Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-02-03
2001-08-14
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06275313
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to optical communication systems and, more particularly, to apparatus for and method of controlling Raman gain tilt in an optical fiber communication system.
BACKGROUND OF THE INVENTION
Stimulated Raman Scattering (SRS) is an important non-linear effect in fibers that can be used to build Raman amplifiers, but that also degrades Wavelength Division Multiplexed (WDM) signals due to channel-to-channel cross talk and spectral distortion (gain tilt) (see e.g., A. R. Chraplyvy, “Optical power limits in Multichannel Wavelength-Division-Multiplexed Systems due to Stimulated Raman Scattering,” Electron. Lett., Vol. 20, 1984, pp. 58-59.). The general case of SRS in a multi-channel system cannot be solved analytically (see S. Tariq, J. C. Palais, “A Computer Model of Non-Dispersion-Limited Stimulated Raman Scattering in Optical Fiber Multiple-Channel Communications,” J. of Lightwave Technology, vol. 11, 1993, 1914-1924). Analytical solutions, however, have been found under the assumptions of negligible signal cross-coupling, negligible pump depletion and/or constant Raman coupling coefficient (see e.g., Y. Aoki, “Properties of Fiber Raman Amplifiers and Their Applicability of Digital Optical Communication Systems,” J. of Lightwave Technology, vol. 6, 1988, 1225-1239). In the case of a Raman amplifier with a strong pump signal and relatively weak signal channels spectrally far removed from the pump, these are certainly valid assumptions. However, for a system with many densely packed WDM channels with a non-uniform power distribution, the above assumptions no longer accurately reflect the physics of the problem.
Since WDM systems are being widely deployed today, what is needed is a technique for compensating for the Raman gain tilt in such WDM systems.
SUMMARY OF THE INVENTION
In accordance with the present invention, we have recognized that the spectral distortion introduced by stimulated Raman scattering (SRS) in an optical fiber transmission system is approximately linear on a dB
m scale and depends solely on the total input power and not on the input power distribution. By maintaining the total input power to the fiber constant, the Raman gain tilt can be compensated for or canceled by using an optical filter.
More particularly, the present invention controls the Raman gain tilt of an optical fiber communication system by sending a plurality of optical signals at a combined power level Ps over the fiber and by sending an optical control signal at a power level Pc over the fiber such that the total power Ps plus Pc is maintained at a predetermined value irrespective of the number of said plurality of optical signals sent over the fiber.
According to another aspect of our invention, an optical fiber communication system is disclosed that comprises (1) a first optical apparatus for transmitting a plurality of optical signals over an optical fiber at a combined power level that does not vary as a function of the number of optical signals transmitted over the fiber and (2) a second optical apparatus for filtering the optical signals using an optical filter having a fixed wavelength response to compensate for a gain tilt introduced to the optical signals transmitted over the fiber.
The present invention can be utilized with WDM or other type of optical communications systems that use optical signals that may be intensity, amplitude, frequency or phase modulated.
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A.R. Chraplyvy, “Limitations on Lightwave Communications Imposed by Optical-Fiber Nonlinearities”, JTL, vol. 8, pp. 1548-1557, 1990.
A.R. Chraplyvy, “Optical power limits in Multichannel Wavelength-Division-Multiplexed Systems due to Stimulated Raman Scattering”, Electron. Lett., vol. 20, 1984, pp. 58-59.
A.K. Srivastava et al., “Fast-Link Control Protection of Surviving Channels in Multiwavelength Optical Networks”, IEEE PTL, vol. 9, Dec. 1997 issue.
Y. Aoki, “Properties of Fiber Raman Amplifiers and Their Applicability of Digital Optical Communication Systems”, J. of Lightwave Technology, vol. 6, 1988, 1225-1239.
M.E. Marhic, F.S. Yang, L.G. Kazovsky, “Cancellation of SRS crosstalk in WDM optical comunication systems by series of parallel techniques”, LEOS'97, paper MC2, 1997.
Denkin Nathan Myron
Zirngibl Martin
Caccuro John A.
Lucent Technologies - Inc.
Pascal Leslie
Singh Dalzid
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