The growth of Internet traf?c in recent years surpassed the prediction
of one decade ago. Data stream in individual countries already reached
terabit/s level. To cope with the petabit class demands of traf?c in
coming years the communication engineers are required to go beyond the
incremental improvement of today's technology. A most promising
breakthrough would be the introduction of modulation f- mats enabling
higher spectral ef?ciency than that of binary on-off keying scheme,
virtually the global standard of ?ber-optic communication systems. In
wireless communication systems, techniques of high spectral density
modulation have been well developed, but the required techniques in
optical frequency domain are much more complicated because of the
heavier ?uctuation levels. Therefore the past trials of coherent optical
modulation/detection schemes were not successful. However, the addition
of high-speed digital signal processing technology is the fundam- tal
difference between now and two decades ago, when trials of optical
coherent communication systems were investigated very seriously. This
approach of digital coherent technology has attracted keen interest
among communication specialists, as indicated by the rapid increase in
the pioneering presentations at the post-deadline sessions of major
international conferences. For example, 32 terabit/s transmission in a
?ber experiment based on this technology was reported in post-deadline
session of Optical Fiber Communication Conference (OFC) 2009. The
advancement of the digital coherent technologies will inevitably affect
the network architecture in terms of the network resource management for
the new generation photonic networks, rather than will simply provide
with huge transmission capacity.
