reproduced signal StZ is obtained.

The specific direction of the perpendicular magnetization direction of the recording medium is (+), the opposite direction is (=), the reflected light vector in the recording bit magnetized in the (+) direction is α, and the recording bit magnetized in the (-) direction. Let β be the reflected light vector and γ be the incident light vector. As shown in FIG. 6, the plane of polarization of the reflected light vector α is rotated by the Kerr rotation angle θa with respect to the incident light vector γ. Further, the plane of polarization of the reflected light vector β is rotated by the Kerr rotation angle −θ with respect to the incident light vector γ.

The reflected light vectors α and β are detected by the analyzer 31 in two mutually orthogonal polarization directions X and Y. With respect to the polarization direction is output. As a result, the reproduction signal Sl corresponding to the recording in which the high level is magnetized in the (-) direction
+ is output from the photodetector 32.

On the other hand, for the polarization direction Y, the component α is detected by the photodetector 32.
When the component α of the reflected light vector α is output as a low level, the photodetector 33 outputs a signal corresponding to the component α as a high level. In addition, component β8 is detected by the photodetector 32.
When the signal corresponding to the component βY of the reflected light vector β is output as a high level, the photodetector 33 outputs a signal as a low level. As a result, a reproduction signal S corresponding to the recording is outputted from the photodetector 32 and the high level is magnetized in the (=) direction, and a reproduction signal S is outputted from the photodetector 33 and the high level is magnetized in the (+) direction. The reproduced signals S12 corresponding to the recorded data are signals whose phases are shifted by half a cycle and whose polarities are reversed.

The reproduced signal Sll and the reproduced signal S12 obtained in this way are signals obtained based on the rotation of the plane of polarization of the reflected light, so they are less affected by dust attached to the optical disc and are less likely to contain disc noise. Further, in order to improve the S/N ratio, these signals are input to a differential amplifier, and information is reproduced based on the output signal thereof.

However, as described above, the reproducing method using the Kerr effect, which is normally performed for magneto-optical recording, requires high precision in the setting of the analyzer 31, and also has the problem of causing an increase in the price of the reproducing device. are doing.

Therefore, as a method that can simplify the optical device for reproduction without using an analyzer and reduce the cost of the reproduction device, it is possible to irradiate the recording medium with circularly polarized light and to correspond to the difference in the perpendicular magnetization direction of the recording medium. A reproduction method that utilizes the so-called circular dichroism effect, which takes advantage of the anisotropy that occurs in the intensity and phase of reflected light, has also been theoretically considered.

As shown in FIG. 7, the refractive index of the medium on the incident light side of the recording medium 34 is no, and the refractive index of the recording bin 34a in the upward magnetization direction of the recording medium 34 is n. , the complex reflectance is r9, the refractive index of the recording bit 34b in the downward magnetization direction is n-1, and the complex reflectance is r-. When such a recording medium 34 is irradiated with clockwise circularly polarized light, for example, directly facing the direction of travel of the light, the reflected light at the recording bin 34a becomes counterclockwise circularly polarized light L12, and the reflected light at the recording bit 34b becomes a counterclockwise circularly polarized light L12. The reflected light becomes counterclockwise circularly polarized light LI3 with low intensity, (r.)2
−(rJ”...The reflected light intensity difference expressed by the formula (1) is obtained. (When the recording medium 34 is irradiated with counterclockwise circularly polarized light, the reflected light becomes clockwise, The relationship between the reflected light intensity at the recording bit 34a and the recording pitch 34b is opposite to the above). Note that r. and r- are no.
, n.

, n-, r −= (no n-)/(no +n, −)...
...(2) r-= (no n-)/(no +
n-)...It is expressed as (3).

[Problem B to be solved by the invention] However, in the reproduction method using the circular dichroism effect, since the magnetization direction of the recording medium is detected by the difference in the intensity of reflected light, foreign matter such as dust attached to the optical disk may be detected. This will affect the intensity of the reflected light, and the reproduced signal will contain noise.

As a result, compared to the conventional reproduction method using the Kerr effect,
This has the problem that signal quality tends to deteriorate.

The purpose of the present invention is to simplify the optical system by using the circular dichroism effect without using the Kerr effect for information reproduction from a magneto-optical recording medium, and to enable differential amplification of the reproduced signal as in the conventional method. An object of the present invention is to provide an optical device for reproducing a magneto-optical recording medium.

[Means to solve 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 light beam onto a magneto-optical recording medium, for example, from a rare earth transition metal alloy thin film, and optically detects the reflected light. In an optical device for reproducing a magneto-optical recording medium, which reads information recorded on the magneto-optical recording medium based on the difference in magnetization direction by detecting the difference in magnetization direction,
A first light source that emits polarized light, such as a semiconductor laser; a second light source, such as a semiconductor laser, that emits P-polarized light that has a different wavelength from the S-polarized light; and 17'4 that converts the S-polarized light and P-polarized light into circularly polarized light. The S-polarized circularly polarized light and the P-polarized circularly polarized light converted by the quarter-wave plate are simultaneously guided to the magneto-optical recording medium, and the intensity of the reflected light on the magneto-optical recording medium of each circularly polarized light is adjusted. It is characterized in that each reproduction signal output from the photodetection means in response to the change is differentially amplified.

[For production]

According to the above configuration, the S-polarized light emitted by the first light source can be used with a quarter-wave plate (if the optical path difference between linearly polarized lights vibrating in directions perpendicular to each other is within 174 wavelengths ±20%) ), the light is converted into, for example, right-handed circularly polarized light and irradiated onto a magneto-optical recording medium. At the same time, the P-polarized light emitted by the second light source is converted into, for example, left-handed circularly polarized light by a quarter-wave plate and irradiated onto 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 right-handed circularly polarized light irradiated onto a magnetic domain whose magnetization direction is vertically downward will have an intensity of It is reflected as highly attenuated left-handed circularly polarized light.

On the other hand, the irradiated left-handed circularly polarized light is reflected as right-handed circularly polarized light whose intensity is less attenuated.

Furthermore, left-handed circularly polarized light irradiated onto a magnetic domain whose magnetization direction is vertically upward is reflected as right-handed circularly polarized light with greatly attenuated intensity. On the other hand, the irradiated right-handed circularly polarized light is reflected as left-handed circularly polarized light whose intensity is less attenuated. As a result, the change in the intensity of the reflected light of the right-handed circularly polarized light irradiated onto the magneto-optical recording medium and the change in the intensity of the reflected light of the left-handed circularly polarized light irradiated with the recording medium are out of phase by half a cycle and have opposite polarities.

The light detection means detects each circularly polarized light beam reflected on the magneto-optical recording medium and outputs it as a reproduction signal. Therefore, the polarities of the reproduced signals are opposite to each other in accordance with the magnetization direction of the recording medium, so differential amplification is possible. Even if each reproduced signal based on the reflected light intensity contains noise due to the influence of dust, etc., the noise is canceled out by differential amplification, so that a practical level S/N ratio can be obtained.

[Embodiment 1] An embodiment of the present invention will be described below based on FIGS. 1 to 3.

As schematically shown in FIG. 1, the main parts of the reproduction optical device of the present invention include a first light source and a second light source that emit laser beams having different wavelengths and whose polarization directions are orthogonal to each other. Semiconductor laser 2, polarizing beam splitter 3
, a half mirror 4, a collimating lens 5 that creates a parallel beam of light, a quarter-wave plate 6 that changes linearly polarized light into circularly polarized light, an objective lens 7, and a wavelength filter 1 that transmits only light of a specific wavelength.
0, and photodetectors 11 and 12 as photodetecting means.

The polarizing beam splitter 3 has a function of totally reflecting S-polarized light whose electric field Hector is perpendicular to the plane of incidence, and totally transmitting P-polarized light whose electric field Hector is parallel to the plane of incidence.

The linearly polarized light LIS emitted by the semiconductor laser 1 becomes S-polarized light with respect to the polarization beam splitter 3, and the semiconductor laser 2
The linearly polarized light I-zp emitted by the polarizing beam splitter 3 becomes P-polarized light (therefore, the polarization directions of the linearly polarized light LIS and the linearly polarized light L2P are orthogonal to each other).

The 1/4 wavelength plate 6 has a function of delaying the phase of the component in the direction of the principal axis M of the electric field vector of the incident light by 1/4 wavelength. As shown in FIG.
The polarization direction N2 of the linearly polarized light LZP is
has an inclination of θ=45° to the right. Such linearly polarized light L Is changes into right-handed circularly polarized light when the phase of the component in the direction of the principal axis M is delayed by 1/4 wavelength by the 1/4 wavelength plate 6 . On the other hand, the linearly polarized light LZF as described above changes to left-handed circularly polarized light when the phase of the component in the direction of the principal axis M is delayed by 1/4 wavelength.

Note that the recording medium 9 of the optical disk 8 is provided perpendicularly 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 Is emitted from the semiconductor laser 1 is S-polarized light, it is totally reflected by the polarizing beam splitter 3 in the optical axis direction of the objective lens 7 . Then, the linearly polarized light LIS passes through the half mirror 4 and is converted into a parallel beam by the collimating lens 5.
As already explained, the quarter-wave plate 6 converts the light into right-handed circularly polarized light. The right-handed circularly polarized light is focused into a beam spot by an objective lens 7, and is irradiated onto a recording medium 9 of an optical disc 8. As explained in FIG. 6 of the conventional example, this right-handed circularly polarized light becomes left-handed circularly polarized light (as seen from the recording medium 9) in the recording medium 9.
It is reflected and becomes a parallel ray bundle by the objective lens 7,
The light returns to the quarter-wave plate 6 again as right-handed circularly polarized light (as viewed from the quarter-wave plate 6). In the 1/4 wavelength plate 6, when the transmitted light of the right circularly polarized light has a phase delay of 1/4 wavelength of the component in the direction of the principal axis M, P
The polarized light becomes linearly polarized light LIF. The linearly polarized light LIF is reflected by the half mirror 4 in the direction of the wavelength filter 10 via the collimating lens 5, selectively transmitted by the wavelength filter IO, and detected by the photodetector 11.

On the other hand, since the linearly polarized light LAF emitted from the semiconductor laser 2 (which has a different wavelength from the linearly polarized light L+s emitted from the semiconductor laser 1) is P-polarized light, it completely passes through the polarization beam splitter 3. And linearly polarized light L2
Similarly to the linearly polarized light LI5, F is converted into left-handed circularly polarized light by a quarter-wave plate 6 via a half mirror 4 and a collimating lens 5. The above-mentioned left-handed circularly polarized light irradiated onto the recording medium 9 through the objective lens 7 is transmitted to the recording medium 9 by right-handed circularly polarized light (
(as seen from the recording medium 9) and is reflected by the objective lens 7.
The light returns to the quarter-wave plate 6 as left-handed circularly polarized light (as viewed from the quarter-wave plate 6). In the quarter-wave plate 6, the transmitted left-handed circularly polarized light becomes S-polarized linearly polarized light LZS. The linearly polarized light L 2g is reflected by the half mirror 4 in the direction of the wavelength filter 1O via the collimating lens 5, totally reflected by the wavelength filter 10, and detected by the photodetector 12.

The change in the signal intensity of the reproduced signal S1 output by the photodetector 11 corresponds to the change in the light intensity of the linearly polarized light LIF that has been reflected on the two recording media 9. As shown in FIG. 3, in the vertically downwardly magnetized magnetic domain 9b of the recording medium 9, the reflected light intensity of the irradiated right-handed circularly polarized light is caused by the circular dichroism effect as explained in FIG. 6 of the conventional example. Attenuates, so the low level of the reproduced signal S1 corresponds to the magnetic domain 9b. Therefore, the reproduced signal S1
The high level corresponds to the magnetic domain 9a.

Further, the change in signal intensity of the reproduced signal S2 outputted by the photodetector 12 is caused by linearly polarized light Lzs after reflection on the recording medium 9.
corresponds to changes in light intensity. In the vertically downwardly magnetized magnetic domain 9b of the recording medium 9, the reflected light intensity of the irradiated left-handed circularly polarized light is attenuated in the magnetic domain 9a, contrary to the case of right-handed circularly polarized light, so the low level of the reproduced signal S2 is caused by the magnetic domain. Corresponds to 9a. Therefore, the high level of the reproduced signal S2 corresponds to the magnetic domain 9a.

As a result, the signal intensities of the reproduced signal S1 and the reproduced signal S2 corresponding to the magnetic domains 9a and 9b have opposite polarities. Therefore, if such reproduced signal S and reproduced signal S2 are input to a differential amplifier, a practical S
A reproduced signal having a /N ratio is obtained. Furthermore, even if the reflected light intensity is affected by foreign matter such as dust attached to the optical disk 80 substrate, and the signal intensity of the reproduced signal S1 becomes ΔS smaller than the original value, the semiconductor laser 1 and the semiconductor laser 2 Since the recording medium 9 is irradiated at the same time, the signal intensity of the reproduced signal S2 is also reduced by ΔS due to the influence of the same foreign matter. Therefore, if the reproduced signal S1 and the reproduced signal S2 are input to the differential amplifier, ΔS will be canceled out. As a result, disc noise other than the reproduction signal of recorded information is reduced.

[Embodiment 2] Another embodiment of the present invention will be described below based on FIG. For convenience of explanation, members having the same functions as those shown in the drawings of the first embodiment will be denoted by the same reference numerals, and the explanation thereof will be omitted.

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

As shown in FIG. 4, the reproducing optical device of this embodiment includes a half mirror 13 disposed between the semiconductor laser 1 and the polarizing beam splitter 3, and a half mirror 13 between the semiconductor laser 2 and the polarizing beam splitter 3. A half mirror 14 is provided. On the optical axis of the objective lens 7, a collimating lens 51/4 wavelength plate 6, an optical disk 8 and its recording medium 9 are provided, as in FIG. In addition, as described later,
After each reflected light beam on the recording medium 9 is separated by a polarizing beam splitter 3, one side is separated by a half mirror 13.
is guided to the photodetector 12 by a half mirror 14, and at the same time, the other one is guided to the photodetector 11 by a half mirror 14.

In the above configuration, since the linearly polarized light LIS emitted by the semiconductor laser 1 is S-polarized light, it is transmitted through the half mirror 13 and then totally reflected by the polarized beam splinter 3, and is then passed through the collimating lens 5, quarter-wave plate 6, and Objective lens 7
7, the light becomes right-handed circularly polarized light and is irradiated onto the recording medium 9 of the optical disc 8.

Then, as in the first embodiment, the reflected light becomes P-polarized linearly polarized light LIF at the 1/4 wavelength plate 6, so it is completely transmitted through the polarizing beam splinter 3, completely reflected at the half-biller 14, and then optically detected. Guided to vessel 11.

Similarly to the first embodiment, the P-polarized linearly polarized light LAF emitted from the semiconductor laser 2 and having a different wavelength from the linearly polarized light LIS is converted into the S-polarized linearly polarized light LZ by the action of the quarter-wave plate 6.
Since the light returns as S, it is totally reflected by the polarizing beam splitter 3, totally reflected by the half mirror 13, and then guided to the photodetector 12.

Hereinafter, descriptions of the reproduced signal S0 outputted by the photodetector 11, the reproduced signal S2 outputted by the photodetector 12, and the reproduced signal obtained by differential amplification are the same as in the first embodiment, and will therefore be omitted.

〔Effect of the invention〕

As described above, the optical device for reproducing a magneto-optical recording medium according to the present invention includes: a first light source that emits S-polarized light; a second light source that emits P-polarized light having a different wavelength from the S-polarized light; a quarter-wave plate for converting the S-polarized light and the P-polarized light into circularly polarized light; the S-polarized circularly polarized light and the P-polarized circularly polarized light converted by the quarter-wave plate are simultaneously guided to the magneto-optical recording medium; This configuration is such that each reproduction signal output from the photodetecting means is differentially amplified in response to a change in the intensity of each circularly polarized beam reflected on the magneto-optical recording medium.

Therefore, due to the circular dichroism effect, the polarities of the reflected light intensities of the circularly polarized lights simultaneously guided to the recording medium change so that they become opposite to each other in accordance with the magnetization direction of the recording medium. It becomes possible to differentially amplify each reproduced signal output from the optical detection means. As a result, the disc noise is canceled out by the differential amplification, making it possible to obtain a practical level S/N ratio with a simple optical device without using an expensive analyzer as in the past. play.

[Brief explanation of drawings]

1 to 3 show one embodiment of the present invention. FIG. 1 is a block diagram of the main parts of an optical device for reproducing a magneto-optical recording medium. FIG. 2 is an explanatory diagram regarding the action 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, showing the components of the main parts of an optical device for reproducing a magneto-optical recording medium. 5 to 7 show conventional examples. FIG. 5 shows the constituent countries of the main parts of an optical device for reproducing magneto-optical recording media. 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 regarding the circular dichroism effect in a magneto-optical recording medium. 1 is a semiconductor laser (first light source), 2 is a semiconductor laser (
6 is a quarter wavelength plate, 9 is a recording medium (magneto-optical recording medium), 11 and 12 are photodetectors (photodetection means) 1
．． L, s are linearly polarized light (S polarized light), L2F is linear polarized light (P
polarization), SI-32 is a reproduced signal.

Claims (1)

[Claims] A magneto-optical recording medium that reads information recorded on the magneto-optical recording medium based on the difference in magnetization direction by irradiating the magneto-optical recording medium with a light beam and detecting the reflected light using a photodetector. A reproduction optical device comprising: a first light source that emits S-polarized light; a second light source that emits P-polarized light having a different wavelength from the S-polarized light; and a first light source that converts the S-polarized light and the P-polarized light into circularly polarized light. /4 wavelength plate, the S-polarized circularly polarized light and the P-polarized circularly polarized light converted by the above-mentioned 1/4 wavelength plate are simultaneously guided to the magneto-optical recording medium, and the reflected light on the magneto-optical recording medium of each circularly polarized light is 1. An optical device for reproducing a magneto-optical recording medium, characterized in that each reproduction signal output from a photodetecting means is differentially amplified in response to a change in the intensity of a magneto-optical recording medium.