High sensitivity balloon-like interferometric optical fiber humidity sensor based on tuning gold nanoparticles coating thickness

We report on the effect of gold nanoparticles (Au-NPs) coating thickness on the sensitivity of a relative humidity (RH) sensor based on a balloon-shaped single-mode fiber (BSBS) structure for the first time. The BSBS structure was established by bending a coated single-mode fiber (SMF) into a balloon shape by using a section of a capillary tube. Three structures with various gold nanocoated sensing have been fabricated to select the optimal thickness. The thicknesses of the Au-NPs were 10, 20, and 30 nm. The performance of each structure towards different RH levels ranged from 35% to 95% was experimentally investigated. Experimentations showed that the RH sensor with a nanocoating thickness of 20 nm exhibits the highest sensitivity of -0.571 nm/RH with an ultrafast response time of 1.41 ms. Overall, the results point out that increasing or decreasing the coating thicknesses than the optimum thickness leads to a decrease in sensor sensitivity.