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Développements de systèmes multipassages pour application à la spectroscopie d'absorption : Cavity Ring Down Spectroscopy multimode et cellules à passages multiples

Abstract : The absorption Spectroscopy technique based on the measure of the free photon lifetime trapped inside a high-finesse optical cavity, also known as Cavity Ring Down Spectroscopy (CRDS), is among the most sensitive optical diagnostic tools. The « mode matched » cavity excitation using a continuous wave laser quickly and continuously swept (~4 THz.s-1 during ~2 ms), i.e. the high-speed monomode cw-CRDS, ensures to the technique its best performances in terms of detection limit “with however the drawback” data-points separated by the Free Spectral Range of the resonator (FSR=c/2L) : then, probing low pressure samples demand long cavities. This original study proposes to take advantage of the resonator's transverse mode structure and then to make possible the high-speed cw-CRDS device miniaturization. The fractionally degenerated cavity multimode excitation (also called "magic length” resonator) is presented here. It provides a wide spectral sampling improvement proportional to the employed reentrant order N. We are developing and testing two multimode cavity injections schemes. The first one is axial while the other one is reentrant, also said to be an off-axis excitation injection of such cavities and we examine the main differences. A detailed investigation of the intracavity field at the N resonances fractioning the FSR is proposed, explaining the detection limits observation associated to these two slightly distinct multimode injection schemes. A second part of this work is dedicated to the detailed characterization of intracavity patterns, which are created in off-axis injection when the cavity has an inbuilt weak astigmatism (ΔR/R~2 %) : referred as Lissajous profile. This study allows accounting for the fundamental mechanisms, such as the particular coincidence of particular modes by modal dispersion, which result in a millimeter degenerate interval. It exists inside this one a succession of symmetrical Lissajous transverse profiles from each other on both sides around a particular cavity length : we specify the causes. The residual astigmatism, highlighted here, explains the disappearance of the Doppler oscillations, usually present during the ring down decays in these high-fineness cavity injection dynamics. Finally, we develop several types of non-resonant multipass cells as an alternative to the CRDS device for a hostile environments analysis.
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Contributor : Jérémie Courtois <>
Submitted on : Sunday, February 28, 2010 - 5:07:53 PM
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Jérémie Courtois. Développements de systèmes multipassages pour application à la spectroscopie d'absorption : Cavity Ring Down Spectroscopy multimode et cellules à passages multiples. Physique Atomique [physics.atom-ph]. Université Paris Sud - Paris XI, 2009. Français. ⟨tel-00458101v2⟩

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