Airborne Hygrometer Using a Diode Laser

Southwest Sciences proposes the development of a diode-laser-based hygrometer for atmospheric studies. The instrument dynamic range will span dew (frost) points of +40¿ C down to -90¿ C and will be suitable for in situ measurements throughout the troposphere and tropopause. Response times will be 1 second, or less. The planned instrument detects absolute water vapor concentrations using optical spectroscopy with a custom-fabricated InGaAsP diode laser operating at 1393 nm. Optical absorbance measurements are made using high frequency wavelength modulation spectroscopy (WMS) which, we anticipate, will be capable of minimum detectable absorbances of 1 x 10-5 providing a signal-to-noise ratio of 3 for the lowest water concentrations. Fiber optics will "pipe" the light from the laser to the measurement point thereby eliminating unwanted absorbance due to water vapor outside of the actual measurement path. The Phase I experiments are expected to demonstrate the feasibility of the diode-laser-based system for detecting water vapor accurately and rapidly over a nearly million-fold range of concentrations. The Phase II program will culminate in the delivery of a fully-automated hygrometer for airborne studies.