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Sub-Doppler dispersion and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy revised
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
2008 (English)In: Journal of the Optical Society of America B, Vol. 25, no 7, 1166-1177 p.Article in journal (Refereed) Published
Abstract [en]

An expression for the peak-to-peak sub-Doppler optical phase shift of two counter-propagating modes of light, to which the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) dispersion signal is proportional, valid for arbitrary degree of saturation, is derived.

For low degrees of saturation it agrees with the expression for weakly saturating (ws) conditions, [(1+S)-1/2-(1+2S)-1/20/2, where S is the degree of saturation and α0 the unsaturated peak absorption.

However, the new expression, which can be written as 0.45S(1+S)-1α0/2, does not predict a distinct maximum as the ws-expression does; instead it predicts an optical phase shift that increases monotonically with S and levels off to 0.45α0/2 for large S. This alters the optimum conditions for the sub-Doppler NICE-OHMS technique and improves its shot-noise-limited detectability.

The new expression is based upon the same explicit assumptions as the ws-expression but not the Kramers-Kronig’s relations, which are not valid for nonlinear responses, and is supported by experimental results up to S = 100. The new expression is expected to be valid for all techniques measuring sub-Doppler dispersion signals

Place, publisher, year, edition, pages
2008. Vol. 25, no 7, 1166-1177 p.
Keyword [en]
line shapes and shifts, lasers, fiber, dispersion, spectroscopy, heterodyne, spectroscopy, modulation, spectroscopy, saturation
National Category
Atom and Molecular Physics and Optics Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:umu:diva-9812DOI: 10.1364/JOSAB.25.001166OAI: oai:DiVA.org:umu-9812DiVA: diva2:149483
Available from: 2008-05-21 Created: 2008-05-21 Last updated: 2009-05-05Bibliographically approved
In thesis
1. 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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