The long-term impact of different fluids on polymer-coated fibers in Rayleigh-based OFDR

This paper investigates the long-term effect of immersion in working fluids of coated optical fibers on the spectral shift measured by Rayleigh optical frequency domain reflectometry (OFDR). The aim of the study was to determine whether and how the surrounding liquid affects the properties of the protective coating of the fiber and thus indirectly the fiber itself. We investigated the behavior of acrylate and polyimide coatings when exposed to four different liquids, including water, acetone, ethanol and methanol, at room temperature. This allowed us to analyze in detail the effects of fiber deformation (compression or elongation) resulting from the interactions between the coating and the liquid. The results led to the determination of a stabilization period after which the influence of the liquid on the coating - indirectly on the fiber - stabilizes, providing accurate and reliable measurements using the Rayleigh-based OFDR method. The presented studies, aimed at the application of optical fibers as distributed temperature sensors in heat exchangers, yield valuable insights into the selection of suitable optical fiber coatings and appropriate working fluids for system operation. Given the variable thermal conditions typical of heat exchanger environments, the temperature sensitivity of optical fibers immersed in selected working fluids was also systematically investigated.