Precise measurements and their analysis of GAWBS-induced depolarization noise in various optical fibers for digital coherent transmission

We undertake precise measurements of guided acoustic wave Brillouin scattering (GAWBS) depolarization noise caused by the TR_2,m mode (torsional and radial mode) in various fibers and analyze the results. And we describe the influence of the noise on digital coherent transmission. We first show that the TR_2,m mode is distributed over a wider bandwidth when the effective core area A_eff of the fiber is smaller. We then describe the strong mode-number dependence of the depolarization power generated from the profile of the refractive index change induced by the TR_2,m mode. We also use two methods to measure the polarization crosstalk (XT) induced by the depolarization, namely, a direct detection method with a photodiode and a conventional power detection method with an optical spectrum analyzer. The results of the two methods agree well, and the XT increase is inversely proportional to the fiber A_eff and proportional to fiber length. Finally, we evaluate the influence of the GAWBS-induced XT on the BER characteristics in a coherent QAM transmission, where we find that the influence of the TR_2,m mode is much weaker than that of the R_0,m mode (radial mode). That is, the error-free transmission distance in standard single-mode fiber is extended to 8600 km for 256 QAM signal assuming hard-decision FEC with a 7% overhead. This distance is seven times longer than that obtained with the R_0,m mode.