Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
1999-09-23
2001-05-01
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
Reexamination Certificate
active
06226091
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
N/A
BACKGROUND OF THE INVENTION
A wavelength division multiplexer/demultiplexer can be fabricated from a fiber optic Mach-Zehnder interferometer (MZI). The MZI is fabricated from a pair of symmetric couplers. Identical fiber Bragg gratings are written in the interfering arms between the couplers. A fiber Bragg grating (FBG) is a change in the refractive index in the fiber core that reflects a selective wavelength on the fiber.
In operation of the MZI to drop or extract a wavelength, for example, as a demultiplexer, a signal carrying several channels or wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
, &lgr;
4
, and &lgr;
5
is input into the first fiber of the first coupler. Other numbers of channels, for example, 4 or 8 channels, may be input into the coupler. The signal is split at the coupler to pass along both arms. The FBGs, which are identical, are resonant at a selected frequency, for example, &lgr;
4
. Thus, at the FBGs, &lgr;
4
is reflected, passes back through the first coupler, and is extracted on the second fiber of the first coupler. The remaining wavelengths, &lgr;
1
, &lgr;
2
, &lgr;
3
, and &lgr;
5
, pass through the second coupler and are output on the second fiber of the second coupler.
In operation to add or insert a wavelength, for example, as a multiplexer, a signal having the wavelength &lgr;
4
is inserted on the first fiber at the second coupler. A signal of various wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
, and &lgr;
5
, is input into the first fiber of the first coupler. As described above with respect to the demultiplexer, &lgr;
4
is reflected at the FBGs. &lgr;
4
is then output on the second fiber of the second coupler. Thus, the output of the second coupler includes all the wavelengths, &lgr;
1
, &lgr;
2
, &lgr;
3
, &lgr;
4
, and &lgr;
5
.
In typical Mach-Zehnder interferometers fabricated from symmetric couplers, 30 dB isolation is limited to spans of ±20 nm about the desired wavelength.
SUMMARY OF THE INVENTION
The present invention provides an optical fiber Mach-Zehnder interferometer fabricated from asymmetrical couplers, which allow 30 dB isolation over a wider wavelength range about a desired wavelength.
More particularly, the asymmetric optical fiber Mach-Zehnder interferometer of the present invention comprises first and second optical fibers connected at a first coupling region and a second coupling region. The first and second optical fibers form two interfering arms between the first and second coupling regions. To form the asymmetry, the propagation constant in a portion of the first optical fiber in one of the coupling regions, for example, the first coupling region, differs from the propagation constant in a portion of the second optical fiber in that coupling region. The propagation constant in the first optical fiber and the propagation constant in the second optical fiber are chosen to provide 30 dB isolation in a through port of the Mach-Zehnder interferometer over a span of more than ±20 nm, and preferably more than ±60 nm, about a desired wavelength. The splitting ratio of the first coupling region is controlled to split the power 50 percent at a desired wavelength.
REFERENCES:
patent: 5841920 (1998-11-01), Lemaire et al.
patent: 5875272 (1999-02-01), Kewitsch et al.
S.J. Madden, P.G. Jacob, J.S. Harradence, C. Robilliard: “High Performance Modular DWDM Implementation Based On Fibre Bragg Grating Mach Zehnder Interferometers,”NFOEC1998, vol. 1, pp. 453-463.
F. Bilodeau, D.C. Johnson, S. Thériault, B. Malo, J. Albert, and K.O. Hill: “An All-Fiber Dense-Wavelength-Division Multiplexer/Demultiplexer Using Photoimprinted Bragg Gratings,”IEEE Photonics Technology Letters,Apr. 1995, vol. 7, No. 4, pp. 388-390.
D.A. Nolan and W.J. Miller: “Wavelength-Tuned Mach-Zehnder Device,”OFC '94 Technical Digest,WF5, pp. 94-95.
F. Gonthier, D. Ricard, S. Lacroix, and J. Bures: “Wavelength-Flattened 2 x 2 Splitters Made of Identical Single-Mode Fibers,”Optics Letters,Aug. 1, 1991, vol. 16, No. 15, pp. 1201-1203.
A. Tervonen, S. Honkanen, S. I. Najafi: “Analysis of Symmetric Directional Couplers and Asymmetric Mach-Zehnder Interferometers As 1.30- and 1.55-&mgr;m Dual-Wavelength Demultiplexers/Multiplexers,”Optical Engineering,Sep. 1993, vol. 32, No. 9, pp. 2083-2090.
Thomas & Betts International Inc.
Turner Samuel A.
Weingarten, Schurgin Gagnebin & Hayes LLP
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