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Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry
Umeå University, Faculty of Science and Technology, Department of Physics. (Laser Physics Group)
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 [en]
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: urn:nbn:se:umu:diva-22269ISBN: 978-91-7264-740-4 (print)OAI: oai:DiVA.org:umu-22269DiVA: diva2:214195
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
List of papers
1. 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, E-ISSN 1520-8540, 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: 2017-12-14
2. 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
Show others...
2007 (English)In: Optics Express, ISSN 1094-4087, E-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: 2017-12-14Bibliographically approved
3. Theoretical description of Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectroscopy under optically saturated conditions
Open this publication in new window or tab >>Theoretical description of Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectroscopy under optically saturated conditions
2008 (English)In: Journal of the Optical Society of America B, Vol. 25, no 7, 1144-1155 p.Article in journal (Refereed) Published
Abstract [en]

A theoretical description of Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) under optically saturated conditions is presented. Expressions for the strength and shape of the Doppler-broadened NICE-OHMS signals are given for both absorption and dispersion phase, in the Voigt regime as well as in the Doppler limit. It is shown that Doppler-broadened NICE-OHMS is affected less by optical saturation than other cavity enhanced techniques; in the Doppler limit the absorption signal decreases by a factor of (1+G+-1)-1/2, where G+-1 is the degree of saturation for one of the frequency modulation sidebands, whereas the dispersion signal is virtually unaffected by optical saturation. In the Voigt regime both signals show additional dependence on optical saturation. The concept of saturation-insensitive detection is introduced and its conditions are identified.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-9809 (URN)10.1364/JOSAB.25.001144 (DOI)
Available from: 2008-05-21 Created: 2008-05-21 Last updated: 2009-05-05Bibliographically approved
4. Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry signals from optically saturated transitions under low pressure conditions
Open this publication in new window or tab >>Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry signals from optically saturated transitions under low pressure conditions
2008 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 25, no 7, 1156-1165 p.Article in journal (Refereed) Published
Abstract [en]

The influence of optical saturation on noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) signals from purely Doppler-broadened transitions is investigated experimentally. It is shown that the shape and the strength of the dispersion signal are virtually unaffected by optical saturation, whereas the strength of the absorption signal decreases as (1+G+-1)-1/2, where G+-1 is the degree of saturation induced by the sideband of the frequency modulated triplet, in agreement with theoretical predictions. This implies, first of all, that Doppler-broadened NICE-OHMS is affected less by optical saturation than other cavity enhanced techniques but also that it exhibits nonlinearities in the power and pressure dependence for all detection phases except pure dispersion. A methodology for assessments of the degree of saturation and the saturation power of a transition from Doppler-broadened NICE-OHMS signals is given. The implications of optical saturation for practical trace species detection by Doppler-broadened NICE-OHMS are discussed.

Keyword
spectroscopy heterodyne, spectroscopy modulation, absorption, trace species detection, cavity enhanced spectrometry
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-9811 (URN)10.1364/JOSAB.25.001156 (DOI)
Available from: 2008-05-21 Created: 2008-05-21 Last updated: 2017-12-14Bibliographically approved
5. Sub-Doppler dispersion and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy revised
Open this publication in new window or tab >>Sub-Doppler dispersion and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy revised
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

Keyword
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:nbn:se:umu:diva-9812 (URN)10.1364/JOSAB.25.001166 (DOI)
Available from: 2008-05-21 Created: 2008-05-21 Last updated: 2009-05-05Bibliographically approved
6. Noise-immune cavity-enhanced optical heterodyne molecular spectrometry: Current status and future potential
Open this publication in new window or tab >>Noise-immune cavity-enhanced optical heterodyne molecular spectrometry: Current status and future potential
2008 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 92, no 3, 313-326 p.Article in journal (Refereed) Published
Abstract [en]

As a result of a combination of an external cavity and modulation techniques, noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is one of the most sensitive absorption techniques, capable of reaching close-to-shot-noise sensitivities, down to 5×10-13 fractional absorption at 1 s averaging. Due to its ability to provide sub-Doppler signals from weak molecular overtone transitions, the technique was first developed for frequency standard applications. It has since then also found use in fields of molecular spectroscopy of weak overtone transitions and trace gas detection. This paper describes the principles and the unique properties of NICE-OHMS. The historical background, the contributions of various groups, as well as the performance and present status of the technique are reviewed. Recent progress is highlighted and the future potential of the technique for trace species detection is discussed.

Place, publisher, year, edition, pages
Berlin / Heidelberg: Springer Berlin/Heidelberg, 2008
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-10593 (URN)10.1007/s00340-008-3126-z (DOI)
Available from: 2008-10-05 Created: 2008-10-05 Last updated: 2017-12-14Bibliographically approved
7. Characterization of fiber-laser-based sub-Doppler NICE-OHMS for quantitative trace gas detection
Open this publication in new window or tab >>Characterization of fiber-laser-based sub-Doppler NICE-OHMS for quantitative trace gas detection
2008 (English)In: Optics Express, Vol. 16, no 19, 14689-14702 p.Article in journal (Refereed) Published
Abstract [en]

The potential of fiber-laser-based sub-Doppler noise-immune cavity-enhanced optical heterodyne molecular spectrometry for trace gas detection is scrutinized. The non-linear dependence of the on-resonance sub-Doppler dispersion signal on the intracavity pressure and power is investigated and the optimum conditions with respect to these are determined. The linearity of the signal strength with concentration is demonstrated and the dynamic range of the technique is discussed. Measurements were performed on C2H2 at 1531 nm up to degrees of saturation of 100. The minimum detectable sub-Doppler optical phase shift was 5 x 10-11 cm-1 Hz-1/2, corresponding to a partial pressure of C2H2 of 1 x 10-12 atm for an intracavity pressure of 20 mTorr, and a concentration of 10 ppb at 400 mTorr.

Identifiers
urn:nbn:se:umu:diva-10592 (URN)doi:10.1364/OE.16.014689 (DOI)
Available from: 2008-10-05 Created: 2008-10-05Bibliographically approved
8. Wavelength modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy signal line shapes in the Doppler limit
Open this publication in new window or tab >>Wavelength modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy signal line shapes in the Doppler limit
2009 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 26, no 7, 1384-1394 p.Article in journal (Refereed) Published
Abstract [en]

A thorough analysis of the shape and strength of Doppler-broadened wavelength modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy signals is presented and their dependence on modulation frequency, modulation amplitude and detection phase is investigated in detail. The conditions that maximize the on-resonance signal are identified. The analysis is based on the standard frequency modulation spectroscopy formalism and the Fourier description of wavelength modulation spectroscopy and verified by fits to experimental signals from C2H2 and CO2 measured at 1531 nm. In addition, the line strengths of two CO2 transitions in the v2→3v1+v2+v3 hot band [Pe(7) and Pe(9)] were found to differ by ~20% from those given in the HITRAN database.

Keyword
heterodyne spectroscopy, modulation spectroscopy, absorption, line shapes and shifts, cavity enhanced spectroscopy
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-22312 (URN)10.1364/JOSAB.26.001384 (DOI)000268542200018 ()
Available from: 2009-05-05 Created: 2009-05-05 Last updated: 2017-12-13Bibliographically approved
9. 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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