High-Sensitivity EFPI-Based Acoustic Sensor Using in-Fiber Collimator and Adjustable 45° Mirror

In this paper, a high-sensitivity cross-axial extrinsic Fabry-Perot interferometric (EFPI) acoustic sensor is proposed and experimentally demonstrated. The cross-axial EFPI is constructed by optical fiber and cantilever's inner surface, assisted with an adjustable 45° mirror. Herein, the 45° mirror is utilized to achieve beam total reflection and flexibly adjust the parallelism of the two reflectors. A quarter-pitch graded index fiber (GIF) is introduced as an in-fiber collimator to reduce beam propagation loss and thus enhance coupling efficiency. Experimental results show that the proposed sensor achieves a high fringe visibility of up to 41 dB, greatly enhancing acoustic sensing performance. Furthermore, the sensitivity and the minimum detectable pressure (MDP) measured at resonance frequency (@2.05 kHz) are significantly improved to be 70.67 mV/Pa that is corresponding to mechanical sensitivity of ∼2490 nm/Pa, and 13.4 μPa/Hz1/2, respectively. The proposed sensor demonstrates the potential for low-frequency vibration monitoring with the advantages of easy fabrication, highsensitivity and flexible manipulability.