Optical Force Sensor Based on Whispering Gallery Mode Resonators

Abstract

This paper discusses a novel micro-optical force sensor based on dielectric microspheres that are excited by coupling light from optical fibers. The technique exploits the morphology-dependent shifts in resonant frequencies that are commonly referred to as the whispering gallery modes (WGM). A small change in the size, shape or optical constants of the microsphere causes a shift in the resonant frequency (or the WGM). For example, a compression force applied to the microsphere will lead to a change in both its shape and its index of refraction distribution. These changes will result in a shift of the WGM. By monitoring this shift, the magnitude of the applied force can be determined. The WGM shifts are observed by scanning a tunable diode laser that is coupled into the optical fiber on one end and monitoring the transmission spectrum by a photo diode on the other end. When the microsphere is in contact with a bare section of the fiber, the optical modes are observed as dips (due to destructive interference) in the intensity of the light transmitted through the fiber. Current results demonstrate the WGM shifts due to compression force applied to micro-spheres along the polar direction. The measurements also indicate a force measurement resolution of ~ 10 N with the current sensor design.