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.
A BOTDA with the capacity of break interrogation is proposed and demonstrated experimentally. In our configuration, coherent detection and double sideband probe method are employed to enhance the signal-to-noise ratio (SNR) and to effectively reduce nonlocal effects, respectively. Without amplification, a 72 km sensing range with 5-meter resolution and an estimated temperature uncertainty of 1.8 °C are obtained. Benefiting from the flexible optical configuration, this sensor system has the capacity of break interrogation as a coherent optical time domain reflectometry (COTDR) if there is a break in the fiber under test (FUT). The sensor achieves a dynamic range of 36 dB with a 100 m spatial resolution, which offers an excellent solution for the requisite of two-end-access in BOTDA, and significantly enhances the robustness of the sensing system.
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