This paper reports on the development of optically based techniques to detect and identify chemical agents. Detection
sensitivity and molecule discrimination are studied. In parallel, efforts are made to develop rugged and compact
experimental designs that can be used for field measurements. Laser Induced breakdown spectroscopy (LIBS) is a
surface analyzing optical technique investigated to measure sarin like molecules deposited on samples coming from the
Parisian subway. On the other hand, Tunable Diode Laser Spectroscopy (TDLS) - Cavity Ring-Down Spectroscopy
(CRDS) or Cavity Enhanced Absorption Spectroscopy (CEAS) - is used to measure traces of the industrial toxic
hydrogen fluoride gas down to the ppb level. Measurements in laboratory are reported and primary results obtained in a
field experiment are described.
We recently demonstrated trace detection using Cavity Ring Down Spectroscopy (CRDS) coupled with telecom DFB diode lasers. Our scheme exploits optical feedback from a V-shaped cavity back to the laser. We built trace-gas detectors for CH4 and HF, characterized by a low cost, simplicity, compactness and sensitivity. Operating wavelength are 1.312 micrometers for HF and 1.65 micrometers for methane. The optical setup includes a distributed feed-back (DFB) diode laser, temperature stabilized by a Peltier, a collimating lens, 2 steering mirrors, a V-shaped optical resonator and a photodiode. The V-cavity is made of three low-cost super mirrors R 99.995%) and contains the air sample to be analyzed (20cm3). In standard atmospheric conditions the detection limits for 1 second integration time are of 50 ppbv for HF and 200 ppbv for methane. We present an analysis of the mechanisms of cavity injection and laser feedback, allowing to estimate the influence of various parameters on the performances of this type of apparatus. Calculations and results are given, with particular emphasis on the detection limit and the dynamic range.
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.
Optical spectroscopic methods based on direct absorption offer a quantitative measurement of the absorbance, which is the product of the concentration, the molar absorption coefficient of the transition being observed and the length of the absorption path. An absorption sensitivity adequate for trace detection may be achieved by increasing the path length. One solution is offered by cavity ringdown spectroscopy (CRDS), attractive for its simplicity. We recently demonstrated that an external cavity diode laser (ECDL) can be conveniently employed for CRDS instead of a pulsed laser, contrary to previous applications. Here we extend this result to distributed feed-back (DFB) diode lasers. Paying special attention to the coupling of the laser source to the cavity, we developed an extremely simplified CRDS scheme with a sensitivity of about 10-8/cm/(root)Hz. We then built detectors for methane and HF, working close to the optical wavelengths 1.65 and 1.31 micrometer, respectively With an optical assembly of about 50 cm length and a response time of about 1 s, these devices accurately measure atmospheric methane concentrations in the range 0.5 to 200 ppmv, and HF concentrations from 0.1 to 50 ppmv.
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