Precise Brillouin gain and phase spectra measurements in coherent BOTDA sensor with phase fluctuation cancellation.

In order to cancel phase fluctuation induced Brillouin gain spectrum (BGS) and Brillouin phase spectrum (BPS) distortions, a new scheme with phase fluctuation cancellation (PFC) is proposed to realize precise BGS and BPS measurements in coherent BOTDA sensors. We present comprehensive and theoretical analysis about the effect of phase fluctuation on the shape of BGS and BPS, and further design a new experimental setup to fully cancel phase fluctuation induced measurement errors. In our new scheme, the two signals sent into the in-phase/quadrature (I/Q) demodulator are almost the same except for the additional amplitude amplification and phase shift induced by Brillouin gain, thus a strict phase synchronization between them has been realized (i.e., PFC). Experimental results show that the biggest BFS decoding error induced by phase fluctuation is reduced from 4.9MHz to 0.4MHz over a 40-km sensing fiber.