Using a Mach-Zehnder-interference-based passive configuration to eliminate the polarization noise in Brillouin optical time domain reflectometry.

We propose a passive polarization depolarizer to overcome the polarization noise in Brillouin optical time domain reflectometry (BOTDR). We also give a theoretical analysis of the depolarizer. The depolarizer is highly stable and has small insertion loss. It is based on a Mach-Zehnder interference scheme and mainly consists of two polarization beam splitters and a piece of delay fiber. Experimental results show that the polarization noise can be reduced by 96% by the depolarizer, and the Brillouin frequency shift induced by heat can be accurately detected by BOTDR with this depolarizer. The pulse width is 30 ns and the length of the sensing fiber is 24 km, which is the longest reported distance achieved with such a short pulse for BOTDR, to the best of our knowledge.