In this book chapter, optical signal processing technology, including optical wavelength conversion, wavelength exchange and wavelength multicasting, for phase-noise-sensitive high-order quadrature-amplitude modulation (QAM) signals will be discussed. Due to the susceptibility of high-order QAM signals against phase noise, it is imperative to avoid the phase noise in the optical signal processing subsystems. To design high-performance optical signal processing subsystems, both linear and nonlinear phase noise and distortions are the main concerns in the system design. We will first investigate the effective monitoring approach to optimize the performance of wavelength conversion for avoiding undesired nonlinear phase noise and distortions, and then propose coherent pumping scheme to eliminate the linear phase noise from local pumps in order to realize pumpphase-noise-free wavelength conversion, wavelength exchange and multicasting for high-order QAM signals. All of the discussions are based on experimental investigation.

Optical 64-QAM transmitter using QPSK modulators in tandem driven by binary electrical signals is proposed. The proposed transmitter eliminates the need for complex electronics required for the generation of multiple-level signals and provides a Gray-coded constellation with high quality eye diagrams. The design could be scaled to higherorder QAM based on the user needs. Conclusion: We have presented a scalable, optical square 64-QAM transmitter using tandem QPSK modulators driven by binary electrical signals. The transmitter designs for 64-QAM have been simulated at 50 Gbaud using OptiSystem. The results demonstrated Gray-coded symbol constellations and high quality eye diagrams.

An eight-phase-shift-keying signal is experimentally deaggregated onto two four-pulse amplitude modulation signals using nonlinear processes in optical elements. Quadraturephase-shift-keying signals are similarly de-multiplexed into two binary phase shift keying signals by mapping the data points onto the constellation axes. De-multiplexing performance is evaluated as a function of the optical signal-to-noise ratio of the incoming signals. The effect of phase noise is also studied.