JPH0237612B2 - KIROKUPATAAN YOMITORIKEI - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a system for reading magnetically recorded information using magneto-optic effects. Conventionally, a method of optically reading out magnetically recorded information using the magneto-optical Kerr effect has been known, and in particular, a method of reading recorded patterns using the polar Kerr effect from a perpendicular magnetic recording medium is known. Widely used. The optical system shown in FIG. 1A is used for optical observation and electrical detection of such recorded patterns. In FIG. 1A, 2 is a polarizing plate, 3 is a semi-transparent mirror, 4 is an objective lens, 5 is a perpendicular magnetic recording medium, 6 is an analyzer, 7 is an eyepiece lens, 8 is a photoelectric detector or a magnetic pattern It is an observation surface. The light beam 1 is linearly polarized by the polarizing plate 2 and enters the perpendicular magnetic recording medium 5 . Here, the semi-transparent mirror used in the conventional method has a transmittance and reflectance of approximately 50% regardless of the polarization direction. Corresponding to the magnetization direction (upward or downward) of the perpendicular magnetic recording body 5, the polarization planes of the light beam are rotated in opposite directions due to the Kerr effect and reflected. For example, if the plane of polarization of the light beam reflected by the downwardly magnetized portion is rotated by θ _K , the plane of polarization of the beam reflected by the upwardly magnetized portion is rotated by −θ _K . When the incident light flux is P-polarized light as shown in FIG _.
When placed in the perpendicular direction (Q direction), the reflected light from the upward magnetization direction part is blocked by the analyzer 6, and the transmitted component Δ of the reflected light from the downward magnetization direction part passes through the analyzer 6. pass. This phenomenon allows perpendicular magnetization patterns to be detected or observed. However, in a conventional optical system using a semi-transparent mirror that does not depend on the polarization direction, the light beam passes through the semi-transparent mirror twice, causing the amplitude to drop to 1/4. Furthermore, the polarization rotation angle θ _K due to the Kerr effect is generally approximately 1° or less,
Considering that the Kerr rotation modulation component obtained by passing through the analyzer 6 is extremely small, the loss of light amount due to the semi-transparent mirror portion reduces the detection sensitivity of the detection signal. Therefore, the conventional method has the following drawbacks. 1 Passing through a semi-transparent mirror that does not have polarization characteristics causes more than half of the light intensity of the modulation component to be lost, resulting in poor utilization efficiency of the detection signal light.
The signal-to-noise ratio decreases. 2. Because the signal-to-noise ratio is low, a complex detection processing system is required to detect the signal, which is undesirable in terms of cost and reliability. The present invention aims to solve the above-mentioned drawbacks of the conventional reading system, improve the effectiveness of the use of detection signal light, reduce the influence of noise, and provide a recorded pattern reading system that can read with a high signal-to-noise ratio. purpose. The above object of the present invention is to provide a light source that irradiates a magnetic recording medium with a light beam polarized in a predetermined direction, and a detector that converts a light beam whose polarization state is modulated according to a recording pattern of the recording medium into an intensity-modulated light beam. In a recording pattern reading system comprising a photodetector having a multiplication effect for photoelectrically detecting photons and a luminous flux converted by the analyzer, and an amplifier including a load resistor for amplifying the signal detected by the photodetector. , the polarization component intensity in the predetermined direction of the light beam received by the photodetector is I _R ,
Similarly, the intensity of the polarized light component in the direction perpendicular to it is _IK , the multiplication factor of the photodetector is G, the multiplication noise figure of the photodetector is x, the amount of charge is e, Planck's constant is h, Boltzmann's constant is k, and the light When the quantum efficiency of the detector is ε, the resistance value of the load resistor is R _L , the equivalent noise temperature is T _e , the bandwidth of the detection signal is ΔB, and the frequency of the light beam is ν, the output signal S of the amplifier is /N ratio S is calculated using the following formula: S=10 log ₁₀ {e ² ε ² /2h ² ν ² G ² I _K ² / (2e ² ε/hνG ^2+x I _R +4kT _e /R _L )ΔB} This is achieved by providing in the optical path of the modulated light beam an optical element that adjusts the ratio of I _K and I _R so as to maximize the value of S. Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows an optical system according to an embodiment of the present invention. Here, a case will be considered in which the polarization direction of the incident light beam a is set to a P polarization state parallel to the paper surface by the polarizer 9. The optical element 10 used in this example has a property that the transmittance t and the reflectance r differ depending on the polarization direction, unlike a conventional semi-transparent mirror that does not have polarization characteristics. That is, the amplitude transmittance t _p of the P polarized light component of the optical element 10,
The optical element generally has an amplitude reflectance r _p , an amplitude transmittance t _s of the S-polarized component (polarized component perpendicular to the plane of the paper in FIG. 2), and an amplitude reflectance r _s as |t _p |≠=|t _s
It is a semi-transparent mirror with the characteristics |, |r _p |≠|r _s |. The polarization characteristics of the optical element 10, perpendicular magnetic recording medium 11 and analyzer 12 in FIG.
Expressed using Jones matrix, it is as follows. First, the transmission Jones matrix and the reflection Jones matrix of the optical element 10 are as follows: = t _p , o o, t _s , = r _p , o o, r _s (1). Regarding the perpendicular magnetic recording medium 11, if the medium amplitude reflectance at perpendicular incidence is R, and the Kerr rotational amplitude reflectance due to the magneto-optic Kerr effect is K, then the medium
Jones matrix 〓 is expressed as 〓=-R, K K, R (2). Regarding the analyzer 12, when the analyzer transmission axis A is tilted in the S polarization direction by an angle θ with respect to the P polarization direction as shown in FIG. 3, and the extinction rate is η, the transmission Jones matrix is becomes. Therefore, in FIG. 2, if the Jones vector representing the polarization state of the detection light f transmitted through the analyzer 12 is D, and the Jones vector of the light beam b is E, it can be expressed as 〓=〓〓〓〓〓 (4). Assuming that the luminous flux b is linearly polarized light in the P polarization direction as described above and its amplitude is V ₀ , the intensity I of the detection light f transmitted through the analyzer 12 is (1), (2),
Using equations (3) and (4), I|V ₀ | ² |t _p | ² [|R| ² |r _p | ² (cos ² θ+ηsin ²
θ)−｜R｜｜K｜｜r _p ｜｜r _s ｜(1−η)cosδsin2
θ〕 (5) Here, R=｜R｜e ⁱ 〓, K=｜K｜e ⁱ 〓, r _p =｜r _p
|e ⁱ 〓 ^p , r _s = | r _s | e ⁱ 〓 ^s , δ=α−β+γ _p −γ _s |
The term K| ² was omitted as a second-order minute amount. The first term on the right side of the above equation (5) is the direct current (DC) component of the detection light, and indicates the non-modulated component light of the incident light beam, that is, the component intensity _{IR in} the polarization direction of the incident light beam. The second term is a modulation (AC) component by the perpendicular magnetic recording medium 11, and means the intensity I _K of the Kerr rotation modulation component light, that is, the component perpendicular to the polarization direction of the incident light beam. From equation (5), when the azimuth angle θ of the analyzer 12 is set to 45°,
_IK becomes maximum value. Further, the extinction rate η of the analyzer 12 is generally η10 ^-2 , and at θ45°, the influence of incomplete extinction of the analyzer 12 on both I _R and I _K is 1% or less. Now, assuming that θ=45° and the absorption of the optical element 10 can be ignored, |t _p | ² = 1− | r _p | ² and from equation (5), I _R 1/2 | V ₀ | ² | R | ² | r _p | ² (1- | r _p | ² ) (6) I _K | V ₀ | ² | R | | K | | | r _p | | r _s | (1-
|r _p | ² ) cosδ(7). The light that has passed through the analyzer in Figure 2 is photoelectrically detected by a photodetector 14 with a multiplier effect, such as a Si-avalanche photodiode (APD), as shown in the detection light 13 in Figure 4, and a photocurrent is generated. After being converted into , the modulated component is read out as a voltage amplified electrical signal by an amplifier 16 including a load resistor 15 . If we consider that the noise sources in signal readout are mainly shot noise from the light receiving element and thermal noise from the amplifier, the SN ratio S is expressed in decibels as follows. S=10 log ₁₀ {e ² ε ² /2h ² ν ² G ² I _K ² /(2e ² ε
/hνG ^2+x I _R +4kT _e /R _L )ΔB}(8) Here, I _R is the non-modulated component light intensity, I _K is the Kerr modulated component light intensity, G is the multiplication factor of the photodetector 14,
e is the amount of charge, h is a blank constant, k is Boltzmann's constant, ε is the quantum efficiency of the photodetector 14, R _L is the resistance value of the load resistor 15, T _e is the equivalent noise temperature, ΔB is the bandwidth of the detection signal, ν is the frequency of the luminous flux. In addition, x is a multiplication noise parameter representing the inherent shot noise accompanying multiplication of the photodetector, for example, Si-
For APD, x=0.3-0.4. The optical element 10 of this embodiment has amplitude transmittances t _p and t _s and amplitude reflectance r _p that maximize the values shown by S in equations (6) and (7) to (8). , r _s . Therefore, in this embodiment, an optical element having an intensity transmittance and an intensity reflectance depending on the polarization direction of the modulated light beam is provided in the optical path of the light beam modulated according to the recording pattern. This maximizes the SN ratio in reading. Furthermore, in the case described above, since x≠o, an optimal multiplication factor Gopt exists. The value is obtained by partially differentiating equation (8) with respect to G: C ₁ =e ² ε ² /2h ² ν ² ,
As C ₂ = 2e ² ε/hν, C ₃ = 4kT _e /R _L , Gopt={4c ₃ /xc ₂ |V ₀ | ² (1−|r _p | ² ) | r _p | ² |
R｜ ² } ^1/(x+2) (9) Substituting this Gopt, equations (6), and equations (7) into equation (8), the SN ratio S' at the optimal multiplication factor Gopt is: becomes. Furthermore, optical element 10 that maximizes S′ in equation (10)
The P-polarized light intensity reflectance |r _p | ² is given by partial differentiation of equation (10) with |r _p | ² as follows: |r _p | ² =x/(2+3x) (11). This result shows that the perpendicular magnetic recording medium 11
Since it does not depend on the R and K of the magnetic film parameters, it can be determined uniquely for the reading system. As mentioned above, x = 0.3 to 0.4 for Si-APD and x for Ge-APD, so depending on the type of light receiving element 13 used, the optical element 10 is in the range of |r _p | ² 0.1 to 0.2.
By giving polarization characteristics to , it is possible to maximize the SN ratio given by equation (10). As a specific example, when the light source is a semiconductor laser (wavelength λ = 850 nm) and the detector has the optimal multiplication factor Gopt, the above
Figure 5 shows the dependence of the SN ratio on |r _p | ² and |r _s | ² calculated using equation (10). Here ε=0.9, T _e =1200
〓, ΔB = 3 × 10 ⁴ Hz, R _L = 10 ⁴ Ω, |V ₀ | ² = 10 ^-4
and |R| ² and θ _K using MnBi as the recording medium,
Known literature (K. Egashira et al., J. Appl. Phys.
45, 3643 (1974)) into equation (10), the case of |R| ² = 0.57, θ _K = 0.7° is expressed as the fifth
Curves b and d in the figure show the case of |R| ² =0.10 and θ _K =3.6° as curves a and c. Furthermore, curves a and b shown by broken lines indicate the case where the intensity reflectance of the optical element 10 in FIG. 2 for the S-polarized light component |r _s | ² =0.9,
Curves c and d drawn as solid lines indicate the case where |r _s | ² =0.5. The vertical axis in FIG. 5 indicates the SN ratio with respect to the center frequency of the detection signal, expressed as a C/N ratio, and the phase difference γ _p -γ _s between orthogonal components is an integral multiple of π. As can be seen from Figure 5, when x = 0.4 in equation (11), the C/N ratio is maximum at the P-polarized light reflectance | r _p | ² = 0.125, and such an optimal reflectance is ,
It does not depend on the magnetic film parameters |R| ² and θ _K. or,
It can be seen that the larger |r _s | ² , the higher the C/N ratio can be achieved. In this way, |r _p | ² = |r _s | ² =
Based on this embodiment, for example, in FIG. 2, the S-polarization degree reflectance of the optical element 10 |
When r _s | ² = 0.9 and the P-polarized light intensity reflectance | r _p | ² is the optimal value obtained by substituting the multiplication noise factor x specific to the photodetector into equation (11), it is approximately 5dBC/N is increased. Furthermore, the optical element used in the above example has an intensity reflectance of 10 to 20% with respect to the polarization direction of the incident light beam, for example.
A polarizing beam splitter having a high intensity reflectance of nearly 100% in the polarization direction perpendicular to the polarization direction can be manufactured by a known method. Further, as mentioned above, the present invention can be applied to any device using a photodetector having a multiplication effect, such as Si-APD or Ge-APD. The above embodiments concerned the system shown in FIG. 6A. Next, for other embodiments of recorded pattern reading systems using optical elements 10, optimal conditions for the characteristics of each optical element 10 will be determined. As shown in FIG. 6B, for the optical element 10,
Consider the case where the light beam enters in the S polarization state. The amount of light passing through the analyzer 12 goes through the same derivation process as in the case of Fig. 6A, and is ^calculated as I _R 1/2 | V ₀ | ² | R | ² | _{r s} ^| 12) I _K | V ₀ | ² | R | | K | r _s | | r _p | (1− | r _p
| ² ) cosδ(13) is obtained. In equations (6) and (7), r _p is replaced by r _s and r _s
If we replace r with _p , both equations match exactly. The intensity reflectance |r _s | ² , |r _p | ² that gives I _R , I _K that maximizes the value of S in equation (8) above is determined from equations (12) and (13), and this An optical element 10 having an intensity reflectance such as
By using this, it is possible to realize a recorded pattern reading system with a high signal-to-noise ratio, similar to the embodiments described above. In this example, if the photodetector has an optimal multiplication factor, |r _p | ² is as large as possible, and |r _s | ² is set to |r _s | ² =x/(2+3x) (14) The maximum SN ratio can be obtained by setting the optimal value of 10 to 20%, which is obtained by substituting x unique to the detector. In the cases shown in FIGS. 6A and 6B, the light beam passes through the optical element, enters the optical element 10 again from the recording medium 11, is reflected, and becomes detected light by the analyzer 12. Next, in the case of the reverse of the above, that is, the case where the light beam is reflected by the optical element 10, further reflected by the recording medium 11, and then transmitted through the optical element 10, in the above example, Jones expressed by equation (4) The vector 〓 becomes 〓＝〓〓〓〓〓 (15). Therefore, as shown in FIG. 6C, when the light beam is incident on the optical element 10 in a P-polarized state,
By a similar idea, in equations (6) and (7), r _p is replaced by t _p
Then, by setting r _s to t _s , I _R 1/2 | V ₀ | ² | R | ² | t _p | ² (1− | t _p | ² ) (16) I _K | V ₀ | ² ｜R｜｜K｜｜t _p ｜｜t _s ｜(1−
|t _p | ² )cos δ(17) is obtained, and the polarized light intensity transmittance that maximizes S in equation (8) from equations (15) and (16) |t _s ² , |t _s | ² is an optical system. By using the element 10, the maximum SN ratio can be obtained. At this time, if the photodetector of this example has the optimum multiplication factor, the multiplication noise figure x of the photodetector can be used to calculate from |t _p | ² = x/(2+3x) (18) By using the optical element 10 having an optimum transmittance of 10 to 20%, the SN ratio can be maximized. As shown in FIG. 6D, when a light beam enters the optical element 10 in the S polarization state, the light intensity passing through the analyzer 12 is _IR 1/2 | V ₀ | ² | R | ² | t _s | ² (1- | t _s | ² ) (19) I _K | V ₀ | ² | R | | K | | t _p | | t _s | (1-
|t _s | ² ) can be derived as cosδ(20). _If r _p and r s in equations (8) and (7) are replaced by t _s and t _p , respectively, they match equations (19) and (20),
The signal-to-noise ratio can be maximized by using an optical element 10 having intensity transmittances |t _p | ² and |t _s | ² that maximize equation (8). In addition, if the photodetector has an optimal multiplication factor, the above intensity transmittance is determined by increasing |t _p | ² , |t _s | ² from the multiplication noise index x of the photodetector, and |t _s | The SN ratio can be maximized as described above by setting the optimal value of 0 to 20% determined according to ² = x/(2 + 3x). Even when the detection light from the optical element 10 is further divided by the optical element 10' as shown in FIGS. 7A and 7B,
The foregoing discussion is applicable. In the case of Figure 7A,
When P-polarized light is incident, the light intensity obtained as 〓a＝〓a〓′〓〓〓〓〓〓〓〓b＝〓b〓′〓〓〓〓〓〓〓〓〓〓〓〓b＝〓b〓′〓〓〓〓〓〓〓〓〓〓〓〓b＝〓b〓′〓〓〓〓〓〓〓 In equations (6) and (7), r _p is defined as r _p r _p ′
, replace r _s with r _s r _s ′ and θ with θa, analyzer 12
For the detected light from b, r _p is r _p t _p ′ and r _s is r _s
In _ts ', θ is replaced with θb, and as in the previous embodiment, the recorded pattern reading system is formed using the formula (8) or optical elements 10 and 10' having reflection and transmission characteristics that provide the maximum SN ratio. Can be configured. For this time-divided detection light, an optical element 10
and the transmission axis directions θa, θb of the analyzers 12a, 12b
By appropriately selecting , it is possible to change the relative values of each non-modulated component intensity and Kerr rotation modulated component light intensity. Generally, the arrangement as shown in Figure 7A is
Normally, each of the light beams divided by the optical element 10' is received by a photodetector, and electrical differential detection is performed. In this case, it is desirable that θa=-θb, and that the relative values of the non-modulated component light intensity and the Kerr rotation modulated component intensity of the divided light beams are equal to each other.
Therefore, in the case of such electrical differential detection, |r _s ′
It is preferable to use a non-polarizing semi-transparent mirror as the optical element 10', which has the properties |=|t _s ′|, |r p ′ _| =|t _p ′|. However, it goes without saying that a semi-transparent mirror having polarization characteristics in advance may be used in consideration of differential imperfections caused by the detection processing system. Also in the case of S-polarized light incidence, r _p ′ is r _s ′, r _s ′ is r _p ′,
Exactly the same argument can be made by replacing t _p ′ with t _s ′ and t _s ′ with t _p ′. FIG. 7B shows a case where the detection light is divided by the optical element 10' in the same optical system as in FIGS. 6C and 6D. At this time, in equations (6) and (7) for incident P-polarized light, t _p for the light passing through the analyzer 12a.
Replace t _p t _p ′, t _s with t _s t _s ′, θ with θa, analyzer 1
For the light passing through 2b, t _p is t _p r _p ′, t _s is t _s
An argument similar to that in Figure 7A can be made by setting r _s ', θ to θb. Also in the case of S-polarized light incidence, t _p ′ is t _s ′, t _s ′ is t _p ′,
Just replace r _p ′ with r _s ′ and r _s ′ with r _p ′. Even in the cases shown in FIGS. 7A and 7B, the optical elements 10 and 10' are adjusted so that S in equation (8) is maximized.
By setting the intensity reflectance and intensity transmission rate depending on the polarization direction, it is possible to read a recorded pattern with a high signal-to-noise ratio. Next, an eighth embodiment will be described in which the recording pattern reading system of the present invention is applied to a magneto-optical recording type disk memory.
This will be explained using figures. In the figure, 17 is a light source such as a semiconductor laser He--Ne laser. Reference numeral 18 denotes a collimating optical system for converting the light beam from the light source into a parallel light beam. Reference numeral 19 denotes a polarizing plate whose axis is arranged with respect to the optical element 23 described in the previous embodiment so that the incident polarization plane becomes P-polarized light. Reference numeral 20 denotes a phase diffraction grating, which performs angular separation of a light beam so that a sub-spot for tracking detection is focused on a perpendicular magnetic recording medium 25 by an objective lens 24. The lens 21 has the function of forming an image of this diffraction grating 20 near the pupil plane of the objective lens 24, so that the angularly separated light beams are focused on the objective lens 24.
It is possible to prevent blockage in the system up to. A mirror 22 bends the optical axis by 90 degrees and directs the light beam toward the objective lens 24. The optical element 23 of the present invention is placed in the position of a conventionally used semi-transparent mirror. The light flux is from objective lens 2
4, a spot is imaged on the rotating perpendicular magnetic recording medium 25. Due to the angular separation effect of the diffraction grating 20, the spot is divided into two spots for tracking signal detection.
There are a total of three spots: one for this spot and one for detecting RF signals. The light beam that has been subjected to Kerr rotation and reflected by the perpendicular magnetic recording body is separated from the incident light beam by an optical element 23, and the polarized light components are separated by an analyzer 26. 27
is an optical system having astigmatism, and is mainly necessary for detecting an automatic focusing signal for controlling the focusing state of the objective lens in the four-segment light receiving element 30. The electrical signal received by the four-division light-receiving element is separated by a frequency separator 34 into an automatic focusing signal and an RF signal in an appropriate frequency band. After the RF signal is amplified by an amplifier 31, an automatic focusing signal sent to a signal demodulation system is sent to a driver 32, which controls the focus state of the objective lens according to the signal. On the other hand, the light beam separated by the optical system 27 is detected by photodetectors 28 and 29, and these signals are sent to a subtractor 33.
After making a difference, the horizontal direction of the objective lens is controlled via a driver to perform tracking. With the above configuration, file memory can be reproduced using a perpendicular magnetic recording medium. In the above embodiment, an example was shown in which reflected light is detected using the Kerr effect, but the present invention can also be used in a reading system that detects modulated light transmitted through a recording medium. As explained above, the present invention has the effect of improving the signal-to-noise ratio in signal detection in a recorded pattern reading system using a conventional semi-transparent mirror without polarization characteristics and a photodetector with a multiplication effect. Furthermore, by achieving a high signal-to-noise ratio, the configuration of the detection processing system is simplified, costs are reduced, and reliability is increased.

[Brief explanation of drawings]

Figures 1A and B are diagrams showing conventional detection examples of perpendicular magnetic recording patterns, Figure 2 is a diagram showing the optical system of the first embodiment of the present invention, and Figure 3 is a diagram showing the direction of the analyzer transmission axis. 4 is a schematic diagram showing the configuration of the photoelectric detection system of the first embodiment, and FIG. 5 shows the C/N ratio in reading the recorded pattern of the present invention, and the polarization of the optical element used in the first embodiment. Figures 6A, B, C, D and 7A and B are diagrams showing optical systems of other embodiments of the present invention, respectively, and Figure 8 is a diagram showing the dependence on characteristics. FIG. 2 is a schematic diagram showing an example in which the recording method is applied to a disk memory. 9...Polarizer, 10...Optical element, 11...Perpendicular magnetic recording medium, 12...Analyzer.

Claims (1)

[Scope of Claims] 1. A light source that irradiates a magnetic recording medium with a light beam polarized in a predetermined direction, and a detector that converts the light beam whose polarization state is modulated according to the recording pattern of the recording medium into an intensity-modulated light beam. a photodetector with a multiplication effect that photoelectrically detects photons and the luminous flux converted by the analyzer;
In a recording pattern reading system comprising an amplifier including a load resistor for amplifying the signal detected by the photodetector, the polarization component intensity in the predetermined direction of the light beam received by the photodetector is I _R , which is also perpendicular to it. The polarization component intensity in the direction is I _K , the multiplication factor of the photodetector is G,
The multiplication noise figure of the photodetector is x, the electric charge is e, Planck's constant is h, Boltzmann's constant is k, the quantum efficiency of the photodetector is ε, the resistance value of the load resistor is R _L , and the equivalent noise temperature is T _e , where the bandwidth of the detection signal is ΔB and the frequency of the light beam is ν, the S/N ratio S of the output signal of the amplifier is expressed by the following formula: S=10 log ₁₀ {e ² ε ² /2h ² ν ² G ² I _K ² / (2e ² ε/hνG ^2+x I _R +4kT _e /R _L )ΔB}, and in the optical path of the modulated light flux, I is set so as to maximize the value of S A recording pattern reading optical system characterized by being provided with an optical element for adjusting the ratio of _K and _IR .