I am trying to simulate the whole long haul optical transmission system in the matlab.
However, I have a doubt about the PM-QPSK-WDM system because I don't know how to include the linewidth in the samples? Also how to make the WDM output is a single vector of samples with spectrum equivalent o WDM channel spectrum.
Create a pair of single channel complex time-domain vectors to represent the optical field in each polarisation state. Use a sufficiently small time step to describe the full optical bandwidth of the WDM array. Transform into the frequency domain by FFT, then Matlab circshift by channel_frequency_offset / frequency_step. IFFT, and add the frequency-shifted channel to the WDM array. Repeat for each WDM channel in turn.
You can also multiply the field by exp(i Delta-f t), but the FFT route is efficient, and maintains cyclic boundary conditions without discontinuity.
What do you know about the laser's frequency and phase excursions? What is the correlation time? One option is to calculate a quasi-random time-domain frequency deviation profile, integrate to derive a vector of phase error as a function of time, then multply the field by exp(i phase). If subsequent FFT processing is needed, AC filter, or apply a linear tilt to the phase profile to ensure any phase discontinuity between start and end of the sequence is a multiple of 2 pi.
Simulate WDM multiplexing system in Matlab is very, very slow. I highly recommend Java.
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