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Wavelength modulation absorption spectrometry from optically pumped collision broadened atoms and molecules
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
School of Engineering, Jönköping University, Jönköping, Sweden.
Umeå University, Faculty of Science and Technology, Department of Physics.
2007 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 108, no 2, 220-238 p.Article in journal (Refereed) Published
Abstract [en]

A theoretical investigation of the influence of optical pumping on wavelength modulation absorption spectrometry (WMAS) signals from collision broadened atoms and molecules is presented. General expressions for the nf-WMAS signal from atomic and molecular systems, modeled as three-level systems that can accommodate both optical saturation and optical pumping, are derived by the use of a previously developed Fourier series-based formalism in combination with rate equations solved under steady-state conditions. The expressions are similar to those describing the nf-WMAS signal from two-level systems that can accommodate optical saturation [Schmidt FM, Foltynowicz A, Gustafsson J, Axner O, WMAS from optically saturated collision-broadened transitions. JQSRT 2005;94:225–54], the difference being the value of the saturation flux, wherefore the general parametric dependence of WMAS signals from optically pumped systems is the same as that from optically saturated systems. The additional effect of optical pumping on the WMAS signal is investigated for three typical cases: molecules or atoms in an ordinary atmosphere, atoms in an inert atmosphere, and atoms or molecules possessing metastable states. The possibility to describe any of these systems with a two-level model is investigated.

Place, publisher, year, edition, pages
Oxford: Pergamon Press , 2007. Vol. 108, no 2, 220-238 p.
Keyword [en]
Wavelength modulation, diode laser, absorption spectrometry, wms, wmas, optical pumping, optical saturation, bleaching
Identifiers
URN: urn:nbn:se:umu:diva-2701DOI: 10.1016/j.jqsrt.2007.03.012OAI: oai:DiVA.org:umu-2701DiVA: diva2:140948
Available from: 2007-11-01 Created: 2007-11-01 Last updated: 2013-01-30
In thesis
1. Laser-based absorption spectrometry: development of NICE-OHMS towards ultra-sensitive trace species detection
Open this publication in new window or tab >>Laser-based absorption spectrometry: development of NICE-OHMS towards ultra-sensitive trace species detection
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Laser-based absorption spectroscopy (AS) is a powerful technique for qualitative and quantitative studies of atoms and molecules. An important field of use of AS is the detection of species in trace concentrations, which has applications not only in physics and chemistry but also in biology and medicine, encompassing environmental monitoring, regulation of industrial processes and breath analysis. Although a large number of molecular species can successfully be detected with established AS techniques, there are some applications that require higher sensitivity, selectivity and accuracy, yet robust and compact instrumentation.

Various approaches have been made during the years to improve on the performance of AS, usually based on modulation spectrometry or external cavities. The most sensitive absorption technique of today is, however, noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS). This technique elegantly combines several approaches: external cavities (for optical path length enhancement), modulation techniques (for noise reduction) and saturation spectroscopy (for enhanced selectivity). However, due to its complexity, the technique has so far not been applied to practical trace species detection.

This thesis provides the background for an understanding of NICE-OHMS and describes the construction of a first compact NICE-OHMS spectrometer based on a narrowband fiber laser. Moreover, it gives theoretical expressions for NICE-OHMS signal lineshapes, measured in various modes of detection, which can be fitted to the experimental data and thereby facilitate the assessment of species concentration. The sensitivity of the instrumentation is demonstrated by detection of acetylene (C2H2) and carbon dioxide (CO2) in the 1.5 μm region. A fractional absorption sensitivity of 3*10-9 (integrated absorption of 5*10-11 cm-1), could be achieved using a cavity with a finesse of 4800 and an acquisition time of 0.7 s. This results in a detection limit for C2H2 of 4.5 ppt (4.5*10-12 atm).

In addition, the thesis revives the idea of using an accurate (frequency) measurement of the free-spectral-range (FSR) of an external cavity for sensitive and calibration-free concentration assessment. A theoretical description of the expected signal lineshapes is given, and in a first experimental demonstration the FSR could be measured with a resolution of 5 Hz, resulting in a fractional absorption sensitivity of 1*10-7, and subsequently in a detection limit for C2H2 of 180 ppt (12.5 s acquisition time).

The thesis, finally, also contributes to the continuously ongoing development of conventional AS and wavelength modulated AS by addressing concepts related to when the light optically saturates the transition.

Place, publisher, year, edition, pages
Umeå: Fysik, 2007. 91 p.
Keyword
absorption spectrometry, trace species detection, fiber laser, modulation, cavity-enhanced spectroscopy, optical saturation, laser frequency stabilization, free-spectral-range
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-1414 (URN)978-91-7264-403-8 (ISBN)
Public defence
2007-11-23, N430, Naturvetarhuset, Umeå Universitet Campus, Umeå, 13:15
Opponent
Supervisors
Available from: 2007-11-01 Created: 2007-11-01 Last updated: 2013-01-31Bibliographically approved

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Foltynowicz, AleksandraSchmidt, Florian M.Axner, Ove

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