Effect of self-phase modulation and cross-phase modulation on OFDM signals in fibre-optic access networks

We consider a passive fibre-optic transmission system using orthogonal subcarriers in channels with quadrature phaseshift keying and quadrature-amplitude modulation formats. A technique is proposed for estimating the signal quality index in orthogonal frequency division multiplexing, based on the transformation of phase distortions of channel carriers into distortions of signal levels. The deterioration in the quality index is found to be due to nonlinear phenomena (self-phase modulation and cross-phase modulation) and noise of the optical amplifier. Relations are given that allow an optimum power level of the optical signal to be chosen as a function of the modulation format of the channels.