High-sensitivity direct-detection DAS system using a UWFBG array based on linear frequency modulation

Microwave photonic (MWP) technology is widely used in signal generation and processing, which helps optical sensors achieve higher measurement resolution and sensitivity. In this Letter, MWP technology is used to improve the performance of distributed acoustic sensors based on an ultra-weak fiber Bragg grating array. Specifically, a linear-frequency-modulation (LFM) pulse is used as a probe signal, effectively improving the excitation threshold of fiber nonlinearity. Distributed positioning and quantitative measurement of vibration are achieved through a simple direct-detection scheme. Based on the non-matching interference, the echo beating frequency after chirp removal is reduced to an integral multiple of the LFM probe repetition rate, which helps to reduce the bandwidth of the receiving end. Compared with existing demodulation methods, the demodulation method combining phase shift integration and empirical mode decomposition reduces the noise floor by at least 7.6 dB.