Suppression of Doppler-shift-induced crosstalk in linear-frequency-swept φ-OTDR by using scattering-enhanced fiber

Linear-frequency-swept (LFS) phase-sensitive optical time domain reflectometry (φ-OTDR) breaks the trade-off between spatial resolution and signal-to-noise ratio. However, the vibration induced Doppler shift of Rayleigh backscattering (RBS) generates serious crosstalk beyond the vibration area. Here, we propose and demonstrate a Doppler-shift-suppression distributed acoustic sensor (DAS) based on LFS φ-OTDR and discrete Rayleigh-enhanced single-mode fiber (eSMF). The relationship between the crosstalk and detection pulses parameters, including pulse width, chirp rate and bandwidth, was investigated. Moreover, a discrete eSMF was used to reduce the phase change rate, and hence could suppress the crosstalk. As a proof of concept, a vibration signal with a frequency of 8 kHz was successfully detected with a spatial resolution of 10 cm, a strain resolution of 110 pε/√Hz and an improved crosstalk suppression of 20 dB. The Doppler-shift-suppression DAS system proposed in this work could potentially be used for ultrasonic structural health monitoring.