Compact source-to-fiber diamond optical coupler enhances absorbances from optical fiber evanescent-wave IR spectroscopy using a simple design

We have shown that a direct-coupling approach can give a high throughput of light from a broadband IR source into a chaicogenide optical fiber. The high light levels in the fiber facilitate sensing based on evanescent-wave absorption. We place one end of a diamond rod in direct contact with an optical fiber of the same diameter, while the other directly contacts a hot JR source. This results in efficient coupling of a wide cone of optical modes into the fiber, including those propagating at nearly the cutoff angle (the critical angle for internal reflection from the fiber-liquid interface). These very high-order modes have a large penetration depth, a high interfacial evanescent wave intensity, and a large number of reflections per unit length. As a result, multimode spectra obtained by using them demonstrate high sensitivity, i.e. very large measured absorbances per unit length of fiber contact with sample. Using the diamond coupler with a 500-pm-dia. fiber, we observe an absorbance coefficient α_e of 0.04 M^-1 cm^-1 for the 1030-cm^-1 band of glucose in water. This sensitivity can be increased even farther (with little or no increase in the noise present in the absorbance spectrum) by tapering the portion of the fiber in contact with the sample. With a tapered fiber diameter of 200 μm, we observe an α_e of nearly 0.2 M^-1 cm^-1 for the glucose absorption band cited above. With either tapered or untapered fiber, it is possible to measure glucose concentrations in the range 0-250 mM with a sensitivity of <25 mM in 2.5 mm. With a 7-mm-long, 200-μm-dia. taper on the fiber, curled into a ∼2-mm-dia. loop, 25 mM glucose can be detected in sample volumes as small as 20 μL.