Loop-synchronous polarization scrambling technique for simulating polarization effects using recirculating fiber loops

A loop-synchronous polarization-scrambling technique has been proposed for the purpose of simulating polarization effects in straight-line systems using recirculating loops. This technique uses a fast polarization controller within a fiber loop. The polarization controller changes its transmission matrix after each round trip of the optical signal circulating through the loop; thus, the periodic polarization transform of the loop is avoided. Moreover, the polarization controller generates a series of random uncorrelated transmission matrixes. Therefore, the mean-square value of differential group delay (DGD) or polarization-dependent loss (PDL) increases linearly with the number of circulations. The matrix expression for a random polarization transform that scatters the state of polarization (SOP) uniformly on the Poincare sphere for any input SOP was also found. Experiments were performed for a 94-km fiber loop that contains a fixed DGD or PDL element. The histograms of polarisation mode dispersion (PMD) induced power penalties at 10^-9 bit error rate (BER) were measured. There is a good agreement between experimental and theoretical results. Using loop-synchronous polarization scrambling, accurate reproduction of the Maxwellian distribution of DGD can be realized when the background PMD of transmission fiber is much smaller than the PMD intentionally introduced into the loop.