Pre-frequency-shift method for random number coding Phi-OTDR lightweight demodulation

Pulse coding technology can effectively improve the performance of the phase-sensitive optical time domain reflectometry (Phi-OTDR) system, but the cross-correlation decoding for mass data along a long sensing distance brings a lengthy extra time cost. In this paper, a pre-frequency-shift method is proposed for the lightweight demodulation in the random number coding Phi-OTDR system. By using the first-order sideband generated by the phase modulation, this method can effectively reduce the frequency of the beating signal, thus reducing the requirement of the data sampling rate. The principle of the cross-correlation decoding algorithm based on sequence interpolation is studied, and the extra computation caused by the cross-correlation operation is analyzed theoretically. In the experiment, the validity of the pre-frequency-shift scheme is verified, and the sampling rate of the system is reduced from 500 MSps to 100 MSps. Comparative experiments using conventional down-sampling techniques with sampling rates of 250 MSps, 125 MSps, and 60 MSps validate the superiority of the proposed method. Finally, the vibrations of different frequencies are successfully located at 62.24 km with a spatial resolution of 5 m. At this distance, the algorithmic time cost of the proposed method is only 13.28 % of that of the traditional scheme.