Modeling of multi-layer fiber-optic Fabry-Perot interferometer as a sensing element of humidity, pressure, and temperature

The work presents a concept of a fiber-optic sensing element, which allows to simultaneously measure humidity, pressure and temperature, and is based on a multi-layer Fabry-Perot interferometer (FPI). The basic element of the proposed FPI is a droplet-shape air-filled cavity obtained using the catastrophic fuse effect of a single-mode fiber. The opening of the cavity is covered with a diaphragm composed of a double-layer film, the first (inner) layer of which has an increased thermo-optical coefficient, while the second (outer) layer’s refractive index is sensitive to humidity of the ambient medium. The outer pressure causes the deflection of the diaphragm, which leads to the change of the length of the air-filled cavity. The work also discusses the requirements for the parameters of the materials constituting the sensor, based on the results of the reflected optical spectrum modeling of the sensing structure at various ambient conditions, which would provide the desired measurement performance.