Rayleigh signature interrogation in time-domain DAS sensors using the short-frequency Fourier transform

We present a general method to determine the dependence of Rayleigh back-reflection in an optical fiber on position and optical frequency. It is based on applying the so-called short-frequency Fourier transform to signals obtained from coherent optical time-domain reflectometry (COTDR) measurements. The proposed method enables the implementation of distributed acoustic sensors (DAS) with high linearity, spatial resolution, and measurement frequency, while being immune to signal fading. Additionally, the technique is compatible with both conventional COTDR setups and those employing pulse compression, including phase-coded compression waveforms. We experimentally demonstrate the technique with DAS measurements of a 50-km sensing fiber, achieving a 2-m spatial resolution and a sensitivity of 137 pε/√Hz using a pulse-compression COTDR with a P4 phase-coded compression waveform.