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 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 mode is distributed over a wider bandwidth when the effective core area A 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 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 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 mode is much weaker than that of the R 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 mode.