Hollow cylindrical waveguides for use as evanescent fluorescence-based sensors: effect of numerical aperture on collected signal.

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Abstract
Hollow cylindrical waveguide sensors permit conventional capillary injection techniques for flowing precise volumes of a liquid sample through the sensor while exciting and collecting fluorescence by use of evanescent fields. Both a ray-optics model and experimental data show that fluorescence collected from bulk or thin-layer fluorescent samples increases strongly as the numerical aperture (N.A.) increases and is maximized when the N.A. of the excitation-collection optics matches the waveguide N.A. The dependence of fluorescence on N.A. closely resembled that reported previously for solid cylindrical waveguides. Mode mixing reduced the strength of this dependence and should be minimized to increase collected fluorescence.