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Laser-based absorption spectrometry: development of NICE-OHMS towards ultra-sensitive trace species detection
Umeå University, Faculty of Science and Technology, Department of Physics.
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 [en]
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: urn:nbn:se:umu:diva-1414ISBN: 978-91-7264-403-8 (print)OAI: oai:DiVA.org:umu-1414DiVA: diva2:140953
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
List of papers
1. Absorption spectrometry by narrowband light in optically saturated and optically pumped collision and doppler broadened gaseous media under arbitrary optical thickness conditions
Open this publication in new window or tab >>Absorption spectrometry by narrowband light in optically saturated and optically pumped collision and doppler broadened gaseous media under arbitrary optical thickness conditions
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2006 (English)In: Applied Spectroscopy, ISSN 0003-7028, Vol. 60, no 11, 1217-1240 p.Article in journal (Refereed) 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.

Place, publisher, year, edition, pages
New York: Society for applied spectroscopy, 2006
Identifiers
urn:nbn:se:umu:diva-2699 (URN)10.1366/000370206778999049 (DOI)
Available from: 2007-11-01 Created: 2007-11-01 Last updated: 2013-01-30
2. Wavelength modulation absorption spectrometry from optically saturated collision-broadened transitions
Open this publication in new window or tab >>Wavelength modulation absorption spectrometry from optically saturated collision-broadened transitions
2005 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 94, no 2, 225-254 p.Article in journal (Refereed) Published
Abstract [en]

A theoretical investigation of the influence of optical saturation on wavelength modulation absorption spectrometry (WMAS) signals from collision-broadened transitions is presented. Expressions are derived for the nth Fourier coefficient of the analytical detector signal, and thereby also for the nth harmonic signal from a WMAS instrumentation (i.e. the nf-WMAS signal), from a wavelength modulated collision-broadened transition exposed to optical saturation. The flux- (or irradiance-) and modulation-amplitude dependences of the nf-WMAS signal on resonance are scrutinized in detail. It is shown that the nth Fourier coefficient of the wavelength modulated analytical detector signal from an optically saturated collision-broadened transition can be written as a product of a flux-dependent (φ) bleaching function, given by (1+φ/φsat)-1 and identical to that appearing for ordinary, unmodulated absorption spectrometry (AS), and a flux-, detuning-, and modulation-amplitude-dependent wavelength modulated peak-normalized saturation-broadened Lorentzian lineshape function, specific for the WMAS technique. It is found that the nf-WMAS signal on resonance decreases faster than an ordinary AS signal as a function of laser flux when smaller-than-optimum modulation amplitudes are used, but slower when larger-than-optimum modulation amplitudes are used. When optimum (or close-to-optimum) modulation amplitudes are being used, on the other hand, the flux dependence of the WMAS signal resembles to a large degree that of ordinary AS. The conditions for when WMAS from collision-broadened transitions has the same flux dependence as ordinary AS are identified.

Place, publisher, year, edition, pages
Oxford: Pergamon Press, 2005
Keyword
Wavelength modulation, Diode laser, Absorption spectrometry, WMS, WMAS, Optical saturation, Bleaching
Identifiers
urn:nbn:se:umu:diva-11933 (URN)10.1016/j.jqsrt.2004.09.010 (DOI)
Available from: 2008-01-16 Created: 2008-01-16 Last updated: 2013-01-30
3. Wavelength modulation absorption spectrometry from optically pumped collision broadened atoms and molecules
Open this publication in new window or tab >>Wavelength modulation absorption spectrometry from optically pumped collision broadened atoms and molecules
2007 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 108, no 2, 220-238 p.Article in journal (Refereed) Published
Abstract [en]

A theoretical investigation of the influence of optical pumping on wavelength modulation absorption spectrometry (WMAS) signals from collision broadened atoms and molecules is presented. General expressions for the nf-WMAS signal from atomic and molecular systems, modeled as three-level systems that can accommodate both optical saturation and optical pumping, are derived by the use of a previously developed Fourier series-based formalism in combination with rate equations solved under steady-state conditions. The expressions are similar to those describing the nf-WMAS signal from two-level systems that can accommodate optical saturation [Schmidt FM, Foltynowicz A, Gustafsson J, Axner O, WMAS from optically saturated collision-broadened transitions. JQSRT 2005;94:225–54], the difference being the value of the saturation flux, wherefore the general parametric dependence of WMAS signals from optically pumped systems is the same as that from optically saturated systems. The additional effect of optical pumping on the WMAS signal is investigated for three typical cases: molecules or atoms in an ordinary atmosphere, atoms in an inert atmosphere, and atoms or molecules possessing metastable states. The possibility to describe any of these systems with a two-level model is investigated.

Place, publisher, year, edition, pages
Oxford: Pergamon Press, 2007
Keyword
Wavelength modulation, diode laser, absorption spectrometry, wms, wmas, optical pumping, optical saturation, bleaching
Identifiers
urn:nbn:se:umu:diva-2701 (URN)10.1016/j.jqsrt.2007.03.012 (DOI)
Available from: 2007-11-01 Created: 2007-11-01 Last updated: 2013-01-30
4. Double modulation diode laser absorption spectrometry by simultaneous wavelength modulation and optically induced population modulation: application to trace element detection in window-equipped graphite furnaces
Open this publication in new window or tab >>Double modulation diode laser absorption spectrometry by simultaneous wavelength modulation and optically induced population modulation: application to trace element detection in window-equipped graphite furnaces
2004 (English)In: Spectrochimica Acta Part B - Atomic Spectroscopy, ISSN 0584-8547, E-ISSN 0031-6987, Vol. 59, no 1, 67-92 p.Article in journal (Refereed) Published
Abstract [en]

A new diode laser-based double modulation absorption spectrometry (DMAS) technique for detection of species in trace amounts/concentrations is presented. The new technique makes use of a simultaneous modulation of the wavelength and population in order to reduce the background signals from multiple reflections in optical components (so-called etalon effects) that normally appear in ordinary wavelength modulation absorption spectrometry (WMAS). The simultaneous wavelength and population modulation are achieved by splitting the light from a wavelength-modulated diode laser into two beams—one strong pump beam and one weak probe beam—that subsequently are overlapped in an interaction region inside a sample compartment. The objective of the pump beam is to periodically transfer population from the state with which the probe beam interacts. The modulation of the population is achieved by modulating the pump beam with a chopper. The transmission of the probe beam is detected and demodulated at a frequency that is a combination of various harmonics of the wavelength modulation and chopping frequencies. The purely optical modulation makes the new technique more generally applicable than other DMAS techniques. The new DMAS technique is thoroughly described by a Fourier series-based theoretical description that previously has shown to be powerful for description of WMAS. The theoretical description is general in the sense that it considers DMAS for a variety of modes of operation and for any sample compartment providing homogeneously broadened transitions. The experiments were carried out on the 780-nm transition in Rb in a window-equipped graphite furnace (GF) used as an atomizer for aqueous solutions of Rb in ppt concentrations. The limit of detection obtained for the DMAS technique applied to a window-equipped GF was more than an order of magnitude better than that for the ordinary WMAS technique applied to the same type of window-equipped GF, and similar to that from an ordinary WMAS instrumentation coupled to a window-less GF, i.e. approximately 10 fg. Since the analytical DMAS signal was found to be approximately one order of magnitude smaller than the corresponding WMAS signal, it could be concluded that the noise from the background signal from the DMAS technique applied to a window-equipped GF was likewise about one order of magnitude smaller than the noise from ordinary WMAS applied to an open GF system. This implies in turn that the results so far published from the ordinary GF–WMAS technique are not shot noise limited and should therefore in principle also be improvable.

Place, publisher, year, edition, pages
Oxford: Pergamon P., 2004
Keyword
double modulation, population modulation, wavelength modulation, diode laser, absorption spectrometry, dmas, wmas, wms, graphite furnace, background signal, etalon effects, trace amounts, rb
Identifiers
urn:nbn:se:umu:diva-2702 (URN)10.1016/j.sab.2003.10.003 (DOI)
Available from: 2007-11-01 Created: 2007-11-01 Last updated: 2013-01-31
5. Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry for Doppler-broadened detection of C2H2 in the parts per trillion range
Open this publication in new window or tab >>Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry for Doppler-broadened detection of C2H2 in the parts per trillion range
2007 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, Vol. 24, no 6, 1392-1405 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:umu:diva-2703 (URN)10.1364/JOSAB.24.001392 (DOI)
Available from: 2007-11-01 Created: 2007-11-01 Last updated: 2013-01-30
6. Doppler-broadened fiber-laser-based NICE-OHMS: improved detectability
Open this publication in new window or tab >>Doppler-broadened fiber-laser-based NICE-OHMS: improved detectability
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2007 (English)In: Optics Express, ISSN 1094-4087, Vol. 15, no 17, 10822-10831 p.Article in journal (Refereed) Published
Abstract [en]

The performance of fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) has been improved by elimination of the technical constraints that limited its first demonstration. Doppler-broadened detection of C2H2 and CO2 at ~1531 nm is demonstrated using a cavity with a finesse of 4800. Frequency and wavelength modulated detection at absorption and dispersion phase are compared and the optimum mode of detection is discussed. A minimum detectable absorption of 8 × 10-11 cm-1, which corresponds to a detection limit of 4.5 ppt (2 ppt·m) for C2H2, was obtained for an acquisition time of 0.7 s by lineshape fitting. The linearity of the pressure dependence of the signal strengths is investigated for both C2H2 and CO2.

National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-2704 (URN)10.1364/OE.15.010822 (DOI)000249336000039 ()19547439 (PubMedID)
Available from: 2007-11-01 Created: 2007-11-01 Last updated: 2015-03-12Bibliographically approved
7. Highly sensitive dispersion spectroscopy by probing the free spectral range of an optical cavity using dual-frequency modulation
Open this publication in new window or tab >>Highly sensitive dispersion spectroscopy by probing the free spectral range of an optical cavity using dual-frequency modulation
2010 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 101, no 3, 497-509 p.Article in journal (Refereed) Published
Abstract [en]

Dual-frequency modulation (DFM) has been used to continuously track the frequency shifts of optical cavity modes in the vicinity of an optical transition of a gas inside the cavity for assessment of the gas concentration. A theoretical description of the size and lineshape of the DFM dispersion spectroscopy (DFM-DS) signal is given. Since the signal is measured in terms of a radio frequency the technique is insensitive to laser intensity fluctuations. The signal strength, which can accurately be obtained by curve fitting, only depends on fundamental parameters (including the line strength), thus enabling quantitative detection without calibration procedure. In a first demonstration, using a compact setup based on a narrowband fiber laser, the change in free spectral range around a value of 379.9 MHz due to an acetylene transition near 1531 nm was measured with a resolution of 6 Hz (i.e. with an accuracy of 1.5 parts in 10(8)) in 12.5 s acquisition time, which corresponds to a minimum detectable integrated absorption (SNR=3) of 3x10(-9) cm(-1)

National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-39222 (URN)10.1007/s00340-010-4120-9 (DOI)000284110800003 ()
Available from: 2011-01-18 Created: 2011-01-18 Last updated: 2017-12-11

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