Tailoring Refractive Index and Adlayer Sensitivity of an Optical Fiber Fabry-Perot Interferometer by a Thin Layer Deposition

This work discusses the capability of tailoring sensitivity of an optical-fiber-based Fabry–Perot interferometric (FPI) sensor to changes in refractive index (volume) and formation of a layer on the sensor surface (adlayer). A simple single-layer approach shows some disadvantages, especially when refractive index (n) sensing is considered, e.g., there is no shift of the interference pattern in the wavelength domain. The considered FPI sensor is based on two transparent thin films deposited on a single-mode optical fiber’s end face. As the first layer, a high-n titanium oxide (TiO₂) was chosen. We show that addition of a second layer of lower n results in the blueshift of spectral pattern with external n (n_ext). Moreover, when an additional layer, e.g., biological one, is formed, a redshift of the pattern appears, what is in contrary to the shift induced by the increase of n_ext. Numerical analysis as well as experiments show that a wide range of materials (with different n and thickness) can be applied as the second layer, influencing both volume and adlayer sensitivities. We have found that when the second layer thickness is tailored to obtain a well spectrally defined pattern, high n contrast between the layers increases the volume sensitivity, while for the moderate n contrast the adlayer sensitivity of the FPI can be enhanced. The proposed approach shows large tuning capability towards desired application, as well as can be easily introduced to large-scale sensor manufacturing.