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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head for writing and/or reading
information on and/or from an optical record medium comprising a
semiconductor laser for emitting a laser beam, a light converging means
for converging said laser beam and projecting the thus converged laser
beam onto the optical record medium, and a photodetecting means for
receiving the laser beam reflected from the optical record medium by means
of said light converging means.
2. Related Art Statement
The optical head of the type mentioned above has been widely used for
writing and/or reading information on and/or from an optical record medium
such as optical disk and optical card.
In Japanese Patent Application Laid-open Publication Kokai Sho 63-32743,
there is disclosed a known optical head having a semiconductor laser. As
shown in FIG. 1, the optical head includes a semiconductor laser 1 for
emitting a diverging laser beam and a lens 2 for converging the laser
beam. The laser beam is then transmitted through a hologram 3 and then is
converted into a parallel laser beam by means of a collimator lens 4. The
parallel laser beam is then made incident upon an objective lens 5 and is
projected onto an optical record medium 6 such as a magneto-optical disk
as a very fine spot.
The laser beam reflected by the magneto-optical disk 6 is made incident
upon the hologram 3 by means of the objective lens 5 and collimator lens
4. Then there are produced plus and minus first order (.+-.1-order)
diffracted laser beams having astigmatism contained therein. These
.+-.1-order diffracted laser beams are then made incident upon
photodetectors 9 and 10 by means of a polarizing lens 7. The
photodetectors 9 and 10 are formed on a substrate 8 and each of them has
four divided light receiving regions. An information signal may be
obtained from a difference between output signals of the photodetectors 9
and 10 and at the same time a focusing error signal can be obtained by the
well-known astigmatism method.
FIGS. 2 and 3 illustrate another known optical head including a
semiconductor laser. A laser beam emitted by a semiconductor laser 1 is
converted into a parallel laser beam by means of a collimator lens 4 and
then is made incident upon an objective lens 5 via a polarizing beam
splitter 11. It should be noted that the incident laser beam is of
P-polarized beams, so that it is transmitted through the polarizing beam
splitter.
The laser beam is then made incident upon a magneto-optical disk 6 by means
of an objective lens 5. The laser beam reflected by the magneto-optical
disk 6 is made incident upon the polarizing beam splitter 11 by means of
the objective lens 5 and is reflected thereby. A polarizing direction of
the laser beam reflected by the polarizing beam splitter 11 is rotated by
45 degrees by means of a half-wavelength plate 16 and is then made
incident upon a trapezoidal prism 13 by means of a lens 12, said prism
having a function for dividing polarized light. The prism 13 has a
polarization beam splitting plane 13a which transmits P-polarized
component but reflects S-polarized component. The P-polarized component
transmitted through the polarization beam splitting plane 13a is made
incident upon a photodetector 15a having three divided light receiving
regions and the S-polarized component reflected by the polarization beam
splitting plane 13a is made incident upon a photodetector 15b also having
three light receiving regions. The photodetectors 15a and 15b are formed
on a common substrate 14. An information signal can be reproduced by a
difference between output signals from the photodetectors 15a and 15b and
the focusing error signal can be obtained by the known beam size method.
In this known optical head, the P-polarized laser beam emitted from the
semiconductor laser 1 is made incident upon the polarizing beam splitter
11, and thus information about the Kerr rotation contained in the laser
beam reflected by the magneto-optical disk 6 is borne by the S-polarized
laser beam. Therefore, in order to obtain the information signal having
large amplitude and high C/N, it is advantageous to introduce in an
efficient manner the S-polarized component into the photodetectors 15a and
15b. Therefore, it is desired to make the reflectance of the polarizing
beam splitter 11 for the S-polarized light 100%. To this end, in this
known optical pick-up head, transmittance T and reflectance R of the
polarizing beam splitter 11 for the P- and S-polarized components are set
to, for instance Tp 80%: Ts 0%: Rp 20%: Rs 100%.
In Japanese Patent Application Laid-open Publication Kokai Hei 5-307759
published on Nov. 19, 1993, there is disclosed an optical pick-up head as
shown in FIGS. 4 to 7. The optical pick-up head comprises a block like
unit 16 of semiconductor laser, photodetectors and holograms and a laser
beam emitted by the unit 16 is made incident upon an optical record medium
19 by means of a stop 17 and an objective lens 18. The laser beam
reflected by the record medium 19 is then made incident upon the unit 16
by means of the objective lens 18 and stop 17.
As shown in FIG. 5, the unit 16 comprises a substrate 20 on which a
semiconductor substrate 21 is provided, and a hologram element 22 is
arranged on the substrate 20 via a spacer 23. On the semiconductor
substrate 21 there is provided a semiconductor laser 24, and as best shown
in FIG. 6 in a surface of the semiconductor substrate there are formed
photodetectors 25, 26 and 27 and 28, 29 and 30 on both sides of the
semiconductor laser 24 viewed in a tracking direction x, each of said two
sets of photodetectors being aligned in a track direction y. The middle
photodetectors 26 and 29 have three light receiving regions 26a, 26b and
26c and 29a, 29b and 29c which are divided along lines extending in the
direction x. As illustrated in FIG. 7, in the front surface of the
semiconductor substrate 21, there is formed a recess 21a with an inclined
side wall 21b by etching, and the semiconductor laser 24 is placed on the
bottom of the recess 21a. The inclined side wall 21b is polished as a
mirror surface, so that a laser beam emitted from the semiconductor laser
24 in a direction parallel with the surface of the semiconductor substrate
21 is reflected by the inclined side wall 21b into a direction which is
normal to the surface of the semiconductor substrate 21.
As depicted in FIG. 5, the hologram element 22 comprises on its one surface
gratings 22a for dividing the laser beam emitted by the semiconductor
laser 24 into three laser beams, i.e. 0-order beam (main beam), +1-order
beam and -1-order beam (sub-beams). The hologram element 22 further
comprises on its other surface a hologram pattern 22b for diffracting an
incident beam and giving the +1-order and -1-order laser beams opposite
focal powers.
The laser beam emitted by the semiconductor laser 24 is reflected by the
inclined side wall 21b and is then divided by the gratings 22a into the
single main beam and two sub-beams, these three beams being made incident
upon the optical record medium 19 by means of the stop 17 and objective
lens 18. The three laser beams reflected by the record medium 19 are made
incident upon the hologram pattern 22b by means of the objective lens 18
and stop 17 and are diffracted thereby. .+-.1-order beams of the main beam
are received by the middle photodetectors 26 and 29, respectively,
.+-.1-order beams of one of the two sub-beams are received by the
photodetectors 25 and 28, respectively and .+-.1-order beams of the other
sub-beam are received by the photodetectors 27 and 30, respectively. The
focusing error signal is derived from outputs of the photodetectors 26 and
29 in accordance with the beam size method, and the tracking error signal
is obtained by output signals from the photodetectors 25, 27, 28 and 30 on
the basis of the three beam method.
In the known optical head shown in FIG. 1, the laser beam reflected by the
magneto-optical record disk 6 is made incident upon the photodetectors 9
and 10 after being diffracted by the hologram 3. The hologram 3 is of a
thin type in which a depth of the recesses is smaller than a wavelength,
and thus the diffraction efficiency for the .+-.1-order diffracted beams
(an amount of .+-.1-order diffracted light/an amount of incident light) is
not dependent upon the polarizing condition, so that a polarized component
containing information about the Kerr rotation could not be completely
introduced onto the photodetectors 9 and 10. Therefore, the
magneto-optical information signal has a small amplitude, and thus C/N of
the signal becomes decreased.
In order to mitigate the above mentioned drawback, it is considered that
the diffraction coefficient for the .+-.1-order diffracted beams of the
hologram 3 is increased, but then the diffraction coefficient for the
0-order beam (an amount of the 0-order light/an amount of incident light)
would be deceased and a necessary laser power for writing information
might become high and a problem of heat generation and so on might occur.
In the known optical head illustrated in FIGS. 2 and 3, the laser beam
reflected by the magneto-optical record medium 6 is made incident upon the
photodetectors 15a and 15b by means of the polarizing beam splitter 11,
and therefore the polarized component bearing the information about the
Kerr rotation could be effectively received by the photodetectors.
However, the optical path is bent at right angles by the polarizing beam
splitter 11, and thus a size of the optical head is liable to be large.
Further, the semiconductor laser 1 and the photodetectors 15a, 15b are
formed separately form each other, they are liable to be deviated from
each other due to temperature variation and secular variation, and thus a
reliability of the optical head is low.
In the optical pick-up head shown in FIGS. 4 to 7, the semiconductor laser
24, photodetectors 25-30 and hologram element 22 are all integrated into
the single unit 16, and thus the optical head can be made small in size
and the reliability can be improved. However, there is not disclosed a
concrete construction for deriving the magneto-optical information signal.
Therefore, a primary object of the present invention is to provide a novel
optical head for mitigating the above mentioned drawbacks of the known
optical head, in which the head can be made small in size, a high
reliability can be attained, the decrease in C/N can be suppressed and the
magneto-optical information can be read out stably.
According to one aspect of the present invention, an optical head
comprises:
a semiconductor substrate having a surface;
a semiconductor laser arranged on said surface of the semiconductor
substrate for emitting a laser beam;
a light receiving means including a plurality of photodetectors formed in
said surface of the semiconductor substrate;
a collimator lens for converting said laser beam emitted by said
semiconductor laser into a substantially parallel laser beam;
an objective lens for converging said substantially parallel laser beam
onto a magneto-optical record medium;
a beam dividing means arranged between said collimator lens and said
objective lens for dividing a laser beam reflected by said magneto-optical
record medium into first and second return beams; and
a polarization beam splitting means arranged between said semiconductor
laser and said collimator lens such that an integral unit is formed
together with said semiconductor substrate, and including gratings for
diving the laser beam emitted by said semiconductor laser into a main beam
which is used to write or read information on or from said magneto-optical
record medium and sub-beams which are used to derive a tracking error
signal, a first hologram pattern for diffracting said first return beam, a
second hologram pattern for diffracting said second return beam, and a
polarization beam splitting plane arranged substantially in parallel with
an optical axis of a zero order beam emanating from said second hologram
pattern for splitting +1-order and/or -1-order diffracted beam emanating
from said second hologram pattern in accordance with a polarizing
direction of the beam; whereby +1-order and -1-order diffracted beams
emanating from said first hologram pattern and beams emanating from said
polarization beam splitting plane are separately received by said
plurality of photodetectors of said light receiving means.
According to another aspect of the invention, an optical head comprises:
a semiconductor substrate having a surface;
a semiconductor laser arranged on said surface of the semiconductor
substrate for emitting a laser beam;
a light receiving means including a plurality of photodetectors formed in
said surface of the semiconductor substrate;
a collimator lens for converting said laser beam emitted by said
semiconductor laser into a substantially parallel laser beam;
an objective lens for converging said substantially parallel laser beam
emanating from said collimator lens onto a magneto-optical record medium;
a beam dividing means arranged between said collimator lens and said
objective lens for dividing a laser beam reflected by said magneto-optical
record medium into first and second return beams; and
a polarization beam splitting means arranged between said semiconductor
laser and said collimator lens such that an integral unit is formed
together with said semiconductor substrate, and including a first hologram
pattern for diffracting said first return beam, a second hologram pattern
for diffracting said second return beam, a third hologram pattern for
diffracting a zero order beam emanating from said first hologram pattern,
and a polarization beam splitting plane arranged substantially in parallel
with an optical axis of a zero order beam emanating from said second
hologram pattern for splitting +1-order and/or -1-order diffracted beams
emanating from said second hologram pattern in accordance with a
polarizing direction of the beam; whereby 1-order and -1-order diffracted
beams emanating from said first hologram pattern, beams emanating from
said polarization beam splitting plane and +1-order and/or -1-order
diffracted beam emanating from said third hologram pattern are separately
received by said plurality of photodetectors of said light receiving
means.
According to another aspect of the invention, an optical head comprises:
a semiconductor substrate having a surface;
a semiconductor laser arranged on said surface of the semiconductor
substrate for emitting a laser beam;
a light receiving means including a plurality of photodetectors formed in
said surface of the semiconductor substrate;
an objective lens converging the laser beam emitted by said semiconductor
laser onto a magneto-optical record medium; and
an optical block arranged to form an integral unit together with said
semiconductor substrate and including a hologram for diffracting a return
beam reflected by said magneto-optical record medium and a polarization
beam splitting plane arranged in substantially parallel with an optical
axis of a zero order beam emanating from said hologram for splitting a
pupil of said hologram; wherein return beams diffracted by said hologram
and split by said polarization beam splitting plane are separately
received by said plurality of photodetectors of said light receiving
means.
In Japanese Patent Application Laid-open Publication Kokai Hei 4-248134
published on Sep. 3, 1992, there is disclosed another known optical head.
As shown in FIGS. 8 and 9, this known optical head comprises a
semiconductor substrate 31 in which first and second photodetectors 32 and
33 are formed. On the semiconductor substrate 31 there are arranged
semiconductor laser 35 and reflection mirror 36. A laser beam emitted by
the semiconductor laser 35 is reflected by the mirror 36 upwardly toward
an optical record medium 40 by means of hologram element 38 and objective
lens 39.
The hologram element 38 has formed thereon a hologram pattern 38a on a
surface facing the objective lens 39 and gratings 38c on a surface facing
the substrate 31. The gratings 38c serve to divide the laser beam emitted
by the semiconductor laser 35 into a main beam and two sub-beams, these
three beams being made incident upon the optical record medium 40 by means
of the objective lens 39. The three laser beam reflected by the optical
record medium 40 are made incident upon the hologram element 38 by means
of the objective lens 39. Each of these beams is divided into .+-.1-order
beams having opposite refraction powers. 1-order beams are received by the
first photodetector 32 and -1-order beams are received by the second
photodetector 33. The photodetector 32 comprises three light receiving
elements 32a, 32b and 32c for receiving the three +1-order beams, and the
middle light receiving element 32b for receiving the +1-oder beam of the
main beam has three light receiving regions 32d, 32e and 32f which are
divided in the same direction as the diffracting direction of the hologram
pattern 38a. Similarly, the photodetector 33 comprises three light
receiving elements 33a, 33b and 33c for receiving the three -1-order
beams, and the middle light receiving element 33b for receiving the
-1-oder beam of the main beam has three light receiving regions 33d, 33e
and 33f which are divided in the diffracting direction of the hologram
pattern 38a.
Then, the focusing error signal can be derived from output signals of the
light receiving regions 32d, 32e and 32f of the light receiving element
32b for receiving the +1-order beams of the main beam and output signals
of the light receiving regions 33d, 33e and 33f of the light receiving
element 33b for receiving the -1-order beams of the main beam on the basis
of the beams size method. Further the tracking error signal is derived
from output signals from the light receiving elements 32a and 32c for
receiving the +1-order beams of the two sub-beams and output signals from
the light receiving elements 33a and 33c for receiving the -1-order beams
of the two sub-beams in accordance with the three beam method. In FIGS. 8
and 9, a direction in which information tracks extend is denoted by x and
a tracking direction perpendicular to the direction x is represented by y.
This known optical head can be constructed from a smaller number of parts,
and further the semiconductor laser 35 and photodetectors 32 and 33 are
provided on the same semiconductor substrate 31, so that the optical head
can be made small in size. Moreover, even if the diffraction angles by the
hologram pattern 38a for the .+-.1-order diffracted beams are changed due
to the variation in the wavelength of the laser beam emitted from the
semiconductor laser 35, the images on the photodetectors 32 and 33 move in
the direction which is parallel with the dividing lines of the
photodetectors, and therefore the focusing error signal is hardly affected
by the fluctuation of the wavelength. Furthermore, even if the spots on
the photodetectors 32 and 33 are shifted due to errors in machining
various parts and in assembling, the +1-order beams and -1-order beams are
shifted in the same manner, and thus it is possible to cancel out off-sets
of the focusing error signal from the +1-order beams and of the focusing
error signal from the -1-order beams. In this manner, the signal detection
can be performed stably and adjustment of various parts during the
assembling can be simplified.
This known optical head could be applied to the compact disk device and
once-write type disk device, but could not be utilized for the
magneto-optical disk device, because the optical head does not comprise
the polarizing splitting means.
In Japanese Patent Application Laid-open Publication Kokai Hei 5-120755
published on May 18, 1993, there is disclosed another optical head which
could be used for the magneto-optical disk. As depicted in FIGS. 10 and
11, this optical head comprises a semiconductor substrate 31 in which
first, second and third photodetectors 32, 33 and 34 are formed. On the
semiconductor substrate 31 there are arranged semiconductor laser 35,
reflection mirror 36 and polarizing beam splitter 37. A laser beam emitted
by the semiconductor laser 35 is reflected by the mirror 36 upwardly
toward an optical record medium 40 by means of hologram element 38 and
objective lens 39. It should be noted that the optical record medium 40 is
formed by the magneto-optical disk. On one surface of the hologram element
38 there are formed hologram patterns 38a and 38b and on the other surface
there are formed gratings 38c. The function of the hologram element 38 is
substantially same as that of the known optical head illustrated in FIGS.
8 and 9. Further the construction of the first and second photodetectors
32 and 33 are similar to that of the optical head shown in FIGS. 8 and 9
and receive the three 1-order beams and three -1-order beams,
respectively.
The third photodetector 34 comprises two light receiving regions 34a and
34b which are inclined by 45 degrees with respect to the directions x and
y. On the third photodetector 34, there is arranged the polarizing beam
splitter 37 such that a polarization beam splitting plane 37a of multiple
coatings of dielectric films is inclined by 45 degrees with respect to the
direction y.
The focusing error signal can be obtained from output signals of the light
receiving regions 32d, 32e and 32f of the light receiving element 32b for
receiving the +1-order beams of the main beam and output signals of the
light receiving regions 33d, 33e and 33f of the light receiving element
33b for receiving the -1-order beams of the main beam on the basis of the
beams size method, and the tracking error signal can be derived from
output signals from the light receiving elements 32a and 32c for receiving
the +1-order beams of the two sub-beams and output signals from the light
receiving elements 33a and 33c for receiving the -1-order beams of the two
sub-beams in accordance with the three beam method. P-polarized component
of the +1-order beam of the main beam which is transmitted through the
plane 37a of the polarizing beam splitter 37 is received by the light
receiving region 34a and S-polarized component reflected by the plane 37a
is received by the light receiving region 34b. Now it is assumed that
output signals from these light receiving regions 34a and 34b are denoted
by Ia and Ib, respectively. Then, the magneto-optical information signal S
is obtained as a difference between these signals, i.e. S=Ia-Ib.
In this optical head, since the polarizing beam splitter 37 is provided, it
is possible to apply to the magneto-optical disk. However, an operation of
mounting the small polarizing beam splitter having a complicated shape on
the photodetector 34 is very cumbersome and requires a human skill and a
long time, which apparently increases a cost of the optical head.
It is another object of the invention to provide a novel and useful optical
head which can be effectively applied to the magneto-optical disk and can
be easily assembled without difficult adjustment in a less expensive
manner.
According to further aspect of the invention, an optical head comprises:
a semiconductor laser for emitting a laser beam;
a converging means for converging said laser beam emitted by the
semiconductor laser onto a magneto-optical record medium;
a light receiving means for receiving a return beam reflected by said
magneto-optical record medium and converged by said converging means;
a diffracting means arranged between said semiconductor laser and said
converging means for diffracting said return beam reflected by said
magneto-optical record medium; and
a polarization beam splitting means having a polarization beam splitting
plane which is substantially in parallel with an optical axis of a zero
order beam of the return beam emanating from said diffracting means and
splits diffracted beams emanating from said diffracting means in
accordance with polarizing directions of said diffracted beams.
In this optical head according to the invention, the polarization beam
splitting plane is arranged to be substantially in parallel with the
optical axis of the zero order beam emanating from the diffracting
element, and thus an incident angle of an incident beam upon the
polarization beam splitting plane can be made large. Therefore, the
polarization beam splitting plane can be manufactured easily and the
positional adjustment of the light receiving elements with respect to the
polarization beam splitting element can be simplified. Moreover, the
diffracting element and the polarization beam splitting element may be
formed into a single unit, so that the optical head can be made compact in
construction and cheap in cost.
The present invention also relates to a light emitting and receiving device
for use in the optical head.
FIG. 12 is a perspective view showing a known light emitting and receiving
device, which is described in Japanese article "Extended Abstracts (40th
Spring Meeting, 1993) 29p-c-12; The Japan Society of Applied Physics and
Related Societies No. 3)" held on March 39 to Apr. 1, 1993 at Tokyo,
Japan. In this known device, in a surface of a silicon semiconductor
substrate 41, there are formed photodetectors 42 and 43 and a recess 44
having a side wall 45 which is inclined by 45 degrees with respect to the
surface of the substrate, and on a bottom of the recess is arranged a
semiconductor laser 46. It should be noted that inclined side wall 45
serves as a micromirror for reflecting a laser beam emitted from the
semiconductor laser 46 in a direction parallel with the surface of the
silicon substrate 41 and the reflected laser beam propagates in a
direction perpendicular to the surface of the substrate. In this manner,
the light emitting and receiving device is realized which comprises a
surface emitting type laser element and a photoelectric element.
In Japanese Patent Application Laid-open Publication Kokai Hei 3-187285,
there is disclosed a multibeam semiconductor laser device in which a
plurality of laser diodes are integrated as a monolithic device. In
Japanese Patent Application Laid-open Publication Kokai Hei 3-112184,
there is shown a semiconductor laser device in which three semiconductor
lasers are integrated on a sub-mount such that light emitting points are
deviated from each other.
In the known light emitting and receiving device illustrated in FIG. 12,
only one semiconductor laser is provided, so that this device could not be
utilized in such a case that a plurality of semiconductor lasers have to
be arranged close to each other. In another known laser device disclosed
in the Japanese Patent Application Laid-open Publication Kokai Hei
3-187285, there is a limitation that the light emitting points have to be
aligned along a line perpendicular to an optical axis. Further, in another
known semiconductor laser device described in the Japanese Patent
Application Laid-open Publication Kokai Hei 3-112184, it is possible to
arrange the light emitting points in a three-dimensional manner, but two
semiconductor lasers have to be positioned with respect to the middle
semiconductor laser, so that if these semiconductor lasers have to be
arranged such that they are separated from each other by comparatively
large distances, a precision of the positioning might be decreased.
Moreover, the three semiconductor lasers on the sub-mount are positioned
with respect to optical axes by utilizing a difference in a thickness of
base substrates, and thus the precession of the positioning in a direction
perpendicular to the optical axis is affected by the precision of
manufacturing the base substrates.
Therefore, the present invention has for its object to provide a novel and
useful light emitting and receiving device, in which light emitting points
of a plurality of semiconductor lasers can be arranged in a
three-dimensional manner, while the precision of positioning of the
semiconductor lasers and the precision of positioning the semiconductor
lasers and light receiving element can be improved.
According to the invention, a light emitting and receiving device
comprises:
a semiconductor substrate having a surface;
a plurality of recesses formed in said surface of the semiconductor
substrate;
at least one photodetector formed in said surface of the semiconductor
substrate at a position outside said recesses; and
a plurality of semiconductor lasers arranged on bottoms of said recesses
for emitting laser beams.
According to one aspect of the invention, a depth of at least one recess
differs from that of the remaining recesses or a distance between at least
one inclined side wall of a recess and an end surface of a semiconductor
laser arranged in the relevant recess differs from a distance between at
least one inclined side wall of other recess and an end surface of a
semiconductor laser arranged in the relevant recess. Then, it is possible
to obtain a surface emitting type laser and in which a plurality of light
emitting points can be precisely arranged in a three-dimensional manner
and can be precisely positioned with respect to the light receiving
element.
According to another aspect of the invention, a light emitting and
receiving device comprises:
a semiconductor substrate having at least one recess formed in one surface
thereof;
at least one photodetectors formed in said surface at a position outside
said recess; and
a plurality of semiconductor lasers arranged on bottoms of said recess for
emitting laser beams.
In this light emitting and receiving device according to the invention, a
semiconductor laser is arranged such that its end surface is not in the
same plane as that of other semiconductor laser, so that a surface
emitting type laser can be realized in which a plurality of light emitting
points can be precisely arranged in a three-dimensional manner and can be
precisely positioned with respect to the light receiving element.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing a known optical head;
FIG. 2 is a schematic view illustrating another known optical head;
FIG. 3 is a plan view depicting a photodetecting means shown in FIG. | | |
