New perspectives in ultrasensitive trace gas monitoring by cavity-enhanced laser absorption spectroscopy

Ultrasensitive optical spectroscopy technologies for environment monitoring, and in general, for gas analysis in the near Infrared are mainly based on the following spectroscopic methods: diode laser spectroscopy with multipass cells, with or without frequency modulation, photoacoustic spectroscopy, and since very recently, difference frequency spectroscopy with diode lasers. One of the most important issues of any monitoring technology, as important as the sensitivity, is its ability to provide absolute absorption coefficients without the need of complicated and cumbersome calibration procedures. Until now, two of the most sensitive optical spectroscopic technologies capable of providing this absolute information, Cavity Ringdown Spectroscopy and Intracavity Laser Absorption Spectroscopy have practically no use in this field. Due to recent advances, these two methods can now provide low-cost very compact field instruments working in the spectral range from 0.8 to 2.5 microns, with the smallest detectable absorption down to 10-10 (one over 10 billions) per one centimeter of the absorption path. This would result in the sub-ppb detection limit for moisture for example. Experimental results obtained with prototype field instruments developed by our group will be presented. Future perspectives will be discussed.