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Characterization of Supported Cylinder–Planar Germanium Waveguide Sensors with Synchrotron Infrared Radiation

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Abstract:

Cylinder-planar Ge waveguides are being developed as evanescent-wave sensors for chemical microanalysis. The only non-planar surface is a cylinder section having a 300-mm radius of curvature. This confers a symmetric taper, allowing for direct coupling into and out of the waveguide's 1-mm2 end faces while obtaining multiple reflections at the central <30-μm-thick sensing region. Ray-optic calculations indicate that the propagation angle at the central minimum has a strong nonlinear dependence on both angle and vertical position of the input ray. This results in rather inefficient coupling of input light into the off-axis modes that are most useful for evanescent-wave absorption spectroscopy. Mode-specific performance of the cylinder-planar waveguides has also been investigated experimentally. As compared to a blackbody source, the much greater brightness of synchrotron-generated infrared (IR) radiation allows a similar total energy throughput, but restricted to a smaller fraction of the allowed waveguide modes. However, such angle-selective excitation results in a strong oscillatory interference pattern in the transmission spectra. These spectral oscillations are the principal technical limitation on using synchrotron radiation to measure evanescent-wave absorption spectra with the thin waveguides.

Keywords: BLACKBODY IR SOURCE; EVANESCENT WAVE ABSORPTION SPECTROSCOPY; SYNCHROTRON IR RADIATION; TAPERED WAVEGUIDES; THROUGHPUT

Document Type: Research Article

DOI: http://dx.doi.org/10.1366/000370204322842869

Affiliations: Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330 Thailand

Publication date: February 1, 2004

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