An IR tunable diode laser absorption spectrometer for trace humidity measurements at atmospheric pressure

There is an increasing demand for the measurement of trace humidity in air at levels down to a few parts in 10/sup 9/ (ppb/sub v/) at atmospheric pressure. This summary describes the development by a European collaboration of a diode laser absorption spectrometer capable of measuring trace humidity with an uncertainty of 5 ppb. The diode laser was a distributed feedback device, tuned to the 3,0,3/spl larr/2,0,2 rotation transition in the /spl nu//sub 1/+/spl nu//sub 3/ vibrational band of water vapour at 1393 nm. Two-tone frequency modulation spectroscopy (TTFMS) was chosen as it combines the advantages of being the most sensitive technique and is relatively straightforward to implement. After a number of refinements to the optical layout, TTFMS features were obtained with no observable background interference effects.