Evanescent Wave Absorption Based Fiber-Optic Sensor - Cascading of Bend and Tapered Geometry for Enhanced Sensitivity

Evanescent wave absorption (EWA) based fiber-optic sensors have found widespread applications ranging from environmental sensing to biosensing. In these sensors, optical and geometrical characteristics such as optical fiber type (single-mode or multi-mode), fiber core diameter, fiber probe geometry, fiber probe length, etc., are very important. These parameters affect the penetration depth and fractional power by modulating the ray propagating in the fiber probe that ultimately influences the sensitivity of the EWA sensors. Various geometries of fiber probe designs, like bent, tapered, coiled, etc., have been explored for improving the sensitivity. This chapter describes the design, development and fabrication of a novel bent-tapered fiber-optic sensor. A combination of bending and tapering acts as a mode converter, which results in high penetration depth of the evanescent field. In addition, tapered region of the probe increases the coupling efficiency at the detector end by V-number matching and thus improves the signal-to-noise ratio. EWA sensitivity of the sensor was compared for different taper ratios. Finally, the optimized geometrical design was used to demonstrate biosensing application.