Tailoring Refractive Index and Adlayer Sensitivity of an Optical Fiber
Fabry-Perot Interferometer by a Thin Layer Deposition
Abstract
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
(next). 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
next. 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.