Suppressing interference fading in Φ-OTDR with intensity-coded pulse based on spectrum-extraction and rotated-vector-sum method

Abstract

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) is able to detect the perturbation through the phase demodulation of the Rayleigh backscattering (RBS) light using ordinary optical fiber. However, the intrinsic interference fading at certain points deteriorates the signal-to-noise ratio (SNR) of RBS light severely, reducing the reliability of vibration detection. In this work, a pulse-intensity-coding Φ-OTDR (using 8-bit Golay complementary codes) with spectrum extraction and the rotated-vector-sum (SERVS) method is proposed to deal with the interference fading as well as maintain a high SNR of the demodulated disturbance signal. We demonstrate that the SERVS method can reduce the percentage of fading points from 2.98% to 0.33% significantly within a sensing distance of 2 km. What’s more, the averaged power spectral density (PSD) level of the differential phase at all locations of the fiber with SERVS method is lower than that without SERVS method, verifying the reduction of interference-fading-induced phase noises along the fiber. Also the SERVS method achieves a SNR of 43.96 dB for the demodulated disturbance signal, about 8.6 dB higher than the single-pulse Φ-OTDR due to the correlation characteristics of Golay complementary sequence.