Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2001-09-18
2003-10-28
Nasri, Javaid H. (Department: 2839)
Optical waveguides
With optical coupler
Plural
Reexamination Certificate
active
06640026
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority of Japanese Patent application number 2000-281927, filed Sep. 18, 2000.
FIELD OF THE INVENTION
This invention relates to an optical multiplexer to multiplex optical signals having different wavelengths and an optical demultiplexer to demultiplex wavelength multiplexed optical signals.
BACKGROUND OF THE INVENTION
To realize high-density wavelength multiplexed optical transmission, an optical multiplexer/demultiplexer to multiplex optical signals having different wavelengths and to demultiplex wavelength multiplexed optical signals into individual wavelengths in a low crosstalk. As one means, a combination of an interleaved optical filter to distribute a plurality of optical signals having wavelengths at constant interval into two ports and an arrayed waveguide grating has been studied.
As shown in
FIG. 3
, an interleaved optical filter consists of multi-stage Mach-Zehnder interferometers connected in serial and has periodic bandpass characteristics and relatively steep loss wavelength characteristics (See H. Arai et al., “Improvement of Isolation of Interleave Multi/Demultiplexers using three-stage MZIs”, Proceedings of The 2000 IEICE General Conference, C-3-85, JAPAN, 2000).
FIG. 4
is a schematic diagram showing insertion loss wavelength characteristics of an interleaved optical filter to reduce interval of 100 GHz of wavelength multiplexed optical signals into interval of 50 GHz.
However, when the wavelength interval become as narrow as 0.3 nm, it becomes difficult to sufficiently suppress the crosstalk. In addition, an optical filter generally has chromatic dispersion characteristics due to a production error etc., and this causes waveform deterioration owing to frequency chirping. As shown in
FIG. 5
, assuming that the transfer function between ports X
0
and Y
0
of an interleaved optical filter is H
1
, the transfer function between ports X
1
and Y
1
is H
1
*, the transfer function between port X
0
and Y
1
is H
2
, and the transfer function between ports X
1
and Y
0
is H
2
*, those transfer functions show chromatic dispersion characteristics and transmission factor characteristics as shown in FIG.
6
.
FIG. 6
is a schematic diagram showing characteristics in one cycle of the straight direction (between the ports X
0
, Y
0
and between the ports X
1
, Y
1
) and the cross direction (between the ports X
0
, Y
1
and between the ports X
1
, Y
0
). In
FIG. 6
, the horizontal axis expresses wavelength, and the vertical axis expresses chromatic dispersion and transmission factor. The transmission characteristics of the transfer functions H
1
and H
1
* are equal each other and the transmission characteristics of the transfer functions H
2
and H
2
* are equal each other. However, the chromatic dispersion characteristics of the transfer functions H
1
and H
1
* vary reversely from each other relative to a wavelength, and the chromatic dispersion characteristics of the transfer functions H
2
and H
2
* vary reversely from each other relative to a wavelength. In other words, when the chromatic dispersion characteristics of the transfer functions H
1
and H
1
* are multiplied, a constant value which does not depend on the wavelength is obtained, and similarly, when the chromatic dispersion characteristics of the transfer functions H
2
and H
2
* are multiplied, a constant value which does not depend on the wavelength is obtained. Since the interleaved optical filter satisfies the principle of reciprocity, its transfer function basically does not depend on the propagation direction of the light.
Although it is possible to obtain the steep loss wavelength characteristics, namely the chromatic dispersion characteristics having little crosstalk by increasing the number of the stages of Mach-Zehnder interferometer, it is difficult to produce such an optical filter and also there is a problem that the element length is increased.
Furthermore, because the chromatic dispersion that each wavelength receives is not even, a dispersion compensating element must be newly installed to equalize the chromatic dispersion for each wavelength.
When two interleaved optical multiplexing/demultiplexing elements are connected in serial so that the optical signal propagates passing through both straight and cross connections, it is theoretically possible to flatten the chromatic dispersion characteristics (See H. Arai et al., “Reduction of Chromatic Dispersion of Wavelength Interleaver using three-stage MZIs”, Proceedings of The 2000 Electronics Society Conference, C-3-15, JAPAN, 2000). However, it is difficult to produce two interleaved optical multiplexing/demultiplexing elements having the identical transmission characteristics, and thus chromatic dispersion depending on a wavelength still remains in a practical situation.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optical multiplexer and demultiplexer having flat chromatic dispersion characteristics.
An optical multiplexer according to the invention consists of an interleaved optical filter having first and second port pairs, a connector to connect between the two ports composing the second port pair of the interleaved optical filter, a first optical circulator having first, second and third ports to output an input light of the first port from the second port and output an input light of the second port from the third port, a second optical circulator having first, second and third ports to output an input light of the first port from the second port and output an input light of the second port from the third port, and a combiner to combine the output light from the third port of the first optical circulator and the output light from the third port of the second optical circulator.
The second port of the first optical circulator connects to one of the two ports composing the first port pair of the interleaved optical filter, and a first signal light enters the first port of the first optical circulator. The second port of the second optical circulator connects to the other port of the two ports composing the first port pair of the interleaved optical filter, and a second optical signal enters the first port of the second optical circulator.
Also, the optical multiplexer according to the invention further comprising an optical filter having first and second port pairs, a connector to connect between the two ports composing the second port pair of the optical filter, a first optical circulator having first, second and third ports to output an input light of the first port from the second port and output an input light of the second port from the third port, a second optical circulator having first, second and third ports to output an input light of the first port from the second port and output an input light of the second port from the third port, and a combiner to combine the output light from the third port of the first optical circulator and the output light from the third port of the second optical circulator.
The optical filter has the optical transmission characteristics in which the chromatic dispersion characteristics of two transfer functions in straight direction between the first and second port pairs are in the opposite relation from each other relative to a wavelength and the chromatic dispersion characteristics of two transfer functions in cross direction between the first and second port pairs are also in the opposite relation from each other relative to a wavelength, and transmission wavelengths which differ according to the straight and cross propagation between the first and second port pairs.
The second port of the first optical circulator connects to one of the two ports composing the first port pair of the optical filter, and a first optical signal enters the first port. The second port of the second optical circulator connects to the other port of the two ports composing the first port pair, and a second optical signal enters the first port.
An optical demultiplexer according to the invention consists
Suzuki Masatoshi
Tsuritani Takehiro
KDD Submarine Cable Systems Inc.
Nasri Javaid H.
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