Optical waveguides – Having nonlinear property
Reexamination Certificate
2002-03-08
2004-07-06
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Having nonlinear property
C385S031000, C359S332000, C398S075000, C398S079000, C398S098000
Reexamination Certificate
active
06760524
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
This invention is based on and claims priority of Japanese patent application 2001-331091, filed on Oct. 29, 2001, the whole contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical signal processing device for carrying out signal processing without converting an optical signal into an electrical signal, an optical demultiplexer, a wavelength converting device, an optical signal processing method, and a wavelength converting method.
2. Description of the Related Art
Recently, a wavelength division multiplexing (WDM) optical communication system has been developed as a broadband optical communication system. Other optical communication systems, such as optical time division multiplexing (OTDM) and time wavelength division multiplexing (TWDM), have also been proposed and studied aiming at broader band optical communication.
In the WDM optical communication system, a plurality of wavelengths of wavelength multiplexed optical signals are assigned to communication channels in a one-to-one relation. For multiplexing a plurality of optical signals, each of original optical signals to be multiplexed must be converted to have the wavelength of a communication channel to which the optical signal is assigned. Hitherto, such wavelength conversion has been performed by first converting the optical into an electrical signal and then converting the electrical signal into an optical signal of a desired wavelength.
In the OTDM optical communication system, signal density is increased by employing optical pulses having the same wavelength and having a very narrow pulse width. Communication rate of a time-division multiplexed optical signal is, for example, 160 Gbits/s or higher.
Response speed of an electrical signal is limited by a moving time of carriers in a semiconductor device and hence lower than the response speed of an optical signal. At present, the speed limit of an electrical signal is thought to be about 40 Gbits/s. To process an OTDM signal having speed higher than that limit, an optical signal must be divided through high-speed optical signal processing and demultiplexed to a bit rate, at which electrical processing is feasible.
The TWDM optical communication system as a combination of the OTDM optical communication system and the WDM optical communication system is able to realize broader band optical communication.
In view of the above-mentioned background, an optical device (optical demultiplexer) has recently been studied which is able to demultiplex an optical signal, as it is, without converting the optical signal into an electrical signal. Hitherto, optical demultiplexers of, e.g., non-linear optical loop mirror (NOLM) type, Mach-Zehnder type and polarization separating type, have been proposed.
FIG. 9A
is a schematic view of a NOLM type optical demultiplexer. An optical signal sig
1
reaches a branch point
102
of an optical fiber loop
101
via an input side optical fiber
100
. At the branch point
102
, the optical signal sig
1
is branched into an optical signal sig
2
propagating in the loop
101
counterclockwise and an optical signal sig
3
propagating in the loop
101
clockwise. The optical signal sig
1
is a signal having four time-division multiplexed channels, i.e., channels #
1
to #
4
.
A non-linear waveguide
103
is inserted in the optical loop
101
at a position asymmetrical to the branch point
102
. The optical signal sig
2
propagating counterclockwise reaches the non-linear waveguide
103
at timing earlier than the optical signal sig
3
propagating clockwise. A control light pulse con is inputted to the non-linear waveguide
103
immediately after the channel #
2
of the optical signal sig
2
has passed the non-linear waveguide
103
. The refractive index of the non-linear waveguide
103
is changed upon the inputting of the control light pulse con, whereby the phase of a pulse light in each channel #
3
and #
4
of the optical signal sig
2
is shifted &pgr;. In
FIG. 9A
, a pulse having phase shifted &pgr; is represented by hatching.
Because the optical signal sig
3
reaches the non-linear waveguide
103
at timing delayed from the optical signal sig
2
, only the channel #
1
of the optical signal sig
3
has passed the non-linear waveguide
103
at the time when the control light pulse con is inputted to the non-linear waveguide
103
. Therefore, the phase of a pulse light in each of the channels #
2
to #
4
of the optical signal sig
3
is shifted &pgr;.
When the optical signals sig
2
and sig
3
return to the branch point
102
, the pulses in those ones #
1
, #
3
and #
4
of the channels of both the signals, which are in phase, propagate in the input side optical fiber
100
, and the pulse in the out-of-phase channel #
2
propagates in an output side optical fiber
105
. Thus, only the signal of one channel can be separated from the time division multiplexed signal sig
1
.
In the NOLM type optical demultiplexer, the time required for the optical signal to pass the optical loop
101
limits the signal speed achievable in signal processing. Also, the use of an optical fiber loop raises a difficulty in reducing the device size.
FIG. 9B
is a schematic view of a Mach-Zehnder type optical demultiplexer. Non-linear waveguides
121
and
122
are inserted respectively in two arms of a Mach-Zehnder interferometer
120
. An optical signal sig
10
is branched into two optical signals sig
11
and sig
12
, which are introduced to the non-linear waveguides
121
and
122
, respectively. A control light pulse con is inputted to the non-linear waveguides
121
and
122
at different timings from each other.
The control light pulse con is inputted to the non-linear waveguide
121
immediately after a pulse in a channel #
1
has passed the non-linear waveguide
121
, and is inputted to the non-linear waveguide
122
immediately after a pulse in a channel #
2
has passed the non-linear waveguide
122
. Therefore, the phase of an optical pulse in each of the channels #
2
to #
4
of the optical signal sig
11
is shifted &pgr; after passing the non-linear waveguide
121
, and the phase of an optical pulse in each channel #
3
and #
4
of the optical signal sig
12
is shifted &pgr; after passing the non-linear waveguide
122
.
When the optical signals sig
11
and sig
12
are combined with each other, the signals in the channels #
1
, #
3
and #
4
are introduced to one output optical fiber
125
, and the signal in the channel #
2
is introduced to the other output optical fiber
126
.
Thus, in the Mach-Zehnder type optical demultiplexer, two arms, in which non-linear waveguides are respectively inserted, must be arranged parallel to each other. The device size is therefore increased.
FIG. 9C
is a schematic view of a polarization separating type optical demultiplexer. An optical signal sig
20
enters a birefringence crystal
130
. The birefringence crystal
130
delays a light in the TM mode by one pulse relative to a light in the TE mode. An optical signal sig
21
having passed the birefringence crystal
130
and a control light pulse con are both inputted to a non-linear waveguide
131
. The control light pulse con is inputted to the non-linear waveguide
131
immediately after a TE-mode pulse in the channel #
2
has passed the non-linear waveguide
131
.
In an optical signal sig
22
having passed the non-linear waveguide
131
, therefore, the phase of the TE-mode optical pulse in each channel #
3
and #
4
is shifted &pgr;, and the phase of the TM-mode optical pulse in each of the channels #
2
to #
4
is shifted &pgr;. The optical signal sig
22
having passed the non-linear waveguide
131
is inputted to another birefringence crystal
132
. The birefringence crystal
132
delays a light in the TE mode by one pulse relative to a light in the TM mode. Accordingly, in an optical signal sig
23
having passed the birefringenc
Pak Sung
Ullah Akm Enayet
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