A Secondary Sensitivity-Enhanced Fiber-Optic Microphone Based on Ultra-Weak Fiber Bragg Grating

Highly sensitive fiber-optic microphone (FOM) is required in a variety of practical applications. In this article, a secondary sensitivity-enhanced FOM based on ultra-weak fiber Bragg grating (uwFBG) is proposed and demonstrated. By conducting theoretical and simulation analyses, the FOM is designed from two aspects. First, a foam mandrel wrapped around by the bending insensitive optical fiber with a diameter of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$165~\mu $ </tex-math></inline-formula>m is employed as the first sensitization structure to enhance the responsivity to acoustic waves. Then, the foam mandrel is fixed in an aluminum shell that can effectively reflect acoustic waves and is employed as the second sensitization structure. Experimental results show that the proposed FOM achieves a high average acoustic pressure sensitivity of −116.0 dB (re 1 rad/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>Pa) from 50 to 2000 Hz, as well as a high signal-to-noise ratio (SNR) of 71.07 dB at 1000 Hz and superior linearity, repeatability, and hysteresis. Moreover, the FOM demonstrates the capability for high-fidelity sound recovery, with a speech recognition accuracy reaching up to 96.25% and an average spectral coherence of 0.9909 within the fault sound concentration frequency of the conveyor belt. The sensitization structure holds great potential in sound detection applications, such as speech recovery and conveyor belt fault monitoring, and it enables a single distributed acoustic sensing (DAS) system to interrogate a series of FOMs via bending insensitive optical fiber with uwFBGs inscribed on it.