Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-05-21
2004-09-28
Lee, John R. (Department: 2881)
Optical waveguides
With optical coupler
Input/output coupler
C385S024000, C385S027000, C385S039000, C385S043000, C385S046000, C385S083000, C385S130000
Reexamination Certificate
active
06798952
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to an optical multiplexer/demultiplexer comprising an arrayed waveguide grating and, more particularly, to an optical multiplexer/demultiplexer having a small-loss structure.
(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. 22
is a view showing the structure of a conventional arrayed waveguide grating.
As shown in
FIG. 22
, an arrayed waveguide grating
10
has the following waveguide structure. A sector slab waveguide
13
is connected to the output side of one or more optical input waveguides
12
arranged. An arrayed waveguide
14
is connected to the output side of the sector slab waveguide
13
. A sector slab waveguide
15
is connected to the output side of the arrayed waveguide
14
. A plurality of optical output waveguides
16
are connected to the output side of the sector slab waveguide
15
. Usually the arrayed waveguide grating
10
is made by forming the above waveguide structure on, for example, a silicon substrate with cores made from siliceous glass or the like.
The sector slab waveguide
13
on the input side has the center of curvature at the end of the middle waveguide of the optical input waveguides
12
. The sector slab waveguide
15
on the output side also has the center of curvature at the end of the middle waveguide of the optical output waveguides
16
. The sector slab waveguides
13
and
15
have a structure in which the optic axes of waveguides in the arrayed waveguide
14
are located radially from the center of curvature. As a result, the optical arrangement of the sector slab waveguide
13
and arrayed waveguide
14
and of the sector slab waveguide
15
and arrayed waveguide
14
is the same as that of a concave mirror. That is to say, they will function the same as a lens. Moreover, in the arrayed waveguide
14
, there is optical path length difference &Dgr;L between any two adjacent waveguides.
For example, the number of the optical input waveguides
12
and optical output waveguides
16
located corresponds to that of signal light beams with different wavelengths which are obtained as a result of demultiplexing by the arrayed waveguide grating
10
or which are to be multiplexed by the arrayed waveguide grating
10
. Moreover, usually the arrayed waveguide
14
includes a large number of waveguides. In
FIG. 22
, for the sake of simplicity, only one optical input waveguide
12
is shown and the number of waveguides included in the arrayed waveguide
14
and optical output waveguide
16
is reduced.
If the arrayed waveguide grating
10
functions as an optical demultiplexer, light with a plurality of wavelengths &lgr;1, &lgr;2, . . . , &lgr;n is multiplexed by a WDM system and is input from the optical input waveguide
12
to the sector slab waveguide
13
. This wavelength-multiplexed light spreads in the sector slab waveguide
13
by diffraction and is spreaded to each of the waveguides of the arrayed waveguide
14
. In this case, the phases of light distributed to the waveguides of the arrayed waveguide
14
are the same. The light beams which propagated through the arrayed waveguide
14
are given phase difference corresponding to optical path length difference &Dgr;L between adjacent waveguides, interfere with one another in the sector slab waveguide
15
on the output side, and are condensed into the optical output waveguides
16
. In this case, phase difference given in the arrayed waveguide
14
depends on the wavelengths, so the wavelengths are dispersed and the signal light beams are condensed into the different optical output waveguides
16
according to their wavelengths. As a result, the wavelength-multiplexed light input from the optical input waveguides
12
is demultiplexed into light with wavelengths of &lgr;1, &lgr;2, . . . , &lgr;n and is output from the different optical output waveguides
16
.
Operation in the arrayed waveguide grating
10
is reversible. That is to say, if the direction in which light travels is inverted, the arrayed waveguide grating
10
will function as an optical multiplexer. Intervals &Dgr;&lgr; between the wavelengths of light obtained by demultiplexing are given approximately by:
&Dgr;&lgr;=(
ns·d·nc
)/(
f·m·ng
)·&Dgr;
x
(1)
where ns is an effective refractive index in the sector slab waveguides
13
and
15
, d is a waveguide pitch at a portion where the arrayed waveguide
14
and sector slab waveguide
13
connect and at a portion where the arrayed waveguide
14
and sector slab waveguide
15
connect, nc is an effective refractive index in each of the waveguides of the arrayed waveguide
14
, f is the focal length of the sector slab waveguides
13
and
15
, m is a diffraction degree, ng is a group index in the arrayed waveguide
14
, and &Dgr;x is an interval between adjacent optical output waveguides
16
. If a center wavelength is &lgr;0, then m=(nc·&Dgr;L)/&lgr;0.
FIG. 23
is a graph showing an example of the passband characteristic of light demultiplexed in the above arrayed waveguide grating
10
.
The passband characteristic of light obtained in each of the optical output waveguides
16
in the case of the arrayed waveguide grating
10
shown in
FIG. 22
being used as an optical demultiplexer is shown in FIG.
23
. In this case, the intensity of light obtained in each optical output waveguide
16
is 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, moderately wide wavelength range R with the center wavelength &lgr;0 as its center will be used and there will be fluctuations in the wavelength of light propagating. As a result, with the above passband characteristic, the intensity of light obtained varies according to its wavelengths. In this case, shift D0 will occur. Therefore, a passband characteristic must be made flat so that the intensity of light obtained in the used wavelength range R will be constant.
FIG. 24
is a graph showing an example in which a passband characteristic is made flat.
On a graph shown in
FIG. 24
, a spectrum is flat in the used wavelength range R with the center wavelength &lgr;0 as its center. The intensity of light obtained is almost constant in this range and shift D1 in the intensity of the light is slight.
Conventionally, a Y branch circuit has been located at a portion where the optical input waveguide
12
and sector slab waveguide
13
connect in order to obtain light of constant intensity in the used wavelength range R.
FIG. 25
is a view showing the structure of a Y branch circuit.
FIG. 26
is a schematic view showing the shape of a mode of light output from the Y branch circuit to the sector slab waveguide
13
. The x-axis in
FIG. 26
is perpendicular to the waveguides of the optical input waveguide
12
or the arrayed waveguide
14
.
As shown in
FIG. 25
, a Y branch circuit
17
has the shape of the letter “Y” and is located at a portion where the optical input waveguide
12
and sector slab waveguide
13
connect. As a result, when single mode light which propagated through the optical input waveguide
12
is radiated into the sector slab waveguide
13
via the Y branch circuit
17
, two peaks as shown in
FIG. 26
will appear side by side in the shape of its mode. Therefore, two peaks also appear in the shape of a mode of light which is input from the sector slab waveguide
13
on the input side to the sector slab waveguide
15
on the output side through the arrayed waveguide
14
and which is condensed.
There is one peak
Fujitsu Limited
Lee John R.
Staas & Halsey
Vanore David A.
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