Maximum achievable sensitivity of the fiber optic evanescent field absorption sensor based on the U-shaped probe

The influence of the bending radius of the probe on the sensitivity of the fiber optic evanescent field absorption sensor based on the U-shaped probe is experimentally investigated. Optical fibers of different core diameters and different numerical apertures are used. It is found that the sensitivity increases with the decrease in the bending radius of the probe. It increases up to a certain value of the bending radius termed as the optimum bending radius and then decreases on further decreasing the bending radius. The upper limit on the sensitivity occurs due to the cross coupling of power between input and output arms of the U-shaped probe. It is found that the optimum bending radius depends on the numerical aperture and the core radius of the fiber. The higher the numerical aperture, the larger the optimum bending radius. Further, the smaller the core diameter of the fiber, the smaller the optimum bending radius. The present study will be useful for deciding the bending radius of the U-shaped probe to achieve maximum sensitivity for a fiber of given core diameter and numerical aperture. (C) 2000 Elsevier Science B.V. All rights reserved.