Curvature sensor based on resonance mode splitting of multimode interference combined with a long-period grating

Optical microfiber/nanowire-based photonic devices have been widely used in a range of sensing applications, including refractive index, stain, humidity, chemical gas and temperature. Optical fiber based refractive index (RI) sensors have attracted extensive attention due to their unique advantages such as immunity to electromagnetic interference, small size, high sensitivity, etc. The techniques used to implement fiber based RI sensing include a fiber Bragg grating (FBG), long period fiber grating (LPFG), surface plasmon resonance, tapered fiber and a singlemode– multimode–singlemode (SMS) fiber structure. Among these techniques, an SMS fiber structure has the advantages of simplicity and ease of fabrication and previous investigations have shown that an SMS fiber structure can excite and couple multiple modes to an SMF. For Bragg-grating-based sensors, which suffer from limited temperature- and strain-induced spectral displacements, is often required that one use complex interferometric techniques to detect these shifts. On the other hand, the LPFG is a promising technique for optical sensing and may exhibit wavelength blue-shift as the RI increases. If the two types of sensors are combined with each other, the sensitivity will be improved by monitoring the separation wavelength shifts between the resonant wavelengths of the LPFG and SMS fiber structures. Our proposed technique has advantages of simple configuration, easy fabrication, and simultaneously measurement of both RI and temperature