I used optisystem to simulate the nonlinearity impacts on PM-QPSK system with single mode fiber, however, the phase correction in DSP at the receiver remove the effect of nonlinearity, which is a pure phase rotation in that case.
Can I study the effect of nonlinearity on coherent systems in optical fiber without the need to optisystem? and how can I isolate the effect of nonlinearity on system performance?
In the litrature,I found that I can simulate the QPSK sequence using the so-called quaternary De Bruijn sequences but I am not sure about the remaining parts of the system (WDM, filter and photodiodes)
SPM produces a pure phase rotation of a QPSK signal only when you have both an ideal square wave modulation, where the transition time between successive symbols is small fraction of the symbol period, and there is negligible chromatic dispersion over the non-linear interaction length. However, if you only sample the waveform once per symbol, you may not be aware of the waveform chirp introduced by SPM. Similarly, with high chromatic dispersion, the waveform shape evolves rapidly within the non-linear interaction length, reducing the magnitude of systematic waveform distortion.
A simple way to visualise the impact of Kerr effect non-linearity is to plot the QPSK constellations after correcting for mean local oscillator phase and frequency drift, chromatic dispersion and polarisation rotation. Depending on the dispersion regime, it may be appropriate to treat the non-linearity as an equivalent noise, as a waveform distortion leading to eye closure, or as a combination of the two. If you omit fibre birefringence and local oscillator phase drift from your model, you can focus on chromatic dispersion, SPM phase shift and the waveform distortion resulting from the two acting together.
Variants of the split step Fourier method are widely used to simulate non-linear propagation in single mode fibres. WDM filters and electrical low-pass filtering are readily simulated as simple linear filters if you know the frequency dependence of transmission.
Photodiodes can be modelled as square law detectors, in which the photocurrent is proportional to the instantaneous optical power. Include the junction and package capacitances when estimating the frequency response of the pre-amplifier. There will be multiple noise sources, but in a well-designed coherent system, you should be dominated by optical noise in the input signal.
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