Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL MULTIPLEXERIDEMULTIPLEXER
BACKGROUND OF THE INVENTION
The present invention relates to an optical
mi~.ple~r/da~ult'Lfied ' to multiplex or demultiplex
wavelength division multiple signals, and more ,
particularly, an optical ~~~'/d~-~
reducing insertion loss and crosstalk.
In the field of optical communication, a
wavelength division multiplexing transmission system is
examined such that light beams of different wavelengths
are loaded individually with a plurality of signals and
information capacity is enlarged by transmitting the
signals by means of one optical fiber. In this
transmission system, optical multiplexers/demultiplexers for
multiplexing or demultiplexing light beams of different
wavelengths play an important role. Among various
other optical multiplexers/demultiplexers, an optical
m~ltip7~r/dan~ltip7~r that uses an arrayed-waveguide
grating (AWG) holds promise, since it can increase
the frequency of multiplexing with short wavelength
intervals.
In one such optical maltiplex?x/dan~1_t,~ r usc~ in
the wavelength division multiplexing transmission
system, it is essential to reduce loss in a wavelength
passband in consideration of the wavelength control
tolerance of a semiconductor Laser beam source, gain
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characteristics of an optical fiber amplifier,
wavelength characteristics of a dispersion compensating
fiber, etc. It is also important to ensure sharp
rising and falling edges in the passband.
Conventionally, there is a proposal to taper an end
portion of an arrayed waveguide in order to reduce loss
in the wavelength passband. ~' e~ple, such an arrayed
waveguide has a tapered end portion on the interface between
an input slab waveguide and the arrayed waveguide.
According to this prior art structure in which the
end portion of the arrayed waveguide is tapered,
however, a loss is caused by the difference between the
respective native modes of the slab waveguide and the
arrayed waveguide, so that reduction of loss is
limited.
It is found that crosstalk can be reduced by
maximizing the width of the slab waveguide and
increasing the number of channel waveguides of the
arrayed waveguide that are connected to the slab
waveguide. If the channel waveguides of the arrayed
waveguide are increased in number, however, they are
easily influenced by the refractive index distribution
and fluctuations of the channel waveguide width. This
leads to adverse results including an increase in loss
and a worsened crosstalk level.
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Crosstalks are calculated in the following manner.
FIG. 22 shows a wavelength characteristic of
a channel No. S out of eight channels of an AWG of
100 GHz as an example. The criterion for the
calculation of crosstalks is not 0 (zero) dB but
insertion loss for the center wavelength.
For example, a in FIG. 22 indicates insertion loss
for the center wavelength of the channel No. 5.
Further, b indicates a crosstalk between channels No. 5
and No. 6; c, crosstalk between channels No. 5 and
No. 4; d, crosstalk between channels No. 5 and No. 7;
e, crosstalk between channels No. 5 and No. 8; f,
crosstalk between channels No. 5 and No. 3;
crosstalk between channels No. 5 and No. 2; and h,
crosstalk between channels No. 5 and No. 1.
The average of all the crosstalks in the channel
No. 5 can be given by (b + c + d + a + f + ~ + h) /7.
In this specification, the value calculated in this
manner is referred to as crosstalk.
As a power splitter for splitting signal light, on
the other hand, a splitter that combines a slab
waveguide and channel waveguides is proposed in place
of a conventional splitter that is composed of
multilayered Y-branches. "An integrated power splitter
with ultra-low loss" (Integrated Photonics Research
1999, Santa Barbara, CA, July 19-21, 1999, pp. 141-143)
is reported as an example of the proposed splitter.
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In this splitter, a semiconductor with a refractive
index of 3.0 or more is used as its material, and a
high-refraction region with a refractive index higher
than that of a core layer is provided in the slab
waveguide.
Since this splitter uses a rib waveguide with a
core width of 1 ~ m as its output waveguide, however,
its mode diameter is as small as about 1 a m. Since
the mode diameter of an ordinary optical fiber ranges
from 9 to 10 ~ m, on the other hand, the mode mismatch
(connection loss) in the output waveguide portion is
substantial. It is feared that this difference in mode
diameter should entail a loss of 13 dB or more. Thus,
the loss of the whole splitter, including the
splitter's own loss of 6 to 7 dB, inevitably amounts to
about 20 dB, a substantial loss.
In addition, the aforesaid high-refraction region
in the slab waveguide measures only 2.5 ~ m by 0,9 a m.
Thus, the individual parts have very fine dimensions,
and the layer structure is complicated, so that the
manufacture of the splitter is subject to variation.
Thus, the quality of the splitter lacks stability and
reproducibility. It is hard, therefore, to improve
insertion loss or the like remarkably by means of a
splitter of this type.
SUMMARY OF THE INVENTION
The object of the present invention is to provide
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an optical multiplexer/demultiplexer enjoying lowered coupling loss and
improved crosstalk that is easy to manufacture.
Accordingly, the present invention provides an optical
multiplexerfderrultiplexer comprising:
an input waveguide to which wavelength division
multiple signals are applied
a plurality of output waveguides for
demultiplexing and outputting the wavelength division
multiple signals;
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an arrayed waveguide provided between the input
waveguide and the output waveguides and including a
plurality of channel waveguides having different
waveguide lengths;
an input slab waveguide formed between the input
waveguide and the arrayed waveguide, said input slab
waveguide including a core layer and a clad layer;
an output slab waveguide formed between the
arrayed waveguide and the output waveguides, said
output slab waveguide including a core layer and a clad
layer, and
two or more island regions having a refractive
index different from that of the core layers of the
input and output slab waveguides are provided in at
least one of the slab waveguides and situated in
positions associated with the channel waveguides of the
arrayed waveguide, each of said island region being tapered so that a width
thereof decreases toward the arrayed waveguide.
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An optical rtultiplexer/demultiplexer of the present
invention therefore crises an input waveguide to which
wavelength division multiple signals are applied;
a plurality of output waveguides for demultiplexing and
outputting the wavelength division multiple signals: an
arrayed waveguide provided between the input waveguide
and the output waveguides and including a plurality of
channel waveguides having different waveguide lengths;
an input slab waveguide formed between the input
waveguide and the arrayed waveguide: an output slab
waveguide formed between the arrayed waveguide and the
output waveguides; and two or more island regions
having a refractive index different from that of core
layers of the input and output slab waveguides,
provided in at least one of the slab waveguides, and
situated in positions associated with the channel
waveguides of the arrayed waveguide.
According to this invention, the island regions
are formed in the slab waveguide, whereby field
distribution at the end point of the input slab
waveguide can be approximated to field distribution in
the arrayed waveguide, so that a low-loss version of
the optical multiplexer/demultiplexer can be obtained.
In the present invention, the refractive index of
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the island regions is lower than that of the core layer
around the island regions, for example. According to
this invention, light that passes through the island
regions can be concentrated on the channel waveguides
of the arrayed waveguide.
Preferably, in the present invention, the island
regions are formed integrally with a clad layer of the
slab waveguide. According to this invention, the
refractive index of the island regions can be made
equal to that of the clad layer. In designing mask
patterns in the process of manufacturing the optical
miltiplexer/demultiplexer,therefore, the island regions can
be formed integrally with the clad layer in a given
position in the slab waveguide by only forming patterns
corresponding to the island regions. Thus, there is no
necessity of changing or adding manufacturing processes
despite the presence of the island regions.
In the case where the refractive index of the
island regions is lower than that of the core layer
around the island regions, the island regions are
preferably located between axes connecting the input
waveguide or the output waveguides and the channel
waveguides of the arrayed waveguide. According to this
invention, the island regions axe provided individually
between the axes that connect the input waveguide or
output waveguides and the channel waveguides of the
arrayed waveguide, so that the light that passes
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through the island regions can be concentrated on the
channel waveguides of the arrayed waveguide.
In the case where the refractive index of the
island regions is higher than that of the core layer
around the island regions, a desired result may
possibly be obtained by forming the island regions on
the axes individually.
Preferably, in the present invention, each of the
island regions is tapered so that the width thereof
decreases toward the arrayed waveguide. If the island
regions are tapered in this manner, the intensity
distribution in the slab waveguide can be controlled,
so that the efficiency of coupling to the arrayed
waveguide can be improved. According to this
invention, the degree of concentration of light on the
channel waveguides of the arrayed waveguide can be
further improved.
Preferably, in the present invention, the width of
that end of each of the island regions which faces the
arrayed waveguide is 5 ~ m or more and is shorter than
the pitch of the channel waveguides of the arrayed
waveguide. Thus, etching can be easily carried out in
forming the slab waveguides, arrayed waveguide, input
waveguide, and output waveguides, and the quality can
be securely stabilized. In forming the clad layer,
moreover, the clad layer can be securely embedded in
the core layer and the island regions with good
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capability.
In the present invention, the width and/or
position of each of the island regions should vary with
regard to distance from the center of the slab
waveguide toward the side portions of the slab
waveguide, in some cases. For example, the respective
widths of the island regions are gradually reduced from
the center of the slab waveguide toward the side
portions thereof. By gradually changing the respective
shapes of the island regions, according to this
invention, generation of side lobes can be restrained,
and the crosstalk properties can be further improved.
In the present invention, a value Q can be
minimized so that there is a relation N X H < 40,000 X
(logo)-5, where N is the number of channels for the
wavelength division multiple signals applied to the
input waveguide, H (GHz) is the frequency interval, and
Q is the number of channel waveguides of the arrayed
waveguide. According to this invention, the array
pitch of the arrayed waveguide can be widened, and the
number of channel waveguides can be reduced.
Preferably, in the present invention, the width of
the wider end of each of the tapered island regions
accounts for 38% to 62% of the array pitch of the
channel waveguides, the width of the narrower end
accounts for 0% to 26% of the array pitch of the
channel waveguides, the product of the length of the
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island region and the relative refractive index
difference ranges from 0.4 to 0.6, and the distance
from the island region and the connecting end of the
arrayed waveguide ranges from 100 a m to 150 ~cm.
According to this invention, loss and crosstalk can be
further lowered by optimizing the respective dimensions
or lengths of the opposite ends of the tapered island
regions or the relative positions of the island regions
and the arrayed waveguide.
Preferably, in the present invention, the array
pitch of the island regions is different from the array
pitch of the channel waveguides. If the array pitch of
the channel waveguides is 25 ,um, for example, the
array pitch of the island regions is adjusted to
24.8 ~ m or 25.2 a m. According to this invention,
field distribution at the focal point can be changed
into one with a low crosstalk by slightly deviating the
respective array pitches of the island regions and the
channel waveguides from each other.
Preferably, in the present invention, deviations
between the respective axes of the island regions and
the respective axes of regions opposite the island
regions and between the channel waveguides gradually
increase with distance from a specific island region or
as the side portions of the slab waveguide are
approached. According to this invention, the field
distribution at the focal point can be changed into
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a desired one by changing the deviation between the
island regions and the channel waveguides in the
direction of arrangement of the island regions.
In this specification and the accompanying
drawings, insular regions of the present invention or
"regions formed in the slab wavegu.ide and having a
refractive index different from that of the core layer
of the slab waveguide" are referred to as "island
regions" or simply as "islands" in some cases.
Additional objects and advantages of the invention
will be set forth in the description which follows, and
in part will be obvious from the description, or may be
learned by practice of the invention. The objects and
advantages of the invention may be realized and
obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
The accompanying drawings, which are incorporated
in and constitute a part of the specification,
illustrate embodiments of the invention, and together
with the general description given above and the
detailed description of the embodiments given below,
serve to explain the principles of the invention.
FIG. 1 is a plan view of a part of an optical
rnzltiplexer/demultiplexer to an embodiment of the
invention;
FIG. 2 is a general perspective view of the
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opt i c al multiplexer/demultiplexer shown in FIG. 1;
FIG. 3 is a sectional view of a part of the
op ti cal ~tiplexer/demultiplexer taken along line F3-F3 of
FIG. 1;
FIG. 4 is a sectional view of a part of the
op ti cal ~ltiplexer/demultiplexer.taken along line F4-F4 of
FIG. 1;
FIG. 5 is an enlarged view showing the principal
part of the optical multiplexer/demultiplexer shown in
FIG. 1;
FIG. 6 is a plan view of an island region of the
op t i c a 1 molt iplex~er/demult iplexer shown in FIG . 1;
FIG. 7 is a plan view of an island region of an
op ti caI multiplexer/demultiplexer according to a second
embodiment of the invention
FIG. 8 is a sectional view of a part of an optical
multiplexer/demultiplexer according to a third ~nbodiment of
the invention;
FIG. 9 is a sectional view of an island region of
the optical multiplexer/demultiplexer shown in FIG. 8;
FIG. 10 is a diagram showing the respective
outputs of the optical multiplexers/demultiplexers of the
first and second embodiments and a conventional optical
irultiplexer/demultiplexer;
FIG. 11 is a diagram showing the relationship
between the number of channel waveguides and the
product of the number of signal beam channels and the
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frequency interval;
FIG. 12 is a diagram showing the relationship
between loss and a distance G between the island region
and an arrayed waveguide shown in FIG. 6;
FIG. 13 is a diagram showing the relationship
between loss and a length L of the island region shown
in FIG . 6;
FIG. 14 is a diagram showing the relationship
between loss and a width W1 of one end of the island
region shown in FIG. 6;
FIG. 15 is a diagram showing the relationship
between loss and a width W2 of the other end of the
island region shown in FIG. 6;
FIG. 16 is a plan view of a part of an optical
1 5 ~ltiplexer/demultiplexer according to a fourth embodiment of
the invention;
FIG. 17 is a diagram showing relationships between
the chirping level of island regions shown in FIG. 16,
loss, and crosstalk;
FIG. 18 is a plan view of a part of an optical
multiplexer/demultiplexer according to a fifth embodiment of
the invention;
FIG. 19 is a plan view showing an island region
according to a sixth embodiment of the invention by
hatching;
FIG. 20 is a plan view showing an island region
according to a seventh embodiment of the invention by
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hatching;
FIG. 21 is a plan view showing an island region
according to an eighth embodiment of the invention by
hatching; and
FIG. 22 is a wavelength characteristic curve for
illustrating crosstaZk.
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment of the present invention will
now be described with reference to FIGS. 1 to 6.
FIG. 2 shows a waveguide-type wavelength
multiplexing rniltiplexer/demultiplexer element (hereinafter
referred to as optical ~tiplexer / d~ltiplexer 10) that
uses an arrayed-waveguide grating.
The optical multiplexer/c~multiplexer 10 c~nprises
a substrate 11 formed of silica glass or silicon,
for example, a plurality of input waveguides 12,
an input slab waveguide 13, an arrayed waveguide 14,
an output slab waveguide 15, a plurality of output
waveguides 16, etc. Optical fibers (not shown) are
connected optically to the input waveguides 12, which
receive wavelength division multiple signals through
the optical fibers.
As shown in FIG. 3, the input slab waveguide 13 is
provided with the substrate 11 of silica glass, a core
layer 20 spread flat on the substrate 11, and a clad
layer 21 covering the core layer 20. The input slab
waveguide 13 is formed between the input waveguides 12
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and the arrayed waveguide 14, and optically connects
the waveguides 12 and 14. The wavelength division
multiple signals delivered from the input waveguides 12
to the input slab waveguide 13 are spread in the core
layer 20 of the waveguide 13 by a diffraction effect,
and land on channel waveguides 25 of the arrayed
waveguide 14.
The arrayed waveguide 14 includes a plurality of
channel waveguides 25. The respective lengths of each
two adjacent channel waveguides 25 are somewhat
different. Therefore, the wavelength division multiple
signals delivered from the one end of the channel
waveguides 25 undergoes an optical phase shift for each
frequency as they propagate to the other end of the
channel waveguides 25. This phase shift depends on the
wavelength of light, and the wave front of converged
light is inclined according to the wavelength.
Thus, the respective positions of convergence of light
beams in the output slab waveguide 15 vary depending on
the respective wavelengths of the beams.
The light beams with different wavelengths
demultiplexed in this manner are fetched individually
from separate output waveguides 16 according to the
wavelengths. Thus, the output slab waveguide 15 is
formed between the arrayed waveguide 14 and the output
waveguides 16. The waveguide 15 optically connects
the waveguides 14 and 16. The output slab waveguide 15
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may be constructed substantially in the same manner as
the input slab waveguide 13.
For convenience, in this specification, the
waveguides (e. g., waveguides 12) on the incidence side
are referred to as input waveguides, and the waveguides
(e. g., waveguides .16) on the emission side as output
waveguides. If the light is incident in the opposite
direction, however, the waveguides 12 serve as the
output waveguides, and the waveguides 16 as the input
waveguides . Thus, the optical multiplexer/demultiplexer 10
can fulfill equal functions with respect to either
direction of signal light transmission.
The optical multiplexer/demultiplexer 10 is provided
with two or more island regions 30 (typically shown in
FIG. 1) in the input slab waveguide 13 and/or the
output slab waveguide 15. The regions 30 have
a refractive index different from that of the core
layer 20 of the slab waveguide 13 or 15. FIG. 1
representatively shows only one of the input
waveguides 12.
In the case of this embodiment, the refractive
index of the island regions 30 is lower than that of
the core layer 20 that surrounds the regions 30.
As shown in FIG. 4, the island regions 30 are formed on
the substrate 11 formed of silica glass or the like,
and preferably, formed integrally with the clad layer
21 of the input slab waveguide 13.
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The island regions 30 are formed in positions that
are associated with the channel waveguides 25 of the
arrayed waveguide 14, individually. More specifically,
if the refractive index of the island regions 30 is
lower than that of the core layer 20, the island
regions 30 are formed individually between axes X that
connect each input waveguide 12 and the channel
waveguides 25 of the arrayed waveguide 14, as shown in
FIG. 1.
These island regions 30 are arranged at spaces
from the center of the input slab waveguide 13 toward
opposite side portions 13a and 13b. Besides, the
island regions 30 of this embodiment are formed so that
the distance from each channel waveguide 25 of the
arrayed waveguide 14 is fixed. The number of island
regions 30 is one more than the number of channel
waveguides 25.
If the island regions 30 are tapered, as in the
case of this embodiment, the optical power distribution
in the input slab waveguide 13 can be more easily
coupled to the channel waveguides 25 of the arrayed
waveguide 14, as mentioned later. Thus, the optical
power can be efficiently concentrated on the arrayed
waveguide 14 even if each array pitch P (shown in
FIG. 1) of the channel waveguides 25 is widened to
about 30 ~cm or more, for example. If the channel
waveguides 25 are reduced in number, therefore, loss in
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the optical multiplexer/demultiplexer 10 as a whole cannot be
increased, and crosstalk cannot be worsened.
As shown in the enlarged views of FIGS. 5 and 6,
each island region 30 of this embodiment is tapered (in
the shape of an elongate trapezoid) so that a width W1
of one end 30a,.as viewed toward the arrayed waveguide
14, is greater than a width W2 of the other end 30b.
The width W2 of that end 30b of each island region 30
which faces the arrayed waveguide 14 is 5 a m or more
and is shorter than the pitch P of the channel
waveguides 25. A length L of each island region 30
ranges from 50 ~ m to 100 a m, for example.
FIG. 7 shows an island region 30' according to
a second embodiment of the present invention.
The island region 30' has a substantially fixed width
W3 (or is rectangular) throughout its length from one
end to the other. Since the island region 30' shares
other configurations with those of the optical
mufti-demultiplexer 10 of the first embodiment,
a description of those configurations is omitted.
The following is a description of a manufacturing
method for the optical ~ltiplexer/demultiplexer 10 of the
first embodiment.
Silica glass is used for the substrate 11.
The respective core layers 20 of the slab waveguides 13
and 15 and the channel waveguides 25 of the arrayed
waveguide 14 are integrally formed of silica glass
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doped with germanium and have a thickness of 6 ~cm.
The glass material may be formed by the chemical vapor
deposition (CVD), flame deposition (FHD), or vapor
deposition. The refractive index (ncl) of the clad
layer 21 was adjusted to ncl = 1.4574, the refractive
index (nc0).of the core layer 20 and each channel
waveguide 25 to nc0 = 1.4684, the relative refractive
index difference to ~ - 0.750, and the sectional area
of each channel waveguide 25 to 6 X 6 a m2.
The respective patterns of the core layer 20 and
the island regions 30 were simultaneously formed by
reactive ion etching. After the patterns were formed,
the clad layer 21 was formed into a given thickness.
Each island region 30 of this embodiment is formed by
partially scooping the core layer 20 and filling the
resulting hollow with a part of the clad layer 21. In
a slab waveguide 13 of an optical multiplexer/demultiplexer
according to a third embodiment shown in FIGS. 8 and 9,
silicon is used for a substrate 11. In the slab
waveguide 13, a lower clad layer 21' is formed between
the substrate 11 and a core layer 20.
The following is a description of the operation of
the optical multiplexer/demultiplexer 10.
Wavelength division multiple signal beams from the
input waveguides 12 incident on the input slab
waveguide 13 spread in the waveguide 13 in the width
direction thereof. In FIG. 5, PH1 designates
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a wavefront of the incident signal beams. Among the
spread signal beams, beams A1 that are directed toward
the channel waveguides 25 of the arrayed waveguide 14
without passing through the island regions 30 advance
straight along the axes X and land on the channel
waveguides 25, individually.
Among the light beams from the input waveguides 12
incident an the input slab waveguide 13, some beams A2
reach the island regions 30, individually. If the
refractive index of the island regions 30 is lower than
that of the core layer 20, the beams A2 that pass
through the island regions 30 tend to have their phases
advance faster than those of the beams Al that never
pass through the island regions 30. Accordingly, the
wavefront is deformed as indicated by PH2, so that the
advancing direction of the beams A2 is slightly
inclined.
The shape and position of each island region 30
are optimized so that the inclination of the advancing
direction of the beams A2 is directed toward the
channel waveguides 25 of the arrayed waveguide 14.
By doing this, the beams A2 can be concentrated on the
channel waveguides 25 of the arrayed waveguide 14.
Thus, light beams that leak to the clad layer 21 from
between the channel waveguides 25 are reduced, so that
coupling loss from the input slab waveguide 13 to the
arrayed waveguide 14 lessens.
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In the case of a conventional optical multiplexer/
demultiplexer that is not provided with the island
regions 30, some of light beams from .input waveguides
incident on an input slab waveguide leak to a clad
layer from between channel waveguides. The beams
leaked to the clad layer are in a radiation mode, not
in a guided mode, and most of them are lost. Some of
these radiated beams may reach an output slab
waveguide, in some cases. Since their phases are not
controlled at all, these beams cause noise in the
output slab waveguide and worsen crosstalk.
The inventors hereof conducted a simulation to
couple light from the input slab waveguide 13 to the
arrayed waveguide 14. in this simulation, the
conventional structure (without the island regions 30)
and the structures of the foregoing embodiments having
the island regions 30 were checked for variation in
coupling loss. FIG. 10 shows the result of this
simulation.
As conditions for this simulation, the refractive
index (ncl) of the clad layer 21 was adjusted to
ncl = 1.4574, the refractive index (nc0) of the core
layer 20 and each channel waveguide 25 to nc0 = 1.4692,
the relative refractive index difference to ~ - 0.75,
the sectional area of each channel waveguide 25 to
6 X 6 ~ m2, the slab waveguide radius to R = 2,380 ,um,
and the demultiplexing interval to 705 GHz in terms of
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frequency (D ~, - 5.64 nm in terms of wavelength).
The simulation indicated that the loss of the
conventional structure was 4 dB. It was confirmed, on
the other hand, that the loss could be reduced to
0.34 dB according to the first embodiment of the
invention (provided with the tapered island regions 30
shown in FIG. 6). It was confirmed, moreover, that the
loss could be reduced to 1.2 dB according to the second
embodiment of the invention (provided with the
rectangular island regions 30' shown in FIG. 7).
The respective lengths of each two adjacent ones
of the channel waveguides 25 that constitute the
arrayed waveguide 14 are somewhat different.
Therefore, the wavelength division multiple signals
applied to the arrayed waveguide 14 undergoes an
optical phase shift for each frequency as they
propagate from the channel waveguide 25 to the output
slab waveguide 15. As this is done, wavelength
multiplexing signal beams are focused on different
points according to the wavelength. In consequence,
light beams having their respective wavelengths are
dividedly incident on the output waveguides 16.
The island regions 30 may possibly be provided on
both in input and output slab waveguides 13 and 15 or
on only one of the slab waveguides 13 and 15. The case
depends on the way of use of the optical ~ltiplexer/
demultiplexer of the waveguide-grating type (e. g.,
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optical multi-demultiplexer 10 of each of the foregoing
embodiments).
In the case where the island regions 30 are not
formed in the output slab waveguide 15, for example,
the slab waveguide 15 is shorter than in the case where
the island regions 30 are not formed. This is because
the point of convergence is settled when light from
the arrayed waveguide 14 reaches the output slab
waveguide 15. If the island regions 30 are formed in
the output slab waveguide 15, light is influenced again
by phase change as it pass through the island
regions 30. In this case, therefore, the distance from
the point of convergence is settled when the passage
through the island regions 30 is finished.
By successively changing the width of the island
regions 30 from the center of the input slab waveguide
13 toward the opposite side portions, for example,
generation of side lobes can be restrained, and
the crosstalk properties can be further improved.
In the case where all the island regions 30 have
the same shape, positions that meet the conditions for
the generation of side lobes approach the main lobe, so
that leakage of light to output waveguides other than
a specific one tends to increase.
By gradually changing the respective shapes of the
island regions 30, therefore, the positions that meet
the conditions for the generation of the side lobes can
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be kept wide apart from the main lobe. Thus, light
beams that are incident on output waveguides other than
the specific one can be reduced.
The inventors hereof conducted a simulation to
analyze the wavelength characteristics of the
conventional optical multiplexer/demultiplexer without the
island regions 30, thereby obtaining insertion loss and
crosstalk values. In consequence, the minimum
insertion loss and the crosstalk level of any other
channels than adjacent ones were found to be -4.46 dB
and 41.38 dB, respectively. The number (N) of channels
for the signal beams was adjusted to 8, the refractive
index (ncl) of the clad layer to ncl = 1.4574,
the refractive index (nc0) of the core layer to
nc0 = 1.4692, the relative refractive index difference
to ~ - 0.750, the sectional area of each channel
waveguide to 6 X 6 ,um2, the slab waveguide radius to
R = 2,380 ~ m, and the demultiplexing interval to
705 GHz in terms of frequency (0 ~, - 5.64 nm in terms
of wavelength).
In the case of the optical multiplexer/demultiplexer 10
of each of the foregoing embodiments in which a
plurality of island regions 30 with the same shape are
situated in equivalent positions with respect to the
directions of the respective axes X of the slab
waveguides, on the other hand, the insertion loss and
the crosstalk were found to be -1.81 dB and 41.39 dB,
CA 02409118 2005-07-06
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respectively. Thus, it was confirmed that the
insertion loss of the optical multiplexer/demultiplexer 10
was about 2.7 dB better than that of the conventional
example.
The inventors hereof also conducted a simulation
for an embodiment such that the respective positions of
the island regions 30 with the same shape are gradually
varied with respect to the directions of the respective
axes X of the slab waveguides. In this embodiment,
the insertion loss and the crosstalk were found to
be -3.27 dB and 50.45 dB, respectively. Thus, it was
confirmed that the crosstalk value of this embodiment
was about 10 dB better than that of the conventional
example without the island regions 30.
Further, the inventors hereof examined the
relation between the channel waveguide number Q of
the arrayed waveguide 14 and the product of the
signal beam channel number N and the frequency
interval H. FIG. 11 is a graph obtained by plotting
the relation. In the case provided with the island
regions 30, there are plots inside a curved border line
N X H = 40,000 X (logo)-5. In the conventional case
without the island regions 30, on the other hand, plots
were found to exist outside the curved border line.
Thus, in the case of the conventional optical
nultlple~'/dan~ltiplexerwithout the island regions 30, the
number of channel waveguides 25 tends to increase if
CA 02409118 2005-07-06
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the channel number N and the frequency interval H are
fixed. This is unavoidable in view of design for the
achievement of a given insertion loss and the
improvement of crosstalk. In the optical multiplexer/
demultiplexer 10 according to each of the embodiments
of the invention, on the other hand, the island regions
30 are formed in at least one of the slab waveguides 13
and 15. In this case, light can be efficiently coupled
to the arrayed waveguide 14, so that the number of
channel waveguides 25 can be made smaller than in the
conventional case.
In each of the tapered island regions 30 shown in
FIGS. 1 and 6, the width of the wider end 30a is
indicated by W1; the width of the other or narrower end
30b by W2, the length of each island region 30 by L,
the array pitch of the channel waveguides 25 by P, and
the distance from the other end 30b of each island
region 30 to a connecting end 14a of the arrayed
waveguide 14 by G. In order to discriminate a target
loss criterion from the loss of the conventional
arrayed-waveguide grating (AwG), according to the
present embodiment, it is adjusted to 1.5 dB,
a practical smaller value. The array pitch P of the
channel waveguides 25 is 25 a m.
FIG. 12 shows a change of loss made when the
distance G was changed. When the distance G ranged
from 100 ~ m to 150 a m, the target value 1.5 dB could
CA 02409118 2002-10-22
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be ensured.
FIG. 13 shows a change of loss made when the
length L was changed. When the length L ranged from
55 ,um to 75 ,um, the target value 1.5 dB could be
ensured. The length L is in inverse proportion to
the change of the relative refractive index difference
D between the core layer 20 and the clad layer 21.
If the relative refractive index difference D changes,
therefore, the product of the length L (~cm) and the
relative refractive index difference 11 is
substantially fixed.
In this embodiment, for example, the refractive
index (nc1) of the clad layer 21 is 1.4574, and the
refractive index (ncp) of the core layer 20 and each
channel waveguide 25 is 1.4684, so that the relative
refractive index difference ~ is 0.0075. In the case
where the relative refractive index difference ~ is
adjusted to 0.0075, the target value 1.5 dB can be
ensured if the product (L X ~) of the length L (u m)
and the relative refractive index difference 1~ ranges
from 0.4 to 0.6.
FIG. 14 shows a change of loss made when the width
W1 of one end 30a of each tapered island region 30 was
changed. When the width W1 ranged from 9.5 ~ m to
15.5 a m, the target value 1.5 dB could be ensured.
Since the array pitch of the channel waveguides 25 is
25 hem, the range from 9.5 ,um to 15.5 ,um is equivalent
CA 02409118 2005-07-06
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to 38~ to 62~ of the array pitch P.
FIG. 15 shows a change of loss made when the width
W2 of the other end 30b of each tapered island region
30 was changed. When the width W2 ranged from 0 a m to
6.5 ~cm, the target value 1.5 dB could be ensured.
Since the array pitch P of the channel waveguides 25 is
25 a m, the range from 0 a m to 6.5 ,um is equivalent to
0~ to 26~ of the array pitch P. If W2 accounts for 0~
of the pitch P, it implies that the other end 30b of
the island region 30 is sharp-pointed like the vertex
of an isosceles triangle.
For these reasons, it is advisable to adjust the
width W1 to 38~ to 62~ of the array pitch P; the width
W2 to 0~ to 26$ of the array pitch P, the product of
the length L (,um) and the relative refractive index
difference ~ to 0.4 to 0.6, and the distance G from
the other end 30b of each island region 30 to the
connecting end 14a of the arrayed waveguide 14 to
100 a m to 150 ~c m.
FIG. 16 shows a slab waveguide 13 and an arrayed
waveguide 14 of an optical multiplexer/demultiplexer
according to a fourth embodiment of the invention.
In order to improve crosstalk, in this embodiment, the
array pitch of island regions 30 and an array pitch P
of channel waveguides 25 are slightly deviated from
each other by 8P. In this specification, the
difference between the pitch of axes C1 of the island
CA 02409118 2002-10-22
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regions 30 and the pitch P of the channel waveguides 25
at a connecting end 14a of the arrayed waveguide 14 is
referred to as a chirping level 8P.
For example, the array pitch P of the channel
waveguides 25 and the chirping level 8P are adjusted
to 25 ~cm and 0.2 a m, respectively. In this case, the
pitch of the island regions 30 is adjusted to 24.8 ~ m
or 25 . 2 ~c m.
More specifically, let it be supposed that the
axis of each island region 30 and the axis of a region
35 between each two adjacent channel waveguides 25 and
opposite the island region 30 are C1 and C2,
respectively, as shown in FIG. 16. In this case, the
island regions 30 are arranged at pitches (P - $P)
such that the deviation between C1 and C2 gradually
increases from a specific island region 301 toward the
opposite side portions 13a and 13b (shown in FIG. 1) of
the slab waveguide 13. Thus, the array pitch of the
island regions 30 is smaller than the array pitch P of
the channel waveguides 25 by bP.
Accordingly, the axis C1 of an nth island region
as counted from the specific island region 301 is
deviated from the axis C2 of the arrayed waveguide 14
by (n X bP). The specific island region 301 is
25 an island region that is situated in the center of
the input slab waveguide 13 with respect to the width
direction, for example. In this case, the deviation
CA 02409118 2002-10-22
29 -
(n X $p) between the axes Cl and C2 increases from the
specific island region 301 toward the opposite side
portions of the input slab waveguide 13.
The field distribution (remote field) at the
optical point of convergence is controlled by deviating
the respective pitches of the island regions 30 and the
channel waveguides 25 from each other in this manner.
By doing this, those components of the optical power
that influence crosstalk can be restrained
satisfactorily.
FIG. 17 shows the results of simulations in which
changes of loss and crosstalk were obtained when the
chirping level ~P was changed. The number of channels
was adjusted to 8, the slab waveguide radius to
9,381 ~cm, the number of channel waveguides to 60, and
the pitch P of the channel waveguides to 25 ~cm.
The respective length difference between each two
adjacent channel waveguides is 126 ~cm. In this
configuration, the wavelength interval is about 0.8 nm,
which is equivalent to the frequency interval of
100 GHz. It was confirmed that the crosstalk level
falls with the increase of the chirping level, as shown
in FIG. 17. Although the insertion loss somewhat
increases as the crosstalk level lowers, it is only
about 7 dB at the maximum. As compared with the level
of the type without any island regions, therefore,
this level is still low enough for practical use.
CA 02409118 2005-07-06
- 30 -
The best result was obtained when the chirping level
was at ~0 . 4 ~c m.
FIG. 18 shows a slab waveguide 13 and an arrayed
waveguide 14 of an optical multiplexer/demultiplexer
according to a fifth embodiment of the invention.
In order to improve crosstalk, in this embodiment, the
width W1 of the one end 30a of each tapered island
region 30 is gradually reduced by 8W at a time from
the center of the input slab waveguide 13 toward the
opposite side portions. The width W2 of the other end
30b of each island region 30 is fixed. The fifth
embodiment arranged in this manner, like the fourth
embodiment, can reduce the crosstalk level.
FIG. 19 shows an island region 30A according to
a sixth embodiment of the invention by hatching.
Opposite side'faces 30e and 30f of this island region
30A have a taper shape such that they individually
extend along opposite half arcs of two adjacent
ellipses V1 and V2.
FIG. 20 shows an island region 30B according to
a seventh embodiment of the invention. Opposite side
faces 30e and 30f of this island region 30B have
a taper shape such that they individually extend along
opposite half arcs of two separate ellipses V1 and V2.
FIG. 21 shows an island region 30C according to
an eighth embodiment of the invention. A length L of
this island region 30C is shorter than a half arc of
CA 02409118 2002-10-22
- 31 -
each of ellipses Vl and V2 in the direction of the
major axis thereof.
With use of the island regions 30A, 30B and 30C
having the side faces 30e and 30f that extend along the
ellipses, light incident on the island regions 30A, 30B
and 30C can be efficiently concentrated on the channel
waveguides 25.
According to the present invention, the insertion
loss and crosstalk can be reduced by increasing the
number of channel waveguides of the arrayed waveguide
and the number of island regions. This effect may be
also obtained with use of the conventional arrayed-
waveguide grating (AWG). With use of the structure of
the present invention that has the island regions in
the slab waveguide, in particular, however, the
insertion loss and crosstalk can be reduced more
effectively.
Additional advantages and modifications will
readily occur to those skilled in the art. Therefore,
the invention in its broader aspects is not limited to
the specific details and representative embodiments
shown and described herein. Accordingly, various
modifications may be made without departing from the
spirit or scope of the general inventive concept as
defined by the appended claims and their equivalents.
