Impact of SPM on dispersion compensated DPSK transmission link under tight optical filtering

The tolerance against self-phase modulation (SPM) of three differential phase-shift keying (DPSK) modulation schemes viz. conventional DPSK (CDPSK), symmetrical DPSK (SDPSK) and pi/4-DPSK in a 40-Gb/s multi-span dispersion compensated single-mode fiber (SMIF) link is studied through detailed numerical simulation. The study takes into account the influence of pulse duty-ratio, tight optical filtering, in-line amplifier noise and the impact of delay mismatch in the delay-demodulation receiver while comparing the variants of DPSK systems. The numerical analysis clearly suggests the need for smaller duty ratio for all DPSK formats under tight optical filtering for achieving minimum eye-opening penalty (EOP) due to SPM. Return-to-zero (RZ)-CDPSK exhibits the highest tolerance towards the SPM induced penalty in noisy and noiseless situation whereas SDPSK shows more robustness to the uncertainty in the interferometer delay mismatch.