Fading-Free Φ-OTDR With Multi-Frequency Decomposition

A novel multi-frequency decomposition (MFD) scheme is proposed to achieve a fading-free phase-sensitive optical time-domain reflectometry (
$\Phi $
-OTDR). By performing short-time Fourier transform (STFT) on the detected signal, the spectrum contents of the normal probe pulse can be fully utilized and decomposed in time-frequency domain without sacrificing more spatial resolution. Through employing the rotated-vector-average (RVA) method to synthesize the decomposed signals, the demodulated phase noise is greatly suppressed and the false phases caused by fading are cleanly eliminated. In experiments, two simultaneous vibrations along the 9.6-kilometer-long fiber are clearly detected without fading for 50 measurements.