Parametric Amplification and Wavelength Conversion of Single- and Dual-Polarization DQPSK Signals

We demonstrate system experiments for polarization-independent parametric amplification and wavelength conversion of single-channel differential quadrature phase-shift keying (DQPSK) signals up to 112 Gbit/s. Dual-pump four-wave mixing in a highly nonlinear fiber is utilized to achieve up to 20 dB ON-OFF gain and conversion efficiency. The investigations are done in a polarization-dependent (straight) as well as in a polarization-independent (loop) configuration using single- and dual-polarization DQPSK. Particular attention is paid to the resulting optical signal-to-noise ratio penalties after the amplification. We ascribe this to pump-induced nonlinear phase noise. It is shown that this type of excess noise is increased for the loop configuration in comparison with the straight configuration due to Rayleigh backscattering and that it depends on the input polarization of the signal. Still, the penalty can be made negligible using high-quality pump sources.