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26 February 2001 Evanescent-wave cavity ring-down spectroscopy for trace water detection
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Proceedings Volume 4205, Advanced Environmental and Chemical Sensing Technology; (2001)
Event: Environmental and Industrial Sensing, 2000, Boston, MA, United States
We explore the use of evanescent wave cavity ring-down spectroscopy (BW-CRDS) for water detection through a signal-to-noise ratio analysis. Cavity ring-down spectroscopy (CRDS) is an emerging optical absorption technique that employs the mean photon decay time ofa high-finesse optical cavity as the absorption-sensitive observable. EW-CRDS is a novel implementation of CRDS that extends the technique to surfaces, films, and liquids by employing optical cavities which incorporate at least one total-internal-reflection (TIR) mirror. The concomitant evanescent wave is then used to probe the absorption ofan ambient medium at the TIR surface also through a change in the photon decay time. By employing miniature monolithic cavities with ultra-smooth surfaces that are fabricated from ultra-high transmission materials, extreme sub-monolayer detection sensitivity is readily achieved. The detection of water by EW-CRDS with a fused-silica resonator provides an interesting and important application, since the nascent hydroxylated Si02 surface is expected to show a high natural affinity for adsorption ofwater through hydrogen-bonding interactions. Furthermore, in the 13 80 nm spectral region where water absorbs strongly, low-OH-content fused silica has extremely high bulk transmission. These factors potentially provide the basis for a novel water sensor.
© (2001) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Andrew C. R. Pipino and Joseph T. Hodges "Evanescent-wave cavity ring-down spectroscopy for trace water detection", Proc. SPIE 4205, Advanced Environmental and Chemical Sensing Technology, (26 February 2001);

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