Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-06-25
2003-02-11
Healy, Brian (Department: 2874)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C385S122000, C385S123000
Reexamination Certificate
active
06519066
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical performance monitoring of an optical fiber network and more specifically to Stimulated Raman Scattering (SRS) error estimation and its effect on estimating the average power for each optical wavelength using the pilot tone method.
BACKGROUND OF THE INVENTION
Today's optical fiber networks carry many channels along their optical fibers. A significant challenge in maintaining these networks is the problem of power level estimation within these channels at every point in the network or in other words optical performance monitoring. A simple tool for optical performance monitoring and channel identification in DWDM (Dense Wave Division Multiplexing) systems is to add small signal sinusoidal dithers (pilot tones) to optical carriers. Consequently, each optical carrier has a unique sinusoidal dither whose amplitude is proportional to the average power of its carrier. These pilot tones are superimposed to the average power of the optical channel and can be separated and analysed easily. The presence of a specific dither at a particular point in the network therefore indicates the presence of its corresponding wavelength and its amplitude will show the average optical power.
This is true when each dither travels solely with its optical carrier. However, an effect known as Stimulated Raman Scattering (SRS) precipitates an inter-channel energy transfer that interferes with the ability to accurately estimate power levels through pilot tones. This inter-channel energy transfer occurs from smaller wavelengths to larger wavelengths causing larger wavelength power levels to increase. SRS not only causes an interaction between the average power of each channel but also brings about a transfer of dithers between different channels. Therefore, some of the dither of each channel is transferred to other channels and hence its amplitude will not be proportional to the power of its carrier any more. This causes inaccuracy in power level estimation using pilot tones. What is needed is a method of calculating the amount of SRS error at every point in the network and therefore provide a means to correct for the inaccuracy in power readings estimated by the pilot tone technique.
One way to correct for the inaccuracy resulting from the SRS on power estimation is to calculate the amount of SRS error in each span of the network prior to the node of interest and then adding all the calculated SRS errors to obtain the total error at that specific point. In other words, to compensate for SRS error by using local information for a given span. This approach is called “card level” compensation because it only needs the span number to estimate the error assuming that the global information of the network (number of wavelengths, fiber type, first span launch power) is already provided. In card level compensation identical characteristics for each span is assumed. Then the evolution of SRS error in one span is calculated. Finally, the total SRS error at a given point is estimated by adding each individual SRS error up to that point.
Although card level compensation provides a simple and reliable error estimation, it is typically inaccurate in the presence of severe SRS error. In order to accurately compensate for SRS error, the characteristics of the system at every span should be incorporated within the compensation process rather than the global information.
For the foregoing reasons, a need exists for a method of SRS error compensation in an optical fiber network that incorporates system configuration characteristics, or “system level compensation”.
SUMMARY OF THE INVENTION
The present invention is directed to a method and apparatus for system level Stimulated Raman Scattering (SRS) error compensation for pilot tone measurements in an optical fiber network, the network characterized in that it comprises the infrastructure required to measure the power levels of all optical channels using a pilot tone monitoring technique, the compensation method comprising the steps of collecting preceding fiber spans system configuration data, transmitting the configuration data to a network component having computer readable-code adapted to receive the configuration data, calculating local SRS error values incorporating the transmitted configuration data and leveraging the calculated local SRS error values to correct power levels.
System level compensation provides for improved compensation for SRS error by providing to the compensation process important configuration information that has previously been unavailable. The inaccuracy in power readings due to SRS error can now be corrected at each amplifier with great accuracy.
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Harley James
Seydnejad Saeid
Healy Brian
Wood Kevin S
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