Laseroptical sensors for in-situ gas analysis

Semiconductor diode lasers were first developed in the mid-1960s and found, immediate application as much needed tunable sources for high-resolution laser spectroscopy commonly referred to as tunable diode laser absorption spectroscopy (TDLAS). In this paper the characteristics of currently available semiconductor lasers for spectroscopy in the near- and mid-infra-red spectral region based upon Gallium-Arsenide, Indium-Phosphite, Antimonides and Lead-salt containing compounds will be reviewed together with the main features of TDLAS. Besides the discussion of direct absorption and modulation spectroscopy the focus will be set on modern high frequency modulation techniques. Substantial improvements in sensitivity and detection speed have been achieved by applying these modulation schemes and an increasing number of diode laser based applications in fundamental spectroscopy and for analytical measurement challenges have been reported. In the context of the discussion of greenhouse gases, the determination of trace gas fluxes between biosphere and atmosphere is such a challenge. A fast laseroptical sensor has been applied within an interdisciplinary reseach project to provide micrometeorological measurements of methane emissions from rice paddy fields. The measurements have been performed to assess the quality of data on methane fluxes and allow a comparison with simultaneously recorded data provided by the conventionally used closed chamber method. Systematic differences between chamber and the eddy correlation techniques have been found and it seems that chamber measurements overestimate the actual emission up to 70%. This finding demonstrates once more that TDLAS, combining modem semiconductor laser devices with high resolution spectroscopy, is a valuable tool for atmospheric research