Impact of self-phase modulation on optical CPFSK system

Abstract

A detail theoretical analysis is presented for direct detection optical CPFSK and heterodyne optical CPFSK system with Mach-zehnder Interferometer (MZI) as an optical frequency discriminator (OFD). The analysis is carried out taking into account the combined effect of Self-phase Modulation and chromatic dispersion of optical fibre. Fluctuation in signal power due to GVD causes a random phase modulation of the signal itself due to nonlinear refractive index of fibre. The effect is termed as selfphase- modulation (SPM) and causes spectral broadening of the transmitted signal and higher bandwidth requirement and degrades receiver performance. The statistics of the signal phase fluctuations at the output of the fibre caused by non-linear filtering due to chromatic dispersion are to be determined in terms of its moments and the probability density function (pdf) of the random phase fluctuation due to SPM and fibre chromatic dispersion. The analysis also includes the effect of GVD and laser phase noise. The method is based on the linear approximation of the output phase of a linearly filtered angle-modulated signal such as the CPFSK signal considered in this work. An equivalent transfer function of a single mode fibre is derived which includes the effects of SPM in presence of GVD. The overall degradation in signal phase is also derived following by an analytical approach. The expression for HER is then derived considering the effects ofGVD and SPM in presence oflaser phase noise and receiver noise. The analysis is carried out for both direct detection receiver and heterodyne delay demodulation receiver. Using the noise statistics and moments, the bit error rate (HER) performance of the receiver is then evaluated for different values of input power at different modulation index at a bit rate of 10 Gbit/s. The penalty suffered by the system due to dispersion and Self-phase modulation is then determined at a HER of 10,9. For different fibre length penalty is measured individually for direct detection and heterodyne CPFSK system. Also for negative dispersion coefficient the penalty is determined. Finally, theoretical results are compared with the results obtained from simulation.