Polarization pulling in Brillouin fiber sensing using orthogonal probes

While birefringence-induced polarization fading in dual-probe Brillouin optical time-domain analysis has been demonstrated to be mitigated by using sub-mW orthogonally polarized probes, this work investigates consequences of using stronger probes. Experiments and simulations show that with probe powers of only a few mW and depending on the initial states of polarization (SOP) of the three waves (pump and dual probes), the Brillouin gain trace for kilometers long fibers may deviate from the expected attenuation-dictated exponential decay: sometimes dipping below and then almost always rising above this shape, indicating pump power amplification toward the fiber-end. Pump depletion may not be compensated either. Furthermore, Brillouin-induced strong polarization forces appear to play an important role in the process by pulling the SOP of the pump pulse towards the conjugate of the SOP of the launched loss probe (i.e., towards its reflected image by the equatorial plane of the Poincare sphere). Implications of these observations are discussed.