Process for reducing interfering signals in optical measurements of water vapor
US Patent #5,804,702, issued September 8, 1998
Water vapor is present in the atmosphere and adheres to almost any surface it contacts, so it should come as no surprise that water is present as an impurity inside lasers, detectors, and other spectrometer components.
When using a laser spectrometer or other absorption spectroscopy system to measure water vapor, the contamination inside the laser adds an interfering signal that makes it very difficult to quantify the water in a sample. This patent describes a simple way to reduce this interference. The laser is exposed to deuterium oxide, D2O, which is identical to water vapor except that the hydrogen atoms are twice as heavy. As a result, the water impurities in the laser are converted to HDO. This change in mass shifts the absorption frequencies, so that the water vapor in the sample region can be detected at H2O frequencies without interference from the water in the laser.
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