Long-Distance Detection for Periodic Vibration Signal in Φ-OTDR System Using Global Phase Demodulation Method

In order to overcome the limitation of weak intensity of Rayleigh backward scattering signal for long-distance Phi-OTDR system, a global phase demodulation method is proposed to extend the sensing distance without additional optical amplification. The localization limitation of conventional difference cumulative average algorithm is firstly analyzed. The global phase demodulation is carried out for all the signals on the whole sensing fiber. The autocorrelation processing is then realized to obtain the autocorrelation curves, on which the difference values between two instants can be used to locate the position of vibration signal. Experimental results show that according to the characteristics of autocorrelation curves, the vibration signal and noise signal can be distinguished. The sinusoidal signal with a frequency of 5 Hz and the third harmonic signal with the frequency of 5 Hz, 10 Hz, 15 Hz can be successfully located on the sensing fiber with the length of 90 km, without additional optical amplification. This method can achieve the long-distance detection for periodic vibration signal in Phi -OTDR system.