Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2001-03-02
2004-04-20
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Plural
38, 38
Reexamination Certificate
active
06724954
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an optical multiplexer/demultiplexer, particularly to an optical multiplexer/demultiplexer which can significantly reduce the wavelength dispersion, and further to an optical multiplexer/demultiplexer which can simultaneously perform multiplexing processing and demultiplexing processing.
BACKGROUND OF THE INVENTION
An interleave system, which is one form of advanced wavelength multiplexing communications, requires an optical multiplexer/demultiplexer having a function such that a signal with certain channel wavelength spacings is demultiplexed to two signals with doubled channel wavelength spacings, or conversely, two signals are multiplexed to one signal.
FIG. 12
is an explanatory view showing one example of a prior art technique for coping with this demand. Since a broad and flat wavelength passband is required of the optical multiplexer/demultiplexer used in the interleave, as shown in
FIG. 12
, the prior art technique has adopted a multistage-connected construction of Mach-Zehnder interference circuits which each comprise four optical couplers (directional couplers)
24
,
25
,
26
,
27
and waveguide pairs each comprising two waveguides with different lengths (
28
,
29
), (
30
,
31
), (
32
,
33
), for connecting the optical couplers to each other (see, for example, U.S. Pat. No. 5,852,505).
FIG. 13
shows a spectral response for an input/output port
22
in the case where a white light source is input through an input/output port
21
, and
FIG. 14
a spectral response for an input/output port
23
. When signals
1
,
2
,
3
,
4
with channel spacings of about 0.4 nm (frequency 50 GHz) are input through the port
21
,
1
and
3
are output through the port
22
while
2
and
4
are output through the port
23
. In this case, the output signals have channel wavelength spacings of about 0.8 nm (frequency 100 GHz). The multistage construction of Mach-Zehnder interference circuits shown in
FIG. 12
has an advantage that, as shown in wavelength loss characteristics in
FIGS. 13 and 14
, a broad and flat passband can be provided.
The optical multiplexer/demultiplexer shown in
FIG. 12
, however, suffers from a problem that the realization of good loss wavelength flatness disadvantageously leads to wavelength dispersion.
FIG. 15
shows wavelength dispersion characteristics for a path wherein optical signals are input through the port
21
and output through the port
22
, and
FIG. 16
wavelength dispersion characteristics for a path wherein optical signals are input through the port
21
and output through the port
23
. For both the drawings, in the abscissa, frequency is used instead of the wavelength, while the ordinate represents only dispersion around passbands (around
1
passband and around
2
passband). As is apparent from the drawings, the dispersion around the passbands is about 30 ps
m. This value, of course, varies depending upon parameters. In theory, however, it is unavoidable that improving the loss wavelength flatness leads to dispersion. This wavelength dispersion is significantly disadvantageous in terms of transmission speed of the system and relay distance.
Further, the conventional optical multiplexer/demultiplexer can have both optical multiplexing and optical demultiplexing functions. However, multiplexing processing and demultiplexing processing of a plurality of optical signals derived from different signal sources could not have been simultaneously performed in a single optical multiplexer/demultiplexer. This has necessitated the provision of at least one optical multiplexer/demultiplexer for multiplexing and at least one optical multiplexer/demultiplexer for demultiplexing. Therefore, a space for the necessary number of optical multiplexer/demultiplexers should be provided.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to solve the above problem of the prior art and to provide an optical multiplexer/demultiplexer which causes no significant wavelength dispersion (theoretically has no wavelength dispersion) and, at the same time, has excellent wavelength flatness characteristics in passband.
It is another object of the invention to realize an optical multiplexer/demultiplexer which can reduce the space necessary for the optical multiplexer/demultiplexer to perform the multiplexing processing and demultiplexing processing of a plurality of optical signals derived from different signal sources.
According to the first feature of the invention, an optical multiplexer/demultiplexer comprises optical multiplexer/demultiplexer circuits connected in multistage, wherein:
the optical multiplexer/demultiplexer circuits each have two input ports and two output ports;
an optical path, through which input optical signals with predetermined wavelengths are output after multiplexing and demultiplexing, varies depending upon the input port;
the optical paths in each of the optical multiplexer/demultiplexer circuits have mutually opposite wavelength dispersion characteristics; and
one of the optical paths in a first optical multiplexer/demultiplexer circuit is connected to one of the optical paths, in a second optical multiplexer/demultiplexer circuit, having wavelength dispersion characteristics opposite to the optical path in the first optical multiplexer/demultiplexer circuit.
This optical multiplexer/demultiplexer may comprise two optical multiplexer/demultiplexer circuits connected to each other, wherein:
said optical multiplexer/demultiplexer circuits each have a waveguide with first and third ports as terminals and a waveguide with second and fourth ports as terminals, and have optical multiplexing/demultiplexing characteristics such that, for optical signals with predetermined wavelengths, the optical signals input through the first port are output through the third port while optical signals input through the second port are output through the fourth port;
the wavelength dispersion characteristics, in the case where the optical signals are input through the first port and output through the third port, are opposite to the wavelength dispersion characteristics in the case where the optical signals are input through the second port and output through the fourth port; and
the third port in the first optical multiplexer/demultiplexer circuit, when the first port in the first optical multiplexer/demultiplexer circuit is used as an input port, is connected to the second port in the second optical multiplexer/demultiplexer circuit while the fourth port in the second optical multiplexer/demultiplexer circuit is used as an output port for optical signals with predetermined wavelengths.
Further, the optical multiplexer/demultiplexer may comprise two optical multiplexer/demultiplexer circuits connected to each other, wherein:
said optical multiplexer/demultiplexer circuits each have a waveguide with first and third ports as terminals and a waveguide with second and fourth ports as terminals, and have optical multiplexing/demultiplexing characteristics such that, for optical signals with predetermined wavelengths, the optical signals input through the first port are output through the fourth port while optical signals input through the second port are output through the third port;
the wavelength dispersion characteristics, in the case where the optical signals are input through the first port and output through the fourth port, are opposite to the wavelength dispersion characteristics in the case where the optical signals are input through the second port and output through the third port; and
the fourth port in the first optical multiplexer/demultiplexer circuit, when the first port in the first optical multiplexer/demultiplexer circuit is used as an input port, is connected to the second port in the second optical multiplexer/demultiplexer circuit while the third port in the second optical multiplexer/demultiplexer circuit is used as an output port for optical signals with predetermined wavelengths.
Furthermore, the optical multiplexer/demultiplexer may comprise three optical multiplex
Arai Hideaki
Chiba Takafumi
Uetsuka Hisato
Font Frank G.
Hitachi Cable, Inc.
Kianni Kaveh
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