Sensitivity degradation due to fibre dispersion-induced PM-AM noise in optically preamplified direct detection systems

It is well known that sensitivities as good as 45 photons/bit can be achieved when direct detection systems use ideal optical preamplifiers. We study the degradation in this sensitivity due to the presence of PM-AM noise that is caused by the laser phase noise and fibre dispersion. The results form a basis for the optimum design of optically preamplified direct detection systems by selection of a suitable laser source and fibre.