Quasi-distributed acoustic sensing based on P4-code modulation

Abstract In this paper, a quasi-distributed acoustic sensing system based on P4-code modulation is proposed for the enhancement of the signal-to-noise ratio (SNR). The P4-code modulated signal is generated by an acousto-optic modulator and injected into an ultra-weak fiber Bragg grating array. At the receiver, coherent detection is adopted for optical-to-electrical conversion. The optical time-domain reflectometry trace is then retrieved using pulse compression technology. The vibration information is obtained following the differential phase demodulation process. Experimental results reveal that the noise floor is suppressed by 15.27 dB ref rad 2 Hz −1 , and the demodulated SNR is improved by 6.97 dB compared with the conventional single-pulse interrogation scheme. The effect of the coding length on the noise floor is investigated. Moreover, the frequency response is measured, and the power spectrum densities of the vibration signals are obtained with high SNR over 24.83 dB. Vibrations with different voltages can be demodulated quantitatively with excellent linearity, and the sensitivity is 1.026 rad V −1 at 3 kHz.