System based control of optical amplifier transmission...

Optical: systems and elements – Optical amplifier – Correction of deleterious effects

Reexamination Certificate

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C359S199200

Reexamination Certificate

active

06313940

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to optical transmission systems and more particularly relates to optical transmission amplifiers.
BACKGROUND OF THE INVENTION
An optical amplifier may be arranged to perform a particular system/amplifier function in response to a change in particular stimuli. For example, an optical amplifier may be arranged to change the level of its optical pump power in response to a change in the power level of an incoming optical signal, which may be due to a change in the number of optical channels carried by the incoming signal or due to a change in span loss.
A change in an incoming optical signal may also be due to an optical fiber nonlinearity. One such nonlinearity is commonly referred to as Raman gain. The Raman gain, or effect, becomes particularly troublesome when an appreciable level of optical power distributed over a certain range of wavelengths is pumped into an optical fiber. In that instance, the Raman gain is tilted in favor of the channels having the longer wavelengths, which degrades the Signal-to-Noise Ratio (SNR) of the signals in the lower wavelength channels and thus seriously degrades their performance. Moreover, the power levels of higher wavelength channels may increase, thereby making those channels more susceptible to non-linearity problems. Raman gain may be dealt with at an amplifier by “tilting” the outputted optical signal in a direction opposite to the Raman gain, as disclosed in copending U.S. patent application of Ser. No. 09/265,943 filed Mar. 8, 1999, which is hereby incorporated by reference.
A problem arises when a downstream optical amplifier in an optical transmission system having a plurality of optical amplifiers disposed along the optical transmission path responds independently in the described manner to a change in the power level of an incoming optical signal. More specifically, a downstream optical amplifier may tilt its output signal in the wrong direction, or exaggerate the tilt, if it performs its tilt adjustment before an upstream optical amplifier has completed its tilt adjustment. Also, the optical transmission system may not stabilize with respect to a change in the input signal, or a change in some other nonlinearity, if each optical amplifier in the transmission path is allowed to proceed independently.
SUMMARY OF THE INVENTION
The foregoing problem is dealt with by arranging an optical amplifier such that, in response to detecting particular stimuli occurring at an input or receipt of an adjustment start message from an upstream amplifier, it starts the aforementioned adjustments slowly using small incremental steps and sends an adjustment start message over the optical transmission media to a next downstream amplifier. The amplifier will complete the adjustment using small steps if it does not receive an adjustment done message from an upstream optical amplifier within a predetermined period of time of starting the adjustment in response to detecting the stimuli. If the optical amplifier receives an adjustment start message, then it continues to perform the adjustment and quickly completes the adjustment using large steps when it receives an adjustment done message from the upstream optical amplifier. At that point, the optical amplifier sends an adjustment done message to the next downstream amplifier if it is not the tail-end amplifier.
As an aspect of the invention, the head-end optical amplifier performs the adjustments using only large steps. As another aspect of the invention, the head-end optical amplifier periodically initiates the adjustments even if it does not detect such stimuli.
These and other aspects of the invention will be appreciated from the following detailed description taken in conjunction with the accompanying drawings.


REFERENCES:
patent: 5440418 (1995-08-01), Ishimura et al.
patent: 6023366 (2000-02-01), Kinoshita et al.
patent: 6151148 (2000-11-01), Harano

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