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United States Patent
5577017
Yamamoto , ; et al.
November 19, 1996
Title
Magnetooptical information recording/reproducing apparatus
Abstract
An optical information recording and/or reproducing apparatus includes an irradiating device for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of tracks, the irradiating device includes an objective lens, having a curved surface, for converging the light irradiated onto the recording medium to form a micro spot, an applying device for applying a predetermined magnetic field to the recording medium and a detecting device for detecting interference light caused by interference of light produced by a magnetooptic effect of the recording medium and light produced by diffraction at the curved surface of the lens, and for reproducing recording information.
Inventors:
Yamamoto; Masakuni
(Yamato,
JP
)
, Matsumura; Susumu
(Kawaguchi,
JP
)
, Hoshi; Hiroaki
(Yokohama,
JP
)
, Yamaguchi; Eiji
(Zama,
JP
)
Assignee:
Canon Kabushiki Kaisha
(Tokyo,
JP
)
Appl. No.:
514477
Filed:
August 11, 1995
Foreign Application Priority Data
Feb 05, 1992 [JP] 4-047789
Feb 05, 1992 [JP] 4-047790
Jun 02, 1992 [JP] 4-165533
Sep 14, 1992 [JP] 4-269096
Current U.S. Class:
369/13.29
369/110.01
369/112.17
Field of Search:
369/100,103,110,109,112,120,44.12,44.23,44.22 360/114
U.S. Patent Documents
4982383
January 1991
Matsushita et al.
5043960
August 1991
Nakao et al.
5153868
October 1992
Fujinaga
Foreign Patent Documents
0218250
Apr., 1987
EP
0346121
Dec., 1989
EP
Other References
Patent Abstracts of Japan, Kokai No. 58-029151, vol. 7, No. 109, May 1983. .
Patent Abstracts of Japan, Kokai No. 02-198044, vol. 14, No. 487, Oct. 1990. .
Levenson, et al., "Edge Detection for Magnetooptical Data Storage," Applied Optics, vol. 30, No. 2, Jan. 1991, pp. 232 through 252. .
Aratani, et al., "Magnetically Induced Super Resolution in Novel Magneto-Optical Disk," Optical Data Storage Topical Meeting, vol. 5, Feb. 1991, pp. 112 through 119..~
Primary Examiner:
Wong; Don
Attorney, Agent or Firm:
Fitzpatrick, Cella, Harper & Scinto
Parent Case Text
This application is a continuation of prior application, Application Ser. No. 08/012,453 filed Feb. 2, 1993, now abandoned.
Claims
What is claimed is:
1. An optical information recording and/or reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of tracks, said irradiating means comprising lens means, having a curved surface;
applying means for applying a predetermined magnetic field to the recording medium;
means for separating light having a polarization component perpendicular to that of the irradiated light, from light reflected from the magnetooptical recording medium;
detecting means for detecting interference light caused by interference of light produced by a magnetooptic effect of the recording medium and light produced by diffraction at the curved surface of said lens means, said detecting means comprising a two-divided photodetector, the division line of which is parallel to the recording medium tracks, for detecting the separated light; and
means for calculating a difference of detection signals from the respective detection surfaces of said two-divided photodetector, and for reproducing recording information.
2. An apparatus according to claim 1, further comprising:
means for separating light having a polarization component perpendicular to that of the irradiated light, from light reflected from the magnetooptical recording medium;
detecting means comprising a four-divided photodetector, the detection surface of which is divided by a line parallel to the recording medium tracks and a line perpendicular to the tracks, for detecting the separated light; and
means for executing a predetermined computation using detection signals from the respective divided detection surfaces of said four-divided photodetector, and for reproducing recording information corresponding to one of a pit position recording and a pit edge recording.
3. An apparatus according to claim 1, further comprising:
DC component detecting means for detecting a DC component superposed with the reproduction signal when said lens means is moved in a tracking direction relative to the magnetooptical recording medium; and
correction means for correcting the reproduction signal on the basis of an output of said DC component detecting means, and for removing the DC component included in the reproduction signal.
4. An optical information recording and/or reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of tracks, said irradiating means comprising lens means having a curved surface;
applying means for applying a predetermined magnetic field to the recording medium;
means, disposed in an optical path of the light irradiated to the magnetooptical recording medium, for rotating a polarization direction of the irradiated light by substantially 45 degrees with respect to the information tracks;
means for separating light having a polarization component perpendicular to that of the irradiated light, from light reflected from the magnetooptical recording medium;
detecting means for detecting interference light caused by interference of light produced by a magnetooptic effect of the recording medium and light produced by diffraction at the curved surface of said lens means, said detecting means comprising a two-divided photodetector, the division line of which is perpendicular to the recording medium information tracks, for detecting the separated light; and
means for calculating a difference of detection signals from the respective divided detection surfaces of said two-divided photodetector, and for reproducing recording information.
5. An optical information recording and/or reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of information tracks, said irradiating means comprising lens means, having a curved surface, for converging the light irradiated to the recording medium to form a micro spot;
applying means for applying a predetermined magnetic field to the recording medium;
splitting means comprising a polarizing beam splitter for splitting at least light produced by a magnetooptic effect of the magnetooptical recording medium and light produced by diffraction at the curved surface of said lens means into two light beams, respectively;
detecting means comprising a two-divided photodetector divided in a direction perpendicular to the recording medium information tracks, for detecting the split light beams; and
means for calculating a difference of detection signals from the respective divided detection surfaces of said two-divided photodetector, and for reproducing recording information.
6. An optical information recording and/or reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of information tracks, said irradiating means comprising lens means, having a curved surface, for converging the light irradiated to the recording medium to form a micro spot;
applying means for applying a predetermined magnetic field to the recording medium;
splitting means comprising a polarizing beam splitter for splitting at least light produced by a magnetooptic effect of the magnetooptical recording medium and light produced by diffraction at the curved surface of said lens means into two light beams, respectively;
detecting means comprising a four-divided photodetector having a detection surface divided into four by crossing lines, one of which is parallel to the recording medium information tracks and the other of which is perpendicular to the information tracks;
means for calculating a difference of detection signals from the respective divided detection surfaces of said four-divided photodetector, and for reproducing recording information; and
means for executing a predetermined computation using detection signals from the four-divided detection surfaces of said four-divided photodetector, and for reproducing a signal corresponding to one of a pit edge recording and a pit position recording.
7. An apparatus according to claim 5, further comprising:
DC component detecting means for detecting a DC component superposed with the reproduction signal when said lens means is moved in a tracking direction relative to the magnetooptical recording medium; and
correction means for correcting the reproduction signal on the basis of an output of said DC component detecting means, and for removing the DC component included in the reproduction signal.
8. An apparatus according to claim 5, wherein said polarizing beam splitter comprises converging lens means for respectively converging the two light beams split by said beam splitter onto said two-divided photodetector at a beam convergence position, and wherein said apparatus further comprises detecting means for detecting one of the light beams split by said beam splitter, disposed ahead of the beam convergence position, and for detecting the other of the light beams split by said beam splitter, disposed behind the beam convergence position.
9. An optical information reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of tracks, said irradiating means comprising lens means having a curved surface;
means for separating light having a polarization component perpendicular to that of the irradiated light, from light reflected from the magnetooptical recording medium;
detecting means for detecting interference light caused by interference of light produced by a magnetooptic effect of the recording medium and light produced by diffraction at the curved surface of said lens means, said detecting means comprising a two-divided photodetector, the division line of which is parallel to the recording medium tracks, for detecting the separated light; and
means for calculating a difference of detection signals from the respective detection surfaces of said two-divided photodetector, and for reproducing recording information.
10. An optical information reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of tracks, said irradiating means comprising lens means having a curved surface;
means, disposed in an optical path of the light irradiated to the magnetooptical recording medium, for rotating a polarization direction of the irradiated light by substantially 45 degrees with respect to the information tracks;
means for separating light having a polarization component perpendicular to that of the irradiated light from light reflected from the magnetooptical recording medium;
detecting means for detecting interference light caused by interference of light produced by a magnetooptic effect of the recording medium and light produced by diffraction at the curved surface of said lens means, said detecting means comprising a two-divided photodetector, the division line of which is perpendicular to the recording medium information tracks, for detecting the separated light; and
means for calculating a difference of detection signals from the respective divided detection surfaces of said two-divided photodetector, and for reproducing recording information.
11. An optical information reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of information tracks, said irradiating means comprising lens means, having a curved surface, for converging the light irradiated to the recording medium to form a micro spot;
splitting means comprising a polarized beam splitter for splitting at least light produced by a magnetooptic effect of the magnetooptical recording medium and light produced by diffraction at the curved surface of said lens means into two light beams, respectively;
detecting means comprising a two-divided photodetector divided in a direction perpendicular to the recording medium information tracks, for detecting the split light beams; and
means for calculating a difference of detection signals from the respective divided detection surfaces of said two-divided photodetector, and for reproducing recording information.
12. An optical information recording and/or reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of tracks, said irradiating means comprising lens means, having a curved surface;
applying means for applying a predetermined magnetic field to the recording medium;
means for separating light having a polarization component perpendicular to that of the irradiated light, from light reflected from the magnetooptical recording medium;
detecting means for detecting interference light caused by interference of light produced by a magnetooptic effect of the recording medium and light produced by diffraction at the curved surface of said lens means, said detecting means comprising a four-divided photodetector, a division line of which is parallel to the recording medium tracks and a division line of which is perpendicular to the recording medium tracks, for detecting the separated light; and
means for calculating a difference of detection signals from the respective detection surfaces of said four-divided photodetector, and for reproducing recording information.
13. An optical information recording and/or reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of tracks, said irradiating means comprising lens means having a curved surface;
applying means for applying a predetermined magnetic field to the recording medium;
means, disposed in an optical path of the light irradiated to the magnetooptical recording medium, for rotating a polarization direction of the irradiated light by substantially 45 degrees with respect to the information tracks;
means for separating light having a polarization component perpendicular to that of the irradiated light, from light reflected from the magnetooptical recording medium;
detecting means for detecting interference light caused by interference of light produced by a magnetooptic effect of the recording medium and light produced by diffraction at the curved surface of said lens means, said detecting means comprising a four-divided photodetector, a division line of which is perpendicular to the recording medium information tracks and a division line of which is parallel to the recording medium information tracks, for detecting the separated light; and
means for calculating a difference of detection signals from the respective divided detection surfaces of said four-divided photodetector, and for reproducing recording information.
14. An optical information recording and/or reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of information tracks, said irradiating means comprising lens means, having a curved surface, for converging the light irradiated to the recording medium to form a micro spot;
applying means for applying a predetermined magnetic field to the recording medium;
splitting means comprising a polarizing beam splitter for splitting at least light produced by a magnetooptic effect of the magnetooptical recording medium and light produced by diffraction at the curved surface of said lens means into two light beams, respectively;
detecting means comprising a four-divided photodetector divided in a direction perpendicular to the recording medium information tracks and divided in a direction parallel to the recording medium information tracks, for detecting the split light beams; and
means for calculating a difference of detection signals from the respective divided detection surfaces of said four-divided photodetector, and for reproducing recording information.
15. An optical information reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of tracks, said irradiating means comprising lens means having a curved surface;
means for separating light having a polarization component perpendicular to that of the irradiated light, from light reflected from the magnetooptical recording medium;
detecting means for detecting interference light caused by interference of light produced by a magnetooptic effect of the recording medium and light produced by diffraction at the curved surface of said lens means, said detecting means comprising a four-divided photodetector, a division line of which is parallel to the recording medium tracks and a division line of which is perpendicular to the recording medium tracks, for detecting the separated light; and
means for calculating a difference of detection signals from the respective detection surfaces of said four-divided photodetector, and for reproducing recording information.
16. An optical information reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of tracks, said irradiating means comprising lens means having a curved surface;
means, disposed in an optical path of the light irradiated to the magnetooptical recording medium, for rotating a polarization direction of the irradiated light by substantially 45 degrees with respect to the information tracks;
means for separating light having a polarization component perpendicular to that of the irradiated light from light reflected from the magnetooptical recording medium;
detecting means for detecting interference light caused by interference of light produced by a magnetooptic effect of the recording medium and light produced by diffraction at the curved surface of said lens means, said detecting means comprising a four-divided photodetector, a division line of which is perpendicular to the recording medium information tracks and a division line of which is parallel to the recording medium information tracks, for detecting the separated light; and
means for calculating a difference of detection signals from the respective divided detection surfaces of said four-divided photodetector, and for reproducing recording information.
17. An optical information reproducing apparatus comprising:
irradiating means for irradiating light polarized in a predetermined direction onto a magnetooptical recording medium having a plurality of information tracks, said irradiating means comprising lens means, having a curved surface, for converging the light irradiated to the recording medium to form a micro spot;
splitting means comprising a polarized beam splitter for splitting at least light produced by a magnetooptic effect of the magnetooptical recording medium and light produced by diffraction at the curved surface of said lens means into two light beams, respectively;
detecting means comprising a four-divided photodetector divided in a direction perpendicular to the recording medium information tracks and divided in a direction parallel to the recording medium information tracks, for detecting the split light beams; and
means for calculating a difference of detection signals from the respective divided detection surfaces of said four-divided photodetector, and for reproducing recording information.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetooptical information recording and/or reproducing apparatus for recording information on a magnetooptical recording medium and/or reproducing the recorded information from the magnetooptical recording medium.
2. Related Background Art
In recent years, a magnetooptical information recording/reproducing apparatus using a magnetooptical disk as a recording medium has received a great deal of attention as the most promising recording/reproducing apparatus because it is portable, has a large storage capacity, and is capable of performing information erasure and updating. FIG. 1 is a view showing an optical system of a conventional magnetooptical information recording/reproducing apparatus. This apparatus includes a semiconductor laser 19 serving as a recording/reproduction light source. A divergent light beam emitted from the semiconductor laser 19 is collimated by a collimator lens 20. The sectional shape of the collimated light beam is shaped into a circular shape by a beam shaping prism 21. The collimated light beam is a beam of linear polarization (to be referred to as a P-polarized light component) having a polarization direction parallel to the drawing surface. The P-polarized light beam is incident on a polarizing beam splitter 22. The characteristics of the polarizing beam splitter 22 are given such that the transmittance for the P-polarized light component is 60%, the reflectance therefor is 40%, the transmittance for a linearly polarized light component (to be referred to as an S-polarized light component) having a polarization direction perpendicular to the polarization direction of the P-polarized light component is 0%, and the reflectance for the S-polarized light component is 100%. The P-polarized light beam passing through-the polarizing beam splitter 22 is focused by an objective lens 23, so that a light spot is formed on the magnetic layer of a magnetooptical disk 24. An external magnetic field is applied from a magnetic head 25 to a portion irradiated with this light spot, thereby recording an information domain on the magnetic layer.
Light reflected by the magnetooptical disk 24 returns to the polarizing beam splitter 22 through the objective lens 23. Part of the reflected light is split and guided to a reproduction optical system. In the reproduction optical system, the split light beam is further split by another polarizing beam splitter 26. The characteristics of the polarizing beam splitter 26 are given such that the transmittance for the P-polarized light component is 20%, the reflectance for the P-polarized light component is 80%, the transmittance for the S-polarized light component is 0%, and the reflectance for the S-polarized light component is 100%. One light beam split by the polarizing beam splitter 26 is guided to a reproduction optical system 27, and a reproduction signal (to be described later) is generated. The other light beam is guided to a half prism 36 through a condenser lens 35. This light beam is split into halves by the half prism 36. One light beam is guided to a photodetector 37, and the other light beam is guided to a photodetector 39 through a knife edge 38. By these control optical systems, servo error signals for autotracking control and autofocusing control of the light beam are generated.
The reproduction optical system 27 comprises a .lambda./2 plate 28 for rotating the polarization direction of the light beam through 45.degree., a condenser lens 29 for condensing the light beam, a polarizing beam splitter 30, and photodetectors
31 and 32 for respectively detecting the light beams split by the polarizing beam splitter 30. The characteristics of the polarizing beam splitter 30 are given such that the transmittance for the P-polarized light component is 100%, the reflectance for the P-polarized light component is 0%, the transmittance for the S-polarized light component is 0%, and the reflectance for the S-polarized light component is 100%. Signals detected by the photodetectors 31 and 32 are differentially detected by a differential amplifier 33 to generate a reproduction signal 34. In the magnetooptical recording medium, information is recorded in accordance with a difference in direction of perpendicular magnetization. Recording schemes are classified into an optical modulation scheme and a magnetic field modulation scheme. The optical modulation scheme is a scheme for radiating a laser beam intensity-modulated in accordance with recording information on a recording medium to record the information while a predetermined external magnetic field is being applied to the recording medium. On the other hand, the magnetic field modulation scheme is a scheme for applying an external magnetic field modulated in accordance with recording information while a recording medium is being irradiated with a laser beam having a predetermined intensity.
When a linearly polarized light component is radiated on a magnetooptical recording medium on which information is recorded depending on a difference in direction of magnetization, the polarization direction of the reflected light is rotated clockwise or counterclockwise depending on the difference in direction of magnetization. For example, as shown in FIG. 2, assume that the polarization direction of the linearly polarized light component incident on the magnetooptical recording medium is a P (ordinate) direction, that reflected light with respect to downward magnetization is R.sub.+ rotated through +.theta..sub.k, and that reflected light with respect to upward magnetization is R.sub.- rotated through -.theta..sub.k. When an analyzer is located in a direction indicated in FIG. 2, light passing through the analyzer becomes A with respect to R.sub.+ and B with respect to R.sub.-. When these light components are detected by the photodetectors, information as a difference in light intensity can be obtained. In the example of FIG. 1, the polarizing beam splitter 30 plays a role as the analyzer for rotating one split light beam through +45.degree. from the P-axis and the other light beam through -45.degree. from the P-axis. That is, the resultant signal components from the photodetectors 31 and 32 have opposite phases. When these signals are differentially detected, a reproduction signal whose noise is reduced is obtained. In order to increase the storage capacity, a recording scheme is recently being shifted from a mark interval recording scheme in which the central position of an information pit is meaningful to a mark length recording scheme in which the length of an information pit is meaningful. According to the mark length recording scheme, the position of an information pit is optically detected with an optical head, and the position information is electrically processed to detect the edge of the information pit.
In a conventional technique, however, light in the P-axis direction and light in the S-axis direction are detected using the polarizing beam splitter 30 shown in FIG. 1 to reproduce information. In this reproduction scheme, the number of components of the optical head is increased to undesirably result in a bulky, complicated arrangement. In the mark length recording scheme, the edge of the information pit (domain) must be detected. In the conventional technique, the edge is not directly optically detected. When the size of the domain becomes equal to that of the light spot, the detection position of the edge is shifted, and information cannot be accurately detected, resulting in inconvenience.
SUMMARY OF THE INVENTION
The present invention has been made to solve the conventional problems described above, and has as its object to provide a magnetooptical information recording/reproducing apparatus capable of reducing the number of components constituting an optical head and accurately detecting the edge position of an information pit.
In order to achieve the above object according to an aspect of the present invention, there is provided a magnetooptical information recording/reproducing apparatus in which a light beam is radiated on a magnetooptical recording medium and information recorded on a recording medium is reproducted from the light reflected by the recording medium, characterized in that light produced by a magnetooptic effect of the magnetooptical recording medium is interfered with light produced by diffraction at a curved surface of a lens for condensing the light beam radiated on the recording medium into a minute light spot, and the interference light is detected by a photodetector, thereby reproducing the recorded information.
In order to achieve the above object according to another aspect of the present invention, there is provided a magnetooptical information recording/reproducing apparatus in which a light beam is radiated on a magnetooptical recording medium and information recorded on a recording medium is reproduced from the light reflected by the recording medium, characterized in that light produced by a magnetooptic effect of the magnetooptical recording medium and light produced by diffraction at a curve surface of a lens for condensing the light beam radiated on the recording medium into a minute light spot are split into light beams by a polarizing beam splitter serving as an analyzer, and the split light beams are respectively detected by multi-divided photodetectors, thereby reproducing information.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a constructional view showing an optical system of a conventional magnetooptical information recording/reproducing apparatus;
FIG. 2 is a view showing the polarization direction of a linearly polarized light component incident on the magnetooptical recording medium, the state of rotation of the reflected light on the medium in correspondence with the direction of magnetization, and the state of change in the reflected light with respect to an analyzer;
FIG. 3 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to an embodiment of the present invention;
FIGS. 4(a-1) to 4(c-3) are views for explaining the function of a .lambda./2 plate used in the embodiment shown in FIG. 3;
FIGS. 5A to 5C are views showing distributions of light components diffracted at the curved surface of an objective lens in the embodiment shown in FIG. 3;
FIG. 6 is a view showing the amplitude of light along the line B-B' in FIG. 5;
FIGS. 7A to 7E are views for explaining an information reproduction operation of the embodiment shown in FIG. 3;
FIG. 8 is a view showing a magnetooptical information recording/reproducing apparatus according to another embodiment of the present invention;
FIGS. 9A and 9B are views showing distributions of light components diffracted at the curved surface of an objective lens in the embodiment shown in FIG. 8;
FIGS. 10A and 10B are views showing the amplitudes of the light components along the lines B-B' and C-C';
FIGS. 11A to 11F are views for explaining an information reproduction operation of the embodiment shown in FIG. 8;
FIG. 12 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to still another embodiment of the present invention;
FIGS. 13A and 13B are views showing the distributions of light components diffracted at the curved surface of an objective lens of the embodiment shown in FIG. 12;
FIGS. 14A to 14G are views for explaining an information reproduction operation of the embodiment shown in FIG. 12;
FIG. 15 is a view showing a recording scheme discrimination mark added to the case of a magnetooptical disk, and a sensor for detecting the mark;
FIG. 16 is a flow chart showing a sequence for controlling switching between signal reproduction operations for a pit position recording disk and a pit edge recording disk;
FIGS. 17A to 17E are views showing pit trains in pit position recording and pit edge recording, and detection signals thereof;
FIG. 18 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to still another embodiment of the present invention;
FIGS. 19A and 19B are a view obtained such that the distribution of P.sub.+ light immediately before incidence on a polarizing beam splitter is projected on the detection surface of a four-divided photodetector, and a view showing the amplitude of light on the section along the line A-A' on the detection surface, respectively;
FIGS. 20A to 20C are views obtained such that the distribution of an S-polarized light component at the curved surface of the objective lens immediately before incidence on the polarized beam splitter is projected on the detection surface of the four-divided photodetector, and showing the amplitudes of the light components on the sections along the lines B-B' and C-C' of the detection surfaces, respectively;
FIGS. 21A to 21I are views for explaining an information reproduction operation of the embodiment shown in FIG. 18;
FIGS. 22A and 22B are views showing the distribution of an S-polarized light component produced by a magnetooptic effect upon shifting the objective lens of FIG. 18 in the tracking direction and the distribution of an S-polarized light component produced at the curved surface of the objective lens, respectively;
FIGS. 23A and 23B are views showing the amplitudes of light components on the sections along the lines B-B' and C-C' of FIG. 22B;
FIGS. 24A to 24D are views showing changes in DC component superposed on a reproduction signal upon shifting the objective lens of FIG. 18 in the tracking direction;
FIG. 25 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to still another embodiment of the present invention;
FIG. 26 is a view showing a lens position sensor used in the embodiment of FIG. 25 and a reflecting plate adhered to the side portion of an objective lens;
FIG. 27 is a plan view showing the lens position sensor and the objective lens shown in FIG. 26;
FIG. 28 is a graph showing the relationship between the shift amount of the objective lens and the output from the lens position sensor;
FIG. 29 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to still another embodiment of the present invention;
FIG. 30 is a front view showing a plane-parallel plate used in the embodiment of FIG. 29;
FIG. 31 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to still another embodiment of the present invention;
FIGS. 32A and 32B are a view obtained such that the distribution of P.sub.+ light immediately before incidence on a polarizing beam splitter is projected on the detection surfaces of a four-divided photodetector, and a view showing the amplitude of light on the section along the line A-A' on the detection surfaces, respectively;
FIGS. 33A to 33C are a view obtained such that the distribution of an S-polarized light component at the curved surface of the objective lens immediately before incidence on the polarized beam splitter is projected on the detection surfaces of the four-divided photodetector, and views showing the amplitudes of the light components on the sections along the lines B-B' and C-C' of the detection surfaces, respectively;
FIGS. 34A to 34I are views for explaining an information reproduction operation of the embodiment shown in FIG. 31;
FIG. 35 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to still another embodiment of the present invention;
FIGS. 36A and 36B are views showing a conventional reproduction scheme for reproducing a domain smaller than a recording light spot;
FIGS. 37A and 37B are views showing another conventional reproduction scheme;
FIG. 38 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to still another embodiment of the present invention;
FIG. 39 is a sectional view showing the structure of a magnetooptical recording medium used in the embodiment of FIG. 38;
FIG. 40 is a graph showing the relationship between the coercive forces of magnetic layers of the magnetooptical recording medium shown in FIG. 39 and the temperature;
FIGS. 41A and 41B are views showing light spots an information track, information domains, and the magnetized states of the respective magnetic layers in information recording of the embodiment shown in FIG. 38;
FIGS. 42A and 42B are views showing light spots on an information track, information domains, and the magnetized states of the respective magnetic layers in information reproduction of the embodiment shown in FIG. 38;
FIGS. 43A to 43I are views for explaining a detection operation of a reproduction signal for verification in information recording of the embodiment shown in FIG.
FIGS. 44A to 44I are views for explaining a detection operation of a reproduction signal in information reproduction of the embodiment shown in FIG. 38; and
FIG. 45 is a constructional view showing still another embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 3 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to an embodiment of the present invention. The same reference numerals as in the conventional apparatus of FIG. 1 denote the same parts or functions in FIG. 3, and a detailed description thereof will be omitted. Referring to FIG. 3, an objective lens 23 has an NA of about 0.55 and has a surface having a large curvature. When a linearly polarized beam is incident from a semiconductor laser 19 to the curvature surface of the objective lens 23, the polarization plane is rotated because the reflectance for the incident light is significantly different from the reflectance for perpendicularly polarized light. When the polarization plane of the incident light and the perpendicularly polarized light are taken into consideration, a diffracted image in the form of a four-leaf clover (to be described later) is obtained. The light beam from the semiconductor laser 19 is a linearly polarized light component (P-polarized light component) having the polarization direction parallel to the drawing surface as in the conventional case. A .lambda./2 plate 1 is arranged between the objective lens 23 and a polarizing beam splitter 22. The characteristics of a polarizing beam splitter 2 are given such that the transmittance for the P-polarized light component is 100%, the reflectance for the P-polarized light component is 0%, the transmittance for the S-polarized light component is 0%, and the reflectance for the S-polarized light component is 100%. The magnetooptical information recording/reproducing apparatus also includes a condenser lens 3, a two-divided photodetector 4, and a differential amplifier 5. The two-divided photodetector 4 is arranged such that its division line is perpendicular to an information track of a magnetooptical disk 24. Other arrangements of FIG. 3 are the same as those of the conventional apparatus shown in FIG. 1. The information recording scheme is the magnetic field modulation scheme, the autofocusing control scheme in the control optical system is knife edge scheme, and the autotracking control scheme is a push-pull scheme.
FIGS. 4(a-1), 4(a-2), and 4(a-3) show the behaviors of the polarized incident component, FIGS. 4(b-1), 4(b-2), and 4(b-3) show the behaviors of the polarized component perpendicular to the polarized incident component, which is produced by information on the magnetooptical disk 24, and FIGS. 4(c-1), 4(c-2), and 4(c-3) show the behaviors of the polarized component perpendicular to the polarized incident component, which is produced by diffraction at the curved surface of the objective lens
23. FIGS. 4(a-1), 4(b-1), and 4(c-1) show the behaviors of incident light before incidence on the .lambda./2 plate 1, FIGS. 4(a-2), 4(b-2), and 4(c-2) show the behaviors of reflected light before incidence on the .lambda./2 plate 1 after the reflected light passes through the objective lens 23, and FIGS. 4(a-3), 4(b-3), and 4(c-3) show the behaviors of the reflected light after it passes through the .lambda./2 plate. In FIGS. 4(a-1) to 4(c-3), P represents the direction of the P-polarized light component, and S represents the direction of the S-polarized light component. Each dotted line indicates the optical axis of the .lambda./2 plate 1. Note that the optical axis of the .lambda./2 plate 1 is located to form an angle of 22.5.degree. with respect to the direction of the P-polarized light component.
The incident light before incidence on the .lambda./2 plate 1 is a P-polarized light component, as shown in FIG. 4(a-1). At this time, as shown in FIGS. 4(b-1) and 4(c-1), an S-polarized light component is not yet produced. The reflected light before incidence on the .lambda./2 plate 1 through the objective lens 23 upon reflection on the magnetooptical disk 24 passes through the .lambda./2 plate 1 once, so that the polarization plane of the polarized incident component is rotated through
45.degree., as shown in FIG. 4(a-2). This light component is a polarized light component having an angle of 45.degree. with respect to the track direction on the magnetooptical disk 24. As shown in FIG. 4(b-2), the polarized light component perpendicular to the polarization direction of the component of FIG. 4(a-2) is produced by magnetooptic effect (a Kerr or Faraday effect). In addition, when the light component of FIG. 4(a-2) is going and returning through the objective lens 23, the polarized light component perpendicular to the polarization direction of the light component of FIG. 4(a-2) is produced, as shown in FIG. 4(c-2). When the reflected light beams of these light components pass through the .lambda./2 plate 1, the polarized incident light component returns to the P-polarized light component, as shown in FIG. 4(a-3). The light component produced by the magnetooptic effect and the light component produced at the curved surface of the objective lens 23 become S-polarized light components. When the intensities of the polarized incident component, the light component produced at the curved surface of the objective lens 23 and the light component produced by magnetooptic effect are defined by I.sub.i, I.sub.c and I.sub.o, respectively, the following relation is given.
The reflected light beams of the P- and S-polarized light components reflected by the magnetooptical disk 24 are split by the polarizing beam splitter 22 toward a reproduction optical system and are incident on the polarizing beam splitter 2. The characteristics of the polarizing beam splitter 2 are given such that the transmittance for the P-polarized light component is 100%, the reflectance for the P-polarized light component is 0%, the transmittance for the S-polarized light component is
0%, and the reflectance for the S-polarized light component is 100%. Therefore, the reflected light beams are split into S- and P-polarized light components. The P-polarized light beam directly passes through the polarizing beam splitter 2, so that a focusing error signal and a tracking error signal for autofocusing control and autotracking control are generated in the control optical system in the same manner as in the conventional case. On the other hand, the S-polarized light beam is reflected by the polarizing beam splitter 2 and is incident on the two-divided photodetector 4 through the condenser lens 3. The incident beam is detected by photodetection surfaces 4-1 and 4-2 of the two-divided photodetector 4, and detection signals therefrom are differentially detected by the differential amplifier 5, thereby generating a reproduction signal 6. As described above, the division line of the two-divided photodetector 4 is perpendicular to the information track of the magnetooptical disk 24. Of all light components, light components incident on the two-divided photodetector 4 are the S-polarized light beam produced by the magnetooptic effect, and the S-polarized light beam produced at the curved surface of the objective lens 23.
The S-polarized light beams produced by the magnetooptic effect are an S.sub.+ beam with respect to upward magnetization and an S.sub.- beam with respect to downward magnetization, as is apparent from FIG. 2. The S.sub.+ beam has a phase difference of .pi. from the S.sub.- beam. When the boundary (edge) between upward magnetization and downward magnetization comes in a light spot, light diffraction equal to a pit having a depth of .lambda./4 (.lambda. is the wavelength of light) occurs, which will be described in detail later. The S-polarized beams produced at the curved surface of the objective lens 23 are given as follows. An arrow A in FIGS. 5A to 5C indicates the direction of information tracks. FIG. 5A shows the distribution of the S.sub.+ light produced by the magnetooptic effect with respect to downward magnetization. FIG. 5C shows the distribution of S-polarized diffracted light produced at the curved surface of the objective lens 23 in the absence of the .lambda./2 plate in the optical head shown in FIG. 3. Although it will be described in detail, superposition of the diffracted light components produced by the magnetooptic effect is poor, and only a DC component is obtained. By arranging the .lambda./2 plate, the distribution of the diffracted light of the S-polarized light component produced at the curved surface of the objective lens 23 is changed, as shown in FIG. 5B. In the distribution in the form of a four-leaf clover, as shown in FIG. 5B, the opposite leaves are in-phase components, and the adjacent leaves have a phase difference of .pi.. The light in FIG. 5B is incident on the two-divided photodetector 4 regardless of the direction of magnetization on the magnetooptical disk
24. The amplitude of the light on the section along the line B-B' in FIG. 5B is shown in FIG. 6. In this embodiment, the light beam produced at the curved surface of the objective lens 23, as shown in FIG. 5B, is interfered with the light beam produced by the magnetooptic effect. A change in resultant light amount distribution is detected to reproduce the information.
The information reproduction operation will be described in detail with reference to FIGS. 7A to 7E. FIG. 7A is a view showing a recorded information pit (domain) and a reproduction light spot. A domain 8 is recorded on an information track 7
in accordance with the magnetic field modulation scheme. The recording scheme may be an optical modulation scheme. In this embodiment, the magnetization of the magnetooptical disk 24 faces downward in initialization, so that the direction of magnetization of the domain 8 recorded on the information track 7 is upward. A light spot 9 scans the information track 7 recorded with the domain 8 in the A direction, as indicated by 10, 11, 12, and 13. FIG. 7B is a view showing light distributions on the two-divided photodetector 4 so as to correspond to the respective light spot positions of FIG. 7A. At the positions of the light spots 9 and 13, since all the directions of magnetization within the light spots are downward, no light diffraction occurs. The distributions on the two-divided photodetector 4 represent circles, as indicated by 14 and 18. At the positions of the light spots 10 and 12, since the edges between upward and downward magnetization components are located within the light spots, light components parallel to the information track 7 are diffracted, so that each distribution is divided laterally, as indicated by 15 or 16. The phase difference between the right and left light components becomes .pi.. In this case, the phase of 15 is opposite to that of 17. In addition, at the position of the light spot 11, downward magnetization slightly enters at the upper and lower portions of the light spot, so that the light component is slightly diffracted in a direction perpendicular to the track, so that a distribution 16 slightly elongated in the vertical direction is obtained. In this case, the phase of light has a difference of .pi. from 14 or 18.
FIG. 7C is a view showing the amplitude distribution of light on the two-divided photodetector shown in FIG. 7B along the section of B-B'. At the positions of the light spots 9, 11, and 13, the intensity distributions are symmetrical about the division line. FIG. 7D is a view showing the intensity distribution of light obtained when the distribution of light (FIG. 7C) produced by the magnetooptic effect is interfered with the intensity distribution of light (FIG. 6) produced by diffraction at the curved surface of the objective lens. At the positions of the light spots 9, 11, and 13, the intensity distributions become symmetrical about the division line as in FIG. 7C. The intensity distributions at the positions of the light spots 10 and 12
which correspond to the edges of the domain 8 become asymmetrical about the division line. FIG. 7E shows a reproduction signal obtained by differentially detecting output signals from the photodetection surfaces 4-1 and 4-2 of the two-divided photodetector 4 by the differential amplifier 5. Since the intensity distributions on the B-B' section of the two-divided photodetector 4 are symmetrical about the division line at the positions of the light spots 9, 11, and 13, differential detection by the differential amplifier 5 results in an output signal of "0", as shown in FIG. 7E. That is, no reproduction signal appears. To the contrary, since the intensity distributions at the positions of the light spots 10 and 12 which correspond to the edges of the domain 8 are asymmetrical about the division line, a reproduction signal having a positive or negative peak, as shown in FIG. 7E, is obtained. By detecting the positive and negative peak positions of the reproduction signal, the edges of the domain 8 can be detected.
In this embodiment, the light produced by the magnetooptic effect of the magnetooptical disk 24 is interfered with the light produced by diffraction at the curved surface of the objective lens 23, and a change in light amount distribution is detected. Therefore, the conventional polarizing beam splitter required for detecting the light components along the P- and S-axes can be omitted, thereby simplifying the arrangement of the optical head accordingly. Since the edge of the domain is optically directly detected, the edge position of the domain can be accurately detected regardless of the size of the domain and the size of the light spot.
FIG. 8 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to another embodiment of the present invention. In this embodiment, the division line of a two-divided photodetector 4 is parallel to information tracks of a magnetooptical disk 24. Detection signals from the photodetection portions of the two-divided photodetector 4 are differentially detected to generate a reproduction signal. The .lambda./2 plate 1 used in the embodiment of FIG. 3
is not used in this embodiment. Other constructions are the same as those of the embodiment of FIG. 3, and a detailed description thereof will be omitted. A light beam from a semiconductor laser 19 is a linearly polarized light component (P-polarized light component) having the polarization direction parallel to the drawing surface as in the embodiment of FIG. 3. This light beam is radiated on the magnetooptical disk 24 through a collimator lens 20, a beam shaping prism 21, a polarizing beam splitter 22, and an objective lens 23. The objective lens 23 has an NA of about 0.55 as in the above embodiment and has a large curvature surface. The incident P-polarized light beam is converged by the objective lens 23 into a minute light spot. This light spot is formed on the magnetooptical disk 24. At this time, the polarization direction of the P-polarized light beam is parallel or perpendicular to the track of the magnetooptical disk 24. When the light beam is incident on the magnetooptical disk 24, a P-polarized light component and an S-polarized light component perpendicular to the P-polarized light component are produced by the magnetooptic effect (a Kerr or Faraday effect) of the disk. The light beam incident on the magnetooptical disk
24 is reflected by the medium surface and passes through the objective lens 23 again, thereby obtaining a collimated beam. This collimated beam contains an S-polarized light component produced at the curved surface of the objective lens 23 in addition to the S-polarized light component produced by the magnetooptical disk 24.
The light beam reflected by the magnetooptical disk 24 is reflected by the polarizing beam splitter 22 and is guided to a polarizing beam splitter 2. The characteristics of the polarizing beam splitter 2 are given such that the transmittance for the P-polarized light component is 100%, the reflectance for the P-polarized light component is 0%, the transmittance for the S-polarized light component is 0%, and the reflectance for the S-polarized light component is 100%. The P-polarized light beam passes through the polarizing beam splitter 2 and is guided to the control optical system. The S-polarized light beam is reflected and guided to a condenser lens 3 in the reproduction optical system. Error signals for autofocusing control and autotracking control are generated in the control optical system. The S-polarized light beam guided to the condenser lens 3 is detected by the photodetection portions of the two-divided photodetector 4. Detection signals from the photodetection portions are differentially detected by a differential amplifier 5 to produce a reproduction signal 6. The division line of the two-divided photodetector 4 is parallel to the track of the magnetooptical disk 24. Light components incident on this photodetector 4 are the S-polarized light beam produced by the magnetooptic effect and the S-polarized light beam produced at the curved surface of the objective lens 23. The S-polarized light beams produced by the magnetooptic effect are the S.sub.+ light beam with respect to upward magnetization and the S.sub.- light beam with respect to downward magnetization. When the boundary (edge) of the upward magnetization and the downward magnetization enters into a reproduction light spot, light diffraction as in a pit having a depth of .lambda./4 (.lambda. is the wavelength of light) occurs.
The S-polarized light beams produced at the curved surface of the objective lens 23 are as follows. As shown in FIGS. 9A and 9B, the two-divided photodetector 4 has photodetection portions having detection surfaces 4-1 and 4-2. An arrow A indicates the track direction. FIG. 9A shows the distribution of the S.sub.+ light beam produced by the magnetooptic effect with respect to downward magnetization, and FIG. 9B shows the distribution of the diffracted light of the S-polarized light component produced at the curved surface of the objective lens 23. The distribution in FIG. 9B has the form of a four-leaf clover. The opposite leaves are in-phase components, and the adjacent leaves have a phase different of .pi.. That is, light components of the opposite leaves within the detection surfaces 4-1 and 4-2 are in-phase components, and light components of the adjacent leaves within the detection surfaces 4-1 and 4-2 have a phase difference of .pi.. This light distribution in FIG.
9B appears on the two-divided photodetector regardless of the direction of magnetization on the magnetooptical disk 24. FIGS. 10A and 10B are views showing the amplitudes of the light components on the sections along the lines B-B' and C-C' in FIG. 9B. As described above, FIG. 10A shows the amplitude along the line B-B', and FIG. 10B shows the amplitude along the line C-C', in accordance with the phase relationship in the form of the four-leaf clover. In this embodiment, the light beam produced by the magnetooptic effect, as shown in FIG. 9A, is interfered with the light beam produced at the curved surface of the objective lens 23, as shown in FIG. 9B, and the change in light amount distribution is detected to reproduce information.
The information reproduction operation will be described in more detail with reference to FIGS. 11A to 11F. FIG. 11A shows a reproduction light spot and a recorded information pit (domain). A domain 8 is recorded on an information track 7 in accordance with the magnetic field modulation scheme. The recording scheme may be an optical modulation scheme. In this embodiment, the magnetization of the magnetooptical disk 24 faces downward in initialization, so that the direction of magnetization of the domain 8 is upward. A light spot 9 scans the information track 7 recorded with the domain 8, in the A direction, as indicated by 10, 11, 12, and 13. FIG. 11B is a view showing light distributions of the S-polarized light components produced by the magnetooptic effect on the two-divided photodetector 4 so as to correspond to the respective light spot positions of FIG. 11A. The two-divided photodetector 4 has the detection surfaces 4-1 and 4-2. At the positions of the light spots 9 and 13, since all the directions of magnetization within the light spots are downward, no light diffraction occurs. The distributions on the two-divided photodetector 4 represent circular shapes. At the positions of the light spots 10 and 12, since the edges of the domain 8 are located within the light spots, and the upward magnetization and downward magnetization are mixed, light components parallel to the information track 7 are diffracted, so that each distribution is divided laterally. The phase difference between the right and left light components becomes .pi.. In this case, the phase of the right component is opposite to that of the left component at the positions of the light spots 10 and 12. In addition, at the position of the light spot
11, the direction of magnetization is almost upward, and no diffraction of light occurs, thereby providing the illustrated light distribution. At this time, the light has a phase difference of .pi. from the positions of the light spots 9 and 13.
FIG. 11C is a view showing the amplitude distribution of light on the two-divided photodetector shown in FIG. 11B along the section of D-D'. FIGS. 11D and 11E are views showing the intensity distributions of the light components obtained by interfering the distribution of light in FIG. 11B with the distribution of the light shown in FIG. 9B on the two-divided photodetector 4. FIG. 11D shows the intensity distribution along the line B-B', and FIG. 11E shows the intensity distribution along the line C-C'. In the distributions of FIGS. 11D and 11E, at the positions of the light spots 9, 11, and 13, the right and left distributions on the two-divided photodetector 4 are asymmetrical, but the amount of light on the detection surface 4-1 is equal to that on the detection surface 4-2. At the positions of the light spots 10 and 12 where the edges of the domain 8 are located, the right and left distributions are symmetrical with each other, but the amount of light on the detection surface 4-1
is different from that on the detection surface 4-2. When detection signals from the detection surfaces 4-1 and 4-2 are differentially detected by the differential amplifier 5, no reproduction signal appears at the positions of the light spots 9, 11, and 13 where the amounts of light on the detection surfaces 4-1 and 4-2 are equal to each other, as shown in FIG. 11F. A reproduction signal having a positive or negative peak appears at only a position of each of the light spots 10, and 12 where the amount of light on the detection surface 4-1 is different from that on the detection surface 4-2. The positive and negative peak positions of the reproduction signals are detected to detect the edges of the domain 8. In this embodiment, the structure of the optical head can be simplified as in the embodiment of FIG. 3, and the edge positions of the domain can be accurately detected.
FIG. 12 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to still another embodiment of the present invention. In this embodiment, a reproduction signal can be obtained from a pit position recording scheme or a pit edge recording scheme by using a four-divided photodetector as a photodetector. Referring to FIG. 12, a four-divided photodetector 50 detects light reflected by a magnetooptical disk 24. Addition amplifiers 51 and 52 add detection signals from diagonal detection surfaces of the four-divided photodetector 50. A differential amplifier 53 differentially detects outputs from the addition amplifiers 51 and 52 to generate a reproduction signal of information of the pit position recording scheme. Addition amplifiers 54 and 55 add detection signals from the adjacent detection surfaces parallel to the track of the four-divided photodetector 50. A differential amplifier 56 differentially detects output signals from the addition amplifiers 54 and 55 to reproduce information of the pit edge recording scheme. Other constructions of this embodiment are the same as those of the embodiment shown in FIG. 8, and a detailed description thereof will be omitted. In this embodiment, the information recording scheme is a magnetic recording scheme, focusing control is performed by a knife edge scheme, and tracking control is performed by a push-pull scheme as in the above embodiment. In addition, a light beam from a semiconductor laser 19 is a linearly polarized light component (P-polarized light component) having the polarization direction parallel to the drawing surface.
The entire detection surface of the four-divided photodetector 50 is divided into four portions in a crossed shape. One of the division lines of the four-divided photodetector 50 is parallel to the track direction of the magnetooptical disk 24, and the other division line is perpendicular thereto. The light beams incident on the four-divided photodetector 50 are the S-polarized light beam produced by the magnetooptic effect as described above and the S-polarized light beam produced at the curved surface of an objective lens 23. The S-polarized light beams produced by the magnetooptic effect have been described with reference to the embodiments shown in FIGS. 3 and 8. The S-polarized light beams produced at the curved surface of the objective lens 23 are basically the same as those of the embodiment of FIG. 8. However, since the number of detection surfaces of this embodiment is different from that of the embodiment shown in FIG. 8, the detection surfaces of the four-divided photodetector 50 and the light distributions on the detection surfaces are shown in FIGS. 13A and 13B. Referring to FIGS. 13A and 13B, the four-divided photodetector 50 has four detection surfaces 50-1 to 50-4. An arrow A indicates the track direction. FIG. 13A shows the distribution of the S.sub.+ light beam produced by the magnetooptic effect with respect to downward magnetization. FIG. 13B shows the distribution of the diffracted light of the S-polarized light component produced at the curved surface of the objective lens 23. In the distribution in the form of a four-leaf clover, as shown in FIG. 13B, opposite leaves are in-phase components, and adjacent leaves have a phase difference of .pi.. The light in the form of a four-leaf clover shown in FIG. 13B is distributed on the four-divided photodetector 50 regardless of the direction of magnetization of the magnetooptical disk 24. The amplitudes of the light components along the lines B-B' and C-C' in FIG. 13B are as shown in FIGS. 10A and 10B. In this embodiment, the light beam produced at the curved surface of the objective lens 23, as shown in FIG. 13B, is interfered with the light beam produced by the magnetooptic effect, and a change in light amount distribution is detected, thereby reproducing the information.
The information reproduction operation will be described in more detail with reference to FIGS. 14A to 14G. FIG. 14A is a view showing a reproduction light spot and an information pit (domain). A domain 8 is recorded on an information track 7
by the magnetic field modulation scheme. However, the recording scheme may be a light modulation scheme. The magnetization of the magnetooptical disk 24 is entirely initialized downward, and the direction of magnetization of the domain 8 is upward. A light spot 9 scans the information track 7 along the A direction, as indicated by 10, 11, 12, and 13. FIG. 14B is a view showing the distribution of the S-polarized light produced by the magnetooptic effect on the four-divided photodetector 50 at the respective positions of the light spots shown in FIG. 14A. The light distribution on the four-divided photodetector 50 is the same as that of the embodiment shown in FIG. 8 (FIGS. 11A to 11F). More specifically, at the positions of the light spots 9
and 13, all the directions of magnetization within the light spots are downward, no diffraction of light occurs, and the distributions on the four-divided photodetectors 50 assume a circular shape, respectively. At the positions of the light spots 10
and 12, since the edges of the domain 8 are located within the light spots, the light is diffracted parallel to the track 7, so that the light distribution is divided into right and left distributions, as shown in FIG. 14B. Since the direction of magnetization within the light spot at the position of the light spot 11 is almost upward, the illustrated distribution is obtained almost free from the diffraction of the light.
FIG. 14C is a view showing the amplitude of the light on the section along the line D-D' of FIG. 14B. FIGS. 14D and 14E are views showing the intensity distributions of light components obtained when the distribution of the light diffracted at the curved surface of the objective lens, as shown in FIG. 13B is interfered with the distribution of the light produced by the magnetooptic effect, as shown in FIG. 14B, on the four-divided photodetector 50. FIG. 14D shows the intensity distribution along the line B-B', and FIG. 14E shows the intensity distribution along the line C-C'. The amplitudes and intensity distributions of these light components are the same as those described with reference to FIGS. 11A to 11F. The intensities of the positions of the light spots 9 and 13 on the detection surfaces 50-1 and 50-3 are higher than those on the detection surfaces 50-2 and 50-4. In this case, the intensity of the detection surface 50-1 becomes equal to that of the detection surface 50-3, and the intensity of the detection surface 50-2 becomes equal to that of the detection surface 50-4. To the contrary, the intensities of the position of the light spot 11 on the detection surfaces 50-2 and 50-4 are higher than those on the detection surfaces 50-1 and 50-3. In this case, the intensity of the detection surface 50-2 becomes equal to that of the detection surface 50-4, and the intensity of the detection surface 50-1 becomes equal to that of the detection surface 50-3. At the position of the light spot 10 where the left edge of the domain 8 is located, the intensities of the detection surfaces 50-1 and 50-2 are higher than those of the detection surfaces 50-3 and 50-4. At this time, the intensity of the detection surface 50-1 becomes equal to that of the detection surface 50-2, and the intensity of the detection surface 50-3 becomes equal to that of the detection surface 50-4. At the position of the light spot 12 where the right edge of the domain 8 is located, the intensities of the detection surfaces 50-3 and 50-4 are higher than those of the detection surfaces 50-1 and 50-2. At this time, the intensity of the detection surface 50-3 becomes equal to that of the detection surface 50-4, and the intensity of the detection surface
50-1 becomes equal to that of the detection surface 50-2.
The detection signals from the detection surfaces of the four-divided photodetector 50 are output to the addition amplifiers 51, 52, 54, and 55, as described with reference to FIG. 12. The detection signals from the diagonal detection surfaces
50-1 and 50-3 and the diagonal detection surfaces 50-2 and 50-4 are added by the addition amplifiers 51 and 52, respectively. The resultant sum signals from the addition amplifiers 51 and 52 are differentially detected by the differential amplifier 53, thereby obtaining a reproduction signal, as shown in FIG. 14F. That is, a positive signal is obtained in the region where the direction of magnetization is downward, while a negative signal is obtained in the region where the direction of magnetization is upward. A reproduction signal which rises or falls at the edge of the domain and corresponds to the pit position recording can be obtained. The detection signals from the detection surfaces 50-1 and 50-2 adjacent to each other in the track direction and the detection surfaces 50-3 and 50-4 adjacent to each other in the track direction are added by the addition amplifiers 54 and 55. The resultant sum signals are differentially detected by the differential amplifier 56, thereby obtaining a reproduction signal, as shown in FIG. 14G. That is, a reproduction signal having a positive or negative peak at the edge of the domain 8 is obtained, thereby obtaining the reproduction signal corresponding to the pit edge recording.
In this embodiment, the information recording/reproducing apparatus corresponding to either pit position recording or pit edge recording has been described. When a recording medium is inserted into the apparatus, it is desirable that reproduction in pit position recording or pit edge recording can be automatically selected in accordance with the type of recording medium. A selective switching control device will be exemplified below. As shown in FIG. 15, a mark 61 representing a pit position recording disk or a pit edge recording disk is added to a case 60 of the magnetooptical disk 24. The mark 61 can be optically detected. For example, a mark having a high reflectance represents a pit position recording disk, while a mark having a low reflectance represents a pit edge recording disk. On the other hand, light-emitting diodes 62 and 63 are provided on the apparatus. When the disk is inserted into the apparatus, light is emitted from the light-emitting diode 62 to the mark
61, and light reflected by the mark 61 is received by the photosensor 63. When the magnetooptical disk 24 is inserted into the apparatus, signal reproduction is Controlled and switched in accordance with the sequence shown in FIG. 16. That is, the magnetooptical disk is inserted into the apparatus (step 1). The mark 61 of the disk case 60 is investigated on the basis of a detection signal from the photosensor 63 (step 2). It is determined in step 3 whether the disk is a pit position recording disk or a pit edge recording disk. Reproduction corresponding to pit position recording (step 4) or pit edge recording (step 5) is selected in accordance with the determination result. When the magnetooptical disk is inserted into the apparatus, the signal reproduction corresponding to the disk recording scheme can be automatically selected.
When information is to be recorded in the form of a pit, a pit position recording scheme in which the central position of the pit represents meaningful information or a pit edge recording scheme in which the edge position of the pit represents meaningful information is available. FIGS. 17A to 17E are views for explaining these two recording schemes. FIG. 17A shows a pit train obtained in pit position recording. The sizes of the pits are almost identical among adjacent pits. FIG. 17B shows a detection signal obtained by optically reproducing the pit train shown in FIG. 17A. FIG. 17C shows a pit train obtained in pit edge recording. FIG. 17D shows a detection signal obtained by optically reproducing the pit train in FIG. 17C. In order to detect an edge position of each pit from the detection signal, for example, a slice level is electrically set, and a position where the detection signal in FIG. 17D crosses the slice level is obtained. FIG. 17E shows an edge detection signal.
According to the embodiment described above, a magnetooptical information recording/reproducing apparatus in which a light beam is radiated on a magnetooptical recording medium and information recorded on the recording medium is reproduced from the light reflected by the recording medium is characterized in that light produced by magnetooptic effect of the magnetooptical recording medium is interfered with light produced by diffraction at a curved surface of a lens for converging the light beam radiated on the recording medium into a minute light spot, and the interference light is detected by a photodetector, thereby reproducing the recorded information.
The magnetooptical information recording/reproducing apparatus includes a .lambda./2 plate, arranged in an optical path of light incident on the magnetooptical recording medium, for rotating the polarization direction through about 45.degree. with respect to an information track of the recording medium, and a polarizing beam splitter for splitting the light reflected by the recording medium into linearly polarized light components having the polarization directions parallel and perpendicular to the polarization direction of the incident light, wherein the split light components are detected by a two-divided photodetector having detection surfaces divided in a direction perpendicular to the information track, and detection signals from the detection surfaces of the photodetector are differentially detected, thereby reproducing the information.
In the magnetooptical information recording/reproducing apparatus, a polarizing beam splitter is arranged to split the light reflected by the magnetooptical recording medium into linearly polarized light components having the polarization directions parallel and perpendicular to the polarization direction of the incident light, the split light components are detected by a two-divided photodetector having detection surfaces divided in a direction parallel to the information track, and detection signals from the detection surfaces of the photodetector are differentially detected, thereby reproducing the information.
In the magnetooptical information recording/reproducing apparatus, a polarizing beam splitter is arranged to split the light reflected by the magnetooptical recording medium into linearly polarized light components having the polarization directions parallel and perpendicular to the polarization direction of the incident light, the split light components are detected by a four-divided photodetector having detection surfaces divided in directions parallel and perpendicular to the information track, and a predetermined analog operation is performed using detection signals from the detection surfaces of the photodetector, thereby performing signal reproduction corresponding to pit position recording or pit edge recording.
In the magnetooptical information recording/reproducing apparatus, the detection signals from the diagonal detection surfaces of the four-divided photodetectors are added to each other, and the resultant sum signals are differentially detected, thereby reproducing information corresponding to pit position recording.
In the magnetooptical information recording/reproducing apparatus, the detection signals from the adjacent detection surfaces of the four-divided photodetectors along the direction of information tracks are added to each other, and the resultant sum signals are differentially detected, thereby reproducing information corresponding to pit edge recording.
In the magnetooptical information recording/reproducing apparatus, a means for detecting medium identification information added to the magnetooptical recording medium is used to determine whether the medium identification information represents a pit edge recording medium or a pit position recording medium, thereby selecting signal reproduction corresponding to pit position recording or pit edge recording in accordance with this determination result.
As described above, this embodiment has the following effects.
(1) The light produced by the magnetooptic effect of the magnetooptical recording medium is interfered with the light produced by diffraction at the curved surface of the objective lens, and a change in light amount distribution is detected, so that the number of components constituting the optical head can be reduced, thereby obtaining a compact, lightweight, low-cost optical head.
(2) A conventional problem posed such that the detection position of the pit edge is shifted when the size of the pit becomes equal to that of the light spot so as to optically and directly detect the edge of the recording pit can be solved. Therefore, the position of the pit edge can be accurately detected regardless of the size of the pit, thereby greatly improving reliability of a reproduction signal according to the pit edge recording scheme.
(3) Light is detected by the four-divided photodetector having detection surfaces divided in directions parallel and perpendicular to the information track, and the predetermined analog operation is performed using the detection signals from the detection surfaces. Therefore, information reproduction can be performed from a pit position recording medium or a pit edge recording medium using a single apparatus.
Still another embodiment of the present invention will be described in detail with reference to the accompanying drawings. FIG. 18 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to this embodiment. More specifically, FIG. 18 shows only the arrangement of a reproduction optical system as the main part of this embodiment. Other constructions of this embodiment are substantially the same as those in FIG. 1, and identical parts are omitted in FIG. 18. In this embodiment, an objective lens 23 has a curved surface having a large curvature corresponding to an NA of about 0.5 or more. Referring to FIG. 18, the reproduction optical system includes a polarizing beam splitter 26, a .lambda./2 plate 28, a condenser lens 29, and a polarizing beam splitter 30 serving as an analyzer. These optical components are identical to those FIG. 1. Each of four-divided photodetectors 101 and 102 has four divided detection surfaces. The detection surfaces of the four-divided photodetectors 101 and 102 are illustrated in FIG. 18, and the directions of the information tracks of a magnetooptical disk 24 are indicated by arrows A and A' so as to clarify the relative positional relationship with the magnetooptical disk 24. The four-divided photodetector 101 has detection surfaces 101-1 to 101-4, and the arrow A represents the direction of the information tracks. The four-divided photodetector 102 has detection surfaces 102-1 to 102-4, and the arrow A' represents the direction of the information track. The right and left detection surfaces of the four-divided photodetector 101 correspond to the left and right detection surfaces of the four-divided photodetector 102 so as to receive light through the polarizing beam splitter 30 serving as an analyzer. That is, the detection surfaces 101-1 and 102-1, the detection surfaces 101-2 and 102-2, the detection surfaces 101-3 and 102-3, and the detection surfaces 101-4 and 102-4 correspond to each other.
Addition amplifiers 103 and 104 add detection signals from the diagonal detection surfaces of the four-divided photodetector 101. Addition amplifiers 106 and 107 add the detection signals from the detection surfaces adjacent to each other along the direction of the information tracks of the four-divided photodetector 101. A differential amplifier 105 differentially detects the sum signals from the addition amplifiers 103 and 104. A differential amplifier 108 differentially detects the sum signals from the addition amplifiers 106 and 107. Addition amplifiers 109 and 110 add the detection signals from the diagonal detection surfaces of the four-divided photodetector 102. Addition amplifiers 112 and 113 add the detection signals from detection surfaces adjacent to each other along the direction of the track of the four-divided photodetector 102. A differential amplifier 111 differentially detects the sum signals from the addition amplifiers 109 and 110. A differential amplifier 114
differentially detects the sum signals from the addition amplifiers 112 and 113. A differential amplifier 115 differentially detects the output signals form the differential amplifiers 105 and 111. A signal obtained upon differential detection serves as a reproduction signal 116 corresponding to pit position recording. An addition amplifier 117 adds the output signals from the differential amplifiers 108 and 114. A sum signal from the addition amplifier 117 serves as a reproduction signal 118
corresponding to pit edge recording.
The operation of this embodiment will be described below. A light beam emitted from a semiconductor laser 19 is a linearly polarized light component (P-polarized light component) having the polarization direction parallel to the drawing surface in the same manner as in the conventional case. When this light beam is incident on the objective lens 23, the polarization plane is rotated because the objective lens 23 has a large curvature and the reflectance for the incident light beam is greatly different from that for the perpendicularly polarized light component. When only the polarized components perpendicular to the polarization plane of the incident light beam are taken into consideration, a diffracted image has the form of a four-leaf clover (to be described later). The light beams incident on the four-divided photodetectors 101 and 102 are the polarized light components having the same polarization direction as that of the incident P.sub.+ light and the polarized light component perpendicular thereto. Of these light components, S.sub.+ and S.sub.- light components produced by the Kerr or Faraday effect of the magnetooptical disk 24, and the light components produced at the curved surface of the objective lens 23 are also included.
FIG. 19A shows P.sub.+ light in the distribution of light immediately before incidence on the polarizing beam splitter 30 serving as an analyzer. FIG. 19B is a view showing the amplitude of the P.sub.+ light on the section along the line D-D' of FIG. 19A. Four squares shown in FIG. 19A represent the detection surfaces of the four-divided photodetector, and the P.sub.+ light is projected on the detection surfaces. In FIG. 19A, the P.sub.+ light is represented by hatched portions equally distributed on the four detection surfaces. The phases of the light components of the four detection surfaces are almost equal to each other. The amplitude of the light on the section along the line D-D' is shown in FIG. 19B. Of all light components immediately before incidence on the polarizing beam splitter 30 serving as the analyzer, S-polarized light produced at the curved surface of the objective lens 23 is shown in FIG. 20A. This light is projected on the four detection surfaces of the four-divided photodetector in the same manner as described above. As is apparent from FIG. 20A, the light distribution has the form of a four-leaf clover. The light components corresponding to the four leaves are projected on the four detection surfaces. The light components corresponding to the diagonal leaves are in-phase components, and the light components corresponding to the adjacent leaves have a phase difference of .pi.. The light component represented by a leaf E in FIG. 20A and the light component represented in FIG. 19A are in-phase components. FIGS. 20B and 20C are views showing the amplitudes of the light components on the sections along the lines B-B' and C-C' in FIG. 20A. The illustrated amplitudes are obtained in accordance with the relationship between the light components corresponding to the four leaves. The light beams in FIGS. 19A and 19B and FIGS. 20A to 20C have predetermined distributions regardless of the direction of magnetization on the magnetooptical disk 24. According to this embodiment, these two light beams and the S.sub.+ and S.sub.- light components changing depending on the magnetization on the magnetooptical recording medium are split by the polarizing beam splitter 30 serving as the analyzer. A change in light amount distribution upon interference between these three light beams is detected on the two four-divided photodetectors 101 and 102, thereby reproducing the information.
Information reproduction will be described in more detail with reference to FIGS. 21A to 21I. FIG. 21A is a view showing a recorded information pit (domain) and a reproduction light spot. A domain 8 is recorded on an information track 7. This embodiment employs a magnetic field modulation scheme. The domain 8 has a shape of the feather of an arrow. An optical modulation scheme may be used in place of the magnetic field modulation scheme. The magnetization of the magnetooptical disk 24
faces downward in initialization, so that the direction of magnetization of the domain 8 is upward. A light spot 9 scans the information track 7 recorded with the domain 8 in the A direction, as indicated by 10, 11, 12, and 13. Arrows A' and A in FIG.
21A correspond to the arrows A' and A shown in FIG. 18. FIG. 21B is a view showing distributions of the S-polarized light components produced by the magnetooptic effect immediately before incidence on the polarizing beam splitter 30 at the respective positions of the light spots in FIG. 21A. Each light spot is projected on the detection surfaces of the four-divided photodetector. At the positions of the light spots 9 and 13, all the directions of magnetization within the light spots are downward, and no diffraction of light occurs, thereby obtaining circular distributions, as indicated by 126 and 130. At this time, the light components are in-phase with the light in FIG. 19A and the light represented by E in FIG. 20A. If the light shown in FIGS. 19A and 19B is the P.sub.+ light indicated in FIG. 2, the light distributions 126 and 130 represent the S.sub.+ light beams.
At the positions of the light spots 10 and 12, the boundaries between the upward magnetization and the downward magnetization, i.e., the edges of the domain 8, are located within the light spots, light is diffracted in a direction parallel to the information track 7, and the distribution is divided into right and left portions, as indicated by each of distributions 127 and 129. The right and left light components have a phase difference of .pi.. The phase of the right light component is opposite to that of the left light component. The left light components are represented by 127a and 129a, and the right light components are represented by 127b and 129b in the distributions 127 and 129. The light components 127a and 129b are S.sub.+ light components, and the light components 127b and 129a are S.sub.- light components. When the edge of the domain 8 is located within a light spot, the S.sub.+ and S.sub.- light components are mixed in the resultant light distribution. At the position of the light spot 11, the direction of magnetization within the light spot is almost upward, and substantially no diffraction of light occurs. An almost circular distribution 128 is obtained. The light in the distribution 128 is the S.sub.- light having a phase difference of .pi. from those of the distributions 126 and 130. FIG. 21C is a view showing the magnitudes of the light components on the section along the line D-D' of FIG. 21B at the respective positions of the light spots. The amplitudes are changed depending on the positions of the light spots.
FIGS. 21D and 21E are views obtained such that the distributions of the light components obtained by interfering the S-polarized light components (FIGS. 20A to 20C) produced at the curved surface of the objective lens 23 with the S-polarized light components (FIG. 21B) produced by the magnetooptic effect are represented in the form of the amplitudes of the light components on the sections along the lines B-B' and C-C' of FIG. 21B. FIGS. 21F and 21G are views showing detection light amount differences on the detection surfaces of the four-divided photodetectors 101 and 102 after the S-polarized light components produced at the curved surface of the objective lens 23, the S-polarized light components produced by the magnetooptic effect, and the P.sub.+ light components shown in FIGS. 19A and 19B are synthesized and split into two components which are respectively incident on the four-divided photodetectors 101 and 102. FIG. 21F shows the detection light amount difference on the detection surfaces of the four-divided photodetector 101, and FIG. 21G shows the detection light amount difference on the detection surfaces of the four-divided photodetector 102. Referring to FIGS. 21F and 21G, the light amounts are represented as LL (L: large), LS (S: small), SL, and SS in order from the larger values. As described with reference to FIG. 18, since the light incident on the four-divided photodetector 102 is reflected by the polarizing beam splitter 30, the right or left detection surfaces of the four-divided photodetector 102 correspond to the left or right detection surfaces of the four-divided photodetector 101. When the light distributions on the four-divided photodetectors at the position of the light spot 9 as the left end position in FIGS. 21F and 21G are taken into consideration, the detection surface 101-1 of the four-divided photodetector 101 corresponds to the detection surface 102-1 of the four-divided photodetector 102. The light incident on these detection surfaces is a sum of the large upward amplitude (the S.sub.+ direction component shown in FIG. 2) on the left represented by F of the light amplitude on the section along the line B-B' in FIG. 21D and the upward amplitude (the P.sub.+ direction component shown in FIG. 2) shown in FIGS. 19A and 19B. That is, the light incident on the above detection surfaces is light rotated in the +.THETA. side. If the light polarized by the polarizing beam splitter 30 serving as a +45.degree. analyzer is incident on the four-divided photodetector 101, as shown in FIG. 21F, and the light polarized by the polarizing beam splitter 30 serving as a -45.degree. analyzer is incident on the four-divided photodetector 102, as shown in FIG. 21G, the amount of light incident on the detection surface 101-1 is larger than that on the detection surface 102-1. The light amounts are represented as LL on the detection surface 101-1 and LS on the detection surface 102-1 (LL >LS).
The detection surface 101-2 corresponds to the detection surface 102-2. Light incident on these detection surfaces is a sum of the small downward amplitude (the S.sub.- direction component in FIG. 2) on the right side represented by G of the light amplitude on the section along the line B-B' in FIG. 21D and the upward amplitude (the P.sub.+ direction component in FIG. 2) shown in FIGS. 19A and 19B. That is, the light incident on the above detection surfaces is light rotated on the -.THETA. side. In this case, the amount of light incident on the detection surface 101-2 is smaller than that on the detection surface 102-2. The light amounts are represented as SS on the detection surface 101-2 and SL on the detection surface 102-2 (SS <SL). The detection surface 101-3 corresponds to the detection surface 102-3. Light incident on these detection surfaces is a sum of the small downward amplitude represented by H of the light amplitude on the section along the line C-C' in FIG. 21E and the upward amplitude shown in FIGS. 19A and 19B. In this case, the amount of light incident on the detection surface 101-3 is larger than that on the detection surface 102-3. The light amounts are represented as LL on the detection surface 101-3
and LS on the detection surface 102-3. Light incident on the detection surfaces 101-4 and 102-4 is a sum of the large upward amplitude represented by H of the light amplitude on the section along the line C-C' in FIG. 21E and the upward amplitude shown in FIGS. 19A and 19B. In this case, the amount of light incident on the detection surface 101-4 is smaller than that on the detection surface 102-4. The light amounts are represented as SS on the detection surface 101-4 and SL on the detection surface
102-4. Similarly, the amounts of light incident on the detection surfaces of the four-divided photodetectors 101 and 102 are represented, as shown in FIGS. 21F and 21G.
The detection signals from the detection surfaces of the four-divided photodetectors 101 and 102 are output to an analog operation circuit constituted by the addition amplifiers and the differential amplifier, as described with reference to FIG.
18. More specifically, the detection signals from the diagonal detection surfaces 101-1 and 101-3 of the four-divided photodetector 101 and the detection signals from the diagonal detection surfaces 101-2 and 101-4 are added by the addition amplifiers
103 and 104, respectively. The resultant sum signals are differentially detected by the differential amplifier 105. The detection signals from the detection surfaces 101-1 and 101-2 adjacent to each other along the direction of the track and the detection signals from the detection surfaces 101-3 and 101-4 adjacent to each other along the direction of the track are added by the addition amplifiers 106 and 107, respectively. The resultant sum signals are differentially detected by the differential amplifier 108. On the other hand, the detection signals from the diagonal detection surfaces 102-1 and 102-3 of the four-divided photodetector 102 and the detection signals from the diagonal detection surfaces 102-2 and 102-4 are added by the addition amplifiers 109 and 110, respectively. The resultant sum signals are differentially detected by the differential amplifier 111. The detection signals from the detection surfaces 102-1 and 102-2 adjacent to each other along the direction of the track and the detection signals from the detection surfaces 102-3 and 102-4 adjacent to each other along the direction of the track are added by the addition amplifiers 112 and 113, respectively. The resultant sum signals are differentially detected by the differential amplifier 114.
Output signals from the differential amplifiers 105 and 111 are differentially detected by the differential amplifier 115 to generate the reproduction signal 116 shown in FIG. 21H. The resultant reproduction signal has a negative level in the region of downward magnetization and a positive level in the region of upward magnetization. In a region where the upward magnetization and the downward magnetization are mixed at the edge of the domain 8 within the light spot, the level is changed from the negative level to the positive level or the positive level to the negative level. That is, the reproduction signal has pulses rising and falling at both edges of the domain 8. Therefore, the reproduction signal in which the central position of the domain represents meaningful information is obtained as the pit position recording reproduction signal. On the other hand, the output signals from the differential amplifiers 108 and 114 are added by the addition amplifier 117, and the reproduction signal 118 shown in FIG. 21I is generated. This reproduction signal has a level "0" in the regions of downward magnetization and upward magnetization and has a positive or negative peak at the edge of the domain 8. By detecting the peak position of the signal, each edge of the domain can be detected, so that information recorded by pit edge recording can be reproduced. Note that a combination of detection surfaces may be changed in analog operation to invert the polarities of the reproduction signals shown in FIGS. 21H and 21I.
In the above embodiment, signal reproduction corresponding to either pit position recording or pit edge recording has been exemplified. When a recording medium is inserted into the apparatus, it is desirable that pit position recording reproduction or pit edge recording reproduction is automatically switched in accordance with the recording medium. This switching control device has been described with reference to FIGS. 15 and 16.
According to the embodiment described above of the present invention, a magnetooptical information recording/reproducing apparatus in which a light beam is radiated on a magnetooptical recording medium and information recorded on the recording medium is reproduced from the light reflected by the recording medium is characterized in that light produced by magnetooptic effect of the magnetooptical recording medium and light produced by diffraction at a curved surface of a lens for converging the light beam radiated on the recording medium into a minute light spot are split into light beams by a polarizing beam splitter serving as an analyzer, and the split light beams are respectively detected by multi-divided photodetectors, thereby reproducing information.
In the magnetooptical information recording/reproducing apparatus, the light produced by the magnetooptic effect has a light amount equal to or smaller than the light produced by the diffraction at the curved surface of the lens.
In the magnetooptical information recording/reproducing apparatus, the multi-divided photodetector comprises a four-divided photodetector having detection surfaces divided in a crossed shape such that division lines of the detection surfaces are parallel and perpendicular to the information track of the magnetooptical recording medium, wherein detection signals from the detection surfaces of the four-divided photodetector are used to perform a predetermined analog operation, thereby performing signal reproduction corresponding to pit position recording or pit edge recording.
In the magnetooptical information recording/reproducing apparatus, the detection signals from the diagonal detection surfaces of each four-divided photodetector are added, resultant sum signals are differentially detected, and differential detection signals derived from the two four-divided photodetectors are differentially detected to reproduce information corresponding to pit position recording.
In the magnetooptical information recording/reproducing apparatus, the detection signals from the detection surfaces adjacent to each other along the direction of the track of each four-divided photodetector are added, resultant sum signals are differentially detected, and differential detection signals derived from the two four-divided photodetectors are differentially detected to reproduce information corresponding to pit edge recording.
In the magnetooptical information recording/reproducing apparatus, a means for detecting medium identification information added to the magnetooptical recording medium is used to determine whether the medium identification information represents a pit edge recording medium or a pit position recording medium, thereby selecting signal reproduction corresponding to pit position recording or pit edge recording in accordance with this determination result.
In the magnetooptical information recording/reproducing apparatus, the multi-divided photodetector comprises a two-divided photodetector having detection surfaces divided by one division line parallel to the track, wherein detection signals from the two detection surfaces of each two-divided photodetector are added, and sum signals derived from the two-divided photodetectors are differentially detected to reproduce information corresponding to pit edge recording.
In the above embodiment, the single apparatus can perform both signal reproduction corresponding to pit position recording and signal reproduction corresponding to pit edge recording. However, a reproduction apparatus may perform either signal reproduction corresponding to pit position recording or signal reproduction corresponding to pit edge recording. When a reproduction apparatus is constituted as an apparatus for pit edge recording, the detection surfaces of each four-divided photodetector which are adjacent along the direction of a track may be integrally formed. That is, the four-divided photodetector is replaced with a two-divided photodetector having a division line along the direction of the track, detection signals from the detection surfaces of each two-divided photodetector are added, and the sum signals from the two-divided photodetectors are differentially detected to obtain a reproduction signal corresponding to pit edge recording. Signal reproduction may be performed using a multi-divided photodetector other than the four-and two-divided photodetectors.
According to this embodiment of the present invention, the light produced by the magnetooptic effect is interfered with the light produced by the diffraction at the curved surface of the objective lens, and the incident beam is split into two light beams. The two split light beams are detected by the multi-divided photodetectors, respectively. Even if the size of the recording pit is smaller than that of the light spot, the recorded information can be accurately detected. Even if the recording density is increased, signal reproduction can be accurately performed. In particular, the edge shift phenomenon caused by degradation of the transfer characteristics of the optical head in reproduction according to conventional pit edge recording can be effectively eliminated. Information can be accurately reproduced even in pit edge recording.
Still another embodiment of the present invention will be described below.
In the magnetooptical information recording/reproducing apparatus shown in FIG. 8, when a target information track is to be accessed, an actuator (not shown in FIG. 8) is driven to perform a high-speed seek operation, and only the objective lens
23 is radially moved along the magnetooptical disk 24. In this case, the DC component of the information reproduction signal varies, and the edge of the domain cannot be accurately detected. More specifically, when the objective lens is radially shifted along the magnetooptical disk, the light amount distribution of the laser beam incident on the objective lens becomes an asymmetrical distribution as a Gaussian distribution whose center is shifted. The distribution of the S-polarized light components produced by the magnetooptic effect is a distribution asymmetrical in the radial direction of the magnetooptical disk, as shown in FIG. 22A. In addition, the distribution of the S-polarized light components produced at the curved surface of the objective lens is given as a distribution asymmetrical in the radial direction of the magnetooptical disk, as shown in FIG. 22B. The distributions on the sections along the lines B-B' and C-C' in FIG. 22B are different from each other, as shown in FIGS. 23A and 23B.
For the above reason, a reproduction signal obtained by differentially detecting detection signals from the detection pieces or photodetection surfaces 4-1 and 4-2 of the two-divided photodetector 4 includes a DC component corresponding to a shift amount of the objective lens 23, thus causing an error in detection of the edge of the domain. FIGS. 24A to 24D show the state of a change in reproduction signal with respect to the shift amount of the objective lens. FIG. 24A is a view showing an information pit (domain) 8 recorded on an information track 7 and light spots 9 to 13 scanned on the information track 7 as in FIG. 11A. FIG. 24B shows a reproduction signal obtained when the objective lens is shifted to the inner track of the magnetooptical disk. In this case, a positive DC component corresponding to the shift amount is contained in the reproduction signal. FIG. 24C shows a reproduction signal obtained when the objective lens is located at the central position. This reproduction signal does not contain any DC component and is a normal signal. FIG. 24D shows a reproduction signal obtained when the objective lens is shifted to an outer track. A negative DC component is contained in the reproduction signal in a manner opposite to the shift of the objective lens toward the inner track. In the resultant reproduction signal, the DC component serves as a signal varying depending on the shift amount of the objective lens. An error occurs in detection of the edge position of the domain. Demand has arisen for improving accurate detection of the edge position.
In the magnetooptical information recording/reproducing apparatus shown in FIG. 18, although the edge position of the domain can be accurately detected regardless of the size of the light spot, as described above, demand has arisen for providing a magnetooptical information recording/reproducing apparatus capable of reproducing information with a high reliability while accurately detecting the edge position of the domain regardless of the size of the light spot.
The following embodiment of the present invention has been made in consideration of the above situation, and aims at providing a magnetooptical information recording/reproducing apparatus capable of eliminating a DC component contained in a reproduction signal upon movement of an objective lens in the tracking direction and accurately detecting the edge position of the pit without any error.
This embodiment also aims at providing a magnetooptical information recording/reproducing apparatus capable of accurately detecting recorded information even if the size of the pit is smaller than that of the light spot and accurately performing signal reproduction even if the recording density is increased.
More specifically, there is provided a magnetooptical information recording/reproducing apparatus in which a linearly polarized light beam having a polarization direction parallel or perpendicular to a track direction is radiated on a magnetooptical recording medium, causing light produced from a reflected light beam from the magnetooptical recording medium by the magnetooptic effect of the magnetooptical recording medium and having a polarization direction perpendicular to the polarization of the incident light beam to interfere with light produced by diffraction at a curved surface of the objective lens, and detecting a change in light amount distribution of the interfered light beams, thereby reproducing information, comprising detecting means for detecting a DC component superposed on the reproduction signal when the objective lens is moved in the tracking direction as the radial direction of the magnetooptical recording medium, and correcting means for correcting the reproduction signal on the basis of an output from the detecting means to eliminate the DC component contained in the reproduction signal.
There is also provided a magnetooptical information recording/reproducing apparatus in which a light beam is radiated on a magnetooptical recording medium and information recorded on the recording medium is reproduced from the light reflected by the recording medium, characterized in that the light reflected by the recording medium is converged by a condenser lens, converged light is split into two light beams by a polarizing beam splitter serving as an analyzer, one of the two split light beams is detected by a multi-divided photodetector located at a position ahead of a convergence position of the condenser lens, the other of the two split light beams is detected by a multi-divided photodetector located behind the convergence position of the condenser lens, and a change in light amount distribution on the two multi-divided photodetectors is detected, thereby reproducing the recorded information.
This embodiment of the present invention will be described in more detail with reference to the accompanying drawings. FIG. 25 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to this embodiment. FIG. 25 shows only the arrangement of an optical system as the main part of this embodiment. Since the basic construction of the magnetooptical information recording/reproducing apparatus of this embodiment is substantially the same as that of the magnetooptical information recording/reproducing apparatus shown in FIG. 8, a detailed description thereof will be omitted. Referring to FIG. 25, a lens position sensor 301 is mounted on the side portion of an objective lens 221. The lens position sensor 301 serves as a position sensor for detecting a shift amount of the objective lens 221 in the radial direction of a magnetooptical disk 205. The lens position sensor 301 (to be described in detail later) comprises a photointerrupter for optically detecting a shift amount of the objective lens 221. An actuator 199 moves the objective lens 221 in the radial direction of the magnetooptical disk 205.
This apparatus also includes a semiconductor laser 200 serving as a recording/reproduction light source, a collimator lens 201, a beam shaping prism 202, a polarizing beam splitter 203, and a magnetic head 206. A light beam emitted from the semiconductor laser 200 is a linearly polarized light component (P-polarized light component) having the polarization direction parallel to the drawing surface. The objective lens has an NA of about 0.55 and has a large curvature. The P-polarized light beam incident on the magnetooptical disk 205 is parallel or perpendicular to the direction of the information track of the magnetooptical disk 205.
The apparatus further includes a condenser lens 209, a half prism 210, photodetectors 211 and 213, and a knife edge 212. These optical elements constitute a control optical system. In the control optical system, a focusing error signal and a tracking error signal are generated on the basis of detection signals from the photodetectors 211 and 213. The autofocusing control in the control optical system employs a knife edge scheme, and autotracking control therein employs a push-pull scheme. The apparatus of this embodiment also includes a polarizing beam splitter 222, a condenser lens 223, and a two-divided photodetector 224 having detection surfaces divided in a direction parallel to the track. These optical elements constitute a reproduction optical system. In the reproduction optical system, detection signals from the detection pieces or surfaces of the two-divided photodetector 224 are differentially detected by a differential amplifier 219 to obtain a reproduction signal of the recorded information.
The information reproduction operation has been described with reference to FIGS. 9A to 11F, and a detailed description thereof will be omitted. The light produced by the magnetooptic effect of the magnetooptical disk is interfered with the light produced by the diffraction at the curved surface of the objective lens, and a change in light amount distribution is detected to reproduce information. That is, as shown in FIG. 11F, a reproduction signal having a positive or negative peak at the edge position of the domain is obtained. On the basis of this reproduction signal, information of pit edge recording can be reproduced. An information recording scheme employs a magnetic field modulation scheme.
FIG. 26 is a view showing the detailed arrangement of the lens position sensor 301 and the side portion of the objective lens 221. The lens position sensor 301 comprises a light-emitting diode 301a and a phototransistor 301b. Light is radiated from the light-emitting diode 301a onto a reflecting plate 302, and light reflected by the reflecting plate 302 is received by the phototransistor 301b. The reflecting plate 302 comprises a white portion 302a for reflecting the light and a black portion
302b for absorbing the light, as shown in FIG. 26. The reflecting plate 302 is adhered to the side portion of a lens barrel 221a of the objective lens 221 so as to oppose the lens position sensor 301, as shown in FIG. 27. FIG. 27 is a plan view showing the positional relationship between the lens position sensor 301 and the objective lens 221. While the lens position sensor 301 is fixed at a predetermined position, the objective lens 221 is moved between the outside and the inside in the tracking direction of the magnetooptical disk 301 upon driving by the actuator 199. Therefore, an output signal corresponding to a shift amount of the objective lens 221 in the tracking direction can be obtained from the phototransistor 301b. That is, the light reception amount of the phototransistor 301b is determined by a ratio of the area of the white portion 302a to the area of the black portion 302b of the reflecting plate 302 within the area irradiated with the light-emitting diode 301a. When the shift position of the objective lens 221 comes close to the inside of the magnetooptical disk, the output from the phototransistor 301b is increased, as shown in FIG. 28. When the shift position comes close to the outside of the magnetooptical disk, the output from the phototransistor 301b is decreased. Therefore, the signal output from the phototransistor 301b is increased/decreased in accordance with the shift amount of the objective lens 221. This signal is supplied to an adder/subtracter circuit (not shown).
On the other hand, the differential amplifier 219 differentially detects the signals from the detection surfaces of the two-divided photodetector 224 to obtain a reproduction signal. This reproduction signal is superposed with a positive or negative DC component in accordance with a shift direction when the objective lens 221 is shifted in the radial direction of the magnetooptical disk 205, as described with reference to FIGS. 24A to 24D. In addition, the DC component changes in accordance with a change in shift amount. The resultant reproduction signal is supplied to the adder/subtracter circuit. The adder/subtracter circuit performs an addition or subtraction to cancel the DC component of the reproduction signal on the basis of the output from the lens position sensor 301, thereby always obtaining a reproduction signal free from a DC component. Even if the objective lens 221 is shifted, the DC component of the reproduction signal can be properly eliminated. Therefore, the edge position of the domain can be accurately detected without any error, and the information can be accurately reproduced.
FIG. 29 is a constructional view showing a magnetooptical information recording/reproducing apparatus according to still another embodiment of the present invention. Referring to FIG. 29, a plane-parallel plate 303 is disposed between an objective lens 221 and a polarizing beam splitter 203. The plane-parallel plate 303 is rotated about its center by a driving means (not shown) in directions indicated by a double-headed arrow. A lens position sensor 301 is arranged on the side portion of the objective lens 221 to detect a shift amount of the objective lens 221 in the tracking direction in the same manner as in the embodiment of FIG. 25. The plane-parallel plate 303 h