| 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.
But now we are slightly moving toward the physical limits of the fiber and,
as it was the case for radio communications, more sophisticated techniques
will be needed to increase the spectral efficiency and counteract the transmission
impairments. At the same time, the evolution of the techniques should be
supported, or better preceded, by an analogous evolution of the theory. Looking
at the literature, contradictions are not unlikely to be found among different
theoretical works, and a lack of standards and common theoretical basis can be
observed. As an example, the performance of an optical system is often given
in terms of different, and sometimes misleading, figures of merit, such as the
error probability, the Q-factor, the eye-opening and so on. Under very strict hypotheses,
there is a sort of equivalence among these figures of merit, but things
drastically change when nonlinear effects are present or different modulation
formats considered. |
