Experimental investigation on encapsulation of whispering gallery mode sensors for smart materials applications

Structural health monitoring applications of composite materials can benefit from various micro-sized optical sensors. Whispering Gallery Mode (WGM) optical sensors are well known for ultra-high sensitivity in force and displacement measurement. Their sensing principle is based on optical coupling between evanescent fields of an optical fiber with a microsphere resonator. Any stimulus causing a change in the radius or the index of refraction of the microsphere can be sensed by a shift in transmission spectrum. Present WGM sensors utilize two separate systems to hold fiber and particle in the optically favorable configuration to obtain WGM resonances. However, use in structural health monitoring of materials requires that these sensors are encapsulated in a solid material system, which is a complex task. Therefore, most of the present studies on WGM sensors conduct measurements in fluidic media such as air or water. The present work explores the possibility of encapsulating WGM sensors as a system inside a solid material so that they can be used as standalone sensors without the requirement of a holding mechanism for the particle. In such a configuration they can be embedded inside composite materials and used for health monitoring applications. The present work demonstrates success in encapsulating the sensor in a solid medium and also presents analysis about the sensitivity of such sensor configurations. The results are useful in developing sensing schemes for composite materials using WGM sensors.