Interferometrian ja valoakustisen spektroskopian soveltaminen taustavapaaseen hivenkaasuanalyysiin

A trace amount of a specific gas in air, breath, or an industrial process can profoundly affect the chemistry and properties of the medium. Therefore, an accurate measurement of the concentration of the trace gas can provide invaluable information. This thesis focuses on the development of trace gas detection methods based on background-free laser absorption spectroscopic techniques. Background-free techniques possess characteristics that greatly benefit the detection of minuscule amounts of gases. These include, for example, scalability with optical power and diminished sensitivity to optical power fluctuations. The thesis deals with two spectroscopic approaches: a novel interferometric method for broadband optical background suppression in absorption spectroscopy, and cantilever-enhanced photo-acoustic spectroscopy. We performed the spectroscopy mainly in the two atmospheric windows of 2000 to 3000 cm^(−1) and 800 to 1200 cm^(−1) found in the mid-infrared region. The employed light sources encompass various broadband and single mode laser devices, including optical parametric oscillators, optical frequency combs, and a quantum cascade laser. The presented results include a demonstration of the interferometric background suppression with a state-of-the-art mid-infrared dual-comb spectrometer. We used the setup to compare the signal-to-noise ratio in direct absorption spectroscopy with and without the background suppression technique. The novel method was found to improve the signal-to-noise ratio by approximately a factor of five. The improvement was limited by the available optical power, and is expected to increase considerably with high power laser light sources. In the cantilever-enhanced photo-acoustic experiments, we investigated the use of high optical power in improving the trace gas detection performance. Using a high power mid-infrared optical parametric oscillator as a laser light source, we reached a record level noise equivalent concentration of 2.5 ppt in 15 s measurement time for hydrogen fluoride. In another work, we reached a record normalised noise equivalent absorption of 1.75×10^(−12) W cm^(−1) Hz^(−1/2) by using an optical build-up cavity to enhance the optical power in the photo-acoustic cell. Lastly, we presented results on hyphenation of the cantilever-enhanced photo-acoustic detector and a gas chromatograph. With the hyphenation, we demonstrated the capability of quantitatively analysing a complex mixture of small to large molecular weight compounds, at a detection sensitivity far better than what can be obtained with a conventional Fourier-transform based infrared detector used in gas chromatography. Quantitative analysis of the sample would have been difficult for laser absorption spectroscopy without the chromatographic separation. The results show a great potential for laser absorption spectroscopy to be used as a detector for gas chromatography in the development of a field deployable multigas analyser.Hivenkaasuilla tarkoitetaan aineita, joita on vain hyvin vähän väliaineessa, kuten ilmassa. Pienistä pitoisuuksista huolimatta hivenkaasuilla voi olla merkittävä vaikutus kaasuseoksen kemiallisiin ominaisuuksiin. Siksi on tärkeää määrittää hivenkaasujen laatu ja pitoisuudet tarkasti. Väitöskirja keskittyy taustavapaiden laserabsorptiospektroskopiaan perustuvien hivenkaasumittausmenetelmien kehittämiseen. Taustavapaalla tarkoitan sitä, että muusta kuin mittauskohteesta tuleva signaali pyritään poistamaan, koska se häiritsee mittausta. Kyseisillä mittausmenetelmillä on erityispiirteitä, jotka sopivat hyvin erittäin pienten pitoisuuksien mittaamiseen. Näihin lukeutuu esimerkiksi havaittavan signaalin voimistuminen mittalaitteen optisen tehon suhteen sekä vähäisempi herkkyys optisen tehon vaihtelulle. Väitöskirjatyössäni hyödynsin kahta spektroskooppista mittausperiaatetta: uudenlaista interferometrista menetelmää laajakaistaisen taustavapaan absorptiospektrin mittaamiseksi sekä läppävahvisteista valoakustista spektroskopiaa. Suoritin mittaukset keski-infrapuna-alueella käyttäen useita erilaisia laservalon lähteitä, kuten optisia parametrivärähtelijöitä, optisia taajuuskampoja sekä kvanttikaskadilasereita. Tutkimustuloksiini lukeutuu kehittämäni uuden interferometrisen taustavapaan mittausmenetelmän havainnollistaminen tämän hetkistä huipputasoa edustavan keski-infrapunakaksoiskampaspektrometrin avulla. Mittauksilla osoitin, että uusi menetelmä parantaa absorptiomittauksen signaali-kohinasuhdetta noin kertoimella viisi verrattuna tavalliseen suoraan absorptiospektroskopiaan. Saavutettua etua rajoitti käytettyjen laserien pieni optinen teho, ja signaali-kohinasuhdetta onkin mahdollista parantaa tulevaisuudessa suurteholasereiden avulla. Tutkimuksissani läppävahvisteisen valoakustisen spektroskopian alalla saavutin ennätyksellisiä havaintoherkkyyksiä hyödyntämällä suuria lasertehoja. Ensimmäisessä tutkimuksessa mittauskohinaa vastaava pitoisuus erittäin haitalliselle fluorivedylle oli 2.5 ppt (2.5 biljoonasosaa) 15 s mittausajalla, kun käytin suuritehoista optista parametrivärähtelijää valonlähteenä. Vastaavasti toisessa tutkimuksessa lasertehoa vahvistavan optisen resonaattorin avulla saavutin 1.75×10^(-12) Wcm^(-1) Hz^(-1/2) suuruisen ennätyksellisen alhaisen normalisoitua mittauskohinaa vastaavan absorption. Erityisen herkkien mittauksien lisäksi kehitin uudenlaisen kaasukromatografian ja läppävahvisteisen valoakustisen spektroskopian yhdistävän menetelmän. Uuden menetelmän avulla on mahdollista analysoida aiempaa luotettavammin monimutkaisia kaasuseoksia, jotka sisältävät sekä pienen että suuren molekyylimassan yhdisteitä. Menetelmä osoittautui jo alustavissa tutkimustuloksissa selvästi herkemmäksi kuin verrattavissa oleva pitkään käytössä ollut kaasukromatografian ja Fourier-muunnos infrapunaspektrometrian yhdistelmä. Tulokset havainnollistavat laserabsorptiospektroskopian soveltuvuuden kehittyneeksi kaasukromatografian ilmaisimeksi etenkin kenttäsovelluksissa, joissa laserien pienestä koosta ja huoltovapaudesta on etua

Laseriin perustuva pitkän matkan lämpötilamittaus

Electronic distance meters are routinely used in many long-distance applications to accurately determine the distance between two points. Currently, the instruments are reaching towards relative uncertainties of 10^-7, the main obstacle being the uncertainty in the refractive index of air. The refractive index is conventionally calculated from the meteorological observations of the ambient air using either Edlen or Ciddor equations. However, the accuracy of these equations is often limited by inaccuracy in the integral meteorological observations. Temperature, which has to be known at ~100 mK level, is by far the most significant uncertainty source in many advanced distance measurements making accurate temperature meters a key for improving the quality of modern ranging instruments. A solution is to use laser absorption spectroscopy for non-invasive measuring of the average temperature of air between the two points of interest. In this thesis, a laser-based thermometer is constructed for accurate determination of the refractive index of air over long distances. The thermometer, which is based on direct absorption spectroscopy of oxygen transition near 770 nm, is demonstrated for the first time in real field conditions up to 864 m in distance in a typical distance measurement configuration. The results prove that the instrument can achieve RMS noise well below 100 mK and accuracy of +/-300 mK using 120 s measurement time, which is to our knowledge the best reported resolution for such a long distance.Elektronisia pituusmittalaitteita käytetään rutiininomaisesti monissa tarkoissa pitkän matkan mittaussovelluksissa. Näistä laitteista parhaimpien suhteelliset mittausepävarmuudet lähestyvät jo suuruusluokkaa 10^-7, jossa merkittävimpänä epävarmuuskomponenttina on usein ilman taitekerroin. Taitekerroin lasketaan yleensä säähavaintojen avulla Edlen- tai Ciddor-yhtälöistä, jolloin ilman lämpötilan mittaustarkkuus on usein rajoittava tekijä. Jotta pituusmittauksessa saavutettaisiin suuruusluokkaa 10^-7 oleva suhteellinen epävarmuus, tulee mitattavan matkan keskimääräinen lämpötila tuntea ~100 mK tarkkuudella. Tarkat lämpötilamittaukset ovat siten avainasemassa modernien pituusmittalaitteiden kehityksessä. Mahdollinen ratkaisu on hyödyntää laserabsorptiospektroskopiaa ilman keskimääräisen lämpötilan määrittämiseksi mitattavalla matkalla. Tässä diplomityössä on toteutettu laseriin perustuva lämpömittari, jonka avulla parannetaan ilman efektiivisen taitekertoimen määrittämistä pitkillä matkoilla. Lämpömittari perustuu suoraan hapen transition absorptiospektroskopiaan lähellä 770 nm aallonpituutta. Instrumentin toiminta havainnollistettiin ensimmäistä kertaa kenttämittauksin aina 864 metrin mittausetäisyyteen asti. Tulokset osoittavat, että laite voi saavuttaa selvästi alle 100 mK kohinatason ja +/-300 mK tarkkuuden käyttäen 120 sekunnin mittausaikaa. Saavutettu resoluutio on tietääksemme paras raportoitu tulos näin pitkälle matkalle

Sub-parts-per-trillion level sensitivity in trace gas detection by cantilever-enhanced photo-acoustic spectroscopy

An exceptional property of photo-acoustic spectroscopy is the zero-background in wavelength modulation configuration while the signal varies linearly as a function of absorbed laser power. Here, we make use of this property by combining a highly sensitive cantilever-enhanced photo-acoustic detector, a particularly stable high-power narrow-linewidth mid-infrared continuous-wave optical parametric oscillator, and a strong absorption cross-section of hydrogen fluoride to demonstrate the ability of cantilever-enhanced photo-acoustic spectroscopy to reach sub-parts-per-trillion level sensitivity in trace gas detection. The high stability of the experimental setup allows long averaging times. A noise equivalent concentration of 650 parts-per-quadrillion is reached in 32 minutes.Peer reviewe

Background-free broadband absorption spectroscopy based on interferometric suppression with a sign-inverted waveform

Background-free methods have potentially superior detection sensitivity because of their ability to take advantage of the full laser power; they are therefore attractive to spectroscopists. We implement background-free Fourier transform spectroscopy based on coherent suppression of the background using an interferometer, whereby the central peak of the interferogram is suppressed without losing molecular absorption signatures. This results in the appearance of peaks rather than dips in the measured spectrum. The technique can be used with a variety of broadband spectroscopies and features advantages such as a reduction in the required detector dynamic range, the capability to perform quantitative measurements, and strongly enhanced sensitivity down to the quantum limit. We validated our method experimentally by performing mid-infrared dual-comb spectroscopy with a mixture of multiple molecular species over a broad wavelength range of 3-5 mu m. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing AgreementPeer reviewe

Cavity-enhanced cantilever-enhanced photo-acoustic spectroscopy

We have improved the sensitivity of a state-of-the-art cantilever-enhanced photo-acoustic trace gas sensor by combining it with an optical power build-up cavity. The build-up cavity enhances the photo-acoustic signal by a factor of approximate to 100, resulting in an exceptionally good normalised noise equivalent absorption (NNEA) value of 1.75 x 10(-12) W cm(-1) Hz(-1/2). We demonstrate the sensor platform in the 1530 nm wavelength range with a simple distributed feedback diode laser, achieving 75 ppt sensitivity for C2H2 with a 10 s integration time.Peer reviewe

Radiocarbon dioxide detection using cantilever-enhanced photoacoustic spectroscopy

Publisher Copyright: © 2021 Optical Society of America.In this Letter, we report on the sub-parts-per-billion-level radiocarbon dioxide detection using cantilever-enhanced photoacoustic spectroscopy. The 14C/C ratio of samples is measured by targeting a 14CO2 absorption line with minimal interference from other CO2 isotopes. Using a quantum cascade laser as a light source allows for a compact experimental setup. In addition, measurements of sample gases with 14CO2 concentrations as low as 100 parts-per-trillion (ppt) are presented. The Allan deviation demonstrates a noise equivalent concentration of 30 ppt at an averaging time of 9 min. The achieved sensitivity validates this method as a suitable alternative to more complex optical detection methods for radiocarbon dioxide detection used so far, and it can be envisioned for future in situ radiocarbon detection.Peer reviewe

Broadband photoacoustic spectroscopy of 14CH4 with a high-power mid-infrared optical frequency comb

We report a photoacoustic spectroscopy setup with a high-power mid-infrared frequency comb as the light source. The setup is used in broadband spectroscopy of radiocarbon methane. Owing to the high sensitivity of a cantilever-enhanced photoacoustic cell and the high-power light source, we can reach a detection limit below 100 ppb in a broadband measurement with a sample volume of only a few milliliters. The first infrared spectrum of 14CH4 is reported and given a preliminary assignment. The results lay a foundation for the development of optical detection systems for radiocarbon methane.We report a photoacoustic spectroscopy setup with a high-power mid-infrared frequency comb as the light source. The setup is used in broadband spectroscopy of radiocarbon methane. Owing to the high sensitivity of a cantilever-enhanced photoacoustic cell and the high-power light source, we can reach a detection limit below 100 ppb in a broadband measurement with a sample volume of only a few milliliters. The first infrared spectrum of (CH4)-C-14 is reported and given a preliminary assignment. The results lay a foundation for the development of optical detection systems for radiocarbon methane. (c) 2019 Optical Society of AmericaPeer reviewe

Broadband Laser-Based Infrared Detector for Gas Chromatography

Cantilever-enhanced photoacoustic spectroscopy coupled with gas chromatography is used to quantitatively analyze a mixture of alcohols in a quasi-online manner. A full identification and quantification of all analytes are achieved based on their spectral fingerprints using a widely tunable continuous-wave laser as a light source. This can be done even in the case of interfering column/septum bleed or simultaneously eluted peaks. The combination of photoacoustic spectroscopy and gas chromatography offers a viable solution for compact and portable instruments in applications that require straightforward analyses with no consumables.Peer reviewe