Abstract
Semiconductor optical amplifier (SOA) is a promising candidate for metro and access networks thanks to its wide amplification bandwidth and potential low cost. However, in wavelength-division-multiplexing (WDM) systems, nonlinear cross gain modulation (XGM) can degrade the transmission performances by inducing strong cross-talk between WDM channels. Recently, differential phase-shift keying (DPSK) modulation has attracted much interest for its enhanced immunity against the effects of XGM nonlinearities [1]. DPSK signals can be obtained by biasing a single Mach-Zehnder Modulator (MZM) at a null, and using a high-power RF driver amplifier with Vp-p ~ 2Vπ to have a phase shift between 0 and π. Employing an intensity modulator, instead of a pure phase one to generate optical DPSK can have the advantage that exact n phase shifts are produced. However, intensity modulation is present in the signal and can degrade the signal amplification through SOA by phase-amplitude coupling, especially if one operates near the SOA saturation power, as this can occur with a complete WDM comb.
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