Evanescent-absorption coefficient for diffuse source illumination: uniform- and tapered-fiber sensors.

A comparative study of evanescent-wave fiber-optic absorption sensors based on uniform and tapered fibers has been carried out. The expressions for an effective evanescent-absorption coefficient have been derived for diffused or Lambertian source illumination. It has been shown that the sensitivity of sensors depends on the numerical aperture of the fiber, the taper ratio, and the refractive index of the absorbing fluid. The higher the sensitivity the smaller the range of functional refractive indices of the fluid. In the case of taper, which fiber (with a low or high numerical aperture) has maximum sensitivity depends on the refractive index of the fluid.