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(degrees) mirror. Herein, the 45(degrees) 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 similar to 2490 nm/Pa, and 13.4 mu Pa/Hz(1/2), respectively. The proposed sensor demonstrates the potential for low-frequency vibration monitoring with the advantages of easy fabrication, high-sensitivity and flexible manipulability.