Newton’s rings for absolute distance measurement

In this study, we build upon our earlier research on distance calibration using Newton's rings [Opt. Lett.47, 6053 (2022)10.1364/OL.474539] in a disc-shaped whispering gallery mode resonator (WGMR). Here, we use a glass trapezoidal prism coupler to evanescently trap light in a disc of calcium fluoride (CaF₂) WGMR. While sweeping the distance between the coupler and the WGMR, the back of the trapezoidal prism is illuminated with a red-green-blue (RGB) lamp. Microscopic color images of the Newton's rings formed between the resonator and the coupler are captured using a camera. The resonator-prism distance is measured by fitting the color images via phase extraction from the Newton's rings. This modified method removes the ambiguity inherent in the interferometric distance measurement with monochromatic light and allows for the determination of distance with a single image, which results in a distance error of a few nanometers. We show that by including a position reference, we can further improve the accuracy to less than a tenth of a nanometer. Additionally, this technique eliminates the need for filters and offers more precision, owing to broad spectrum coverage of the RGB imaging, better signal-to-noise ratio, and three distinct color channels.