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Cavity enhanced optical sensing
Umeå University, Faculty of Science and Technology, Department of Physics.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Kavitetsförstärkt optisk detektion (Swedish)
Abstract [en]

An optical cavity comprises a set of mirrors between which light can be reflected a number of times. The selectivity and stability of optical cavities make them extremely useful as frequency references or discri­mi­nators. With light coupled into the cavity, a sample placed inside a cavity will experience a significantly increased interaction length. Hence, they can be used also as amplifiers for sensing purposes. In the field of laser spectroscopy, some of the most sensitive techniques are therefore built upon optical cavities. In this work optical cavities are used to measure properties of gas samples, i.e. absorption, dispersion, and refractivity, with unprecedented precision.

The most sensitive detection technique of all, Doppler-broadened noise-immune cavity enhanced optical heterodyne molecular spectrometry (Db NICE-OHMS), has in this work been developed to an ultra-sensitive spectroscopic technique with unprecedented detection sensitivity. By identifying limiting factors, realizing new experimental setups, and deter­mining optimal detection conditions, the sensitivity of the technique has been improved several orders of magnitude, from 8 × 10-11 to 9 × 10-14 cm-1. The pressure interval in which NICE-OHMS can be applied has been extended by deri­vation and verification of dispersions equations for so-called Dicke narrowing and speed dependent broadening effects. The theoretical description of NICE-OHMS has been expanded through the development of a formalism that can be applied to the situations when the cavity absorption cannot be considered to be small, which has expanded the dynamic range of the technique. In order to enable analysis of a large number of molecules at their most sensitive transitions (mainly their funda­mental CH vibrational transitions) NICE-OHMS instrumentation has also been developed for measurements in the mid-infrared (MIR) region. While it has been difficult to realize this in the past due to a lack of optical modulators in the MIR range, the system has been based on an optical para­metric oscillator, which can be modulated in the near-infrared (NIR) range.

As the index of refraction can be related to density, it is possible to retrieve gas density from measurements of the index of refraction. Two such instru­men­tations have been realized. The first one is based on a laser locked to a measure­ment cavity whose frequency is measured by compassion with an optical frequency comb. The second one is based on two lasers locked to a dual-cavity (i.e. one reference and one measurement cavity). By these methods changes in gas density down to 1 × 10-9 kg/m3 can be detected.

All instrumentations presented in this work have pushed forward the limits of what previously has been considered measurable. The knowledge acquired will be of great use for future ultrasensitive cavity-based detection methods.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2015. , 124 p.
Keyword [en]
Optical resonators, Fiber Laser, Parametric oscillators, Optical frequency comb, Infrared, Spectroscopy heterodyne, Spectroscopy molecular, Absorption, Dispersion, Lineshapes, Optical standards and testing, Refractivety measurements
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:umu:diva-110278ISBN: 978-91-7601-338-0 (print)OAI: oai:DiVA.org:umu-110278DiVA: diva2:861948
Public defence
2015-11-13, KBC-huset, KB3A9 (lilla hörsalen i KBC-huset), Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilThe Kempe Foundations
Available from: 2015-10-23 Created: 2015-10-19 Last updated: 2015-10-29Bibliographically approved
List of papers
1. Reduction of background signals in fiber-based NICE-OHMS
Open this publication in new window or tab >>Reduction of background signals in fiber-based NICE-OHMS
2011 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 28, no 11, 2797-2805 p.Article in journal (Refereed) Published
Abstract [en]

Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) based on a fiber-coupled electro-optic modulator (EOM) provides a compact and versatile experimental setup. It has, however, been limited by background signals originating from an imbalance of the phase modulated triplet created by a cross-coupling between the principal axes of the polarization maintaining fibers and the extraordinary axis of the EOM. Two strategies for reducing these background signals are investigated: (i) using an EOM with a titanium diffused waveguide, in which the balance of the triplet is controlled by active feedback, and (ii) using an EOM with a proton exchanged waveguide that does not support light propagation along the ordinary axis. It is shown that both approaches significantly reduce drifts and noise in the system. Using a cavity with a finesse of 5700, an absorption sensitivity of 3: 2 x 10(-12) cm(-1) in 1 min of integration time (i.e., 1: 8 x 10(-11) cm(-1) Hz(-1/2)) is demonstrated for Doppler-broadened detection, the lowest reported so far for Doppler-broadened NICE-OHMS. For sub-Doppler detection, a minimum detectable optical phase shift of 1: 3 x 10(-12) cm(-1) in 400s of integration time is obtained. (C) 2011 Optical Society of America

Place, publisher, year, edition, pages
Washington, D.C.: Optical Society of America, 2011
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-50420 (URN)10.1364/JOSAB.28.002797 (DOI)000296710300030 ()
Available from: 2011-12-12 Created: 2011-12-08 Last updated: 2017-12-08Bibliographically approved
2. 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
3. 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
4. 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
Show others...
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
5. 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
6. 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
7. 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
8. Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry down to 4 x 10-13 cm-1 Hz-1/2: implementation of a 50,000 finesse cavity
Open this publication in new window or tab >>Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry down to 4 x 10-13 cm-1 Hz-1/2: implementation of a 50,000 finesse cavity
Show others...
2015 (English)In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 40, no 9, 2004-2007 p.Article in journal (Refereed) Published
Abstract [en]

We report on the realization of a Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) instrumentation based on a high-finesse (50,000) cavity with a detection sensitivity of 4 x 10(-13) cm(-1) Hz(-1/2). For the P-e(11) transition targeted at 1.5316 mu m, this corresponds to a C2H2 concentration of 240 ppq (parts-per-quadrillion) detected at 100 Torr. The setup was originally affected by recurrent dips in the cavity transmission, which were attributed to excitation of high-order transverse mode by scattering from the mirrors. The effect of these was reduced by insertion of a small pinhole in the cavity.

Place, publisher, year, edition, pages
Optical Society of America, 2015
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-106284 (URN)10.1364/OL.40.002004 (DOI)000353924600031 ()25927769 (PubMedID)
Available from: 2015-07-10 Created: 2015-07-09 Last updated: 2017-12-04Bibliographically approved
9. Model for in-coupling of etalons into signal strengths extracted from spectral line shape fitting and methodology for predicting the optimum scanning range: demonstration of Doppler-broadened, noise-immune, cavity-enhanced optical heterodyne molecular spectroscopy down to 9  ×  10−14 cm−1
Open this publication in new window or tab >>Model for in-coupling of etalons into signal strengths extracted from spectral line shape fitting and methodology for predicting the optimum scanning range: demonstration of Doppler-broadened, noise-immune, cavity-enhanced optical heterodyne molecular spectroscopy down to 9  ×  10−14 cm−1
2015 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 32, no 10, 2104-2114 p.Article in journal (Refereed) Published
Abstract [en]

Expressions for the in-coupling of white noise and etalons into fitted signal strengths are derived. These show that the amount of noise picked up is affected by the scanning range. A methodology for finding the optimum scanning range from a single set of measurements has been developed. This was used to estimate the optimum conditions of a noise-immune, cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) setup. The methodology was validated by measurements. This resulted in a spectral noise equivalent absorption per unit length of 2.6 × 10−13 cm−1 Hz−1∕2 and a minimum Allan deviation of 9 × 10−14 cm−1 at 30 s, which are, to our knowledge, the lowest reported for Doppler-broadened NICE-OHMS

Place, publisher, year, edition, pages
Optical Society of America, 2015
Keyword
Absorption, Spectroscopy - heterodyne, Spectroscopy - modulation, Spectroscopy - molecular
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-110081 (URN)10.1364/JOSAB.32.002104 (DOI)000367235700012 ()
Available from: 2015-10-15 Created: 2015-10-15 Last updated: 2017-12-01Bibliographically approved
10. Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – I. Theoretical Description
Open this publication in new window or tab >>Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – I. Theoretical Description
2016 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 168, 217-244 p.Article in journal (Other academic) Published
Abstract [en]

Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) is conventionally described by an expression (here referred to as the CONV expression) that is restricted to the case when the single-pass absorbance, α0L, is much smaller than the empty cavity losses, π/F [here termed the conventional cavity-limited weak absorption (CCLWA) condition]. This limits the applicability of the technique, primarily its dynamic range and calibration capability. To remedy this, this work derives extended descriptions of Db NICEOHMS that are not restricted to the CCLWA condition. First, the general principles of Db NICEOHMS are scrutinized in some detail. Based solely upon a set of general assumptions, predominantly that it is appropriate to linearize the Beer–Lambert law, that the light is modulated to a triplet, and that the Pound–Drever–Hall sidebands are fully reflected, a general description of Db NICE-OHMS that is not limited to any specific restriction on α0L vs. π/F, here referred to as the FULL description, is derived. However, this description constitutes a set of equations to which no closed form solution has been found. Hence, it needs to be solved numerically (by iterations), which is inconvenient. To circumvent this, for the cases when α0Loπ/F but without the requirement that the stronger CCLWA condition needs to be fulfilled, a couple of simplified extended expressions that are expressible in closed analytical form, referred to as the extended locking and extended transmission description, ELET, and the extended locking and full transmission description, ELFT, have been derived. An analysis based on simulations validates the various descriptions and assesses to which extent they agree. It is shown that in the CCLWA limit, all extended descriptions revert to the CONV expression. The latter one deviates though from the extended ones for α0L around and above 0.1π/F. The two simplified extended descriptions agree with the FULL description for a larger range of α0L than the CONV expression, viz. for the ELET description for α0L up to 0.3π/F and for ELFT for α0L up to 0.6 or 1.0 π/F (depending on the mode of detection). It is then demonstrated that the conventional view of Db NICE-OHMS, which states that the out-of-phase and the in-phase signals can be referred to as a pure absorption and dispersion signal, respectively, breaks down when the CCLWA condition does not hold. In this case, the out-of-phase signal is additionally affected by the phase shifts of the laser components (i.e. dispersion) while the in-phase signal is also influenced by their attenuation. Access to new descriptions broadens considerably the dynamic range of Db NICE-OHMS and facilitates calibration using standard references samples, and thereby its applicability

Keyword
NICE-OHMS, Cavity enhanced spectroscopy, Dynamic range, Frequency modulation spectroscopy, Theoretical model
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-110271 (URN)10.1016/j.jqsrt.2015.09.007 (DOI)000366007000018 ()
Available from: 2015-10-19 Created: 2015-10-19 Last updated: 2017-12-01Bibliographically approved
11. Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – II: experimental verification
Open this publication in new window or tab >>Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – II: experimental verification
2016 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 168, 245-256 p.Article in journal (Refereed) Published
Abstract [en]

Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) is normally described by an expression, here termed the conventional (CONV) description, that is restricted to the conventional cavity-limited weak absorption condition (CCLWA), i.e. when the single pass absorbance is significantly smaller than the empty cavity losses, i.e. when α0L<<π/F. To describe NICE-OHMS signals beyond this limit two simplified extended descriptions (termed the extended locking and extended transmission description, ELET, and the extended locking and full transmission description, ELFT), which are assumed to be valid under the relaxed cavity-limited weak absorption condition (RCLWA), i.e. when α0L<π/Fα0L<π/F, and a full description (denoted FULL), presumed to be valid also when the α0L<π/Fα0L<π/F condition does not hold, have recently been derived in an accompanying work (Ma W, et al. Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition - I. Theoretical Description. J Quant Spectrosc Radiat Transfer, 2015, http://dx.doi.org/10.1016/j.jqsrt.2015.09.007, this issue). The present work constitutes an experimental verification and assessment of the validity of these, performed in the Doppler limit for a set of Fα0L/πFα0L/π values (up to 3.5); it is shown under which conditions the various descriptions are valid. It is concluded that for samples with Fα0L/πFα0L/π up to 0.01, all descriptions replicate the data well. It is shown that the CONV description is adequate and provides accurate assessments of the signal strength (and thereby the analyte concentration) up to Fα0L/πFα0L/π of around 0.1, while the ELET is accurate for Fα0L/πFα0L/π up to around 0.3. The ELFT description mimics the Db NICE-OHMS signal well for Fα0L/πFα0L/π up to around unity, while the FULL description is adequate for all Fα0L/πFα0L/π values investigated. Access to these descriptions both increases considerably the dynamic range of the technique and facilitates calibration using certified reference gases, which thereby significantly broadens the applicability of the Db NICE-OHMS technique.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
NICE OHMS, Frequency modulation spectroscopy, Cavity enhanced spect, Experimental verification
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-110273 (URN)10.1016/j.jqsrt.2015.09.008 (DOI)000366007000019 ()
Available from: 2015-10-19 Created: 2015-10-19 Last updated: 2017-12-01Bibliographically approved
12. Doppler-broadened mid-infrared noise-immune cavity-enhanced optical heterodyne molecular spectrometry based on an optical parametric oscillator for trace gas detection
Open this publication in new window or tab >>Doppler-broadened mid-infrared noise-immune cavity-enhanced optical heterodyne molecular spectrometry based on an optical parametric oscillator for trace gas detection
Show others...
2015 (English)In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 40, no 4, 439-442 p.Article in journal (Refereed) Published
Abstract [en]

An optical parametric oscillator based Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) system suitable for addressing fundamental vibrational transitions in the 3.2-3.9 mu m mid-infrared (MIR) region has been realized. An Allan-Werle analysis provides a detection sensitivity of methane of 1.5 x 10-9 cm(-1) with a 20 s integration time, which corresponds to 90 ppt of CH4 if detected at the strongest transition addressed at 40 Torr. This supersedes that of previous Db MIR NICE-OHMS demonstrations and suggests that the technique can be suitable for detection of both the environmentally important (CH4)-C-13 and CH3D isotopologues. It also opens up for detection of many other molecular species at ppt and sub-ppt concentration levels.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-101400 (URN)10.1364/OL.40.000439 (DOI)000349848400001 ()25680119 (PubMedID)
Available from: 2015-07-02 Created: 2015-03-30 Last updated: 2017-12-04Bibliographically approved
13. Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry in the mid-IR region down to 10−10 cm−1 Hz−1/2
Open this publication in new window or tab >>Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry in the mid-IR region down to 10−10 cm−1 Hz−1/2
(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-110274 (URN)
Available from: 2015-10-19 Created: 2015-10-19 Last updated: 2015-10-22
14. 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
15. 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
16. 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
17. 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
18. Optical measurement of the gas number density in a Fabry-Perot cavity
Open this publication in new window or tab >>Optical measurement of the gas number density in a Fabry-Perot cavity
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2013 (English)In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 24, no 10, 105207- p.Article in journal (Refereed) Published
Abstract [en]

An optical method for measuring the gas density by monitoring the refractive index inside a high-finesse Fabry-Perot cavity is presented. The frequency of a narrow linewidth Er:fiber laser, locked to a mode of the cavity, is measured with the help of an optical frequency comb while the gas density inside the cavity changes. A resolution of 1.4 x 10(-6) mol m(-3) is achieved in 3 s for nitrogen, which allows measurement of a relative gas density change of 3.4 x 10(-8) at atmospheric pressure.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2013
Keyword
laser refractometry, gas density measurement, metrology, optical frequency comb, Fabry-Perot cavity
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-82812 (URN)10.1088/0957-0233/24/10/105207 (DOI)000324621900031 ()
Funder
Swedish Research Council, 621-2011-5123, 621-2012-3650
Available from: 2013-11-11 Created: 2013-11-11 Last updated: 2017-12-06Bibliographically approved
19. A dual Fabry-Perot cavity for fast assessments of gasnumber density
Open this publication in new window or tab >>A dual Fabry-Perot cavity for fast assessments of gasnumber density
(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-110276 (URN)
Available from: 2015-10-19 Created: 2015-10-19 Last updated: 2015-10-22
20. 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
21. 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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