Studies of tapered single-mode optical fiber devices for hybridization-based biosensing and other applications

Tapered single-mode optical fibers are investigated for use in hybridization-based biosensing and other applications. Theoretical modeling and experimental studies of both evanescent wave excitation and collection of fluorescence from molecules located at refractive-index discontinuity are presented. Device design issues that come out of the experiments and modeling are addressed. The evanescent field distribution surrounding the taper region has been measured directly using a scanning near field optical microscope and results are found to be in good agreement with predictions made using a finite difference beam propagation method. The results of detailed theory of electric dipole radiation at an interface show a strong anisotropy of the emitted fluorescence. I have used these results in a model for numerical calculations of fluorescence collection efficiency of the tapers I fabricated. The calculated collection efficiencies were compared with the measured efficiencies using the taper of the same shape to collect fluorescence from molecules within the effective evanescent penetration thickness in free dye solutions of different concentrations. The agreement of measured and calculated collection efficiency suggests that the main design issue to be considered when tailoring the taper shape to maximize the efficiency of fluorescence collection, is the angular distribution of radiation from dipoles placed near a dielectric interface. All experiments are performed using an all-fiber optics biosensor instrument that was designed and built for this work.