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 microfiber with a diameter of 2 μm, the MKR with a loop diameter of 70 μm 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.