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