Nonlinearity mitigation of DQPSK signal by frequency-shift free spectral inversion using counter-propagating dual pump four-wave mixing in a semiconductor optical amplifier

We present a scheme of nonlinearity mitigation of DQPSK signal in transmission fiber using frequency-shift free mid-span spectral inversion (MSSI). MSSI is obtained by using counter-propagating dual pump four-wave mixing in a semiconductor optical amplifier (SOA). Frequency-shift between signal and conjugate is avoided by keeping two pump frequencies symmetrical about the signal frequency. We examine the performance of MSSI by scrutinizing the improvement of the Q-factor of a 100 Gbps DQPSK signal transmitted over standard single mode fiber, as a function of launched power for different span lengths and number of spans. We demonstrate a 7.8 dB improvement in input power dynamic range and an almost 92 % increase in transmission length for optimum MSSI parameters of -2 dBm pump power and 400 mA SOA current.