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

Roy E. Jonas, Mark S. Braiman

Research output: Contribution to journalConference Articlepeer-review

2 Scopus citations


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 αe 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 αe 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.

Original languageEnglish (US)
Pages (from-to)9-14
Number of pages6
JournalProceedings of SPIE - The International Society for Optical Engineering
StatePublished - May 21 1993
Externally publishedYes
EventFiber Optic Sensors in Medical Diagnostics 1993 - Los Angeles, United States
Duration: Jan 17 1993Jan 22 1993

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics
  • Computer Science Applications
  • Applied Mathematics
  • Electrical and Electronic Engineering


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