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Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry for Doppler-broadened detection of C2H2 in the parts per trillion range
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
2007 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 24, no 6, 1392-1405 p.Article in journal (Refereed) Published
Place, publisher, year, edition, pages
2007. Vol. 24, no 6, 1392-1405 p.
Identifiers
URN: urn:nbn:se:umu:diva-2703DOI: 10.1364/JOSAB.24.001392OAI: oai:DiVA.org:umu-2703DiVA: diva2:140950
Available from: 2007-11-01 Created: 2007-11-01 Last updated: 2017-12-14
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
2. Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry
Open this publication in new window or tab >>Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Noise-immune cavity-enhanced optical heterodyne molecular spectro-metry (NICE-OHMS) is one of the most sensitive laser-based absorption techniques. The high sensitivity of NICE-OHMS is obtained by a unique combination of cavity enhancement (for increased interaction length with a sample) with frequency modulation spectrometry (for reduction of noise). Moreover, sub-Doppler detection is possible due to the presence of high intensity counter-propagating waves inside an external resonator, which provides an excellent spectral selectivity. The high sensitivity and selectivity make NICE-OHMS particularly suitable for trace gas detection. Despite this, the technique has so far not been often used for practical applications due to its technical complexity, originating primarily from the requirement of an active stabilization of the laser frequency to a cavity mode.

The main aim of the work presented in this thesis has been to develop a simpler and more robust NICE-OHMS instrumentation without compro-mising the high sensitivity and selectivity of the technique. A compact NICE-OHMS setup based on a fiber laser and a fiber-coupled electro-optic modulator has been constructed. The main advantage of the fiber laser is its narrow free-running linewidth, which significantly simplifies the frequency stabilization procedure. It has been demonstrated, using acetylene and carbon dioxide as pilot species, that the system is capable of detecting relative absorption down to 3 × 10-9 on a Doppler-broadened transition, and sub-Doppler optical phase shift down to 1.6 × 10-10, the latter corresponding to a detection limit of 1 × 10-12 atm of C2H2. Moreover, the potential of dual frequency modulation dispersion spectrometry (DFM-DS), an integral part of NICE-OHMS, for concentration measurements has been assessed.

This thesis contributes also to the theoretical description of Doppler-broadened and sub-Doppler NICE-OHMS signals, as well as DFM-DS signals. It has been shown that the concentration of an analyte can be deduced from a Doppler-broadened NICE-OHMS signal detected at an arbitrary and unknown detection phase, provided that a fit of the theoretical lineshape to the experimental data is performed. The influence of optical saturation on Doppler-broadened NICE-OHMS signals has been described theoretically and demonstrated experimentally. In particular, it has been shown that the Doppler-broadened dispersion signal is unaffected by optical saturation in the Doppler limit. An expression for the sub-Doppler optical phase shift, valid for high degrees of saturation, has been derived and verified experimentally up to degrees of saturation of 100.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2009. 145 p.
Keyword
absorption spectrometry, frequency modulation, cavity enhancement, NICE-OHMS, Laser frequency stabilization, fiber laser, Fabry-Perot cavities, sub-Doppler spectroscopy, trace gas detection
National Category
Physical Sciences Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-22269 (URN)978-91-7264-740-4 (ISBN)
Distributor:
Institutionen för fysik, 90187, Umeå
Public defence
2009-05-29, N430, Naturverarhuset Umeå universitet, Umeå, 09:15 (English)
Opponent
Supervisors
Available from: 2009-05-08 Created: 2009-05-04 Last updated: 2013-01-31Bibliographically approved

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