Microstructured Optical Fibers Based Hybrid Fabry–Perot Interferometer Structure for Improved Strain Sensing by Vernier Effect

We present a novel strain sensor based on a hybrid Fabry–Perot interferometer (HFPI), which is mainly constructed by a cascade of a suspended-core fiber (SCF) and a hollow-core fiber (HOF) between two single-mode fibers (SMFs). When the optical path length (OPL) is matched to some extent, the reflection spectrum of the proposed HFPI demonstrates dense periodic Fabry–Perot interferometer (FPI) fringes tailored with a widely expanded envelope, the dip or peak of which responds swiftly to the strain due to the Vernier effect and the strain response difference between the SCF and the HOF. Through optimizing the parameters, one can achieve a magnified strain sensitivity up to −91.41 pm/
$\mu \varepsilon $
ranging from 0 to
$1800~\mu \varepsilon $
. The strain sensitivity is much higher than that of the single HOF FPI structure or the single SCF FPI structure. Moreover, a theoretical model has been established to explain the principle of the proposed HFPI based on the Vernier effect and the ways to further improve the sensitivity. By adding a fiber Bragg grating (FBG), the temperature can also be interrogated by demodulating the envelope peak of the HFPI and the peak of the FBG. The proposed structure has the advantages of high sensitivity, large dynamic range, temperature discrimination ability, and compact size, providing potential from personal human health perception to public structure health monitoring.