Electronic Mitigation of Polarization Mode Dispersion
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Polarization Mode Dispersion induces polarization dependent propagation. Consequently it generates a multiple imaging of the light pulse carrying the information. Its first order appears as a dual path fading channel of Maxwellian statistics. It results in harmful impairments that prevent the upgrade and installation of high bit-rate systems. The random process PMD exhibits a strong frequency dependence, so that its amelioration requires channel by channel, non-linear, adaptive mitigation. Electronic mitigation appears as a very attractive solution to overcome the limit set by the PMD. Consequently, we considered the implementation of these solutions at the receiver in the electrical domain. We verified that these linear and non-linear equalization techniques can greatly reduce the power penalty due to PMD. Equalization's performance depends highly on the type of systems considered. For the two main types of systems: thermal noise limited systems and systems exhibiting ASE (systems using optical amplifiers), we demonstrated and quantified the induced improvement (measured as power penalty reduction). The most sophisticated technique that we considered (NLC+FDE) handles any kind of first order PMD within a 4 dB margin in the thermal noise limit. This extended to a 11 dB margin in the presence of ASE. This comes from the limitation set by the signal dependence of the noise. In fact, these DSP techniques do a better job at reducing very high penalty. Consequently, for a power and ISI limited link, it may be required to associate to electronic solutions optical compensation in order to reach acceptable performance. On the other hand, for links having large power margin or exhibiting reasonable PMD, electronic techniques appear as an easy, inexpensive and convenient solution. We derived in this work the bounds to NLC performance in the presence of ASE. Therefore, we extended the usual results of the thermal noise limit to the particular case of signal dependent noise. We also made clear that optical systems, because of their noise specificities can not be studied or designed as others links. Notions such as eye opening, SNR and ISI need to be carefully defined and adapted to this case. We have provided in this work PMD dependent power penalty map for known systems. Given the link's statistics and characteristics, one can determine, following our structure, which mitigation techniques allow upgrade.
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