Theory of polarization spreading techniques-Part II

This paper, together with a companion one by Poggiolini and Benedetto (see IEEE Trans. Commun., vol.42, p. 2105-2118, May 1994), constitute a thorough presentation and analysis of a series of techniques grouped under the denomination “polarization spreading”, the intent of which is that of making the performance of a coherent optical receiver insensitive to the polarization transformations occurring along the optical fiber, without resorting to polarization compensation or control. The principle on which polarization spreading operates is that of spreading the signal power over different states of polarization, so that heterodyning of at least half of the optical incident bit energy is always ensured. The main obtained results concern a condition that guarantees the maximum heterodyned signal energy, some classes of spreading waveforms satisfying the above condition, the derivation of the structure and performance of optimum receivers in Gaussian noise; and, presented in this paper, the analysis of a suboptimum but easily implementable receiver and a detailed comparative analysis of the spectral broadening induced by the different spreading techniques. The results encompass most of the binary optical coherent modulation schemes proposed and experimented, such as ASK, FSK and DPSK