JPH04153938A - Optical information recording and reproducing device - Google Patents

Abstract

PURPOSE:To obtain an optical head with high capability of edge detection by executing the image forming of only a polarization component newly generated by a magneto-optical effect on a light detector by means of a light receiving optical system and executing operation by means of the output of each signal of the divided light detector. CONSTITUTION:Linear polarizing fluxes from a semiconductor laser 1 is emitted via an objective lens so that they can be irradiated on an information recording surface as spot light. The image of only the reflection light of the spot light or only the polarization component generated from the magneto-optical effect out of the polarization component of the transmitting light on the information recording surface is formed on the light detector divided in the direction of a track by the light receiving optical system. The operation is executed by means of the output signal from the divided light detector and the edge detection for the information zone recorded on an information track 6 is executed. Thus, the optical head with high capability of edge detection can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical information recording/reproducing device for recording and/or reproducing information on an optical information recording carrier. More specifically, the present invention relates to the configuration of an optical head for a magneto-optical disk device that reproduces information recorded on an optical information recording carrier using the magneto-optic effect.

[Conventional technology]

Magneto-optical disk devices have large data storage capacity;
Furthermore, it is attracting attention because the data can be rewritten. Although magneto-optical recording and reproducing devices have already been put into practical use,
The data recording method in these devices is a method called inter-mark recording, and in recent years, a mark length recording (edge recording) method has been studied to increase the data capacity of these magneto-optical disk devices.

In order to accurately reproduce information recorded from an optical disc recorded with mark lengths, the optical head is also required to accurately read edge positions of information bits recorded with mark lengths.

In conventional optical heads for magneto-optical disks, the light beam from a semiconductor laser is turned into a minute spot light by an objective lens, and data is recorded using the mark-to-mark recording method using this spot light. Information was reproduced by so-called differential detection of changes in the amount of light reflected from the bits.

In such a conventional differential detection method, a polarizing beam splitter whose polarization axis direction is at an angle of 45 degrees with the polarization direction of the incident linearly polarized light is used to separate the reflected light into two signal lights, and these two light beams are split into two signal lights. The two signals were detected and a differential signal was created from them.

[Problem that the invention is trying to solve]

As already mentioned, in the mark length recording method that can increase data capacity, the edge position of the information bit has an important meaning. In the conventional method, when reproducing information recorded with a merging length, a single minute spot light having a light intensity distribution in the shape of a Gaussian distribution is projected onto the information bit, and changes in the light intensity of the light reflected from this information bit are differentiated. In the dynamic detection method, the light intensity distribution of the projected spot light is a Gaussian distribution with a certain spread, and the method of detecting changes in the light intensity of the entire reflected light beam has a problem in that edge detection ability is low.

[Means to solve the problem]

The present invention projects a linearly polarized light beam from a semiconductor laser through an objective lens to become a spot light on an information recording surface, and includes the reflected light of the spot light or the polarized light component of the transmitted light. Only the polarized light component generated by the magneto-optical effect on the information recording surface is imaged on a photodetector divided in the track direction by the light receiving optical system including the objective lens, and an output signal from the divided photodetector is generated. By performing calculations using the above information and detecting edges of information magnetic domains recorded on the information track, an optical head with high edge detection ability is realized.

〔Example〕

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

FIG. 1 is a diagram for explaining a first embodiment of the present invention.

FIG. 1 is a schematic diagram of a magneto-optical recording/reproducing head of the present invention. In the figure, l is a semiconductor laser that emits linearly polarized light with wavelength λ (λ = 830 nm, ) (the electric field vector direction of this light flux is indicated by E), 2 is a collimator lens that converts this light flux into a parallel light flux, and 3 is E The first polarizing beam splitter 14 is an objective lens that transmits most of the polarized light components in the direction and reflects 100% of the polarized light components in the direction perpendicular to this direction. 5 is a light spot projected by an objective lens, 6 is one information track provided on a magneto-optical disk, and 7 is a guide groove provided for tracking the light spot. 8 is a second polarizing beam splitter, 19 is a transmitted wavefront, 10 is a sensor lens, and 11 is a photoelectric conversion element.

The light beam emitted from the semiconductor laser 1 is linearly polarized light, and the direction of its electric field vector is the direction indicated by E in the figure. The direction of E is the p direction, and the direction perpendicular to this is S
direction. This light beam is converted into a parallel light beam by the collimator lens 2.

This light flux passes through the first polarizing beam splitter 3,
The light passes through a transparent substrate of a magneto-optical disk (not shown) through an objective lens 4 and is imaged as a light spot 5 on an information track 6 having a magneto-optical recording film such as TbFeCo.

Due to this magneto-optical recording film, the reflected light beam subjected to the magnetic Kerr effect has a polarization component in the S direction orthogonal to the E direction.

This polarized light component is entirely reflected by the first polarizing beam splitter 3, while a portion of the original polarized light component in the p direction is reflected and enters the second polarizing beam splitter 8. The second polarizing beam splitter 8 has a characteristic of reflecting 100% of the polarized light component in the S direction and transmitting 100% of the polarized light component in the p direction. Therefore, the luminous flux 9 after passing through the second polarizing beam splitter 8 is entirely a p-polarized component, and is guided to a focus detection optical system for autofocus control and a tracking detection optical system for autotracking control (not shown). Various conventionally known systems can be used for each of these detection optical systems.

On the other hand, the S-polarized light beam reflected by the second polarizing beam splitter 8 is imaged by the sensor lens 10, and is incident on the photoelectric conversion element 11 provided near the imaging plane.

FIG. 2 shows a light spot 5 on one information magnetic domain 12 (magnetization direction M) recorded in an information track on a magneto-optical recording film.
The figure shows the case where .

Let Ei be the electric field vector on the incident light beam 13 that forms this light spot. This direction is the p direction, as described in the explanation of FIG. 1, and is indicated by 14. When this incident light flux is reflected by the magneto-optical recording film as a light spot 5, it is subjected to the magnetic Kerr effect, and the polarization direction of the linearly polarized light is rotated by θ according to the direction of magnetization of the information magnetic domain 12, and the reflected light flux 1
The electric field vector on 5 becomes as shown at 16. The S polarization component of this electric field vector is indicated by 17. In the case of this figure, there is no edge of the information magnetic domain within the light spot, and the reflected light beam does not have edge information. In this example, only the S-polarized light component 17 is imaged on the photoelectric conversion element, and in this case, the reflected light beam does not have edge information of the information magnetic domain, so there is no phase distribution within its wavefront. Instead, the S-polarized component of the reflected light beam is imaged by the sensor lens onto the photoelectric conversion element as a single spot light.

FIG. 3 shows a case where a spot light 5 is incident on the edge of an information magnetic domain recorded in an information track on a magneto-optical recording film.

The direction of the electric field vector on the incident light beam 13 is the same as in the case of FIG. 2 above, and is indicated by 16. The information magnetic domains on which the spot light 5 is incident differ from each other in their magnetization directions with the edge 17 as a boundary, as shown by 18.19. In the wavefront of the incident spot light 5, a region affected by the magnetization 18 is designated as 5-1, and a region affected by the magnetization 19 is designated as 5-2. The light in the region 5-1 is subjected to the magnetic Kerr effect due to the magnetization 18, and the light 20-1 in the portion of the reflected light beam 20 corresponding to this region
undergoes Kerr rotation in the same direction as in FIG.

On the other hand, the light with the wavefront 5-2 is subjected to the magnetic Kerr effect due to the magnetization 19, and the light 20 of the portion of the reflected light flux 20 corresponding to this region is
-2 has a polarization plane rotated in the opposite direction to 20-1, and 23
The electric field vector is as shown by , and its S polarization component is 24.

As you can see by comparing this S polarization component 22.24,
The direction is reversed. In other words, the reflected luminous flux 2
The phase distribution of the S-polarized light component of 0 has a phase difference of π with the information magnetic domain edge 17 as a boundary.

FIG. 4 is a diagram showing the imaging relationship of the wavefront 20 in FIG. 3 with respect to S-polarized light. In the figure, a light spot 5 incident on the magneto-optical recording film is reflected, and a reflected light beam 20 has a phase difference π between 20-1 and 20-2 regarding the S-polarized component on its wavefront. This light flux is the sensor lens 10
When an image is formed by , due to this phase difference, it does not become a single light spot, but becomes an amplitude distribution 25 with zero at the center, and the intensity distribution becomes a dark distribution at the center as shown in 26.

FIG. 5 shows the imaging relationship by the light receiving optical system for S-polarized light in the case of FIG. 2 above.

In this case, as explained in Figure 2 above, the reflected light flux 1
There is no phase difference in the S-polarized light component on the wavefront of 5, and the same intensity distribution 27 as the normal imaging spot light distribution is formed.

FIG. 6 is a diagram showing the light amount distribution of the imaging spot of the light receiving optical system in FIGS. 4 and 5. FIG.

FIG. 6(a) is a schematic diagram showing the spot shape,
When the positional relationship between the magnetic domain and the light spot is as shown in FIG. 4, it is divided into two spots 52-1 and 52-2, and in the case of FIG. 5, it becomes a spot 51 shown by a broken line. In addition, the 6th
Figure (a) shows the approximate shape of the spot; in reality, there is a light intensity distribution within the spot, and in addition to the main spot shown in the figure, there are annular zones and minute spots that have a small light intensity. Spots occur, but they are omitted. FIG. 6(b) shows the light amount distribution in the X' direction in the same figure. In the figure, a curve 26 shows the light intensity distribution of the two spots in the case of FIG. 4, and a broken line 27 shows the light intensity distribution of the spots in the case of FIG. 5, and both are normalized.

FIG. 7 is a diagram showing the relationship between the imaging spot of the light receiving optical system and the sensor.

FIG. 7(a) is a reproduction of FIG. 6(b).

FIGS. 7(b) to 7(f) show different embodiments of the present invention, and each is a schematic diagram of the division of the sensor. Figure 7 (
In b), the sensor is divided into three parts in the track direction X', and independent outputs are obtained from each divided sensor 53, 54, 55. In this way, the sensor is arranged so that the dividing line is perpendicular to the track direction. The sensor 54 has a sensor width in the X' direction that is intermediate to the cross region of the light quantity distributions 27 and 26 so that the sensitivity for detecting changes in the light quantity on the y' axis is high. This is approximately the full half-value of the light amount distribution 27. Similarly, sensors 53 and 55 are provided corresponding to two peaks of the light amount distribution 26, respectively.

However, outside both peaks of the light intensity distribution 26, the light intensity distribution 2
7 is extremely small and can be ignored, that is, the Talostalk is small, so the widths of the sensors 53 and 55 are expanded outward so that they can detect a large amount of light at both peaks.

The output of the sensor 53 and the output of the sensor 55 are subtracted by a subtraction circuit 56 to obtain a difference output 58.

Also, the output 57 of the sensor 54 is obtained.

The output of each sensor and edge detection will be explained using FIG. 8.

FIG. 8(a) is a schematic cross-sectional view of a magneto-optical recording film, showing information magnetic domains 18 and 19 (each having a different magnetization direction M) recorded on an information track, and an edge 17.

8(b), (c), (d,) correspond to the output of each sensor when the recording film of FIG. 8(a) is read out by the embodiment of FIG. .53.5
It represents the output of 5. The output waveform 59 shows the output waveform of the sensor 54, and since the light amount distribution 27 of FIG. 7(a) is incident at approximately the center of the information magnetic domain 18.19, it takes the maximum value
At edge 17 of the magnetic domain, the light intensity distribution 26 in FIG. 7(a)
Therefore, the sensor output 57 becomes small. On the other hand, the 8 forces of sensors 53 and 54 each have an output waveform of 60
．． It will be like 61. It is minimum at the center of magnetic domains 18 and 19 and maximum at edge 17. When the two output waveforms 60 and 61 are subtracted by a subtracter 56, a waveform 62 is obtained. magnetic domain 18
．． At the center of the sensor 19 and at the edge 17, there are two light intensity distributions 27 and 26 shown in FIG.
becomes. Therefore, as shown in waveform 62, magnetic domains 18.
The waveform crosses zero at edge 19 and edge 17.

A possible method for detecting the edge 17 is to use only the sensor 54 and find the minimum value peak of its output waveform 59. At the edge 17, the light amount distribution on the sensor surface becomes the distribution 26 shown in FIG. 7(a), so it is possible to take advantage of the fact that the amount of light incident on the sensor 54 is small. This enables erroneous detection.

However, since this method requires detection of peaks that are not very steep, jitter is likely to occur, and the detection circuit must be devised. Since it is modulated, the maximum and minimum values change depending on the MTF characteristics of the optical system.
A differentiation circuit (high-pass filter circuit) used to detect a peak caused by fluctuations in the DC component of the waveform 59;
If the characteristics of the integrating circuit (low-pass filter circuit) are not modified, there will be problems such as peak shift and jitter.

Another possible edge detection method is to use the sum of the outputs of the sensors 53 and 55. Since the light amount distribution is 26 at the edge 17, the sum of the outputs of the sensors 53 and 55 takes the maximum value. As can be seen from FIGS. 8(C) and 8(d), the waveform is a waveform obtained by inverting the output waveform 59 of the sensor 54 centering on the DC component. That is, sensor 5
Since the sum of the output of sensor 4 and the sum output of sensor 53.55 is approximately the amount of light, it is approximately a constant value, and the two are in a complementary relationship. Therefore, this method is basically the same as the method of detecting the peak from the output of the sensor 54 described above, and has the same advantages and disadvantages.

The peak detection embodiment shown in FIG. 8 is an edge detection method that overcomes the shortcomings of the two detection methods described above. sensor 5
A difference output 58 between 3 and 55 is obtained as a cellocross waveform 62, from which the above-mentioned DC component fluctuation has been removed. Therefore, the waveform 62 can be binarized at the zero crossing point to obtain a binary signal 64. The rising portion of this pulse waveform 64 corresponds to the edge 17, and for example, it is possible to obtain a pulse waveform 65 that corresponds to the position of the edge 17 and corresponds to the length of the magnetic domain 18.19.

At this time, if the signal waveform 63 obtained by binarizing the output waveform 59 of the sensor 54 is used as a gate, the edge 17
Since the center of the magnetic domain 18, 19 can be clearly distinguished from the center of the magnetic domain 18, 19, the reliability of edge detection can be further improved. It also has the effect of preventing a decrease in the reliability of edge detection due to noise such as defects in the medium and noise in the circuit.

Further, as mentioned above, since the output of the sensor 54 and the sum output of the sensors 53 and 55 are in a complementary relationship with each other, an adder circuit is added to calculate the sum of the outputs of the sensors 53 and 55, although not particularly shown. The output can be used in place of the output of the sensor 54 described above, in which case the sensor 54 is basically unnecessary, resulting in a simpler sensor configuration as described below.

Next, the embodiment shown in FIG. 7(C) will be described.

The figure shows an example in which edge detection sensitivity is improved by narrowing the width of each sensor 53, 54, 55. The area of each sensor is limited in order to detect a portion where the crosstalk between the light amount distributions 26 and 27 is small and the change is large. Although the output of each sensor becomes smaller,
It becomes possible to obtain an edge detection waveform with less crosstalk. In addition, when the gap between these sensors is large and a large amount of light enters the non-sensor part, for example in PIN-PD, unnecessary carriers are generated, accumulated, and leak into the sensor part. Since problems such as poor frequency response may occur, such gaps should not be masked to prevent light from entering them. Or, remove part of the dummy sensor and electrode,
Measures such as taking out the generated carriers are required.

FIG. 7(d) shows an example in which the sensor shape is two-dimensionally optimized in order to reduce crosstalk between the light quantity distributions 27 and 26. The two-dimensional distribution of light amount distribution is schematically as follows.
As shown in Fig. 6(a), it is better to optimize each sensor shape according to the two-dimensional distribution of light intensity, which reduces crosstalk and sensitivity, rather than simply dividing it into one dimension in the X' force direction. However, for two-dimensionally complex shapes,
Difficulties may arise in sensor creation. Sometimes it is better to approximate the curve shape to a polygon.

The embodiment shown in FIG. 7(e) is an embodiment in which a two-part sensor is used. Basically, this is an example in which the middle sensor 54 of the embodiment of FIG. 7(b) is removed, and the sensors 53 and 5 are
By calculating the difference and sum of the outputs of 5, edge detection becomes possible as described above. However, since the crosstalk between the light quantity distributions 27 and 26 is large, the amplitude of the difference signal is small, the signal quality is poor, and the degree of modulation of the sum signal is also low. However, the sensor has the advantage of being simple in configuration.

FIG. 7(f) is an improvement of the above embodiment, and is aimed at reducing crosstalk. In the figure, sensors 53 and 55 corresponding to the two peaks of the light amount distribution 26 are arranged, and the gap between the two is made large, so that the light amount distribution 27
In other words, the crosstalk is reduced so that the light intensity distribution at the center of the magnetic domains 18 and 19 is not detected.

By using the sensor described above, it is possible to realize an optical head that can detect edges with high accuracy without making major changes to the conventional optical system.

FIG. 9 shows another embodiment of the invention. The same members as in FIG. 1 above are designated by the same numbers.

FIG. 9 shows a phase plate 2 after the collimator lens 2 in FIG.
8. The phase plate 28 is divided into two regions 28-1 and 28-2 by a straight line 29 including the optical axis.
An optically transparent dielectric film having a refractive index n and a thickness D is provided in the region 28-1. The relationship between the thickness, the refractive index, and the wavelength λ of the light beam from the semiconductor laser is as follows: 2π(n-1)D/λ=π. Therefore, the projected light beams have a phase difference of π on the wavefronts corresponding to these two regions.

The light flux that has passed through this phase plate passes through the first polarizing beam splitter 3, is imaged by the objective lens 4, and is projected onto the information track 6 as a spot light 5. Unlike the case shown in FIG. 1, in this case, a phase difference π due to the phase plate 28 exists in the projected light spot.

FIG. 10 shows a case where the projected light spot 5 of FIG. 9 exists on one information magnetic domain 30. Since there is a phase difference given by the phase plate in the projected light beam, the wavefront 31 of the projected light beam 31 corresponds to the region 28-1, 28-2 of the phase plate.
There is a phase difference π between -1.31-2. Therefore, when the electric field vector on the wave surface 31-1 is shown as 32, the electric field vector on the wave surface 31-2 is shown as 33.

There is also a phase difference on the wavefront of the projected light spot 5, and a phase difference π exists between the wavefronts 5-1, 5-2. Therefore, this phase difference also exists on the light beam 34 reflected by one information magnetic domain 30 and subjected to the magnetic Kerr effect,
The electric field vectors of wavefronts 34-1 and 34-2 are 35
．． It can be shown as shown in 37. These S-polarized light components are 36.37, and there is a phase difference π between them.

FIG. 11 is a diagram showing the case where edges 41 of information magnetic domains 39 and 40 exist within the projected light spot 5. In FIG. Projection wavefront 31-
1.3 The electric field vector on the wavefront of 1-2 is expressed in the same way as in Fig. 10 above. In Figure 11, the projection spot 5
The magnetization at -1 is the same as in Fig. 10, and is subject to the same magnetic Kerr effect, and the electric field vector at the reflected wavefront 42-1 from now on can be expressed in the same way as in Fig. 10, and becomes 35. The S polarization component is 36.

On the other hand, in the projected light spot 5-2, since the direction of magnetization is different, the electric field vector of the reflected wavefront 42-2 becomes as shown by 43. This S polarization component is 44.

Therefore, in the embodiment shown in FIG. 9, due to the phase difference π existing in the projected light spot, contrary to the embodiment shown in FIG.
When an information magnetic domain edge exists within the projected light spot, there is no phase difference between the S-polarized light components on the reflected wavefront. Therefore, the intensity distribution of the spot light on the photoelectric conversion element created by the imaging effect in the light receiving optical system becomes similar to that shown in FIG. 5.

That is, in the embodiment of FIG. 9, the light intensity distribution shown in FIGS. 6 and 7(a) is opposite to that of the embodiment of FIG. 1, and when there is no information magnetic domain edge, the spot is 52. -1
．． 52-2, and the light amount distribution has two peaks as shown in 26.

Therefore, each of the eight divided sensors shown in FIGS. 7(b) to 7(f) has a waveform whose phase is shifted by π with respect to FIG. 8(b) to (h).

That is, waveform 59 has its maximum value at edge 17, and waveform 6
0.61 has a maximum value near the center of magnetic domain 18.19.

The falling portion of the binary signal 64 created from the difference signal waveform 62 corresponds to the edge 17. With this configuration, the gate signal 63 generated by the waveform 59
becomes more selective with respect to the edge 17, which improves the reliability of edge detection.

However, in the embodiment shown in FIG. 9, since a phase difference is imparted to the projected light beam, the projected spot size becomes larger compared to the embodiment shown in FIG. 1, resulting in a decrease in resolution. There are drawbacks.

In the following embodiment, this drawback is eliminated. FIG. 12 is another embodiment of the invention. In the same figure, the first
Components similar to those in the figures are designated by the same numbers. 45 is a phase plate provided within the light receiving optical system. The phase plate includes the optical axis of the light-receiving optical system and is divided into two regions 45-1 and 45-2 by a boundary line 46 in a direction perpendicular to the information track direction. A phase film is provided to give a phase difference of π to each other. That is, it is a member similar to the phase plate 28 shown in FIG. 9 above.

In this embodiment, the projected light beam is the same as that shown in FIG.
It is similar to FIG. However, in this embodiment, since the phase plate 45 is provided in the light-receiving optical system, the phase difference on the wavefront of the reflected light beam in the light-receiving optical system is is compensated, and the imaging spot light has a normal spot light intensity distribution. In other words, when there is an information magnetic domain edge on the optical axis of the projected spot light, the reflected light beam becomes a light spot with a single intensity peak like a normal light spot, similar to the embodiment shown in FIG. You can get output.

Moreover, in this embodiment, unlike the previous embodiment shown in FIG. The light spot size does not increase, and the resolution of information reading does not deteriorate.

In the above description, it is assumed that the spot light is imaged on the light-receiving surface of the photoelectric conversion element, but in this case, the influence of uneven sensitivity and the like depending on the location on the light-receiving surface becomes large. As a countermeasure against this, it is sufficient to place the light receiving surface of the photoelectric conversion element at a position slightly defocused from the image forming surface.

〔Effect of the invention〕

As explained above, the present invention utilizes the S-polarized light component of the magnetic Kerr effect produced by the magneto-optical recording film to provide information with high precision without significantly changing or complicating the configuration of the conventional magneto-optical head. The objective is to realize an optical head for a magneto-optical disk that can detect magnetic domain edges.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram of an optical head showing an embodiment of the present invention, Figs. 2 to 5 are explanatory diagrams showing polarization and amplitude of light showing the principle of the invention, and Fig. 6 is an embodiment of the invention. FIG. 7 is an explanatory diagram showing the divided sensor according to the embodiment of the present invention. FIG. 8 is a schematic diagram showing the output of each sensor and edge detection waveform according to the embodiment of the present invention. 9 is a configuration diagram of an optical head showing an embodiment of the present invention, and FIG.
11 is an explanatory diagram showing the polarization and amplitude of light showing the principle of the present invention, and FIG. 12 is a configuration diagram of an optical head showing an embodiment of the present invention. 1... Semiconductor laser 5... Optical spot on disk 11... Photoelectric conversion element 12.18.19... Information recording magnetic domain 7... Magnetic domain edge 26. 27...Light amount distribution on light receiving surface 53. 54. 55... split sensor

Claims (1)

[Claims] (1) A light beam from a laser is guided by a projection optical system to an information track provided on an information recording surface as a minute spot through an objective lens, and the reflected light from the information recording surface/
Alternatively, in an optical information recording and reproducing apparatus that guides transmitted light to a photodetector by a light receiving optical system through the objective lens and reproduces information recorded on the information recording surface using the magneto-optic effect, The photodetector is provided on the optical axis of the light-receiving optical system and is divided in a direction corresponding to the information track direction, and detects the polarized light of the reflected light from the information track of the light projection spot or the polarized light of the transmitted light. Among them, only the polarized light component newly generated by the magneto-optic effect is imaged as a light spot on the photodetector by the light receiving optical system, and calculation is performed using each signal output of the divided photodetector. An optical information recording/reproducing apparatus characterized in that an edge of an information magnetic domain recorded on the information track is detected.