Fading Suppression of Φ-OTDR With the New Signal Processing Methodology of Complex Vectors Across Time and Frequency Domains

To suppress coherent fading in Φ-OTDR distributed vibration sensing (DVS) with frequency-division-multiplexing (FDM), we propose an efficient methodology named the double averaging approach that processes complex vectors of Rayleigh scattered light on IQ plane. The double averaging approach rotates and averages complex vectors across the time domain as well as the optical frequency domain; this yields the final phase representing the vibration with less phase noise than the conventional method. This allows the proposal to prevent the phase unwrapping failures caused by the noise and improve the vibration detection sensitivity without incurring penalties in detection bandwidth or system complexity. The proposal is also effective in suppressing the vibration waveform distortion permitted by the conventional method. We confirm the effectiveness of the proposal through experiments in which we apply nϵ-level vibrations to an over 5 km sensing fiber and perform Φ-OTDR DVS by using FDM pulses composed of three optical frequencies.