Since the advent of optical communications, a great technological effort has
been devoted to the exploitation of the huge bandwidth of optical fibers. Starting
from a few Mb/s single channel systems, a fast and constant technological
development has led to the actual 10 Gb/s per channel dense wavelength division
multiplexing (DWDM) systems, with dozens of channels on a single
fiber. Transmitters and receivers are now ready for 40 Gb/s, whereas hundreds
of channels can be simultaneously amplified by optical amplifiers.
Nevertheless, despite such a pace in technological progress, optical communications
are still in a primitive stage if compared, for instance, to radio
communications: the widely spread on-off keying (OOK) modulation format
is equivalent to the rough amplitude modulation (AM) format, whereas the
DWDM technique is nothing more than the optical version of the frequency division
multiplexing (FDM) technique. Moreover, adaptive equalization, channel
coding or maximum likelihood detection are still considered something
“exotic” in the optical world. This is mainly due to the favourable characteristics
of the fiber optic channel (large bandwidth, low attenuation, channel
stability, ...), which so far allowed us to use very simple transmission and
detection techniques.