| 1. |
- Johansson, Alexandra C., et al.
(författare)
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Optical frequency comb Faraday rotation spectroscopy
- 2018
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Ingår i: Applied physics. B, Lasers and optics (Print). - : Springer. - 0946-2171 .- 1432-0649. ; 124:5
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate optical frequency comb Faraday rotation spectroscopy (OFC-FRS) for broadband interference-free detection of paramagnetic species. The system is based on a femtosecond doubly resonant optical parametric oscillator and a fast-scanning Fourier transform spectrometer (FTS). The sample is placed in a DC magnetic field parallel to the light propagation. Efficient background suppression is implemented via switching the direction of the field on consecutive FTS scans and subtracting the consecutive spectra, which enables long-term averaging. In this first demonstration, we measure the entire Q- and R-branches of the fundamental band of nitric oxide in the 5.2–5.4 μm range and achieve good agreement with a theoretical model.
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| 2. |
- Johansson, Alexandra C., 1987-, et al.
(författare)
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Optical frequency comb faraday rotation spectroscopy
- 2018
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Ingår i: 2018 Conference on Lasers and Electro-Optics, CLEO 2018 - Proceedings. - : Institute of Electrical and Electronics Engineers (IEEE). - 9781943580422 - 9781538657331
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Konferensbidrag (refereegranskat)abstract
- By combining Faraday rotation spectroscopy with an optical frequency comb Fourier transform spectrometer, we measure background- and calibration-free spectra of the entire Q- and R-branches of the fundamental band of nitric oxide at 1850-1920 cm-1.
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| 3. |
- Westberg, Jonas, et al.
(författare)
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Optical frequency comb Faraday rotation spectroscopy
- 2018
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Ingår i: Light, Energy and the Environment. - : Optica Publishing Group (formerly OSA). - 9781943580477
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Konferensbidrag (refereegranskat)abstract
- We present measurements of the entire Q- and R-branches of the fundamental band of nitric oxide at 5.2 - 5.4 μm by the background- and calibration-free optical frequency comb Faraday rotation spectroscopy technique.
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| 4. |
- Johansson, Alexandra C., et al.
(författare)
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Faraday rotation spectroscopy using an optical frequency comb
- 2017
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Ingår i: 2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC). - : IEEE. - 9781509067367
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Konferensbidrag (refereegranskat)abstract
- Summary form only given. The mid-infrared (MIR) part of the optical spectrum (3-12 μm) houses the fundamental absorption bands of a multitude of environmentally important molecules, but the abundance of water absorption often causes interference with the target species and makes concentration measurement inaccurate. The broad spectral coverage of optical frequency comb spectroscopy (OFCS) provides access to entire ro-vibrational bands and allows more accurate concentration quantification and retrieval of sample temperature. To further improve detection sensitivity of paramagnetic species in the presence of interfering species, we combine a MIR optical frequency comb with the Faraday rotation spectroscopy (FRS) technique [I], which is insensitive to interferences from diamagnetic molecules, such as H 2 O, CO 2 , and CO. In FRS, the rotation of the polarization induced by an external magnetic field in the vicinity of paramagnetic molecular transitions is translated to an intensity change by the use of a polarization analyzer, which effectively removes the influence of any non-paramagnetic species. In the proof of principle demonstration of OFC-FRS we detect nitric oxide (NO) in the presence of water at 5.3 μm using a Fourier transform spectrometer.
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| 5. |
- Westberg, Jonas, 1983-, et al.
(författare)
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Lineshape asymmetries in Faraday modulation spectroscopy
- 2014
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Ingår i: Applied physics. B, Lasers and optics (Print). - Berlin/Heidelberg : Springer. - 0946-2171 .- 1432-0649. ; 116:2, s. 467-476
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Tidskriftsartikel (refereegranskat)abstract
- Faraday modulation/rotation spectroscopy (FAMOS/FRS) is a spectroscopic technique for detection of paramagnetic species in gas phase. Although the prevailing theoretical description predicts fully symmetrical lineshapes, experiments do not in general provide such. This work shows that asymmetries in FAMOS can have at least two origins; (i) a frequency dependent laser intensity and (ii) polarization imperfections, which both are scrutinized here. A general analytical description for the latter, derived under the assumption that both the polarization imperfections and the relative absorption are small, is presented, conveniently expressed in terms of 1st Fourier coefficients of modulated dispersion and absorption lineshape functions. The resulting expression, which is thus an extension to the conventional FAMOS expression, can thereby be swiftly evaluated and allows for on-line fitting to measured asymmetric FAMOS signals. Curve fits to experimentally obtained data from nitric oxide measured both in the ultra-violet and the mid-infrared region demonstrate the applicability of the methodology.
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| 6. |
- Westberg, Jonas, et al.
(författare)
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Methodology for fast curve fitting to modulated Voigt dispersion lineshape functions
- 2014
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Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier. - 0022-4073 .- 1879-1352. ; 133, s. 244-250
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Tidskriftsartikel (refereegranskat)abstract
- Faraday rotation spectroscopy (FAMOS) as well as other modulated techniques that rely on dispersion produce lock-in signals that are proportional to various Fourier coefficients of modulated dispersion lineshape functions of the molecular transition targeted. In order to enable real-time curve fitting to such signals a fast methodology for calculating the Fourier coefficients of modulated lineshape functions is needed. Although there exist an analytical expression for such Fourier coefficients of modulated Lorentzian absorption and dispersion lineshape functions, there is no corresponding expression for a modulated Voigt dispersion function. The conventional computational route of such Fourier coefficients has therefore so far either consisted of using various approximations to the modulated Voigt lineshape function or solving time-consuming integrals, which has precluded accurate real-time curve fitting. Here we present a new methodology to calculate Fourier coefficients of modulated Voigt dispersion lineshape functions that is significantly faster (several orders of magnitude) and more accurate than previous approximative calculation procedures, which allows for real-time curve fitting to FAMOS signals also in the Voigt regime.
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| 7. |
- Westberg, Jonas, 1983-
(författare)
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Faraday modulation spectroscopy : Theoretical description and experimental realization for detection of nitric oxide
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Faraday modulation spectroscopy (FAMOS) is a laser-based spectroscopic dispersion technique for detection of paramagnetic molecules in gas phase. This thesis presents both a new theoretical description of FAMOS and experimental results from the ultra-violet (UV) as well as the mid-infrared (MIR) regions. The theoretical description, which is given in terms of the integrated linestrength and Fourier coefficients of modulated dispersion and absorption lineshape functions, facilitates the description and the use of the technique considerably. It serves as an extension to the existing FAMOS model that thereby incorporates also the effects of lineshape asymmetries primarily originating from polarization imperfections. It is shown how the Fourier coefficients of modulated Lorentzian lineshape functions, applicable to the case with fully collisionally broadened transitions, can be expressed in terms of analytical functions. For the cases where also Doppler broadening needs to be included, resulting in lineshapes of Voigt type, the lineshape functions can be swiftly evaluated (orders of magnitude faster than previous procedures) by a newly developed method for rapid calculation of modulated Voigt lineshapes (the WWA-method). All this makes real-time curve fitting to FAMOS spectra feasible. Two experimental configurations for sensitive detection of nitric oxide (NO) by the FAMOS technique are considered and their optimum conditions are determined. The two configurations target transitions originating from the overlapping Q22(21=2) and QR12(21=2) transitions in the ultra-violet (UV) region (227nm) and the Q3=2(3=2)-transition in the fundamental rotational-vibrational band in the mid-infrared (MIR) region (5.33 µm). It is shown that the implementations of FAMOS in the UV- and MIR-region can provide detection limits in the low ppb range, which opens up the possibility for applications where high detection sensitivities of NO is required.
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| 8. |
- Wang, Junyang, et al.
(författare)
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Speed-dependent Voigt dispersion line-shape function : applicable to techniques measuring dispersion signals
- 2012
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Ingår i: Journal of the Optical Society of America. B, Optical physics. - Washington : Optical Society of America. - 0740-3224 .- 1520-8540. ; 29:10, s. 2971-2979
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Tidskriftsartikel (refereegranskat)abstract
- 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
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| 9. |
- Westberg, Jonas, et al.
(författare)
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Fast and non-approximate methodology for calculation of wavelength-modulated Voigt lineshape functions suitable for real-time curve fitting
- 2012
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Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer. - Oxford : Elsevier. - 0022-4073 .- 1879-1352. ; 113:16, s. 2049-2057
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Tidskriftsartikel (refereegranskat)abstract
- Wavelength modulation (WM) produces lock-in signals that are proportional to various Fourier coefficients of the modulated lineshape function of the molecular transition targeted. Unlike the case for the Lorentzian lineshape function, there is no known analytical expression for the Fourier coefficients of a modulated Voigt lineshape function; they consist of nested integrals that have to be solved numerically, which is often time-consuming and prevents real-time curve fitting. Previous attempts to overcome these limitations have so far consisted of approximations of the Voigt lineshape function, which brings in inaccuracies. In this paper we demonstrate a new means to calculate the lineshape of nf-WM absorption signals from a transition with a Voigt profile. It is shown that the signal can conveniently be expressed as a convolution of one or several Fourier coefficients of a modulated Lorentzian lineshape function, for which there are analytical expressions, and the Maxwell-Boltzmann velocity distribution for the system under study. Mathematically, the procedure involves no approximations, wherefore its accuracy is limited only by the numerical precision of the software used (in this case similar to 10(-16)) while the calculation time is reduced by roughly three orders of magnitude (10(-3)) as compared to the conventional methodology, i.e. typically from the second to the millisecond range. This makes feasible real-time curve fitting to lock-in output signals from modulated Voigt profiles. (C) 2012 Elsevier Ltd. All rights reserved.
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| 10. |
- Kluczynski, P., et al.
(författare)
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Detection of acetylene impurities in ethylene and polyethylene manufacturing processes using tunable diode laser spectroscopy in the 3-mu m range
- 2011
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Ingår i: Applied physics. B, Lasers and optics (Print). - Berlin : Springer-Vlg. - 0946-2171 .- 1432-0649. ; 105:2, s. 427-434
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Tidskriftsartikel (refereegranskat)abstract
- Using recently developed GaInAsSb/AlGaInAsSb DFB lasers, tunable diode laser spectroscopy (TDLS) has been extended into the 3-mu m wavelength region for the detection of acetylene impurities in hydrocarbon compounds encountered in ethylene manufacturing. Measurements of acetylene in pure polymer grade ethylene and in a gas mixture of ethylene and ethane typical of the process stream around a hydrogenation reactor have been performed. Using a procedure incorporating subtraction of a hydrocarbon background spectrum a detection limit of 5 ppb m was achieved under ordinary laboratory conditions. Under forced temperature cycling conditions, the detection limit deteriorated to 180 ppb m, due to temperature drift caused by optical interferences generated by reflections in the laser TO8 can.
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