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Further development of NICE-OHMS: – an ultra-sensitive frequency-modulated cavity-enhanced laser-based spectroscopic
 technique for detection of molecules in gas phase
Umeå University, Faculty of Science and Technology, Department of Physics. (Laser physics group)
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 [en]
NICE-OHMS, Frequency Modulation, Cavity Enhancement, Molecular Spectroscopy
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
URN: urn:nbn:se:umu:diva-92510ISBN: 978-91-7601-107-2 (print)OAI: oai:DiVA.org:umu-92510DiVA: diva2:741220
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
List of papers
1. Distributed-feedback-laser-based NICE-OHMS
in the pressure-broadened regime
Open this publication in new window or tab >>Distributed-feedback-laser-based NICE-OHMS
in the pressure-broadened regime
2010 (English)In: Optics Express, 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.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-36156 (URN)10.1364/OE.18.018580 (DOI)000282107900009 ()
Available from: 2010-09-21 Created: 2010-09-21 Last updated: 2014-08-27Bibliographically approved
2. Dicke narrowing in the dispersion mode of detection and in noise-immune cavity-enhanced optical heterodyne molecular spectroscopy-theory and experimental verification
Open this publication in new window or tab >>Dicke narrowing in the dispersion mode of detection and in noise-immune cavity-enhanced optical heterodyne molecular spectroscopy-theory and experimental verification
2011 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 28, no 10, 2390-2401 p.Article in journal (Refereed) Published
Abstract [en]

Dicke narrowing in both the absorption and dispersion modes of detection have been scrutinized by noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) using an isolated transition in the v(1) + v(3) + v(4)(1) - v(4)(1) band of acetylene [P(e)(33) at 6439.371 cm(-1)] at room temperature. The results represent the first (to our knowledge) demonstration of Dicke narrowing detected in dispersion, as well as by NICE-OHMS, and the paper provides thereby the first comparison of the Dicke narrowing phenomenon for the two modes of detection. It is shown that Dicke narrowing in dispersion can be described by the dispersive counterparts to the conventional Galatry and Rautian absorption line-shape functions, which are explicitly given. Spectroscopic parameters for the targeted transition were extracted in both absorption and dispersion and found to be in agreement with those previously reported for other lines and bands. The shortcomings of the Galatry model to provide physically relevant parameters in this pressure range are discussed.

Place, publisher, year, edition, pages
Washington, D. C.: The Optical Society of America, 2011
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-49570 (URN)10.1364/JOSAB.28.002390 (DOI)000296045400014 ()
Available from: 2011-11-22 Created: 2011-11-14 Last updated: 2017-12-08Bibliographically approved
3. Frequency modulation background signals from fiber-based electro optic modulators are caused by crosstalk
Open this publication in new window or tab >>Frequency modulation background signals from fiber-based electro optic modulators are caused by crosstalk
2012 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 29, no 5, 916-923 p.Article in journal (Refereed) Published
Abstract [en]

Frequency modulated spectroscopy (FMS) performed by the use of fiber-coupled electro optic modulators (FC-EOMs) is often plagued by background signals that bring in noise and, by their temperature dependence, cause severe drifts. These signals cannot be zeroed out by the conventional technique of using a carefully adjusted polarizer that can be applied to free space electro optic modulators (EOMs). This can limit the use of FC-EOMs in high performance detection techniques. Here we provide an explanation to these background signals that is based upon crosstalk between various polarization directions of light in the fixed mounted polarization-maintaining fibers and the electro optic crystal. The description provides a basis for the previously demonstrated technique that utilizes an EOM regulated simultaneously by temperature and DC voltage to eliminate background signals from systems encompassing FC-EOMs. (C) 2012 Optical Society of America

Place, publisher, year, edition, pages
Optical Society of America, 2012
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-55672 (URN)10.1364/JOSAB.29.000916 (DOI)000303544000007 ()
Available from: 2012-05-30 Created: 2012-05-28 Last updated: 2017-12-07Bibliographically approved
4. Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry instrumentation for Doppler-broadened detection in the 10-12 cm-1 Hz-1/2 region
Open this publication in new window or tab >>Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry instrumentation for Doppler-broadened detection in the 10-12 cm-1 Hz-1/2 region
2012 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 29, no 6, 1305-1315 p.Article in journal (Refereed) Published
Abstract [en]

A fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry (FL-NICE-OHMS) system for white-noise-limited Doppler-broadened detection down to 5.6 x 10(-12) cm(-1) Hz(-1/2) is demonstrated. The system is based on a previous FL-NICE-OHMS instrumentation in which the locking of the laser frequency to a cavity mode has been improved by the use of an acousto-optic modulator (AOM) and provision of a more stable environment by the employment of a noise-isolating enclosed double-layer table, a temperature regulation of the laboratory, and an ultra-high-vacuum (UHV) gas system. White-noise behavior up to 10 s provides the instrument with a minimum detectable on-resonance absorbance per unit length of 1.8 x 10(-12) cm(-1) and a relative single-pass absorption (Delta I/I) of 7.2 x 10(-11). The system was applied to detection of acetylene on a transition at 1531.588 nm, yielding a detection sensitivity of C2H2 in atmospheric pressure gas of 4 ppt (measured over 10 s). (C) 2012 Optical Society of America

Place, publisher, year, edition, pages
Optical Society of America, 2012
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-56987 (URN)10.1364/JOSAB.29.001305 (DOI)000305029900023 ()
Available from: 2012-07-03 Created: 2012-07-02 Last updated: 2017-12-07Bibliographically approved
5. Speed-dependent Voigt dispersion line-shape function: applicable to techniques measuring dispersion signals
Open this publication in new window or tab >>Speed-dependent Voigt dispersion line-shape function: applicable to techniques measuring dispersion signals
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2012 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 29, no 10, 2971-2979 p.Article in journal (Refereed) Published
Abstract [en]

An analytical expression for a Voigt dispersion line-shape function that incorporates speed-dependent effects (SDEs) on the collision broadening, applicable to spectroscopic techniques that measure dispersion signals, is presented. It is based upon a speed-dependent Voigt (SDV) model for absorption spectrometry that assumes that the molecular relaxation rate has a quadratic dependence on molecular speed. The expression is validated theoretically in the limit of small SDEs by demonstration that it reverts to the ordinary Voigt dispersion line-shape function and experimentally in a separate work by experiments performed by the noise-immune cavity-enhanced optical heterodyne molecular spectrometry technique. A comparison is given between the SDEs in the SDV absorption and dispersion line-shape functions. It is shown that both line shapes are affected significantly but differently by SDEs. The expression derived provides, for the first time to our knowledge, a possibility also for the techniques that measure dispersion signals to handle SDEs. (c) 2012 Optical Society of America

Place, publisher, year, edition, pages
Washington: Optical Society of America, 2012
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-61566 (URN)10.1364/JOSAB.29.002971 (DOI)000309588200045 ()
Available from: 2012-11-28 Created: 2012-11-20 Last updated: 2017-12-07Bibliographically approved
6. Speed-dependent effects in dispersion mode of detection and in noise-immune cavity-enhanced optical heterodyne molecular spectrometry: experimental demonstration and validation of predicted line shape
Open this publication in new window or tab >>Speed-dependent effects in dispersion mode of detection and in noise-immune cavity-enhanced optical heterodyne molecular spectrometry: experimental demonstration and validation of predicted line shape
2012 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 29, no 10, 2980-2989 p.Article in journal (Refereed) Published
Abstract [en]

Speed-dependent effects (SDEs) in both the absorption and dispersion modes of detection have been detected and scrutinized by the noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) technique. The present paper achieves four objectives: (i) it provides the first demonstration of SDEs detected in dispersion, (ii) it validates the expression for a speed-dependent Voigt (SDV) dispersion line-shape function that is derived in an accompanying paper, (iii) it illustrates the influence of SDEs on the NICE-OHMS technique, and (iv) it gives the first experimental comparison of SDEs for the absorption and dispersion modes of detection. Experiments were performed using an isolated transition in the v(1) + v(3) + v(4)(1) - v(4)(1) band of acetylene [P-e (33) at 6439.371 cm(-1)] in the 100-250 Torr range at room temperature. It is shown that SDEs appear in both the absorption and dispersion modes of detection, that they can be well described by the suggested SDV dispersion line-shape function, and that they need to be taken into account if NICE-OHMS signals detected under optimal pressures are to be properly assessed. (c) 2012 Optical Society of America

Place, publisher, year, edition, pages
Washington: Optical Society of America, 2012
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-61567 (URN)10.1364/JOSAB.29.002980 (DOI)000309588200046 ()
Available from: 2012-11-28 Created: 2012-11-20 Last updated: 2017-12-07Bibliographically approved
7. NICE-OHMS – frequency modulation cavity-enhanced spectroscopy: principles and performance
Open this publication in new window or tab >>NICE-OHMS – frequency modulation cavity-enhanced spectroscopy: principles and performance
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2014 (English)In: Cavity-Enhanced Spectroscopy and Sensing / [ed] Gianluca Gagliardi and Hans-Peter Loock, Berlin: Springer Berlin/Heidelberg, 2014, 221-251 p.Chapter in book (Refereed)
Abstract [en]

Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is a sensitive technique for detection of molecular species in gas phase. It is based on a combination of frequency modulation for reduction of noise and cavity enhancement for prolongation of the interaction length between the light and a sample. It is capable of both Doppler-broadened and sub-Doppler detection with absorption sensitivity down to the 10−12 and 10−14 Hz−1/2 cm−1 range, respectively. This chapter provides a thorough description of the basic principles and the performance of the technique.

Place, publisher, year, edition, pages
Berlin: Springer Berlin/Heidelberg, 2014
Series
Springer Series in Optical Sciences, ISSN 0342-4111 ; 179
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-84914 (URN)10.1007/978-3-642-40003-2_6 (DOI)978-3-642-40002-5 (ISBN)978-3-642-40003-2 (ISBN)
Available from: 2014-01-22 Created: 2014-01-22 Last updated: 2015-10-22Bibliographically approved
8. Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry incorporating an optical circulator
Open this publication in new window or tab >>Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry incorporating an optical circulator
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2014 (English)In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 39, no 2, 279-282 p.Article in journal (Refereed) Published
Abstract [en]

To reduce the complexity of fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry, a system incorporating a fiber-coupled optical circulator to deflect the cavity-reflected light for laser stabilization has been realized. Detection near the shot-noise limit has been demonstrated for both Doppler-broadened and sub-Doppler signals, yielding a lowest detectable absorption and optical phase shift of 2.2 x 10(-12) cm(-1) and 4.0 x 10(-12) cm(-1), respectively, both for a 10 s integration time, where the former corresponds to a detection limit of C2H2 of 5 ppt. (C) 2014 Optical Society of America

Place, publisher, year, edition, pages
Optical Society of America, 2014
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-85701 (URN)10.1364/OL.39.000279 (DOI)000329906900026 ()
Funder
Swedish Research Council, 621-2011-4216Carl Tryggers foundation
Available from: 2014-02-20 Created: 2014-02-10 Last updated: 2017-12-06Bibliographically approved
9. On the accuracy of the assessment of molecular concentration and spectroscopic parameters by frequency modulation spectrometry and NICE-OHMS
Open this publication in new window or tab >>On the accuracy of the assessment of molecular concentration and spectroscopic parameters by frequency modulation spectrometry and NICE-OHMS
2014 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 136, 28-44 p.Article in journal (Refereed) Published
Abstract [en]

Frequency modulation spectrometry (FMS), and thereby also noise immune cavity enhanced optical heterodyne molecular spectrometry (NICE-OHMS), can detect both absorption and dispersion signals, and can therefore, by curve fitting, extract molecular parameters from both these types of signals. However, parameters evaluated from the two modes of detection have been previously shown not to be identical. Their accuracy is affected by both the type of lineshape used by the fit and the accuracy of the detection phase. A thorough study is presented of the influence of three lineshape functions [Voigt, Rautian, and speed-dependent Voigt (SDV)] and errors in the detection phase on the retrieval of various molecular parameters, in particular the signal strength, which provides information about the concentration of molecules in a gas, from reference spectra in the 10-260 Torr region. It was found that for data detected and evaluated at pure absorption or dispersion phase by a system calibrated in the Doppler limit the signal strength can be underestimated at higher pressures by up to 45% if the evaluation is made using the Voigt profile. If the detection is plagued by phase errors additional inaccuracies, often in the order of percent per degree phase error (%/deg), can occur. More reliable parameters can be obtained if an appropriate lineshape function is used and the detection phase is considered a free parameter. However, despite this, none of the evaluation procedures can retrieve the molecular parameters fully correctly; the most accurate assessments of the signal strength, obtained when the data is detected close to absorption phase and evaluated by the SDV lineshape function, are still associated with an error of a few percent. (C) 2013 Elsevier Ltd. All rights reserved.

Keyword
Frequency modulation spectrometry, Lineshapes, Absorption, Dispersion, Detection phase, Parameter assessments
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-88332 (URN)10.1016/j.jqsrt.2013.12.017 (DOI)000332814300003 ()
Note

Included in thesis in manuscript form with the title "On the accuracy of the assessment of concentration and spectroscopic parameters by frequency modulation spectrometry and NICE-OHMS"

Available from: 2014-06-16 Created: 2014-04-30 Last updated: 2017-12-05Bibliographically approved
10. Doppler broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry: optimum modulation and demodulation conditions, cavity length, and modulation order
Open this publication in new window or tab >>Doppler broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry: optimum modulation and demodulation conditions, cavity length, and modulation order
2014 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 31, no 9, 2051-2060 p.Article in journal (Refereed) Published
Abstract [en]

Doppler broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry (Db-NICE-OHMS) has been scrutinized with respect to modulation and demodulation conditions (encompassing the modulation frequency,nu(m), the modulation index, beta, and the detection phase, theta), the cavity length, L, and the modulation order, k (defined as nu(m)/nu(FSR), where nu(FSR) is the free-spectral range of the cavity), primarily in the Doppler limit but also for two specific situations in the Voigt regime (for equal Doppler and homogeneous width and for purely Lorentzian broadened transitions), both in the absence and presence of optical saturation (the latter for the case in which the homogeneous broadening is smaller than the modulation frequency). It is found that, for a system with a given cavity length, the optimum conditions (i.e., those that produce the largest NICE-OHMS signal) for an unsaturated transition in the Doppler limit comprise nu(m)/Gamma(D) = 1.6 (where Gamma(D) is the half-width at half-maximum of the Doppler width of the transition), beta = 1.3, and theta = 0.78 pi. It is also found that the maximum is rather broad; the signal takes 95% of its maximum value for modulation frequencies in the entire 0.4 less than or similar to nu(m)/Gamma(D) less than or similar to 2.4 range. When optical saturation sets in, theta shifts toward the dispersion phase. The optimum conditions encompass k > 1 whenever L > 0.35L(D) and 2.6L(D) for the dispersion and absorption modes of detection, respectively [where L-D is a characteristic length given by c/(2 Gamma(D))]. Similar conditions are found under pressure broadened conditions. 

Place, publisher, year, edition, pages
The Optical Society of America, 2014
Keyword
(120.6200) Spectrometers and spectroscopic instrumentation; (140.3518) Lasers, frequency
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-92499 (URN)10.1364/JOSAB.31.002051 (DOI)000343242700006 ()19547439 (PubMedID)
Available from: 2014-08-27 Created: 2014-08-27 Last updated: 2017-12-05Bibliographically approved
11. Doppler broadened NICE-OHMS beyond the triplet formalism: assessment of optimum modulation index
Open this publication in new window or tab >>Doppler broadened NICE-OHMS beyond the triplet formalism: assessment of optimum modulation index
2014 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 31, no 7, 1499-1507 p.Article in journal (Refereed) Published
Abstract [en]

The dependence of Doppler broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) on the modulation index, beta, has been investigated experimentally on C2H2 and CO2, both in the absence and the presence of optical saturation. It is shown that the maximum signals are obtained for beta that produce more than one pair of sidebands: in the Doppler limit and for the prevailing conditions (unsaturated transition and the pertinent modulation frequency and Doppler widths) around 1 and 1.4 for the dispersion and absorption detection phases, respectively. The results verify predictions given in an accompanying work. It is also shown that there is no substantial broadening of the NICE-OHMS signal for beta < 1. The use of beta of unity has yielded a Db-NICE-OHMS detection sensitivity of 4.9 x 10(-12) cm(-1) Hz(-1/2), which is the lowest (best) value so far achieved for NICE-OHMS based on a tunable laser. The number of sidebands that needs to be included in fits of the line-shape function to obtain good accuracy has been assessed. It is concluded that it is enough to consider three pairs of sidebands whenever the systematic errors in a concentration assessment should be below 1% when beta < 2 are used and <1 parts per thousand for beta < 1.5.

Place, publisher, year, edition, pages
Optical Society of America, 2014
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-91852 (URN)10.1364/JOSAB.31.001499 (DOI)000338931700013 ()
Note

Ingår i Patrick Ehlers doktorsavhandling som delarbete nr XI med titeln: NICE-OHMS beyond the triplet formalism: assessment of the optimum modulation index.

Available from: 2014-08-26 Created: 2014-08-18 Last updated: 2017-12-05Bibliographically approved
12. Use of etalon-immune distances to reduce the influence of background signals in frequency-modulation spectroscopy and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy
Open this publication in new window or tab >>Use of etalon-immune distances to reduce the influence of background signals in frequency-modulation spectroscopy and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy
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2014 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 31, no 12, 2938-2945 p.Article in journal (Refereed) Published
Abstract [en]

The detection sensitivity of phase-modulated techniques such as frequency-modulation spectroscopy (FMS) and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is often limited by etalon background signals. It has previously been shown that the impact of etalons can be reduced by the use of etalon-immune distances (EIDs), i.e., by separating the surfaces that give rise to etalons by a distance of q. L-m, where L-m is given by c/2n nu(m), where, in turn, n and nu(m) are the index of refraction between the components that make up the etalon (thus most often that of air) and the modulation frequency, respectively, and where q is an integer (i.e., 1, 2, 3,.) or half-integer (i.e., 1/2, 1, 3/2,.) for the dispersion and absorption modes of detection, respectively. An etalon created by surfaces separated by an EID will evade detection and thereby not contribute to any background signal. The concept of EIDs in FMS and NICE-OHMS is in this work demonstrated, scrutinized, and discussed in some detail. It is shown that the influence of EIDs on the absorption and dispersion modes of detection is significantly different; signals detected at the dispersion phase are considerably less sensitive to deviations from exact EID conditions than those detected at the absorption phase. For example, the FM background signal from an etalon whose length deviates from an EID by 2.5% of L-m (e.g., by 1 cm for an L-m of 40 cm), detected in dispersion, is only 9% of that in absorption. This makes the former mode of detection the preferred one whenever a sturdy immunity against etalons is needed or when optical components with parallel surfaces (e.g., lenses, polarizers, or beam splitters) are used. The impact of the concept of EIDs on NICE-OHMS is demonstrated by the use of Allan-Werle plots.

National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-92503 (URN)10.1364/JOSAB.31.002938 (DOI)000345901500002 ()
Funder
Swedish Research Council
Note

Included in theses in manuscript form with the title: "On the use of etalon-immune-distances to reduce the influence of background signals in frequency modulation spectroscopy and NICE-OHMS"

Available from: 2014-08-27 Created: 2014-08-27 Last updated: 2017-12-05Bibliographically approved
13. Noise-immune cavity-enhanced analytical atomic spectrometry — NICE-AAS: a technique for detection of elements down to zeptogram amounts
Open this publication in new window or tab >>Noise-immune cavity-enhanced analytical atomic spectrometry — NICE-AAS: a technique for detection of elements down to zeptogram amounts
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2014 (English)In: Spectrochimica Acta Part B - Atomic Spectroscopy, ISSN 0584-8547, E-ISSN 1873-3565, Vol. 100, 211-235 p.Article in journal (Refereed) Published
Abstract [en]

Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is a powerful technique for detection of molecular compounds in gas phase that is based on a combination of two important concepts: frequency modulation spectroscopy (FMS) for reduction of noise, and cavity enhancement, for prolongation of the interaction length between the light and the sample. Due to its unique properties, it has demonstrated unparalleled detection sensitivity when it comes to detection of molecular constituents in the gas phase. However, despite these, it has so far not been used for detection of atoms, i.e. for elemental analysis. The present work presents an assessment of the expected performance of Doppler-broadened (Db) NICE-OHMS for analytical atomic spectrometry, then referred to as noise-immune cavity-enhanced analytical atomic spectrometry (NICE-AAS). After a description of the basic principles of Db-NICE-OHMS, the modulation and detection conditions for optimum performance are identified. Based on a previous demonstrated detection sensitivity of Db-NICE-OHMS of 5×10−12 cm−1 Hz−1∕2 (corresponding to a single-pass absorbance of 7×10−11 over 10 s), the expected limits of detection (LODs) of Hg and Na by NICE-AAS are estimated. Hg is assumed to be detected in gas phase directly while Na is considered to be atomized in a graphite furnace (GF) prior to detection. It is shown that in the absence of spectral interferences, contaminated sample compartments, and optical saturation, it should be feasible to detect Hg down to 10 zg/cm3 (10 fg/m3 or 10-5 ng/m3), which corresponds to 25 atoms/cm3, and Na down to 0.5 zg (zg = zeptogram = 10-21 g), representing 50 zg/mL (parts-per-sextillion, pps, 1:1021) in liquid solution (assuming a sample of 10 µL) or solely 15 atoms injected into the GF, respectively. These LODs are several orders of magnitude lower (better) than any previous laser-based absorption technique previously has demonstrated under atmospheric pressure conditions. It is prophesied that NICE-AAS could provide such high detection sensitivity that the instrumentation should not, by itself, be the limiting factor of an assessment of elemental abundance; the accuracy of an assessment would then instead be limited by concomitant species, e.g. originating from the handling procedures of the sample or the environment.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS), Frequency modulation spectroscopy (FMS), Cavity enhanced (CE) spectroscopy, Ultra-trace element detection, Individual atom detection
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-92508 (URN)10.1016/j.sab.2014.08.016 (DOI)000343853400028 ()
Funder
Swedish Research Council, 621-2008-3674Swedish Research Council, 621-2011-4216
Available from: 2014-08-27 Created: 2014-08-27 Last updated: 2017-12-05Bibliographically approved

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Ehlers, Patrick

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