Low-Frequency Vibration Detection Enhancement in Dual-Pulse DAS With Single AOM

Low-frequency vibration detection has always been a crucial application of distributed acoustic sensing (DAS), particularly in hydroacoustic sensing, and seismic wave observation. However, the inevitable frequency drift of lasers poses a persistent challenge to subhertz vibration detection in DAS. In this work, we present a novel phase-sensitive optical time-domain reflectometry (
$\boldsymbol {\varphi } $
-OTDR)-based DAS scheme to substantially enhance low-frequency vibration detection. Using just a single acoustic optical modulator (AOM), the probe signal in the form of a dual-pulse with specific delays and distinct carrier frequencies can be generated, thereby eliminating the need for an extra modulator and two additional couplers. Through a process of weight average and reference subtracting, the Rayleigh backscattering (RBS) signals resulting from the dual-pulse can be recombined, significantly improving low-frequency vibration detection. The experimental results confirm that the proposed scheme achieves a signal-to-noise ratio (SNR) improvement of up to 23 dB at a frequency of 1 Hz, corresponding to a noise level decrease of one order of magnitude and a sensitivity of
$220~p\varepsilon /\sqrt {\text {Hz}}\text{@}60$
mHz~10 Hz. The minimum detectable vibration frequency verified here is as low as 80 mHz with an SNR exceeding 70 dB. In addition, the
$R {^{{2}}}$
value shows an improvement after low-frequency enhancement in the 1-Hz linearity test. This work offers a cost-effective and superior performance DAS scheme that has the potential to facilitate DAS applications where low-frequency vibration monitoring is required.