Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-04-22
2004-05-25
Bruce, David V. (Department: 2882)
Optical waveguides
With optical coupler
Input/output coupler
C385S014000
Reexamination Certificate
active
06741772
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to an optical multiplexer/demultiplexer including an arrayed waveguide grating and a waveguide type optical coupler including a sector slab waveguide.
(2) Description of the Related Art
With an explosive increase in data traffic on networks, in recent years attention has been riveted to photonic networks on which a large amount of data can be transferred. To realize such networks, wavelength division multiplexing (WDM) optical communication networks are being built. An arrayed waveguide grating (AWG) in which the technology of a planar lightwave circuit (PLC) is adopted is a likely candidate for an optical wavelength multiplexer/demultiplexer essential to these WDM transmission systems.
FIG. 17
is a view showing the structure of a conventional arrayed waveguide grating.
As shown in
FIG. 17
, an arrayed waveguide grating
200
has the following waveguide structure. A sectors lab waveguide
203
is connected to the output side of one or more optical input waveguides
202
arranged via a mode conversion waveguide
207
described later. An arrayed waveguide
204
is connected to the input side of the sector slab waveguide
203
. A sector slab waveguide
205
is connected to the output side of the arrayed waveguide
204
. A plurality of optical output waveguides
206
are connected to the output side of the sector slab waveguide
205
. Usually the arrayed waveguide grating
200
is made by forming the above waveguide structure on, for example, a silicon substrate with cores made from, for example, siliceous glass.
The sector slab waveguide
203
on the input side has the center of curvature at the end of the middle waveguide of the optical input waveguides
202
. The sector slab waveguide
205
on the output side also has the center of curvature at the end of the middle waveguide of optical output waveguides
206
. The sector slab waveguides
203
and
205
have a structure in which the optic axes of waveguides in the arrayed waveguides
204
are located radially from the center of curvature. As a result, the optical arrangement of the sector slab waveguide
203
and arrayed waveguides
204
and of the sector slab waveguide
205
and arrayed waveguides
204
are the same as that of a concave mirror. That is to say, they will function the same as a lens. Moreover, in the arrayed waveguides
204
, there is optical path difference &Dgr;L between any two adjacent waveguides.
If the arrayed waveguide grating
200
functions as an optical demultiplexer, optical signal with a plurality of wavelengths is multiplexed by a WDM system and is input from the optical input waveguides
202
to the sector slab waveguide
203
via the mode conversion waveguide
207
. This wavelength-multiplexed optical signal spreads in the sector slab waveguide
203
by diffraction and is spreaded to each of the waveguides in the arrayed waveguide
204
. In this case, the phases of optical signal spreaded to the waveguides in the arrayed waveguide
204
are the same.
The optical signal beams which propagated through the arrayed waveguide
204
are given phase difference corresponding to optical path difference &Dgr;L between adjacent waveguides, interfere with one another in the sector slab waveguide
205
on the output side, and are condensed into the optical output waveguides
206
. In this case, phase difference given in the arrayed waveguide
204
depends on wavelengths, so the optical signal beams are condensed into the different optical output waveguides
206
according to their wavelengths. As a result, the wavelength-multiplexed optical signal input from the optical input waveguides
202
is demultiplexed according to wavelengths and is output from the different optical output waveguides
206
.
Operation in the arrayed waveguide grating
200
is reversible. That is to say, if the direction in which optical signal travels is inverted, the arrayed waveguide grating
200
will function as an optical multiplexer. Moreover, for example, the sector slab waveguide
203
alone can be used. In this case, the sector slab waveguide
203
will function as an optical coupler for spreading optical signal input from the optical input waveguides
202
into a plurality of optical output waveguides connected to its exit.
For example, the number of the optical input waveguides
202
and optical output waveguides
206
located corresponds to that of signal light beams with different wavelengths which are obtained as a result of demultiplexing by the arrayed waveguide grating
200
or which are to be multiplexed by the arrayed waveguide grating
200
. Moreover, usually the arrayed waveguide
204
includes a large number of waveguides. In
FIG. 17
, for the sake of simplicity, only one optical input waveguide
202
is shown and the number of waveguides included in the arrayed waveguides
204
and optical output waveguide
206
is reduced.
By the way, if the arrayed waveguide grating
200
is used as an optical demultiplexer, the passband characteristic of optical signal obtained in each optical output waveguide
206
are as follows. The intensity of optical signal obtained in each optical output waveguide
206
is the highest at center wavelength &lgr;
0
and becomes significantly lower at a wavelength farther from the center wavelength &lgr;
0
. In actual optical communication systems, however, the passband characteristic of propagated signal must be flat with, for example, fluctuations in the wavelength of the light taken into consideration so that the intensity of the signal will be constant in a moderately wide wavelength range with the center wavelength &lgr;
0
as its center. In the conventional arrayed waveguide grating
200
, therefore, the mode conversion waveguide
207
is located between the optical input waveguides
202
and sector slab waveguide
203
.
FIG. 18
is an enlarged view of portions around the mode conversion waveguide
207
. In
FIG. 18
, the shape of modes of optical signal propagating through the sector slab waveguide
203
is also shown.
As shown in
FIG. 18
, the mode conversion waveguide
207
is a waveguide in the shape of a paraboloid in which the width of a core widens in the direction of the exit, and connects the exit of the optical input waveguides
202
and the entrance of the sector slab waveguide
203
. The mode conversion waveguide
207
includes a core in the shape of a paraboloid, so optical signal as shown by a curve
181
, the shape of a mode of which has two peaks, will be output to the sector slab waveguide
203
. When this optical signal propagates through the sector slab waveguide
203
, the width of the shape of its mode will widen. At this time the shape of its mode can be shown by a curve
182
. Then the optical signal is input to the arrayed waveguide
204
.
Now, the principles underlying flattening a passband characteristic by locating the mode conversion waveguide
207
will be described with reference to
FIGS. 19 and 20
. FIGS.
19
(A) and
19
(B) are schematic views showing how optical signal output from the mode conversion waveguide
207
propagates through the arrayed waveguide grating
200
. FIG.
19
(A) shows how optical signal output from the mode conversion waveguide
207
propagates through portions around the sector slab waveguide
203
on the input side. FIG.
19
(B) shows how optical signal output from the mode conversion waveguide
207
propagates through portions around the sector slab waveguide
205
on the output side.
In FIGS.
19
(A) and
19
(B), peaks P
1
and P
2
appear in the shape of a mode of optical signal with a wavelength of &lgr;
1
which propagated through the optical input waveguides
202
, on which a mode conversion was performed in the mode conversion waveguide
207
, and which was output to the sector slab waveguide
203
. The peaks P
1
and P
2
are input to the arrayed waveguide
204
at different positions at the exit of the sector slab waveguide
203
. Therefore, after the peaks P
1
and P
2
are input to the sector slab waveguide
205
on the output side via
Artman Thomas R
Bruce David V.
Fujitsu Limited
Staas & Halsey , LLP
LandOfFree
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