Phase fluctuation cancellation for coherent-detection BOTDA fiber sensors based on optical subcarrier multiplexing

The employment of coherent detection in a Brillouin optical time domain analysis (BOTDA) fiber sensor brings benefits, including signal-to-noise ratio enhancement, non-local effect reduction, and sensing speed improvement. Recently, it was found that the performance of a coherent-detection BOTDA fiber sensor suffers from phase fluctuations introduced by the fiber group delay jitter. Here, we propose a phase fluctuation cancellation approach based on optical subcarrier multiplexing. In a proof-of-concept experiment, the phase stability for in-phase/quadrature demodulation reaches a standard deviation value of as small as 0.4 mrad. The variations in the Brillouin gain and phase spectra caused by the phase fluctuation are then effectively alleviated, resulting in an enhancement of the Brillouin frequency shift measurement certainty along the whole sensing fiber.