Waveguiding properties of fiber-optic capillaries for chemical sensing applications

The optical properties of a novel fused silica fiber-optic capillary (FOCap) waveguide for absorbance spectroscopy were evaluated. Absorbance within the tubing was measured by optically coupling the FOCap to a spectrophotometer. The FOCap operated evanescently or as a liquid core waveguide (LCW) depending upon the refractive index of the solution within the capillary core. Evanescent absorbance was linear with concentration of a non-polar dye but non-linear with ionic dyes due to adsorption to the capillary wall. Evanescent absorbance was linear with FOCap length up to 50 m. Evanescent field depth of penetration ratios determined from peak absorbances at λ_max for a series of thiacyanine dyes show increasing penetration depth with increasing absorbance wavelength as predicted by theory. Evanescent absorbance measurements in 50, 150, and 250 μm inner diameter (ID) FOCaps show that greater sensitivity is achieved in thinner-walled tubing with more internal reflections. The evanescent effective pathlength ratio (EPLR) of 150 μm ID FOCaps is on the order of ∼10^-6 to 10^-5 m/m of tubing length. FOCaps operating as LCWs have shorter effective pathlengths than conventional polyimide-coated glass capillaries. A FOCap that determines pH from the evanescent absorbance spectrum of immobilized fluorescein demonstrates the utility of the device as an optical chemical sensor.