Whispering-gallery-mode evanescent-wave microsensor for trace-gas detection

A prototype evanescent-wave sensor using resonant whispering-gallery modes of a fused-silica microsphere has been developed and is being applied to the detection of trace amounts of carbon dioxide, carbon monoxide, ammonia, and acetylene in the 1530 - 1580 nm wavelength region. Its sensitivity is comparable to that of typical multipass-cell systems, but our sensor is much more compact because of resonant cavity enhancement. It represents a significant improvement over current total-internal-reflection or fiber evanescent-wave sensors, owing to the longer effective absorption path length (tens of meters in a sphere less than a millimeter in diameter). The present sensitivity is about a hundred parts per million, equivalent to the level detected by a household carbon monoxide sensor. This sensitivity is for direct absorption measurements; wavelength-modulation spectroscopy is being implemented and should provide two or three orders of magnitude improvement. By extending the same techniques, using fluorozirconate- glass microspheres and stronger transitions in the mid- infrared, the sensitivity is expected to improve to about a part per billion.

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