Interference Fading Suppression in DAS System Using a Frequency-modulated Optical Loop

In this paper, we propose a interference fading suppression method for distributed acoustic sensing (DAS) systems by using a frequency-modulated optical loop. In this proposed system, continuous light in the form of an equal-amplitude frequency comb is generated by a frequency-modulated optical loop, and the number of frequencies is increased with the number of modulated. Then, it is modulated into a multi-frequency probe pulse light, and the number of frequencies can be accurately controlled by setting an appropriate delay. Finally, the phase information is demodulated by aggregated different frequency Rayleigh backscattering (RBS) signals. The statistical analysis along the sensing fiber of the DAS systems is reduced significantly by using multi-frequency probe pulse light generated by the frequency-modulated optical loop. The proposed new interference fading suppression method without the aid of special fiber or large bandwidth modulator, has the advantages of flexible control of frequency number, high modulation efficiency, and low cost.