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Distributed-feedback-laser-based NICE-OHMS
in the pressure-broadened regime
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
2010 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 18, no 18, 18580-18591 p.Article in journal (Refereed) Published
Abstract [en]

A compact noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) system based on a narrow linewidth distributed-feedback laser and fiber-coupled acousto-optic and electro-optic modulators has been developed. Measurements of absorption and dispersion signals have been performed at pressures up to 1/3 atmosphere on weak acetylene transitions at 1551 nm. Multiline fitting routines were implemented to obtain transition parameters, i.e., center frequencies, linestrengths, and pressure broadening coefficients. The signal strength was shown to be linear with pressure and concentration, and independent of detection phase. The minimum detectable on-resonance absorption with a cavity with a finesse of 460 was 2 × 10−10 cm−1 for 1 minute of integration time.

Place, publisher, year, edition, pages
2010. Vol. 18, no 18, 18580-18591 p.
National Category
Physical Sciences
Research subject
Physics
Identifiers
URN: urn:nbn:se:umu:diva-36156DOI: 10.1364/OE.18.018580ISI: 000282107900009OAI: oai:DiVA.org:umu-36156DiVA: diva2:352416
Available from: 2010-09-21 Created: 2010-09-21 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Dicke narrowing and speed-dependent effects in dispersion signals: Influence on assessment of concentration and spectral parameters by noise-immune cavity-enhanced optical heterodyne molecular spectrometry
Open this publication in new window or tab >>Dicke narrowing and speed-dependent effects in dispersion signals: Influence on assessment of concentration and spectral parameters by noise-immune cavity-enhanced optical heterodyne molecular spectrometry
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Dicke-avsmalning och hastighetsberoende effekter hos dispersionssignaler : Påverkan på bestämning av koncentration och spektrala parametrar genom brusimmun kavitetsförstärkt optisk heterodyn molekylär spektrometri
Abstract [en]

Laser spectroscopic techniques have, during the last decades, demonstrated an extraordinary capability for sensitive detection of molecular constituents in gas phase. Since spectra from such techniques constitute unique and characteristic signatures for each type of species, these techniques enable investigations of molecular structures as well as detection of the presence of species in a gas mixture. They are therefore used for a variety of application, from fundamental studies to the assessment of gas concentrations. In fact, quantitative assessments of gas concentrations by laser-based techniques are constantly gaining in popularity, primarily due to properties such as high sensitivity and selectivity and an ability to perform non-invasive measurement. Moreover, investigations of isolated molecular transitions under different conditions provide excellent means to obtain a comprehensive understanding of spectral broadening mechanisms, which is of importance for, for example, environmental sciences and remote sensing applications. In fundamental studies, spectroscopic parameters are often retrieved from fits of a model function of the technique used, which in turn is based upon a suitable lineshape function. In order to obtain parameter values with highest possible accuracy, it is of importance to use the lineshape model that most correctly can predict the measured spectra. Even though the Voigt function is the most commonly used lineshape model when both Doppler and collision broadenings are present, it is not always suitable when spectroscopic parameters are to be assessed with high precision.

This thesis represents a thorough investigation of Dicke narrowing and speed-dependent effects, which are phenomena that are not accounted for by the conventional Voigt profile. For the first time, it is demonstrated that both these effects take place not only in absorption but also in the dispersion mode of detection. Their dispersion lineshape functions are first theoretically presumed and explicitly given before they are validated experimentally by the noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS). By using the models developed, it is also shown that although the two modes of detection, absorption and dispersion, both can provide good quality of fits, they do not always provide identical spectroscopic parameters. A detailed analysis under which conditions they do so, and subsequent recommendations of their use, are presented.

It also describes the instrumental implementation of a distributed-feed-back (DFB) laser-based NICE-OHMS instrumentation, which constitutes an important step towards the further development of this technique. Due to the wide tunability of the DFB laser, the setup is capable of extending the working range of NICE-OHMS into the collision broadening region, which, in turn, allows for precise spectroscopic studies. The use of a fiber-coupled DFB laser also provides a compact NICE-OHMS system. The minimum detectable on-resonance absorption was assessed to 2× 10-10 cm-1 for a 70 s integration time.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet, 2013. 89 p.
Keyword
Dicke narrowing, Speed-dependent effects, Dispersion signals, Noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS), accuracy
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-70235 (URN)978-91-7459-628-1 (ISBN)978-91-7459-627-4 (ISBN)
Public defence
2013-06-04, Naturvetarhuset, N420, Umeå Universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2013-05-14 Created: 2013-05-08 Last updated: 2014-06-16Bibliographically approved
2. Further development of NICE-OHMS: – an ultra-sensitive frequency-modulated cavity-enhanced laser-based spectroscopic
 technique for detection of molecules in gas phase
Open this publication in new window or tab >>Further development of NICE-OHMS: – an ultra-sensitive frequency-modulated cavity-enhanced laser-based spectroscopic
 technique for detection of molecules in gas phase
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, NICE-OHMS, is a laser-based spectroscopic detection technique that comprises the concepts of frequency modulation (FM, for reduction of 1/f-noise by detecting the signal at a high frequency) and cavity enhancement (CE, for a prolongation of the optical path length) in a unique way. Properly designed, this gives the technique an intrinsic immunity against the frequency-to-noise conversion that limits many other types of CE techniques. All this gives it an exceptionally high sensitivity for detection of molecular species. Although originally developed for frequency standard purposes in the late 1990s, soon thereafter development of the technique towards molecular spectroscopy and trace gas detection was initiated. This thesis focuses on the further development of Doppler- broadened NICE-OHMS towards an ultra-sensitive detection technique. A number of concepts have been addressed. A few of these are: i) The detection sensitivity of fiber-laser-based NICE- OHMS has been improved to the 10−12 cm−1 range, which for detection of C2H2 corresponds to a few ppt (parts-per-trillion, 1:1012) in gas phase, by improving the locking of the laser to a cavity mode by use of an acousto-optic modulator. ii) It is shown that the system can be realized with a more compact footprint by implementation of a fiber-optic circulator. iii) A systematic and thorough investigation of the experimental conditions that provide maximum signals, referred to as the optimum conditions, e.g. modulation and demodulation conditions and cavity length, has been performed. As a part of this, an expression for the NICE-OHMS line shape beyond the conventional triplet formalism has been proposed and verified. iv) To widen the applicability of NICE-OHMS for detection of pressure broadened signals, also a setup based upon a distributed-feedback (DFB) laser has been realized. v) In this regime, the Voigt profile cannot model signals with the accuracy that is needed for a proper assessment of analyte concentrations. Therefore, the thesis demonstrates the first implementations of line profiles encompassing Dicke narrowing and speed-dependent effects to NICE-OHMS. While such profiles are well-known for absorption, there were no expressions available for their dispersion counterparts. Such expressions have been derived and validated by accompanying experiments. vi) The applicability of the technique for elemental detection, then referred to as NICE-AAS, has been prophesied. 

Abstract [sv]

Brusimmun kavitetsförstärkt optisk-heterodyndetekterad molekylärspektroskopi (NICE-OHMS) är en laser-baserad spektroskopisk teknik som förenar frekvensmodulation (för reducring av 1/f-brus genom detektion vid en hög frekvens) och kavitetsförstärkning (KF, för en förlängning av den optiska väglangden) på ett unikt sätt. Korrekt realiserad uppvisar tekniken en inneboende immunitet mot omvandling av frekvensbrus till intensitetsbrus som många andra KF-tekniker är begränsade av. Allt detta ger tekniken en exceptionellt hög känslighet för molekyldetektion. Ursprungligen utvecklad för frekvensstandardändamål i slutet av 1990, har den sedan dess utvecklats för molekylspektroskopi och spårgasdetektering. Denna avhandling fokuserar på vidareutvecklingen av NICE-OHMS mot en tillämpbar, ultrakänslig detektionsteknik. Ett antal koncept har adresserats. Några av dessa är: i) Detektionskänsligheten hos fiberlaserbaserad NICE-OHMS har förbättrats till 10-12 cm-1 området, vilket för detektion av C2H2 i gasfas motsvarar några få ppt (parts per biljon, 1:1012), genom att förbättra låsningen av lasern till en kavitetsmod med hjälp av en akustooptisk modulator. ii) Det har demonstrerats att NICE-OHMS kan realiseras mer kompakt med hjälp av en fiber-kopplad optisk cirkulator. iii) En systematisk och grundlig utredning av de experimentella förhållanden som ger maximala signaler, betecknade de optimala förhållanden, t.ex. modulering och demodulering och kavitetslängden, har utförts. Som ett led i detta har ett uttryck för NICE-OHMS linjeform bortom den konventionella triplett formalismen föreslagits och verifierats. iv) För att bredda tillämpbarheten av NICE-OHMS för detektering av tryckbreddade signaler har även en instrumentering baserad på en distribuerad-återkopplad (eng. distributed feedback, DFB) laser realiserats. v) I detta område kan inte Voigt profilen modellera signalen med den noggrannhet som krävs för en korrekt bedömning av analytkoncentrationer. Därför visar avhandlingen de första implementeringarna i NICE-OHMS av linjeprofiler som inkluderar Dicke avsmalning (eng. Dicke narrowing) och hastighetsberoende effekter (eng. speed-dependent effects). Emedan sådana profiler är välkända för absorption, fanns det inga uttryck för deras dispersiva motparter. Sådana uttryck har därför härletts och validerats av medföljande experiment. vi) Tillämpbarheten av tekniken för detektion av atomer, NICE-AAS, har diskuterats och förutspåtts. 

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2014. 108 p.
Keyword
NICE-OHMS, Frequency Modulation, Cavity Enhancement, Molecular Spectroscopy
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-92510 (URN)978-91-7601-107-2 (ISBN)
Public defence
2014-09-25, Naturvetarhuset (N450), Johan Bures väg, Umeå, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 621-2008-3674Swedish Research Council, 621-2011-4216
Note

Ytterligare forskningsfinansiär: Kempestiftelserna

Available from: 2014-09-04 Created: 2014-08-27 Last updated: 2014-09-05Bibliographically approved

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