A bandpass optical channel contains four degrees of freedom (DOF) corresponding to two quadrature phase and two quadrature polarization components. If independent information is sent on each of these four components, the spectral efficiency is nearly four times that achieved when utilizing a single component. For a given spectral efficiency, the minimum number of required photons per bit N_p diminishes with increasing number of degrees of freedom (DOF), but the lower limit on N_p, obtained when the spectral efficiency goes to zero, is 0.693 independent of DOF. For the case of an incoherent channel with square law detection and without polarization filters an upper bound to spectral efficiency and a lower bound to the number of photons per bit is obtained by assuming parallel coherent channels, but in which the same signal is transmitted in each channel. An even tighter bound on performance of the incoherent channel is obtained by computing the mutual information for a specific choice of input distribution utilizing a Markov chain approach.

Date Published: 25 October 2005
PDF: 9 pages
Proc. SPIE 6012, Optical Transmission Systems and Equipment for WDM Networking IV, 60120K (25 October 2005); doi: 10.1117/12.630187

Published in SPIE Proceedings Vol. 6012:
Optical Transmission Systems and Equipment for WDM Networking IV
Ken-ichi Sato; Werner Weiershausen; Achyut K. Dutta; Benjamin B. Dingel, Editor(s)