Theoretical investigations for combination optical fiber tip sensors

For the optical fiber biosensor made of combination optical fiber tip, the effects of the refractive index (RI) of outer medium and the geometrical parameters of combination optical fiber tip on two important quantities have been investigated. One factor is the total intensity of the evanescent wave into fluorescent layer through core-medium interface; the other is the intensity of the fluorescence signal transmitted from fluorescent layer to measurement end. Theoretical investigations show that the two quantities are dependent upon the RI of outer medium, and the total intensity of the evanescent wave into fluorescent layer increases with the increase of taper angle, and the fluorescence signal arrived at the measurement end gets larger value for shorter fiber tip when the RI of outer medium is smaller than the RI of fiber core.