Integrated fiber-optic Fabry–Perot vibration/acoustic sensing system based on high-speed phase demodulation

A fiber-optic Fabry–Perot (F-P) vibration/acoustic sensing system based on high-speed phase demodulation was developed. The demodulation part is mainly composed of a super luminescent diode (SLD), a miniature high-speed spectral module, and a field programmable gate array (FPGA) based spectral signal acquisition and processing circuit. The spectral acquisition rate is up to 20 kHz. Real-time spectral acquisition, fast Fourier transform (FFT), frequency estimation, phase calculation, and mode hopping suppression are all performed by FPGA to obtain absolute cavity length of a F-P interferometric sensor. As a result, the integrated design significantly improves the speed and stability of the phase demodulation. The performance of the system was verified by a fiber-optic F-P cantilever microphone. Experimental results show that the sensing system can realize highly sensitive detection of acoustic signals with the frequency between 20 Hz and 8 kHz, which basically covers the main range of human voice frequencies. The noise equivalent minimum detection limit of sound pressure reaches 2.8 μPa/Hz1/2 at 1 kHz. The designed fiber-optic acoustic sensing system has the features of resistance to electromagnetic interference, intrinsic safety, remote detection and small size. It can be used for vibration monitoring of transformers and large structures, voice communication in coal mines and nuclear magnetic resonance rooms.