Design and realization of a side-polished single-mode fiber optic high-sensitive temperature sensor

A high sensitive temperature sensor based on evanescent field coupling between a side-polished fiber half-coupler (SPFHC) and a thermo-optic multimode overlay waveguide (MMOW) is designed and demonstrated. Such a structure essentially functions as an asymmetric directional coupler with a band-stop characteristic attributable to the wavelength-dependent resonant coupling between the mode of the SPFHC and one or more modes of the MMOW. A slight change in temperature leads to a significant shift in the phase resonance-coupling wavelength (lambda(r)) between the MMOW and SPFHC lambda(r), which is easily measurable. The wavelength sensitivity of the device is measured to be similar to 5.3 nm/ degrees C within the measurement range of 26-70 degrees C; this sensitivity is more than 5 times higher compared to earlier reported temperature sensors of this kind. The SPFHC was fabricated by selective polishing of the cladding from one side of a bent telecommunication standard single-mode fiber and the MMOW was formed on top of the SPFHC through spin coating. A semi-numerical rigorous normal mode analysis was employed at the design stage by including the curvature effect of the fiber lay in the half-coupler block and the resultant z-dependent evanescent coupling mechanism. An excellent agreement between theoretical and experimental results is found.