Very High Sensitivity Spectroscopy using Tunable Diode Lasers

In recent years, tunable diode lasers have become widely used in the study of laboratory infrared spectra. The high resolution provided by the diode lasers is employed to obtain accurate line positions, and to a lesser extent lineshapes and linestrengths. While this type of research is now undertaken in many laboratories, few researchers are currently exploiting the potential of the diode lasers to provide very high sensitivity measurements in the infrared. In this paper, we describe diode modulation techniques which enable one to detect absorption coefficients as small as 10-9 cm-1. This sensitivity, combined with the tunability and resolution of the diode laser, allow one to perform a wide range of experiments relevant to atmospheric spectroscopy. In the laboratory one can detect very weak absorption lines in stable molecules [1], or investigate the spectra of unstable molecules such as free radicals [2]. The instrumentation is port able, and we have used it to make in situ field measurements on a wide range of gases and pollutants in an urban atmosphere [3]. Similar instruments have been flown in aircraft to monitor tropospheric CO concentrations, and balloon flights are planned to simultaneously determine stratospheric NO2 and NO concentrations [4].