Microsphere-Augmented PDMS integration in tapered FBG small-scale sensors for enhanced temperature sensitivity

This work presents a novel high-sensitivity temperature sensor based on a fiber Bragg grating inscribed on a tapered optical fiber which terminates in a microsphere, all embedded within a PDMS-filled silica capillary. The fabricated microsphere at the taper's end enhances PDMS traction, improving strain transfer between the polymer and the fiber during temperature changes. Different waist diameters for the tapered fiber were considered for the design of the sensor. The sensor's response was analyzed over a temperature range of 20 °C to 90 °C for taper waist diameters ranging from 60 μm to 20 μm. Experimental results demonstrate a wavelength temperature sensitivity of 47.19 pm °C⁻¹ for a 60 μm waist diameter and 221.2 pm °C⁻¹ for a 20 μm waist diameter, achieving up to 22 times the sensitivity of a bare FBG. The experimental results were supported by finite element analysis simulations, which showed a clear correlation between the enhanced sensitivity and the increase in axial strain applied by the PDMS on the embedded fiber. This enhancement in sensitivity was demonstrated by housing the fiber in a capillary, integrating the microsphere at the end of the fiber, and diminishing the fiber taper's diameter. Moreover, unlike traditional techniques aimed at enhancing the thermal sensitivity of fiber Bragg gratings, the sensor developed through this innovative approach exhibits enhanced performance regarding both dimensions and sensitivity.