Characterization of long pathlength capillary waveguides for evanescent chemical sensing applications

Light-guiding fused silica capillary tubing has promising applications in the field of chemical sensors. Light propagation in these waveguides occurs within the fused silica capillary wall as opposed to liquid core waveguides where light traverses the solution-filled inner core. The electric field of light within the capillary wall extends slightly into the capillary inner diameter as an evanescent wave. An evaluation of the optical absorbance characteristics of these novel capillaries was performed by coupling the light source from a spectrophotometer into a capillary waveguide flow cell. A series of four thiacyanine dyes in methanol solution provided distinct absorbance peaks throughout the visible spectrum (400-800 nm). Evanescent absorbance values show a linear dependence upon capillary length up to ∼10 m (with 150 μm inner diameter). Evanescent absorbance measurements were nonlinear with solution concentration for all capillary lengths and may be attributed to adsorption of the ionic dye on the capillary fused silica inner surface. The waveguide effective pathlength ratio (EPLR) was determined by comparing evanescent absorbance values to conventional measurements in a 0.1 mm pathlength cuvette. The EPL is on the order of 10^-5 - 10^-6 m per meter of capillary tubing and also shows wavelength dependence. The evanescent field penetration depth ratios from experiment were d_p ratio (424 nm) = 0.60 ± 0.07, d_p ratio (557 nm) = 0.78 ± 0.10, and d_p ratio (652 nm) = 0.73 ± 0.07 when normalized to d _p ratio (758 nm) and compare reasonably with predicted values of 0.56, 0.73, 0.86, and 1.00 respectively. Absorbance sensitivity was investigated with 50, 150, and 250 μm inner diameter capillaries (360 μm outer diameter) that have theoretical inner reflection ratios of 1, 1.7, and 6.9 respectively. Experimentally measured ratios were 1, 2.0 ± 0.9, and 6.5 ± 1.2.