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Doppler-broadened mid-infrared noise-immune cavity-enhanced optical heterodyne molecular spectrometry based on an optical parametric oscillator for trace gas detection
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
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2015 (English)In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 40, no 4, p. 439-442Article 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.

Place, publisher, year, edition, pages
2015. Vol. 40, no 4, p. 439-442
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:umu:diva-101400DOI: 10.1364/OL.40.000439ISI: 000349848400001PubMedID: 25680119OAI: oai:DiVA.org:umu-101400DiVA, id: diva2:839397
Available from: 2015-07-02 Created: 2015-03-30 Last updated: 2018-11-06Bibliographically approved
In thesis
1. Cavity enhanced optical sensing
Open this publication in new window or tab >>Cavity enhanced optical sensing
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Kavitetsförstärkt optisk detektion
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. p. 124
Keywords
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:nbn:se:umu:diva-110278 (URN)978-91-7601-338-0 (ISBN)
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: 2018-06-07Bibliographically approved
2. Mid- and near-infrared NICE-OHMS: techniques for ultra-sensitive detection of molecules in gas phase
Open this publication in new window or tab >>Mid- and near-infrared NICE-OHMS: techniques for ultra-sensitive detection of molecules in gas phase
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) is a technique for ultra-sensitive detection of molecular absorption and dispersion. For highest performance, the technique combines cavity enhancement (CE) with frequency modulation (FM); while the former increases the effective interaction length between the light and the analyte by several orders of magnitudes, the latter removes the in-coupling of 1/f noise and makes the signals background free. The combination of CE and FM also gives the technique an immunity to amplitude noise caused by the jitter of the laser frequency relative to the cavity resonance frequencies. All these properties make the technique suitable for ultra sensitive trace gas detection in the sub-parts-per-trillion (ppt) range. The aim of this thesis is to improve the performance of the NICE-OHMS technique and to increase its range of applications.

The work in this thesis can be divided into three areas:Firstly, a mid-infrared (MIR)-NICE-OHMS instrumentation was developed. In a first realization an unprecedented white-noise equivalent absorption limit for Doppler broadened (Db) detection in the MIR of 3×10-9 cm-1Hz-1/2was demonstrated. This was subsequently improved to 2.4×10-10 cm-1Hz-1/2allowing for detection methane and its two main isotopologues (CH3D and 13CH4) at their natural abundance.Secondly, further development of an existing near-infrared NICE-OHMS system was performed. This resulted in an improved longtime stability and the first shot-noise limited NICE-OHMS system for Db detection with a noise equivalent absorption limit of 2.3×10-14 cm-1detected over 200 s. Thirdly, models and theoretical descriptions of NICE-OHMS signals under strong absorption conditions and from methane under high laser power were developed. It was experimentally verified that the models allow for a more accurate evaluation of NICE-OHMS signals under a wide range of conditions.

Abstract [sv]

Brusimmun kavitetsförstärkt optisk-heterodyndetekterad molekylärspektroskopi (eng.Noise-immune cavity-enhanced optical heterodyne molecular spectrometry, NICE-OHMS) är en teknik för ultrakänslig detektion av molekylär absorption och dispersion. NICE-OHMS-tekniken kombinerar kavitetsförstärkning (eng. CE) med frekvensmodulering (FM); emedan den första väsentligt ökar den effektiva interaktionslängden mellan ljuset och analyten vilket ökar teknikens känslighet, tar den senare bort inkopplingen av 1/f-brus och gör signalerna bakgrundsfria. Kombinationen av CE och FM ger också tekniken en immunitet mot amplitudstörning som orsakas av jitter hos laserljusets frekvens i förhållande till kavitetsresonansfrekvenserna. Alla dessa egenskaper gör tekniken lämplig för ultrakänslig spårgasdetektering i och under ppt (eng. parts-per-trillion) - området. Syftet med denna avhandling är att förbättra prestandan hos NICE-OHMS-tekniken och att öka dess tillämpningspotential.

Avhandlingen kan delas in i tre delar: Inom den första utvecklades en mid-infraröd (MIR)-NICE-OHMS instrumentering. Vid en första realisering påvisades en aldrig tidigare uppnådd vitt-brus-ekvivalent absorptionsgräns för Dopplerbreddad (Db) detektering i MIR området på 3 × 10-9 cm-1Hz-1/2. Detta förbättrades därefter till 2,4 x 10-10 cm-1Hz-1/2, vilket möjliggör detektering av metan och dess två huvudsakliga isotopologer (CH3D och 13CH4) vid deras naturliga förekomst. Inom det andra området utfördes vidareutveckling av ett existerande NICE-OHMS-system verksamt i det när-infraröda (NIR) området. Detta resulterade i en förbättrad långtidsstabilitet och en brus-ekvivalent absorptionsgräns för Db detektion på 2,3 × 10-14 cm-1 mätt över 200 s. Inom den tredje utvecklades modeller och teoretiska beskrivningar av NICE-OHMS under starka absorptionsförhållanden och från metan under hög laserintensitet. Det var experimentellt verifierat att modellerna möjliggör en mer noggrann utvärdering av NICE-OHMS-signalerunder ett stort antal förhållanden.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2018. p. 139
Keywords
spectrometry, NICE-OHMS, trace gas detection, acetylene, methane, isotopologues, near-infrared, mid-infrared, shot-noise, optical parametric oscillator
National Category
Atom and Molecular Physics and Optics
Research subject
Technical Physics
Identifiers
urn:nbn:se:umu:diva-153068 (URN)978-91-7601-977-1 (ISBN)
Public defence
2018-11-30, N450, Naturvetarhuset, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2018-11-09 Created: 2018-11-06 Last updated: 2018-11-13Bibliographically approved

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