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

In this paper we discuss the development of an online monitoring technique of diaphragm etching through hydrofluoric acid (HF) exposure; the tip of the extrinsic Fabry-Perot interferometer (EFPI) under test i s immersed in 20 ml HF. The EFPI i s interrogated by means of a low-cost white-light system, based on a 1550-nm spectrometer that records the Fabry-Perot spectrum at 1 kHz rate. During the HF exposure phase, the diaphragm i s linearly etched at approximately 1.48 μm/min for 48% HF, with smaller etching rates for smaller concentrations; the initial d length, obtained after λ/20 surface polishing, i s usually 7-15 μm and i s reduced to 1.0-1.5 μm.