Optical Nyquist Filtering for Elastic OTDM Signals: Fundamentals and Demonstrations

Optical Nyquist filtering offers an efficient method to generate ultra-high speed optical time division multiplexing (OTDM) signals with bandwidth equal to the symbol rate without intersymbol interference. Since the Nyquist filtering can be simply implemented in a passive optical device such as a liquid crystal on silicon-based switching element, optical Nyquist filtering is potentially advantageous over digital filtering method in terms of energy efficiency and capacity. Especially in extremely coarse granular optical networks based on high baud rate Nyquist-OTDM signals, it would offer better spectral efficiency and elasticity through efficient add/drop multiplexing with small guard band and flexibility in bandwidth allocation. In this paper, we will discuss important roles of the optical Nyquist filtering as a platform to realize ultra-coarse granular elastic networks based on wavelength division multiplexing of Nyquist OTDM signals. While concepts of basic building blocks, such as elastic multi-Tbit/s transceiver and add/drop multiplexing, are presented, we will introduce proof-of-concept experimental demonstrations placing emphasis on the elasticity and the network aspect operations. We will also argue that spectral defragmentation is a critical network functionality at elastic add/drop nodes to improve the spectral utilization, and the realization of the spectral defragmentation by means of an all-optical wavelength converter is superior to the optical-electrical/electrical-optical approach thanks to the modulation format independent property of the employed optical parametric processes.