Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2001-06-20
2004-02-10
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Plural
C385S011000, C385S129000, C385S141000
Reexamination Certificate
active
06690852
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical arrayed waveguide grating type multiplexer/demultiplexer which is employed in the field of optical communication. The present specification is based upon Japanese Patent Application S.N. 2000-188217, and hereby incorporates the subject matter of that application by reference.
2. Background Art
In an optical communication system, in order to transmit a large quantity of information, a wavelength multiplex transmission method is proposed in which light at different wavelengths is multiplexed and transmitted. Further, since light is made up from two polarizations which are perpendicular, a polarization/wavelength multiplex transmission method is also proposed in which, when multiplexing, transmitting and outputting light at different wavelengths, the light is separated for each wavelength and also for each polarization and is outputted. It should be noted that one of the polarizations will hereinafter be referred to as the “TE polarization” and the other as the “TM polarization”.
FIG. 14
is a block diagram which shows an example of a prior art polarization/wavelength multiplex transmission circuit. With this circuit, when multiplexed light of four wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
, and &lgr;
4
is inputted to an input side optical fiber
1
which is positioned at the left side of the figure, this light is inputted to an input port
2
a
of a polarization beam splitter
2
, is divided into polarizations in this polarization beam splitter
2
, and is outputted from output ports
2
b
and
2
c
as, respectively, a TE polarization and a TM polarization of the four wavelengths.
Next, the multiplexed light of the TE polarization is inputted via an optical fiber
1
which is connected to the output port
2
b
to an input port
3
a
of an optical multiplexer/demultiplexer
3
, and is separated into its various wavelengths by this optical multiplexer/demultiplexer
3
, so that four TE polarizations of the four wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
, and &lgr;
4
are output to optical fibers
1
which are connected to its output ports
3
b
,
3
c
,
3
d
and
3
e
, respectively. On the other hand, the multiplexed light of the TM polarization is also, in the same manner, inputted via an optical fiber
1
which is connected to the output port
2
c
to an input port
4
a
of an optical multiplexer/demultiplexer
4
, and is separated into its various wavelengths by this optical multiplexer/demultiplexer
4
, so that four TM polarizations of the four wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
, and &lgr;
4
are output to optical fibers
1
which are connected to its output ports
4
b
,
4
c
,
4
d
and
4
e
, respectively. As a result, it is possible to separate the multiplexed light of the four wavelengths by wavelength and by polarization, into a total of eight parts.
In this manner, with a prior art polarization/wavelength multiplex transmission circuit, they are required both a single polarization beam splitter for separation into polarizations, and two optical multiplexer demultiplexers for separation into different wavelengths. However, since the efficiency of space utilization is deteriorated and the loss is increased when the number of devices which make up the circuit increases, there has been a demand for a technique for building a polarization/wavelength multiplex transmission circuit with a smaller number of devices. In concrete terms, a device would be desirable which could perform separation of polarizations and separation of wavelengths simultaneously.
On the other hand an arrayed waveguide grating type optical multiplexer/demultiplexer is often used as an optical multiplexer/demultiplexer, because its wavelength separation performance is high.
FIG. 15
shows an example of an arrayed waveguide grating type optical multiplexer/demultiplexer: in this figure, an waveguide array
8
is formed by providing a plate shaped cladding layer
6
made from silica based glass upon a silicon substrate
5
, and by arranging waveguides
7
,
7
. . . made from silica based glass on this cladding layer generally in parallel and in a letter-U configuration. It should be understood that the lengths of these waveguides
7
,
7
. . . which make up the waveguide array
8
differ from one another in steps of &Dgr;L.
At both the ends of this waveguide array
8
, i.e. its input side and its output side, respective slab waveguides
9
and
10
are provided. To the input side of this slab waveguide
9
there are provided plural input waveguides
11
,
11
. . . in parallel. On the opposite side, to the output side of the slab waveguide
10
there are provided plural input waveguides
12
,
12
. . . in parallel. Each of the waveguides
7
through
12
, in order for it to propagate light, is made from a material having a refractive index which is higher than that of the cladding layer
6
which is provided around each of the waveguides
7
through
12
.
And when multiplexed light consisting of light of plural wavelengths is inputted to one of the input waveguides
11
, this multiplexed light is distributed via the slab waveguide
9
between the plurality of waveguides
7
,
7
. . . which make up the waveguide array
8
in roughly equal proportions, and is propagated through these waveguides
7
,
7
. . . with optical path length differences occurring by &Dgr;L. And wavelengths are selected by these lights interfering in the slab waveguide
10
on the output side, and these lights of the wavelengths are outputted from the output waveguides
12
,
12
. . . respectively.
Moreover, such an arrayed waveguide grating type optical multiplexer/demultiplexer is an optical product which uses a planar waveguide, and just as it is the refractive index differs according to polarization (there is an anisotropy in the refractive index), a polarization dependency exists. Since a planar waveguide, as described above, generally comprises a cladding layer made from silica based glass and a waveguide upon a silicon substrate, during manufacture, due to the difference in coefficient of thermal expansion between silicon and silica based glass, a slight residual stress is engendered in the waveguide in the process of cooling of the cladding layer and the waveguide down from a high temperature to a low temperature, and this residual stress causes a polarization dependency.
Accordingly, just as it is, even at the same wavelength, the focal positions at the output end of the slab waveguide
10
are different for the different polarizations.
Since this characteristic is an inconvenience when separating the wavelengths, in the prior art, as shown in
FIG. 15
, a half wavelength plate
13
is inserted in the center of the waveguide array
8
, and thereby characteristics which do not depend upon polarization are obtained.
Or the method is employed of setting the position of the input end of each of the respective output waveguides
12
,
12
. . . of the slab waveguide
10
of the output side at the central point between the focusing positions of the two polarizations which make up the light of the wavelength which is distributed to this output waveguide
12
, so as to output these two polarizations from a single output waveguide
12
.
On the other hand, a proposal has been made to use an arrayed waveguide grating type optical multiplexer/demultiplexer as a polarization beam splitter by taking advantage of this polarization dependence.
However, no proposal has ever yet been made for an arrayed waveguide grating type optical multiplexer/demultiplexer which simultaneously performs wavelength separation and polarization separation.
SUMMARY OF THE INVENTION
The present invention has as its subject the provision of an art which is capable of constructing a polarization/wavelength multiplex transmission circuit from a small number of comprised devices. In concrete terms, its objective is to provide an optical multiplexer/demultiplexer which is capable of simultaneously performing separation by wavelength and separation by polarization
Asano Kenichiro
Hosoya Hideyuki
Kohmoto Katsutoshi
Ogawa Hirokuni
Ohura Kouji
Bell Boyd & Lloyd LLC
Font Frank G.
Fujikura Ltd.
Mooney Michael P.
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