Phase noise sensitivity and compensation techniques in long-haul coherent optical links

In coherent optical systems, the phase noise due to transmit and receive lasers has a significant impact on the receiver performance - the sensitivity to phase noise and the capability of electronic processing to compensate for its effects are reduced for increasing lasers' linewidths. However, a detailed analysis of the effects of phase noise on the system performance cannot leave other aspects out of consideration. In fact, the picture is completely different for optical systems using single-carrier or orthogonal frequency division multiplexing, in the presence or absence of inline dispersion compensation, or when an unmodulated carrier is also transmitted for phase noise compensation purposes. In addition, transmit and receive phase noise components may have a different impact. All these aspects are analyzed and discussed in this paper. A novel digital coherence enhancement technique, able to significantly reduce the phase noise of transmit or receive lasers by using an interferometric device plus a very simple electronic processing, is also proposed.