Short-Range Non-Bending Fully Distributed Water/Humidity Sensors

Abstract

Existing sensing technologies lack the ability to spatially resolve multiple sources of water or humidity without relying on the deployment of numerous inline sensors. A fully distributed approach has the potential to unlock a diverse range of applications, such as humidity mapping and liquid-depth measurements. We have explored a new direction toward what is, to the best of our knowledge, the first non-bending fully distributed water/humidity sensors. This new class of sensors was made possible from the first combination of small-core exposed-core fiber, a hydrophilic polyelectrolyte multilayer coating, and coherent optical frequency-domain reflectometry. Their non-bending nature enables deployment in a wider range of environments compared to the bending type based on water-induced fiber bending. The sensing mechanism involves monitoring back-reflected optical signals created by changes in the local reflectivity due to water-induced reduction in the local refractive-index of the coating. The demonstrated average sensitivity of the sensing fiber with 10.0 bilayer polyelectrolyte multilayer coating to relative humidity varies from 0.060 to 0.001/%RH (0–38 cm distance) within a dynamic range of 26–95%RH. The distance-dependent detection limit varies between 0.3–10.0%RH (0–38 cm distance), and the spatial resolution of 4.6 mm is the smallest demonstrated for exposed-core fibers and can be vastly improved by simply broadening the swept range. The response time is 4–6 s, and the recovery time is 3–5 s. The sensing range (i.e., distance) is −0.5 m, which is more suitable for water-depth monitoring.