Suppression of Doppler-induced crosstalk in linear-frequency-swept Φ-OTDR using scattering-enhanced fiber

Linear-frequency-swept (LFS) phase-sensitive optical time domain reflectometry (Φ-OTDR) can overcome the constraints between spatial resolution and signal-to-noise ratio. However, the Doppler shift induced by vibration in the Rayleigh backscattering (RBS) signal generates significant crosstalk beyond the vibration area. Here, we propose and demonstrate a Doppler-crosstalk-suppressed quasi-distributed acoustic sensor (QDAS) based on an LFS Φ-OTDR, utilizing a discrete Rayleigh-enhanced single-mode fiber (eSMF). The mechanism of Doppler-induced crosstalk was analyzed, and the effects of pulse parameters, including pulse width, sweep rate, and bandwidth, were studied. Moreover, a discrete eSMF was proposed to suppress the crosstalk by reducing the change rate in the local phase. 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 190 pε/√Hz, and an improved crosstalk suppression of 20 dB. The proposed QDAS system with Doppler-induced crosstalk suppression is promising for ultrasonic structural health monitoring, such as aerial vehicles and deep-sea submersibles.