U.S. patent application number 09/734729 was filed with the patent office on 2001-08-23 for magneto-optical recording and reproducing apparatus and method.
This patent application is currently assigned to Sanyo Electric Co. Ltd. Invention is credited to Sumi, Satoshi, Suzuki, Yoshihisa, Tanase, Kenji, Yamaguchi, Atsushi.
Application Number | 20010015937 09/734729 |
Document ID | / |
Family ID | 18133693 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010015937 |
Kind Code |
A1 |
Yamaguchi, Atsushi ; et
al. |
August 23, 2001 |
Magneto-optical recording and reproducing apparatus and method
Abstract
A magneto-optic recording medium reproduction device includes a
multi-clad, step index optical fiber, a semiconductor laser
arranged on one end surface side of the optical fiber for providing
laser beam to one end surface of the optical fiber, a photodetector
arranged on one end surface side of the optical fiber and receiving
laser beam from one end surface of the optical fiber, and a
magnetic head for applying an alternating field to the
magneto-optic recording medium. Since the other end surface of the
optical fiber is arranged adjacent to the magneto-optic recording
medium, an accurate signal can be reproduced even from an extremely
small recorded domain. Furthermore, since an alternating field is
applied to the magneto-optic recording medium, a domain transferred
from a recording layer of the magneto-optic recording medium to a
reproducing layer is expanded and a large reproduced signal can be
obtained from the expanded domain.
Inventors: |
Yamaguchi, Atsushi; (Gifu,
JP) ; Sumi, Satoshi; (Gifu, JP) ; Suzuki,
Yoshihisa; (Aichi, JP) ; Tanase, Kenji; (Gifu,
JP) |
Correspondence
Address: |
ARMSTRONG,WESTERMAN, HATTORI,
MCLELAND & NAUGHTON, LLP
1725 K STREET, NW, SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
Sanyo Electric Co. Ltd,
Moriguchi-shi
JP
|
Family ID: |
18133693 |
Appl. No.: |
09/734729 |
Filed: |
December 13, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09734729 |
Dec 13, 2000 |
|
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08976010 |
Nov 21, 1997 |
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6205092 |
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Current U.S.
Class: |
369/13.01 ;
G9B/11.016; G9B/11.029; G9B/11.034 |
Current CPC
Class: |
G11B 11/10543 20130101;
G11B 11/10515 20130101; G11B 7/1387 20130101; G11B 7/123 20130101;
G11B 7/1384 20130101; G11B 7/1356 20130101; G11B 11/10554 20130101;
G11B 7/127 20130101; B82Y 10/00 20130101 |
Class at
Publication: |
369/13 |
International
Class: |
G11B 011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 2, 1996 |
JP |
8-321539 (P) |
Claims
What is claimed is:
1. A magneto-optic recording medium reproduction device for
reproducing a signal from a magneto-optic recording medium with a
recording layer and a reproducing layer, comprising: laser means
for oscillating a laser beam; optical means having an end surface
adjacent to said magneto-optic recording medium, for irradiating
said magneto-optic recording medium with the laser beam from said
laser means via said end surface and receiving a laser beam
reflected from said magneto-optic recording medium via said end
surface; detection means for detecting the laser beam received by
said optical means; and magnetic field application means for
applying an alternating field to said magneto-optic recording
medium to expand and shrink a domain created in said reproducing
layer.
2. The magneto-optic recording medium reproduction device according
to claim 1, wherein said optical means includes an optical fiber
including: a core having a first refractive index; a first clad
formed around said core and having a second refractive index
smaller than said first refractive index; and a second clad formed
around said first clad and having a third refractive index smaller
than said second refractive index.
3. The magneto-optic recording medium reproduction device according
to claim 1, wherein said optical means includes: a first optical
fiber including a first core having a first diameter and a first
refractive index, and a first clad formed around said first core
and having a second refractive index smaller than said first
refractive index; and a second optical fiber including a second
core having a second diameter larger than said first diameter and a
third refractive index, and a second clad formed around said second
core and having a fourth refractive index smaller than said third
refractive index.
4. The magneto-optic recording medium reproduction device according
to claim 1, wherein said optical means includes: a solid immersion
lens having said end surface and a curved surface opposite to said
end surface; an objective lens arranged on a said curved surface
side of said solid immersion lens, coaxial with said solid
immersion lens; and an optical system forming first laser beam with
a first diameter and a second laser beam with a second diameter
larger than said first diameter and allowing said first and second
laser beams coaxially incident on said objective lens.
5. The magneto-optic recording medium reproduction device according
to claim 4, wherein: said laser means includes a first laser, and a
second laser; and said optical system includes a first lens forming
said first laser beam, based on a laser beam from said first laser,
a second lens forming said second laser beam, based on a laser beam
from said second laser, and an optical mixing element for mixing
said first laser beam and said second laser beam.
6. The magneto-optic recording medium reproduction device according
to claim 4, wherein: said laser means includes one laser; and said
optical system includes an optical splitting element for splitting
a laser beam from said laser into two laser beams, a first lens
forming said first laser beam, based on one laser beam split from
the laser beam by said optical splitting element, a second lens
forming said second laser beam, based on the other laser beam split
from the laser beam by said optical splitting element, and an
optical mixing element for mixing said first laser beam and said
second laser beam.
7. The magneto-optic recording medium reproduction device according
to claim 1, further comprising diffraction means for transmitting a
laser beam from said laser means straight and diffracting a laser
beam received by said optical means towards said detection
means.
8. The magneto-optic recording medium reproduction device according
to claim 7, wherein said diffraction means includes a hologram.
9. A magneto-optic recording medium reproduction device for
reproducing a signal from a magneto-optic recording medium with a
recording layer and a reproducing layer, comprising: an optical
fiber having one end surface adjacent to said magneto-optic
recording medium and the other end surface; a laser arranged on the
other end surface side of said optical fiber for providing a laser
beam to the other end surface of said optical fiber; a
photodetector arranged on the other end surface side of said
optical fiber and receiving a laser beam from the other end surface
of said optical fiber; and a magnetic head arranged for applying an
alternating field to said magneto-optic recording medium to expand
and shrink a domain created in said reproducing layer.
10. The magneto-optic recording medium reproduction device
according to claim 9, wherein said optical fiber includes: a core
having a first refractive index; a first clad formed around said
core and having a second refractive index smaller than said first
refractive index; and a second clad formed around said first clad
and having a third refractive index smaller than said second
refractive index.
11. A magneto-optic recording medium reproduction device for
reproducing a signal from a magneto-optic recording medium with a
recording layer and a reproducing layer, comprising: a first
optical fiber having one end surface adjacent to said magneto-optic
recording medium and the other end surface; a laser arranged on the
other end surface side of said first optical fiber for providing a
laser beam to the other end surface of said first optical fiber; a
second optical fiber having one end surface adjacent to said
magneto-optic recording medium and the other end surface; a
photodetector arranged on the other end surface side of said second
optical fiber and receiving a laser beam from the other end surface
of said second optical fiber; and a magnetic head arranged for
applying an alternating field to said magneto-optic recording
medium to expand and shrink a domain created in said reproducing
layer.
12. The magneto-optic recording medium reproduction device
according to claim 11, wherein: said first optical fiber includes a
first core having a first diameter and a first refractive index,
and a first clad formed around said first core and having a second
refractive index smaller than said first refractive index; and said
second optical fiber includes a second core having a second
diameter larger than said first diameter and a third refractive
index, and a second clad formed around said second core and having
a fourth refractive index smaller than said third refractive
index.
13. A magneto-optic recording medium reproduction device for
reproducing a signal from a magneto-optic recording medium with a
recording layer and a reproducing layer, comprising: a solid
immersion lens having a plane adjacent to said magneto-optic
recording medium and a curved surface opposite to said plane; an
objective lens arranged on the curved surface side of said solid
immersion lens, coaxial with said solid immersion lens; a laser; an
optical system for forming from a laser beam from said laser a
first laser beam with a first diameter and a second laser beam with
a second diameter larger than said first diameter and allowing said
first and second laser beams coaxially incident on said objective
lens; a photodetector receiving a laser beam reflected from said
magneto-optic recording medium and transmitted through said solid
immersion lens and said objective lens; and a magnetic head
arranged for applying an alternating field to said magneto-optic
recording medium to expand and shrink a domain created in said
reproducing layer.
14. A magneto-optic recording medium reproduction device for
reproducing a signal from a magneto-optic recording medium with a
recording layer and a reproducing layer, comprising: a solid
immersion lens having a plane adjacent to said magneto-optic
recording medium and a curved surface opposite to said plane; an
objective lens arranged on the curved surface side of said solid
immersion lens, coaxial with said solid immersion lens; a first
laser; a first lens forming a first laser beam with a first
diameter, based on a laser beam from said first laser; a second
lens forming a second laser beam with a second diameter larger than
said first diameter, based on a laser beam from said second laser;
an optical system for allowing said first and second laser beams
coaxially incident on said objective lens; a photodetector
receiving a laser beam reflected from said magneto-optic recording
medium and transmitted through said solid immersion lens and said
objective lens; and a magnetic head arranged for applying an
alternating field to said magneto-optic recording medium to expand
and shrink a domain created in said reproducing layer.
15. A magneto-optic recording medium reproduction method for
reproducing a signal from a magneto-optic recording medium having a
recording layer and a reproducing layer magnetized in a
predetermined direction, comprising the steps of: irradiating said
magneto-optic recording medium with a laser beam having a mixed
intensity distribution of first and second intensity distributions
different in beam diameter; applying an alternating field to said
magneto-optic recording medium to expand and shrink a domain
created in said reproducing layer when said magneto-optic recording
medium is irradiated with said laser beam; and detecting a laser
beam reflected from said magneto-optic recording medium.
16. The magneto-optic recording medium reproduction method
according to claim 15, wherein said step of detecting includes
detecting said laser beam when a reproduced signal based on an
intensity of the laser beam reflected from said magneto-optic
recording medium is a greatest reproduced signal.
17. The magneto-optic recording medium reproduction method
according to claim 16, wherein said step of detecting includes
detecting said laser beam when a magnetic field is applied in a
direction opposite to said predetermined direction.
18. A magneto-optic recording medium reproduction method for
reproducing a signal from a magneto-optic recording medium having a
recording layer and a reproducing layer magnetized in a
predetermined, first direction, comprising the steps of:
irradiating said magneto-optic recording medium with a laser beam
having an intensity distribution with a first diameter of beam to
transfer a domain in said recording layer to said reproducing
layer; applying a magnetic field to said magneto-optic recording
medium in a second direction opposite to said first direction to
expand a domain transferred to said reproducing layer; irradiating
said magneto-optic recording medium with a laser beam having an
intensity distribution with a second diameter of beam larger than
said first diameter of beam; detecting a laser beam reflected from
said magneto-optic recording medium when said magneto-optic
recording medium is irradiated with the laser beam having the
intensity distribution with said second diameter of beam; and
applying a magnetic field to said magneto-optic recording medium in
said first direction to shrink said expanded domain in said
reproducing layer.
19. A magneto-optic recording medium, comprising: a substrate; a
recording layer positioned on said substrate and formed of a
magnetic material; and a reproducing layer positioned on said
recording layer and formed of a magnetic material.
20. The magneto-optic recording medium according to claim 19,
further comprising an intermediate layer positioned between said
recording layer and said reproducing layer and formed of a
non-magnetic material.
21. The magneto-optic recording medium according to claim 19,
wherein a minimum size of stable domain of said reproducing layer
is larger than a minimum size of stable domain of said recording
layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a magneto-optic recording
medium reproduction device, and more specifically to that employs
near-field light and domain expansion.
[0003] 2. Description of the Background Art
[0004] Magneto-optic recording medium has been noted as a highly
reliable recording medium with rewritability and high storage
capacity and has been put to practical use as the memory for
computer and the like. However, further high density recording and
reproducing technique has been sought for as the amount of
information is further increased and the device is further
miniaturized.
[0005] High density recording and reproducing technique is
constituted by medium technique and device technique. The former
technique includes a narrowed pitch of medium, an improved
resolution for reproduction by means of magnetic multilayered film,
and the like. The technique by means of magnetic multilayered film
employs the fact that the intensity of laser spot forms Gaussian
distribution to selectively transfer the magnetized state of a
recording layer to a reproducing layer and read the magnetized
state of the reproducing layer, and at present mainly has the three
types of FAD (Front Aperture Detection), RAD (Rear Aperture
Detection) and CAD (Center Aperture Detection). According to these
techniques, the front or rear side or the vicinity of the center of
a laser spot serves as a reproduction aperture to reduce the
substantial diameter of a laser spot and thus increase reproduction
density. The latter technique includes optical super-resolution
technique for obtaining a condensation spot which exceeds the
diffraction limit of laser beam, reduction in wavelength of laser
beam and the like. Furthermore, near-field light recording and
reproducing technique is provided for positioning one end surface
of an optical fiber adjacent to a magneto-optic recording medium
and irradiating the magneto-optic recording medium with laser beam
from the optical fiber to record and reproduce signals. This
technique allows formation of a recording domain of approximately
0.06 .mu.m.
[0006] For reproduction by near-field light recording and producing
technique, however, the reproduced signals which are detected are
small due to the small domain, sufficient C/N ratio cannot be
obtained, and reproduced signals are disadvantageously lost.
SUMMARY OF THE INVENTION
[0007] Therefore, an object of the present invention is to provide
a magneto-optic recording medium reproduction device capable of
accurately reproducing a signal recorded in a small domain, and a
method of reproducing the same.
[0008] Another object of the present invention is to provide a
magneto-optic recording medium suitable for the reproduction device
and reproduction method mentioned above.
[0009] According to one aspect of the present invention, a
magneto-optic recording medium reproduction device for reproducing
a signal from a magneto-optic recording medium with a recording
layer and a reproducing layer includes a laser device, an optical
device, a detector and a magnetic field application device. The
laser device oscillates laser beam. The optical device has an end
surface adjacent to the magneto-optic recording medium, and
irradiates the magneto-optic recording medium with the laser beam
from the laser device via the end surface and receives the laser
beam reflected from the magneto-optic recording medium via the end
surface. The detector detects the laser beam received by the
optical device. The magnetic field application device applies an
alternating magnetic field to the magneto-optic recording medium to
expand and shrink a domain created in the reproducing layer.
[0010] Preferably, the optical device includes an optical fiber
which has a core, a first clad and a second clad. The core has a
first refractive index. The first clad is formed around the core
and has a second refractive index smaller than the first refractive
index. The second clad is formed around the first clad and has a
third refractive index smaller than the second refractive
index.
[0011] Preferably, the optical device includes a first optical
fiber and a second optical fiber. The first optical fiber has a
first core and a first clad. The first core has a first diameter
and a first refractive index. The first clad is formed around the
first core and has a second refractive index smaller than the first
refractive index. The second optical fiber has a second core and a
second clad. The second core has a second diameter larger than the
first diameter, and a third refractive index. The second clad is
formed around the second core and has a fourth refractive index
smaller than the third refractive index.
[0012] Preferably, the optical device a solid immersion lens, an
objective lens and an optical system. The solid immersion lens has
an end surface and a curved surface opposite to the end surface.
The objective lens is arranged on the curved surface side of the
solid immersion lens and is coaxial with the solid immersion lens.
The optical system forms a first laser beam with a first diameter
and a second laser beam with a second diameter larger than the
first diameter coaxially incident on the objective lens.
[0013] Preferably the reproduction device further includes a
diffraction grating element for transmitting the laser beam from
the laser device straight and diffracting the laser beam received
by the optical device towards the detector. The diffraction grating
element further preferably includes a hologram.
[0014] According to another aspect of the present invention, a
magneto-optic recording medium reproduction method for reproducing
a signal from a magneto-optic recording medium having a recording
layer and a reproducing layer magnetized in a predetermined
direction includes the steps of: irradiating a magneto-optic
recording medium with laser beam having a mixed intensity
distribution of first and second intensity distributions each
having a different beam diameter; applying an alternating magnetic
field to the magneto-optic recording medium to expand and shrink a
domain created in a reproducing layer when the magneto-optic
recording medium is irradiated with the laser beam; and detecting a
laser beam reflected from the magneto-optic recording medium.
[0015] Preferably, the step of detecting includes detecting the
laser beam when a reproduced signal based on the intensity of the
laser beam reflected from the magneto-optic recording medium is the
greatest reproduced signal.
[0016] Still preferably, the step of detecting includes detecting
the laser beam at the timing of applying a magnetic field in the
direction opposite to the predetermined direction in which the
reproducing layer is magnetized.
[0017] According to still another aspect of the present invention,
a magneto-optic recording medium includes a substrate, a recording
layer and a reproducing layer. The recording layer is positioned on
the substrate and formed of a magnetic material. The reproducing
layer is positioned on the recording layer and formed of a magnetic
material.
[0018] The magneto-optic recording medium still preferably includes
an intermediate layer positioned between the recording layer and
the reproducing layer and formed of a non-magnetic material.
[0019] Preferably, minimum size of stable domain of the reproducing
layer is larger than that of the recording layer.
[0020] The magneto-optic recording medium reproduction device is
miniaturized since it uses an optical device having an end surface
adjacent to the magneto-optic recording medium, rather than an
objective lens, to irradiate the magneto-optic recording medium
with laser beam. Furthermore, the reproduction device transfers a
domain of the recording layer to the reproducing layer and expands
the transferred domain size to reproduce a signal so that the
intensity of the reproduced signal is improved and consequently a
sufficient high C/N ratio can be obtained.
[0021] Furthermore, a so-called multi-clad, step index optical
fiber is used as the optical device in the reproduction device to
irradiate the magneto-optic recording medium with laser beam. Thus,
the intensity of the beam spot is significantly increased only at
the center, only a desired domain within the recording layer is
transferred to the reproducing layer, and consequently a precise
reproduced signal can be obtained.
[0022] Alternatively, a so-called single-clad, step index optical
fiber, and a single-clad step index optical fiber having a larger
core than that of the other optical fiber are used as the optical
device in the reproduction device to irradiate the magneto-optic
recording medium with laser beam. Thus, only the intensity of the
beam spot is significantly increased only at the center and
consequently a precise reproduced signal can be obtained as
well.
[0023] Alternatively, a solid immersion lens is used as the optical
device in the reproduction device to allow two laser beams
different in diameter incident coaxially on the objective lens.
Thus, only the intensity of the beam spot is significantly
increased only at the center and consequently a precise reproduced
signal can be obtained as well.
[0024] Furthermore, the reproduction device employs a hologram to
transmit laser beam from the laser device straight and diffract the
laser beam reflected from the magneto-optic recording medium. Thus,
the laser device and the detector can be arranged in a same plane
and this can reduce the size of the optical system formed of the
laser device and the detector.
[0025] According to the magneto-optic recording medium reproduction
method, a laser beam reflected from an expanded domain is detected
at the timing of applying a magnetic field in the direction
opposite to that in which the reproducing layer is initially
magnetized, and a sufficiently large reproduced signal can thus be
obtained.
[0026] Furthermore, since the recording layer and the reproducing
layer are positioned successively from the substrate side of the
magneto-optic recording medium, laser beam can be radiated from the
side opposite to the substrate. This allows an end surface of the
optical fiber or the like to be arranged more adjacent to the
recording layer and thus a signal to be reproduced from a smaller
recording domain.
[0027] The foregoing and other objects, features, aspects and
advantages of the present invention will become more apparent from
the following detailed description of the present invention when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 shows a cross section showing one example of a
magneto-optic recording medium suitable for a reproduction device
according to a first embodiment of the present invention.
[0029] FIG. 2 shows a cross section showing another example of the
magneto-optic recording medium.
[0030] FIG. 3 shows a configuration of the magneto-optic recording
medium reproduction device according to the first embodiment
together with its reproduction principle.
[0031] FIG. 4A represents the refractive index of the optical fiber
shown in FIG. 3,
[0032] FIG. 4B shows a cross section showing a configuration of the
optical fiber, and
[0033] FIG. 4C represents intensity distributions of laser beams
emanating from the optical fiber.
[0034] FIGS. 5A and 5B are front and bottom views showing a
configuration of the receiving/emitting unit shown in FIG. 3,
respectively.
[0035] FIG. 6 shows a view for illustrating the reproduction
operation of the reproduction device shown in FIG. 3.
[0036] FIG. 7 shows a view for illustrating the transfer and
expansion of a domain of the magneto-optic recording medium shown
in FIG. 1.
[0037] FIGS. 8A-8D show views of the steps for illustrating the
reproduction principle of the magneto-optic recording medium shown
in FIG. 2.
[0038] FIG. 9 shows still another example of the magneto-optic
recording medium together with its reproduction principle.
[0039] FIGS. 10A and 10B show waveform diagrams representing a
magnetic field applied and reproduced signals obtained in the
reproduction device shown in FIG. 3.
[0040] FIGS. 11A and 11B show a configuration of a magneto-optic
recording medium according to a second embodiment of the present
invention together with its reproduction principle.
[0041] FIG. 12 shows a configuration of a magneto-optic recording
medium reproduction device according to a third embodiment of the
present invention.
[0042] FIG. 13 shows an optical path passing through the solid
immersion lens and objective lens shown in FIG. 12.
[0043] FIG. 14A shows more specifically an optical path of laser
beam transmitted through the solid immersion lens shown in FIG. 13,
and FIG. 14B represents the intensity distribution of the laser
beam in plane A-A' in FIG. 14A.
[0044] FIG. 15 shows a configuration of a magneto-optic recording
medium reproduction device according to a fourth embodiment of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0045] Embodiments of the present invention will now be
specifically described with reference to the drawings. Identical or
corresponding portions in the figures are labeled by identical
reference characters and a description thereof is not repeated.
[0046] First Embodiment
[0047] A configuration of a magneto-optic recording medium will
first be described suitable for a magneto-optic recording medium
reproduction device according to a first embodiment of the present
invention.
[0048] Referring to FIG. 1, a magneto-optic recording medium 10
includes a transparent substrate 1, a recording layer 2 formed on
transparent substrate 1, a reproducing layer 4 formed on recording
layer 2, and a protection layer 5 formed on reproducing layer 4.
Transparent substrate 1 is formed of glass, polycarbonate or the
like. Recording layer 2 is formed of a magnetic material, such as
TbFeCo. Reproducing layer 4 is formed of a magnetic material, such
as GdFeCo. Protection layer 5 is formed of a transparent dielectric
material, such as SiN.
[0049] Recording layer 2, reproducing layer 4 and protection layer
5 are successively deposited by magnetron sputtering. Recording
layer 2 has a film thickness of 500 to 3000 .ANG., reproducing
layer 4 has a film thickness of 50 to 1000 .ANG., and protection
layer 5 has a film thickness of 180 to 220 .ANG..
[0050] Magneto-optic recording medium 10 has recording layer 2
closer to transparent substrate 1 and reproducing layer 4 closer to
protection layer 5. Accordingly, radiation of laser beam for
recording and reproduction is provided from the protection layer 5
side, rather than the transparent substrate 1 side.
[0051] It should be noted that recording layer 2 is not limited to
the TbFeCo mentioned above, and may be a single- or multi-layered
magnetic film formed of an element selected among Tb, Dy and Nd and
Fe, Co and Ni, or a single- or multi-layered magnetic film formed
of one element of Pt and Pd and one element selected among Fe, Co
and Ni.
[0052] Reproducing layer 4 is not limited to the GdFeCo mentioned
above, and may be a magnetic film formed of one element selected
among GdFe, GdCo and TbCo or among Ho, Gd, Tb and Dy and one
element selected among Fe, Co and Ni.
[0053] Furthermore, as shown in a magneto-optic recording medium
shown in FIG. 2, an intermediate layer 3 may be inserted between
recording layer 2 and reproducing layer 4. Intermediate layer 3 is
formed of a non-magnetic material (i.e. a dielectric material),
such as SiN, AlN, TiN, SiO.sub.2, Al.sub.2O.sub.3, SiC, TiC, ZnO,
SiAlON, ITO (idium tin oxide) and SnO.sub.2, and has a film
thickness of 30 to 300 .ANG.. By inserting intermediate layer 3, a
domain with a stable shape can be formed in reproducing layer 4 in
enlarging and reproducing the domain, as described later.
[0054] Desirably, minimum size of stable domain of reproducing
layer 4 is larger than that of recording layer 2, since the process
is not required for enlarging a domain transferred from recording
layer 2 to reproducing layer 4 and accordingly the necessity of
applying an alternating magnetic field to magneto-optic recording
medium 10 or 11, as described later, can be dispensed with.
Magnetic materials with a large the minimum size of stable domain
is applicable to reproducing layer 4 of either of magneto-optic
recording medium 10 or 11.
[0055] A configuration of the magneto-optic recording medium
reproduction device according to the first embodiment of the
present invention will now be described for reproducing a signal
from magneto-optic recording medium 10 or 11.
[0056] Referring to FIG. 3, the reproduction device includes a
receiving/emitting unit 6, an optical fiber 7 and a magnetic head
9.
[0057] Optical fiber 7 has an end surface 77 arranged adjacent to
magneto-optic recording medium 10. The distance between end surface
77 and a surface of magneto-optic recording medium 10 is, for
example, 0.2 .mu.m (with a tolerance of .+-.0.1 .mu.m).
[0058] Emitting/receiving unit 6 arranged on the surface 78 side of
optical fiber 7 and includes a semiconductor laser 6a, a
photodetector 6b and a hologram plate 6c. Semiconductor laser 6a
provides oscillation of laser beam with a wavelength of 635 nm
(with a tolerance of .+-.15 nm) or a wavelength of 680 nm (with a
tolerance of .+-.15 nm), which is provided to end surface 78 of
optical fiber 7. Photodetector 6b is arranged next to semiconductor
laser 6a and receives laser beam from end surface 78 of optical
fiber 7. Hologram plate 6c includes a glass substrate and a
hologram formed on the glass substrate, and splits incident laser
beam into the 0th-order, .+-.first-order, . . . .+-.nth-order
diffracted beams. Accordingly, 0th-order diffracted beam (i.e., a
beam transmitted straight through hologram plate 6c without
diffraction) of laser beam radiated from semiconductor laser 6a and
transmitted through hologram plate 6c enters end surface 78 of
optical fiber 7. Meanwhile, +first-order or -first-order diffracted
beam of laser beam radiated from end surface 78 of optical fiber 7
and transmitted through hologram plate 6c enters photodetector
6b.
[0059] Optical fiber 7 receives laser beam from semiconductor laser
6a via end surface 78, guides the laser beam in the UM direction in
the figure, and irradiates magneto-optic recording medium 10 with
the laser beam via end surface 77. Optical fiber 7 also receives
laser beam reflected from magneto-optic recording medium 10 via end
surface 77, guides the laser beam in the MU direction in the
figure, and irradiates hologram plate 6b with the laser beam via
end surface 78. Formed in optical fiber 7 is a polarizing filter 8
which transmits only laser beam deflecting in a specific direction.
In this example, polarizing filter 8 is formed to transmit only
laser beam which deflects in the direction perpendicular to the
plane of the drawing. Semiconductor laser 6a is arranged so that
laser beam radiated therefrom is polarized in the direction
perpendicular to the plane of the drawing. Thus, laser beam
radiated from semiconductor laser 6a will not be blocked by
polarizing filter 8.
[0060] Magnetic head 9 includes an electromagnetic coil 9a and an
magnetic head driving circuit 9b for supplying alternating current
to electromagnetic coil 9a. Thus, electromagnetic head 9 applies an
alternating field AF to magneto-optic recording medium 10, expands
a domain transferred into reproducing layer 4 depending on a
recording domain 21 in recording layer 2, and thus forms and
shrinks a domain 41 which is larger than recording domain 21. The
magnitude of alternating field AF ranges from 50 to 3000 e. The
pulse width of alternating field AF ranges from 20 to 500 nsec.
[0061] Optical fiber 7 is a multi-clad, step index optical fiber as
shown in FIGS. 4A and 4B, and includes a core 7c with a refractive
index n1 (e.g., 1.50 to 1.70), an inner circumferential clad 7b
formed around core 7c and having a refractive index n2 (e.g., 1.45
to 1.65), and an outer circumferential clad 7a formed around inner
circumferential clad 7b and having a refractive index n3 (e.g.,
1.40 to 1.60). Refractive index n3 is smaller than refractive index
n2, and refractive index n2 is smaller than refractive index n1.
Suitable materials for core 7c, inner circumferential clad 7b and
outer circumferential clad 7a are multi-component glass, plastic
and the like of SiO.sub.2 which contains Na.sub.2O, CaO and GeO
each having a different component ratio from each other.
[0062] Furthermore, a tip of optical fiber 7 is desirably tapered
79 such that the diameter of the tip of optical fiber 7 is smaller
than that of the body of optical fiber 7. In this example, a tip of
inner circumferential clad 7b has a diameter of 0.1 .mu.m (with a
tolerance of .+-.0.05 .mu.m) and the body has a diameter of 300
.mu.m (with a tolerance of .+-.100 .mu.m).
[0063] For optical fiber 7 as described above, laser beam proceeds
through core 7c as well as inner circumferential clad 7b. Laser
beam which has proceeded through core 7c forms on magneto-optic
recording medium 10 a beam spot which has a sharp intensity
distribution 13 with a narrow beam diameter W1, as shown in FIG.
4C. Meanwhile, a laser beam which has proceeded through inner
circumferential clad 7b forms on magneto-optic recording medium 10
a beam spot which has a gentle intensity distribution 14 with a
wide beam diameter W2, as shown in FIG. 4C. Thus, the mixed
intensity distribution of these intensity distributions is
extremely high only at the intensity of the center of the beam
spot.
[0064] In emitting/receiving unit 6, semiconductor laser 6a and
photodetector 6b are arranged in a same plane, as shown in FIG. 5A.
Mounted on the light receiving surface of photodetector 6b is a
polarizing filter 6d which transmits one of p polarization
component and s polarization component of a +first-order or
-first-order diffracted beam L.sub.211. Emitting/receiving unit 6
has three cuts k1-k3 each formed for every 90.degree..
Semiconductor laser 6a is arranged such that semiconductor laser 6a
has an emitting point 61 positioned on line k2-k3.
[0065] The following equations (1) and (2) are established:
sin .theta.=.lambda./p (1)
Z.sub.1=L tan .theta. (2)
[0066] wherein L represents the distance between semiconductor
laser 6a and hologram plate 6c, p represents the pitch of a fine
corrugated structure (indicated by vertical stripe in the figure)
which forms a hologram of hologram plate 6c, .theta. represents
.+-.first-order diffraction angle caused by hologram plate 6c,
.lambda. represents the wavelength of laser beam, and Z.sub.1
represents the distance between semiconductor laser 6a and
photodetector 6b.
[0067] Furthermore, the following equation (3) is established from
equations (1) and (2):
Z.sub.1=L.lambda./(p.sup.2-.lambda..sup.2).sup.1/2 (3).
[0068] Thus, as wavelength .lambda. of laser beam is increased,
.+-.first-order diffraction angle .theta. caused by hologram plate
6c is also increased. Consequently, distance Z.sub.1 between
semiconductor laser 6a and photodetector 6b is also increased.
Distance Z.sub.1 is also changed depending on distance L between
semiconductor laser 6a and hologram plate 6c. Accordingly, with
pitch p of hologram plate 6c and distance L between semiconductor
laser 6a and hologram plate 6c as parameters, Table 1 shows
calculated distances Z.sub.1 between semiconductor laser 6a and
photodetector 6b. When pitch p ranges from 1.5 to 35 .mu.m and
distance L ranges from 3 to 25 mm for a laser beam wavelength of
635 nm, distance Z.sub.1 ranges from 0.45 to 2.2 mm. Accordingly,
distance Z.sub.1 is here set ranging from 0.45 to 2.2 mm and
distance L ranging from 3 to 25 mm.
1TABLE 1 Distance Distance between Distance between between
emission point emission point Pitch of emitting point and detection
and detection hologram: and hologram: point for beam of point for
beam of p(.mu.m) L(mm) 635 nm:Z.sub.1 (mm) 680 nm:Z.sub.2 (nm) 1.5
3 1.4018 1.5258 2 3 1.0044 1.0846 3 5 1.0828 1.1636 3 4.5 0.9745
1.0473 3 4 0.8683 0.9309 5 5 0.6402 0.6864 4.5 15 2.1381 2.2930 5
15 1.9205 2.0591 8 15 1.1944 1.2796 10 10 0.6360 0.6816 10 15
0.9544 1.0224 10 8 0.5090 0.5453 8 10 0.7963 0.8531 5 10 1.2804
1.3728 3 10 2.1657 2.3272 8 25 1.9906 2.1327 10 25 1.5907 1.7039 15
25 1.0593 1.1345 20 25 0.7942 0.8505 25 25 0.6352 0.6803 30 25
0.5293 0.5668 35 25 0.4536 0.4858
[0069] Table 1 also shows a distance Z.sub.2 between semiconductor
laser 6a and photodetector 6b with use of semiconductor laser 6a
which provides oscillation of laser beam with a wavelength of 680
nm. When pitch p of hologram plate 6c ranges from 1.5 to 35 .mu.m
and distance L between semiconductor laser 6a and hologram plate 6c
ranges from 3 to 25 mm, distance Z.sub.2 ranges from 0.48 to 2.3
mm. Accordingly, distance Z.sub.2 in this example is set ranging
from 0.48 to 2.3 mm and distance L ranging from 3 to 25 mm.
[0070] Although hologram plate 6c is arranged in emitting/receiving
unit 6 such that hologram plate 6c is integrated with semiconductor
laser 6a and photodetector 6b, hologram plate 6c may be arranged
separately from emitting/receiving unit 6. Although
emitting/receiving unit 6 is provided with hologram plate 6c, as
shown in FIG. 5A, a half mirror may be provided between end surface
78 of the optical fiber and semiconductor laser 6a to reflect laser
beam L.sub.21 reflected from magneto-optic recording medium 10 such
that laser beam L.sub.21 is perpendicular to photodetector 6b.
[0071] An operation of the magneto-optic recording medium
reproduction device configured as above will now be described.
[0072] As shown in FIG. 6, a laser beam with a wavelength of 635 nm
that is radiated from emitting/receiving unit 6 at semiconductor
laser 6a and is deflected in the direction perpendicular to the
plane of the drawing diffuses and thus enters hologram plate 6c.
The laser beam incident on hologram plate 6c is diffracted at
hologram plate 6c and split into 0th-order, .+-.first-order, . . .
.+-.nth-order diffracted beams. 0th-order diffracted beam is
transmitted straight through hologram plate 6c without diffracting
at hologram plate 6c, and enters end surface 78 of optical fiber 7.
Since the laser beam radiated from semiconductor laser 6a is
deflected in the direction perpendicular to the plane of the
drawing, it is transmitted through polarizing filter 8 in optical
fiber 7. Thus, a laser beam L.sub.1 incident on core 7c proceeds
through core 7c and is radiated from end surface 77 toward magnetic
recording medium 10. Meanwhile, a laser beam L.sub.2 incident on
inner circumferential clad 7b proceeds through inner
circumferential clad 7b and is radiated from end surface 77 towards
magneto-optic recording medium 10.
[0073] Laser beam L.sub.1, which has proceeded through core 7c,
forms a beam spot with a sharp intensity distribution, as shown in
FIG. 4C, and the diameter of the spot is 600 .ANG. (with a
tolerance of .+-.200 .ANG.), which is almost the same size as
recording domain 21 in recording layer 2. Thus, laser beam L.sub.1
radiated onto magneto-optic recording medium 10 raises the
temperature of only the area of recording domain 21 to a
predetermined temperature or more.
[0074] When the temperature of recording domain 21 exceeds the
predetermined temperature, recording domain 21 in recording layer 2
is transferred to reproducing layer 4 and a domain 40 which is
approximately same in size as recording domain 21 is formed in
reproducing layer 4, as shown in FIG. 7. A direction 40a in which
domain 40 is magnetized is the same as a direction 21a in which
recording domain 21 is magnetized.
[0075] When the direction of alternating field AF applied from
magnetic head 9 is then the same as direction 40a in which the
transferred domain 40 is magnetized, domain 40 is expanded in the
plane of magneto-optic recording medium 10 to form a domain 41,
which is larger than recording domain 21.
[0076] A laser beam L.sub.11 of laser beam L.sub.1 proceeding
through core 7c that is reflected by the expanded domain 41 is
diffused outward and will not return to end surface 77 of optical
fiber 7. Meanwhile, a laser beam L.sub.21 of laser beam L.sub.2
proceeding through inner circumferential clad 7b that is reflected
by the expanded domain 41 is not diffused outwards and will return
to end surface 77 of optical fiber 7. Thus, laser beam L.sub.21
incident on end surface 77 proceeds through core 7c and inner
circumferential clad 7b in the opposite direction. Since the plane
of polarization of laser beam L.sub.21 reflected at the expanded
domain 41 is slightly rotated due to Kerr effect, polarizing filter
8 transmits only a polarization component of laser beam L.sub.21
proceeding through core 7c and inner circumferential clad 7b in the
opposite direction that is the same in polarization direction as
polarizing filter 8.
[0077] Laser beam L.sub.21 transmitted through polarizing filter 8
is radiated from end surface 78 of optical fiber 7 towards hologram
plate 6c to allow a +first-order or -first-order diffracted beam
L.sub.211 of laser beam L.sub.21 incident on photodetector 6b via
polarizing filter 6d. Photodetector 6b produces a reproduced signal
according the incident, diffracted beam L.sub.211. Since Kerr
rotation angle is changed depending on the direction in which
reproducing layer 4 is magnetized, the reproduced signal changes
depending on the magnetization direction in reproducing layer
4.
[0078] Expanded domain 41 is shrunk when the direction of
alternating field AF is opposite to magnetized direction 40a after
detection of the reproduced signal. Repetition of the operation as
described above allows signals to be successively reproduced from
recording domains of recording layer 2.
[0079] In order to obtain a reproduced signal with high C/N ratio,
a signal should be reproduced from a domain in reproducing layer 2
when the domain is expanded.
[0080] As shown in FIG. 7, reproducing layer 4 is magnetized in one
predetermined direction (in this example, the downward direction in
the figure) before domain 21 is transferred from recording layer 2.
When magnetization direction 21a for recording domain 21 in
recording layer 2 is transferred to domain 40 in reproducing layer
4 by exchanging coupling and the direction of alternating field AF
is the same as magnetization direction 40a, domain 40 is expanded
to domain 41. Thus, in order to obtain the maximum reproduced
signal, photodetector 6b need only detector laser beam L.sub.21
reflected from magneto-optic recording medium 10 when alternating
field AF is applied in the direction opposite to the initial
magnetization direction for reproducing layer 4 (i.e., in the
upward direction in the figure).
[0081] In reproducing a signal from recording domain 22 with
magnetization direction 21a and an opposite magnetization direction
22a for recording domain 21, a recording layer has already been
substantially transferred and expanded, since the initial
magnetization direction for reproducing layer 4 is the same as
magnetization direction 22a for domain 22 to be reproduced.
Accordingly, the transfer, expansion and erasure of magnetization
are observed only with respect to a recording domain which is
magnetized in a direction opposite to the initial magnetization
direction for reproducing layer 4. Consequently, photodetector 6b
need only detect laser beam L.sub.21 reflected from magneto-optic
recording medium 10 when alternating field AF is applied in a
direction opposite to the initial magnetization direction for
reproducing layer 4.
[0082] Note that for an initial magnetization direction for
reproducing layer 4 opposite to that shown in FIG. 7, transfer of
magnetization and expansion of a domain are observed when a signal
is reproduced from recording domain 22. Accordingly, to obtain the
maximum reproduced signal, photodetector 6b need only detect laser
beam L.sub.21 reflected from magneto-optic recording medium 10
while alternating field AF is applied in the same direction as
magnetization direction 22a for recording domain 22.
[0083] The description hereinabove has been provided with respect
to reproducing a signal from magneto-optic recording medium 10
shown in FIG. 1. The description hereinafter is provided with
respect to reproducing a signal from magneto-optic recording medium
11 shown in FIG. 2.
[0084] As shown in FIG. 8A, reproducing layer 4 is magnetized in
one predetermined direction (i.e., the upward and downward
directions in the figure).
[0085] As shown in FIG. 8B, when laser beam L.sub.1 with a sharp
temperature distribution then illuminates magneto-optic recording
medium 11, magnetization direction 21a in recording domain 21 as
the illuminated position is transferred to reproducing layer 4 to
form domain 40 with a magnetization direction opposite to an
initial magnetization direction. The transfer is provided due to
magnetostatic coupling rather than exchange coupling, since
non-magnetic intermediate layer 3 is formed between recording layer
2 and reproducing layer 4.
[0086] As shown in FIG. 8C, when the direction of alternating field
AF is then same as magnetization direction 40a in domain 40, domain
40 is expanded as shown in FIG. 8C to form domain 41, which is
larger than recording domain 21. Laser beam L.sub.21 reflected from
the expanded domain 41 is detected by photodetector 6b to generate
a reproduced signal.
[0087] It should be noted that since intermediate layer 3 is formed
between recording layer 2 and reproducing layer 4, the magnetic
wall of domain 41 is not fixed by recording domain 22, which is
adjacent to recording domain 21 and has the opposite magnetization
direction 22a. Thus, a domain can be expanded more stably than in
magneto-optic recording medium 10 without intermediate layer 3.
[0088] Then, as shown in FIG. 8D, the expanded domain 41 is shrunk
when the direction of alternating field AF is the same as the
initial magnetization direction for reproducing layer 4.
[0089] With recording layer 2 and reproducing layer 4 of
magneto-optic recording media 10 and 11 shown in FIGS. 1 and 2
formed successively from the substrate 1 side, laser beam is
radiated from the opposite, protection layer 5 side, since
radiating a laser beam from the thin protection layer 5 side allows
end surface 77 of optical fiber 7 to be arranged closer to
recording and reproducing layers 2 and 4 of magneto-optic recording
medium 10 and 11 than radiating a laser beam from the thick
substrate 1 side.
[0090] As shown in FIG. 9, however, a magneto-optic recording
medium 12 with reproducing layer 4 and recording layer 2 formed
successively from the substrate 1 side can also provide
reproduction by the reproduction device. Magneto-optic recording
medium 12 has an optical interference layer 15 of SiN or the like
formed between substrate 1 and reproducing layer 4 to enhance the
reproducing signal. Formed between intermediate layer 3 and
recording layer 2 is a shield layer 16 of AlTi or the like for
separating recording layer 2 more completely from reproducing layer
4.
[0091] According to the first embodiment, optical fiber 7 has end
surface 77 arranged adjacent to magneto-optic recording medium 10
which is then irradiated with laser beam via end surface 77. Thus,
a signal can be accurately reproduced even from small recording
domain 21 of e.g., 600 .ANG..
[0092] Furthermore, since alternating field AF is applied to
recording medium 10, domain 40 transferred from recording layer 2
to reproducing layer 4 is expanded and a signal is reproduced from
the expanded domain 41. Consequently, a large reproduced signal can
be obtained. When an alternating field is not applied (H=0), an
obtained reproduced signal is extremely small, as shown in FIG.
10A. By contrast, when an alternating field is applied, a large
reproduced signal can be obtained, as shown in FIG. 10B.
[0093] Furthermore, since optical fiber 7 is a multi-clad, step
index optical fiber, the intensity distribution of laser beam
L.sub.1 proceeding through core 7c has a sharp shape and thus only
an extremely small recording domain 21 can be transferred to
reproducing layer 4. Consequently, precise reproduced signal can be
obtained.
[0094] Since hologram plate 6c is provided for diffracting a laser
beam reflected from a magneto-optic recording medium, semiconductor
laser 6a and photodetector 6b can be arranged in a same plane.
Furthermore, the use of emitting/receiving unit 6 into which
semiconductor laser 6a, photodetector 6b and hologram plate 6c are
integrated together allows reduction of the size of the entire
reproduction device.
[0095] Second Embodiment
[0096] The single optical fiber 7 described above may be replaced
with two optical fibers 61 and 62, as shown in FIGS. 11A and 11B.
Optical fiber 61 irradiates magneto-optic recording medium 10 with
laser beam for the transfer and expansion of a domain. Optical
fiber 62 irradiates magneto-optic recording medium 10 with laser
beam and also receives laser beam reflected from magneto-optic
recording medium 10.
[0097] Optical fiber 61 for radiation has a core 61c with a
refractive index ranging from 1.50 to 1.70, and a clad 61a which is
formed around core 61c and has a refractive index ranging from 1.40
to 1.60. The refractive index of clad 61a is smaller than that of
core 61c. Thus, optical fiber 61 is a single-clad, step index
optical fiber.
[0098] Optical fiber 62 for reception has a core 62b with a
refractive index ranging from 1.50 to 1.70, and a clad 62a which is
formed around core 62b and has a refractive index ranging from 1.40
to 1.60. The refractive index of clad 62a is smaller than that of
core 62b. Thus, optical fiber 62 is also a single-clad, step index
optical fiber.
[0099] The diameter of core 62b of the receiving optical fiber 62
is larger than that of core 61c of the radiating optical fiber 61.
Thus, optical fiber 61 radiates laser. beam L.sub.1 which has a
sharp intensity distribution 13 with a small, beam diameter W1, as
shown in FIG. 4C. Optical fiber 62 radiates laser beam L.sub.2
which has a broad intensity distribution with a large, beam
diameter W2 shown in FIG. 4C.
[0100] A reproduction process of the reproduction device will now
be described which employs the two optical fibers 61 and 62 as
described above.
[0101] As shown in FIG. 11A, when laser beam L.sub.1 proceeds
through core 61c of optical fiber 61 and is then radiated to
magneto-optic recording medium 10 via an end surface 661, the
temperature of only recording domain 21 in recording layer 2 is
raised to at least a predetermined temperature and recording domain
21 is transferred to reproducing layer 4 by exchange coupling to
create domain 40 which is expanded to domain 41 by alternating
field AF.
[0102] As shown in FIG. 11B, while domain 40 is expanded to domain
41, magneto-optic recording medium 10 is rotated and domain 40 (41)
is thus moved under optical fiber 62. Thus, the expanded domain 41
is irradiated with laser beam which proceeds through core 62b of
optical fiber 62 and is radiated via an end surface 662. Reflected
laser beam L.sub.22 of laser beam L.sub.2 radiated to the expanded
domain 41 returns to end surface 662 of optical fiber 62 and is
detected by photodetector 6b, as in the first embodiment. The
expanded domain 41 is shrunk by alternating field AF, as described
above.
[0103] Third Embodiment
[0104] A magneto-optic recording medium reproduction device
according to a third embodiment of the present invention includes a
solid immersion lens 80, an objective lens 81, a semiconductor
laser 82, a collimator lens 83, a semiconductor laser 84, a
collimator lens 85, a half mirror 86, a photodetector 87 and a
magnetic head--88.
[0105] Solid immersion lens 80 has a plane 80a adjacent to
magneto-optic recording medium 10, and a spherical surface 80b
opposite to plane 80a. Objective lens 81 is arranged on the curved
surface 80b side of solid immersion lens 80 such that the optical
axis of solid immersion lens 80 matches that of objective lens 80.
Semiconductor laser 82 provides oscillation of laser beam with a
wavelength of 635 nm. Collimator lens 83 forms parallel laser beam
L81 based on the laser beam radiated from semiconductor laser 82.
Collimator lens 85 forms parallel laser beam L82 based on the laser
beam from semiconductor laser 84. The diameter D82 of laser beam
L82 is larger than the diameter D81 of laser beam L81. Half mirror
86 mixes laser beams L81 and L82 to allow laser beams L81 and L82
coaxially incident on objective lens 81. Photodetector 87 receives
laser beam reflected from magneto-optic recording medium 10 and
transmitted through solid immersion lens 80 and objective lens 81.
Magnetic head 88 applies alternating field AF to magneto-optic
recording medium 10 and expands and shrinks a domain created in
reproducing layer 4 of magneto-optic recording medium 10.
[0106] The reproduction device further includes a half mirror 89, a
Wollaston prism 90, a condenser lens 91 and a differential
amplifier 92. Half mirror 89 transmits laser beam L81 from
collimator lens 83 straight and reflects laser beam reflected from
magneto-optic recording medium 10 towards photodetector 87.
Wollaston prism 90 splits laser beam reflected from half mirror89
into p polarization component, s polarization component and laser
beam mixed with s and p polarization components. Condenser lens 91
condenses laser beam transmitted through Wollaston prism 90 onto
photodetector 87. Differential amplifier 92 generates a reproduced
signal, depending on a signal generated by photodetector 87 based
on the p polarization component and a signal generated by
photodetector 87 based on the s polarization component. Note that
photodetector 87 also generates an error signal for focusing
control or tracking control based on the laser beam mixed with s
and p polarization components.
[0107] A reproduction process of the reproduction device configured
as described above will now be described.
[0108] Laser beam radiated from semiconductor laser 82 is rendered
parallel by collimator lens 83 to form laser beam L81 with a small
diameter D81. Meanwhile, laser beam radiated from semiconductor
laser 84 is rendered parallel by collimator lens 85 to form laser
beam L82 with a large diameter D82. Laser beam L81 is transmitted
through half mirror 86 straight to allow laser beam L81 incident on
objective lens 81. Meanwhile, laser beam L82 is reflected by half
mirror 86 to allow laser beam L82 incident on object lens 81. Thus,
the optical system formed of collimator lenses 83 and 85 and half
mirror 86 forms laser beam L81 with diameter D81 and laser beam L82
with diameter D82 larger than diameter D81 and allows laser beams
L81 and L82 coaxially incident on objective lens 81.
[0109] Since two laser beams L81 and L82 different in diameter
enter objective lens 81, as shown in FIG. 13, two laser beams L81
and L82 each enter the spherical surface of solid immersion lens 80
at a different angle. Thus, the spot diameter of laser beam L81
formed by solid immersion lens 80 is larger than that of laser beam
L82 formed by solid immersion lens 80, as shown in FIG. 14A.
Accordingly, the mixed intensity distribution of laser beams L81
and L82 in plane A-A' in FIG. 14A is as shown in FIG. 14B. This
intensity distribution is similar to that shown in FIG. 4C, with
the intensity of laser beam significantly larger at the center than
at the outer circumference.
[0110] Consequently, laser beam from objective lens 81 is further
condensed and only an extremely small recording domain of e.g., no
more than 0.1 .mu.m can be transferred to reproducing layer 4.
Furthermore, since alternating field AF is applied to magneto-optic
recording medium 10 by magnetic head 88, a domain created in
reproducing layer 4 is expanded and the expanded domain is
irradiated with laser beam L81 with a broad intensity distribution.
Thus, the intensity of a reproduced signal detected by
photodetector 87 is increased as well as the first and second
embodiments.
[0111] Fourth Embodiment
[0112] As shown in FIG. 15, a magneto-optic recording medium
reproduction device according to a fourth embodiment of the present
invention includes the configuration of the third embodiment plus a
beam splitter 93, a collimator lens 94 and reflecting mirrors 95
and 96. It should be noted, however, that the device is not
provided with semiconductor laser 84 shown in FIG. 12 and is only
provided with a single semiconductor laser 82. Collimator lens 94
replaces collimator lens 85 shown in FIG. 12. Although half mirrors
86 and 89 are opposite in arrangement to those shown in FIG. 12,
half mirrors 86 and 89 in the fourth embodiment may also be
arranged as shown in FIG. 12.
[0113] Beam splitter 93 splits laser beam radiated from
semiconductor laser 82 into two by transmitting the laser beam
straight and also reflecting the laser beam perpendicularly.
Collimator lens 83 forms laser beam L81 with a small diameter D81,
based on the laser beam transmitted straight through beam splitter
93. Collimator lens 94 forms laser beam L82 with a large diameter
D82, based on the laser beam reflected from beam splitter 93
perpendicularly. Reflecting mirror 95 perpendicularly reflects
laser beam L82 from collimator lens 94, and reflecting mirror 96
further reflects the laser beam reflected perpendicularly by
reflecting mirror 95 towards half mirror 86 perpendicularly. Thus,
the optical system formed of collimator lenses 83 and 94, half
mirror 86, beam splitter 93 and reflecting mirrors 95 and 96 forms
laser beam L81 with small diameter D81 and laser beam L82 with
diameter D82 larger than diameter D81 and allows laser beams L81
and L82 coaxially incident on objective lens 81.
[0114] In the reproduction device configured as above, laser beam
radiated from a single semiconductor laser 82 is split into two by
beam splitter 93, based on a laser beam reflected perpendicularly
by beam splitter 93, collimator lens 94 forms laser beam L82 with
large diameter D82. perpendicularly. Thus, the reproduction device
dispenses with semiconductor laser 84 as shown in FIG. 12.
[0115] It should be noted that it is desirable that the distance
between end surface 77 of optical fiber 7 or plane 80a of solid
immersion lens 80 and magneto-optic recording medium 10, 11, and 12
is always fixed and thus tracks are desirably formed in a same
plane, rather than of the land and groove type. In this case, a
signal for tracking control may be previously recorded in a
magneto-optic recording medium and side beam as well as main beam
may be radiated to read the recorded signal to provide tracking
control.
[0116] Although the present invention has been described and
illustrated in detail, it is clearly understood that the same is by
way of illustration and example only and is not to be taken by way
of-limitation, the spirit and scope of the present invention being
limited only by the terms of the appended claims.
* * * * *