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Description  |
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FIELD OF THE INVENTION
This invention relates to a magneto-optical disk head system, and more
particularly to, a magneto-optical disk head system for recording
(writing), reading and erasing data to or from a magneto-optical disk.
BACKGROUND OF THE INVENTION
A first conventional magneto-optical disk head system has been disclosed in
the text 29p-N-2 of Extended Abstracts (The 51th Autumn Meeting, 1990):
The Japan Society of Applied Physics. The first conventional
magneto-optical disk head system includes a semiconductor laser, an
optical system including a collimator lens and an objective lens for
focusing the laser light on a magneto-optical disk, a beam splitter, a
half-wave plate, a polarizing beam splitter and a photodetector.
In operation, a laser light emitted from the semiconductor laser is
transmitted through an optical system consisting of the beam splitter and
the collimator and objective lenses to be focused on a magneto-optical
disk. A reflection light of the laser light reflected at the
magneto-optical disk reaches the beam splitter at which the reflection
light changes its direction toward the half-wave plate. The half-wave
plate rotates the polarization plane of the reflection light by 45 degrees
to spatially separate the reflection light to P-polarized and S-polarized
lights which have orthogonal polarization directions. The P-polarized and
S-polarized lights are detected separately by the photodetector via the
polarizing beam splitter.
Tile photodetector of the first conventional magneto-optical disk head
system includes an N-layer grown by epitaxy on an N.sup.+ -silicon
substrate, and two P-layers formed within the N-layer. The two P-layers
compose two islanded segment regions for detecting the P-polarized and
S-polarized lights, respectively. Each islanded segment region consists of
several photodetecting segments.
Tile first conventional magneto-optical disk head system includes a signal
detecting circuit for detecting an information signal and focusing and
tracking error signals of the magneto-optical disk. In the signal
detecting circuit, each of the photodetecting segments of the
photodetector consists of a photodiode having a cathode corresponding to
an electrode of a top surface and an anode corresponding to an electrode
of a bottom surface. Cathodes of the photodetecting segments are connected
in common to a bias power supply by which the signal detecting circuit is
applied with a bias voltage. Low frequency components of signals supplied
from the anodes of the photodetecting segments give the focusing and
tracking error signals, respectively. On the other hand, high frequency
components of signals supplied from the anodes thereof give the
information signal by which data recorded on the magneto-optical disk are
able to be playbacked.
second conventional magneto-optical disk head system has been disclosed in
the text 30a-G-5 of Extended Abstracts (The 37th Autumn Meeting, 1990);
The Japan Society of Applied Physics. The second conventional
magneto-optical disk headsystem includes a semiconductor laser, a
collimator lens, an objective lens, a beam splitter, a lens, a holographic
element and a photodetector.
In operation, a laser light emitted from the semiconductor laser is
transmitted through an optical system consisting of the collimator lens,
the b earn splitter and the objective lens to be focused on a
magneto-optical disk. A reflection light of the laser light reflected at
the magneto-optical disk reaches the beam splitter at which the reflection
light changes its direction toward the photodetector through the lens and
the holographic element. The reflection light is divided by the
holographic element to three lights, first one being a non-diffracted
light (zeroth order diffracted light) whose polarization direction is
orthogonal to an optical axis of a crystal composing the holographic
element, and second and third ones being .+-.1st order diffracted lights
whose polarization directions are parallel to the optical axis of the
crystal. These three lights are detected by the photodetector.
The photodetector of the second conventional magneto-optical disk head
system includes an N-layer grown by epitaxy on an N.sup.+ -silicon
substrate, and three P-layers formed within the N-layer. The three
P-layers compose three islanded segment regions for detecting the zeroth
order and .+-.1st order diffracted lights, respectively. Each islanded
segment region consists of several photodetecting segments.
The second conventional magneto-optical disk head system includes a signal
detecting circuit for detecting an information signal and focusing and
tracking error signals of the magneto-optical disk. In the signal
detecting circuit, each of the photodetecting segments of the
photodetector consists of a photodiode having a cathode corresponding to
an electrode of a top surface and an anode corresponding to an electrode
of a bottom surface. Cathodes of the photodetecting segments are connected
in common to a bias power supply by which the signal detecting circuit is
applied with a bias voltage. Low frequency components of signals supplied
from anodes of the photodetecting segments give the focusing and tracking
error signals, respectively. On the other hand, high frequency components
of signals supplied from the anodes thereof give the information signal.
According to the first and second conventional magneto-optical disk head
systems, however, there is a disadvantage in that the information signal
and the focusing and tracking error signals are badly affected by each
other by noises generated by the circuits for detecting these signals,
because these three signals are detected through the common anodes of the
photodetecting segments of the photodetector. As a result, the quality of
the signals become deteriorated.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a magneto-optical
disk head system in which the high quality of an information signal and
focusing and tracking error signals can be obtained by avoiding the
interference of these signals due to noises generated by circuits for
detecting these signals.
According to a feature of the invention, a magneto-optical disk head system
comprises:
a laser light source for emitting a laser light;
an optical system for focusing the laser light emitted from the laser light
source on a magneto-optical disk;
means for separating a reflection light of the laser light reflected at the
magneto-optical disk to a plurality of polarized lights having orthogonal
polarization directions; and
a photodetector for detecting the plurality of polarized lights, the
photodetector comprising a plurality of photodetecting segments each of
which consisting of a photodiode isolated from each other;
wherein an information signal of the magneto-optical disk is detected
through cathodes of the plurality of photodetecting segments; and
focusing and tracking error signals of the magneto-optical disk are
detected through anodes of the plurality of photodetecting segments.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained in more detail in conjunction with appended
drawings wherein:
FIG. 1 is a schematic diagram illustrating a first conventional
magneto-optical disk head system;
FIGS. 2A to 2C are explanatory plan views illustrating beam spots on
photodetecting segments of a photodetector of the first conventional
magneto-optical disk head system shown in FIG. 1;
FIG. 3 is a cross-sectional view illustrating the photodetector of the
first conventional magneto-optical disk head system shown in FIG. 1;
FIG. 4 is a block diagram of a signal detecting circuit of the first
conventional magneto-optical disk head system shown in FIG. 1;
FIG. 5 is a schematic diagram illustrating a second conventional
magneto-optical disk head system;
FIG. 6 is an explanatory plan view illustrating a holographic element of
the second conventional magneto-optical disk head system shown in FIG. 5;
FIG. 7 is a cross-sectional view illustrating the holographic element of
the second conventional magneto-optical disk head system shown in FIG. 5;
FIGS. 8A to 8C are explanatory plan views illustrating beam spots on
photodetecting segments of a photodetector of the second conventional
magneto-optical disk head system shown in FIG. 5;
FIG. 9 is a cross-sectional view illustrating the photodetector of the
second conventional magneto-optical disk head system shown in FIG. 5;
FIG. 10 is a block diagram of a signal detecting circuit of the second
conventional magneto-optical disk head system shown in FIG. 5;
FIG. 11 is a cross-sectional view illustrating a photodetector of a
magneto-optical disk head system in a first preferred embodiment according
to the invention;
FIG. 12 is a block diagram of the signal detecting circuit of the
magneto-optical disk head system in the first preferred embodiment
according to the invention;
FIG. 13 is a cross-sectional view illustrating a photodetector of a
magneto-optical disk head system in a second preferred embodiment
according to the invention;
FIG. 14 is a block diagram of a signal detecting circuit of the
magneto-optical disk head system in the second preferred embodiment
according to the invention;
FIG. 15 is a cross-sectional view illustrating a photodetector of a
magneto-optical disk head system in the second preferred embodiment
according to the invention; and
FIG. 16 is a cross-sectional view illustrating a photodetector of a
magneto-optical disk head system in a fourth preferred embodiment
according to the invention,
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before describing a magneto-optical disk head system in preferred
embodiments according to the invention, the conventional magneto-optical
disk head systems described before will be explained in conjunction with
FIGS. 1 to 10.
FIG. 1 shows a first conventional magneto-optical disk head system. The
magneto-optical disk head system includes a semiconductor laser 1, a
collimator lens 2, an objective lens 3, a beam splitter 5, a half-wave
plate 6, a polarizing beam splitter 7 and a photodetector 11.
In operation, a laser light emitted from the semiconductor laser 1 is
transmitted through the beam splitter 5 and reaches the collimator lens 2.
The laser light becomes a parallel light at the collimator lens 2 and then
reaches the objective lens 3. The parallel light is focused by the
objective lens 3 and supplies to a magneto-optical disk 4. A reflection
light of the laser light reflected at the magneto-optical disk 4 is
transmitted through the objective lens 3 and the collimator lens 2, and
reaches the beam splitter 5. The reflection light reflects at the beam
splitter 5 to change its direction to the half-wave plate 6. The half-wave
plate 6 rotates the polarization plane of the reflection light by 45
degrees to spatially separate the reflection light to P-polarized and
S-polarized lights which have orthogonal polarization directions. The
P-polarized light is transmitted through a front surface 7a of the
polarizing beam splitter 7 and reach a total reflection surface 7b
thereof. Tile P-polarized light reflects at the total reflection surface
7b and is transmitted through the front surface 7a where the P-polarized
light changes the direction toward the photodetector 11 to be detected.
The S-polarized light directly reflects at the front surface 7a and
changes the direction toward the photodetector 11 to be detected.
FIGS. 2A to 2C show beam spots of the P-polarized and S-polarized lights on
the photodetector pattern of the photodetector 11. The photodetector 11
includes two islanded segment regions 30a and 3Ob. The islanded segment
region 30a for detecting the P-polarized light consists of photodetecting
segments 31 to 36, while the islanded segment region 30b for detecting the
S-polarized light consists of photodetecting segments 37 to 42. Two beam
spots 20 and 21 corresponding to the P-polarized and S-polarized lights
are focused on the islanded segment regions 30a and 30b, respectively.
Sizes of the beam spots 20 and 21 change in accordance with the change in
the focus state on the magneto-optical disk 4. The two beam spots 20 and
21 have almost the same size, when the disk 4 is in the focus position, as
shown in FIG. 2B. However, the beam spot 20 becomes larger than the beam
spot 21, when the disk 4 is over the focus position, as shown in FIG. 2A.
On the other hand, the beam spot 20 becomes smaller than the beam spot 21,
when the disk 4 is under the focus position, as shown in FIG. 2C.
Therefore, appropriate combinations of the outputs of the photodetecting
segments 81 to 42 give a focusing error signal, a tracking error signal
and an information signal. The focusing error signal is given by
(V(31)+V(32)+V(35)+V(36)+V(39)+V(40))-(V(33)+V(34)+V(37)+V(38)+V(41)+V(42)
), according to the spot size detection method, where V(31) to V(42) are
outputs of the photodetecting segments 31 to 42, respectively. The
tracking error signal is given by either or a sum of
(V(31)+V(33)+V(35))-(V(32)+V(34)+V(36)) and
(V(37)+V(39)+V(41))-(V(38)+V(40)+V(42)), according to the push-pull
method. The information signal is given by an optical power difference
between the two beam spots 20 and 21, that is
(V(31)+V(32)+V(33)+V(34)+V(35)+V(36))-(V(37)+V(38)+V(39)+V(40)+V(41)+V(42)
).
FIG. 3 shows a cross-section of the photodetector 11. The photodetector 11
includes an N-layer 56 which is a conductive layer grown on an N.sup.+
-silicon substrate 63 by epitaxy, and P-layers 51 and 52 formed within the
N-layer 56 in the vicinity of the surface thereof. The P-layers 51 and 52
correspond to the islanded segment regions 30a and 30b, respectively.
FIG. 4 shows a signal detecting circuit for detecting the information
signal and the focusing and tracking error signals in the first
conventional magneto-optical disk head system. In the signal detecting
circuit, each of the photodetecting segments 31 to 42 consists of a
photodiode having a cathode corresponding to an electrode of a top surface
and an anode corresponding to an electrode of a bottom surface. Cathodes
of the photodetecting segments 31 to 42 are connected in common to a
positive terminal of a bias power supply 77 by which the signal detecting
circuit is applied with a bias voltage. Anodes of the photodetecting
segments 31, 32, 35, 36, 39 and 40 are connected to input terminals of an
adder 78 through resistors 96 to 101, respectively. Anodes of the
photodetecting segments 33, 34, 37, 38, 41 and 42 are connected to input
terminals of an adder 79 through resistors 102 to 107, respectively.
Anodes of the photodetecting segments 31, 33, 35, 37, 39 and 41 are
connected to input terminals of an adder 80 through resistors 108 to 113,
respectively. Anodes of the photodetecting segments 32, 34, 36, 38, 40 and
42 are connected to input terminals of an adder 81 through resistors 114
to 119, respectively. Output terminals of the adders 78 and 79 are
connected to input terminals of a differential amplifier 84. Output
terminals of the adders 80 and 81 are connected to input terminals of a
differential amplifier 85. Anodes of the photodetecting segments 31 to 36
are connected to one of two input terminals of a differential amplifier 86
through capacitors 126 to 131, respectively. Anodes of the photodetecting
segments 37 to 42 are connected to the other of the two input terminals of
the differential amplifier 86 through capacitors 132 to 137, respectively.
Low frequency components of outputs of the differential amplifiers 84 and
85 give the focusing and tracking error signals, respectively. On the
other hand, high frequency components of outputs of the differential
amplifier 86 give the information signal.
FIG. 5 shows a second conventional magneto-optical disk head system. The
magneto-optical disk head system includes a semiconductor laser 1, a
collimator lens 2, an objective lens 3, a beam splitter 8, a lens 9, a
holographic element 10 and a photodetector 12.
In operation, a laser light emitted from the semiconductor laser 1 becomes
a parallel light at the collimator lens 2, and then is transmitted through
the beam splitter 8. The parallel light is focused by the objective lens 3
and suppliedto a magneto-optical disk 4. A reflection light of the laser
light reflected at the magneto-optical disk 4 is transmitted through the
objective lens 3 and reaches the beam splitter 8. The reflection light
reflects at the beam splitter 8 to change the direction toward the lens 9
and reaches the holographic element 10 through the lens 9. The holographic
element 10 consists of birefringent crystal whose optical axis forms an
angle of 90 degrees with the polarization direction of a polarized light
irradiated from outside, so that the reflection light is divided to three
lights, first one being a non-diffracted light (zeroth order diffracted
light) whose polarization direction is orthogonal to the optical axis of
the crystal, and second and third ones being .+-.1th order diffracted
lights whose polarization directions are parallel to the optical axis of
the crystal. These three lights are detected by the photodetector 12.
FIG. 6 shows a plan view of the holographic element 10. The holographic
element 10 is divided to four regions 13 to 16, so that the .+-.1th order
diffracted lights generated by the holographic element 10 are divided to
four beams each of which corresponds to each of the regions 13 to 16. The
regions 13 and 14 are divided by a line vertical to the tracking direction
of the magneto-optical disk 4. The regions 15 and 16 each of which is a
boat-shaped region are located symmetrical to each other with the track of
the magneto-optical disk 4.
FIG. 7 shows a cross-section of the holographic element 10. The holographic
element 10 includes a plurality of proton exchanged regions 18 formed
periodically within a lithium niobate substrate 17 consisting of a
birefringent crystal in the vicinity of the surface thereof, and phase
compensation films 19 formed on each of the proton exchanged regions 18.
When an ordinary light whose polarization direction is vertical to the
optical axis of the lithium niobate substrate 17 of the holographic
element 10 is irradiated to the holographic element 10, the ordinary light
will not diffract thereat and become a zeroth order diffracted light
therein, because a phase difference between the light which transmitted
through an area in the holographic 10 including the proton exchanged
regions 18 and the light which is transmitted through an area therein not
including the proton exchanged regions 18 is 0.degree.. On the other hand,
an extraordinary light whose polarization direction is parallel to the
optical axis of the lithium niobate substrate 17 of the holographic
element 10 is irradiated to the holographic element 10, the extraordinary
light will completely diffract thereat and become .+-.1th order diffracted
lights.
FIGS. 8A to 8C show beam spots of the zeroth order and .+-.1st order
diffracted lights on the photodetector patterns of the photodetector 12.
The photodetector 12 includes three islanded segment regions 12a to 12c.
The islanded segment region 12a for detecting the +1st order diffracted
light consists of photodetecting segments 43 to 48, the island segment
region 12b for detecting the zeroth order diffracted light consists of a
photodetecting segment 49, and the islanded segment region 12c for
detecting the -1th order diffracted light consists of a photodetecting
segment 50. A beam spot 22 on the islanded segment region 12b corresponds
to the zeroth order diffracted light, beam spots 23 to 26 on the islanded
segment region 12a correspond to the +1st order diffracted light being
transmitted through the regions 13 to 16 of the holographic element 10,
and beam spots 27 to 30 on the islanded segment region 12c correspond to
the -1st order diffracted light being transmitted through the regions 13
to 16 of the holographic element 10, respectively.
Sizes and locations of the beam spots 22 to 30 change in accordance with
the change in the focus state of the magneto-optical disk 4. When the disk
4 is in the focus position, the beam spot 22 is at the center of the
photodetecting segment 49 (the islanded segment region 12b), the beam spot
23 is on the line separating the photodetecting segments 43 and 44, the
beam spot 24 is on the line separating the photodetecting segments 45 and
46, the beam spots 25 and 26 are at the centers of the photodetecting
segments 47 and 48 respectively, and the beam spots 27 to 30 are within
the photodetecting segment 50 (the islanded segment region 12c), and all
the beam spots 22 to 30 have narrow pointed areas, as shown in FIG. 8B.
When the disk 4 is over the focus position, the beam spots become larger.
The beam spot 22 becomes a disk-shaped spot, each of the beam spots 23,
24, 27 and 28 becomes a half-moon-shaped spot, and each of the beam spots
25, 26, 29 and 30 becomes an oval-shaped spot, as shown in FIG. 8A. The
beam spots 22 and 25 to 30 remain within the corresponding photodetecting
segments as they are in FIG. 8B, however, the beam spots 23 and 24 shift
to locate within the photodetecting segments 44 and 46, respectively. When
the disk 4 is under the focus position, each of the beam spots 22 to 30
has the same shape as in FIG. 8A, however, the beam spots 23, 24, 27 and
28 change their locations, as shown in FIG. 8C.
The beam spots 27 and 28 remain within the photodetecting segment 50,
however, the beam spots 23 and 24 move from the photodetecting segments 44
and 46 to the photodetecting segments 43 and 45, respectively.
Therefore, the focusing error signal is given by
(V(43)+V(45))-(V(44)+V(46)), according to the Foucault's method. The
tracking error signal is given by V(47)-V(48), according to the push-pull
method. The information signal is given by an optical power difference
between the beam spot 22 and a sum of the beam spots 23 to 30, that is
V(49)-(V(43)+V(44)+V(45)+V(46) +V(47)+V(48)+V(50)). Where V(43) to V(50)
are outputs of the photodetecting segments 43 to 50, respectively.
FIG. 9 shows a cross-section of the photodetector 12. The photodetector 12
includes an N-layer 57 which is a conductive layer grown on an N.sup.+
-silicon substrate 64 by epitaxy, and P-layers 53 to 55 formed within the
N-layer 57 in the vicinity of the surface thereof. The P-layers 53 to 55
correspond to the islanded segment regions 12a to 12c, respectively.
FIG. 10 shows a signal detecting circuit of the second conventional
magneto-optical disk head system. In the signal detecting circuit, each of
the photodetecting segments 43 to 50 consists of a photodiode having a
cathode corresponding to an electrode of a top surface and an anode
corresponding to an electrode of a bottom surface. Cathodes of the
photodetecting segments 43 to 50 are connected in common to a bias power
supply 77 by which the signal detecting circuit is applied with a bias
voltage. Anodes of the photodetecting segments 43 and 45 are connected to
input terminals of an adder 82 through resistors 120 and 121,
respectively. Anodes of the photodetecting segments 44 and 46 are
connected to input terminals of an adder 83 through resistors 122 and 123,
respectively. Output terminals of the adders 82 and 88 are connected to
input terminals of a differential amplifier 87. Anodes of the
photodetecting segments 47 and 48 connected to input terminals of a
differential amplifier 88 through resistors 124 and 125, respectively.
Anodes of the photodetecting segments 43 to 48 and 50 are connected to one
of two input terminals of a differential amplifier 89 through capacitors
143 to 138 and 145, respectively. An anode of the photodetecting segment
49 is connected to the other of the two input terminals of the
differential amplifier 89 through a capacitor 144, respectively. The
anodes of the photodetecting segments 49 and 50 are connected to ground
through resistors 90 and 91, respectively. Low frequency components of
outputs of the differential amplifiers 87 and 88 give the focusing and
tracking error signals, respectively. On the other hand, high frequency
components of outputs of the differential amplifier 89 gives the
information signal.
Next, a magneto-optical disk head system in a first preferred embodiment
will be explained. The basic structure of the magneto-optical disk head
system in the first preferred embodiment is the same as that in the first
conventional magneto-optical disk head system shown in FIG. 1, except that
the structure of a photodetector in the first preferred embodiment is
different from that in the first conventional magneto-optical disk head
system.
FIG. 11 shows a cross-section of the photodetector of the magneto-optical
disk head system in the first preferred embodiment. The photodetector
includes N-layers 58 and 59 grown by epitaxy on a polysilicon substrate 70
which is an insulation material. The N-layers 58 and 59 are isolated from
each other by an isolation recess 74 whose bottom is located in the middle
of the polysilicon substrate 70 in depth. P-layers 51 and 52 are formed
within the N-layers 58 and 59 in the vicinity of the surface thereof,
respectively. The P-layers 58 and 59 correspond to the islanded segment
regions 30a and 30b of the photodetector 11 shown in FIGS. 2A to 2C,
respectively.
The photodetector includes two islanded segment regions 30a and 30b
consisting of photodetecting segments 31 to 36 and 37 to 42, respectively,
which is the same as the photodetector 11 of the magneto-optical disk head
system in the first preferred embodiment shown in FIGS. 2A to 2C. Figures
of beam spots of laser lights emitted from a semiconductor laser 1 are the
same as those shown in FIGS. 2A to 2C, and sizes of the beam spots change
in accordance with the change in the focus state on the magneto-optical
disk 4, as like in the first conventional magneto-optical disk head
system, so that the explanation will not be repeated again.
FIG. 12 shows a signal detecting circuit of the magneto-optical disk head
system in the first preferred embodiment. Cathodes of the photodetecting
segments 31 to 36 are connected in common to both a first terminal of a
resister 92 and a first input terminal of a differential amplifier 86,
while cathodes of the photodetecting segments 37 to 42 are connected in
common to both a first terminal of a resistor 93 and a second input
terminal of the differential amplifier 86. Second terminals of the
resistors 92 and 93 are connected to a positive terminal of a bias power
supply 77 by which the signal detecting circuit is applied with a bias
voltage. Anodes of the photodetecting segments 31, 32, 35, 36, 39 and 40
are connected to input terminals of an adder 78, anodes of the
photodetecting segments 33, 34, 37, 38, 41 and 42 are connected to input
terminals of an adder 79, anodes of the photodetecting segments 31, 33,
35, 37, 39 and 41 are connected to input terminals of an adder 80, and
anodes of the photodetecting segments 32, 34, 36, 38, 40 and 42 are
connected to input terminals of an adder 81. Output terminals of the
adders 78 and 79 are connected to input terminals of a differential
amplifier 84, and output terminals of the adders 80 and 81 are connected
to input terminals of a differential amplifier 85. Outputs of the
differential amplifiers 84 and 85 give the focusing and tracking error
signals, respectively. On the other hand, an output of the differential
amplifier 86 gives the information signal.
In the magneto-optical disk head system in the first preferred embodiment,
the N-layers 58 and 59 of the photodetector each of which corresponds to a
cathode of the photodiode composing the photodetector are isolated from
each other by the isolation recess 74, and the N-layers 58 and 59 are
formed on the polysilicon substrate 70 which is an insulation material, so
that signals from the islanded segment regions 30a and 30b are completely
separated from each other. Consequently, it is possible to get the
information signal from the cathodes of the photodetecting segments 31 to
42 by amplifying a difference of a sum of signals front the cathodes 31 to
36 and a sum of signals from the cathodes 37 to 42, as shown in FIG. 12.
The focusing and tracking error signals are given from the anodes of the
photodetecting segments 31 to 42, which are the same as in the
conventional magneto-optical disk head systems, so that the information
signal and the focusing and tracking error signals are prevented from
interferences due to noises generated by the circuits for detecting these
signals.
Next, a magneto-optical disk head system in a second preferred embodiment
will be explained. The basic structure of the magneto-optical disk head
system in the second preferred embodiment is the same as that in the
second conventional magneto-optical disk head system shown in FIG. 5,
except that the structure of a photodetector in the second preferred
embodiment is different from that in the second conventional
magneto-optical disk head system.
FIG. 13 shows a cross-section of the photodetector of the magneto-optical
disk head system in the second preferred embodiment. The photodetector
includes N-layers 60 to 62 grown by epitaxy on a polysilicon substrate 71
which is an insulation material. The N-layers 60 to 62 are isolated from
each other by isolation recesses 75 and 76 whose bottoms are located in
the middle of the polysilicon substrate 71 in depth. P-layer 53 to 55 are
formed within the N-layers 60 to 62 in the vicinity of the surface
thereof, respectively. The P-layers 53 to 55 correspond to the islanded
segment regions 12a to 12c of the photodetector 11 shown in FIGS. 8A to
8C, respectively.
The photodetector includes three islanded segment regions 12a to 12c
consisting of photodetecting segments 43 to 48, 49 and 50, respectively,
which is the same as the photodetector 11 of the magneto-optical disk head
system in the second preferred embodiment shown in FIGS. 8A to 8C. Figures
of beam spots of laser lights emitted from a semiconductor laser 1 are the
same as those shown in FIGS. 8A to 8C. Sizes, shapes and locations of the
beam spots change in accordance with the change in the focus state on the
magneto-optical disk 4, as like in the second conventional magneto-optical
disk head system. Therefore, the explanation will not be repeated again.
FIG. 14 shows a signal detecting circuit of the magneto-optical disk head
system in the second preferred embodiment. Cathodes of the photodetecting
segments 43 to 48 and 50 are connected in common to both a first terminal
of a resister 94 and a first input terminal of a differential amplifier
89, while a cathode of the photodetecting segment 49 is connected to both
a first terminal of a resistor 95 and a second input terminal of the
differential amplifier 89. Second terminals of the resistors 94 and 95 are
connected to a positive terminal of a bias power supply 77 by which the
signal detecting circuit is applied with a bias voltage. Anodes of the
photodetecting segments 43 and 45 are connected to input terminals of an
adder 82, and anodes of the photodetecting segments 44 and 46 are
connected to input terminals of an adder 83. Output terminals of the
adders 82 and 83 are connected to input terminals of a differential
amplifier 87. Anodes of the photodetecting segments 47 and 48 are
connected to input terminals of a diff | | |
