U.S. patent application number 09/886576 was filed with the patent office on 2001-11-22 for optical waveguide device, and light source device and optical apparatus including the optical waveguide device.
Invention is credited to ichi Kasazumi, Ken?apos, Kitaoka, Yasuo, Mizuuchi, Kiminori, Yamamoto, Kazuhisa.
Application Number | 20010043773 09/886576 |
Document ID | / |
Family ID | 17527073 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010043773 |
Kind Code |
A1 |
Kasazumi, Ken?apos;ichi ; et
al. |
November 22, 2001 |
Optical waveguide device, and light source device and optical
apparatus including the optical waveguide device
Abstract
An optical waveguide device includes a dielectric substrate; and
an optical waveguide formed in the dielectric substrate, the
optical waveguide having a longitudinal axis and an outgoing
surface disposed at an angle other than a right angle relative to a
plane perpendicular to the longitudinal axis.
Inventors: |
Kasazumi, Ken?apos;ichi;
(Osaka, JP) ; Kitaoka, Yasuo; (Osaka, JP) ;
Mizuuchi, Kiminori; (Osaka, JP) ; Yamamoto,
Kazuhisa; (Osaka, JP) |
Correspondence
Address: |
Ratner & Prestia
P.O. Box 980
Valley Forge
PA
19482
US
|
Family ID: |
17527073 |
Appl. No.: |
09/886576 |
Filed: |
June 21, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09886576 |
Jun 21, 2001 |
|
|
|
09406953 |
Sep 28, 1999 |
|
|
|
Current U.S.
Class: |
385/31 ; 385/33;
G9B/7.098 |
Current CPC
Class: |
G02F 1/377 20130101;
G11B 7/125 20130101; G02B 6/42 20130101; G02B 6/4207 20130101 |
Class at
Publication: |
385/31 ;
385/33 |
International
Class: |
G02B 006/26 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 28, 1998 |
JP |
10-273378 |
Claims
What is claimed is:
1. An optical waveguide device, comprising: a dielectric substrate;
and an optical waveguide formed in the dielectric substrate, the
optical waveguide having a longitudinal axis and an outgoing
surface disposed at an angle other than a right angle relative to a
plane perpendicular to the longitudinal axis.
2. An optical waveguide device according to claim 1, wherein the
dielectric substrate has an outgoing surface disposed in a plane
within which the outgoing surface of the optical waveguide is
disposed.
3. An optical waveguide device according to claim 1, wherein the
outgoing surface is angled so that light going out from the optical
waveguide in a first direction and reflected back to the outgoing
surface by an external object is directed by the outgoing surface
in a second direction different from the first direction.
4. An optical waveguide device according to claim 1, wherein the
optical waveguide has an incident surface disposed at an angle
other than the right angle relative to the plane perpendicular to
the longitudinal axis of the optical waveguide.
5. An optical waveguide device according to claim 4, wherein the
dielectric substrate has an incident surface disposed in a plane
within which the incident surface of the optical waveguide is
disposed.
6. An optical waveguide device according to claim 5, wherein the
dielectric substrate has an outgoing surface disposed in a plane
within which the outgoing surface of the optical waveguide is
disposed, and the incident surface of the dielectric substrate and
the optical waveguide is substantially parallel to the outgoing
surface of the dielectric substrate and the optical waveguide.
7. An optical waveguide device according to claim 1, wherein the
dielectric substrate and the optical waveguide form a second
harmonic generation element.
8. An optical waveguide device according to claim 1, further
comprising a reflection reducing layer for reducing a reflection of
a harmonic wave.
9. An optical waveguide device according to claim 8, wherein the
reflection reducing layer is disposed on an incident surface of the
optical waveguide.
10. An optical waveguide device according to claim 1, further
comprising a harmonic wave absorption element for absorbing a
harmonic wave in a portion of the optical waveguide in the vicinity
of an incident surface of the optical waveguide.
11. An optical waveguide device according to claim 7, further
comprising a grating element having a periodicity of .LAMBDA. in a
portion of the optical waveguide in the vicinity of the incident
surface: wherein the second harmonic generation element receives
light having a wavelength of .lambda. as a fundamental wave in a
vacuum; and the periodicity .LAMBDA., an effective refractive index
n of the optical waveguide, and the wavelength .lambda. fulfill the
relationship of .lambda./(4.times.n)<.-
LAMBDA.<.lambda./(2.times.n).
12. A light source device, comprising; an optical waveguide device
according to claim 1, and a collimator lens for substantially
collimating outgoing light from the optical waveguide, wherein the
collimator lens is located at a center of distribution of the
outgoing light from the optical waveguide.
13. An optical apparatus, comprising: an optical waveguide device
according to claim 1, and a collection optical system for
collecting outgoing light from the optical waveguide on a target of
detection, wherein the optical waveguide device and the target of
detection are located so as to be confocal with each other.
14. An optical apparatus according to claim 13, wherein an angle
.theta. made by a plane perpendicular to the outgoing surface and
the optical waveguide, an effective refractive index n of the
optical waveguide, and a substantial numerical aperture NA of a
portion of the collection optical system which opposes the outgoing
surface, fulfill the relationship of
NA.ltoreq.sin(.theta..times.n).
15. An optical apparatus according to claim 13, wherein the target
of detection includes an optical disk.
16. An optical waveguide device, comprising: a dielectric
substrate; and an optical waveguide formed in the dielectric
substrate, wherein light going out from the optical waveguide in a
first direction and reflected back to the outgoing surface by an
external object is directed by the outgoing surface in a second
direction different from the first direction.
17. An optical waveguide device according to claim 16, wherein the
dielectric substrate and the optical waveguide form a second
harmonic generation element.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an optical waveguide device
used for optical information processing and optical measurement
control performed utilizing a coherent light source, and a light
source device and an optical apparatus including the optical
waveguide device.
[0003] 2. Description of the Related Art
[0004] In the fields of optical information recording and
reproduction, a higher density of recording and reproduction is
realized by using a light source for emitting light having a
shorter wavelength. For example, whereas conventional compact disk
apparatuses use near infrared light having a wavelength of about
780 nm, digital versatile disk (DVD) apparatuses for recording and
reproducing information at a higher density use red semiconductor
laser light having a wavelength of about 650 nm. In order to
realize a next-generation optical disk apparatus for recording and
reproducing information at a still higher density, development of
blue laser light source devices have been actively developed. For
example, a wavelength conversion element using a non-linear optical
substance has been developed to be included in a compact and stable
blue laser light source device.
[0005] FIG. 14 is a schematic view illustrating an exemplary blue
light source device using a second harmonic generation element
(hereinafter, referred to as an "SHG element") 117. With reference
to FIG. 14, the SHG element 117 will be described.
[0006] The SHG element 117 includes a dielectric substrate 114 and
a high refractive index area having a width of about 3 .mu.m and a
depth of about 2 .mu.m formed by a proton exchange method. The high
refractive index area acts as an optical waveguide 115. Infrared
light emitted from a semiconductor laser 111 having a wavelength of
about 850 nm is collected on an incident surface 139 of the SHG
element 117 through a collection lens 112 and then propagated
through the optical waveguide 115 in the SHG element 117 to form a
fundamental guided wave.
[0007] Lithium niobate crystals forming the dielectric substrate
114 have a non-linear optical constant. As a result of a
sufficiently large non-linear optical constant, a harmonic guided
wave having a wavelength of about 425 nm is obtained by wavelength
conversion of the infrared light, and excited from the electric
field of the fundamental guided wave.
[0008] In order to compensate for a propagation constant difference
between the fundamental guided wave and the harmonic guided wave,
domain inversion areas 116 are periodically formed in the optical
waveguide 115. The harmonic guided waves which are excited
throughout the optical waveguide 115 are coherently added together
and then come out from an outgoing surface 138 of the SEG element
117.
[0009] In order to correctly compensate for the propagation
constant difference between the fundamental guided wave and the
harmonic guided wave, the wavelength of the fundamental guided wave
needs to be maintained at a certain value. Accordingly, as the
semiconductor laser 111, a DBR laser is used for its very small
wavelength fluctuation in accordance with the temperature or the
like. A DBR laser has another feature in that since light is
oscillated at a single wavelength, the light has a satisfactorily
high coherency and a satisfactorily low RIN (relative intensity
noise).
[0010] FIG. 15 is a schematic view of an optical disk pickup
including the SEG element 117 shown in FIG. 14 for providing blue
light. With reference to FIG. 15, an operation of the optical disk
pickup will be described.
[0011] Harmonic blue light output by the SEG element 117 passes
through a collimator lens 113, a polarization beam splitter 120, a
{fraction (1/4)} wave plate 121 and an objective lens 122 and then
is collected to an optical disk 124.
[0012] The light modulated by the optical disk 124 is reflected by
the polarization beam splitter 120 and guided to a light detector
125 by a collection lens 123. Thus, a reproduction signal is
obtained.
[0013] The SHG element 117 outputs linearly polarized light in a
direction parallel to the page. This light passes through the
{fraction (1/4)} wave plate 121 and returns through the {fraction
(1/4)} wave plate 121 to become a polarized light which is in a
direction perpendicular to the page. Thus, the light reflected by
the optical disk 124 is all reflected by the polarization beam
splitter 120 and does not return toward the SHG element 117
theoretically.
[0014] However, the optical disk 124 includes a material having a
birefringence. Accordingly, in actuality, an unnecessary polarized
component returns toward the SHG element 117 through the
polarization beam splitter 120.
[0015] While data stored in the optical disk 124 is reproduced, the
objective lens 122 is positionally controlled to focus the light
accurately to the optical disk 124. Accordingly, the outgoing
surface 138 of the SHG element 117 and the optical disk 124 form a
confocal optical system. Thus, the light reflected by the optical
disk 124 is accurately collected at the optical waveguide 115 on
the outgoing surface 138 of the SHG element 117.
[0016] In an optical system including a semiconductor laser as a
light source, the light component which returns toward a light
source after being reflected induces noise (mode hop noise).
Conventionally, various proposals have been made for avoiding the
mode hop noise.
[0017] For example, oscillation in a plurality of longitudinal
modes is caused by modulating light from the semiconductor laser
with a harmonic signal or by causing self-oscillation of the
semiconductor laser.
[0018] In the field of optical communication, for collecting light
from a semiconductor laser to an optical fiber, a light isolator
utilizing a magneto-optical effect is commonly inserted between the
semiconductor laser and the optical fiber.
[0019] Japanese Laid-Open Publication No. 5-323404 discloses a
method, by which an incident surface of an optical fiber or an
optical waveguide is obliquely polished, so that the returning
light is obliquely reflected and does not return to the
semiconductor laser.
[0020] These technologies are for reducing the mode hop noise
induced by the light returning to inside the semiconductor laser as
a light source.
[0021] The present inventors performed experiments on data
reproduction by an optical pickup including the SHG element 117
shown in FIG. 15. As a result, the present inventors found a noise
which is generated by the following mechanism, which is different
from induction by the returning light.
[0022] The returning light collected at the optical waveguide 115
on the outgoing surface 138 of the SHG element 117 is reflected by
the outgoing surface 138 and interferes with the light coming out
from the optical waveguide 115. Thus, an interference noise is
generated.
[0023] Due to such an interference noise, the optical power output
from the SHG element 117 appears to have been changed from the
optical disk 124, and thus a reproduction signal from the optical
disk 124 is modulated with a low frequency noise, resulting in
signal deterioration.
[0024] Whereas noise induced by the returning light is generated by
the interaction of the light inside the semiconductor laser 111 and
the returning light reflected by the incident surface 139 of the
SHG element 117, the interference noise is generated by the
interference of the light from the SHG element 117 and the
returning light reflected by the outgoing surface 138 of the SHG
element 117.
[0025] The present inventors found another cause of the
interference noise as a result of a further research. A portion of
the returning light from an external optical system external to the
optical waveguide device (including, for example, collimator lens
113) is re-excited in the optical waveguide 115 as a guided wave
and reflected by the incident surface 139 of the SHG element 117.
The light reflected by the incident surface 139 is interfered with
the light from the semi conductor laser 111. Such an interference
also causes the interference noise.
[0026] As described above, an optical system including an optical
waveguide device involves two different types of noises. One is a
low frequency interference noise caused by the interference, in an
external optical system, of (1) light emitted by the light source
and propagating through the optical waveguide device toward the
external optical system and (2) the light reflected by an outgoing
surface or an incident surface of the optical waveguide device
after propagating through the optical waveguide device and being
reflected by an external object (e.g., the optical disk). The other
is the mode hop noise caused inside the semiconductor laser.
[0027] Various proposals have been made in order to reduce the mode
hop noise, but the interference noise caused in the external
optical system has not been a target of attention and no proposals
have been made for solving the problem of the interference
noise.
SUMMARY OF THE INVENTION
[0028] According to the present invention, an optical waveguide
device includes a dielectric substrate; and an optical waveguide
formed in the dielectric substrate, the optical waveguide having a
longitudinal axis and an outgoing surface disposed at an angle
other than a right angle relative to a plans perpendicular to the
longitudinal axis.
[0029] In one embodiment of the invention, the dielectric substrate
has an outgoing surface disposed in a plans within which the
outgoing surface of the optical waveguide is disposed.
[0030] In one embodiment of the invention, the outgoing surface is
angled so that light going out from the optical waveguide in a
first direction and reflected back to the outgoing surface by an
external object is directed by the outgoing surface in a second
direction different from the first direction.
[0031] In one embodiment of the invention, the optical waveguide
has an incident surface disposed at an angle other than the right
angle relative to the plane perpendicular to the longitudinal axis
of the optical waveguide.
[0032] In one embodiment of the invention, the dielectric substrate
has an incident surface disposed in a plane within which the
incident surface of the optical waveguide is disposed.
[0033] In one embodiment of the invention, the dielectric substrate
has an outgoing surface disposed in a plane within which the
outgoing surface of the optical waveguide is disposed, and the
incident surface of the dielectric substrate and the outgoing
surface of the dielectric substrate are substantially parallel to
each other.
[0034] In one embodiment of the invention, the dielectric substrate
and the optical waveguide form a second harmonic generation
element.
[0035] In one embodiment of the invention, the optical waveguide
device further includes a reflection reducing layer for reducing a
reflection of a harmonic wave.
[0036] In one embodiment of the invention, the reflection reducing
layer is disposed on an incident surface of the optical
waveguide.
[0037] In one embodiment of the invention, the optical waveguide
device further includes a harmonic wave absorption element for
absorbing a harmonic wave in a portion of the optical waveguide in
the vicinity of an incident surface of the optical waveguide.
[0038] In one embodiment of the invention, the optical waveguide
device further includes a grating element having a periodicity of
.LAMBDA. in a portion of the optical waveguide in the vicinity of
the incident surface. The second harmonic generation element
receives light having a wavelength of .lambda. as a fundamental
wave in a vacuum; and the periodicity .LAMBDA., an effective
refractive index n of the optical waveguide, and the wavelength
.lambda. fulfill the relationship of
.lambda./(4.times.n)<.LAMBDA.</(2.times.n).
[0039] According to another aspect of the invention, a light source
device includes any of the above-described optical waveguide
devices and a collimator lens for substantially collimating
outgoing light from the optical waveguide. The collimator lens is
located at a center of distribution of the outgoing light from the
optical waveguide.
[0040] According to still another aspect of the invention, an
optical apparatus includes any of the above-described optical
waveguide devices, and a collection optical system for collecting
outgoing light from the optical waveguide on a target of detection.
The optical waveguide device and the target of detection are
located so as to be confocal with each other.
[0041] In one embodiment of the invention, an angle .theta. made by
a plane perpendicular to the outgoing surface and the optical
waveguide, an effective refractive index n of the optical
waveguide, and a substantial numerical aperture NA of a portion of
the collection optical system which opposes the outgoing surface,
fulfill the relationship of NA.ltoreq.sin(.theta..times.n).
[0042] In one embodiment of the invention, the target of detection
includes an optical disk.
[0043] According to still another aspect of the invention, an
optical waveguide device includes a dielectric substrate; and an
optical waveguide formed in the dielectric substrate. Light going
out from the optical waveguide in a first direction and reflected
back to the outgoing surface by an external object is directed by
the outgoing surface in a second direction different from the first
direction.
[0044] In one embodiment of the invention, the dielectric substrate
and the optical waveguide form a second harmonic generation
element.
[0045] According to the present invention, a simple structure of an
optical waveguide device, in which an outgoing surface is disposed
at an angle other than a right angle with respect to a longitudinal
axis of an optical waveguide, effectively reduces the influence of
the light returning from an optical system external from the
optical waveguide device (e.g., a pickup optical system).
[0046] In the embodiment in which the optical waveguide device has
both an outgoing surface and an incident surface disposed at an
angle other than a right angle with respect to a longitudinal axis
of the optical waveguide, the reflection of the light by both
surfaces is reduced and thus the influence of the interference of
the light returning from the external optical system and the light
propagating through and going out from the optical waveguide device
is eliminated substantially completely. Since the incident surface
is disposed at an angle as described above, the light emitted by a
semiconductor laser and reflected by the incident surface is
substantially inhibited toward returning from the semiconductor
laser. Thus, the mode hop noise is also reduced as well as the
interference noise.
[0047] In the embodiment in which the outgoing surface and the
incident surface are parallel to each other, the optical waveguide
devices are easy to produce.
[0048] Also according to the present invention, a light source
device including the above-described optical waveguide device and
an optical apparatus including such a light source are provided. A
collimator lens in the light source device is located at the center
of distribution of outgoing light from an optical waveguide of the
optical waveguide device. Therefore, the light returning toward the
light source device from the external optical system is
reduced.
[0049] Due to the outgoing surface disposed at an angle as
described above, light returning from an external optical system
and reflected by the outgoing surface is prevented from interfering
with the outgoing light from the optical waveguide. Therefore, a
stable light source device with substantially no interference noise
is provided.
[0050] In the embodiment in which an incident surface is disposed
at an angle with respect to a longitudinal axis of the optical
waveguide as well as the outgoing surface, the light returning from
the external optical system is prevented from being reflected by
the incident surface. Thus, the interference noise is further
reduced. Furthermore, the light from a light source is prevented
from returning toward the light source as a result of being
reflected by the incident surface. Thus, the hop mode noise is also
reduced.
[0051] Thus, the invention described herein makes possible the
advantages of providing (1) an optical waveguide device for
reducing an interference noise in an optical apparatus, and (2) a
light source device and an optical apparatus including such an
optical waveguide device.
[0052] These and other advantages of the present invention will
become apparent to those skilled in the art upon reading and
understanding the following detailed description with reference to
the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] FIG. 1 is a schematic top view of an optical apparatus
including a light source device which includes an optical waveguide
device in an example according to the present invention;
[0054] FIG. 2 is a partial schematic top view of a light source
device in another example according to the present invention;
[0055] FIG. 3 is a cross-sectional view of a far-field image of
light from the light source device shown in FIG. 2;
[0056] FIGS. 4A, 4B and 4C each show examples of different
apertures which can be provided in the light source device shown in
FIG. 2:
[0057] FIG. 5 is a partial schematic side elevational view of a
light source device in still another example according to the
present invention;
[0058] FIG. 6 is a partial schematic top view of a light source
device in still another example according to the present
invention;
[0059] FIG. 7 is a schematic top view of a light source device in
still another example according to the present invention;
[0060] FIGS. 8A and 8B each show an exemplary arrangement of a
plurality of optical waveguide devices during production;
[0061] FIG. 9 is a partial schematic top view of a light source
device in still another example according to the present
invention;
[0062] FIG. 10 is a partial schematic top view of a light source
device in still another example according to the present
invention;
[0063] FIG. 11 is a partial schematic view of a light source device
in still another example according to the present invention;
[0064] FIG. 12 is a vector diagram with respect to a harmonic wave
obtained in the light source device shown in FIG. 11;
[0065] FIG. 13 is a vector diagram with respect to a fundamental
wave obtained in the optical apparatus shown in FIG. 11;
[0066] FIG. 14 is a partial schematic view of a conventional light
source device; and
[0067] FIG. 15 is a schematic view of a conventional optical
apparatus including the light source device shown in FIG. 14.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0068] Hereinafter, the present invention will be described by way
of illustrative examples with reference to the accompanying
drawings.
[0069] In this specification, the term "outgoing surface" refers to
a surface of the dielectric substrate and the optical waveguide
from which the light goes out unless otherwise described. Thus, the
dielectric substrate may itself have an outgoing surface different
from that of the optical waveguide, or the dielectric substrate and
the optical waveguide may have an outgoing surface in the same
plane. The term "incident surface" refers to a surface of the
dielectric substrate and the optical waveguide on which light is
incident from a light source unless otherwise described. Thus, the
dielectric substrate may itself have an incident surface different
from that of the optical waveguide, or the dielectric substrate and
the optical waveguide may have an incident surface in the same
plane. The term "outgoing light" refers to the light going out from
the optical waveguide.
EXAMPLE 1
[0070] FIG. 1 shows a schematic view illustrating an exemplary
structure of an optical apparatus 100 including an optical
waveguide device 10 in a first example according to the present
invention. The optical apparatus 100 is, for example, an optical
pickup for detecting data stored in an optical disk 24.
[0071] The optical apparatus 100 includes a light source device 101
and a collection optical system 102. The light source device 101
includes, for example, a semiconductor laser 11 as a light source,
a collection lens 12, the optical waveguide device 10, and a
collimator lens 13. The collection optical system 102 includes, for
example, a polarization beam splitter 20, a {fraction (1/4)} wave
plate 21 and an objective lens 22. The optical waveguide device 10
includes a dielectric substrate 14 and an optical waveguide 15
formed therein. In the following examples, an SHG element includes
the dielectric substrate 14 and the optical waveguide 15. According
to the present invention, other types of elements may be used as a
dielectric substrate and an optical waveguide.
[0072] The optical apparatus 100 operates in, for example, the
following manner.
[0073] Light having a wavelength of about 850 nm is emitted by the
semiconductor laser 11. While the light is propagated through the
optical waveguide 15, the light is converted to blue light having a
wavelength which is about half of that of the infrared light
emitted by the semiconductor laser 11. The blue light goes out from
an outgoing surface 38 of an SHG element 17.
[0074] The light from the SHG element 17 passes through a
collimator lens 13 and the polarization beam splitter 20 and then
is collected to the optical disk 24 by the objective lens 22.
[0075] The light reflected by the optical disk 24 is reflected by
the polarization beam splitter 20 and thus is guided to a light
detector 25 through a collection lens 23. When the optical disk 24
includes a material having birefringence, an unnecessary polarized
light component passes through the polarization beam splitter 20
and thus returns to the SHG element 17.
[0076] Since the objective lens 22 is positionally controlled to
focus light accurately to the optical disk 24, the outgoing surface
38 and the optical disk 24 form a confocal optical system.
Accordingly, returning light 31 returning from a pickup optical
system 103 is collected on the outgoing surface 38. The pickup
optical system 103 includes the collimator lens 13, the
polarization beam splitter 20, and the objective lens 22.
[0077] The outgoing surface 38 shown in FIG. 1 is disposed so that
a plane 50 perpendicular thereto has an angle of with respect to a
longitudinal axis A of the optical waveguide 15. In other words,
the outgoing surface 38 has an angle of .theta. relative to a plane
perpendicular to the longitudinal axis A of the optical waveguide
15. In this specification, such a positional relationship between
the outgoing surface and the optical waveguide may also be
represented as being oblique with respect to the optical
waveguide.
[0078] Outgoing light 30 from the optical waveguide 15 goes out at
an angle of .theta.1 with respect to the plane 50 perpendicular to
the outgoing surface 38. The angle .theta.1 is represented by
expression (1) by Snell's law of refraction.
.theta.1=n.times.sin .theta. (1)
[0079] As can be appreciated from expression (1), the angle
.theta.1 is defined by the refractive index n of the optical
waveguide 15 and the angle .theta. made by the optical waveguide 15
and the plane 50.
[0080] Since the light from the optical waveguide 15 goes out with
an angle other than a right angle with respect to the optical
waveguide 15, the pickup optical system 103 is located at an angle
other than a right angle with respect to the optical waveguide 15
as shown in FIG. 1. Due to such a structure, a reflected light
Component 32 of the returning light 31 which is reflected by the
outgoing surface 38 is offset relative to the outgoing light 30 by
2.times..theta.1. As a result, the reflected light component 32 is
not directed to the collimator lens 13 and thus is not interfered
with the outgoing light 30.
[0081] As shown in FIG. 1, the collimator lens 13 is located at an
angle other than a right angle relative to the optical waveguide
15. More specifically, the collimator lens 13 is located at the
center of distribution of the outgoing light 30 which goes out at
the angle defined by the Snell's law of refraction.
[0082] Due to such a structure, the angle made by the returning
light 31 and the reflected light component 32 is larger than that
in a structure where the collimator lens 13 is located at a right
angle with respect to the optical waveguide 15. Accordingly, the
outgoing light 30 is utilized most efficiently.
[0083] The angle .theta. made by the optical waveguide 15 and the
plane 50 does not need to be limited to any specific angle for
reducing an interference noise so long as it is not a right angle.
In the case where the angle .theta.1 of the outgoing light 30 with
respect to the plane 50 and a numerical aperture (NA) of a portion
of the pickup optical system 103 (the portion opposing the outgoing
surface 38) fulfill the conditions represented by expression (2),
the outgoing light 30 and the reflected light component 32 are
substantially completely separated from each other, so that the
interference of the outgoing light 30 and the reflected light
component 32 is substantially completely eliminated. The
above-mentioned numerical aperture is also obtained at a portion of
the collection optical system 102 (the portion opposing the
outgoing surface 38).
NA<sin(0.times.n) (2)
[0084] The numerical aperture of the above-mentioned portion of the
pickup optical system 103 does not simply refer to a numerical
aperture of the collimator lens 13. For example, in the pickup
optical system 103 shown in FIG. 1, the collimator lens 13 has a
larger effective radius than that of the objective lens 22, and the
effective beam radius of the pickup optical system 103 is defined
by the effective radius of the objective lens 22.
[0085] In this example, the numerical aperture for substantially
completely eliminating the influence of the reflected light
component 32 is also represented by expression (3):
NA-sin(r/f) (3)
[0086] where r is the effective beam radius of the pickup optical
system 103, i.e., the effective radius of the objective lens 22,
and f is the focal length of the collimator lens 13. The angle
.theta. made by the optical waveguide 15 and the plane 50 is set to
be in a range which fulfills expressions (2) and (3).
[0087] In the case where the optical apparatus 100 is used for a
DVD reproduction apparatus or CD reproduction apparatus, the
effective beam radius r is about 2 mm and the focal length f of the
collimator lens 13 is about 15 mm. When the optical waveguide
device 10 having the optical waveguide 15 is formed of lithium
niobate and has a refractive index of 2.2, the angle .theta. made
by the optical waveguide 15 and the plane 50 is preferably set to
be 3.5 degrees or more. The upper limit of the angle .theta. is
determined by a critical angle of total internal reflection.
However, since the dependency of the transmittance of the outgoing
surface 38 on the angle .theta. increases as the angle .theta.
increases, it is practically preferable to set the angle .theta. at
about 20 degrees or less.
[0088] FIG. 2 shows another optical waveguide device 10A usable in
the optical apparatus 100 (FIG. 1). The optical waveguide device
10A includes an SHG element 17A. The SHG element 17A includes a
proton exchange optical waveguide 15A. A plane 51 perpendicular to
an outgoing surface 38A of the SHG element 17A has a smaller angle
with respect to a longitudinal axis A of the optical waveguide 15A
as compared to the outgoing surface 38 in FIG. 1.
[0089] When the optical waveguide device LOA has the proton
exchange optical waveguide device 15A as shown in FIG. 2, a
far-field image of outgoing light 30A from the optical waveguide
15A often has an elliptical shape as shown in FIG. 3. FIG. 3 is a
cross-sectional view of the outgoing light 30A in FIG. 2 taken
along line 3-3 of FIG. 2. As shown in FIG. 3, the height h of the
far-field image is greater than the width d thereof.
[0090] In this case, as shown in FIG. 2, an aperture plate 37A
having an aperture matching the far-field image of the outgoing
light 30A shown in FIG. 3 can be provided between the collimator
lens 13 and the optical waveguide device LOA but in the vicinity of
the collimator lens 13. FIG. 4A is a cross-sectional view of the
aperture plate 37A shown in FIG. 2 taken along line 4A-4A of FIG.
2. As shown in FIG. 4A, the height h of the aperture of the
aperture plate 37A is greater than the width d thereof, which
matches the far-field image of the outgoing light 30A shown in FIG.
3. By providing the aperture plate 37A, the numerical aperture of a
portion of the pickup optical system (the portion opposing the
outgoing surface 38A) in the direction of width d (FIG. 3) can be
reduced without losing a significant amount of light. (Only the
collimator lens 13 of the pickup optical system is shown in FIG.
2.) In the structure shown in FIG. 2, the angle .theta. for
substantially eliminating the interference of the reflected light
component 32 needs to fulfill expressions (2) and (4):
NA=sin{(d/2)/f} (4)
[0091] where d is the width of the aperture of the aperture plate
37A. Accordingly, the angle .theta. can be decreased as compared to
that in the structure shown in FIG. 1.
[0092] A far-field image of light going out from a representative
proton exchange optical waveguide extends in the direction of
height h (FIG. 3) twice as large as in the direction of the width
d. Accordingly, the angle .theta. made by the plane 51 (FIG. 2) and
the optical waveguide 15A can be reduced to about half of the angle
.theta. in FIG. 1. When the optical waveguide device 10A is
combined with the pickup optical system 103 shown in FIG. 1, the
angle .theta. is 1.8 degrees or more. In actuality, the angle
.theta. is reasonably set to be 2 degrees or more in consideration
of positional errors of the lens, the light source and the like.
The upper limit of angle .theta. is appropriately set to be about
20 degrees.
[0093] In the example shown in FIG. 2, the far-field image of the
outgoing light 30A is larger in the direction of the height h (FIG.
3) than in the direction of the width d. In the case where a
far-field image of the light is larger in the direction of the
width d than in the direction of the height h due to, for example,
a structure of the optical waveguide, an aperture plate having an
aperture which is larger in the direction of the width d than in
the direction of the height h can be provided.
[0094] The aperture of the aperture plate to be provided is not
limited to have an elliptical shape but can be rectangular (37B) as
shown in FIG. 4B or strip-like (37C) as shown in FIG. 4C. The same
effect is obtained as long as the aperture is sized differently in
the direction of the width d from in the direction of the height
h.
[0095] A technology for forming the outgoing surface of the optical
waveguide device to be oblique with respect to the optical
waveguide is disclosed in, for example, Japanese Laid-Open
Publication No. 61-22311. The technology disclosed in this
publication, by which an end surface of an optical fiber is
oblique, provides an effect of preventing reflection of the guided
wave by the end surface. According to the present invention, the
optical waveguide device has an oblique outgoing surface for
reflecting the light returning from the pickup optical system to be
directed to a different direction from the light going out from the
optical waveguide device. Therefore, the light returning from an
external object and reflected by the outgoing surface is not
interfered with the outgoing light from the optical waveguide. The
present invention is different from the above-mentioned technology
for preventing the reflection of the guided wave in both the
structure and the effect.
[0096] In FIG. 2, a top view of the optical waveguide device 10A is
shown. An axis A extends through the optical waveguide 15A, and a
plane B is perpendicular to the longitudinal axis A. The outgoing
surface 38A is disposed at a non-parallel angle relative to the
plane B and about an axis C extending into the page and
perpendicular to the longitudinal axis A. The outgoing surface 38
shown in FIG. 1 is also disposed in the same manner.
[0097] FIG. 5 shows a side elevational view of an optical waveguide
device 10B including an SHG element 17B having an outgoing surface
38B which is disposed in a different direction. The outgoing
surface 38B is disposed at a non-parallel angle relative to the
plane B and about an axis D extending into the page and
perpendicular to the longitudinal axis A. The optical waveguide
device 10B having such an outgoing surface 38B also avoids the
interference of the outgoing light 30 and the reflected light
component 32.
[0098] In the examples shown in FIGS. 1, 2 and 5, the entirety of
the outgoing surface of the SHG element is disposed at an angle
other than a right angle relative to a plane perpendicular to a
longitudinal axis of the optical waveguide. Such an outgoing
surface is easily formed by polishing the entire outgoing surface
obliquely. FIG. 6 shows an optical waveguide device 10C having an
alternative outgoing surface 38C. The outgoing surface 38C of an
SHG element 17C of the optical waveguide device 10C is disposed at
an angle other than a right angle relative to a plane perpendicular
to a longitudinal axis of an optical waveguide 15C only at a
portion corresponding to an end of the optical waveguide 15C. The
optical waveguide device 10C having such an outgoing surface 38C
provides similar effects as those provided by the optical waveguide
devices 10, 10A and 10B.
[0099] The outgoing surface 38C is formed by, for example, a dicing
method. For example, use of a blade having a surface roughness of
#6000 realizes an outgoing surface which almost appears to be
obtained by optical polishing. The outgoing surface 38C is obtained
by making a cut-out portion having a width 38w which is equal to or
greater than a width 15w of the optical waveguide 15C, for example,
10 .mu.m in a flat outgoing surface of the optical waveguide device
10C. Since dicing is performed to merely about 10 .mu.m, the
surface can easily be mirror-processed, and the blade is not
significantly worn out. Thus, the processing yield is
satisfactorily high.
[0100] In the first example, optical waveguide devices each having
an outgoing surface which is disposed at an angle other than a
right angle relative to a plane perpendicular to a longitudinal
axis of the optical waveguide are described. Such a structure
reduces the interference noise caused by the influence of the
outgoing light from the optical waveguide and the light returning
from the pickup optical system and reflected by the outgoing
surface. Although such a structure is sufficient to reduce the
interference noise, a reflection by an incident surface of the
optical waveguide device needs to be considered in order to more
completely reduce the interference noise.
[0101] As indicated by dashed line 33R in FIG. 1, the returning
light 31 from the pickup optical system 103 is partially incident
on the optical waveguide 15 and reflected by an incident surface 39
of the SHG element 17 to be a refracted waveguide component 33. The
reflected waveguide component 33 goes out from the outgoing surface
38. The reflected waveguide component 33 cannot be removed by
forming the outgoing surface 38 obliquely with respect to the
optical waveguide 15 and causes noise as a result of interfering
with the outgoing light 30.
EXAMPLE 2
[0102] In a second example according to the present invention, an
exemplary optical waveguide device 10D shown in FIG. 7 for
preventing or substantially inhibiting the reflection by an
incident surface 39D of an SHG element 17D of the optical waveguide
device 10D will be described. The incident surface 39D is disposed
at an angle other than a right angle with respect to a plane
perpendicular to a longitudinal axis of an optical waveguide 15D as
well as an outgoing surface 38D of the SHG element 17D. In this
specification, such a positional relationship between the incident
surface and the optical waveguide may also be represented as being
oblique with respect to the optical waveguide.
[0103] Although not shown in the drawings, the incident surface 39D
can be disposed at an angle other than a right angle relative to a
plane perpendicular to the longitudinal axis A of the optical
waveguide 15D only at a portion corresponding to an end of the
optical waveguide 15D, such as was described, for example, with
respect to the outgoing surface of the example of FIG. 6. Such a
structure provides similar effects as those provided by the optical
waveguide device 10D.
[0104] Returning light 31 from the pickup optical system (only the
collimator lens 13 is shown in FIG. 7) is incident on the optical
waveguide 15D and reflected by the incident surface 39D to be a
reflected waveguide component 33D. The reflected waveguide
component 33D is not incident on the optical waveguide 15D and is
directed to the inside of the optical waveguide device 10D while
being scattered. Thus, the reflected waveguide component 33D does
not reach the outgoing surface 38D.
[0105] In such a structure, the reflection by both the incident
surface 39D and the outgoing surface 38D is reduced, and thus the
influence of the interference of the returning light 31 and the
outgoing light 30 is substantially completely eliminated.
[0106] Since the incident surface 39D is oblique with respect to
the optical waveguide 15D, light returning to the semiconductor
laser 11 as a result of being reflected by the incident surface 39D
is also suppressed. Thus, the mode hop noise caused by the
semiconductor laser 11 is also reduced.
EXAMPLE 3
[0107] Waveguide devices having both an incident surface and an
outgoing surface formed obliquely with respect to an optical
waveguide can be produced without lowering the productivity in the
case where the incident surface and the outgoing surface are
parallel to each other.
[0108] Waveguide devices are usually produced as follows. A number
of optical waveguide devices are formed on a large-area dielectric
substrate and cut into individual optical waveguide devices. End
surfaces of the individual optical waveguide devices are then
smoothed by polishing.
[0109] FIG. 8A shows an exemplary arrangement of a plurality of the
optical waveguide devices 10H on a dielectric substrate 14.
[0110] The optical waveguide devices 10H having an outgoing surface
and an incident surface parallel to each other can be arranged at a
high density with no wasted space. Furthermore, each of lines of
optical waveguide devices 10H can be cut off in one procedure and
end surfaces thereof can be polished in one procedure.
Specifically, each line of optical waveguide devices 10H is cut off
along a line 48 (the vertical line shown in FIG. 8A), and end
surfaces of the optical waveguide devices 10H are polished in one
procedure. Then, each line of optical waveguide devices 10H is cut
into individual optical waveguide devices 10H along a line 47 (the
horizontal line shown in FIG. 8A). In this manner, polishing of end
surfaces, which is relatively time-consuming, does not need to be
repeated for each optical waveguide device 10H.
[0111] FIG. 8B shows an exemplary arrangement of optical waveguide
devices 10J on the dielectric substrate 14A. The optical waveguide
devices 10J have an incident surface and an outgoing surface which
are not parallel to each other. As can be appreciated from FIG. 8B,
each line of optical waveguide devices 10J cannot be cut off along
one line. Accordingly, end surfaces of each line of optical
waveguide devices 10J cannot be polished in one procedure. This
requires the end surfaces of the optical waveguide devices 10J to
be polished individually, which significantly lowers the
productivity as compared to optical waveguide devices having an
incident surface and an outgoing surface formed at a right angle
with respect to the optical waveguide.
[0112] Such a problem is avoided by forming the incident surface
and the outgoing surface parallel to each other.
[0113] In fourth through sixth examples according to the present
invention, other structures of optical waveguide devices will be
described with reference to FIGS. 9 through 11. The structures in
the fourth through the sixth examples are specifically effective
when a second harmonic generation (SHG) element is used in the
optical waveguide device, a fundamental wave having a relatively
long wavelength is incident on an incident surface of an optical
waveguide of the optical waveguide device and a harmonic wave
having a shorter wavelength goes out from an outgoing surface of
the optical waveguide.
EXAMPLE 4
[0114] FIG. 9 schematically shows an optical waveguide device 10E
having an incident surface 39E and an outgoing surface 38E of an
SHG element 17E. A fundamental wave 61E is incident on the incident
surface 39E. The optical waveguide device 10E further includes a
reflection reducing layer 34 disposed on the incident surface 39S.
The reflection reducing layer 34 is provided for reducing a
reflection of a returning harmonic wave 31E returning from the
pickup optical system (only the collimator lens 13 is shown in FIG.
9). A usual SHG element is provided with a reflection reducing
layer for reducing reflection of a fundamental wave by an incident
surface thereof in order to prevent the fundamental wave from
returning to a semiconductor laser (not shown). In the example
shown in FIG. 9, reflection of the fundamental wave 61E does not
need to be prevented since the incident surface 39E oblique with
respect to an optical waveguide 15E prevents the light from
returning to the semiconductor laser.
[0115] The reflection of the returning harmonic wave 31B returning
from the pickup optical system (including the collimator lens 13)
is not completely eliminated by the oblique incident surface 39B.
The reflection reducing layer 34 provided on the incident surface
39E further reduces the reflection of the returning harmonic wave
31E.
[0116] The reflection reducing layer 34 can alternatively be
disposed with a gap from the incident surface 393.
EXAMPLE 5
[0117] FIG. 10 schematically shows an optical waveguide device 10F
for reducing a returning harmonic wave 31F returning from the
pickup optical system (only the collimator lens 13 is shown in FIG.
10) by a harmonic wave absorption element 35 included in the
optical waveguide device 10F. The harmonic wave absorption element
35 is provided in a portion of the optical waveguide 15F in the
vicinity of the incident surface 39F of an SHG element 17F of the
optical waveguide device 10F.
[0118] When near infrared light having a wavelength of about 860 nm
used as a fundamental wave 61F is converted into blue light having
a wavelength of about 425 nm as a harmonic wave, the harmonic wave
absorption element 35 can be formed of, for example, titanium
oxide, zinc selenide, gallium phosphide, or amorphous silicon.
These substances are transmissive with respect to light in the
infrared region and have spectral characteristics absorbing blue
light. These substances can also be formed on an optical waveguide
15F in the optical waveguide device 10F in the form of a thin film
by sputtering or other technologies.
[0119] In a device using light of other wavelength ranges from
above, for example, in a device in which red light as the
fundamental wave is converted into ultraviolet light as the
harmonic wave, other substances can be used as the harmonic wave
absorption element 35.
EXAMPLE 6
[0120] FIG. 11 schematically shows an optical waveguide device 10G
for preventing the interference of a harmonic returning wave 31G
returning from the pickup optical system (only the collimator lens
13 is shown in FIG. 11) and the outgoing light from an optical
waveguide 15G by diffracting and scattering the harmonic returning
wave 31G using a grating element 36 included in the optical
waveguide device 10G.
[0121] As shown in FIG. 11, the grating element 36 having a short
periodicity .LAMBDA. is formed in a portion of an optical waveguide
15G in the vicinity of an incident surface 39G of an SHG element
17G. The periodicity .LAMBDA. is set to a value fulfilling
expression (5):
.lambda./(4.times.n)<.LAMBDA.</(2.times.n) (5)
[0122] where .lambda. is a wavelength of the fundamental wave in a
vacuum and n is the effective refractive index of the optical
waveguide 15G. By setting the periodicity .LAMBDA. to such a value,
the harmonic wave is diffracted and the fundamental wave is not
diffracted.
[0123] FIG. 12 is a vector diagram with respect to the harmonic
wave, and FIG. 13 is a vector diagram with respect to the
fundamental wave.
[0124] The vector diagram shown in FIG. 12 represents a wave number
vector 40 of the harmonic guided wave, a wave number vector 42 of
the grating element 36, a wave number vector 43 of light diffracted
into air, and a wave number vector 44 of light diffracted into the
dielectric substrate.
[0125] The vector diagram shown in FIG. 13 represents a wave number
vector 41 of the fundamental guided wave and a wave number vector
42 of the grating element 36. The direction of each vector
corresponds to the propagation direction, and the size of the
vector corresponds to the wave number.
[0126] The direction of the diffracted light obtained by the
grating element 36 is found by the vector diagram in the following
manner. The wavelength of the light radiating in the air or the
dielectric substrate is determined at a certain value. Accordingly,
the wave number vectors of the radiating value (i.e., diffracted
light to the air and the dielectric substrate) terminate on
semicircles R1 and R2 in FIG. 12 and on semicircles R3 and R4 in
FIG. 13.
[0127] A guided wave is diffracted into the dielectric substrate or
air by the grating element 36 only when a horizontal component of
the difference between the wave number vector of the radiating
light and the wave number vector 40 or 41 of the guided wave
matches the wave number vector 42 of the grating element 36.
Accordingly, the wave number vectors of the radiating light
terminate on dashed straight line DL1 in FIG. 12 and dashed
straight line DL2 in FIG. 13.
[0128] In other words, when the harmonic guided wave is diffracted
(FIG. 12), the radiating light is generated toward an intersection
43A of the semicircle R1 and the dashed straight line DL1 and an
intersection 44A of the semicircle R2 and the dashed straight line
DL1.
[0129] In FIG. 13, the dashed straight line DL2 does not intersect
with the semicircle R3 or R4. Accordingly, the guided wave does not
become radiating light and thus is propagated through the grading
element 36 without lose.
[0130] The reflection of the returning light at the incident
surface 39G of the optical waveguide device 10G (FIG. 11) can also
be reduced by using the grating element 36 having a periodicity
fulfilling expression (5).
[0131] In the above examples, an SHG element is used as the
dielectric substrate in the optical waveguide device. An optical
waveguide device according to the present invention is not limited
to such a structure. Waveguide devices having various functions and
structures, such as, for example, a high-speed modulation element,
a phase shifter, a frequency shifter and a polarization control
element are usable according to the present invention. An optical
waveguide device according to the present invention is usable in
any optical system using a coherent light source.
[0132] In the case where an optical waveguide device according to
the present invention used in a light source device, the optical
waveguide device is especially effective when including an SHG
element for the following reason. When the SHG element is included
in the optical waveguide device, the light source device often
uses, as a light source, a semiconductor device having a high
interference possibility to generate harmonic wave having a high
interference possibility. As an interference noise is thus
generated easily, the optical waveguide device according to the
present invention is more effective.
[0133] In the above examples, an optical apparatus including a
pickup optical system is used as the confocal optical system is
described. The present invention is applicable to other coherent
optical systems such as, for example, a laser scanning microscope
or a laser printer.
[0134] An optical waveguide device according to the present
invention is especially effective for an optical pickup including a
pickup optical system. The reasons for this is that an optical disk
from which the data is detected by the pickup optical system has a
high reflectance, that an objective lens is positionally controlled
so as to collect light to the optical disk to maintain a confocal
optical system, and that an upward and downward movement of the
optical disk causes the interference conditions to be changed
continuously and thus the interference noise is easily
generated.
[0135] As described above, an optical waveguide device according to
the present invention has an outgoing surface formed oblique to an
optical waveguide. Due to such a structure, light returning from an
optical system external to the optical waveguide device such as,
for example, a pickup optical system is reflected by the outgoing
surface in such a manner to avoid interference of the reflected
light and the outgoing light. Thus, a stable light source device
with substantially no interference noise is provided.
[0136] In an embodiment where the incident surface is also oblique
with respect to the optical waveguide, the light returning from the
optical system external to the optical waveguide device is
prevented from being reflected by the incident surface, and also
light returning to a semiconductor laser is suppressed.
[0137] Various other modifications will be apparent to and can be
readily made by those skilled in the art without departing from the
scope and spirit of this invention. Accordingly, it is not intended
that the scope of the claims appended hereto be limited to the
description as set forth herein, but rather that the claims be
broadly construed.
* * * * *