Relative Humidity Sensor Based on a Few-Mode Microfiber Knot Resonator by Mitigating the Group Index Difference of a Few-Mode Microfiber

A highly sensitive relative humidity sensor based on a few-mode microfiber knot resonator (FM-MKR) incorporating a polyvinyl alcohol overlay is investigated. The sensitivity of the proposed FM-MKR to the relative humidity is successfully enhanced by mitigating the difference in effective refractive group indices between the HE11 and HE12 modes. The proposed FM-MKR is structurally composed of two sensing probes, such as an FM-microfiber modal interferometer and an FM-MKR. In the FM-microfiber modal interferometer, two modes (HE11 and HE12) are excited and interfered in a nonadiabatically up-tapered region of the FM-microfiber, resulting in an inline modal interferometer. After making a tie with the FM-microfiber, the FM-MKR is fabricated and two modes must be circulated within the optical knot and cross-coupled independently with a phase delay. By optimizing the diameter of the microfiber (~4 μm), the difference in effective refractive group indices between HE 11 and HE12 is suppressed, resulting in an improved relative humidity sensitivity of the proposed FM-MKR. By using the fast Fourier transform algorithm, optical transmission spectra of the FM-MKR are converted to spatial frequency spectra to precisely measure the concentration of relative humidity. The absorption of relative humidity in the proposed FM-MKR-based sensing probe effectively changes the resonant wavelength of the FM-microfiber modal interferometer and the spatial frequencies of the FM-MKR depending on the radial mode number (m ) and the mode order (k).