Refractive Index Sensing in Rectangular Glass Micro-Capillaries by Spectral Reflectivity Measurements

We report the application of rectangular glass micro-capillaries with cross-section of 20 μm x 200 μm and 50 μm x 500 μm as capillary-driven liquid-core optical cavities for fluid refractive index sensing. Reflected power spectra in two wavelength bands (around 1.3 μm and 1.55 μm) are detected with readout broadband radiation crossing the capillary in orthogonal direction with respect to the flat sides. Using a fiberoptic scheme for remote, non-contact measurements and inserting glucose or bovine serum albumin solutions in water at different concentrations as test fluids, we have revealed the shift of the wavelength position of the reflectivity minima due to refractive index variations. The combination of these low-cost devices with a spectral readout method allows achieving sensitivities higher than 250 nm/RIU and limits of detection better than 1.3·10-3 RIU.