Laser absorption spectrometry

Laser absorption spectrometry (LAS) refers to techniques that use lasers to assess the concentration or amount of a species in gas phase by absorption spectrometry (AS). Optical spectroscopic techniques in general, and laser-based techniques in particular, have a great potential for detection and monitoring of constituents in gas phase. They combine a number of important properties, e.g. a high sensitivity and a high selectivity with non-intrusive and remote sensing capabilities. Laser absorption spectrometry has become the foremost used technique for quantitative assessments of atoms and molecules in gas phase. It is also a widely used technique for a variety of other applications, e.g. within the field of optical frequency metrology or in studies of light matter interactions. The most common technique is tunable diode laser absorption spectroscopy (TDLAS) which has become commercialized and is used for a variety of applications. The most appealing advantages of LAS is its ability to provide absolute quantitative assessments of species. Its biggest disadvantage is that it relies on a measurement of a small change in power from a high level; any noise introduced by the light source or the transmission through the optical system will deteriorate the sensitivity of the technique. Direct laser absorption spectrometric (DLAS) techniques are therefore often limited to detection of absorbance ~10−3, which is far away from the theoretical shot noise level, which for a single pass DAS technique is in the 10−7 – 10−8 range. This detection limit is insufficient for many types of applications. The detection limit can be improved by (1) reducing the noise, (2) using transitions with larger transition strengths or (3) increasing the effective path length. The first can be achieved by the use of a modulation technique, the second can be obtained by using transitions in unconventional wavelength regions, whereas the third by using external cavities.