Polymer optical fiber tapers for biosensing applications

We report the fabrication and the characterization of perfluorinated cladded multimode polymer optical fibers tapers. The fibers employed was a graded-index polymer optical fibers with core and cladding diameters of 62.5/90 and 120/160 and very low refractive index (nCore = 1.356, nCladding = 1.342). The tapers were fabricated using the heat-and-pull technique. This approach permits a good prediction of the taper shape and waist dimension. Despite to the fact that taper core is not in direct contact with the external medium, these fiber tapers can be used for sensing applications. In fact, some of the guided modes are no more confined in the core region but can still be guided by the fiber in the cladding region. Therefore, there is an evanescent wave in the external medium, related to bound and tunneling rays that can pass the taper region in the cladding. In particular, the very low refractive indexes of the core-cladding perfluorinated polymers permit a strong enhancement of the power fraction in the evanescent wave in aqueous environments (n=1.33). We carried out experiments evanescent wave spectroscopy by immerging the tapered fiber water containing a dissolved organic blue dye (Methylene blue) for a concentration range 5e-8-1e-5 M.