Mode Coupling Theory for Evanescent Wave Optical Fiber Sensors

Optical fiber based sensors have potential in the composites processing industry as well as many other industries, but cost and robustness often mitigate the advantages of optical fiber sensors. The least expensive type of optical fiber sensor is a length of fiber either embedded in the part or in distal contact with the part surface. In the case of embedded evanescent mode sensors, important issues hinge on interpreting the evanescent signal obtained during processing or in service inspection. An optical model based on mode coupling theory is presented below to provide a description of the sensing volume around the fiber from which signal is collected during evanescent measurements. The model is developed for a fluorescence sensor but the optical theory could be equally applied for infrared or other types of optical fiber sensors.