 |
Description  |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a thin-film optical function
element, and more particularly to such a thin-film optical function
element adapted to separate two different optical modes which have field
patterns whose vibrating directions are perpendicular to each other. This
optical function element is suitably used for an optical head for a
magnetooptical data storage disk, wherein a light beam produced by a light
source is guided through a thin-film optical waveguide toward the
magnetooptical disk, such that the light beam maintains a constant plane
of polarization, and wherein one or both of the two transverse modes of
the light beam reflected by the disk is/are converted into an electric
signal or signals.
2. Discussion of the Prior Art
There is shown in FIG. 9 a conventional optical head for writing and
reading information on and from a magnetooptical memory or storage disk.
This optical head is adapted such that a laser beam emitted from a
semi-conductor laser element 41 is collimated by a collimator lens 42,
transmitted through a half mirror 43, and focused by an objective or
converging lens 44 on the surface of a magnetooptical disk 45. The laser
beam reflected by the surface of the disk 45 is reflected by the half
mirror 43 toward a polarizing beam splitter 46. A component P of the laser
beam incident upon the beam splitter 46 is transmitted through the beam
splitter 46, while a component S of the beam is reflected by the beam
splitter 46. These two components P and S are received by respective two
light-sensitive elements 47, 48. Information stored at a specific reading
spot on the magnetooptical disk 45 which is irradiated with the laser beam
is read by obtaining a difference between output levels of the two
light-sensitive elements 47, 48. The principle of this information reading
or retrieving operation is illustrated in FIG. 10. If the laser beam was
reflected by the surface of the disk 45 without being influenced by the
information stored thereon, the plane of polarization of the reflected
laser beam incident upon the polarizing beam splitter 46 would be inclined
at 45.degree. with respect to the axes of the beam splitter 46 along which
components P and S are polarized. The plane of polarization of the
reflected laser beam from the disk 45 is rotated by an angle of +.theta.k
or -.theta.k with respect to the plane of polarization of the incident
laser beam, depending upon the direction in which the irradiated reading
spot is magnetized, i.e., depending upon the stored data "0" or "1". When
the plane of polarization of the reflected laser beam is rotated by
+.theta.k, a difference between the outputs of the two light-sensitive
elements 47, 48 which receive the P and S components of the reflected
laser beam is equal to (S.sub.+ -P.sub.+). When the plane of polarization
is rotated by -.theta.k, the above difference is equal to (S.sub.-
-P.sub.-)=-(S.sub.+ -P.sub.+). Thus, the differential outputs .+-.(S.sub.+
-P.sub.+) corresponding .+-..theta.k are obtained by a differential
amplifier which receive the outputs of the light-sensitive elements 47,
48. When information is written on the disk 45, the intensity of the
produced laser beam is increased to heat the relevant spot on the disk 45
to a temperature in the neighborhood of the Curie point or above the
compensation temperature of the material of the disk. The heated spot is
then magnetized in the appropriate direction corresponding to the
information to be written, within an externally produced magnetic field.
When the information on the disk is erased, the relevant spots on the disk
are unidirectionally magnetized in the predetermined direction.
The conventionally available optical head as described above uses optical
components made of glasses or other suitable materials, such as lenses,
half mirror and prism. These optical components require fine adjustments
of their optical axes. Further, the optical head using these optical
components tends to be large-sized and heavy, and consequently the overall
arrangement of the information reading and writing system tends to be
large-sized. Moreover, the relatively heavy optical head leads to a
comparatively long access time.
SUMMARY OF THE INVENTION
It is therefore a first object of the present invention to provide a
thin-film optical function element which is suitably used to provide a
comparatively small-sized and lightweight, inexpensive optical head for a
magnetooptical data storage disk, which assures a reduced access time.
A second object of the present invention is to provide an optical head for
a magnetooptical data storage disk, which uses such a thin-film optical
function element.
The first object may be attained according to one aspect of the present
invention, which provides a thin-film optical function element, comprising
a substrate formed of a dielectric material, a first optical waveguide
consisting of a thin film of a dielectric material formed on the
substrate, and a second optical waveguide consisting of another thin film
of another dielectric material formed on the substrate. The first optical
waveguide has a substantially same propagation constant with respect to
two different modes which have field patterns whose vibrating directions
are perpendicular to each other. The second optical waveguide is optically
coupled to the first optical waveguide, and guiding therethrough only one
of the two different modes.
In the optical function element of the present invention constructed as
described above, the above-indicated one mode is emitted from the end of
the second waveguide, while the other mode is emitted from an end face of
the substrate or from the end of another waveguide. By detecting the
intensity or intensities of the one or both of the two different modes
thus emitted from the optical function elements, information corresponding
to the intensity or intensities of these modes may be obtained. For
improved accuracy of the information, it is recommended to obtain a
difference between the intensities of the two modes.
Although the thin-film optical function element according to the principle
of the present invention is suitably used for an optical head for a
magnetooptical data storage disk, it is to be understood that the instant
optical function element may be applied to various other fields of
technology which involve a change in the polarization plane of a light
beam.
In one form of the optical function element of the invention, the substrate
consists of an anisotropic crystal, and the first optical waveguide
permits a TE mode and a TM mode to be propagated therethrough at a same
velocity such that a linearly polarized light beam maintains a
predetermined constant plane of polarization. In this case, the second
optical waveguide has a refractive index which is intermediate between a
first refractive index of the substrate with respect to ordinary rays of
light, and a second refractive index of the substrate with respect to
extraordinary rays of light. In this form of the optical element, one of
the TE and TM modes is emitted as an output from the end of the second
waveguide, while the other mode is emitted as another output from the end
face of the substrate.
The first and second optical waveguides may be coupled to each other by
mutual contact, or alternatively by a directional coupler. In the latter
case, the optical function element may be adapted such that the second
optical waveguide has two different propagation constants with respect to
the two different modes, respectively, while one of the different
propagation constants is equal to a propagation constant of the first
optical waveguide. In this case wherein the directional coupler is used,
one of the two modes is obtained from the end of the first waveguide,
while the other mode is obtained from the end of the second waveguide.
In another form of the optical function element of the invention, the first
optical waveguide consists of an anisotropic crystal and is partially
covered by a clad layer having a refractive index which is intermediate
between a first refractive index of the first optical waveguide with
respect to ordinary rays of light, and a second refractive index of the
first optical waveguide with respect to extraordinary rays of light. The
clad layer functions as the second optical waveguide. In this case, one of
the two modes is obtained from the end of the clad layer, while the other
mode is obtained from the end of the first waveguide.
The second object may be achieved according to another aspect of the
present invention, which provides an optical head for a magnetooptical
disk, incorporating the thin-film optical function element constructed
according to the first aspect of the invention described above, wherein
the first optical waveguide consists of an Y-branching waveguide having a
common path and two branch paths which merge with each other into the
common path, and the second optical waveguide is coupled to one of the two
branch paths of the Y-branching waveguide. The optical head comprises (a)
a light emitting-element coupled to the other branch path of the
Y-branching waveguide and operable to emit a linearly polarized light
beam, (b) a converging lens coupled to the common path of the Y-branching
waveguide, for converging the linearly polarized light beam which is
guided through the first optical waveguide, and (c) at least one
light-sensitive element each coupled to a corresponding one of the second
optical waveguide and a portion of the substrate adjacent to the second
optical waveguide.
In the thin-film optical function element of the present invention
constructed as described above, a light beam produced by a suitable
light-emitting element attached to the instant optical function element is
guided through the first optical waveguide, and is emitted from the first
optical waveguide so as to irradiate an appropriate spot on a
magnetooptical disk. The light beam incident upon the disk is reflected by
the disk surface, and the reflected light beam is first guided through the
first optical waveguide and then admitted into the second optical
waveguide. Only one of the two different modes of the reflected light beam
is guided and transmitted through the second optical waveguide, and is
emitted from the end of the second optical waveguide. Information stored
at the irradiated spot on the magnetooptical disk can be read based on an
output of a light-sensitive element which receives the mode emitted from
the second optical waveguide or the other mode emitted from the substrate,
or based on a difference between the outputs of the two light-sensitive
elements which receive the separated two modes.
Since the instant thin-film optical function element is alone capable of
separating the light beam reflected by the magneto-optical data storage
disk into the two different modes to be received by the respective
light-sensitive elements, the optical head which uses this optical
function element does not require other optical components such as lenses
and a half mirror as indicated at 42, 43, 44 in FIG. 9, and can be
accordingly reduced in size and weight. The elimination of the optical
components contributes to shortening the access time of the optical head,
and eliminates otherwise required adjustments of the optical axes of these
components, thereby assuring increased operating reliability of the
optical head.
In one form of the optical head of the invention, the light-emitting
element emits the linearly polarized light beam such that the light beam
has a plane of polarization which is inclined at 45.degree. with respect
to a plane of the first optical waveguide, and the two light-sensitive
elements which are coupled to the second optical waveguide and the portion
of the substrate, respectively. In this case, the information stored at
the relevant spot on the magnetoooptical disk can be suitably read based
on a difference between the outputs of the two light-sensitive elements.
In another form of the optical head of the invention, the light-emitting
element emits the linearly polarized light beam such that the light beam
has a plane of polarization which inclined at an angle between 45.degree.
and 90.degree. with respect to a plane of the first optical waveguide. In
this case, only one light-sensitive element is coupled to the second
optical waveguide, and the information at the relevant spot on the disk
can be read based on the output of this light-sensitive element.
Although the light-emitting element and the converging lens may be
indirectly coupled to the first optical waveguide via optical fibers or
other optical coupling elements, it is preferable that the light-emitting
element and the converging lens are directly coupled to the first optical
waveguide. Similarly, a light-sensitive element may be directly coupled to
the second optical waveguide, for example. In this case wherein the
light-emitting and/or light-sensitive elements, and/or the converging lens
are directly coupled to the waveguides, the optical head may be made
relatively compact and lightweight.
The second object may also be achieved according to a further aspect of the
present invention, which provides an optical head for a magnetooptical
disk, incorporating the thin-film optical function element constructed
according to the first aspect of the present invention, wherein the first
optical waveguide consists of a first portion and a second portion which
are coupled to each other by a directional coupler. The optical head
comprises (a) a light-emitting element coupled to the first portion of the
first optical waveguide and operable to emit a linearly polarized light
beam, (b) a converging lens coupled to an end of the second portion of the
first optical waveguide, for converging the linearly polarized light beam
which is guided through the first and second portions, the other end of
the second portion being coupled to the second optical waveguide, and (c)
at least one light-sensitive element each coupled to a corresponding one
of an end portion of the second optical waveguide remote from the second
portion of the first optical waveguide, and a portion of the substrate
adjacent to the end portion of the second optical waveguide.
The second object may also be achieved according to a still further aspect
of the present invention, which provides an optical head for a
magnetooptical disk, comprising: a substrate consisting of an anisotropic
crystal; a first optical waveguide formed on the substrate and having a
substantially same propagation constant with respect to two different
modes which have field patterns whose vibrating directions are
perpendicular to each other; a light-emitting element coupled to one end
of the first optical waveguide and operable to emit a linearly polarized
light beam; a lens for collimating the linearly polarized light beam into
parallel rays of light; a converging lens coupled to the other end of the
first optical waveguide, for converging the parallel rays of light which
have been guided through the first optical waveguide, so that the
converged rays of light are focused on a surface of the magnetooptical
disk, whereby the converged rays of light is reflected by the surface of
the disk and is admitted into the first optical waveguide through the
converging lens; a grating disposed in the first optical waveguide, for
reflecting the rays of light admitted into the first optical waveguide, in
a direction different from that in which the parallel rays of light are
guided through the first optical waveguide toward the converging lens; a
second optical waveguide formed on the substrate and coupled to a portion
of the first optical waveguide which guides the light reflected by the
grating, the second optical waveguide having a refractive index which is
inermediate between a first refractive index of the substrate with respect
to ordinary rays of light, and a second refractive index of the substrate
with respect to extraordinary rays of light; a first light-sensitive
element coupled to an end of the second optical waveguide which is remote
from the first optical waveguide; and a second light-sensitive element
coupled to a portion of the substrate adjacent to the second optical
waveguide. In this case, the information at the relevant spot on the
magnetooptical disk can be read based on a difference between the outputs
of the first and second light-sensitive elements.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present
invention will be better understood by reading the following detailed
description of presently preferred embodiments of the invention, when
considered in connection with the accompanying drawings, in which:
FIG. 1 is a perspective view showing one embodiment of a thin-film optical
function element of the present invention;
FIG. 2 is a graph showing a relationship between the thickness and the
propagation constant of a waveguide of the optical function element;
FIG. 3 is a perspective view of an optical head for a magnetooptical disk,
which uses the thin-film optical function element of FIG. 1;
FIG. 4 is a plan view of another embodiment of the otpical head of the
invention;
FIG. 5 is a perspective view of a further embodiment of the optical head of
the invention;
FIG. 6 is a cross sectional view of a still further embodiment of the
invention;
FIG. 7 is a view explaining the principle of a data reading operation by
the optical head according to one form of the invention;
FIGS. 8(a) and 8(b) are illustrations showing modified embodiments of the
invention;
FIG. 8(c) is a perspective view of a yet further embodiment of the
invention;
FIG. 9 is a schematic view depicting an arrangement of a known optical head
for a magnetooptical disk; and
FIG. 10 is a view explaining the principle of a data reading operation by
the known optical head of FIG. 9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 1, there is shown a planar rectangular dielectric
substrate 1 formed of an anisotropic crystal, a crystal of LiTaO.sub.3 in
this specific example. On one of opposite major surfaces of this substrate
1, there is formed a first optical waveguide in the form of a dielectric
thin-film Y-branching waveguide 2 which generally extends from one of
opposite sides of the substrate 1 to the other. The Y-branching waveguide
2 has a common path 2a, and a pair of branch paths 2b, 2c which merge with
each other into the common path 2a. The branch path 2c has a shorter
length than the other branch path 2b. This first optical waveguide 2 has a
refractive index N.sub.fl. On the same major surface of the substrate 1,
there is also formed a second optical waveguide in the form of a
dielectric thin-film waveguide 3 which is coupled at its one end to the
end of the branch path 2c of the first or Y-branching waveguide 2 which is
remote from the common path 2a. This second optical waveguide 3 has a
refractive index N.sub.f2.
The LiTaO.sub.3 substrate 1 has a refractive index N.sub.o of 2.175 with
respect to ordinary rays of light, and a refractive index N.sub.e of 2.180
with respect to extraordinary rays of light. The LiTaO.sub.3 crystal is
cut such that the plane of the major surfaces of the substrate 1 is
perpendicular to the C axis of the crystal. Namely, the refractive index
N.sub.e is taken in the direction perpendicular to the plane of the
substrate 1, while the refractive index N.sub.o is taken in the direction
which is parallel to the plane of the substrate 1.
The refractive index N.sub.fl of the first, Y-branching waveguide 2 is
determined such that the index N.sub.fl is larger than the refractive
index N.sub.e of the substrate 1, which is larger than the refractive
index N.sub.o. That is, the refractive index N.sub.fl is determined so as
to satisfy an inequality N.sub.fl >N.sub.e >N.sub.o.
For example, it is assumed that a light beam to be guided or propagated
through the waveguide 2 includes a TE.sub.0 mode and a TM.sub.0 mode which
have field patterns in a plane transverse to the direction of propagation
of the light beam through the waveguide 2. The directions of vibration of
the field patterns of the TE.sub.0 and TM.sub.0 modes are perpendicular to
each other. Assuming, for example, that the refractive index N.sub.fl of
the first optical waveguide 2 is 2.20, there exists a relationship as
shown in FIG. 2, between a thickness k.sub.0 W of the first waveguide 2 as
normalized by P/k.sub.0, and a propagation constant P/k.sub.0 of the first
waveguide 2, where k.sub.0 =2.pi./.lambda., where .lambda. is the
wavelength of the light beam. As indicated in FIG. 2, when the normalized
thickness k.sub.0 W is 12.5 (W=1.26 microns, for =0.6328 microns), the
propagation constant with respect to the TE.sub.0 mode is equal to that
with respect to the TM.sub.0 mode, whereby the phases of the two modes
match each other. Accordingly, when a linearly polarized light beam having
a given plane of polarization is incident upon an end face 4 of the branch
path 2b of the Y-branching waveguide 2, the TE.sub.0 mode whose plane of
polarization is parallel to the plane of the substrate 1, and the TM.sub.0
mode whose polarization plane is perpendicular to the plane of the
substrate 1 are propagated at the same phase velocity, through the
waveguide 2, and are emitted from an end face 5 of the common path 2a.
There arises no phase difference between the TE.sub.0 and TM.sub.0
components of the linearly polarized light beam at the end face 5. Thus,
the linearly polarized light beam incident upon the end face 4 is
identical with the linearly polarized light beam emitted from the end face
5.
It is also noted that when the linearly polarized light beam emitted from
the end face 5 is reflected by a magnetooptical data storage disk, for
example, the reflected light beam is incident upon the end face 5 and is
propagated through the common path 2a and the branch path 2c, toward an
interface 6 between the first and second optical waveguides 2, 3, such
that the linearly polarized light beam maintains the predetermined planes
of polarization while travelling the first waveguide 2.
The refractive index N.sub.f2 of the second optical waveguide 3 is
determined such that the refractive index N.sub.f2 is intermediate between
the refractive indices N.sub.e and N.sub.o of the substrate 1, i.e., so as
to satisfy an inequality N.sub.e >N.sub.f2 >N.sub.o. Thus, the refractive
index N.sub.f2 of the second waveguide 3 is larger than the refractive
index N.sub.o with respect to the TE.sub.0 mode, whereby the TE.sub.0 mode
is properly propagated or guided through the waveguide 3. On the other
hand, the refractive index N.sub.f2 is smaller than the refractive index
N.sub.e with respect to the TM.sub.0 mode, whereby the TM.sub.0 mode leaks
from the waveguide 3 toward a portion of the substrate 1 underlying the
waveguide 3. As a result, the TE.sub.0 mode of the linearly polarized
light beam reaching the interface 6, that is, the component of the beam
whose field pattern is parallel to the plane of the substrate 1 reaches an
end face 7 of the second waveguide 3. However, the TM.sub.0 mode of the
light beam whose field pattern is perpendicular to the plane of the
substrate 1 is propagated into the portion of the substrate 1 underlying
the second waveguide 3.
Referring to FIG. 3, there is shown an example of an optical head for a
magnetooptical data storage disk, which employs the thin-film optical
function element of FIG. 1. The waveguide assembly 2, 3 is covered by a
clad layer 30. Further, a semi-conductor laser element 31 is attached to
the end face 4 of the first, Y-branching waveguide 2. The laser element 31
is adapted to produce a laser beam whose plane of polarization is inclined
at 45.degree. with respect to the plane of the waveguide 2 or the plane of
the major surfaces of the substrate 1. Consequently, the TE.sub.0 and
TM.sub.0 modes having the same intensity are propagated through the branch
path 2b and common path 2a of the first waveguide 2, and are emitted from
the end face 5. Since the propagation constants of the first waveguide 2
with respect to the TE.sub.0 and TM.sub.0 modes are equal to each other,
the plane of polarization of the linearly polarized light beam emitted
from the end face 5 is inclined at 45.degree., like the light beam
incident upon the end face 4.
A converging lens 33 is attached to the end face 5 of the first waveguide
2, so that the linearly polarized light beam reaching the end face 5 is
converged by the lens 33 so that the converged light beam is focused on an
appropriate spot on the surface of the magnettooptical storage disk (not
shown). The light beam incident upon the magnetooptical disk is reflected
by the same and is incident upon the converging lens 33 and the end face
5. The plane of polarization of the light beam reflected by the disk with
respect to that incident upon the disk is rotated in the positive or
negative direction depending upon the direction of magnetization of the
irradiated spot on the disk. The reflected light beam is then transmitted
through the common path 2a, and is split into two parts at the merging
point of the two branch paths 2b, 2c of the Y-branching arrangement of the
waveguide 2. The part of the light beam which is guided into the branch
path 2c reaches the interface 6 between the first and second waveguides 2,
3. As described above, the TE.sub.0 component of the light beam reaching
the interface 6 is further propagated through the second waveguide 3 (as
indicated at 35 in FIG. 3) and is received by a suitable light-sensitive
element 36 such as a PIN photodiode attached to the end face 7 of the
second waveguide 3. On the other hand, the TM.sub.0 mode of the light beam
reaching the interface 6 is not be guided through the second waveguide 3,
that is, leaks off the waveguide 3 and travels through the portion of the
substrate 1 underlying the waveguide 3, as indicated at 37 in FIG. 3, such
that the TM.sub.0 mode is emitted from a portion of the end face of the
substrate 1 which is spaced apart from the light-sensitive element 36. The
thus emitted TM.sub.0 mode 37 is received by a second light-sensitive
element 38 disposed below the first light-sensitive element 36, as shown
in FIG. 3.
The TE.sub.O and TM.sub.O compoments 35, 37 received by the light-sensitive
elements 36, 38 correspond to S and P components taken along the vertical
and horizontal axes of a graph in FIG. 10. It will be understood from this
graph that information stored at the relevant spot irradiated by the light
beam can be read based on a difference between the outputs of the two
light-sensitive elements 36, 38 which receive the TE.sub.O and TM.sub.O
components 35, 37, as practiced in a known optical reading apparatus for a
magnetooptical disk as illustrated in FIG. 9. Information writing and
erasing operations on the magnetooptical disk can be effected in the same
manner as practiced in the known arrangement. Namely, the relevant spots
on the disk are heated with an increased intensity of the light beam
produced by the laser element 31, and are subjected to an externally
produced mangetic field in an appropriate direction.
The waveguides 2 and 3 are prepared by first forming on the substrate 1
thin films of suitable dielectric materials, by a suitable method usually
practiced for ordinary optical waveguides, such as sputtering or vacuum
vapor deposition, and then shaping the formed thin films into desired
configurations by photolithography teachnique. The dielectric materials
used for the waveguides 2, 3 may be selected from transparent dielectric
materials such as SiO.sub.2, ZnO, ZnS, TiO.sub.2 and SiO.sub.x, or a
mixture thereof. In the case where the mixture of SiO.sub.2 and TiO.sub.2
is employed for the waveguide films, the refractive indices of the
obtained waveguides 2, 3 may be adjusted as needed, by changing the
proportions of SiO.sub.2 and TiO.sub.2. The clad layer 30 may be prepared
in a similar manner, by sputtering or vacuum vapor deposition. However,
the clad layer 30 is not essential and may be eliminated. The converging
lens 33 is formed to a part-spherical shape, by using a photoresist, or by
utilizing a surface tension of a molten glass or similar material.
It will be understood that the present invention is not limited to the
details of the above-illustrated embodiments, but may be embodied with
various changes and modifications within the spirit of the present
invention.
For example, the first, Y-branching waveguide 2 may be replaced by a
directional coupler as indicated at 61 in FIG. 4. Further, the waveguides
may be ridge waveguides. A further modification is shown in FIG. 5,
wherein a thin-film optical function element of an optical head uses a
two-dimensional waveguide assembly, which consists of a first waveguide 71
and a second waveguide 72. The first waveguide 71 has a same propagation
constant with respect to the TE and TM modes. The second waveguide 72 is
adapted such that the TE mode or component is guided through the waveguide
72, while the TM mode or component deviates off the waveguide 72 and
travels through the portion of the substrate underlying the waveguide 72.
The laser beam produced rays of light by a geodesic lens 73, for example,
and is propagated through the first waveguide 71 to the converging lens
33, from which the light beam is emitted toward a magnetooptical disk. The
light beam reflected by the disk surface is incident upon the lens 33 and
is guided through the first waveguide 71. At a portion of the first
waveguide 71 between the lenses 73, 33, there is formed a grating 74 which
reflects the light beam from the convering lens 33, in a direction
perpendicular to the direction of propagation of the light beam between
the lenses 73, 33. Consequently, the light beam reflected by the grating
74 is incident upon the second waveguide 72, which separates the received
light beam into the TE and TM components. The light beam may have the same
plane of polarization with respect to the plane of the substrate 1, as in
the embodiment of FIG. 3.
While the semi-conductor laser element 31, converging lens 33 and
light-sensitive element 36 are directly coupled to the end faces of the
waveguides 2, 3, 71, 72, these elements may be coupled to the waveguides
via a suitable intermediate element such as an optical fiber, optical
isolator or rod lens.
The substrate 1 may be formed of a LiNbO.sub.3 crystal, rather than the
LiTaO.sub.3 crystal. Further, the crystallographic axis orientation may be
suitably modified. Although the anisotropic crystal is used for the
substrate 1 of the embodiments illustrated above, a modification as
indicated in FIG. 6 is possible, for example. In this modified arrangement
of FIG. 6, a thin film 92 of LiNbO.sub.3 is formed on a LitaO.sub.3
substrate 91. LiNbO.sub.3 has a refractive index N.sub.e of 2.20 with
respect to the ordinary rays of light, and a refractive index N.sub.o of
2.29 with respect to the extraordinary rays of light. Therefore, the thin
film 92 should be oriented such that the C axis is parallel to the plane
of the thin film 92 ans perpendicular to the direction of propagation of
the light beam through the thin film 92, so as to obtain a phase matching
between the TE and TM modes 94, 95. To separate these two different modes
from each other, a portion of the thin film 92 adjacent to the
light-sensitive element 38 is covered by a clad layer 93 which has a
refractive index N.sub.c which is determined so as to satisfy an
inequality N.sub.e <N.sub.c <N.sub.o. That is, the refractive index
N.sub.c of the clad layer 93 is smaller than the refractive index No with
respect to the TM mode 94, but is larger than the refractive index N.sub.e
with respect to the TE mode 95. Accordingly, only the TM mode 94 is
continuously propagated through the thin film 92, but the TE mode 95
deviates into the clad layer 93.
In the embodiments illustrated above, the plane of polarization of the
linearly polarized laser beam from the semi-conductor laser element 31 is
inclined at 45.degree. with respect to the plane of the first optical
waveguide 2, 61, 71, 92. However, it is possible that the plane of
polarization of the laser beam is inclined at an angle between 90.degree.
(0.degree.) and 45.degree., for example, an angle considerably close to
90.degree. or 0.degree., with respect to the plane of the waveguide. In
this case, only one of the TE and TM modes is detected by a
light-sensitive element, and information reading is effected based on the
output of this light-sensitive element. A graph of FIG. 7 illustrates an
angle of rotation of the polarization plane of the light beam reflected by
a magnetooptical disk with respect to the polarization plane of the
incident light beam, where the polarization plane of the incident light
beam is inclined at an angle close to 90.degree.. When the polarization
plane of the reflected light beam is rotated by .+-..theta.k, the output
of the light-sensitive element receiving the TE mode is changed to
respective amounts S+ and S-. Thus, the information reading is effected
based on these output amounts S+ and S-.
While the first waveguide used in the illustrated embodiments has a same
propagation constant with respect to both the TE mode and the TM mode, in
order to maintain the predetermined polarization plane of the linearly
polarized light beam being gui | | |
