Efficient source-to-fiber coupling method using a diamond rod: Theory and application to multimode evanescent-wave IR absorption spectroscopy

The use of a diamond rod to couple a blackbody source optically to a chalcogenide glass fiber is described and shown to be useful for evanescent- wave IR absorbance measurements. One end of a short 0.5-mmdiameter diamond rod abuts an optical fiber of the same diameter; the other end is in direct contact with a glowing silicon carbide IR source. The close match of the refractive indices of SiC, diamond, and AsSeTe fiber reduces optical losses at the interfaces, allowing a high overall throughput as well as transmission into the fiber of a wider range of transverse optical modes than is available with the use of through-air focusing methods. Use of the higher-order modes for evanescent-wave spectroscopy leads to increases in sensitivity, due both to increased evanescent pathlength and to a larger number of internal reflections per unit length.