All-Optical Pulse Shaping for Highest Spectral Efficiency
Pulse shaping gives communications engineers another degree of freedom in designing a link. It holds promise to allow extending transmission reach, achieve optical multiplexing at highest spectral efficiency or to limit nonlinear distortions. A variety of pulse shapes—rectangular, sinc, raised cosine to cite just a few—have been investigated but the important question is how optical transmitters can generate such pulses at the necessary speed. Should the transmitter be realized in the digital domain, the all-optical domain or can it be implemented as a hybrid? In this chapter, the fundamentals for pulse shaping in transmitters and receivers are reviewed. A particular emphasis is on orthogonal frequency division multiplexing (OFDM) where the system’s data are encoded onto subcarriers with a rectangularly shaped impulse response, and Nyquist pulse shaping where the symbols are carried by Nyquist pulses. Electronic, digital and optical processors are described and recent experimental demonstrations are reported.