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Polarization Mode Dispersion (PMD) Induced Penalties in High Bit-Rate Systems
Tool Used: OptSim
This example demonstrates the effect that Polarization Mode Dispersion (PMD) has on signal propagation in a fiber and on system performance. The OptSim setup is shown below:
For this setup, a transmitter consists of a 40-Gbps PRBS generator, CW Laser source at 1550 nm, electrical driver, external modulator, and optical power normalizer. A 40-Gbps RZ-modulated signal then is launched into a fiber span. The output from the fiber span is inserted into a receiver. PMD is a statistical effect caused by randomly varying fiber birefringence, therefore the simulation results will be different for different settings of the random seed parameter. PMD causes differential group delay (DGD) between x- and y- polarization components during propagation in fiber, and, hence, eye distortion at the receiver. One can run a parameter scan to obtain DGD and BER/Q for different values of the PMD coefficient and different random seeds. After the simulation run is finished one can double-click on various analysis blocks to view the signal plot, eye diagram, DGD, Stokes parameters on Poincare Sphere, and BER/Q values as shown in the following series of figures:
Let us take a look at two particular runs:
(a) best case for low-PMD fiber; Figure below shows corresponding signal plots and eye diagrams. Corresponding Q-factor (and BER) is 16.0 dB (1.0E-10).
(b) worst case for high-PMD fiber. Figure below shows corresponding signal plots and eye diagrams. Corresponding Q-factor (and BER) is 9.0 dB (2.5E-3).
In conclusion, the PMD-induced differential group delay degrades system performance with penalties more severe at higher bit rates and higher PMD coefficients in fibers.