This work demonstrates the feasibility of the evanescent field perturbation sensor approach for both three-dimensional and planar resonator geometries. An optical fiber-coupled silica sphere resonator and an integrated waveguide-ring resonator are used with a smaller sphere inserted in their evanescent tail. In both experiments, the motion of the perturber across the evanescent tail leads to a measurable shift in the resonators’ whispering-gallery-modes. The results show a consistent relationship between the mode shift and the position of the perturber.
The proposed sensor utilizes a whispering gallery mode (WGM) resonator to measure velocity-induced non-coherent Doppler shifts in Mie-scattered light from particles moving with the air-flow. The resonator replaces the typical Fabry-Perot instrument to measure the Doppler shift. A prototype sensor was developed and experiments were carried out in an atomizing nozzle with a 25μm-diameter water-droplet-seeded air jet. Individual velocity measurements were made at the center of the nozzle, switching the flow on and off. Preliminary results show promise for the WGM-based velocity sensor concept, which would significantly reduce the size and weight of future Direct-Detection-Doppler systems for air-speed measurement, with possible applications in airborne-platform weather-monitoring and planetary studies.
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