Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, NICE-OHMS, is a laser-based spectroscopic detection technique that comprises the concepts of frequency modulation (FM, for reduction of 1/f-noise by detecting the signal at a high frequency) and cavity enhancement (CE, for a prolongation of the optical path length) in a unique way. Properly designed, this gives the technique an intrinsic immunity against the frequency-to-noise conversion that limits many other types of CE techniques. All this gives it an exceptionally high sensitivity for detection of molecular species. Although originally developed for frequency standard purposes in the late 1990s, soon thereafter development of the technique towards molecular spectroscopy and trace gas detection was initiated. This thesis focuses on the further development of Doppler- broadened NICE-OHMS towards an ultra-sensitive detection technique. A number of concepts have been addressed. A few of these are: i) The detection sensitivity of fiber-laser-based NICE- OHMS has been improved to the 10−12 cm−1 range, which for detection of C2H2 corresponds to a few ppt (parts-per-trillion, 1:1012) in gas phase, by improving the locking of the laser to a cavity mode by use of an acousto-optic modulator. ii) It is shown that the system can be realized with a more compact footprint by implementation of a fiber-optic circulator. iii) A systematic and thorough investigation of the experimental conditions that provide maximum signals, referred to as the optimum conditions, e.g. modulation and demodulation conditions and cavity length, has been performed. As a part of this, an expression for the NICE-OHMS line shape beyond the conventional triplet formalism has been proposed and verified. iv) To widen the applicability of NICE-OHMS for detection of pressure broadened signals, also a setup based upon a distributed-feedback (DFB) laser has been realized. v) In this regime, the Voigt profile cannot model signals with the accuracy that is needed for a proper assessment of analyte concentrations. Therefore, the thesis demonstrates the first implementations of line profiles encompassing Dicke narrowing and speed-dependent effects to NICE-OHMS. While such profiles are well-known for absorption, there were no expressions available for their dispersion counterparts. Such expressions have been derived and validated by accompanying experiments. vi) The applicability of the technique for elemental detection, then referred to as NICE-AAS, has been prophesied. / Brusimmun kavitetsförstärkt optisk-heterodyndetekterad molekylärspektroskopi (NICE-OHMS) är en laser-baserad spektroskopisk teknik som förenar frekvensmodulation (för reducring av 1/f-brus genom detektion vid en hög frekvens) och kavitetsförstärkning (KF, för en förlängning av den optiska väglangden) på ett unikt sätt. Korrekt realiserad uppvisar tekniken en inneboende immunitet mot omvandling av frekvensbrus till intensitetsbrus som många andra KF-tekniker är begränsade av. Allt detta ger tekniken en exceptionellt hög känslighet för molekyldetektion. Ursprungligen utvecklad för frekvensstandardändamål i slutet av 1990, har den sedan dess utvecklats för molekylspektroskopi och spårgasdetektering. Denna avhandling fokuserar på vidareutvecklingen av NICE-OHMS mot en tillämpbar, ultrakänslig detektionsteknik. Ett antal koncept har adresserats. Några av dessa är: i) Detektionskänsligheten hos fiberlaserbaserad NICE-OHMS har förbättrats till 10-12 cm-1 området, vilket för detektion av C2H2 i gasfas motsvarar några få ppt (parts per biljon, 1:1012), genom att förbättra låsningen av lasern till en kavitetsmod med hjälp av en akustooptisk modulator. ii) Det har demonstrerats att NICE-OHMS kan realiseras mer kompakt med hjälp av en fiber-kopplad optisk cirkulator. iii) En systematisk och grundlig utredning av de experimentella förhållanden som ger maximala signaler, betecknade de optimala förhållanden, t.ex. modulering och demodulering och kavitetslängden, har utförts. Som ett led i detta har ett uttryck för NICE-OHMS linjeform bortom den konventionella triplett formalismen föreslagits och verifierats. iv) För att bredda tillämpbarheten av NICE-OHMS för detektering av tryckbreddade signaler har även en instrumentering baserad på en distribuerad-återkopplad (eng. distributed feedback, DFB) laser realiserats. v) I detta område kan inte Voigt profilen modellera signalen med den noggrannhet som krävs för en korrekt bedömning av analytkoncentrationer. Därför visar avhandlingen de första implementeringarna i NICE-OHMS av linjeprofiler som inkluderar Dicke avsmalning (eng. Dicke narrowing) och hastighetsberoende effekter (eng. speed-dependent effects). Emedan sådana profiler är välkända för absorption, fanns det inga uttryck för deras dispersiva motparter. Sådana uttryck har därför härletts och validerats av medföljande experiment. vi) Tillämpbarheten av tekniken för detektion av atomer, NICE-AAS, har diskuterats och förutspåtts. / <p>Ytterligare forskningsfinansiär: Kempestiftelserna</p>

Laser spectroscopic techniques have, during the last decades, demonstrated an extraordinary capability for sensitive detection of molecular constituents in gas phase. Since spectra from such techniques constitute unique and characteristic signatures for each type of species, these techniques enable investigations of molecular structures as well as detection of the presence of species in a gas mixture. They are therefore used for a variety of application, from fundamental studies to the assessment of gas concentrations. In fact, quantitative assessments of gas concentrations by laser-based techniques are constantly gaining in popularity, primarily due to properties such as high sensitivity and selectivity and an ability to perform non-invasive measurement. Moreover, investigations of isolated molecular transitions under different conditions provide excellent means to obtain a comprehensive understanding of spectral broadening mechanisms, which is of importance for, for example, environmental sciences and remote sensing applications. In fundamental studies, spectroscopic parameters are often retrieved from fits of a model function of the technique used, which in turn is based upon a suitable lineshape function. In order to obtain parameter values with highest possible accuracy, it is of importance to use the lineshape model that most correctly can predict the measured spectra. Even though the Voigt function is the most commonly used lineshape model when both Doppler and collision broadenings are present, it is not always suitable when spectroscopic parameters are to be assessed with high precision. This thesis represents a thorough investigation of Dicke narrowing and speed-dependent effects, which are phenomena that are not accounted for by the conventional Voigt profile. For the first time, it is demonstrated that both these effects take place not only in absorption but also in the dispersion mode of detection. Their dispersion lineshape functions are first theoretically presumed and explicitly given before they are validated experimentally by the noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS). By using the models developed, it is also shown that although the two modes of detection, absorption and dispersion, both can provide good quality of fits, they do not always provide identical spectroscopic parameters. A detailed analysis under which conditions they do so, and subsequent recommendations of their use, are presented. It also describes the instrumental implementation of a distributed-feed-back (DFB) laser-based NICE-OHMS instrumentation, which constitutes an important step towards the further development of this technique. Due to the wide tunability of the DFB laser, the setup is capable of extending the working range of NICE-OHMS into the collision broadening region, which, in turn, allows for precise spectroscopic studies. The use of a fiber-coupled DFB laser also provides a compact NICE-OHMS system. The minimum detectable on-resonance absorption was assessed to 2× 10-10 cm-1 for a 70 s integration time.

Noise-immune cavity-enhanced optical heterodyne molecular spectro-metry (NICE-OHMS) is one of the most sensitive laser-based absorption techniques. The high sensitivity of NICE-OHMS is obtained by a unique combination of cavity enhancement (for increased interaction length with a sample) with frequency modulation spectrometry (for reduction of noise). Moreover, sub-Doppler detection is possible due to the presence of high intensity counter-propagating waves inside an external resonator, which provides an excellent spectral selectivity. The high sensitivity and selectivity make NICE-OHMS particularly suitable for trace gas detection. Despite this, the technique has so far not been often used for practical applications due to its technical complexity, originating primarily from the requirement of an active stabilization of the laser frequency to a cavity mode. The main aim of the work presented in this thesis has been to develop a simpler and more robust NICE-OHMS instrumentation without compro-mising the high sensitivity and selectivity of the technique. A compact NICE-OHMS setup based on a fiber laser and a fiber-coupled electro-optic modulator has been constructed. The main advantage of the fiber laser is its narrow free-running linewidth, which significantly simplifies the frequency stabilization procedure. It has been demonstrated, using acetylene and carbon dioxide as pilot species, that the system is capable of detecting relative absorption down to 3 × 10-9 on a Doppler-broadened transition, and sub-Doppler optical phase shift down to 1.6 × 10-10, the latter corresponding to a detection limit of 1 × 10-12 atm of C2H2. Moreover, the potential of dual frequency modulation dispersion spectrometry (DFM-DS), an integral part of NICE-OHMS, for concentration measurements has been assessed. This thesis contributes also to the theoretical description of Doppler-broadened and sub-Doppler NICE-OHMS signals, as well as DFM-DS signals. It has been shown that the concentration of an analyte can be deduced from a Doppler-broadened NICE-OHMS signal detected at an arbitrary and unknown detection phase, provided that a fit of the theoretical lineshape to the experimental data is performed. The influence of optical saturation on Doppler-broadened NICE-OHMS signals has been described theoretically and demonstrated experimentally. In particular, it has been shown that the Doppler-broadened dispersion signal is unaffected by optical saturation in the Doppler limit. An expression for the sub-Doppler optical phase shift, valid for high degrees of saturation, has been derived and verified experimentally up to degrees of saturation of 100.