A new phase demodulation method for φ-OTDR based on coherent detection

Phase-sensitive optical time-domain reflectometry (φ-OTDR) has been widely used in many situations. In order to simplify the system structure and reduce costs, a new phase demodulation method is developed for φ-OTDR based on coherent detection. It works by extracting the time domain signals at two points in the disturbance area, and that could construct a pair of IQ component containing the disturbance phase information. Compared with most commonly used digital quadrature demodulation, the digital coherent signal is not required. And the need for high speed sampling frequency is reduced. The PZT induced vibration is applied to a certain section of fiber, which is used to introduce phase disturbance. Experimental results show that the phase change caused by external disturbance can be successfully demodulated in a sensing fiber of 22km, and a simple system structure is realized. When the central frequency of the beat signal is 200 MHz, the need for sampling rate can be reduced to 156MSa/s or less. Amplitude modulated disturbance tests show a reasonable linear response of the system with the R-square of 0.963.