Research on the intrinsic phase difference of Rayleigh scattering in distributed acoustic sensing

The differential cross-phase multiply (DCM) demodulation algorithm based on a 3 x 3 coupler is commonly employed in distributed acoustic systems (DAS) to recover phase information along the sensing fiber. This study investigates the amplitude-spatial resolution response characteristics of the DCM algorithm using 3 x 3 coupler demodulation. Firstly, we theoretically establish the variation of demodulation amplitude in DAS systems due to the intrinsic phase of the signal. Subsequently, we analyze the amplitude-pulse width response curves of the DCM algorithm based on the distribution of demodulated phase error obtained from both experimental and simulation data. The findings reveal a double peaks phenomenon in the amplitude response curves of the DCM demodulation algorithm based on 3 x 3 couplers. We also explore the amplitude response curves for multiple pulse widths and observe that, under the same arm length difference conditions, the double peaks persist. However, this double peak phenomenon is eliminated when the pulse width is less than 21 ns. Therefore, when the width determined by the pulse is greater than 40% of the difference between the light paths, the demodulation will appear double peaks phenomenon.