Diaphragm etching in extrinsic Fabry-Perot interferometric fiber optic pressure sensors

Extrinsic Fabry-Perot interferometric (EFPI] pressure sensors are a key asset in a number of applications, including medical devices, life science, oil&gas, and energy plants [1]. EFPI probes, as in Fig. 1A, are based on creating an air-gap cavity L within an optical fiber structure, acting as a Fabry-Perot resonator; the fiber tip d acts as a diaphragm, compressing the air gap as a function of the pressure applied on the fiber surface. The sensitivity of EFPI probes is strongly dependent upon diaphragm thickness, which is approximately proportional to d∼3, and ranges from 0.01 nm/kPa to 10 nm/kPa. An accurate optimization of the diaphragm length is required to match target performances, in terms of sensitivity, pressure accuracy, and working range [2]. Hydrofluoric acid [HF] is employed for diaphragm shrinking, as it allows controlled glass fiber etching while maintaining polishing quality; however, HF etching rate is poorly repeatable, leading to a poorly predictable diaphragm length. Industrially, this problem has highly affected probes performance repeatability.