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Absorption spectrometry by narrowband light in optically saturated and optically pumped collision and doppler broadened gaseous media under arbitrary optical thickness conditions
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
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2006 (Engelska)Ingår i: Applied Spectroscopy, ISSN 0003-7028, E-ISSN 1943-3530, Vol. 60, nr 11, s. 1217-1240Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

This work examines absorption spectrometry by narrowband light in gaseous media with arbitrary optical thickness when the light induces optical saturation or optical pumping. Two quantities are defined: the observed absorbance, Aobs, and the true absorbance, Atrue. The former is the absorbance that is measured under the existing conditions, whereas the latter represents the absorbance one would measure if the light acted solely as a probe of the populations of the various levels, and it is therefore directly proportional to the concentration or density of absorbers. A general integral equation for the propagation of light in media of arbitrary optical thickness in which the light influences the populations of the levels involved is derived. This expression is transcendental in the observed absorbance and cannot be solved analytically. It is shown that an analytical expression can be derived by investigating the inverse relationship, i.e., Atruef(Aobs). Inasmuch as collision and Doppler broadened media react differently to optical saturation, they are considered separately. It is shown that a nonlinear response results if the medium is optically saturated (or pumped) and not optically thin. Expressions for the error introduced if the technique of standard additions is uncritically applied to such a system are derived.

Ort, förlag, år, upplaga, sidor
New York: Society for applied spectroscopy , 2006. Vol. 60, nr 11, s. 1217-1240
Identifikatorer
URN: urn:nbn:se:umu:diva-2699DOI: 10.1366/000370206778999049OAI: oai:DiVA.org:umu-2699DiVA, id: diva2:140946
Tillgänglig från: 2007-11-01 Skapad: 2007-11-01 Senast uppdaterad: 2018-06-09
Ingår i avhandling
1. Laser-based absorption spectrometry: development of NICE-OHMS towards ultra-sensitive trace species detection
Öppna denna publikation i ny flik eller fönster >>Laser-based absorption spectrometry: development of NICE-OHMS towards ultra-sensitive trace species detection
2007 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Umeå: Fysik, 2007. s. 91
Nyckelord
absorption spectrometry, trace species detection, fiber laser, modulation, cavity-enhanced spectroscopy, optical saturation, laser frequency stabilization, free-spectral-range
Nationell ämneskategori
Fysik
Identifikatorer
urn:nbn:se:umu:diva-1414 (URN)978-91-7264-403-8 (ISBN)
Disputation
2007-11-23, N430, Naturvetarhuset, Umeå Universitet Campus, Umeå, 13:15
Opponent
Handledare
Tillgänglig från: 2007-11-01 Skapad: 2007-11-01 Senast uppdaterad: 2018-06-09Bibliografiskt granskad

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Axner, OveSchmidt, Florian M.Foltynowicz, Aleksandra

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