Characteristics and Explanations of Interference Fading of a $\phi$ -OTDR With a Multi-Frequency Source

The characteristics of interference fading in a phase sensitive optical time domain reflectometer (φ-OTDR) system with a three optical frequency source are described in this paper. It is observed in the experiments that the fading appearance is quite stable over 400 different pulse scans for each of frequencies, but different for the three frequencies. A model is proposed to explain the experimental phenomena, in which the fading waveform is expressed by an integral of propagation phase factor, taking non-uniform index distributions along the sensing fiber into account, multiplied by the random Rayleigh scattering coefficient summed within the detector response period. Simulations show that the basic characteristics of experimental observations can be explained by the model.