Membrane-free acoustic sensing based on an optical fiber Mach–Zehnder interferometer

Traditional optical fiber acoustic sensors are mostly based on mechanical diaphragms and use indirect coupling between the acoustic and optical signals. The detectable frequency range and sound pressure range of such a sensor have limitations because they are influenced by the membrane or a mechanically deformable material. In this paper, a Mach-Zehnder interferometer-based membrane-free acoustic sensing method is developed. The sensing principle relies on direct detection of sound-pressure-induced changes of the refractive index in the open cavity. This enables an inherently flat frequency response over a broad bandwidth. Simulation and experiment were carried out to verify and demonstrate the idea. The results show that the membrane-free acoustic sensor has a flat frequency response from 500 Hz to 20 kHz.