Transmission aspects on broadband coherent optical access networks

This paper addresses transmission aspects on very high aggregate wavelength-division multiplexing (WDM) based passive optical networks (PON) capable of delivering, up to 1000 wavelengths/users separated by a few Gigahertz broadband connections at 1-10Gb/s through a single optical fiber. These networks rely on employing high-order modulation formats such as M-ary PSK (phase-shift keying) or M-ary QAM (quadrature amplitude modulation) along with coherent detection to provide high receiver sensitivity and wavelength selectivity. The first part of the paper focus on the most relevant technical challenges as to comply with the already populated wavelength spectrum by several PON technologies. This study covers the analysis of the most relevant fiber nonlinearities such as self-phase modulation, cross-phase modulation and four-wave mixing and their dependence on power, transmission distance and modulation format. The second part of the paper deals with optimizing the overall system performance by using a channel frequency - tuning scheme for mitigating inter-channel four-wave mixing. We found that the limitations imposed by inter-channel four-wave mixing can be reduced allowing by 3.5 dB improvement on the signal to noise ratio.