Relative Humidity Sensor Based on an Optical Microfiber Knot Resonator With a Polyvinyl Alcohol Overlay

A highly sensitive relative humidity (RH) sensor based on a microfiber knot resonator (MKR) with a polyvinyl alcohol (PVA) overlay is investigated. To realize a miniature fiberoptic RH sensing probe, we designed a MKR with a PVA coating. After making a tie using the microfiberwith a diameter of 2 mu m, the MKR with a loop diameter of 70 mu m is realized and coated with PVA. The MKR with the PVA overlay effectively responds to variations in the concentration of RH because of a strong evanescent field from the microfiber. For precise measurement of the concentration of RH, the optical spectra of the MKR-based RH sensor are converted to spatial frequency spectra by using the fast Fourier transform algorithm. The absorption of humidity in the proposed MKR-based sensing probe effectively changes the spatial frequency. The RH sensitivity of the proposed MKR-based RH sensing probe with a higher order mode dramatically improved by a factor of four compared to that of lower order mode.This work was supported by the Basic Science Research Program through the National Research Foundation of Korea, funded by the Ministry of Education, Science and Technology (NRF-2015R1D1A1A09060875)