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9LD-O-10
LASER DIAGNOSTICS AND SPECTROSCOPY
ALT'22
Determination of particle concentrations from absorption spectra with fast
frequency tuning
V. Lagunov1, V.Ochkin1, A.Volkova1,2
1- P NLebedev Physics Institute, Russian Academy of Sciences, Moscow, Russian Federation 2 - Department of Physics, M.V. Lomonosov Moscow State University, Russian Federation
lagunoww@yandex.ru
When the frequency of probing lasers is rapidly tuned, distortions can appear in the absorption spectra, the nature of which depends on the measurement scheme. For single-pass schemes, the effect was observed in [1] and is associated with the interference of the incident radiation and the coherent radiation of the medium induced by the incident radiation. Later, it was repeatedly studied, especially with the development of quantum-cascade laser technology (eg, [2-4]). Features of the spectrum dynamics in measurements with long optical paths inside multi-pass resonators were recently noted in [5]. The question of the possibility of determining particle concentrations from such contours remained open. It is especially relevant for measurements at low particle density, because with its increase the coherence of the interaction of light with particles disappears. In this paper, this issue is investigated for both schemes.
It is shown that in single-pass measurements errors arise due to not taking into account the saturation of the optical transition when interpreting the spectra. Figure 1 shows distortions of the contour of the absorption line of ethylene (a) and the results of measurements of the concentration of these particles (b). Accounting for saturation leads to results close to true concentrations not only at high but also at low pressures of the buffer gas.
Fig.1. (a) Ethylene absorption contours at a density of 1.55-1014 cm-3 with various additions of nitrogen, scanning speed 300 MHz/ns. The solid lines are the measurements from [4], the dashed lines are the correction taking into account saturation. (b) Ethylene concentration measurements with and without nitrogen additions. The dashed line is the true concentration. Squares are direct integration of experimental contours, triangles are Voigt contour approximation,
circles are integration of contours corrected for saturation.
For a laser system with an external multi-pass resonator, dynamic distortions already occur atfrequency tuning rates approximately two orders of magnitude lower than for measurements on relatively short paths in open space. This is because, at a high quality factor of the resonator, the length of the optical path can exceed the coherence length of the laser source. In this case, the deformations of the contour are of a different nature, not demonstrating areas of negative absorption (Fig. 1a), and the classical Beer-Bouguer-Lambert law is satisfied^for the integral absorption. This work was supported by a grant from the Russian Science Foundation (project No.19-12-00310).
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[2] J. H. Van Helden, S. J. Horrocks, G. A. D. Ritchie, Application of quantum cascade lasers in studies of low-pressure plasmas: Characterization of rapid passage effects on density and temperature measurements, Applied Physics Letters, 92(8), 081506, (2008).
[3] G. Duxbury, et al., Rapid passage induced population transfer and coherences in the 8 micron spectrum of nitrous oxide, Molecular Physics, 105.5-7, 741-754, (2007).
[4] M. T. McCulloch, G. Duxbury, N. Langford, Observation of saturation and rapid passage signals in the 10.25 micron spectrum of ethylene using a frequency chirped quantum cascade laser, Molecular Physics, 104.16-17, 2767-2779, (2006).
[5] Lagunov, V. V., I. V. Nikolaev, V. N. Ochkin, High-resolution and high-sensitivity absorption spectroscopy in external optical resonators with fast frequency tuning, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 246, 119060, (2021).
