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Laser absorption spectroscopy in the mid infrared region : Investigation of optical saturation and solutions for its circumvention

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Scientific context :

Recently, developments in ultrasensitive laser spectroscopy have been carried out in the mid-infrared (MIR) spectral region, based on a new type of compact and low-power semiconductor lasers (the Interband Cascade Laser - ICL) compatible with field measurements. The MIR spectral region is very attractive for ultrasensitive detection of different molecules as strong absorption lines can be addressed, allowing to obtain a sub-ppb (part per billion) sensitivity [1]. It opens the field toward new applications in different fields such as in global and paleo climatology, or breath gas analysis for medical diagnostics.

In order to reach extreme sensitivities, laser optical spectroscopy techniques at the state of the art are required. Our group at LIPhy is well known in France and abroad for being at the fore-front in the development of ultrasensitive spectroscopic techniques for the selective and quantitative measurements of molecules present in a gas at trace levels. Specifically we developed an original technique (Optical Feedback-Cavity Enhanced Absorption Spectroscopy - OF-CEAS) to enhance the effective absorption length up to tens of kilometers, while the actual sample cavity length is only about 50 cm, allowing for a compact set-up for in-situ measurements [2]. This absorption length enhancement factor is provided by a high finesse resonant optical cavity used as the sample cell.

Optical saturation :

As it turns out, the high absorption intensity of the transitions in the MIR combined with the intense laser field inside a resonant optical cavity induce an optical saturation phenomenon : a significant fraction of molecules is transferred to the excited energy state of the molecular transition. These molecules contribute to stimulated emission rather than to absorption, leading to an underestimation of the absorption and thus of the molecular concentration. It is therefore mandatory to correct the measurements for this effect in new MIR analyzers. To date, we have formally identified the limiting effect of saturation on concentration measurements and an empirical correction method is applied [1].

Internship objective :

This internship will focus on the optical saturation effect. The first step will be to model this effect for different molecules and different experimental conditions (modelling for different pressure regimes, studying effects of laser intensity, molecular concentration, presence of other molecules...). The model will be compared with measurements which will be carried out with different analyzers already developed by the team in the MIR region (between 4.3 and 5.6 µm). The second step will be to investigate new laser spectroscopy methods derived from OF-CEAS, which will reduce or even eliminate the saturation effect. In particular, we are considering heterodyning techniques between different modes of the resonant cavity.

Expected skills :

We are looking for a person motivated by experimental research and theoretical modeling (methodical, organized, persevering). The candidate should have a good knowledge in optics and quantum mechanics, notions in laser spectroscopy and molecular spectroscopy.

Supervisors :


1. L. Richard, D. Romanini, and I. Ventrillard, Sensors 18, 1997 (2018).
2. J. Morville, S. Kassi, M. Chenevier, and D. Romanini, Appl. Phys. B 80, 1027 (2005).