Micromachined extrinsic Fabry-Pérot cavity for low-frequency acoustic wave sensing.

In this work we demonstrate a low-frequency acoustic sensor structure based on extrinsic Fabry-Pérot interferometer (EFPI) cavity. The cavity is fabricated through micromachining techniques in a square silicon substrate with 4 mm side length and 400 µm thickness, which gives the sensor relative compact size. In the assembling process of the lead-in fiber, a D-shaped ceramic ferrule is designed to achieve the open cavity structure, which can balance the environmental pressure inside and outside the cavity and thus giving the sensor potentials of resisting strong pressure variations in some harsh application environments. Experimentally, sensor response to low-frequency acoustic waves from 0.1 Hz to 250 Hz is measured and demonstrated. A flat response region between 0.5 Hz to 250 Hz with sensitivity fluctuation of 0.8 dB is realized. Pressure resistant test of 25 MPa is also conducted on the sensor and exhibited to prove the function of the open cavity structure.