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The effect of the fast polarization scrambling on the nonlinear distortions of the signal propagated in the fiber optic link

Keywords:

R.V. Kutluyarov - Ph.D. (Eng.), Senior Research Scientist, Ufa State Aviation Technical University A.K. Sultanov - Dr.Sc. (Eng.), Professor, Ufa State Aviation Technical University V.K. Bagmanov - Dr.Sc. (Eng.), Professor, Ufa State Aviation Technical University


Polarization scrambling is a useful tool for avoiding of multiple errors related to polarization-dependent gain in erbium amplifiers and polarization-dependent loss in fiber optic communications. It was shown in recent articles that polarization scrambling may be effectively used for mitigating penalties related to undesirable polarization effects as PMD and for monitoring of such effects. We show that fast polarization scrambling at transmitting end of the fiber link may be used for mitigating penalties related to Kerr-nonlinearities. Fast polarization scrambling efficiency was defined by simulation. We have simulated propagation of two 40Gbps WDM signals by numerical solving system of coupled nonlinear Schrödinger equations. For solving equations we have used split-step Fourier-method and coarse-step method. Model that we used includes chromatic dispersion, fiber loss, Kerr-nonlinearities and random birefringence. Simulated link includes five spans, consisting of non-zero dispersion shifting fiber, dispersion compensating fiber and erbium amplifier. Receiver consists of pass-band optical filter and square-law detector. To model polarization scrambler we used rotating linear birefringent wave plate. Frequency of rotation is assumed as scrambling frequency. We have investigated efficiency of the scrambler consisting of the only half-wave plate. This kind of scrambler rotates Stokes vector of the signal through the Poincare sphere equator. For scrambling frequency range 1 – 60 000 kHz we have found out the local maximum of Q-factor improvement function. Optimum scrambling frequency is 32,5 MHz. Q-factor improvement is 0,9 – 1,1 dB at this frequency. Scrambler, consisting of the concatenation of the quarter-wave plate, half-wave plate and other quarter-wave plate is more complex and more effective because it rotates Stokes vector on the whole Poincare sphere. Each of the three wave plates has its own rotation frequency. Simulation shows that this kind of scrambler providing for better signal improvement than the only half-wave plate. Timing jitter due to polarization scrambling is the main factor limiting its employment in fiber optics. The origin of jitter is that signal polarization moving between birefringent axis of the fiber. Therefore polarization scrambling may be used only for links with small total PMD.
References:

 

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