Compensation of Raman scattering and EDFA's nonuniform gain in ultra-long-distance WDM links

Two physical mechanism effects which cause nonuniform spectral gain in a long-distance WDM system are Raman scattering and EDFA's, with the Raman gain increasing with wavelength and the EDFA gain profile in a cascaded-amplifier system having a peak at 1560 nm. Both effects limit system capacity by causing a wide variation in signal powers and SNR among several WDM channels. We analyze the scenario in which these two deleterious effects partially compensate each other so that a more uniform overall spectral gain can be achieved. By adjusting the link-loss between amplifiers and by strategically placing six WDM channels between 1560 and 1563 nm, the SNR differential among these channels is reduced from 10 dB to 0.5 dB for a 10 000 km link, thereby significantly enhancing the system performance.