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Optical Frequency Comb Fourier Transform Spectroscopy
Umeå University, Faculty of Science and Technology, Department of Physics. (Optical Frequency Comb Spectroscopy)
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Fourier transform spectroscopy (FTS) based on optical frequency combs is an excellent spectroscopic tool as it provides broadband molecular spectra with high spectral resolution and an absolutely calibrated frequency scale. Moreover, the equidistant comb mode structure enables efficient coupling of the comb to enhancement cavities, yielding high detection sensitivity. This thesis focuses on further advances in comb-based FTS to improve its performance and extend its capabilities for broadband precision spectroscopy, particularly in terms of i) spectral resolution, ii) accuracy and precision of molecular parameters as well as concentrations retrieved from fitting models to spectra, and iii) species selectivity.

To improve the spectral resolution we developed a new methodology to acquire and analyze comb-based FTS signals that yields spectra with a resolution limited by the comb linewidth rather than the optical path difference of the FTS, referred to as the sub-nominal resolution method. This method enables measurements of narrow features, e.g. low-pressure absorption spectra and modes of enhancement cavities, with frequency scale accuracy and precision provided by the comb. Using the technique we measured low-pressure spectra of the entire 3ν13 carbon dioxide (CO2) band at 1575 nm with sufficient signal-to-noise ratio and precision to observe collision narrowing of the absorption lineshape, which was for the first time with a comb-based spectroscopic technique. This allowed retrieval of spectral line parameters for this CO2 band using the speed-dependent Voigt profile.

Using the sub-nominal resolution method, we measured the transmission modes of a Fabry-Perot cavity over 15 THz of bandwidth with kHz resolution and characterized the cavity modes in terms of their center frequency, linewidth, and amplitude. From the mode center frequencies, we retrieved the group delay dispersion of cavity mirror coatings and intracavity gas with an unprecedented combination of spectral bandwidth and resolution. By measuring both the mode broadening and frequency shift simultaneously we performed broadband cavity-enhanced complex refractive index spectroscopy (CE-CRIS), which allows for simultaneous and calibration-free assessment of the absorption and dispersion spectra of intracavity gas. In this first demonstration we measured the absorption and dispersion spectra of three combination bands of CO2 in the 1525 to 1620 nm range.

Another comb-based FTS technique is noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS), which combines phase modulation and cavity-enhancement to obtain broadband and highly sensitive absorption spectra. In this thesis we improved the NICE-OFCS technique in terms of stability, sensitivity and modeling of the NICE-OFCS signal. We implemented a model of the NICE-OFCS signal with multiline fitting for assessment of gas concentration. We also identified the optimum operating conditions of the NICE-OFCS systems for accurate gas concentration assessment.

Finally, to improve the species selectivity we combined comb-based FTS with the Faraday rotation spectroscopy (FRS) technique. In this first demonstration of optical frequency comb Faraday rotation spectroscopy (OFC-FRS), we measured background and interference-free spectra of the entire Q- and R-branches of the fundamental vibrational band of nitric oxide at 5.3 μm showing good agreement with the theoretical model.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet , 2018. , p. 155
Keywords [en]
Optical frequency comb spectroscopy, Fourier transform, molecular absorption & dispersion
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
URN: urn:nbn:se:umu:diva-153239ISBN: 978-91-7601-983-2 (print)OAI: oai:DiVA.org:umu-153239DiVA, id: diva2:1262989
Public defence
2018-12-07, N430, Naturvetarhuset, Umeå, 09:15 (English)
Opponent
Supervisors
Available from: 2018-11-16 Created: 2018-11-13 Last updated: 2018-11-14Bibliographically approved
List of papers
1. Noise-immune cavity-enhanced optical frequency comb spectroscopy: a sensitive technique for high-resolution broadband molecular detection
Open this publication in new window or tab >>Noise-immune cavity-enhanced optical frequency comb spectroscopy: a sensitive technique for high-resolution broadband molecular detection
2015 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 119, no 1, p. 87-96Article in journal (Refereed) Published
Abstract [en]

Noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS) is a recently developed technique that utilizes phase modulation to obtain immunity to frequency-to-amplitude noise conversion by the cavity modes and yields high absorption sensitivity over a broad spectral range. We describe the principles of the technique and discuss possible comb-cavity matching solutions. We present a theoretical description of NICE-OFCS signals detected with a Fourier transform spectrometer (FTS) and validate the model by comparing it to experimental CO2 spectra around 1,575 nm. Our system is based on an Er:fiber femtosecond laser locked to a cavity and phase-modulated at a frequency equal to a multiple of the cavity free spectral range (FSR). The NICE-OFCS signal is detected by a fast-scanning FTS equipped with a high-bandwidth commercial detector. We demonstrate a simple method of passive locking of the modulation frequency to the cavity FSR that significantly improves the long-term stability of the system, allowing averaging times on the order of minutes. Using a cavity with a finesse of ~9,000, we obtain absorption sensitivity of 6.4 × 10−11 cm−1 Hz−1∕2 per spectral element and concentration detection limit for CO2 of 450 ppb Hz−1/2, determined by multiline fitting.

Place, publisher, year, edition, pages
Springer Berlin/Heidelberg, 2015
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-114664 (URN)10.1007/s00340-015-6010-7 (DOI)000352292500011 ()
Conference
4th International Conferene on Field Laser Applications in Industry and Research, MAY 05-09, 2014, Firenze, ITALY
Funder
Carl Tryggers foundation , CTS12:131Swedish Research Council, 621-2012-3650Swedish Foundation for Strategic Research , ICA12-0031
Note

Special Issue.

This project was supported by the Swedish Research Council (621-2012-3650), Swedish Foundation for Strategic Research (ICA12-0031), the Carl Trygger’s Foundation (CTS12:131), and the Faculty of Science and Technology, Umeå University.

Available from: 2016-01-25 Created: 2016-01-25 Last updated: 2018-11-13Bibliographically approved
2. Surpassing the path-limited resolution of Fourier-transform spectrometry with frequency combs
Open this publication in new window or tab >>Surpassing the path-limited resolution of Fourier-transform spectrometry with frequency combs
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2016 (English)In: Physical Review A, ISSN 2469-9926, Vol. 93, no 2, article id 021802Article in journal (Refereed) Published
Abstract [en]

We overcome the resolution limit of Fourier-transform spectrometry and measure instrumental line-shape-free broadband molecular spectra with lines narrower than the optical path-limited resolution. We do this by using an optical frequency comb and precisely matching the maximum delay range of the spectrometer to the comb line spacing to measure the intensities of the individual comb lines. This method allows measurements of undistorted high-resolution spectra with acquisition time and interferometer length reduced by orders of magnitude and with frequency scale accuracy given by the comb.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-117379 (URN)10.1103/PhysRevA.93.021802 (DOI)000369724700001 ()
Available from: 2016-03-21 Created: 2016-02-29 Last updated: 2018-11-13Bibliographically approved
3. Signal line shapes of Fourier-transform cavity-enhanced frequency modulation spectroscopy with optical frequency combs
Open this publication in new window or tab >>Signal line shapes of Fourier-transform cavity-enhanced frequency modulation spectroscopy with optical frequency combs
2017 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 34, no 2, p. 358-365Article in journal (Refereed) Published
Abstract [en]

We present a thorough analysis of the signal line shapes of Fourier-transform-based noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS). We discuss the signal dependence on the ratio of the modulation frequency, f(m), to the molecular linewidth, G. We compare a full model of the signals and a simplified absorption-like analytical model that has high accuracy for low f(m)/G ratios and is much faster to compute. We verify the theory experimentally by measuring and fitting the NICE-OFCS spectra of CO2 at 1575 nm using a system based on an Er: fiber femtosecond laser and a cavity with a finesse of similar to 11000. 

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-133206 (URN)10.1364/JOSAB.34.000358 (DOI)000394028400019 ()
Available from: 2017-04-13 Created: 2017-04-13 Last updated: 2018-11-13Bibliographically approved
4. Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution
Open this publication in new window or tab >>Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution
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2017 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 25, no 18, p. 21711-21718Article in journal (Refereed) Published
Abstract [en]

Optical cavities provide high sensitivity to dispersion since their resonance frequencies depend on the index of refraction. We present a direct, broadband, and accurate measurement of the modes of a high finesse cavity using an optical frequency comb and a mechanical Fourier transform spectrometer with a kHz-level resolution. We characterize 16000 longitudinal cavity modes spanning 16 THz of bandwidth in terms of center frequency, linewidth, and amplitude. Using the center frequencies we retrieve the group delay dispersion of the cavity mirror coatings and pure N2 with 0.1 fs2 precision and 1 fs2 accuracy, as well as the refractivity of the 3ν13 absorption band of CO2 with 5 × 10‒12 precision. This opens up for broadband refractive index metrology and calibration-free spectroscopy of entire molecular bands.

Place, publisher, year, edition, pages
Optical Society of America, 2017
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-140651 (URN)10.1364/OE.25.021711 (DOI)000411529000067 ()
Available from: 2017-10-17 Created: 2017-10-17 Last updated: 2018-11-13Bibliographically approved
5. Optical frequency comb Fourier transform spectroscopy with sub-nominal resolution and precision beyond the Voigt profile
Open this publication in new window or tab >>Optical frequency comb Fourier transform spectroscopy with sub-nominal resolution and precision beyond the Voigt profile
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2018 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 204, p. 63-73Article in journal (Refereed) Published
Abstract [en]

Broadband precision spectroscopy is indispensable for providing high fidelity molecular parameters for spectroscopic databases. We have recently shown that mechanical Fourier transform spectrometers based on optical frequency combs can measure broadband high-resolution molecular spectra undistorted by the instrumental line shape (ILS) and with a highly precise frequency scale provided by the comb. The accurate measurement of the power of the comb modes interacting with the molecular sample was achieved by acquiring single-burst interferograms with nominal resolution matched to the comb mode spacing. Here we describe in detail the experimental and numerical steps needed to achieve sub-nominal resolution and retrieve ILS-free molecular spectra, i.e. with ILS-induced distortion below the noise level. We investigate the accuracy of the transition line centers retrieved by fitting to the absorption lines measured using this method. We verify the performance by measuring an ILS-free cavity-enhanced low-pressure spectrum of the 3ν1 + ν3 band of CO2 around 1575 nm with line widths narrower than the nominal resolution. We observe and quantify collisional narrowing of absorption line shape, for the first time with a comb-based spectroscopic technique. Thus retrieval of line shape parameters with accuracy not limited by the Voigt profile is now possible for entire absorption bands acquired simultaneously.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Optical frequency combs, Fourier transform spectroscopy, High resolution spectroscopy, Line shapes
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-142443 (URN)10.1016/j.jqsrt.2017.09.001 (DOI)000414880500009 ()
Available from: 2017-12-06 Created: 2017-12-06 Last updated: 2018-11-13Bibliographically approved
6. Optical frequency comb Faraday rotation spectroscopy
Open this publication in new window or tab >>Optical frequency comb Faraday rotation spectroscopy
2018 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 124, no 5, article id 79Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Springer, 2018
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-148020 (URN)10.1007/s00340-018-6951-8 (DOI)000431906400001 ()
Available from: 2018-05-30 Created: 2018-05-30 Last updated: 2018-11-13Bibliographically approved
7. Broadband calibration-free cavity-enhanced complex refractive index spectroscopy using a frequency comb
Open this publication in new window or tab >>Broadband calibration-free cavity-enhanced complex refractive index spectroscopy using a frequency comb
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2018 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 26, no 16, p. 20633-20648Article in journal (Refereed) Published
Abstract [en]

We present broadband cavity-enhanced complex refractive index spectroscopy (CE-CRIS), a technique for calibration-free determination of the complex refractive index of entire molecular bands via direct measurement of transmission modes of a Fabry-Perot cavity filled with the sample. The measurement of the cavity transmission spectrum is done using an optical frequency comb and a mechanical Fourier transform spectrometer with sub-nominal resolution. Molecular absorption and dispersion spectra (corresponding to the imaginary and real parts of the refractive index) are obtained from the cavity mode broadening and shift retrieved from fits of Lorentzian profiles to the individual cavity modes. This method is calibration-free because the mode broadening and shift are independent of the cavity parameters such as the length and mirror reflectivity. In this first demonstration of broadband CE-CRIS we measure simultaneously the absorption and dispersion spectra of three combination bands of CO2 in the range between 1525 nm and 1620 nm and achieve good agreement with theoretical models. This opens up for precision spectroscopy of the complex refractive index of several molecular bands simultaneously. 

Place, publisher, year, edition, pages
Optical Society of America, 2018
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-151397 (URN)10.1364/OE.26.020633 (DOI)000440803600079 ()30119372 (PubMedID)
Funder
Swedish Research Council, 2016-03593Swedish Research Council, 2015-04374Knut and Alice Wallenberg Foundation, KAW 2015.0159
Available from: 2018-09-04 Created: 2018-09-04 Last updated: 2018-11-13Bibliographically approved
8. CO2 Line Parameter Retrieval Beyond the Voigt Profile Using Comb-Based Fourier Transform Spectroscopy
Open this publication in new window or tab >>CO2 Line Parameter Retrieval Beyond the Voigt Profile Using Comb-Based Fourier Transform Spectroscopy
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2018 (English)In: Conference on Lasers and Electro-Optics, Optical Society of America, 2018Conference paper, Published paper (Refereed)
Abstract [en]

We measure absorption spectra of the CO213 band at 1.57 μm using optical frequency comb Fourier transform spectroscopy with sub-nominal resolution and retrieve line shape parameters using multiline fitting with the speed-dependent Voigt profile.

Place, publisher, year, edition, pages
Optical Society of America, 2018
Series
OSA Technical Digest
Keywords
Spectroscopy, Fourier transforms (300.6300), Absorption (300.1030), Line shapes and shifts (020.3690).
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-153175 (URN)10.1364/CLEO_SI.2018.STu3P.6 (DOI)
Conference
Conference on Lasers and Electro-Optics (CLEO, Science and innovations), San Jose, CA, USA, May 13-18, 2018.
Funder
Knut and Alice Wallenberg Foundation, KAW 2015.0159Swedish Research Council, 2016-03593
Note

Paper STu3P.6

Available from: 2018-11-12 Created: 2018-11-12 Last updated: 2018-11-14Bibliographically approved

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