  We propose and experimentally demonstrate a technique to extend the dynamic
sensing range of phase sensitive optical time domain reflectometry system based
on the frequency interrogation. Benefitting from the range Doppler coupling
feature, the frequency modulation pulse is capable of measuring the frequency
shift induced by the dynamic strain, thus the large dynamic strain can be
recovered. The performance of the proposed method is experimentally evaluated
by comparing it with phase unwrapping. The strain sensing rang can at least be
increased by factor of hundreds, and fast dynamic strain with peak to peak 130
microstrain vibration frequency of 20 kHz is measured.
