Guard-Band-Less and Polarization-Insensitive Tunable Wavelength Converter for Phase-Modulated Signals: Demonstration and Signal Quality Analyses

We have designed a practical, tunable wavelength converter, using cascaded two-stage single-pump degenerated four-wave mixing processes in polarization-maintaining highly nonlinear fibers, employed in conjunction with a polarization-diversity scheme. This made it possible to achieve stable and high-quality operation of guard-band-less and polarization-insensitive tunable wavelength conversion for an 86-Gbit/s dual-polarization quadrature phase-shift keying signal over the operating bandwidth of 1535-1565 nm. Two factors of signal quality degradation during the conversion process were analyzed. First, the total amount of the amplified spontaneous emission (ASE) noise accumulated in the converter was studied theoretically. This leads to some important converter design parameters to improve the suppression of the signal quality degradation due to the ASE noise. Second, the increased phase error of the converted signal was investigated using a theoretical prediction, which was confirmed by an experimental study.