JPS63100646A - Magneto-optical information reproducing device - Google Patents

Abstract

PURPOSE:To improve the C/N of a signal detection by setting an angle from the incident luminous flux polarizing direction of the photodetector optical axis of a magneto-optical information reproducing device to an optimum angle. CONSTITUTION:A P polarizing luminous flux from a semiconductor laser 21 irradiates through a half mirror 23 a recording media 26 as the optical spot of intensity I0. The luminous flux reflected by the recording medium 26 is guided to a photodetector 27, intensity-modulated and photodetected by a photodetecting device 29. When the average of a polarizing component intensity entering the photodetecting device 29 is the inverse of IR, the squared average at the intensity fluctuation in a magneto-optical signal observing frequency is DELTAIR<2>, xsi is a formula I, the light quantity of the incident light flux on the recording medium 26 is I0, the amplitude reflectance of the recording medium 26 is R, an optical type light utilization ratio to come from the recording medium 26 to the photodetecting device 29 excluding a photodetecting means is epsilon, the photoelectric converting efficiency of the photodetecting device 29 is K, a charge quantity is (e), the thermal noise of an amplifying means in the magneto- optical signal observing frequency is T, the band width of the detecting signal is DELTAB, the amplitude transmission rate of the photodetecting means is tA and the quenching ratio of the photodetecting means is etaA, an angle thetaA formed by the optical axis of the photodetecting means and the prescribed direction satisfies a formula II.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a magneto-optical information reproducing device that reproduces information magnetically recorded on a recording medium by utilizing the magneto-optical effect.

[Prior art]

2. Description of the Related Art In recent years, optical memories that perform recording and reproduction using semiconductor laser light have been actively researched and developed for practical use as high-density O-memories. Among these, in addition to read-only optical disks typified by compact disks and DRAW type optical disks that have already been commercialized, magneto-optical disks that are erasable and rewritable are particularly promising. Magneto-optical disks magnetically record information using the local temperature rise of a magnetic thin film caused by laser spot irradiation, and reproduce the information using the magneto-optic effect (particularly the Kerr effect). The Kerr effect here refers to a phenomenon in which the plane of polarization rotates when light is reflected by a magnetic recording medium.

The basic configuration of a conventional magneto-optical disk casing is shown in FIG. In FIG. 7, 1 is a semiconductor laser, 2 is a collimator lens, 11 is a half mirror, 14 is an objective lens, 6 is a magneto-optical recording medium, 7 is an analyzer, 8 is a condensing lens, 9 is a photodetector, P The polarization direction is parallel to the plane of the paper, and the S polarization direction is perpendicular.

Next, the case where magneto-optical information is reproduced in the above device will be explained. The light beam emitted from the semiconductor laser 1 as linearly polarized light in the P polarization direction is converted into a parallel light beam by the collimator lens 2, and passes through the half mirror 11. If the P polarization component amplitude transmittance is tp and the s polarization component amplitude transmittance is ts, then in 11, 1tpl"=1tsl"=0
．． It is 5. The light beam is imaged as a minute spot on the magneto-optical recording medium 6 by the objective lens 4 . If magnetic domains (pits) are already formed on the medium 6,
As shown in FIG. 8, the reflected light from the medium 6 undergoes rotation of the plane of polarization by ±θ depending on whether the magnetization direction of the spot irradiation area is upward or downward due to the Kerr effect. here,
If the P polarization component of the amplitude reflectance of the recording medium 6 is R and the S polarization component is K, the following equation holds true.

The magneto-optically modulated reflected light is again converted into a parallel beam by the objective lens 4, reflected by the half mirror 11, and then passed through the analyzer 7.
is converted into an intensity-modulated luminous flux. That is, in FIG. 8, the reflected light flux is analyzed as an orthogonal projection of its amplitude onto the optical axis of the analyzer, so the intensity of light incident on the magneto-optical medium is expressed as IO.
1 If the angle of the optical axis of the analyzer from the P polarization direction is θA, then the intensity of the luminous flux transmitted through the analyzer is I + θ according to the Kerr rotation angle ±θK, and 1−θ is calculated as shown in equation (2). make it rain.

Since θ is ~1°, IRI">IK+" holds true, so equation (2) can be expressed as i^. The second term in the parentheses in equation (3) is the magneto-optical modulated component, and the first term is the non-modulated component, and the intensity of each is O +R~-l Rl ``cos'' θA
(5) Such a detection light flux passes through the condensing lens 8,
It is detected by the photodetector 9 as a magneto-optical signal.

Here, the polarization plane rotation angle OK due to the Kerr effect is generally 1
0, and considering that the magneto-optical modulation component obtained by passing through the analyzer 7 is extremely small, the azimuth angle θA of the optical axis of the analyzer is equal to the C/N (carrier and carrier wave) of the detection signal. It is necessary to set the optimum position so that the ratio (to noise) is maximized.

Therefore, in the conventional magneto-optical information recording/reproducing device, the azimuth angle θA of the optical axis of the analyzer 7 is set relative to the polarization direction of the incident light beam in order to maximize the intensity of the magneto-optical modulation component calculated by equation (4). It was set at 45°. However, considering the noise superimposed on the detection signal, θA=
45' does not necessarily mean that a C/N of 11 can be obtained.

[Summary of the invention]

The purpose of the present invention is to improve the above-mentioned drawbacks of the prior art, and to make it possible to reproduce a magneto-optical signal with a good C/N with a simple configuration using an inexpensive photodetector that does not have an amplification effect, such as a pin photodiode. The object of the present invention is to provide a magneto-optical information reproducing device.

The above-mentioned object of the present invention is to provide a magneto-optical information reproducing device with a means for irradiating a light beam polarized in a predetermined direction onto a recording medium on which information is magnetically recorded; An analyzing means for analyzing the modulated reflected or transmitted light beam from the recording medium, a photodetector without an amplification effect for photoelectrically detecting the light beam transmitted through the analyzing means, and a detection signal of the photodetector. and an amplification means for reproducing the information, and the average polarization component intensity that is not modulated by the magneto-optic effect incident on the photodetector is calculated as 2 of this intensity fluctuation at the TR1 magneto-optical signal measurement frequency. The root mean is △I2R1ξ=△FR/I'R1 The light intensity of the incident light beam on the recording medium is Io, and the amplitude reflectance of the recording medium is R1 The above analysis means (optical system from the recording medium to the photodetector) The light utilization efficiency is ε, the photoelectric conversion efficiency of the photodetector is 1, the amount of electric charge is e1, the thermal noise of the amplification means at the optomagnetic signal observation frequency is T1, the bandwidth of the detection signal is ΔB1, the amplitude transmittance of the detection means tA, the same (when the extinction ratio of the analyzer is 6, the angle θA between the optical axis of the analyzer and the predetermined direction is as follows, provided that FR=ξ・(Kε l tAl' l Rl
"Ioa・ΔBS=zetsce lt^l" l
This is achieved by setting so as to satisfy R1''lo-ΔB.

〔Example〕

Hereinafter, the present invention will be explained in detail using the drawings.

1 and 2 show a first embodiment of a magneto-optical information reproducing apparatus based on the present invention, respectively. FIG. 1 is a schematic configuration diagram of an optical system, and FIG. 2 is a schematic configuration diagram of a signal processing circuit. be. In FIG. 1, 21 is a semiconductor laser, 22 is a collimator lens, 23 is a half mirror, 124 is an objective lens, 26 is a magneto-optical recording medium, 27 is an analyzer, 28 is a condensing lens, 29
is a photodetector, the P polarization direction is parallel to the plane of the paper, and the S polarization direction is perpendicular. Further, 13 indicates a light flux transmitted through an analyzer 27, and this detection light flux 13 is detected by a photodetector 29 as shown in FIG.
The signal is photoelectrically converted, voltage amplified by an amplifier 15 including a load resistor 16, and outputted from a terminal 14 as a reproduced signal.

The half mirror 23 has a beam shaping function, whereby the light beam of the semiconductor laser 21 having an elliptical far-field pattern can be efficiently imaged as a circular spot on the recording medium 26. Further, the surface a is inclined at a predetermined angle so that stray light does not enter the photodetector 29. A tracking groove (not shown) is formed on the recording medium 26 in a direction perpendicular to the plane of the paper, and the light beam focused on the recording medium 26 by the objective lens 24 is diffracted by this groove. 25 is
This is a photodetector for detecting the imbalance of ±1st-order diffracted light caused by track deviation, and is fixed to the periphery of the aperture of the objective lens 24. For this reason, the objective lens 24
This has the advantage of not causing an offset in the tracking error signal even if it moves in the direction perpendicular to the track groove. Photodetector 2
Reference numeral 9 denotes a photodetector having no amplification effect, such as a Si-pin photodiode, which detects a magneto-optical signal and a focus error signal. A known method is used for focus error detection, but since it has no direct relation to the present invention, detailed explanation will be omitted.

In the above device, the semiconductor laser 21 emits a P-polarized light beam. This emitted light flux becomes parallel light by the collimator lens 22, passes through the half mirror 23, and passes through the objective lens 24.
The light beam is irradiated onto the recording medium 26 as a light spot with an intensity of IO. The light beam reflected by the recording medium 26 is modulated in its polarization state according to the information magnetically recorded on the recording medium 26, passes through the objective lens 24 again, is reflected by the half mirror 23, and is passed through the analyzer. Guided by 27. The detection light 13 that has passed through the analyzer 27 is intensity-modulated and is received by a photodetector 29 via a condenser lens 28 .

The intensity-modulated light beam 13 passing through the analyzer 27 is converted into a photocurrent by a photodetector 29 shown in FIG. The photoelectric conversion efficiency is given by the following equation, where e is the amount of charge, h is a blank constant, ρ is the quantum efficiency of the photodetector, and ν is the frequency of the luminous flux.

eρ 1(=−(6) hν) Here, the following four types of noise can be considered as noise sources in signal readout.

l) Noise caused by root mean square intensity fluctuation ΔI2R of non-modulated component light IR.

2) Noise caused by the root mean square intensity fluctuation △l''K of the modulated component light IK.

3) Photodetector shot noise.

4) Thermal noise due to amplifier.

The noise due to ΔI2R in l) and the noise due to Δ1''K in 2) are caused by the surface roughness and non-uniformity of the recording medium, the intensity fluctuation of the semiconductor laser, etc., and each has a constant determined by the noise source such as the medium and the semiconductor laser. If the averages of the effective values of ξ, ζ, the non-modulated component, and the modulated component are TR and TK, respectively, the following equation holds true.

△I2R; ξPRΔB (7) △I2 to one ζpx△B (8) However, ΔB is the bandwidth of the detection signal.

The noise caused by △I2R, the noise caused by △I2K, shot noise, and thermal noise are respectively FR, FK, S, and T.
Then, it can be calculated by the following formula.

FR = ξ to 2FRΔB (9) FK = ζ to 2T' △B
(10) S=ZeKTR△B
(11) where k is the Boltzmann constant, Te is the equivalent noise temperature, and Rt is the resistance value of the load resistor 16.

From equations (4) and (5), the analyzer optical axis azimuth θA
Since the magneto-optical modulated component strength IK has a dependence on 5in2OA and the non-modulated component strength IR has a dependence on cos''θA, the dependence of each noise on θA can be expressed as follows.

Ficcos'θA (13) FK"sin"2θA (14) Scccos'θA (15) T=const (16)
If you use these to express C/N in decibels,
It becomes like the following formula.

Since the C/N in equation (17) is a function of the analyzer optical axis azimuth angle θ, the extreme value is determined by partially differentiating (17) with respect to θA.

If we find the extreme value of θA, we get the following.

5=eK l Rl I 0cos”θA”ΔB
(20) t = T
(21) If the optical axis azimuth angle of the analyzer is set to satisfy equations (18) to (21), the C/N can be maximized.

In the explanation of Fig. 7, it is assumed that the signal level drop does not occur due to the recording medium and optical system, but in an actual optical system, C/C/
In accurately predicting N, it must be taken into account. The following two points can be considered as causes for the decrease in signal level.

l) Optical loss (decrease in amplitude due to absorption and vignetting) 2)
l) and 2) contribute to the decrease in the intensity of the P-3 phase difference magneto-optical modulation component,
Only l) contributes to the reduction of the unmodulated component strength.

In order to evaluate the decrease in the intensity of the magneto-optical non-modulated component (loss of light amount), the light utilization efficiency εR is defined. It should be noted that the present invention focuses on the ratio of the amount of light on the recording medium to the amount of light reaching the photodetector as light utilization efficiency. In this embodiment, the following points were taken into consideration when determining εR.

l) Tracking groove (pitch 1.6 μm, depth λ/8
．． The rate at which the diffracted light from λ=835 nm enters the entrance pupil of the objective lens is defined as the light utilization efficiency εO.

2) Consider the product along the optical path of the square of the amplitude transmittance (or reflectance) in the P polarization direction of n optical elements excluding the analyzer in the optical path from the medium to the photodetector, and calculate the light utilization. Efficiency ε1
shall be. If the amplitude transmittance of the first optical element is tpi and the reflectance is rp-, ε1 can be calculated by the following equation.

In equation (22), when the luminous flux is reflected by the i-th optical element, l rpi i instead of l jpi 12
Just substitute 2. Note that the transmittance of the analyzer is treated as a change amount when calculating the C/N, so it is excluded from ε1.

From 1) and 2), the light utilization efficiency εR of the magneto-optical non-modulated component
is expressed by the following formula.

εR=ε0εI (23) Next, consider the decrease in the intensity of the magneto-optical modulation component. For this purpose, it is necessary to consider the phase difference between the P-3 polarized lights in addition to the light amount loss.

For example, as shown in Figure 3, the reflected light from the recording medium is generally not linearly polarized light as shown in Figure 8. It is known that the light becomes elliptically polarized light tilted only at the rotation angle θ.

That is, the P and S polarization components R and K of the amplitude reflectance of the recording medium are (
24) It will appear like the formula.

However, α. , R0 is the phase component of each amplitude reflectance.

In this case, the Kerr rotation angle θ is expressed as IKI θK = −CO8ΔO(25) IRl. △. If = nπ (n = integer), the reflected light from the recording medium becomes linearly polarized light, but in other cases, θK decreases, which is undesirable.

The same thing can be said about optical elements, and in this example, the light utilization efficiency εK was defined in order to evaluate the decrease in the intensity of the magneto-optical modulation component, and the following points were taken into consideration when calculating εK.

That is, for the magneto-optical modulation component, P of n optical elements excluding the analyzer in the optical path from the recording medium to the photodetector,
The product of the amplitude transmittance (or reflectance) in the S polarization direction along the optical path is considered and is defined as light utilization efficiency ε2.

The amplitude transmittance of the i-th optical element in the P and S polarization directions is tp
i, ts+ (reflectance rI"+rsl), the following equation holds true.

Using (29), ε2 can be expressed as follows.

In (30), when the luminous flux is reflected by the i-th optical element, I rpi instead of 1 tpil and 1 tsl
11 rsi l can be substituted. Note that the transmittance of the analyzer is treated as a change amount when calculating C/N, so ε2
It is excluded from.

From this, the light utilization efficiency ε of the magneto-optical modulation component is given by the following equation.

ε=ε0ε2 (28) We will also perform a more accurate evaluation of the analyzer.

If the amplitude transmittance of the analyzer is tA and the extinction ratio is nA, then (2
), cos13A is expressed as l tAl・(cosθA
10v'ηAs1nθA), 5inoA itAl
-(sinθA+fηACO8OA).

Assuming IRI">IKI", for the magneto-optical modulation component intensity, the product of ε and the transmittance ε3 of the analyzer given by the following equation may be calculated.

ε3=ltAl”(1nA) sin2θA (29)
However, the amplitude transmittance of the analyzer was made equal in the P-8 polarized acid direction, and no phase difference was given between the P-S polarized light.

For the magneto-optical non-modulated component, it is sufficient to calculate the product of εR and the analyzer transmittance ε4 given by the following equation.

5, = 1tAl”(cos”θA+77Asin
”nA) (30) From the above, if the intensities of the magneto-optical modulation component and non-modulation component are IK and IR, respectively, then □I(1εoEo' IRI IKI 1t
Al 2 (1-nA) sin2 (7 people IR-1oεoε
, l R1”lt^l”(cos”OA+ηAs1n
2OA)=IoεRIRl”lt^l”(cos2θ
A + y7A sin” θ, \).

Substituting equations (31) and (32) into equation (17), C/N
The incident light beam polarization direction of the analyzer optical axis that maximizes f'R
: ξ(K εR1t, ~ l” l Rl 21
0)”−△I3 (34)S'
=2eKεRl t A +21 R121o ・ΔB
(35) t' = T
(36) The calculation conditions are shown below.

The semiconductor laser 21 has a wavelength λ=835 nm, and its output is adjusted so that the incident light on the recording medium 26 is QI o =2X 10-'W without changing the transmittance l tp 12 of the half mirror 23. .

Gd, Tb, Fe, and Co are used for the recording medium 26,
l Rl” = 0.12. θK = 0.74°,
p, s Phase component α of polarization direction amplitude reflectance. ,β. Phase difference △.

is △. =20°.

Light use efficiency ε. is a tracking groove (pitch 1.6 μm
, depth λ/8) is received by an objective lens with N, A = 0.5, ε. =0.6.

The light utilization efficiency ε1 is ε=0.39, considering the product of the transmittance (reflectance in the case of a half mirror) of optical elements other than the analyzer in the optical path from the recording medium to the photodetector.

The light utilization efficiency ε2 can be calculated by considering the product of the P and S amplitude transmittances (reflectances in the case of a half mirror) of optical elements other than the analyzer in the optical path from the recording medium to the photodetector. The half mirror 23 used in this example has a polarization of Δ+-+M between P-8 polarized light.
= 160° phase difference. Therefore, the phase difference △ that occurs in the recording medium. There is a relationship: Δ0+ΔHM−π (37), which prevents a decrease in the intensity of the magneto-optical modulation component. In the Ikemi Example, there is no optical element that provides a phase difference between p and s polarized light during transmission, so ε2=0.
It becomes 39.

Photodetector 25 is an 81-pin photodiode with a photoelectric conversion rate of =0.54. Constants ξ and ζ determined by noise sources such as the recording medium and the semiconductor laser are given as follows.

ξ= 2 x 10-” (R, 1, N) ζ= I
X 10-” (R, 1, N) Thermal noise T is Boltzmann constant = 1.38X10-1, equivalent noise temperature Te =
300 [k], load resistance Rr= I
Ω], signal detection bandwidth ΔB = 3 x 10'
[1/Hz], given as T = 5 x 10-''.

Note that the thermal noise T is (12
) may not be possible to write in a simple form such as an expression, so in such cases there is no need to follow this.

The amplitude transmittance tp of the analyzer is l tP l”=0.84,
Extinction ratio ηA = t x t O-” Tear 6° Figure 4 is a diagram showing the relationship between the angle θA of the optical axis of the analyzer from the polarization direction of the incident light beam and C/N. (33) to (36) ) It can be seen that the C/N of this example is maximum at the optimal θA = 79.4' given by the equation.Compared with the conventional device where θA = 45', this example has a C/N of 8 dB. The C/N is improved as described above.If θA is set to 700 to 85°, a sufficiently good C/N can be obtained.

Figure 5 shows the values of the constant ξ that determines the root mean square fluctuation of the magneto-optical non-modulated component intensity, ξ = lxlQ-'', 10-'', 10-
”.

10 is a diagram showing the relationship between θA and C/H when changed to 10-14. FIG.

Equation (33) shows that when the noise FR caused by △I3R and the shot noise S are relatively larger than the thermal noise T, the optimal value of θA approaches 90''. C/N at and C/N at θA=45°
Comparing the above, it can be seen that when FR and S become relatively larger than T, the C/N is further improved. For example, ξ=
In the case of I x 10-'', the C/N was improved by 18 dB or more, and the present invention is very effective.

FIGS. 6(A) and 6(B) are schematic views showing a fourth embodiment of the present invention, and FIG. 6(B) shows a view of FIG. 6(A) viewed from the six directions of arrows. In FIGS. 6(A) and 6(B), the same members as in FIG. 1 are denoted by the same reference numerals, and detailed explanations will be omitted. In this embodiment as well, the signal processing system after the photodetector 29 is constructed as shown in the second diagram. This embodiment uses a half mirror 10 instead of the half mirror 23 of the first embodiment, and is configured to detect light transmitted through the half mirror 1O. The face of the half mirror 10 is the photodetector 2
9 is tilted at a predetermined angle to prevent stray light from entering.

In this example, P is used in the explanatory text of FIG.

The S polarization direction may be replaced with each other.

Further, although the half mirror 23 and IO are used in FIGS. 1 and 6, the present invention is effective even if these are replaced with a polarizing beam splitter. For example, JP-A-58-1896
Polarizing beam splitter known as No. 12 (1rpl”=0
．． 2. If 1rsI”=1) is used in Figure 1, C/
N can be improved. In Figure 6, for example, l t
If a polarizing beam splitter having the characteristics s l''=0.2. l tp l''=1 is used, the C/N can be further improved.

The present invention can be applied in various ways in addition to the embodiments described above. For example, in the embodiment, the reflected light from the magneto-optical recording medium is detected, but it may be configured to detect a light flux that passes through the magneto-optical recording medium and is modulated by the Faraday effect.

〔Effect of the invention〕

As explained above, in a magneto-optical information reproducing device, the present invention improves the C/N of signal detection by setting the angle of the analyzer optical axis from the incident light beam polarization direction to an appropriate angle from the conventional 45°. It has the effect of improving.

Furthermore, since a high C/N can be obtained according to the present invention, a complicated detection system as in conventional devices is not required, thereby increasing the reliability of the device and reducing manufacturing costs.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an optical system according to an embodiment of the present invention, and FIG.
The first figure is a schematic diagram showing the signal processing system of the embodiment shown in the third figure.
The figure shows the polarization state of reflected light from a magneto-optical recording medium.
4 and 5 are diagrams showing the relationship between the optical axis azimuth angle and C/N of the analyzer in the present invention, and FIGS. 6(A) and 6(B) respectively.
) are schematic diagrams showing other embodiments of the present invention, FIG. 7 is a schematic diagram showing an example of a conventional magneto-optical information reproducing device, and FIG. 8 is a diagram showing the principle of general magneto-optical signal detection. be. 13... Detection light, 21... Semiconductor laser, 22... Collimator lens, 23... Half mirror 124... Objective lens, 26... Magneto-optical recording medium, 27... Analyzer, 28... Condensing lens, 29... Photodetector. % θACdec3] (A) Procedural formalities (spontaneous) October 5, 1988 1. Indication of the case 1986 Patent Application No. 246616 2. Name of the invention Light 61-ki information reproducing device 3. Amendment. Patent applicant address Canon Co., Ltd., 3-30-2 Shimomaruko, Ota-ku, Tokyo (telephone 75B-2111) 5. Specification subject to amendment 6. The full text of the specification of the amendment is as attached. correct. Full text correction Description 1, Name of the invention Magneto-optical information reproducing device 2, Claims (1) Means for irradiating a recording medium on which information is magnetically recorded with a light flux polarized in a predetermined direction; An analyzing means for analyzing the reflected or transmitted light flux from the recording medium whose polarization state has been modulated according to the information by an optical effect, and light without an amplification effect for photoelectrically detecting the light flux transmitted through the analyzing means. It consists of a detector and an amplification means for amplifying the detection signal of the photodetector and reproducing the information, and the average of the polarization component intensity that is not modulated by the magneto-optic effect incident on the photodetector is determined as TR1 magneto-optical. The root mean square of this intensity at the signal measurement frequency △l2R1E = △PR7'rIR1
The amount of light incident on the recording medium is IO1 The amplitude reflectance of the recording medium is R1 The light utilization efficiency of the optical system from the recording medium to the photodetector excluding the analyzing means is ε The photoelectric conversion of the photodetector Efficiency is 1 charge amount e1 Thermal noise of the amplifying means at the optomagnetic signal observation frequency T1 Bandwidth of the detection signal ΔB1 Amplitude transmittance of the analyzing means tA Similarly, the extinction ratio of the analyzing means is ηA When the angle θA between the optical axis of the analyzer and the predetermined direction is as follows, provided that FR=ξ(ε1tAl"lR1"lo)"・Δ
BS = 2e to t 1tAl” lR1”Io−Δ
A magneto-optical information reproducing device characterized by satisfying B. 3. Detailed Description of the Invention [Technical Field] The present invention relates to a magneto-optical information reproducing device that reproduces information magnetically recorded on a recording medium by utilizing the magneto-optical effect. [Prior Art] In recent years, there has been active research and development into practical use of optical memories that perform recording and reproduction using semiconductor laser light as high-density recording memories. Among these, in addition to read-only optical disks and DRAW type optical disks, typified by already commercialized compact disks, magneto-optical disks that are erasable and rewritable are particularly promising. The magneto-optical disk is
Information is magnetically recorded using the local temperature rise of a magnetic thin film caused by laser spot irradiation, and the information is reproduced using the magneto-optical effect (particularly the Kerr effect). The Kerr effect here refers to a phenomenon in which the plane of polarization rotates when light is reflected by a magnetic recording medium. FIG. 7 shows the basic configuration of a conventional magneto-optical disk device. In FIG. 7, l is a semiconductor laser, 2 is a collimator lens, 11 is a half mirror, 14 is an objective lens, 6 is a magneto-optical recording medium, 7 is an analyzer, 8 is a condensing lens, 9 is a photodetector, P The polarization direction is parallel to the plane of the paper, and the S polarization direction is perpendicular. Next, the case where magneto-optical information is reproduced in the above device will be explained. The light beam emitted from the semiconductor laser 1 as linearly polarized light in the P polarization direction is converted into a parallel light beam by the collimator lens 2, and passes through the half mirror 11. If the P polarization component amplitude transmittance is tp and the S polarization component amplitude transmittance is ts, then in 11, 1tp12=1tsl"=0
．． It is 5. The light beam is imaged as a minute spot on the magneto-optical recording medium 6 by the objective lens 4 . If magnetic domains (pits) are already formed on the medium 6,
As shown in FIG. 8, the reflected light from the medium 6 undergoes rotation of the plane of polarization by ±θ depending on whether the magnetization direction of the spot irradiation area is upward or downward due to the Kerr effect. here,
If the P polarization component of the amplitude reflectance of the recording medium 6 is R and the S polarization component is K, the following equation holds true. The magneto-optically modulated reflected light is again converted into a parallel beam by the objective lens 4, reflected by the half mirror 11, and then passed through the analyzer 7.
is converted into an intensity-modulated luminous flux. That is, in FIG. 8, the reflected light flux is analyzed as the orthogonal projection of its amplitude onto the optical axis of the analyzer, so the intensity of the light incident on the magneto-optical medium can be expressed as io
, if the angle of the optical axis of the analyzer from the P polarization direction is θA, then the intensity of the luminous flux transmitted through the analyzer is I+θ and I−θ according to the Kerr rotation angle ±θK, respectively, as shown in equation (2). make it rain. Since θ is ~10, lRI")lK+" holds true, so equation (2) can be expressed as follows. The second term in the parentheses in equation (3) is the magneto-optical modulated component, and the first term is the non-modulated component, and the intensity of each is T. IR～-l Rl "cos" θA
(5) Such a detection light flux passes through the condensing lens 8,
It is detected by the photodetector 9 as a magneto-optical signal. Here, the polarization plane rotation angle θ due to the Kerr effect is generally expressed as P
Considering that the amount of the magneto-optical modulation component obtained by passing through the analyzer 7 is very small, the azimuth angle θA of the optical axis of the analyzer is the C/N (carrier and noise) of the detected signal. It is necessary to set it at the optimal position so that the ratio of Therefore, in the conventional magneto-optical information recording/reproducing device, the azimuth angle θA of the optical axis of the analyzer 7 is set relative to the polarization direction of the incident light beam in order to maximize the intensity of the magneto-optical modulation component calculated by equation (4). It was set at 45°. However, considering the noise superimposed on the detection signal, θA=
It is not necessarily the case that the maximum C/N can be obtained by setting the angle to 45°. [Summary of the Invention] The purpose of the present invention is to improve the above-mentioned drawbacks of the prior art, and to provide a magneto-optical device with a simple configuration and good C/N using an inexpensive photodetector without amplification such as a pin photodiode. An object of the present invention is to provide a magneto-optical information reproducing device capable of reproducing signals. The above-mentioned object of the present invention is to provide a magneto-optical information reproducing device with a means for irradiating a light beam polarized in a predetermined direction onto a recording medium on which information is magnetically recorded; An analyzing means for analyzing the modulated reflected or transmitted light beam from the recording medium, a photodetector without an amplification effect for photoelectrically detecting the light beam transmitted through the analyzing means, and a detection signal of the photodetector. and an amplification means for reproducing the information, and TR is the average intensity of the polarized light component that is not modulated by the magneto-optic effect incident on the photodetector, and this intensity and fluctuation at the magneto-optical signal observation frequency is TR. The root mean square is ΔI”R, ξ=ΔI2R/I2R1 The amount of light incident on the recording medium is 10 s The amplitude reflectance of the recording medium is R1 The optical system from the recording medium to the photodetector excluding the analyzing means The light utilization efficiency of the system is ε, the photoelectric conversion efficiency of the photodetector is e1, the total charge is e1, the thermal noise of the amplification means at the optomagnetic signal observation frequency is T1, the bandwidth of the detection signal is Δ
B1 The amplitude transmittance of the detection means is t^, the same (when the extinction ratio of the analysis means is 0, the angle θA between the optical axis of the analysis means and the predetermined direction is as follows, However, FR=ξ・(εlt^l"lR1"1o)2・ΔB
This is achieved by setting S=3e to satisfy ε1tAl''lR1''lo·ΔB. [Example] Hereinafter, the present invention will be explained in detail using the drawings. 1 and 2 show a first embodiment of a magneto-optical information reproducing apparatus based on the present invention, respectively. FIG. 1 is a schematic configuration diagram of an optical system, and FIG. 2 is a schematic configuration diagram of a signal processing circuit. be. In FIG. 1, 21 is a semiconductor laser, 22 is a collimator lens, 23 is a half mirror, 124 is an objective lens, 26 is a magneto-optical recording medium, 27 is an analyzer 1, 28 is a condenser lens, 2
9 is a photodetector, the P polarization direction is parallel to the plane of the paper, and the S polarization direction is perpendicular. Further, 13 indicates the light flux transmitted through the analyzer 27, and this detection light flux 13 is detected by the photodetector 2 as shown in FIG.
The signal is photoelectrically converted at 9, voltage amplified by an amplifier 15 including a load resistor 16, and outputted from a terminal 14 as a reproduced signal. The half mirror 23 has a beam shaping function, whereby the light beam of the semiconductor laser 21 having an elliptical far-field pattern can be efficiently imaged as a circular spot on the recording medium 26. Further, the surface a is inclined at a predetermined angle so that stray light does not enter the photodetector 29. A tracking groove (not shown) is formed on the recording medium 26 in a direction perpendicular to the plane of the paper, and the light beam focused on the recording medium 26 by the objective lens 24 is diffracted by this groove. 25 is
This is a photodetector for detecting the imbalance of ±1st-order diffracted light caused by track deviation, and is fixed to the periphery of the aperture of the objective lens 24. For this reason, the objective lens 24
This has the advantage of not causing an offset in the tracking error signal even if it moves in the direction perpendicular to the track groove. Photodetector 2
Reference numeral 9 denotes a photodetector having no amplification effect, such as a Si-bin photodiode, which detects a magneto-optical signal and a focus error signal. A known method is used for focus error detection, but since it has no direct relation to the present invention, detailed explanation will be omitted. In the above device, the semiconductor laser 21 emits a P-polarized light beam. This emitted light flux becomes parallel light by the collimator lens 22, passes through the half mirror 23, and passes through the objective lens 24.
A light spot with an intensity of 10 is irradiated onto the recording medium 26. The light beam reflected by the recording medium 26 is modulated in its polarization state according to the information magnetically recorded on the recording medium 26, passes through the objective lens 24 again, is reflected by the half mirror 23, and is passed through the analyzer. Guided by 27. The detection light 13 that has passed through the analyzer 27 is intensity-modulated and is received by a photodetector 29 via a condenser lens 28 . The intensity-modulated light beam 13 passing through the analyzer 27 is converted into a photocurrent by a photodetector 29 shown in FIG. The photoelectric conversion efficiency is given by the following equation, where e is the amount of charge, h is a blank constant, ρ is the quantum efficiency of the photodetector, and ν is the frequency of the luminous flux. Here, the following four types of noise can be considered as noise sources in signal readout. l) Root mean square intensity fluctuation of non-modulated component light IR △I”++
noise caused by. 2) Noise caused by root mean square intensity fluctuation △I2K in modulated component light ■. 3) Photodetector shot noise. 4) Thermal noise due to amplifier. The noise due to △I2H in l) and the noise due to ΔI2K in 2) are caused by the surface roughness and non-uniformity of the recording medium, the intensity fluctuation of the semiconductor laser, etc., and each has a constant determined by the noise source such as the medium and the semiconductor laser. If the averages of the effective values of ξ, ζ, the non-modulated component, and the modulated component are TR and TK, respectively, the following equation holds true. ΔI'R-ξPRΔB (7) ΔI2=ζpgΔB (8) However, ΔB is the bandwidth of the detection signal. The noise caused by △I'R, the noise caused by △I''K, shot noise, and thermal noise are respectively FR, 'FK, S,
If T, it can be calculated using the following formula. FR=ξ"FatΔB(9) F x = ζ" TIK △B
(10) S=Ze pc TR△B
(11) R+ where k is Portzmann constant, Te is equivalent noise temperature, R+
is the resistance value of the load resistor 16. (4) and (5), the analyzer optical axis azimuth OA
, the magneto-optical modulation component intensity IK is sin2θA
, the unmodulated component intensity IR has a dependence on cos2θA, so the dependence of each noise on θA can be expressed as follows. FRoc cos″θA (
13) FKocsin”2θA
(14) S”CO8”θA
(15) T=const, (16) Using these, if C/N is expressed in decibels, then
It becomes like the following formula. Since C/N in equation (17) is a function of the analyzer optical axis azimuth angle θA, the extreme value is determined by partially differentiating (17) with respect to θ8. If we find the extreme value of θA, we get the following. fR-ξ(-lR12Iocos2θA)2・ΔB
(19) 1RIIOcos”θA・ for S=e
ΔB (20)t = T
(21) If the optical axis azimuth angle of the analyzer is set to satisfy equations (18) to (21), the C/N
can be the maximum value. In the explanation of Fig. 7, it is assumed that the signal level drop does not occur due to the recording medium and optical system, but in an actual optical system, C/C/
In accurately predicting N, it must be taken into account. The following two points can be considered as causes for the decrease in signal level. 1) Light loss (decreased amplitude due to absorption and vignetting) 2) P
−3 l) and 2) contribute to the decrease in the intensity of the magneto-optical modulation component due to the phase difference between polarizations,
Only l) contributes to the reduction of the unmodulated component strength. In order to evaluate the decrease in the intensity of the magneto-optical non-modulated component (loss of light amount), the light utilization efficiency εR is defined. It should be noted that the present invention focuses on the ratio of the amount of light on the recording medium to the amount of light reaching the photodetector as light utilization efficiency. In this embodiment, the following points were taken into consideration when determining εR. l) Tracking groove (pitch 1.6 μm, depth λ/8
The rate at which the diffracted light from λ=835 nm enters the entrance pupil of the objective lens is defined as the light utilization efficiency ε0. 2) Considering the product along the optical path of the square of the amplitude transmittance (or reflectance) in the P polarization direction of n optical elements excluding the analyzer in the optical path from the recording medium to the photodetector, calculate the light utilization efficiency. Let ε1. If the amplitude transmittance of the i-th optical element is jpi and the reflectance is rpi, ε1 can be calculated by the following equation. In equation (22), when the luminous flux is reflected by the i-th optical element, l rpi 1 instead of l t, i l''
' can be substituted. Note that the transmittance of the analyzer is treated as a change amount when calculating the C/N, so it is excluded from ε1. From 1) and 2), the light utilization efficiency εR of the magneto-optical non-modulated component can be calculated by the following equation. ε R= ε 0 ε r
(23) Next, consider the decrease in the intensity of the magneto-optical modulation component. For this purpose, in addition to the complete loss, consideration must be given to the phase difference between the P-3 polarized lights. For example, as shown in Figure 3, the reflected light from the recording medium is generally not linearly polarized light as shown in Figure 8 (due to the phase difference between the P-polarized component and the S-polarized component, the long axis It is known that the light becomes elliptically polarized light tilted only at the Kerr rotation angle θ.In other words, the P and S polarization components R and K of the amplitude reflectance of the recording medium are (
24) It will appear like the formula. R= l Rl ello K= l Kle"'
(24) △0=α. −β. However, α. ,β. is the phase component of each amplitude reflectance. In this case, the Kerr rotation angle θ is expressed as follows. △. = nπ (n = integer), the reflected light from the recording medium becomes linearly polarized light, but in other cases, θK decreases, which is undesirable. The same thing can be said about optical elements, and in this example, the light utilization efficiency εK was defined in order to evaluate the decrease in the intensity of the magneto-optical modulation component, and the following points were taken into consideration when determining θK. That is, for the magneto-optical modulation component, P of n optical elements excluding the analyzer in the optical path from the recording medium to the photodetector,
The product of the amplitude transmittance (or reflectance) in the S polarization direction along the optical path is considered and is defined as light utilization efficiency ε2. The amplitude transmittance of the i-th optical element in the P and S polarization directions is rp
l, tsi (rGII+rsl for reflectance)
Then, the following formula holds true. t pi = l t pi le"'ts+ =
l ts+ l elJ' (2
6) Using △i=αi-β1 (29), calculate ε2 as shown in the following equation. In (30), when the luminous flux is reflected by the i-th optical element, l rpl instead of 1tptllts+l
Just substitute ll rsi l. Note that the transmittance of the analyzer is treated as a change amount when calculating C/N, so ε2
It is excluded from. From this, the light utilization efficiency ε of the magneto-optical modulation component is given by the following equation. ε=ε0ε2 (28) We will also perform a more accurate evaluation of the analyzer. If the amplitude transmittance of the analyzer is tA and the extinction ratio is ηA, then (2
), CO5θA is 1t^1・(COSθA+
r sinθA), sinθA as l tAl ・
(sinθA+l cosθA). Assuming IRI">IKI", for the magneto-optical modulation component intensity, the product of ε and the transmittance ε3 of the analyzer given by the following equation may be calculated. εs = l tAl” (1-77A) 5in2θ
A (29) However, the amplitude transmittance of the analyzer was equal in the P-8 polarized acid direction, and no phase difference was given between the P-8 polarized lights. For the magneto-optical non-modulated component, it is sufficient to calculate the product of εR and the analyzer transmittance ε4 given by the following equation. ε4=1t^l 2(c o s”θA+ηA sin”
θK) (30) From the above, if the intensities of the magneto-optical modulation component and non-modulation component are I and IR, respectively, then ■-TI・'・“・' IRI IKI 1txl” (
1-η・) 5in2θ・IR"-1oεoε11
R121t^l”(cos”θA+77 A sin”
θK)=Io εRIRl"lt ^l"(cos"θ
A + 77 A sin” θK). Substituting equations (31) and (32) into equation (17), C/N
The angle θA of the analyzer optical axis from the polarization direction of the incident light beam, which maximizes the angle θA, is determined as follows. fR'=ξ(niεR1t^l"lR1"lo)'・ΔB
(34) S' = 2e to gR1t^l"
1RIJo・ΔB (35)t'=T
(36) The calculation conditions are shown below. The semiconductor laser 21 has a wavelength λ=835 nm, and the amount of light incident on the recording medium 26 is ■. The output is adjusted so that = 2 x 10 -3 W, regardless of the transmittance of the half mirror 23, 1 tp 12. Gd, Tb, Fe, and Co are used for the recording medium 26,
]Rl'=o, 12. θ = 0.74°, I), S
Phase component α of polarization direction amplitude reflectance. ,β. The phase difference △. is △. -20°. Light use efficiency ε. is a tracking groove (pitch 1.6 μm
, depth λ/8) is received by an objective lens with N, A = 0.5, ε. =0,6. The light utilization efficiency ε1 is ε=0.39, considering the product of the transmittance (reflectance in the case of a half mirror) of optical elements other than the analyzer in the optical path from the recording medium to the photodetector. The light utilization efficiency ε2 can be calculated by considering the product of the P and S amplitude transmittances (reflectances in the case of a half mirror) of optical elements other than the analyzer in the optical path from the recording medium to the photodetector. The half mirror 23 used in this example has a polarization of Δ11 to +
= 160° phase difference. Therefore, the phase difference △ occurring in the n medium. Between, △. +I factor = π (37) This relationship prevents a decrease in the intensity of the magneto-optical modulation component. In the shrub example, there is no optical element that provides a phase difference between the P-8 polarized lights during transmission, so Σ3' = co
s (ΣΔ1)=1. Also, since Np1l=ltsl and -Q, ε2=0.39. The photodetector 25 is a Si-pin photodiode with a photoelectric conversion rate of =0.54. Constants ξ and ζ determined by noise sources such as the recording medium and the semiconductor laser are given as follows. ξ=2xlO-" (R, 1, N) ζ= 1 x 10-1l (R, 1, N) Thermal noise T is Boltzmann's constant = 1.38X10-", equivalent noise temperature Te = 300 [K] , load resistance R+=IXIO-
'[Ωko, signal detection bandwidth △B = 3 X I
O' [1/Hz, T = 5 x 10-
”. The thermal noise T is given by (12
) may not be possible to write in a simple form such as an expression, so in such cases there is no need to follow this. The analyzer has an amplitude transmittance tp of 1tpl"=0.84 and an extinction ratio ηA-1xlO-1. Figure 4 shows the relationship between the angle θA of the analyzer's optical axis from the polarization direction of the incident light beam and C/H. (33) to (3).
It can be seen that the C/N of this example is maximized at the optimum θA=79.4° given by equation 6). Compared to the conventional device where θA = 45°, in this example, the angle is 8d.
C/N is improved over B. θA=70'~85°
If so, a sufficiently good C/N can be obtained. Figure 5 shows the root mean square fluctuation of the magneto-optical non-modulated component intensity and the value of the six constants ξ, ξ = I X 10-'', 10-
", 10-". 10-" is a diagram showing the relationship between θA and C/N. According to equation (33), when the noise FR and shot noise S caused by △I2R are relatively larger than the thermal noise T. , the optimal value for 0 people approaches 90°.Also, the C/N at 1 suitable value of θ input and the C/N at θ = 45°
Comparing the above, it can be seen that when FR and S become relatively larger than T, the C/N is further improved. For example, ξ=
In the case of I x 10-'', the C/N is improved by more than 18 dB, which shows that the present invention is very effective. FIG. 6B is a schematic view of FIG. 6A viewed from the arrow direction. In FIGS. 6A and 6B, the same members as in FIG. , and a detailed explanation will be omitted. In this embodiment as well, the signal processing system after the photodetector 29 is configured as shown in the second figure. This embodiment is similar to the half mirror 23 of the first embodiment. Half mirror 1O instead of
The half mirror 10 is configured to detect the transmitted light using the half mirror 10. The face of the half mirror 1O is inclined at a predetermined angle to prevent stray light from entering the photodetector 29. In this example, P is used in the explanatory text of FIG. The S polarization direction may be replaced with each other. Further, although the half mirror 23 and IO are used in FIGS. 1 and 6, the present invention is effective even if these are replaced with a polarizing beam splitter. For example, JP-A-58-1896
Polarizing beam splitter (l rr l”
=0.2. The C/N can be further improved by using l rs l"=1) in FIG. 1. In FIG. If you use a splitter, it will be more C.
/N can be improved. The present invention can be applied in various ways in addition to the embodiments described above. For example, in the embodiment, the reflected light from the magneto-optical recording medium is detected, but it may be configured to detect a light flux that passes through the magneto-optical recording medium and is modulated by the Faraday effect. [Effects of the Invention] As explained above, the present invention improves signal detection in a magneto-optical information reproducing device by setting the angle of the analyzer optical axis from the incident light beam polarization direction to an optimal angle from the conventional 45°. It has the effect of improving C/N. Furthermore, since a high C/N can be obtained according to the present invention, there is no need for a complicated detection system as in conventional devices, thereby increasing the reliability of the device and reducing manufacturing costs. 4. Brief explanation of the drawings Figure 1 is a schematic diagram showing an optical system according to an embodiment of the present invention, Figure 2 is a schematic diagram showing an optical system according to an embodiment of the present invention.
The first figure is a schematic diagram showing the signal processing system of the embodiment shown in the third figure.
The figure shows the polarization state of reflected light from a magneto-optical recording medium.
FIG. 4 and FIG. 5 are diagrams showing the relationship between the optical axis azimuth angle and C/N of the analyzer in the present invention, and FIG. 6 (A),
CB) is a schematic diagram showing other embodiments of the present invention, FIG. 7 is a schematic diagram showing an example of a conventional magneto-optical information reproducing device, and FIG. 8 is a diagram showing the principle of general magneto-optical signal detection. It is. 13... Detection light, 21... Semiconductor laser, 22... Collimator lens, 23... Half mirror 124... Objective lens, 26... Magneto-optical recording medium, 27... Analyzer, 28... Condensing lens, 29... Photodetector.

Claims (1)

[Claims] (1) A means for irradiating a recording medium on which information is magnetically recorded with a light beam polarized in a predetermined direction, and reflection from the recording medium whose polarization state is modulated according to the information by the magneto-optic effect. Alternatively, an analysis means for analyzing the transmitted light beam, a photodetector without an amplification effect for photoelectrically detecting the light beam transmitted through the analysis means, and an amplification for amplifying the detection signal of the photodetector and reproducing the information. The mean of the intensity of the polarized light component that is not modulated by the magneto-optic effect incident on the photodetector is @I@_R, and the root mean square of this intensity fluctuation at the magneto-optical signal observation frequency is ΔI^2_R, ξ =ΔI^2_R
/@I@_R, the light intensity of the incident light beam on the recording medium is I_O, the amplitude reflectance of the recording medium is R, and the light utilization efficiency of the optical system from the recording medium to the photodetector excluding the analyzing means is ε , the photoelectric conversion efficiency of the photodetector is K, and the amount of charge is e.
, where T is the thermal noise of the amplification means at the magneto-optical signal observation frequency, ΔB is the bandwidth of the detection signal, t_A is the amplitude transmittance of the analysis means, and η_A is the extinction ratio of the analysis means. The angle θ_A between the optical axis of the analyzer and the above-mentioned predetermined direction is under the following conditions, ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ However, FR = ξ・(Kε | t_A | ＾ 2 | R | ＾ 2 I_O) ^
2・ΔBS=zeκε｜t_A｜＾2｜R｜＾2I_O
- A magneto-optical information reproducing device characterized by satisfying ΔB.