A theoretical model combined with finite element simulation and numerical analysis is presented to design and optimize of the fiber-wrapped mandrel optic microphone based on distributed acoustic sensing (DAS). To increase the acoustic pressure sensitivity, the optimized fiber-wrapped mandrel microphone with engraved uniform grooves is fabricated and tested as the acoustic transducer. The average sensitivity is -136.915dB re:1rad/μPa between 50 Hz to 2450Hz, which is 25.306dB higher than the same fiber optic microphone without uniform grooves. The experimental results are in good agreement with the theoretical results, which shows this theoretical approach is effective to design and optimize the fiber-wrapped mandrel optic microphone based on DAS.
A series of theoretical study of HWC for distributed acoustic sensing system is proposed to optimize the gaugh length. With the considition of SNR and spatial resolution, the optimal gaugh length is 0.4/(sinαcosθ) to 0.5/(sinαcosθ) times of the Ricker wavelet’s spatial wavelength, when the Ricker wavelet seismic wave travelles to the HWC with the wrapping angle α and the incident angle θ. Similar with the situation in DAS with straight fiber, the optimal gaugh length can achieve the SNR bigger than 90% of the maximum, and the deviation of detected temporal wavelength is less than 14%. Additional coefficient of 1/sinαcosθ is provided to any imping angle of the seismic wave and wrapping angle for the normal working environment.
A real-time distributed acoustic sensing system with ten thousand channels is proposed to detect dynamic signal along the fiber. The phase-sensitive optical domain reflectometry and phase-generated carrier algorithm are used to acquire the phase information of Rayleigh backscattering along the whole fiber. The sensing length of this system could be 10km with the sample resolution of 0.4m, which means that up to 25000 channels signal processing is realized in real time with Field-Programmable Gate Array module and host computer. The working principle of coherent Rayleigh backscattering interference, phase-generated carrier algorithm and the signal processing flow are introduced, and the experimental results are given in this paper.