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.
