JPS6163946A - Optical information detector - Google Patents

Abstract

PURPOSE:To secure the stable detection of signals even to the temperature change with high sensitivity and a compact and light-weight structure of an optical information detector and to improve the reliability, by using a ferromagnetic garnet crystal having the composition of components within a limited range to a modulator for plane of polarization. CONSTITUTION:A semiconductor laser is used to a light source 1, and the parallel plates of TiO2 are used to polarizers 2, 6 and 7 respectively. The modulator 3 for plate of polarization uses a crystal (Tb0.19Y0.81)3Fe5O12. The light transmitted through a mirror 4 is modulated by a magneto-optical recording medium 5. This modulated light is reflected by the mirror 4 and made incident on a detector 8. While the light reflected by the mirror 4 is made incident on a detector 9. Each signal is delivered after undergoing an operation through a differential amplifier 10. Here a garnet crystal having a specific composition of components is used to the modulator 3. This secures the stable detection of signals regardless of the temperature change. In addition, the fluctuation of light quantity can be suppressed by using fibers for reservation of plate of polarization to optical paths 11-a-11-c. This improves the reliability of an optical information detector.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is directed to a device for optically detecting information on a magneto-optical recording medium. This invention relates to an optical information detection device using a modulated differential method.

Conventional Structure and Problems Recently, in a method for optically detecting information on a magneto-optical recording medium, a polarization plane modulation method has been used as a means for improving the SNN rate. Figure 1 shows a diagram of the principle of the polarization plane modulation method. Figure 1 (al indicates that the light passes through the magneto-optical recording medium (transmission type), and Figure 1 (bl indicates that the light is reflected (reflection type). Since they are basically the same, the light passes through the magneto-optical recording medium. A conventional example will be explained below.

The light emitted from the light source 1 has a width of photons 2 after passing through the optical path 11-a.
The light is converted into linearly polarized light and enters the polarization plane modulator 3.

The incident light is modulated by rotation of the plane of polarization by 80 degrees. The modulated light is divided into two optical paths by the mirror 4. By passing through the magneto-optical recording medium 5, the straight light beam becomes φ
[degrees] ff1-r! Receives wave face E.

The light passing through the magneto-optical recording medium 5 is 1lii! After transmitting the photon 6, it passes through the optical path 11-C and enters the detector 8.

The signal photoelectrically converted by the detector 8 is sent to a differential amplifier 51.
0f is guided. On the other hand, the reflected light from the mirror 4 is reflected by the polarizer 7.
After passing through, the light enters the detector 9 via the optical path 11-b. The signal photoelectrically converted by the detector 8 is also guided to the differential amplifier 10.

The purpose of differential amplification is as follows. The output of the detector 8 is expressed by the following equation.

I8(φ, t)=PiK,sxn (φ+&o sin
wo t ) (1) Here, t is time, Pl is the output of the light source 1, K1 is the loss of the optical system, and w(, is the modulation frequency of the polarization plane modulator 3.

When φ and aO are small, equation (1) can be approximated as follows.

Is(φ, t) = PiK, ao shi2 + 2P
IK1 Loφsinw,) t-(PiKI ao2
//2) C082Wo t (21 Also, the output of the detector 9 is expressed by the following formula.

I9(φ, t)=PiK2sln (aOslnwot
)'::PiKzao/2-(PIK2ao/2)CO
52Wot (31 now, polarizer 2 and polarizer 6, and
When the polarizer 2 and the polarizer are placed in an extinction arrangement, ao becomes considerably smaller, and the noise of the light source and the shot noise of the photodetector become as follows.
This is negligible with respect to the total output. Differential amplification is performed to eliminate terms that do not include the signal φ in the magneto-optical recording medium 6 and that oscillate at two intervals. Therefore, the output of the differential amplifier can be calculated as follows: (φ, i) - +'Is (φt) K2'I9(
φt) = 2PiK, 'KI a,) φS engineering nwo
t (41 is obtained.

Therefore, according to this method, it is possible to eliminate the dominant noise due to PiK (width 2/2), shot noise, and light source noise, and also to eliminate the component term of 2Wo that does not include a signal component.

Although this method has the above-mentioned features, the sensitivity is determined by the modulation reaction of the polarization plane modulator 3, as can be seen from equation (4). Furthermore, when this method is used in an actual detection device, &0 needs to be stable against changes in ambient temperature from the viewpoint of reliability. Furthermore, if the device itself becomes large, there will be a limit to its driving ability when reading information from a recording medium continuously. Currently, lead glass is a typical polarization plane modulating element with good temperature characteristics. However, this lead glass has a Werde constant of 0. Q8X
For example, in order to obtain the normal aO = 1°, the length of the lead glass must be 1 (for In, the AC magnetic field is 12500
Even if the length is increased to, for example, 10 ctn, an AC magnetic field of 12 s Oe is required. Therefore, the coil for generating the AC magnetic field will be larger, or the lead glass will be longer and the device will be larger. Y1 workers who do this cannot be avoided.

On the other hand, set the device to 1. ! If the coil is made smaller and the lead glass is made shorter in order to increase the width, the width becomes smaller and therefore the sensitivity becomes smaller.

Purpose of the Invention The present invention has been made in view of the above-mentioned drawbacks, and it is an object of the present invention to provide an optical information detection device that is highly sensitive, has good temperature characteristics, is small and lightweight, and has a large driving force. be.

Structure of the Invention The present invention uses a ferromagnetic garnet crystal as a polarization plane modulator that vibrates the azimuth angle of a polarization plane, and is used as a differential amplification means.

Specifically, a first polarizer that converts incident light from a light source into linearly polarized light, a first optical path that couples between the light source and the first analyzer, a polarization plane modulator that vibrates the linearly polarized light, and a polarization plane modulator that vibrates the linearly polarized light. A second mirror having a mirror that separates the light emitted from the polarization plane modulator into light that passes through two optical paths, and allows the light that passes through the second optical path to pass through the magneto-optical information medium after being transmitted or reflected. The analyzer and the first detection means for detecting the light after passing through the second analyzer, 1) transmit the light that is reflected or transmitted through the mirror described in II and transmitted through the optical path @3. Third
and a second detecting means for detecting the light after passing through the polarizer and the third polarizer, and the output from the first detecting means and the second detecting means are connected to each other.
In this apparatus, a ferromagnetic garnet crystal is used for the polarization plane modulator, and preferably, for example, a ferromagnetic garnet crystal is -X)3F8sO+
2, and the value of X satisfies 0.1≦x≦0.3. Alternatively, a ferromagnetic garnet crystal grown on a crystal substrate by epitaxial/yellow growth is used. moreover,
Preferably, polarization maintaining optical fibers are used as the first, second and third optical paths.

DESCRIPTION OF EMBODIMENTS The present invention is applied to a polarization plane modulator using a ferromagnetic garnet crystal (Tb
xY+-x)3FesO+2(X-0,19), and an embodiment using polarization-maintaining optical fibers in the first, second, and third optical paths will be described below. FIG. 2 shows a second embodiment of the present invention. In the second embodiment, 1 is a light source, and from the viewpoint of miniaturization, a semiconductor laser is used. Polarizers 2.6 and 7 are T
iQz parallel plate is used, 1.5 thickness, 2sJ
It is about l. This is a spatial polarization splitting plate that utilizes the birefringence of TlO2.

The polarization plane modulator 3 has the above (Tbo, +9Yo, a+)
3FesOtz crystal is used, thickness 2.5mm, 2
ml+lφ. Set aside this length, aQ=j
In order to obtain .degree., the alternating current magnetic field may be about 5000.degree.

4 is a mirror, and the light transmitted through the mirror is modulated φ by the magneto-optical recording medium 5, reflected by the mirror 4, and then enters the detector 8. On the other hand, the light reflected by the mirror 4 enters the detector 9.The respective (signs) are calculated and output by the differential amplifier 10.

12-all 2-b, 12-C, and 12-d are rod lenses for condensing light, each having a diameter of 2 mm and a length of about 3 mm. Also, 1l-al 1l-bl 11-C is
A polarization-maintaining fiber is used in the optical path, and is used to suppress light fluctuations due to the optical path between the light source 1, detector 8.9, and polarizer 2.6.7.

When polarization maintaining fiber is used, by increasing the length of the fiber, it becomes possible to make the detection section of the optical information prevention device independent, effectively reducing the size of the detection section, and when using it. can also be given three-dimensional freedom. Also, in the polarization plane modulator (Tbo, t9Yo, s
+) By using 3Fs5O12, the temperature characteristics of ao are improved and stable detection can be performed against changes in ambient temperature.As an actual measurement example, a(.

=1°, φ=15', and ambient temperature -20℃~+80
℃, a sensitivity with an SNR of 50 dB or more and a measurement accuracy of φ within 10 I) were obtained.

In addition, in the embodiment, the polarization plane modulator 3 (Tbo19Yo
, g+ )3 Fe5O12 was used, but the effect is even greater if a ferromagnetic garnet crystal with a large Welde's constant, especially a BL-containing Carnet crystal grown epitaxially on this crystal substrate, is used. b
, 11-C was used with a polarization maintaining fiber, but if a small device such as a semiconductor laser or a semiconductor photodiode is used, one photon can be added directly to 2.6°7, and the optical path fiber can be used. becomes unnecessary.

Effects of the Invention As is clear from what has been described above, the optical information detection device of the present invention is highly sensitive, small and lightweight, and has a
Stable 1g'+ detection is performed by changing the ratio by A degree, and it is highly reliable and has great industrial value.

4, Figure January 1; 4゛j Single f, i Explanation The 11th item is the Kaido style of the drawing 'Kl Xu out ke 11〆 Iζ
FIG. 2 is a diagram for explaining an optical information detection device according to an embodiment of the present invention.

1...Light source, 2,6.7...Polarizer, 3...
...Polarization plane modulator, 4-Mirror, 5...Magneto-optical recording medium, 8.9...Detector, 1o...Differential amplifier, 11-&, b, C... Optical path , 12-z,
b, c.

d...Rod lens,

Claims (4)

[Claims] (1) A polarization plane modulator that vibrates the azimuth angle of the polarization plane and means for differential amplification are provided to detect magneto-optical information, and the polarization plane modulator is characterized by using a ferromagnetic garnet crystal. Optical information detection device. (2) As a ferromagnetic garnet crystal, the general formula (TbxY_
1-x)_3Fe_5O_1_2, and the value of x is (
The optical information detection device according to claim 1, characterized in that an optical information detection device is used that satisfies the following: 0.1≦x≦0.3. (3) The optical information detection device according to claim 1, wherein a crystal epitaxially grown on a crystal substrate is used as the ferromagnetic garnet crystal. (4) The optical information detection device according to claim 1, characterized in that a polarization maintaining optical fiber is used in the optical path.