JPH0526173B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical system for reading magnetically recorded information using the magneto-optic effect.

Conventionally, using the magneto-optical Kerr effect,
Methods for optically reading magnetically recorded information are well known, and in particular, a method for reading recorded patterns using the Polar Kerr effect from a perpendicular magnetic recording medium is 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 magnetic This is the pattern 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 direction of magnetization is blocked by the analyzer 6, and the transmitted component Δ of the reflected light from the downward direction of magnetization passes through the analyzer 6. pass. This phenomenon allows perpendicular magnetization patterns to be detected or observed.

However, in conventional optical systems that use semi-transparent mirrors that do not depend on the polarization direction, the light beam passes through the semi-transparent mirror twice, causing its amplitude to drop to 1/4. Furthermore, considering that the polarization rotation angle θ _K due to the Kerr effect is generally approximately 1° or less, and that the Kerr rotation modulation component obtained by passing through the analyzer 6 is extremely small, The loss of light amount caused by this decreases the detection sensitivity of the detection signal light. Therefore, the conventional method has the following drawbacks.

1 By passing through a semi-transparent mirror that does not have polarization characteristics, more than half of the light intensity of the polarization direction component of the incident light flux and the modulation component due to the magneto-optic Kerr effect is lost, resulting in poor utilization efficiency of the detection signal light and the recording pattern. Hard to detect.

2. Due to the loss of signal light intensity due to the semitransparent mirror, in order to separate the polarized light component containing information from this signal light, an analyzer with an extremely high extinction ratio must be aligned and placed with high precision, which increases the cost. It is also unfavorable from the viewpoint of durability. On the other hand, JP-A-57-
Japanese Patent No. 44241 provides a magneto-optical reproducing device that uses a polarizing beam splitter instead of the semi-transparent mirror to apparently increase the polarization rotation angle. However, in this device,
Due to the focus on increasing the rotation angle of the polarization plane, the characteristics of the polarization beam splitter have not been optimized. That is, in order to read the recorded pattern with a high signal-to-noise ratio, it is necessary to maximize the amount of light of the force-rotation modulation component rather than the force-rotation angle, but such consideration was not taken in the above example.

It is an object of the present invention to solve the problems of the conventional reading optical system described above, and to provide a recorded pattern reading optical system that can obtain a large amount of Kerr rotation modulation component light and detect a bright magnetic pattern. .

The above object of the present invention is to provide a means for causing a light beam deflected in a predetermined direction to enter a recording medium on which a pattern is magnetically recorded, and a means for causing reflected light from the recording medium whose polarization state is modulated in accordance with the pattern to be reflected from the recording medium. It consists of a beam splitter that separates the incident light beam, and means that converts the polarization state of the reflected light separated by the beam splitter into a change in light intensity and detects the polarization state, and the beam splitter converts the reflected light in a predetermined direction. In the recording pattern reading optical system having a characteristic of relatively increasing a polarization component in a direction perpendicular to the polarization component in a predetermined direction of the reflected light, the beam splitter increases polarization components in a predetermined direction of the reflected light. This is achieved by configuring so that 52% of the reflected light is guided to the detection means, and 90% or more of the polarized light component in the direction perpendicular to the predetermined direction of the reflected light is guided to the detection means.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 shows an embodiment of the optical system of the present invention. Here, a case will be considered in which the polarization direction of the incident light beam a is made into a P polarization state parallel to the plane of the paper by the polarizer 9. The optical element 10 used in this example differs from a conventional semi-transparent mirror that does not have polarization characteristics, and has a transmittance t and a reflectance r.
has different properties depending on the polarization direction. That is, the amplitude transmittance t _P of the P-polarized light component of the optical element 10, the amplitude reflectance r _P , the amplitude transmittance t S of the S-polarized light component (the polarized light component perpendicular to the plane of the paper in FIG. 2 ₎ , and the amplitude reflectance r _S , the optical element generally has |t _P |≠|t _S |, |r _P |
It is a semi-transparent mirror with the characteristic ≠｜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 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. cosθ, sinθ, −sinθ −cosθ1, 0 0, ηcosθ, −sinθ, sinθ cosθ=cos ² θ+√sin ² θ, 1/2(1−√) sin2θ, 1/2(1−√) sin2θ sin ² θ+ √cos ² θ (3). Therefore, in FIG. 2, if the Jones vector representing the polarization state of the detection light f transmitted through the analyzer 12 is 〓, and the Jones vector of the luminous flux b is 〓, 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 of the detection light f transmitted through the analyzer 12 is (1), (2),
Using equations (3) and (4), |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| ^ei 〓, K=|K| ^ei 〓, r _P = |r _P
| ^irp , r _S = | r _S | e ^irs , δ = α − β + r _P − r _S | K
| The term ² has been omitted as a second-order minute amount.

The first term on the right side of equation (5) above is the direct current (DC) component of the detection light, that is, the polarization direction component light intensity I _R of the incident light flux, and the second term is the modulation (AD) by the perpendicular magnetic recording medium 11. ) component, which means the Kerr rotation modulation component light intensity I _K.

By considering the transmission axis azimuth angle θ of the analyzer 12 from equation (5), I _K can be changed and the incomplete extinction of the analyzer can be compensated for. Here, I _K is maximized by setting the analysis transmission axis azimuth θ to 45°. Furthermore, when making the magnetic pattern visible and observing it with the naked eye, it is necessary to increase the visibility υ [≡(I _nax − I _nio )/(I _nax + I _nio )]. In this case, I _K ≠ 0, that is,
As sin2θ≠0, υ∞[|R||r _P |/2|K|sonδ(1/tanθ+ηta
nθ) + (1−η) | r _S | ] ^-1 (6). The orientation θ ₀ of the analyzer 12 that maximizes υ at this time is determined as θ ₀ =tan ⁻¹ (1/√) (7). Extinction rate η of commonly used polarizers
is 10 ^-4 to 10 ^-2 , and θ ₀ is in the range of 84° < θ ₀ < 89.5°, as shown in Figure 4. The transmission axis azimuth can be set.

Furthermore, from equation (5), it is possible to change the relative values of I _R and I _K by giving polarization characteristics to the transmittance and reflectance of the optical element 10. As a result, I _K can be increased in appearance to compensate for the decrease in I _K due to incomplete extinction of the analyzer 12. Assuming that the absorption of the optical element 10 can be ignored, |t _p | ² =
From equation (5) as 1−|r _P | ² , I _R ∞ (1−|r _P | ² ) | r _P | ² (8) I _K ∞ (1− | r _P | ² ) | r _P | |r _s | (9) From this, for I _R , the maximum is at |r _P | ² = 0.5, regardless of |r _P | ² , and for I _K , |r _{P |}
It can be seen that the maximum value is ｜ ² = 0.33. Also |r _P | ²
I _K is maximum when = 1.0.

FIG. 5 shows the dependence of I _R and I _K on the polarization characteristics of the optical element 10 using equations (8) and (9) above. The solid line indicates the polarization direction component light intensity I _R , and the broken line indicates the Kerr rotation modulation component light intensity I _K , where a is |r _s | ² = 0.9, b is |r _s | ² = 0.7,
This is the case when c is |r _s | ² =0.5.

From FIG. 5, it is clear that the same |r _P
For the value of | ² _, |r _P | ² and |r _s | ² are both 50% when using a semi-transparent mirror ^.
When using 90% optical elements, approximately 1.4 times more _IX can be obtained and bright magnetic patterns can be detected. As mentioned above, I _K is maximum at |r _s | ² = 1.0, so |r _s | ²
should be 90% or more. Furthermore, ｜r _P ｜ ² is about 33%
When using an optical element of 1.5 compared to the conventional case,
A maximum value of I _K that is more than twice as large can be obtained. here,
If 0.18＜｜r _P ｜ ² ＜0.52, I _K is 90% of the above maximum value
Since the above values can be obtained, a sufficient effect can be obtained by setting |r _P | ² within the above range. Furthermore, in order to increase the aforementioned visibility υ, it is desirable to set |r _s | ² as large as possible and |r _P | ² as small as possible. An optical element having such polarization characteristics is manufactured in the same manner as a general polarization beam splitter.

The observation or detection method in the above embodiments was for the system shown in FIG. 6A. Next, for other examples of observation or detection methods using the optical element 10, optimal conditions for the characteristics of each optical element 10 are 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. _P | ² ) | r _P | | r _S | (11) is obtained. In equations (8) and (9), r _P is replaced by r _S , r _P
If we replace rS with _rS , the two equations match exactly. From this result, we can draw the dependence of I _R and I _K on |r _P | ^{2 and} |r _S | ² , and the _larger | _{r P} ^| I _K can reach its maximum value when it is about 33%. Furthermore, in order to increase the visibility υ, it is preferable to decrease |r _S | ² .

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, regarding 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, it is expressed by equation (4). The JoneS vector 〓 becomes 〓＝〓〓〓〓〓 〓.

Therefore, as shown in FIG. 6C, when the light beam is incident on the optical element 10 in a P-polarized state,
Based on similar considerations, in equations (8) and (9), r _P is t _P
Then, by setting r _S to t _S , I _R ∝ (1−|t _P | ² ) | t _P | ² (13) I _K ∝ (1− | t _P | ² ) | t _P | | t _S | (14) is obtained, and by increasing |t _S | ² and setting |t _P | ² to about 33%, I _K becomes maximum and a bright magnetic pattern can be detected. Similarly to the above, the visibility υ should be as small as |t _P | ² , the better.

As shown in FIG. 6D, when the polarization state of the light beam enters the optical element 10 as S-polarized light, the light intensity passing through the analyzer 12 is I _R ∝(1−|t _S | ² ) in exactly the same way. |t _S | ² (15) I _K ∝(1−|t _S | ² ) |t _S | |t _P | (16) If we replace r _P with t _{S and} r _S with t _P in equations (8) and (9), they match equations (15) and (16), respectively, and by increasing |t _P | ² , |t _P | If ² is about 33%, a bright magnetic pattern can be detected. Furthermore, in order to increase the visibility υ, |t _S | ² can be decreased.

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 7 A, P
When polarized light is incident, the light intensity obtained as 〓a＝〓a〓′〓〓〓〓 〓 〓b＝〓b〓′〓〓〓〓 〓 For the detected light from the detector 12a, the light intensity is obtained as (8) and (9) ), let r _P be r _P r _P ′
, replace r _S with r _S r _S ′ and θ with θ _a , analyzer 12
For the detected light from b, let r _P be r _P t _P ′ and r _S be r _S
In t _S ′, θ can be replaced with θ _b .

For this time-divided detection light, an optical element 10
By appropriately selecting the transmission axis directions θ _a and θ _b of the analyzers 12a and 12b, the relative values of the respective polarization direction component light intensities and Kerr rotation modulation component light intensities can be changed. Generally, in the arrangement as shown in FIG. 7A, each light beam divided by the optical element 10' is received by a photodetector, and electrical differential detection is performed. In this case, it is desirable that θ _a −θ _b be set, and that the relative values of the polarization direction component light intensity and the Kerr rotation modulation component light intensity of the divided light beams are equal. Therefore, in the case of such electrical differential detection, |
r _S ′|=|t _S ′|, |r _P ′|=|t _P ′|,
Preferably, a semi-transparent mirror without polarizing properties is used for optical element 10'. 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. In the case of S-polarized light incident, 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 an optical system similar to that shown in FIGS. 6C and 6D. In this case, (8) and (9) for P-polarized light incidence.
Regarding the light passing through the analyzer 12a in the formula,
Replace t _P with t _P t _P ′, t _S with t _S t _S ′, and θ a with θ _a , and for the light passing through the analyzer 12b, replace t _P with t _P r _P ′ and t _S with
The same argument as in Figure 7A can be made with t _S r _S ' and θ being θ _b . Also in the case of S-polarized light incident, t _P ′ is t _S ′, t _S ′ is t _P ′,
Just replace r _P ′ with r _S ′ and r _S ′ with r _P ′.

In this way, in the embodiments of the present invention, the transmittance or reflectance of the optical elements disposed in the reading optical system is made different for each polarized light component to control the amount of light transmitted or reflected. A bright magnetic pattern is obtained by relatively increasing the Kerr rotation modulation component due to the magneto-optic Kerr effect compared to the polarization direction component of the incident light beam.

Next, an embodiment in which the optical element of the present invention is applied to a magneto-optical recording type disk memory, video, disk, etc. will be described with reference to FIG.

In the figure, 13 is a light source such as a semiconductor laser He--Ne laser. 14 is a collimating optical system for converting the light beam from the light source into a parallel light beam. Reference numeral 15 denotes a polarizing plate, and its axis is arranged with respect to the optical element 19 described in the previous embodiment so that the incident polarization plane becomes P-polarized light. Reference numeral 16 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 21 by an objective lens 20. The lens 17 has the function of forming an image of this diffraction grating 16 near the pupil plane of the objective lens 20, so that the angularly separated light beams are focused on the objective lens 20.
It is possible to prevent blockage in the system up to. A mirror 18 bends the optical axis by 90 degrees and directs the light beam toward the objective lens 20. The optical element 19 of the present invention is placed in the position of a conventionally used semi-transparent mirror. The light flux is from objective lens 2
0, a spot is imaged on the rotating perpendicular magnetic recording medium 21. Due to the angular separation effect of the diffraction grating 16, the spots are divided into two spots for tracking signal detection and three spots for RF signal detection.
This is the spot.

The light beam subjected to Kerr rotation and reflected by the perpendicular magnetic recording body is separated from the incident light beam by an optical element 19, and the polarized light components are separated by an analyzer 22. 23
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-part light receiving element 26.

The electrical signal received by the four-division light receiving element is separated by a frequency separator 30 into an automatic focusing signal and an RF signal in an appropriate frequency band. After the RF signal is amplified by an amplifier 27, an automatic focusing signal sent to the signal demodulation system is sent to a driver 28, 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 23 is detected by photodetectors 24 and 25, and the signals are sent to a subtractor 29.
After the difference is made, tracking is performed by controlling the horizontal direction of the objective lens via a driver.

With the above configuration, playback of file memories, video disks, etc. using perpendicular magnetic recording bodies can be performed.

As explained above, the present invention is capable of detecting a bright magnetic pattern by (1) amplifying the amount of Kerr rotation modulation component light in a conventional magnetic pattern detection system using a semi-transparent mirror without polarization characteristics. is possible.

(2) Increase visibility and make it possible to observe magnetic patterns with strong contrast.

(3) By reducing the loss in the amount of signal light and compensating for imperfections in the analyzer, it is possible to use an inexpensive analyzer.

It has the following effects.

[Brief explanation of the drawing]

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. Figure 4 is a diagram showing the optimum orientation of the analyzer transmission axis to obtain maximum visibility with respect to the analyzer extinction rate, and Figure 5 is a diagram showing the polarization characteristics with respect to the polarization characteristics of the optical element used in the first example. A diagram showing the dependence of directional component light intensity and Kerr rotation modulation component intensity, FIG. 6A,
B, C, D and FIGS. 7A and 7A and 7B are diagrams showing optical systems of other embodiments of the present invention, respectively, and FIG. 8 is a diagram showing an embodiment applied to a reproduction system of a disk memory. 9...Polarizer, 10...Optical element, 11...Perpendicular magnetic recording body, 12...Analyzer.

Claims (1)

[Scope of Claims] 1. A means for causing a light beam polarized in a predetermined direction to enter a recording medium on which a pattern is magnetically recorded; It consists of a beam splitter that separates the incident light beam, and means that converts the polarization state of the reflected light separated by the beam splitter into a change in light intensity and detects the polarization state, and the beam splitter converts the reflected light in a predetermined direction. In a recording pattern reading optical system having a characteristic of relatively increasing a polarization component in a direction perpendicular to the polarization component in a direction perpendicular to the polarization component in a predetermined direction of the reflected light, 52% of the reflected light is guided to the detection means, and the polarized light component in the direction perpendicular to the predetermined direction of the reflected light is detected.
A recording pattern reading optical system characterized by being configured to guide 90% or more of the recorded pattern to a detection means.