Carrier recovery and filtering in optical BPSK heterodyne systems disturbed by phase noise from external cavity semiconductor lasers

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Abstract
The performance of an optical BPSK heterodyne system using external cavity semiconductor lasers and a squaring loop carrier synchroniser is theoretically analysed by means of an analytical method. The method is based on the Gram-Charlier series and includes the effect of filtering on the baseband signal disturbed by phase error due to imperfect carrier recovery and additive Gaussian noise. A detailed modelling of the phase error term is also performed, taking into account the influence of non-Lorentzian laser phase noise. Numerical results for a typical 565 Mb/s BPSK system are presented. It is found that the filter decreases the impact of laser phase noise, namely by reducing the degradation due to relaxation oscillations of the semiconductor lasers.