JPH07101524B2 - Magneto-optical information reproducing device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a magneto-optical information reproducing apparatus for reproducing information magnetically recorded on a recording medium by utilizing a magneto-optical effect.

[Prior art]

2. Description of the Related Art In recent years, an optical memory for recording / reproducing with a semiconductor laser beam has been actively researched and developed for practical use as an optical density recording memory. Among them, magneto-optical disks that can be erased and rewritten are particularly promising, as well as read-only optical disks and DRAW-type optical disks, which are typified by already commercialized compact disks. The magneto-optical disk magnetically records information by utilizing the local temperature rise of a magnetic thin film due to laser spot irradiation, and reproduces information by a magneto-optical effect (especially Kerr effect). Here, the Kerr effect refers to a phenomenon in which a plane of polarization rotates when light is reflected by a magnetic recording medium.

FIG. 9 shows the basic structure of a conventional magneto-optical disk device. In FIG. 9, 1 is a semiconductor laser, 2 is a collimator lens, 11 is a half mirror, 4 is an objective lens, 6
Is a magneto-optical recording medium, 7 is an analyzer, 8 is a condenser lens, and 9 is a photodetector. The P polarization direction is parallel to the paper surface and the S polarization direction is perpendicular.

Next, a case of reproducing magneto-optical information in the above device will be described. The light beam emitted from the semiconductor laser 1 as linearly polarized light in the P polarization direction is collimated by the collimator lens 2 and passes through the half mirror 11. Assuming that the P-polarized component amplitude transmittance is t _P and the S-polarized component amplitude transmittance is t _S , then | t _P | ² = | t _S | ² = 0.5 in 11. The luminous flux is
An image is formed as a minute spot on the magneto-optical recording medium 6 by the objective lens 4. When magnetic domains (pits) are formed on the medium 6 in advance, the reflected light from the medium 6 is directed in the magnetization direction (upward or downward) of the spot irradiation region by the Kerr effect as shown in FIG. Depending on each ±
It undergoes rotation of the polarization plane of θ _K. Here, if the P-polarized component of the amplitude reflectance of the recording medium 6 is R and the S-polarized component is K,
The following equation holds.

The reflected light subjected to the magneto-optical modulation is made into a parallel light flux again by the objective lens 4, reflected by the half mirror 11, and then converted into a light flux whose intensity is modulated by the analyzer 7. That is, the reflected light beam in FIG. 10, since the light detecting a orthogonal projection to the analyzer optical axis of the amplitude, the incident light intensity on the magneto-optical medium 1 _0, from P polarization direction of the optical axis of the analyzer Let θ _A be the angle of
Depending on the Kerr rotation angle ± theta _K, the intensity of the transmitted light beam of the analyzer _{I + θ K, I - θ} K are each expressed as equation (2).

Since θ _K ≈1 °, | R | ² >> | K | ² holds,
Equation (2) is Express. The second term in the parentheses of the equation (3) is the magneto-optical modulation component, and the first term is the non-modulation component. The luminous flux converted into the intensity modulation in this way passes through the condenser lens 8 and is detected by the photodetector 9 as a magneto-optical signal.

However, such an optical system using the conventional half mirror 11 having no polarization characteristic has the following drawbacks.

1) The Kerr rotation angle θ _K is about 1 °, and the magneto-optical modulation component due to this is a very small amount. Therefore, the light amount of the magneto-optical modulation component passes through a half mirror having no polarization characteristic. More than half is lost and the C / N (carrier to noise ratio) of the detected signal is reduced.

2) Since the C / N is low, the conventional device must perform detection using a complicated detection system for detecting the magneto-optical signal, such as a differential detection or a photodetector having an amplifying action (avalanche photodiode, etc.). It is disadvantageous in terms of cost and reliability.

[Outline of Invention]

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

The above object of the present invention is to provide a magneto-optical information reproducing device with means for irradiating a recording medium on which information is magnetically recorded with a light beam polarized in a predetermined direction, and a polarization state according to the information by a magneto-optical effect. A polarized beam splitter that reflects or transmits a reflected or transmitted light beam from the recording medium that has been modulated to a predetermined ratio according to its polarization component, and an amplifying action for photoelectrically detecting the light beam from the polarized beam splitter. A photodetector without a detector and an amplifying means for amplifying a detection signal of the photodetector and reproducing the information, so that the polarized light reflectance-transmittance characteristic of the polarized beam splitter satisfies the following expression. It is achieved by setting to. That is, when the photodetector detects the reflected light of the polarized beam splitter, Also, when the photodetector detects the transmitted light of the polarized beam splitter, Is. However, here, the amplitude reflectance and the amplitude transmittance for the polarization component in the predetermined direction of the polarization beam splitter are respectively r _P and t _S , and the amplitude reflectance and the amplitude transmittance for the polarization component in the predetermined direction and the vertical direction are Each of r _S and t _{P is} the average of the polarization component intensities that are not modulated by the magneto-optical effect incident on the photodetector, and I _{R is} the mean square of this intensity fluctuation at the magneto-optical signal observation frequency. The light quantity of the incident light flux on the recording medium is I ₀ , the amplitude reflectance of the recording medium is R, the light utilization efficiency of the optical system from the recording medium excluding the polarization beam splitter to the photodetector is ε, and the light detection is performed. The photoelectric conversion efficiency of the device is κ, the thermal noise of the amplification means at the magneto-optical signal observation frequency is T, and the bandwidth of the detection signal is ΔB.

〔Example〕

 Hereinafter, a detailed description will be given with reference to the drawings of the present invention.

1 and 2 show a first embodiment of a magneto-optical information reproducing apparatus according to the present invention. FIG. 1 is a schematic configuration diagram of an optical system, and FIG. 2 is a schematic configuration diagram of a signal processing circuit. is there. In FIG. 1, 21 is a semiconductor laser, 22 is a collimator lens,
12 is a polarized beam splitter, 24 is an objective lens, 26 is a magneto-optical recording medium, 27 is an analyzer, 28 is a condenser lens, 29 is a photodetector with no amplification function such as a pin photodiode, and the P polarization direction is It is parallel to the paper surface and the S polarization direction is vertical. Reference numeral 13 denotes a light beam that has passed through the analyzer 27, and this detected light beam 13 is the second light beam.
As shown in the figure, it is photoelectrically converted by the photodetector 29, voltage-amplified by the amplifier 15 including the load resistor 16, and output from the terminal 14 as a reproduction signal.

In the above device, the semiconductor laser 21 emits a P-polarized light flux. This emitted light beam is collimated by the collimator lens 22, passes through the polarized beam splitter, and then passes through the objective lens 24.
The light is irradiated onto the recording medium 26 as an optical spot having an intensity I _O. Then, the light flux reflected by the recording medium 26 is modulated into a polarized state in accordance with the information magnetically recorded on the recording medium 26, passes through the objective lens 24 again, and is reflected by the polarized beam splitter 12. Guided by analyzer 27. The detection light 13 that has passed through the analyzer 27 is intensity-modulated and is received by the photodetector 29 via the condenser lens 28. Here, the amplitude transmittances of the P-polarized light and the S-polarized light of the polarized beam splitter are respectively t _P , t _S ,
Assuming that the amplitude reflectances are r _P and r _S , the intensity of the detection light 13 can be expressed by the following equation (4).

Considering that | R | ² >> | K | ² , equation (4) is Express.

In the equation (5), the second term in the parentheses is the magneto-optical modulation component, the first term is the modulation component, and the respective intensities are I _K and I _R.

I _R ≈I _O | R | ² | r _P | cos ³ θ _A (7) Note that the incident light I ₀ has a predetermined light intensity regardless of the amplitude transmittances t _P and t _s of the polarized beam splitter. The output of the semiconductor laser shall be adjusted.

The light beam thus intensity-modulated is converted into a photocurrent by the photodetector 29 shown in FIG. The photoelectric conversion efficiency κ is the charge amount of e, the Planck constant of h, the quantum efficiency of the photodetector, ν
Is given by the following equation, where is the frequency of the luminous flux.

Here, the following four types of noise can be considered as noise sources in signal reading.

1) the mean square of the non-modulated component light I _R is due to the fluctuation △ I ² _R.

2) noise due to the square mean intensity fluctuation △ I ² _K of the modulated component light I _K.

3) Shot noise of the photodetector.

4) Thermal noise due to the amplifier.

The noise caused by ΔI ² _{R in} 1) and the noise caused by ΔI ² _{K in} 2) are caused by the surface roughness and inhomogeneity of the recording medium, the fluctuation of the semiconductor laser intensity, etc. If the constants to be determined are ξ and ζ, and the averages of the effective values of the non-modulated component and the modulated component are _R and _K , respectively, the following equation holds.

However, ΔB is the bandwidth of the detection signal.

If the noise due to ΔI ² _R , the noise due to ΔI ² _K , the shot noise, and the thermal noise are F _R , F _K , S, and T, they can be expressed by the following equations.

S = 2eκ _R △ B (13) Here, k is the Boltzmann constant, Te is the equivalent noise temperature, and Rf is the resistance value of the load resistor 16.

If these are used to express C / N in decibels, the following equation is obtained.

C / N in equation (15) is the amplitude reflectance r of the polarized beam splitter 12.
Since it is a function of _P , r _s , (15) can be partially differentiated with | r _P |, | r _S | to obtain the maximum value. The extreme value for | r _P | ² is as follows.

The extreme values of 0 | r _P | ² 1 (21) | r _S | ² are as follows.

∂ (C / N) / ∂ (| r _s | ² )> 0 and | r _S | ² = 1 (18) That is, it has polarization characteristics that satisfy (16), (17), and (18). The C / N can be maximized by using the polarized beam splitter described above.

FIG. 3 is a schematic configuration diagram of an optical system showing a second embodiment of the present invention. This embodiment is a modification of the above-described one embodiment so as to detect the transmitted light flux of the polarized beam splitter 12. In FIG. 3, the same members as those in FIG. Detailed description is omitted. Further, the same signal processing circuit as that shown in FIG. 2 can be used.

In the case of this embodiment, the polarization direction of the semiconductor laser 21 is the S polarization direction perpendicular to the paper surface, and P, which is used in the first description,
It may be considered by replacing the S polarization directions with each other. However,
In the expressions (4) to (7), it is necessary to replace r _P with t _S and r _S with t _P. That is, I _R ≈I _O | R | ² | t _S | ² cos θ _A (20) C / N in Eq. (15) is the amplitude transmittance t _S , of the polarized beam splitter.
Since it is a function of t _P , (15) can be partially differentiated by | t _S | ² and | t _P | ² to find the maximum value. The extreme value of | t _S | ^{2 is} For 0 | t _S | ² 1 (22) | t _p | ² , the extreme values are as follows.

∂ (C / N) / ∂ (| t _p | ² )> 0 | t _p | ² = 1 (23) (21), (22), (23) C / N can be maximized by using a printer.

FIG. 4 is a schematic diagram showing a third embodiment of the present invention. 4, the same members as those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted. Also in this embodiment, the signal processing system after the photodetector 29 is constructed as shown in FIG.

In this embodiment, a polarized beam splitter 23 having a beam shaping function is used instead of the polarized beam splitter 12 of the first embodiment. As a result, the light flux of the semiconductor laser 21 having an elliptical far-field pattern can be efficiently formed as a circular spot on the recording medium 26. Also,
The surface a is inclined by a predetermined angle so that stray light does not enter the photodetector 29.

A groove (not shown) for tracking is formed on the recording medium 26 in the direction perpendicular to the paper surface, and the light beam focused on the recording medium 26 by the objective lens 24 is diffracted by this groove.
Reference numeral 25 is a photodetector for detecting the imbalance of ± first-order diffracted light caused by the track shift, and the objective lens
It is fixed to the periphery of 24 openings. Therefore, the objective lens 24
Has the advantage of not causing offsets in the tracking error signal, even if it moves in the direction perpendicular to the track groove.

The photodetector 29 is a photodetector having no amplifying action, such as a Si-pin photodiode, and 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 description thereof will be omitted.

In the explanation of FIG. 1, it is assumed that the signal level reduction does not occur due to the recording medium and the optical system, but in the actual optical system, C / N
Must be taken into account in accurately predicting.
The following two points can be considered as the causes of the signal level reduction.

1) Loss of light amount (fixed value of amplitude due to absorption or vignetting) 2) Phase difference between PS polarization phase difference 1) and 2) contribute to the reduction of the intensity of the magneto-optical modulation component,
Only 1) contributes to the reduction of the intensity of the non-modulation component. In order to evaluate the decrease in the intensity of the magneto-optical modulation component (loss of light quantity), the light use efficiency ε _R is defined. Note 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 the light utilization efficiency. In this example, ε _R
The following points were considered when calculating

1) Tracking groove (Pitch 1.6 μm, depth λ / 8, λ = 83
The ratio of the diffracted light from 5 nm) entering the entrance pupil of the objective lens is defined as the light utilization efficiency ε ₀ .

2) Consider the product along the square optical path of the P polarization direction amplitude transmittance (or reflectance) of the n optical elements excluding the polarization beam splitter in the optical path from the recording medium to the photodetector, The light utilization efficiency is ε ₁ . If the amplitude transmittance of the i-th optical element is t _Pi and the reflectance is r _pi , ε ₁ can be expressed by the following equation, When the light flux is reflected by the i-th optical element in the equation (24), | r _pi | ² may be substituted instead of | t _Pi | ² .
The polarization characteristic | r _P | ² of the polarized beam splitter is treated as a change amount in the C / N calculation and is therefore excluded from ε ₁ .

3) Consider the light utilization efficiency ε ₂ depending on the angle θ _A from the P polarization direction of the optical axis of the analyzer. Assuming that the extinction ratio of the analyzer is η _A , cos θ _A in equation (4) is Since sin θ _A can be considered by replacing sin θ _A + η _A cos θ _A , ε ₂ is represented by the following equation as | R | ² >> | K | ² .

ε ₂ = cos ² θ _A + η _A sin ² θ _A (25) From 1) to 3), the light utilization efficiency of the magneto-optical non-modulated component ε
_R is given by the following equation.

ε _R = ε ₀ ε ₁ ε ₂ (26) Next, consider a decrease in the intensity of the magneto-optical modulation component. For that purpose, it is necessary to consider the phase difference between the P and S polarized lights in addition to the loss of light quantity. For example, as shown in FIG. 5, the reflected light from the recording medium is generally not the linearly polarized light as shown in FIG. 10, but the phase difference generated between the P-polarized component and the S-polarized component causes It is known that the major axis becomes elliptically polarized light whose Kerr rotation angle θ _{K is} tilted. That is, the P and S polarized components of the amplitude reflectance of the recording medium, and R and K are expressed as in equation (27).

However, α ₀ and β ₀ are the phase components of each amplitude reflectance.

In this case, the car rotation angle θ _K is Express. If Δ ₀ = nπ (n is an integer), the reflected light from the recording medium becomes linearly polarized light, but in other cases, θk is decreased, which is not preferable.

The same applies to the optical element. In this example, in order to evaluate the intensity decrease of the magneto-optical modulation component, the light utilization efficiency ε _K was defined and the following points were taken into consideration when obtaining ε _K. 1) For the magneto-optical modulation component, the P, S polarization direction amplitude transmittance (or reflectance) of the n optical elements excluding the polarization beam splitter in the optical path from the recording medium to the photodetector Considering the product along the optical path, the light utilization efficiency is ε ₃ .
The amplitude transmittances of the i-th optical element in the P and S polarization directions are t _Pi and t, respectively.
_{If Si} (r _Pi , r _{Si for} reflectivity), the following equation holds.

Express ε ₃ as in the following equation using equation (29).

In the formula (30), when the light beam is reflected by the i-th optical element, | r _Pi |, | r _Si | may be substituted instead of | t _Pi |, | t _Si |. Characteristics of polarized beam splitter | r _Pi
Since |, | r _Si | is treated as a variation in C / N calculation, it is excluded from ε ₃ .

For the polarized beam splitter, if the amplitude reflectances in the P and S polarization directions are r _P and r _S respectively, Can be expressed as However, γ and δ are phase components of each amplitude reflectance.

2) Consider the light utilization efficiency ε ₄ depending on the angle θ _A from the P polarization direction of the optical axis of the analyzer. If the extinction ratio of the analyzer is η _A , then cos θ A in equation (4) is Since sin θ _A can be considered by replacing sin θ _A + η _A cos θ _A , ε ₄ is represented by the following equation with | R | ² >> | K | ² .

From ε ₄ = (1-η _A ) sin2θ _A (32) 1), 2), the light utilization efficiency ε _K of the magneto-optical modulation component is expressed by the following equation.

ε _K = ε ₀ ε ₃ ε ₄ (33) From the above, the intensities of the magneto-optical modulation component and the non-modulation component are I _K and I, respectively.
_{If R} , I _R ≈I _O ε ₀ ε ₁ · | r _P | ² | R | ² (cos ² θ _A + η _A sin ² θ _A ) ＝ I _O ε _R | r _P | ² | R | ² (35) Be done.

Substituting equations (34) and (35) into equation (15), the polarization characteristics of the polarized beam splitter that maximizes C / N are obtained as follows.

0 | r _P | ² 1 (37) | r _S | ² = 1 (38) 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 amount I ₀ = 2 × 10 ⁻³ W on the recording medium 26 regardless of the polarized beam splitter transmittance | t _P | ^2. ing.

GdTbFeCo is used for the recording medium 26, and | R | ² = 0.12, θ _K
The phase difference Δ0 between the phase components α0 and β0 of the amplitude reflectance of 0.74 ° P, S polarization direction is Δ0 = 20 °.

Light utilization efficiency ε ₀ is a groove for tracking (Pitch 1.6 μm,
When the diffracted light from the depth λ / 8) is received by the objective lens with NA = 0.5, ε ₀ = 0.6. The light utilization efficiency ε ₁ is ε ₁ = 0.66 considering the product of the transmittances of the optical elements other than the polarization beam splitter in the optical path from the recording medium to the photodetector.

Regarding the light utilization efficiencies ε ₂ and ε ₄ , C / N was calculated for θ _A = 45 ° and 60 °. Further, the extinction ratio η _A = 1 × 10 ^-3 .

The light utilization efficiency ε ₃ may be considered as the product of the P and S amplitude transmissivities of the optical elements excluding the polarization beam splitter and the analyzer in the optical path from the recording medium to the photodetector. In this embodiment, since there is no optical element that gives a phase difference of PS polarized light at the time of transmission, Since | t _Pi | = | t _Si |, ε ₃ = 0.66.

The photodetector 25 is a Si-pin photodiode having a photoelectric conversion efficiency κ = 0.54. The constants ξ and ζ determined by the noise source such as the recording medium and the semiconductor laser are given as follows.

ξ = 2 × 10 ^-13 (RIN) ζ = 1 × 10 ^-11 (RIN) Further, the thermal noise T is the Boltzmann constant k = 1.38 × 10 ^-23 ,
Equivalent noise temperature Te = 300 [k], load resistance R _f = 1 × 10
Assuming that ⁴ [Ω] and signal detection bandwidth ΔB = 3 × 10 ⁴ [1 / Hz], T = 5 × 10 ⁻²⁰ . Note that the thermal noise T may not be described in a simple form like the formula (14) due to the capacity of the photodetector, and in such a case, it is not necessary to comply with this.

6 and 7 show the case of using a polarization beam splitter having the polarization characteristics given by the formulas (36) to (38) (shown by a solid line) and the case of using a half mirror (a chain line). It is a comparison of C / N. Figure 6 shows θ _A
= 45 °, Fig. 7 shows θ _A = 60 °,
The vertical axis represents C / N, and the horizontal axis represents the P-polarization direction reflectance | r _P | ² of the polarized beam splitter. In both cases, the reflectance in the S-polarized direction of the polarized beam splitter is | r _S | ² = 1. Here, the half mirror has a characteristic of | r _P | ² = | r _S | ² = 0.5.

When θ _A = 45 °, | r _P | ² = 0.16 and C / N becomes maximum,
Compared with the case of the conventional device using the half mirror, 4.5dBC / N is improved in this embodiment. | r _P | ² = 0.08 to 0.
If a polarized beam splitter with 4, | r _S | ² = 1 is used, a sufficiently good C / N can be obtained with respect to the conventional half mirror.

When θ _A = 60 °, | r _P | ² = 0.32 and C / N becomes maximum,
Compared to the case of the conventional device using the half mirror, 3dBC / N is improved in this embodiment. | r _P | ² = 0.2 to 0.5, | r
_If a polarized beam splitter with _S | ² = 1 is used, a sufficiently good C / N can be obtained with respect to the conventional half mirror.

In this embodiment, P-S polarizations phase difference caused by the polarization beam splitter △ _PBS is in any case △ _PBS = 16
0 has become °, between the phase difference △ ₀ occurring in the recording _{medium, △ 0 + △ PBS = π} (39) becomes relevant. This prevents the intensity of the magneto-optical modulation component from decreasing. It is easy to manufacture a polarized beam splitter having such a polarization characteristic.

8 (A) and 8 (B) are schematic views showing a fourth embodiment of the present invention, respectively, and FIG. 8 (B) is a view of FIG. 8 (A) as seen from the direction of arrow A. 8 (A) and 8 (B), the same members as those in FIG. 4 are designated by the same reference numerals, and detailed description thereof will be omitted. Also in this embodiment, the signal processing system after the photodetector 29 is configured as shown in the second diagram. In the present embodiment, a polarized beam splitter 10 is used in place of the polarized beam splitter 23 of the third embodiment, and the transmitted light of the polarized beam splitter 10 is detected. The surface b of the polarized beam splitter 10 is inclined by a predetermined angle so that stray light does not enter the photodetector 29.

In the present embodiment, the P and S polarization directions used in the description of FIG. 4 may be replaced with each other. However, (34) (3
In equation (5), it is necessary to replace r _P with t _S and r _S with t _P. That is, each of the magneto-optical modulation component and the non-modulation component
If I _K , I _R I _R ≈I _O ε ₀ ε ₁ | t _S | ² | R | ² (cos ² θ _A + η _A sin ² θ _A ) (41)

Substituting Eqs. (40) and (41) into Eq. (15), the polarization characteristics of the polarized beam splitter that maximizes C / N are obtained. 0 | t _S | ² 1 (43) | t _P | ² = 1 (44) If the calculation conditions are the same, FIG.
Results similar to those shown in FIG. 7 are obtained.

However, the horizontal axis is | t _S |.

It should be noted that it is easy to manufacture a polarization beam splitter having polarization characteristics that compensate for the P-S polarization phase difference that occurs in the recording medium.

The present invention can be applied in various ways other than the embodiments described above. For example, although the reflected light of the magneto-optical recording medium is detected in the embodiment, it may be configured to detect the light flux which is transmitted through the magneto-optical recording medium and is modulated by the Faraday effect.

〔The invention's effect〕

As described above, the present invention has the effect of improving the C / N of signal detection by using the polarization beam splitter having the optimum polarization characteristics in the conventional magneto-optical information reproducing apparatus. Furthermore, since a high C / N can be obtained by the present invention, a complicated detection system like the conventional device is unnecessary,
The reliability of the device can be improved and the manufacturing cost can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing an optical system according to an embodiment of the present invention, and FIG.
FIG. 3 is a schematic diagram showing the signal processing system of the embodiment shown in FIG. 1, and FIG.
FIG. 4 and FIG. 4 are schematic views showing another embodiment of the present invention, respectively.
FIG. 5 is a diagram showing the polarization state of the reflected light from the magneto-optical recording medium, and FIGS. 6 and 7 are diagrams showing the relationship between the polarization characteristics and the C / N of the polarization beam splitter according to the present invention. 8A and 8B are schematic views showing still another embodiment of the present invention, FIG. 9 is a schematic view showing an example of a conventional magneto-optical information reproducing apparatus, and FIG. 10 is a general optical system. It is a figure which shows the principle of magnetic signal detection. 12 …… Polarized beam splitter, 13 …… Detection light, 21 …… Semiconductor laser, 22 …… Collimator lens, 24 …… Objective lens, 26 …… Optical recording medium, 27 …… Analyzer, 28 …… Collection Optical lens, 29 ... Photodetector.

Claims (2)

[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 a recording medium which is modulated in a polarization state according to the information by a magneto-optical effect. A reflected or transmitted light beam, a polarization beam splitter that reflects and transmits the light beam at a predetermined ratio according to its polarization component, a photodetector having no amplification effect that photoelectrically detects the light beam reflected by the polarization beam splitter, and the light Amplification means for amplifying the detection signal of the detector and reproducing the information, and the average of the polarization component intensities that are not modulated by the magneto-optical effect incident on the photodetector is _{R 1} , and this intensity at the magneto-optical signal observation frequency The root mean square of the fluctuation The light quantity of the incident light flux on the recording medium is I ₀ , the amplitude reflectance of the recording medium is R, the light utilization efficiency of the optical system from the recording medium excluding the polarization beam splitter to the photodetector is ε, and the photodetector is , K, the thermal noise of the amplification means at the magneto-optical signal observation frequency is T, and the bandwidth of the detection signal is ΔB, the amplitude reflectance r _{P of the} polarization beam splitter with respect to the polarization component in the predetermined direction. And the amplitude reflectance r for the polarization component in the direction perpendicular to the predetermined direction
_S is the following conditions, A magneto-optical information reproducing device characterized by satisfying the requirements. 2. A means for irradiating a recording medium on which information is magnetically recorded with a light beam polarized in a predetermined direction, and the recording medium modulated to a polarization state according to the information by a magneto-optical effect. A polarized beam splitter that reflects and transmits the reflected or transmitted luminous flux at a predetermined ratio according to its polarization component, a photodetector without amplification effect that photoelectrically detects the luminous flux transmitted through the polarized beam splitter, and the photodetection Amplification means for amplifying the detection signal of the optical detector and reproducing the information, and the average of the polarization component intensities which are not modulated by the magneto-optical effect incident on the photodetector is _{R 1} , and the intensity of this intensity at the magneto-optical signal observation frequency The root mean square of the fluctuation The light quantity of the incident light flux on the recording medium is I ₀ , the amplitude reflectance of the recording medium is R, the light utilization efficiency of the optical system from the recording medium excluding the polarization beam splitter to the photodetector is ε, and the photodetector is Where ω is the photoelectric conversion efficiency, T is the thermal noise of the amplifying means at the observation frequency of the magneto-optical signal, and ΔB is the bandwidth of the detection signal, the amplitude transmittance t _{S of the} polarization beam splitter with respect to the polarization component in the predetermined direction. And the amplitude transmittance t _P for the polarized component in the direction perpendicular to the predetermined direction
Are the following conditions, A magneto-optical information reproducing device characterized by satisfying the requirements.