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Enhanced temperature sensing based on birefringence induced Vernier effect in a symmetrical metal-cladding waveguide

A temperature sensor with high sensitivity based on the birefringence induced Vernier effect in a symmetrical metal-cladding waveguide (SMCW) structure is theoretically proposed. Due to the guiding layer of SMCW is extended to a sub-millimeter scale, the excited ultra-high order modes is highly sensitive to the variation of thermally modulated refractive index (RI) and the obtained comb-like resonance reflectivity spectrum shifts accordingly as changing the ambient temperature. There is a little difference between the RI of the ordinary light and the extraordinary light of the filled nematic liquid crystal (LC), the resulted slight different free spectral range of two orthogonal polarizations makes their superposition of reflectivity spectrum metamorphose into a period envelope with multiple sub-peaks. The dips in the fitting upper envelope possesses a much enhanced temperature dependent spectrum shift which can be measured by a low-cost integrated-type micro-spectrometer. Such a sensor has no reference resonant cavity and its sensing range can be easily controlled by filling with different kinds of LC.