Reducing false alarming of vibration and shortening data processing time are one of the key problems of phase sensitive optical time-domain reflectometer (φ-OTDR). Generally speaking, there are two demodulation methods to locate vibrations: phase demodulation and amplitude demodulation. At present, an often-used method is phase-based crosscorrelation, which shows a comparatively reliable detection performance. Compared with phase cross-correlation, energy/power cross-correlation between different positions is simpler and has certain advantages in practical applications. In this paper, we use φ-OTDR to collects periodic vibration signals (power signals) and transient vibration signals (energy signals). Amplitude differentiation is firstly calculated along slow time axis for the Rayleigh backscattering trajectories. For periodic vibration, power spectrum is then obtained at each position, and cross-correlation coefficients between any two spectrums are computed. If the vibration is transient, average energy is calculated along fast time axis and average energy cross-correlation is performed between any two locations. With the cross-correlation values, we are able to determine whether there is vibration on the optical fiber. In the experiment, periodic vibration is simulated by a sine-driven PZT and transient vibration is mimicked by pencil-break. These results demonstrate that power (energy) cross-correlation coefficients work well to locate periodic (transient) vibrations.

Phase-sensitive optical time-domain reflectometry (φ-OTDR) is highly sensitive to strain changes of sensing fiber caused by external vibration, by which we are able to locate the vibration. In practice, interference fading will inevitably occur in backscattered Rayleigh traces of φ-OTDR due to the use of highly coherent light source, which increase the possibility of failure detection. In order to reduce the influence of interference fading on vibration detection, both frequency-division multiplexing (FDM) and rotated-vector-sum (RVS) over both time-and frequency-domain are employed in our method. Based on the method, we perform φ-OTDR experiment to locate vibrations. By extracting 3 frequency components of the beating signals (~200 MHz) and carrying out dual rotation, interference fading can be suppressed to a large extent, the vibration-induced phase changes are precisely recovered. One point should be noted is that we found that there is a certain correlation between each frequency component extracted from the beating signal, resulting in interference fading points cannot be completely removed.

Phase-sensitive optical time domain reflectometer (φ-OTDR) has been extensively investigated in fields of intrusion detection and structural health monitoring. It should be noted that phase noises would keep accumulating during pulse transmission. By subtracting an initial phase at the input point from demodulated phases at other positions, the noises related to the laser itself except random noises can be considerably reduced. In order to further decrease the impact of random noises on waveform retrieval of external vibrations, it is necessary to eliminate the accumulated noises before vibration position as much as possible. In this work, a sliding root mean square method (SRMS) is firstly applied to locate vibration events. By the SRMS, the demodulated phase at ~10 m before vibration point is regarded as the modified reference. Then, the vibration waveform can be retrieved after phase subtraction. For comparison purpose, both the input and modified references are employed to retrieve temporal vibration signals. Experimental results show that the SRMS shows good noise performance for vibration location. In terms of signal retrieval, the vibration waveform can be recovered with better noise suppression by the modified reference compared to the input one.