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Sensitivity Analysis Of Evanescent Fiber Optic Sensors

Evanescent fiber optic sensors are being developed for remote in situ immunoassay. The single reflection total internal reflection fluorescence (TIRF) geometry can serve as a well-defined model against which evanescent waveguide devices can be compared and evaluated. This paper addresses the problem of optimizing the sensitivity of an evanescent fiber optic sensor (EFOS). Two aspects are discussed: (1) the modes of exciting laser light in the fiber have an effect on the sensor efficiency and signal-to-noise ratio; (2) in a fiber biosensor, there is generally a protein layer attached to the core surface; the thickness of the layer is at least 5nm. If the refractive index of the protein layer can be made equal to the refractive index of the core, we can get a new fiber waveguide in which the core also contains the protein layer. The fluorescent emission sources are thus inside the core region and generate the highest signal collection efficiency. We also discuss the situation when the refractive index of the protein layer is larger or smaller than that of the optical fiber core.