| 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. |