The paper introduces the DAS system based on advanced phase-sensitive optical time domain reflectometry (φ-OTDR) with fading noise suppressed. Besides, the DAS system based on time-gated digital optical frequency domain reflectometry (TGD-OFDR) and its improved systems are introduced. The application of DAS in railway perimeter security is introduced at the end of the paper.
The frequency division multiplexing (FDM) technique is firstly introduced into a direct-detection phase-sensitive OTDR to improve the distributed fiber acoustic sensing performance by using a frequency step sweeping laser source and a dual probe pulse scheme. By using FDM technique, a 40 kHz sampling rate to vibration is realized with a 10 km measurement range, which implies the tradeoff between the frequency response and the measurement range is broken. In experiment, a 6 kHz vibration is successfully measured.