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Fourier transform and Vernier spectroscopy using optical frequency combs
Umeå University, Faculty of Science and Technology, Department of Physics. (Optical Frequency Comb Spectroscopy)ORCID iD: 0000-0002-0212-7617
2017 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Fouriertransform- och Vernierspektroskopi med optiska frekvenskammar (Swedish)
Abstract [en]

Optical frequency comb spectroscopy (OFCS) combines two previously exclusive features, i.e., wide optical bandwidth and high spectral resolution, enabling precise measurements of entire molecular bands and simultaneous monitoring of multiple gas species in a short measurement time. Moreover, the equidistant mode structure of frequency combs enables efficient coupling of the comb power to enhancement resonant cavities, yielding high detection sensitivities. Different broadband detection methods have been developed to exploit the full potential of frequency combs in spectroscopy, based either on Fourier transform spectroscopy or on dispersive elements.There have been two main aims of the research presented in this thesis. The first has been to improve the performance of mechanical Fourier transform spectrometers (FTS) based on frequency combs in terms of sensitivity, resolution and spectral coverage. In pursuit of this aim, we have developed a new spectroscopic technique, so-called noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS), and achieved a shot-noise-limited sensitivity and low ppb (parts-per-billion, 10−9) CO2 concentration detection limit in the near-infrared range using commercially available components. We have also realized a novel method for acquisition and analysis of comb-based FTS spectra, a so-called sub-nominal resolution method, which provides ultra-high spectral resolution and frequency accuracy (both in kHz range, limited only by the stability of the comb) over the broadband spectral range of the frequency comb. Finally, we have developed an optical parametric oscillator generating a frequency comb in the mid-infrared range, where the strongest ro-vibrational molecular absorption lines reside. Using this mid-infrared comb and an FTS, we have demonstrated, for the first time, comb spectroscopy above 5 μm, measured broadband spectra of several species and reached low ppb detection limits for CH4, NO and CO in 1 s.The second aim has been more application-oriented, focused on frequency comb spectroscopy in combustion environments and under atmospheric conditions for fast and sensitive multispecies detection. We have demonstrated, for the first time, cavity-enhanced optical frequency comb spectroscopy in a flame, detected broadband high temperature H2O and OH spectra using the FTS in the near-infrared range and showed the potential of the technique for flame thermometry. For applications demanding a short measurement time and high sensitivity under atmospheric pressure conditions, we have implemented continuous-filtering Vernier spectroscopy, a dispersion-based spectroscopic technique, for the first time in the mid-infrared range. The spectrometer was sensitive, fast, robust, and capable of multispecies detection with 2 ppb detection limit for CH4 in 25 ms.

Abstract [sv]

Optisk frekvenskamspektroskopi (OFCS) kombinerar två tidigare icke förenliga egenskaper, dvs. ett brett optiskt frekvensområde med en hög spektral upplösning, vilket möjliggör noggranna mätningar av hela molekylära absorptionsband och detektion av flera gaser samtidigt med en kort mättid. Eftersom frekvenskammar har en regelbunden struktur med jämnt separerade laser moder kan man effektivt koppla kammen till en optisk kavitet och därmed möjliggöra frekvenskamsdetektion med hög känslighet. Olika metoder har utvecklats för att utnyttja frekvenskammarnas fulla potential för spektroskopi, baserad på antingen Fouriertransform-spektroskopi eller dispersiva element.Forskningen som presenteras i denna avhandling har haft två huvudmål. Det första har varit att förbättra prestandan hos mekaniska Fourier-transformspektrometrar (FTS) baserat på frekvenskammar med avseende på känslighet, upplösning och spektral täckning. I strävan efter detta har vi utvecklat en ny spektroskopisk teknik, benämnd brusimmun kavitetsförstärkt optisk frekvenskamspektroskopi (NICE-OFCS), och uppnått en hagelbrusbegränsad känslighet och detektionsgränser ner till låga ppb koncentrationer (miljarddelar, 10−9) för CO2 i det när-infraröda frekvensområdet enbart med användning av kommersiellt tillgängliga komponenter. Vi har också utvecklat en ny metod för insamling och analys av kambaserade FTS-spektra, som betecknas ha sub-nominell upplösning. Metoden gör det möjligt att uppnå ultrahög spektral upplösning och hög frekvensnoggrannhet (båda i kHz-området, endast begränsad av kammens stabilitet) över kammens hela frekvensområde. Slutligen har vi utvecklat en optisk parametrisk oscillator som genererar en frekvenskam i det mid-infraröda frekvensområdet, där de starkaste rotations-vibrationsmolekylära absorptionslinjerna finns. Med hjälp av denna kam och en FTS har vi för första gången demonstrerat frekvenskamspektroskopi över 5 μm. Vi har detekterat bredbandsspektra av flera molekylära gaser och har, för mättider på 1 s, uppnått detektionsgränser ner till låga ppb halter för CH4, NO och CO.Det andra syftet har varit mer applikationsorienterat: att använda frekvenskamspektroskopi i förbränningsmiljö och under atmosfäriska förhållanden för snabb och känslig multiämnesdetektion. Vi har för första gången demonstrerat kavitetsförstärkt optisk frekvenskamspektroskopi i en flamma, där vi har detekterat högtemperaturspektra av H2O och OH i det när-infraröda området med användning av FTS och visat teknikens potential för termometrisk karakterisering av flammor. För applikationer som kräver en kort mättid och hög känslighet under atmosfäriska förhållanden har vi utvecklat ett detektionssystem baserat på Vernier-spektroskopi med kontinuerlig filtrering, vilket är en dispersionsbaserad teknik, för första gången i det mid-infraröda frekvensområdet. Det befanns att spektrometern var känslig, snabb, robust och kapabel till multiämnesdetektion med en detektionsgräns på 2 ppb för CH4 för korta mättider (25 ms).

Place, publisher, year, edition, pages
Umeå: Umeå University , 2017. , 164 p.
Keyword [en]
optical frequency comb spectroscopy, molecular absorption, resonant cavity, Fourier transform spectroscopy, Vernier spectroscopy
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
URN: urn:nbn:se:umu:diva-134439ISBN: 978-91-7601-671-8 (print)OAI: oai:DiVA.org:umu-134439DiVA: diva2:1093488
Public defence
2017-06-02, KB.E3.03, Stora Hörsalen, KBC building, Umeå, 13:15 (English)
Opponent
Supervisors
Available from: 2017-05-12 Created: 2017-05-07 Last updated: 2017-05-11Bibliographically approved
List of papers
1. Noise-immune cavity-enhanced optical frequency comb spectroscopy
Open this publication in new window or tab >>Noise-immune cavity-enhanced optical frequency comb spectroscopy
2014 (English)In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 39, no 17, 5034-5037 p.Article in journal (Refereed) Published
Abstract [en]

We present a new method of optical frequency comb spectroscopy that combines cavity enhancement with frequency modulation to obtain immunity to laser frequency-to-amplitude noise conversion by the cavity modes and, thus, high absorption sensitivity over a broad spectral range. A frequency comb is locked to a cavity with a free spectral range (FSR) equal to 4/3 times the repetition rate of the laser, and phase-modulated at a frequency equal to the cavity FSR. The transmitted light is analyzed by a Fourier transform spectrometer with a high bandwidth detector. Phase-sensitive detection of the interferogram yields a noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS) signal. In the first demonstration, we record NICE-OFCS signals from the overtone CO2 band at 1575 nm with absorption sensitivity of 4.3 x 10(-10) cm(-1) Hz(-1/2) per spectral element, close to the shot noise limit.

Place, publisher, year, edition, pages
Optical Society of America, 2014
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-94151 (URN)10.1364/OL.39.005034 (DOI)000341100300016 ()25166067 (PubMedID)
Available from: 2014-10-08 Created: 2014-10-06 Last updated: 2017-05-07Bibliographically approved
2. Cavity-enhanced optical frequency comb spectroscopy of high-temperature H2O in a flame
Open this publication in new window or tab >>Cavity-enhanced optical frequency comb spectroscopy of high-temperature H2O in a flame
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2014 (English)In: Optics Express, ISSN 1094-4087, Vol. 22, no 11, 13889-13895 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate near-infrared cavity-enhanced optical frequency comb spectroscopy of water in a premixed methane/air flat flame. The detection system is based on an Er:fiber femtosecond laser, a high finesse optical cavity containing the flame, and a fast-scanning Fourier transform spectrometer (FTS). High absorption sensitivity is obtained by the combination of a high-bandwidth two-point comb-cavity lock and auto-balanced detection in the FTS. The system allows recording high-temperature water absorption spectra with a resolution of 1 GHz and a bandwidth of 50 nm in an acquisition time of 0.4 s, with absorption sensitivity of 4.2 x 10 (9) cm(-1) Hz(-1/2) per spectral element.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-91203 (URN)10.1364/OE.22.013889 (DOI)000337501600113 ()
Available from: 2014-07-23 Created: 2014-07-21 Last updated: 2017-05-07Bibliographically approved
3. 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, 87-96 p.Article 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: 2017-08-08Bibliographically approved
4. 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, 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: 2017-05-07Bibliographically approved
5. Fourier transform and Vernier spectroscopy using an optical frequency comb at 3-5.4 μm
Open this publication in new window or tab >>Fourier transform and Vernier spectroscopy using an optical frequency comb at 3-5.4 μm
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2016 (English)In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 41, no 11, 2541-2544 p.Article in journal (Refereed) Published
Abstract [en]

We present a versatilemid-infrared frequency comb spectroscopy system based on a doubly resonant optical parametric oscillator tunable in the 3-5.4 mu m range and two detection methods: a Fourier transform spectrometer (FTS) and a continuous-filtering Vernier spectrometer (CF-VS). Using the FTS with a multipass cell, we measure high precision broadband absorption spectra of CH4 at 3.3 mu m and NO at 5.25 mu m, the latter for the first time with comb spectroscopy, and we detect atmospheric species (CH4, CO, CO2, and H2O) in air in the signal and idler ranges. Multiline fitting yields minimum detectable concentrations of 10-20 ppbHz-1/2 for CH4, NO, and CO. For the first time in the mid-infrared, we perform CF-VS using an enhancement cavity, a grating, and a single detector, and we measure the absorption spectrum of CH4 and H2O in ambient air at similar to 3.3 m mu, reaching a 40 ppb concentration detection limit for CH4 in 2 ms. 

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-123438 (URN)10.1364/OL.41.002541 (DOI)000377466000038 ()27244409 (PubMedID)
Available from: 2016-07-20 Created: 2016-07-04 Last updated: 2017-05-07Bibliographically approved
6. Detection of OH in an atmospheric flame at 1.5 μm using optical frequency comb spectroscopy
Open this publication in new window or tab >>Detection of OH in an atmospheric flame at 1.5 μm using optical frequency comb spectroscopy
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2016 (English)In: Photonics Letters of Poland, ISSN 2080-2242, E-ISSN 2080-2242, Vol. 8, no 4, 110-112 p.Article in journal (Refereed) Published
Abstract [en]

We report broadband detection of OH in a premixed CH4/air flat flame at atmospheric pressure using cavity-enhanced absorption spectroscopy based on an Er:fiber femtosecond laser and a Fourier transform spectrometer. By taking ratios of spectra measured at different heights above the burner we separate twenty OH transitions from the largely overlapping water background. We retrieve from fits to the OH lines the relative variation of OH concentration and flame temperature with a height above the burner and compare them with the 1D simulations of flame structure.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-131041 (URN)10.4302/plp.2016.4.07 (DOI)000391182800007 ()
Available from: 2017-02-03 Created: 2017-02-03 Last updated: 2017-05-07Bibliographically approved
7. 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, 358-365 p.Article 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: 2017-05-07Bibliographically approved
8. Optical Frequency Comb Fourier Transform Spectroscopy with Sub-Nominal Resolution - Principles and Implementation
Open this publication in new window or tab >>Optical Frequency Comb Fourier Transform Spectroscopy with Sub-Nominal Resolution - Principles and Implementation
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(English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352Article in journal (Refereed) Submitted
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-134438 (URN)
Available from: 2017-05-07 Created: 2017-05-07 Last updated: 2017-05-11
9. Mid-infrared continuous-filtering Vernier spectroscopy using a doubly resonant optical parametric oscillator
Open this publication in new window or tab >>Mid-infrared continuous-filtering Vernier spectroscopy using a doubly resonant optical parametric oscillator
2017 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 123, no 210Article in journal (Refereed) Published
Abstract [en]

We present a continuous-filtering Vernier spectrometer operating in the 3.15-3.4 mu m range, based on a femtosecond doubly resonant optical parametric oscillator, a cavity with a finesse of 340, a grating mounted on a galvo scanner, and two photodiodes. The spectrometer allows acquisition of one spectrum spanning 250 nm of bandwidth in 25 ms with 8 GHz resolution, sufficient to detect molecular lines at atmospheric pressure. An active lock ensures good frequency and intensity stability of the consecutive spectra and enables continuous signal acquisition and efficient averaging. The relative frequency scale is calibrated using a Fabry-Perot etalon or, alternatively, the galvo scanner position signal. We measure spectra of a calibrated CH4 gas sample as well as dry and laboratory air and extract CH4 and -H2O concentrations by multiline fitting of model spectra. The figure of merit of the spectrometer is 1.7 x 10(-9) cm(-1) Hz(-1/2) per spectral element and the minimum detectable concentration of CH4 is 360 ppt Hz(-1/2), averaging down to 90 ppt after 16 s.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-134435 (URN)000405506200015 ()
Available from: 2017-05-07 Created: 2017-05-07 Last updated: 2017-08-16Bibliographically approved
10. Broadband cavity characterization using a Fourier transform spectrometer with kHz resolution
Open this publication in new window or tab >>Broadband cavity characterization using a Fourier transform spectrometer with kHz resolution
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(English)In: Optica, ISSN 2334-2536Article in journal (Refereed) Submitted
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-134436 (URN)
Available from: 2017-05-07 Created: 2017-05-07 Last updated: 2017-05-11

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