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

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.