Efficient Source-To-Fiber Coupling Method Using a Diamond Rod: Theory and Application to Multimode Evanescent-Wave IR Absorption Spectroscopy

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Abstract
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-mm-diameter 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.

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