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

Abstract

<p>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 (<em>n</em>) 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 (TiO<sub>2</sub>) was chosen. We show that addition of a second layer of lower<em> n</em> results in the blueshift of spectral pattern with external <em>n</em> (<em>n</em><sub><em>ext</em></sub>). 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<em> n</em><sub><em>ext</em></sub>. Numerical analysis as well as experiments show that a wide range of materials (with different <em>n</em> 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 <em>n</em> contrast between the layers increases the volume sensitivity, while for the moderate <em>n</em> 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. </p>