Power penalties due to Brillouin and Rayleigh scattering in a bidirectional coherent transmission system

In case of a large number of channels and a limited available optical bandwidth (limited laser tuning range) it may be necessary to chose a channel spacing near 0 or 11 GHz, where Rayleigh-and Brillouin backscattering introduce crosstalk in a bidirectional system. Although there is much literature on stimulated Brillouin scattering, that occurs at high powers, there is hardly any attention paid to what happens at relatively low input powers. Furthermore there are no system measurements which thoroughly investigate these effects. The authors performed extensive bidirectional system experiments to investigate the crosstalk from Brillouin scattering in more detail, including its polarization properties. They found, that, while for high powers most of the energy is backscattered to a 11 GHz lower frequency, for low powers there is no difference for the crosstalks at 11 GHz higher or 11 GHz lower than the signal frequency. The power budget is limited to 40 dB for both cases. The predicted 33.3% degree of polarization of low-power Brillouin scattering is experimentally confirmed for the first time.<>