| Light propagates through an optical fiber by total internal reflection. The electromagnetic wave is not confined to the fiber core, but extends into the fiber cladding (the evanescent field), and thus can probe the region outside of the fiber core. If an absorbing, scattering, or fluorescing species is present in the evanescent region, energy can be coupled in and out of the fiber core. A fiber-optic sensor can utilize this interaction to measure a physical or chemical parameter optically. Multiple sensors can be monitored simultaneously through pulsed excitation and time-resolved detection, a scheme known as Optical Time-of-Flight Detection (OTOFD).;An array with four sensing regions demonstrated that sensors can be located within a few centimeters of each other by evanescently coupling light to a second fiber with an appropriate length required by OFOTD, and that a poly(ethylene) glycol film embedded with fluorescein yields a response similar to fluorescein in solution. An array with 100 sensing regions showed that signals are still detectable even under significant attenuation due to a fiber length of one kilometer.;Small displacements, however, between the fibers can result in large signal changes due to the exponential decay of evanescent fields away from the fiber core/cladding interface. Hydrogel resins are unsuitable for sensing in aqueous environments as polymer swelling causes separation of the junction, resulting in weak, inconsistent signals. Here, microsphere templating was used to create a network of pores in a cross-linked poly(ethylene) glycol (PEG) based polymer that allows analyte passage to the evanescent region where sensing occurs. Covalent attachment of the fluorosensor minimizes leaching. The structures of these porous junctions were characterized with SEM imaging. Fluorescence measurements show a dramatic improvement in the sensor response time and improved consistency for replicate measurements using fluorescein-based dyes.;Also, fluorescence lifetime measurements were investigated as an alternate pH sensing scheme. The fluorescence lifetime is relatively insensitive to source fluctuations and to photobleaching of the fluorosensor, and thus less calibration is required. Fluorescence lifetime techniques can be readily incorporated to our sensor architecture. Both Time-Correlated Single Photon Counting (TCSPC) and Stroboscopic or Time-Gated Detection were investigated for lifetime determination. Both methods allowed for successful remote measurement of pH via fluorescence lifetimes. |
