Mesure de traces de gaz à l'aide de lasers à semi-conducteur

Infrared spectroscopy using semiconductor lasers is a promising technique for trace gas detection. It allows a continuous and real time monitoring of several species, with a high sensitivity and a good selectivity. Among all the possible methods two are particularly suitable for this application: wavelength modulation spectroscopy (WMS) based on an optical detection of the transmitted light intensity and photoacoustic spectroscopy, which detects an acoustic wave produced by the energy absorbed in the sample. Semiconductor lasers are easily modulated through their injection current and are thus well suited for these applications. The laser modulation is a key issue for these techniques and is first described in detail, both theoretically and experimentally. Experimental methods used to determine the modulation parameters are presented. Then, a theoretical model describing the interaction of a modulated optical wave with an absorption line is exposed. Two different cases are considered. In the first, the modulation frequency is higher than the width of the absorption line, whereas it is smaller in the second. In this latter case, the developed model takes into account the simultaneous intensity (IM) and frequency (FM) modulation of the laser. This establishes a generalization of existing models, which consider only the FM modulation and thus imperfectly describe the real laser behavior. A more exact model was obtained by taking into account the laser intensity modulation and the results are confirmed with a remarkable precision by experimental measurements. The effect of several modulation parameters on the measured signals was studied from this theoretical model and is validated by numerous experimental results. A new experimental method was developed on the basis of this model to measure the modulation parameters of a laser. The obtained results correspond with a good precision to the values supplied by other traditional methods, which demonstrates the validity of this new technique. The two experimental techniques for trace gas detection were implemented using a DFB semiconductor laser emitting at λ = 2 µm for CO2 monitoring. In the WMS method, the laser emission frequency was actively stabilized on a CO2 absorption line, which reduced its relative frequency fluctuations below 10-7. The photoacoustic spectroscopy was implemented using a resonant acoustic cavity, operated in its first radial mode in order to optimize the photoacoustic effect efficiency. Both intensity and frequency modulations of the laser were applied and their performances were compared. In our experimental configurations, the WMS method showed the best sensitivity, with a detection limit lower than 1 ppm × m. Several solutions are proposed to increase the performances of each method, taking into account their own specificity. It turns out that the WMS method has a better sensitivity in the near infrared, where very sensitive photodetectors are available. But, its performances decrease with increasing wavelength, as the photodetectors sensitivity strongly decreases. On the other hand, photoacoustic spectroscopy has the advantage to be totally wavelength-independent, as the detection is made acoustically. Thus, it becomes more attractive than WMS in the mid-infrared, especially when taking advantage of high power lasers available in this spectral range. Finally, the potential of a new kind of semiconductor lasers is estimated for trace gas detection. These so-called quantum cascade lasers were invented some years ago and operate at room-temperature in the mid-infrared. A characterization of their main optical properties is exposed and some spectroscopic measurements are shown.

Robert, Philippe Alain
Lausanne, EPFL

 Record created 2005-03-16, last modified 2018-01-27

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