Waveguiding properties of fiber-optic capillaries for chemical sensing applications

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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 λmax 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.

Original languageEnglish
Pages (from-to)360-371
Number of pages12
JournalSensors and Actuators, B: Chemical
Issue number2
StatePublished - Aug 8 2007


This research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-04-2-0043. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the US Government. The US Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon.

FundersFunder number
United States Army Research LaboratoryW911NF-04-2-0043


    • Attenuated total reflection spectroscopy
    • Capillary waveguide
    • Evanescent absorbance spectroscopy
    • Liquid core waveguide


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