Improved Temperature Sensing in Upconversion Fiber-Optic Probes via Spectral Modulation by Cladding Removal

Optical-fiber sensors lie at the very core of detection technologies due to their small footprint, lightweight, versatility, chemical inertness, immunity to electromagnetic interference, and application to long-range distributed sensing. In recent times, the search for improved temperature sensors has led to the inclusion of temperature-sensitive nanoparticles in optical-fiber settings. In this work, we use NaYF4:Yb3+/Er3+ upconversion nanoparticles to fabricate a ratiometric fiber-optic temperature sensor. We propose a scheme consisting of pumping at 980 nm to excite coatings of nanoparticles along one end of the optical fibers. We use standard single-mode fibers and multi-mode fibers and explore the effect of removing the fiber cladding in order to allow for the interaction of the nanoparticles with higher-order modes. The modified optical fibers were tested as thermal probes by analyzing the emission spectra. A temperature resolution of 3 and 7 K was achieved for single-mode and multi-mode probes, respectively, by keeping the fiber cladding. However, removal of the cladding allowed the resolution to significantly improve to 0.4 and 0.5 K for single-mode and multi-mode operation, respectively. This is the result of a significantly improved contrast between measured spectral bands, resulting in sharper profiles that can be conveniently measured at reduced optical powers. By removing the fiber cladding, light can interact with the nanoparticles over an extended area, mitigating high-intensity effects that may occur at the fiber output end. A fiber-optic sensor (FOS) design consisting of a modified optical fiber (OF) and a collection fiber within a metallized glass capillary tube was successfully built and tested as a proof-of-concept. These results underscore the capabilities of optical fibers as cutting-edge temperature sensors for applications in diverse areas such as environmental monitoring, industry, and healthcare.