Coherent Optical Communications: Historical Perspectives and Future Directions

Coherent optical fiber communications were studied extensively in the 1980s mainly because high sensitivity of coherent receivers could elongate the unrepeated transmission distance; however, their research and development have been interrupted for nearly 20 years behind the rapid progress in high-capacity wavelength-division multiplexed (WDM) systems using erbium-doped fiber amplifiers (EDFAs). In 2005, the demonstration of digital carrier phase estimation in coherent receivers has stimulated a widespread interest in coherent optical communications again. This is due to the fact that the digital coherent receiver enables us to employ a variety of spectrally efficient modulation formats such as M-ary phaseshift keying (PSK) and quadrature amplitude modulation (QAM) without relying upon a rather complicated optical phase-locked loop. In addition, since the phase information is preserved after detection, we can realize electrical post-processing functions such as compensation for chromatic dispersion and polarization-mode dispersion in the digital domain. These advantages of the born-again coherent receiver have enormous potential for innovating existing optical communication systems. In this chapter, after reviewing the 20-year history of coherent optical communication systems, we describe the principle of operation of coherent detection, the concept of the digital coherent receiver, and its performance evaluation. Finally, challenges for the future are summarized. 2.1 History of Coherent Optical Communications This section describes the history of coherent optical communications. Technical details will be summarized in Sect. 2.2, and readers should refer to them appropriately. K. Kikuchi (B) Department of Electrical Engineering and Information Systems, University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan e-mail: [email protected] M. Nakazawa et al. (eds.), High Spectral Density Optical Communication Technologies, Optical and Fiber Communications Reports 6, DOI 10.1007/978-3-642-10419-0_2, C © Springer-Verlag Berlin Heidelberg 2010 11