Breaking through the Frequency Response Limit in DAS Using UWFBG Arrays and DSTDM Method

Abstract

Typical fiber-optic distributed acoustic sensing (DAS) systems face a trade-off between frequency response and fiber length. Overcoming this trade-off without adding system complexity is a significant challenge. Furthermore, in practical applications, the demodulation equipment is often far from the sensing end, necessitating the use of a leading fiber that further constrains the frequency response. To date, this issue has not been adequately addressed in the literature. In this paper, we propose a novel solution by employing an ultra-weak fiber Bragg grating (UWFBG) array as the sensing fiber, distinct from the SMF used as the leading fiber, thereby enabling a time-division multiplexing (TDM) sensing principle. By optimizing the pulse width and double-pulse periodicity, effective sensing signals corresponding to different positions and probe pulses are interleaved in an orderly manner, facilitating a second stage of TDM. This dual-stage timedivision multiplexing (DSTDM) technique effectively overcomes the trade-off between fiber length and frequency response without requiring system modifications. Experimental results demonstrate a vibration sampling rate of 200 kS/s with a 10.18 km SMF leading fiber and a 1.02 km UWFBG array sensing fiber, achieving a 24- fold enhancement in sampling rate. This technique eliminates the influence of the leading fiber on the frequency response for the first time, significantly expanding the potential applications of DAS, particularly in scenarios involving long leading fibers.