Detection of two identical frequency vibrations by phase discrimination in polarization-OTDR

In this paper, a new detection method for two identical frequency vibrations along optic fiber is proposed, by discrimination of their phase relationship in polarization optical time-domain reflectometer (POTDR). A vibration on fiber would modulate its index and birefringence, then it can consequently change the state of polarization (SOP) of the scattering signal. However, multiple simultaneous vibrations at different positions would result in random fluctuating SOP of the scattering signal, which make them very difficult to be identified. In our proposed method, the phase of the vibration signal along the fiber is obtained by Fast Fourier Transform. Therefore, two vibration events with the same frequency could be distinguished effectively by using the pattern of the phase distribution. The principle of the method is analyzed in detail. Both simulation and experiment results are presented to demonstrate the validity and limitation of this method. It could be widely used for safety monitoring of long distance perimeters, electrical transmission line, and so on.