Analysis of immersed silica optical microfiber knot resonator and its application as a moisture sensor

An embedded silica optical microfiber knot resonator humidity sensor is presented. As silica has a poor response to environmental humidity changes, a surrounding layer of Nafion is used as a transducer. Spectral characterization and also a procedure to determine the coupling and total loss coefficients are presented. Sensitivity as high as (0.29±0.01)  nm/% relative humidity has been noticed. Possible issues that emerge from the use of Nafion such as bulk swelling, refractive index hysteresis, as well as a saturation process, are discussed.