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Summary form only given. Next-generation undersea communications systems plan on using wavelength-division multiplexing (WDM) to increase information capacity. New techniques are needed to monitor the health of these systems. This paper summarizes experimental results using a separate side-tone wavelength to monitor small degradations in a 2100-km bi-directional, 4×2.5 Gbit/s WDM optically amplified system with negligible transmission impairments.