Tailoring Refractive Index and Surface 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 sensitivity) and formation of a layer on the sensor surface (surface sensitivity). 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 (TiO
₂
) was chosen. We show that addition of a second layer of lower
n
than the first one results in a 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 surface 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 surface 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.