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A long range phase-sensitive optical time domain reflectometer (phi-OTDR)with a multi-frequency non-linear frequency modulation (NLFM) optical pulse isproposed in this Letter. To boost the pulse energy while suppressing theoptical nonlinear effects, the distortion of the amplified pulse is rectified,and a three-tone pulse is used. Combining with the NLFM technic which provides42.7 dB side lobe suppression ration (SLSR), these two approaches guaranteethat a sensing distance of 80 km is achieved in the experiment with 2.5 mspatial resolution, 49.6 dB dynamic range, and 45 dB phase signal-to-noiseratio (SNR). To the best of our knowledge, this is the first time that aphase-demodulated phi-OTDR over such a long sensing range has been reportedwith un-pumped sensing fiber.

Long range fading free phase-sensitive reflectometry based on multi-frequency NLFM pulse