U.S. patent application number 09/750281 was filed with the patent office on 2002-07-04 for optical multiplexer/demultiplexer.
This patent application is currently assigned to Sumitomo Electric Industries, Ltd. Invention is credited to Katayama, Makoto, Nishimura, Masayuki, Tanaka, Shigeru.
Application Number | 20020085800 09/750281 |
Document ID | / |
Family ID | 25017202 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020085800 |
Kind Code |
A1 |
Katayama, Makoto ; et
al. |
July 4, 2002 |
OPTICAL MULTIPLEXER/DEMULTIPLEXER
Abstract
The present invention relates to an optical
multiplexer/demultiplexer which ameliorates the deterioration in
crosstalk characteristics between adjacent signal channels in a
simpler configuration with a better reproducibility. In this
optical multiplexer/demultiplexer, a space filled with cladding
glass having a size which is at least three times the width or
thickness of each channel waveguide is provided between the slab
and channel waveguides. This configuration effectively improves the
deterioration in crosstalk between adjacent signal channels on a
par with the case where the slab and channel waveguides are
directly connected to each other.
Inventors: |
Katayama, Makoto; (Kanagawa,
JP) ; Nishimura, Masayuki; (Kanagawa, JP) ;
Tanaka, Shigeru; (Kanagawa, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
Sumitomo Electric Industries,
Ltd
Osaka-Shi
JP
|
Family ID: |
25017202 |
Appl. No.: |
09/750281 |
Filed: |
December 29, 2000 |
Current U.S.
Class: |
385/24 ;
385/37 |
Current CPC
Class: |
G02B 6/12009
20130101 |
Class at
Publication: |
385/24 ;
385/37 |
International
Class: |
G02B 006/293 |
Claims
What is claimed is:
1. An optical multiplexer/demultiplexer comprising: a substrate;
first and second slab waveguides, each having a predetermined slab
length, disposed on said substrate; at least one input waveguide,
disposed on said substrate, having an optical output end optically
connected to an optical input end face of said first slab
waveguide; a plurality of output waveguides two-dimensionally
arranged on said substrate while in a state where respective
optical input ends thereof are optically connected to an optical
output end face of said second slab waveguide, said output
waveguides being provided so as to correspond to respective signals
having channel wavelengths set as signal channels at a
predetermined wavelength interval; and a plurality of channel
waveguides two-dimensionally arranged on said substrate while in a
state where an optical input end of each channel waveguide is
optically connected to an optical output end face of said first
slab waveguide so as to sandwich said first slab waveguide together
with said input waveguide whereas an optical output end of each
channel waveguide is optically connected to an optical input end
face of said second slab waveguide so as to sandwich said second
slab waveguide together with said output waveguides, said channel
waveguides having respective lengths different from each other;
wherein each of said channel waveguides is arranged on said
substrate while in a state where a space between said optical
output end face of said first slab waveguide and said optical input
end of said channel waveguide is at least three times a width or
thickness of each of said channel waveguides so as to ameliorate
deterioration in a crosstalk characteristic between adjacent signal
channels caused upon separating said channel waveguide and said
first slab waveguide from each other.
2. An optical multiplexer/demultiplexer according to claim 1,
wherein at least said channel waveguides have a relative refractive
index difference of 0.75% or more with respect to said
substrate.
3. An optical multiplexer/demultiplexer according to claim 1,
wherein said channel waveguides are arranged such that the
respective optical input ends thereof oppose said optical output
end face of said first slab waveguide over 90% or more of the area
of said optical output end face in a direction perpendicular to
said substrate.
4. An optical multiplexer/demultiplexer according to claim 1,
wherein each of said channel waveguides is disposed on said
substrate while in a state where said space between said optical
output end face of said first slab waveguide and said optical input
end of said channel waveguide is 2M or more but 6M or less, where M
is a mode field diameter of light propagating through said channel
waveguide.
5. An optical multiplexer/demultiplexer according to claim 1,
wherein said input waveguide is arranged on said substrate such
that said optical output end thereof is spaced from said optical
input end face of said first slab waveguide by 1/2 or more of a
thickness of said input waveguide.
6. An optical multiplexer/demultiplexer according to claim 1,
wherein said channel waveguides are arranged at an interval of 15
.mu.m or less.
7. An optical multiplexer/demultiplexer according to claim 1,
wherein said output waveguides are arranged at an interval of 20
.mu.m or less.
8. An optical multiplexer/demultiplexer according to claim 1,
wherein each of said first and second slab waveguides has a slab
length of 15 mm or less.
9. An optical multiplexer/demultiplexer according to claim 1,
wherein said optical multiplexer/demultiplexer comprises thirty or
more output waveguides.
10. An optical multiplexer/demultiplexer according to claim 1,
wherein said channel wavelength interval is 100 GHz or less.
11. An optical multiplexer/demultiplexer comprising: a substrate;
first and second slab waveguides, each having a predetermined slab
length, disposed on said substrate; at least one input waveguide,
disposed on said substrate, having an optical output end optically
connected to an optical input end face of said first slab
waveguide; a plurality of output waveguides two-dimensionally
arranged on said substrate while in a state where respective
optical input ends thereof are optically connected to an optical
output end face of said second slab waveguide, said output
waveguides being provided so as to correspond to respective signals
having channel wavelengths set as signal channels at a
predetermined wavelength interval; and a plurality of channel
waveguides two-dimensionally arranged on said substrate while in a
state where an optical input end of each channel waveguide is
optically connected to an optical output end face of said first
slab waveguide so as to sandwich said first slab waveguide together
with said input waveguide whereas an optical output end of each
channel waveguide is optically connected to an optical input end
face of said second slab waveguide so as to sandwich said second
slab waveguide together with said output waveguides, said channel
waveguides having respective lengths different from each other;
wherein each of said channel waveguides is arranged on said
substrate while in a state where a space between said optical input
end face of said second slab waveguide and said optical output end
of said channel waveguide is at least three times a width or
thickness of each of said channel waveguides so as to ameliorate
deterioration in a crosstalk characteristic between adjacent signal
channels caused upon separating said channel waveguide and said
second slab waveguide from each other.
12. An optical multiplexer/demultiplexer according to claim 11,
wherein at least said channel waveguides have a relative refractive
index difference of 0.75% or more with respect to said
substrate.
13. An optical multiplexer/demultiplexer according to claim 11,
wherein said channel waveguides are arranged such that the
respective optical output ends thereof oppose said optical input
end face of said second slab waveguide over 90% or more of the area
of said optical output end face in a direction perpendicular to
said substrate.
14. An optical multiplexer/demultiplexer according to claim 11,
wherein each of said channel waveguides is disposed on said
substrate while in a state where said space between said optical
output end face of said second slab waveguide and said optical
input end of said channel waveguide is 2M or more but 6M or less,
where M is a mode field diameter of light propagating through said
channel waveguide.
15. An optical multiplexer/demultiplexer according to claim 11,
wherein each said output waveguide is arranged on said substrate
such that said optical input end thereof is spaced from said
optical output end face of said second slab waveguide by 1/2 or
more of a thickness of said output waveguide.
16. An optical multiplexer/demultiplexer according to claim 11,
wherein said channel waveguides are arranged at an interval of 15
.mu.m or less.
17. An optical multiplexer/demultiplexer according to claim 11,
wherein said output waveguides are arranged at an interval of 20
.mu.m or less.
18. An optical multiplexer/demultiplexer according to claim 11,
wherein each of said first and second slab waveguides has a slab
length of 15 mm or less.
19. An optical multiplexer/demultiplexer according to claim 11,
wherein said optical multiplexer/demultiplexer comprises thirty or
more output waveguides.
20. An optical multiplexer/demultiplexer according to claim 11,
wherein said channel wavelength interval is 100 GHz or less.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an arrayed waveguide
grating (AWG) type optical multiplexer/demultiplexer which is
employable as a wavelength-selecting device in a wavelength
division multiplexing (WDM) transmission system.
[0003] 2. Related Background Art
[0004] AWG type optical multiplexer/demultiplexers (hereinafter
referred to as AWG circuits) are widely in use as a wavelength
filter, which can take out or insert a specific wavelength upon
interference, for a wavelength-selecting device in WDM transmission
systems. Also, since the AWG circuits can be realized by general
fine processing procedures such as lithography or etching without
necessitating the machining as precise as that of diffraction
gratings or the forming of multilayer films as precise as that of
interference films, they are expected to develop as a main optical
device in future WDM transmission systems together with their
capability of assembling with other optical waveguide devices.
[0005] Such an AWG circuit has a structure in which an input
waveguide, an input slab waveguide, a plurality of channel
waveguides having respective lengths different from each other
(phased array), an output slab waveguide, and an output waveguide
are integrally formed on a single substrate and are covered with
cladding glass. For lowering loss in a conventional AWG circuit, in
particular, it is necessary for the channel waveguides to be
processed such that each has a rectangular cross-sectional
structure, and that they are disposed closer to each other. Between
adjacent waveguides in which the waveguides are disposed closer to
each other as in a connecting portion between the slab and channel
waveguides, however, a void may occur without being filled with
cladding glass, whereby the AWG circuit may not be made with a
favorable reproducibility as designed.
[0006] In order to prevent the incomplete filling of cladding glass
from occurring as mentioned above and yield an AWG circuit which is
easy to make with a favorable reproducibility, Japanese Patent
Application Laid-Open No. HEI 7-63934 discloses a structure in
which adjacent waveguides such as those between slab and channel
waveguides are separated from each other by about 1 to 10 .mu.m,
whereas thus formed gap is filled with cladding glass.
SUMMARY OF THE INVENTION
[0007] The inventors have studied the conventional AWG circuits
and, as a result, have found a problem as follows.
[0008] In the AWG circuit disclosed in Japanese Patent Application
Laid-Open No. HEI 7-63934, adjacent waveguides are separated from
each other by about 1 to 10 .mu.m in order to suppress the excess
loss caused by diffraction to a maximum of 0.1 dB which is fully
negligible. However, the conventional AWG circuits have been
problematic in that crosstalk characteristics between adjacent
signal channels remarkably deteriorate even when the space between
the adjacent waveguides is set such that the excess loss caused
upon filling with cladding glass can be suppressed to a fully
negligible level.
[0009] In order to overcome the problem mentioned above, it is an
object of the present invention to provide an optical
multiplexer/demultiplexer which ameliorates the deterioration in
crosstalk characteristics between adjacent channels in a simpler
configuration with a better reproducibility.
[0010] The optical multiplexer/demultiplexer according to the
present invention is an AWG type optical multiplexer/demultiplexer,
employable as a wavelength-selecting device in a WDM transmission
system, comprising a substrate, and at least one input waveguide, a
first slab waveguide, a plurality of channel waveguides, a second
slab waveguide, and a plurality of output waveguides provided for
respective signal channels, which are disposed on the
substrate.
[0011] In the optical multiplexer/demultiplexer according to the
present invention, the first and second slab waveguides have
respective predetermined slab lengths. In general, a slab length
corresponds to the focal length of the optical input end
functioning as the lens surface of the respective slab waveguide.
The input waveguide is a waveguide for guiding to the first slab
waveguide individual signals having respective channel wavelengths
set at predetermined wavelength intervals as signal channels, and
has an output end optically connected to an optical input end face
of the first slab waveguide. The plurality of channel waveguides
are waveguides having lengths different from each other, and are
two-dimensionally arranged on the substrate while in a state where
an optical input end of each channel waveguide is optically
connected to an optical output end face of the first slab waveguide
so as to sandwich the first slab waveguide together with the input
waveguide whereas an optical output end of each channel waveguide
is optically connected to an optical input end face of the second
slab waveguide so as to sandwich the second slab waveguide together
with the output waveguides. The output waveguides are waveguides
two-dimensionally arranged on the substrate while in a state where
respective optical input ends thereof are optically connected to an
optical output end face of the second slab waveguide, and are used
for separately taking out signals having respective channel
wavelengths set at predetermined wavelength intervals.
[0012] In particular, the inventors have found the fact that the
deterioration in crosstalk between adjacent signal channels is
remarkably ameliorated when adjacent waveguides are separated from
each other by a predetermined value or more in a portion where the
waveguides are disposed closer to each other, e.g., between the
slab and channel waveguides, thereby accomplishing the optical
multiplexer/demultiplexer according to the present invention.
[0013] Namely, in order to ameliorate the deterioration in
crosstalk between adjacent signal channels caused upon separating
adjacent waveguides from each other, at least one of the space
between the optical input end of each channel waveguide and the
optical output end face of the first slab waveguide, and the space
between the optical output end of each channel waveguide and the
optical input end face of the second slab waveguide is set to at
least three times the width or thickness of each channel waveguide
in the optical multiplexer/demultiplexer according to the present
invention.
[0014] Preferably, at least the channel waveguides have a relative
refractive index difference of 0.75% or more with respect to the
substrate. It is because of the fact that if the relative
refractive index difference of the channel waveguides with respect
to the substrate is made greater, then the light confining effect
improves, whereby the channel waveguide intervals can be set
narrower.
[0015] Preferably, in the optical multiplexer/demultiplexer
according to the present invention, the channel waveguides are
arranged such that the respective optical input ends thereof oppose
the optical output end face of the first slab waveguide over 90% or
more of the area of the optical output end face in a direction
perpendicular to the substrate. Preferably, the channel waveguides
are arranged such that the respective optical output ends thereof
oppose the optical input end face of the second slab waveguide over
90% or more of the area of optical input end face in a direction
perpendicular to the substrate on the second slab waveguide side as
well. It is because of the fact that a greater light capturing
angle is further effective in ameliorating the crosstalk between
adjacent signal channels.
[0016] The gap between the adjacent waveguides (e.g., slab and
channel waveguides) somewhat fluctuates depending on the relative
refractive index differences of these waveguides with respect to
the substrate. Therefore, in order to ameliorate the deterioration
in crosstalk between adjacent signal channels caused upon
separating the adjacent waveguides from each other in the optical
multiplexer/demultiplexer according to the present invention, it is
further preferred that at least one of the space between the
optical input end of each channel waveguide and the optical output
end face of the first slab waveguide, and the space between the
optical output end of each channel waveguide and the optical input
end face of the second slab waveguide be set to 2M or more but 6M
or less, where M is the mode field diameter of light propagating
through the channel waveguide.
[0017] In order to improve the effect of buried cladding glass and
ameliorate the crosstalk deterioration between adjacent signal
channels in the optical multiplexer/demultiplexer according to the
present invention, each input waveguide is preferably arranged on
the substrate such that the optical output end thereof is separated
from the optical input end face of the first slab waveguide by 1/2
or more of the thickness of the input waveguide. Similarly, each of
the output waveguides is preferably arranged on the substrate such
that the optical input end thereof is separated from the optical
output end face of the second slab waveguide by 1/2 or more of the
thickness of the output waveguide.
[0018] In a specific embodiment, the optical
multiplexer/demultiplexer according to the present invention
comprises thirty or more output waveguides (i.e., thirty or more
signal channels to be multi/demultiplexed), and makes it possible
to multi/demultiplex signal channels having a wavelength interval
of 100 GHz or less. Therefore, in the optical
multiplexer/demultiplexer, it is preferred that the channel
waveguides be arranged at an interval of 15 .mu.m or less.
Preferably, the output waveguides are arranged at an interval of 20
.mu.m or less. Preferably, each of the first and second slab
waveguides is designed to have a slab length of 15 mm or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a plan view showing the schematic configuration of
the optical multiplexer/demultiplexer according to the present
invention;
[0020] FIG. 2 is a view showing the cross-sectional structure of
the optical multiplexer/demultiplexer taken along the line I-I
shown in FIG. 1;
[0021] FIG. 3 is a plan view for schematically explaining the
waveguide structure of a sample manufactured as an embodiment of
the optical multiplexer/demultiplexer according to the present
invention;
[0022] FIG. 4 is a graph showing results of measurement of
crosstalk concerning the sample manufactured as an embodiment of
the optical multiplexer/demultiplexer according to the present
invention at its center channel (CH20) when the gap x between slab
and channel waveguides is changed; and
[0023] FIG. 5 is a graph showing results of measurement of
insertion loss concerning the sample manufactured as an embodiment
of the optical multiplexer/demultiplexer according to the present
invention at its center channel (CH20) when the gap x between slab
and channel waveguides is changed.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] In the following, embodiments of the optical
multiplexer/demultiplexer according to the present invention will
be explained in detail with reference to FIGS. 1 to 5. Among the
drawings, parts identical to each other will be referred to with
numerals identical to each other without repeating their
overlapping explanations.
[0025] FIG. 1 is a plan view showing the configuration of an AWG
circuit as the optical multiplexer/demultiplexer according to the
present invention. As depicted, this optical
multiplexer/demultiplexer is an optical component in which optical
waveguide parts are integrally formed on a silica glass substrate
100. Namely, at least one input waveguide 110, a first slab
waveguide 120 (input slab waveguide), a plurality of channel
waveguides 130, a second waveguide 140 (output slab waveguide), and
output waveguides 150 corresponding to respective signal channels
CH1, CH2, . . . , CH39, and CH40 are disposed on the substrate
100.
[0026] Each of the waveguide parts is doped with GeO.sub.2, whereas
the doping amount of GeO.sub.2 is such that the relative refractive
index difference between the substrate 100 and the waveguide parts
is 0.75% or more in order to make it possible to lower the radius
of curvature of channel waveguides 130 (improve the light
confinement efficiency). The substrate 100 is not restricted to the
silica glass substrate, and may be constituted by a silicon
substrate and a glass layer having a thickness of ten to several
tens of micrometers formed on the silicon substrate. Similar
operations and effects are also obtained when waveguides doped with
GeO.sub.2 are formed on this glass layer. FIG. 2 is a view showing
the cross-sectional structure of AWG circuit taken along the line
I-I of FIG. 1, in which a core 101 (having a width W and a
thickness (height) H) to become a waveguide and a cladding 102
covering the core 101 are disposed on the substrate 100.
[0027] The first slab waveguide 120 has a flat optical output end
face 120a, disposed at an angle .theta. with respect to the
incident angle of light fed to the optical
multiplexer/demultiplexer, to which the optical input ends of
channel waveguides 130 are optically connected; and an optical
input end face 120b to which the optical output end of input
waveguide 110 is optically connected. The second slab waveguide 140
has a flat optical input end face 140a to which the optical output
ends of channel waveguides 130 are optically connected, and an
optical output end face 140b to which the optical input ends of
output waveguides 150 are optically connected. Each of the first
and second slab waveguides 120, 140 has a slab length f. Here, the
slab length corresponds to the focal length of the convex lens
surface located at the optical input end face in each of the first
and second slab waveguides 120, 140.
[0028] The input waveguide 110 is a waveguide for guiding to the
first slab waveguide 120 individual signals having respective
channel wavelengths which are set at predetermined wavelength
intervals as signal channels, and has an output end optically
connected to the optical input end face 120b of first slab
waveguide 120. The channel waveguides 130 are waveguides having
respective lengths different from each other, and are
two-dimensionally arranged on the substrate 100. The channel
waveguides 130 are optically connected to the optical output end
face 120a of first slab waveguide 120 so as to sandwich the first
slab waveguide 120 together with the input waveguide 110, and are
optically connected to the optical input end face 140a of second
slab waveguide 140 so as to sandwich the second slab waveguide 140
together with the output waveguides 150. The output waveguides 150
are waveguides two-dimensionally arranged on the substrate 100
while in a state where respective optical input ends are optically
connected to the optical output end face 140b of second slab
waveguide 140, so as to correspond to individual signals having
respective channel wavelengths set at predetermined wavelength
intervals, i.e., so as to correspond to the respective signal
channels.
[0029] Though the optical multiplexer/demultiplexer shown in FIG. 1
is explained as an AWG circuit, in which light successively
propagates through the input waveguide 110, first slab waveguide
120, channel waveguides 130, second slab waveguide 140, and output
waveguides 150, enabling 40 channels of signals to be separated
from each other, a plurality of input waveguides may be provided so
as to correspond to the respective signal channels, thereby
realizing an AWG circuit which enables wavelength multiplexing.
[0030] In particular, the inventors have found the fact that the
deterioration in crosstalk between adjacent signal channels is
remarkably ameliorated when adjacent waveguides are separated from
each other by a predetermined value or more in a portion where the
waveguides are disposed closer to each other, e.g., between the
slab and channel waveguides, thereby accomplishing the optical
multiplexer/demultiplexer according to the present invention.
[0031] Therefore, in order to effectively ameliorate the
deterioration in crosstalk between adjacent signal channels caused
upon separating adjacent waveguides such as the slab waveguide 140
and the channel waveguides 130 from each other, at least one of the
space between the optical input ends of channel waveguides 130 and
the optical output end face 120a of first slab waveguide 120, and
the space between the optical output ends of channel waveguides 130
and the optical input end face 140a of second slab waveguide 140 is
set to at least three times the width or thickness of each channel
waveguide (so as to yield a space x shown in FIG. 1) in the optical
multiplexer/demultiplexer according to the present invention. Here,
the gap between the adjacent waveguides somewhat fluctuates
depending on the relative refractive index differences of these
waveguides with respect to the substrate 100. Therefore, at least
one of the space between the optical input ends of channel
waveguides 130 and the optical output end face 120a of first slab
waveguide 120, and the space between the optical output ends of
channel waveguides 130 and the optical input end face 140a of
second slab waveguide 140 in the optical multiplexer/demultiplexer
according to the present invention is set to 2M or more but 6M or
less, where M is the mode field diameter of light propagating
through the channel waveguides 130.
[0032] In the optical multiplexer/demultiplexer according to the
present invention, each of the waveguides including the channel
waveguides 130 has a relative refractive index difference of 0.75%
or more with respect to the substrate 100. It is because of the
fact that if the relative refractive index difference of each of
the waveguides 110 to 150 is made greater, then the light confining
effect improves, whereby the waveguide intervals can be set
narrower.
[0033] Preferably, in the optical multiplexer/demultiplexer
according to the present invention, the channel waveguides 130 are
arranged such that the respective optical input ends thereof oppose
the optical output end face 120a of first slab waveguide 120 over
90% or more of the area of optical output end face 120a in a
direction perpendicular to the substrate 100. Also, the channel
waveguides 130 are arranged such that the respective optical output
ends thereof oppose the optical input end face 140a of second slab
waveguide 140 over 90% or more of the area of optical input end
face 140a in a direction perpendicular to the substrate on the
second slab waveguide 140 side. It is because of the fact that a
greater light capturing angle is further effective in ameliorating
the crosstalk between adjacent signal channels.
[0034] In order to improve the effect of buried cladding glass
(corresponding to the cladding 102 in FIG. 2) and ameliorate the
crosstalk deterioration between adjacent signal channels in the
optical multiplexer/demultiplexer according to the present
invention, the input waveguide 110 is arranged on the substrate 100
such that the optical output end thereof is separated from the
optical input end face 120b of first slab waveguide 120 by 1/2 or
more of the thickness of input waveguide 110. Similarly, each of
the output waveguides 150 is preferably arranged on the substrate
100 such that the optical input end thereof is separated from the
optical output end face 140b of second slab waveguide 140 by 1/2 or
more of the thickness of output waveguide 150 (so as to yield a gap
y in FIG. 1).
[0035] Preferably, as a specific mode in practical use, the optical
multiplexer/demultiplexer is an optical device comprising thirty or
more output waveguides 150 (i.e., having thirty or more signal
channels CH to be multi/demultiplexed) and enabling signal channels
having a wavelength interval of 100 GHz or less to be
multi/demultiplexed. Here, in the optical
multiplexer/demultiplexer, the channel waveguides 130 are arranged
at an interval d1 of 15 .mu.m or less, the output waveguides 150
are arranged at an interval d2 of 20 .mu.m or less, and each of the
first and second slab waveguides 120, 140 has a slab length f of 15
mm or less.
[0036] The inventors designed an AWG circuit capable of separating
40 channels of signals having a signal wavelength interval
.DELTA..lambda. of 100 GHz and a center channel wavelength (CH20)
of 1550.918 nm so as to satisfy the above-mentioned specific
mode.
[0037] In thus designed AWG circuit, the relative refractive index
difference between the substrate 100 and each of the waveguide
parts 110 to 150 is 1.5%, whereas each of the waveguides 110, 130,
150 has a core width W of 4.3 .mu.m and a core thickness H of 4.3
.mu.m. In this case, the mode field diameter is 5.5 .mu.m. Each of
the first and second slab waveguides 120, 140 has a slab length f
of 4800 .mu.m, the substrate 100 has a size of 20 mm.times.20 mm
with a thickness of 0.5 mm, the channel waveguides 130 have an
interval d1 of 6.0 .mu.m, the number of channel waveguides 130 is
80, the installation angle .theta. of second slab waveguide 120 is
80 degrees, and the output waveguides 150 have an interval d2 of 15
.mu.m. The individual channel waveguides 130 have an optical path
length difference .DELTA.L of 36.702 .mu.m and a radius of
curvature of 2 mm.
[0038] In the sample designed as the AWG circuit according to the
present invention, as mentioned above, each of the space between
the optical output end face 120a of first slab waveguide 120 and
the optical input ends of channel waveguides 130, and the space
between the optical output end face 140a of second slab waveguide
140 and the optical output ends of channel waveguides 130 has at
least a predetermined length x (.mu.m). Also, the channel
waveguides 130 are arranged such that the respective optical input
ends thereof oppose the optical output end face 120a of first slab
waveguide 120 over 90% or more of the area of output end face 120a
in a direction perpendicular to the substrate 100, whereas the
respective optical output ends thereof oppose the optical input end
face 140a of second slab waveguide 140 over 90% or more of the area
of the optical input end face 140a in a direction perpendicular to
the substrate on the second slab waveguide 140 side.
[0039] The inventors measured the change in crosstalk and the
insertion loss concerning the above-mentioned sample centered at
the channel CH20 when the gap x between each of the first and
second slab waveguides 120, 140 and the channel waveguides 130 was
changed. FIG. 4 is a graph showing the results of measurement of
crosstalk concerning the above-mentioned sample at the channel CH20
when the gap x was changed. FIG. 5 is a graph showing the results
of measurement of insertion loss concerning the above-mentioned
sample at the channel CH20 when the gap x was changed.
[0040] As can be seen from FIG. 4, the crosstalk between adjacent
signal channels once increases as the gap x is made greater and
then, at a predetermined gap or greater, decreases to a level on a
par with that obtained when the slab waveguides 120, 140 are
directly connected to the channel waveguides 130 (x=0 .mu.m). The
same tendency can hold for the insertion loss.
[0041] Though this phenomenon cannot be explained definitely, it
may be presumed as a hypothesis that the probability of a part of
the light propagated as a core mode through a channel waveguide
directly connected to the slab waveguides 120, 140 (x=0 .mu.m)
propagating as a core mode of an adjacent channel waveguide after
propagating through a cladding layer between the slab and channel
waveguides increases in region A where the gap x between the slab
and channel waveguides is relatively small, thereby deteriorating
crosstalk characteristics. If the gap x between the slab and
channel waveguides exceeds a certain value (region B in FIG. 4), by
contrast, then a part of light propagated as a core mode through
the channel waveguide propagates through a cladding layer between
the slab and channel waveguides over a considerably long distance,
thereby remarkably increasing the probability of a part thereof
becoming a radiation mode. It is presumed that, as a result, the
probability of light propagating as a core mode of an adjacent
channel waveguide decreases, whereby crosstalk characteristics
improve.
[0042] In accordance with the foregoing studies, in order to
ameliorate the deterioration in crosstalk between adjacent signal
channels caused upon separating adjacent waveguides from each
other, at least one of the space between the optical input ends of
channel waveguides 130 and the optical output end face 120a of
first slab waveguide 120, and the space between the optical output
ends of channel waveguides 130 and the optical output end face 140a
of second slab waveguide 140 is set to at least three times the
width or thickness of each of the channel waveguides 130 in the
optical multiplexer/demultiplexer according to the present
invention. In other words, at least one of the space between the
optical input ends of channel waveguides 130 and the optical output
end face 120a of first slab waveguide 120, and the space between
the optical output ends of channel waveguides 130 and the optical
input end face 140a of second slab waveguide 140 is set to 2M or
more but 6M or less, where M is the mode field diameter of light
propagating through the channel waveguides 130.
[0043] Though the above-mentioned sample illustrates a
configuration in which the first and second slab waveguides 120,
140 and the channel waveguides 130 are separated from each other by
a predetermined distance, the input waveguide 110 and first slab
waveguide 120, and the output waveguides 150 and second slab
waveguide 140 may also be separated from each other by way of
cladding glass in order to further improve the effect of buried
cladding.
[0044] In the present invention, as in the foregoing, a space
filled with cladding glass having a size which is at least three
times the width or thickness of each channel waveguide is provided
between each of the first and second slab waveguides and the
channel waveguides. The present invention is accomplished by the
fact found by the inventors, which has not been attainable from the
prior art at all, and is effective in that the deterioration in
crosstalk between adjacent signal channels can effectively be
improved on a par with the case where each slab waveguide and
channel waveguides are directly connected to each other in a
simpler configuration having a better reproducibility.
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