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1
Ma, Tongmei, and 馬彤梅. "Cavity ringdown laser absorption spectroscopy of free radicals." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2004. http://hub.hku.hk/bib/B30137342.
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Stringer, M. R. "Laser-induced transient absorption spectroscopy of phthalocyanine dyes." Thesis, Open University, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.354989.
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Foo, James. "Laser absorption spectroscopy and tomography of gas flows." Thesis, University of Manchester, 2017. https://www.research.manchester.ac.uk/portal/en/theses/laser-absorption-spectroscopy-and-tomography-of-gas-flows(47a30c34-4290-4b28-bcb4-bbfa94cc5859).html.
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This research focuses on developing optical sensing systems for 2D and 3D spatial monitoring of temperature and concentration distribution profiles of complex or reacting gas flows. Non-invasive, species specific and sensitive nature of monitoring allows spatial information to be extracted from harsh environments with poor physical access, allowing validation of computational models or process monitoring. This is suitable for processes like combustion engines or sealed atmospheric cloud chambers. A novel line-of-sight (LOS) Tunable Diode Laser Absorption Spectroscopy(TDLAS) system using a preselected laser diode centred at 7212.88 cm-1 was first designed to monitor the change of relative humidity (water vapour concentration) during an expansion process within the Manchester Ice Cloud Chamber (MICC), operating from atmospheric pressure, down to 0.7 atm. The experimental results were validated with an Aerosol Cloud Precipitation Interaction Model (ACPIM) simulation, feasible for tomography applications. The MICC shares similar combustion monitoring challenges such as minimal optical access or reactive gas flows. The TDLAS system developed for the MICC was then used as a foundation design for a TDLAS tomography setup capable of conducting temporal two-dimensional (2D) and three-dimensional (3D) concentration and temperature imaging. This system uses the principle of two-line thermometry, centred within the near infrared (NIR) region of 7181.93cm-1 and 7179.8 cm-1. The laser was divided into 4 simultaneous parallel beams using a 1 × 4 fiber coupler (4 LOS). Using a motorised platform, the beams were projected at 0.5° interval, from 0° to 179° angle within 3.6 s, around the exhaust of two asymmetrical shaped flame burners. A total of 360 projection slices comprised of 1440 integrated absorbance data were used per tomogram reconstruction. By solving for the spatial distribution of temperature first, the concentration distribution of water vapour could be then calculated. Reconstruction algorithms (Filtered Back Projection, Fourier Slice Reconstruction and Direct Fourier Reconstruction (DFR)) were compared using a range of criteria. The DFR method was selected as the best method at 700 zero padding, with a spatial in-plane resolution of 1-2 lp/cm, pixel resolution of 128 by 128, thermocouple temperature validations of ±5°C and a relative mean error performance of 8.12%. The concentration could not be validated due to the lack of a mass spectrometer.3D volumetric monitoring results took 36 seconds to complete, and was constructed using 10 interpolated parallel, 1 cm height interval spaced tomograms. Independent vertical slices along the x-axis and y-axis could also be extracted. The temporal results were also successfully conducted and consisted of a quick succession of 16 experiments at a temporal resolution of 0.28 frames per second. A tomographic system that performs 3D and 2D temporal sensing was successfully developed and validated. Although 3D work was conducted using planar imaging or hyperspectral tomography, no work has been conducted so far using NIR TDLAS systems to date.
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Cocola, Lorenzo. "Tunable diode laser absorption spectroscopy for oxygen detection." Doctoral thesis, Università degli studi di Padova, 2012. http://hdl.handle.net/11577/3422063.
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The evolution of diode laser sources for optical communications during the last years led to commercial availability of devices which are suitable for gas absorption spectroscopy in the near and mid infrared. In this work it is shown
how the traditional limits of Tunable Diode Laser Absorption Spectroscopy are addressed with digital signal processing techniques and careful optical design towards the realization of gas sensing instruments with the stability, robustness and reliability that are required in an industrial environment. Being one of the most challenging gases to be sensed with this technique, oxygen was considered under many measurement aspects such as:
• Non invasive monitoring;
• Gas in scattering media sensing;
• Sensing with back-scattering targets;
• Pressure measurement techniques for weak absorption signals;
• Time resolved, dynamic sensing;
• Temperature measurement through absorption spectroscopy.
Many of these aspects were considered together, leading to the developement of instruments tailored for real life industrial applications such as:
• Oxygen sensing in partially transparent containers such as wine or soft
drink bottles;
• Monitoring of double glazing insulating glass gas filling machines;
• Oxygen sensing in containers with backscattering targets such as food
packagings.
Other applications for the technique and experiments involving Gas in Scattering Media Absorption Spectroscopy were explored during a 6 months period at the Lunds Universitet - Lunds Tekniska Högskola - Atomfysik (Sweden) under the supervision of Prof. S. Svanberg:
• Gas probing into porous fruit samples;
• Gas sensing inside the human body as a medical diagnosis technique;
• Oxygen measurement in fully scattering food containers;
• Multi-line absorption spectroscopy as a temperature measurement.<br>L’evoluzione delle sorgenti laser a diodo per le comunicazioni ottiche negli ultimi anni ha portato ad una disponibilità commerciale di dispositivi che si prestano alla spettroscopia di assorbimento di gas nel vicino e medio infrarosso. In questo lavoro si mostra come i limiti tradizionali della spettroscopia di assorbimento a diodi laser sintonizzabili vengano affrontati con tecniche di elaborazione numerica di segnali ed una attenta progettazione ottica rivolta alla realizzazione di strumenti per il rilevamento di gas caratterizzati dalla stabilità, robustezza ed affidabilità necessari per un ambiente industriale. Trattandosi di uno dei gas più critici per il rilevamento con questa tecnica, l’ossigeno è stato affrontato sotto molteplici aspetti di misura come:
• Monitoraggio non invasivo;
• Rilevazione di gas in mezzi diffondenti;
• Rilevazione tramite bersagli retrodiffondenti;
• Tecniche di misura di pressione per deboli segnali di assorbimento;
• Rilevazione dinamica con risoluzione temporale;
• Misure di temperatura attraverso spettroscopia di assorbimento.
Molti di questi aspetti sono stati considerati simultaneamente portando allo
sviluppo di strumenti appropriati ad un uso nel mondo reale in applicazioni
industriali quali:
• Rilevazione di ossigeno in contenitori parzialmente trasparenti come bottiglie di vino e bibite;
• Controllo di macchine per il riempimento di pannelli isolanti in vetrocamera;
• Rilevazione di ossigeno in contenitori con bersagli retrodiffondenti, quali
confezioni alimentari.
Altre applicazioni della tecnica ed esperimenti sulla spettroscopia di assorbimento di gas in mezzi porosi sono stati esplorati durante un periodo di 6 mesi presso Lunds Universitet - Lunds Tekniska Högskola - Atomfysik (Svezia) sotto la supervisione del Prof. S. Svanberg:
• Analisi di gas in campioni porosi di frutta;
• Rilevazione di gas all’interno del corpo umano come tecnica per la diagnostica medica;
• Misura di ossigeno in contenitori completamente diffondenti per alimenti;
• Spettroscopia di assorbimento multi-riga come misura di temperatura.
5
Medhi, Gautam. "Intracavity laser absorption spectroscopy using quantum cascade laser and Fabry-Perot interferometer." Doctoral diss., University of Central Florida, 2011. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/4800.
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Intracavity Laser Absorption Spectroscopy (ICLAS) at IR wavelengths offers an opportunity for spectral sensing of low vapor pressure compounds. We report here an ICLAS system design based on a quantum cascade laser (QCL) at THz (69.9 micrometers]) and IR wavelengths (9.38 and 8.1 micrometers]) with an open external cavity. The sensitivity of such a system is potentially very high due to extraordinarily long effective optical paths that can be achieved in an active cavity. Sensitivity estimation by numerical solution of the laser rate equations for the THz QCL ICLAS system is determined. Experimental development of the external cavity QCL is demonstrated for the two IR wavelengths, as supported by appearance of fine mode structure in the laser spectrum. The 8.1 micrometers] wavelength exhibits a dramatic change in the output spectrum caused by the weak intracavity absorption of acetone. Numerical solution of the laser rate equations yields a sensitivity estimation of acetone partial pressure of 165 mTorr corresponding to ~ 200 ppm. The system is also found sensitive to the humidity in the laboratory air with an absorption coefficient of just 3 x 10[super -7] cm[super -1] indicating a sensitivity of 111 ppm. Reported also is the design of a compact integrated data acquisition and control system. Potential applications include military and commercial sensing for threat compounds such as explosives, chemical gases, biological aerosols, drugs, banned or invasive organisms, bio-medical breath analysis, and terrestrial or planetary atmospheric science.<br>ID: 030646266; System requirements: World Wide Web browser and PDF reader.; Mode of access: World Wide Web.; Thesis (Ph.D.)--University of Central Florida, 2011.; Includes bibliographical references (p. 87-95).<br>Ph.D.<br>Doctorate<br>Physics<br>Sciences<br>Physics
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Nadeau, Patrice. "Measurement of residence time distribution by laser absorption spectroscopy." Thesis, McGill University, 1995. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=22666.
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The residence time distribution was measured at ambient temperature and pressure in a tubular reactor with radial injection at very short space times (0.04-0.7 s). A technique using infrared laser absorption spectroscopy was developed and used to provide the required rapid response for concentration measurements. The equipment comprised an infrared He-Ne laser emitting at a wavelength of 3.39$ mu m$ and a lead selenide detector. Methane, which absorbs strongly at the laser wavelength, was used as the tracer. The absorption of the laser light was related to the tracer concentration by Beer-Lambert law. The laser beam passed through the diameter of the reactor at different axial locations. The residence time distributions were obtained from the response to quasi-step inputs. An axial dispersion model was used to describe the reactor.
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Witonsky, Scott Kenneth 1975. "Kinetics and dynamics measured using IntraCavity Laser Absorption Spectroscopy." Thesis, Massachusetts Institute of Technology, 2002. http://hdl.handle.net/1721.1/8045.
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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2002.<br>Vita.<br>Includes bibliographical references (p. 133-138).<br>IntraCavity Laser Absorption Spectroscopy (ICLAS) is a high-resolution, high sensitivity spectroscopic method capable of measuring line positions, linewidths, lineshapes, and absolute line intensities with a sensitivity that far exceeds that of a traditional multiple pass absorption cell or Fourier Transform spectrometer. From the fundamental knowledge obtained through these measurements, information about the underlying spectroscopy, dynamics, and kinetics of the species interrogated can be derived. The construction of an ICLA Spectrometer will be detailed, and the measurements utilizing ICLAS will be discussed, as well as the theory of operation and modifications of the experimental apparatus. Results include: i) Line intensities and collision-broadening coefficients of the A band of oxygen and previously unobserved, high J, rotational transitions of the A band, hot-band transitions, and transitions of isotopically substituted species. ii) High-resolution (0.013 cm-1) spectra of the second overtone of the OH stretch of trans-nitrous acid recorded between 10,230 and 10,350 cm-1. The spectra were analyzed to yield a complete set of rotational parameters and an absolute band intensity, and two groups of anharmonic perturbations were observed and analyzed. These findings are discussed in the context of the contribution of overtone-mediated processes to OH radical production in the lower atmosphere.<br>(cont.) iii) The implementation of Correlated Double Sampling (CDS) for time-resolved studies of CN fragments generated by the excimer laser photolysis of acrylonitrile. iv) The extension of ICLAS to study the kinetics of a test system. Nitrosyl hydride, HNO, was reacted with oxygen in a flow cell, and the subsequent chemistry was monitored using an electronic transition of HNO. Analysis of the rate equations and time integrated measured signal yielded a preliminary value for the rate constant of the reaction, HNO + 02 [right arrow] products.<br>by Scott Kenneth Witonsky.<br>Ph.D.
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Ekvall, Karin. "Time resolved laser spectroscopy." Doctoral thesis, KTH, Physics, 2000. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3063.
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O'Hagan, Seamus. "Multi-mode absorption spectroscopy for multi-species and multi-parameter sensing." Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:6f422683-7c50-47dd-8824-56b4b4ea941d.
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The extension of Multi-mode Absorption Spectroscopy (MUMAS) to the infra-red spectral region for multi-species gas sensing is reported. A computationally efficient, theoretical model for analysis of MUMAS spectra is presented that avoids approximations used in previous work and treats arbitrary and time-dependent spectral intensity envelopes, thus facilitating the use of commercially available Interband Cascade Lasers (ICLs) and Quantum Cascade Lasers (QCLs). The first use of an ICL for MUMAS is reported using a multi-mode device operating at 3.7 μm to detect CH<sub>4</sub> transitions over a range of 30 nm. Mode-linewidths are measured using the pressure-dependent widths of an isolated absorption feature in HCl. Multi- species sensing is demonstrated by measurement of partial pressures of CH<sub>4</sub>, C<sub>2</sub>H<sub>2</sub> and H<sub>2</sub>CO in a low-pressure mixture with uncertainties of around 10%. Detection of CH<sub>4</sub> in N<sub>2</sub> at 1 bar is demonstrated using a shorter-cavity ICL to resolve spectral features in pressure-broadened and congested spectra. The first use of a QCL for MUMAS is reported using a commercially available device operating at 5.3 μm to detect multiple absorption transitions of NO at a partial pressure of 2.79 μbar in N<sub>2</sub> buffer gas. The revised model is shown to enable good fits to MUMAS data by accounting for the time-variation of the spectral intensity profile during frequency scanning. Individual mode-linewidths are derived from fits to pressure- dependent MUMAS spectra and features from background interferences due to H<sub>2</sub>O in laboratory air are distinguished from those of the target species, NO. Data obtained at scan rates up to 10 kHz demonstrate the potential for achieving short measurement times. The development of a balanced ratiometric detection scheme for MUMAS with commercially available multi-mode lasers operating at 1.5 μm is reported for simultaneous detection of CO and CO<sub>2</sub> showing improved SNR performance over previous direct transmission methods and suitability for a compact field-employable instrument. In addition, MUMAS spectra of CO<sub>2</sub> are used to derive gas temperatures with an uncertainty of 3.2% in the range 300 - 700 K.
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Northern, Jonathen Henry. "Multi-species detection using Infrared Multi-mode Absorption Spectroscopy." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:10f3bd62-4c81-4eaf-854d-1f388af73be9.
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This thesis reports work extending the scope of a recently developed gas sensing technique, multi-mode absorption spectroscopy (MUMAS). The ability of MUMAS to simultaneously detect multiple species from a mixture is demonstrated for the first time. The technique is subsequently extended to mid-infrared wavelengths, realising large gains in sensitivity. A solid-state, multi-mode laser has been developed to provide a high-performance comb source for use with MUMAS. This in-house constructed, diode-pumped, Er/Yb:glass laser operates on 10 longitudinal modes, separated by 18 GHz and centred close to 1565 nm. The extensive development and prototyping work leading to this final laser design is described. Multi-species detection with MUMAS is reported for the first time, thus demonstrating the ability of this technique to perform multi-gas sensing using a single laser and simple detection scheme. The previously described Er/Yb multi-mode laser was used to record MUMAS signals from a sample containing CO, C<sub>2</sub>H<sub>2</sub>, and N<sub>2</sub>O. The components of the mixture were detected simultaneously by identifying multiple transitions in each of the species. Temperature- and pressure-dependent modelled spectral fits to the data were used to determine the partial pressures of each species in the mixture with an uncertainty better than +/-2%. Multi-mode radiation has been successfully generated at 3.3 μm using quasi phase matched difference frequency generation (QPM-DFG). A mid-infrared laser comb was produced by optically mixing the near-infrared, multi-mode comb produced by the previously developed Er/Yb:glass laser with the single-mode output of a Nd:YAG laser operating at 1064 nm. This multi-frequency laser source was characterised to verify performance, and subsequently used to perform proof-of-principle MUMAS measurements on the strong transitions found in this spectral region. Spectra were recorded of NH<sub>3</sub> and CH<sub>4</sub> both individually and as components of a mixture. A minimum detection level for this system was determined to be 4.3 μbar m<sup>-1</sup> for CH<sub>4</sub>, a sensitivity increase of 300 over similar measurements performed in the near-IR.
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陳潔瑩 and Kit-ying Anna Chan. "Near-ir tunable diode laser absorption spectroscopy of gaseous pollutants." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1998. http://hub.hku.hk/bib/B31214940.
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Tütüncü, Erhan [Verfasser]. "Miniaturized gas sensors based on laser absorption spectroscopy / Erhan Tütüncü." Ulm : Universität Ulm, 2019. http://d-nb.info/1200470060/34.
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Goodman, D. A. "X-ray absorption spectroscopy of dense, shock compressed, laser plasma." Thesis, University of Essex, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.303516.
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Liu, Zhuan. "Infrared diode laser absorption spectroscopy of gas phase molecular ions." Thesis, University of Cambridge, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627231.
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Chan, Kit-ying Anna. "Near-ir tunable diode laser absorption spectroscopy of gaseous pollutants /." Hong Kong : University of Hong Kong, 1998. http://sunzi.lib.hku.hk/hkuto/record.jsp?B19905014.
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Leal, Luis Carlos Avila. "Intracavity absorption spectroscopy with Er3+-doped fiber lasers." Master's thesis, Universidade de Aveiro, 2011. http://hdl.handle.net/10773/7371.
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Mestrado em Engenharia Física<br>Espectroscopia de absorção de intracavidade com um laser de banda larga de
fibra dopada com Er3+ é aplicada para medidas resolvidas no tempo de
moléculas de CO2 revelando informações quantitativas sobre a concentração
do gás exalado na respiração humana. A gama espectral das medições
estende-se de 1.52 μm – 1.61 μm através da deslocação da lente de
intracavidade. Com um laser pulsado aplicado nesta experiência, a
sensibilidade à absorção corresponde a um comprimento do percurso de
absorção efectiva de 6 km assumindo que a cavidade está completamente
preenchida com a amostra. O aumento da sensibilidade é alcançada através
da construção de um laser de configuração em anel unidirecional. O
comprimento do percurso de absorção efectiva é aumentado por um factor de
três comparando com uma configuração linear com o mesmo comprimento da
cavidade.<br>Intracavity absorption spectroscopy with a broadband Er3+-doped fiber laser is
applied for time-resolved measurements of CO2 molecules revealing
quantitative information about the gas concentration in exhaled human breath.
The spectral range of measurements extends from 1.52 to 1.61 μm by moving
an intracavity lens. With a pulsed laser applied in this experiment, the sensitivity
to absorption corresponds to an effective absorption path length of 6 km
assuming the cavity is completely filled with the sample. Sensitivity
enhancement is achieved by employing an unidirectional ring laser. The
effective absorption path length is enhanced by a factor of three compared to a
linear configuration with the same cavity length.
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Hassan, Karim H. "Absorption and laser induced fluorescence spectra of some aromatic molecules." Thesis, University of Reading, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.278083.
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Lopez, Mirna Maritza. "Sensitive absorption-based laser wave-mixing spectroscopic methods in microsystems for biomedical applications /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2004. http://wwwlib.umi.com/cr/ucsd/fullcit?p3144304.
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Cummings, Beth L. "Applications of infrared laser spectroscopy to breath analysis." Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:5b6e0624-5982-457c-b13c-61484bace371.
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The work presented in this thesis is concerned with development of spectroscopic detection methods based on absorption spectroscopy using semiconductor lasers, with particular ref- erence to the field of medical diagnostics through breath analysis. The first part of this thesis deals with the design and testing of a prototype analyser for simultaneous monitoring of the exchange gases O<sub>2</sub> , CO<sub>2</sub> and H<sub>2</sub>O in breath. The aim of this analyser is to provide information required to monitor respiration, with potential use in intensive care monitoring or during anaesthesia. The relatively high concentrations of these gases in breath and read- ily available diode laser sources make detection in the near-infrared (NIR) ideal. However, the relatively weakly absorbing A-band O<sub>2</sub> transitions at 760 nm require the application of a sensitive spectroscopic method, cavity enhanced absorption spectroscopy (CEAS). In contrast, CO<sub>2</sub> and H<sub>2</sub>O are monitored using direct single pass absorption spectroscopy, with transitions arising from the 2ν<sub>1</sub> + ν<sub>3</sub> band at 2 μm and ν<sub>1</sub> + ν<sub>3</sub> band at 1.3 μm, respectively. It has been demonstrated that these gases can be detected simultaneously over a short pathlength (2.74 - 4 cm) in the respiratory flow by combining various spectroscopic methodologies and real-time data analysis. This analyser is shown to offer a viable alter- native for monitoring respiration, exhibiting absolute detection limits of changes of 0.26 % O<sub>2</sub> , 0.02 % CO<sub>2</sub> and 0.003 % H<sub>2</sub>O with a 10 ms time resolution, which are comparable to current mass spectrometry based methods, but without their inherent delays. Following this, investigations into the detection of the main gas constituents in breath in the NIR employing noise-reduction modulation based spectroscopic techniques, namely wavelength and frequency modulation (WMS and FMS respectively) are also reported. The described WMS studies on water at 1.37 μm provide a demonstration of conventional WMS detection, as well as a “proof-of-principle” example of a relatively new approach to calibrating the non-absolute information obtained from a WMS absorption signal. Typically WMS spectra are calibrated using mixtures of known gas concentrations or an absolute direct absorption spectrum where possible. In this work however, a self-calibrating method, the phasor decomposition method (PDM), is employed and the returned concentration from this calibration is compared to direct absorption measurement. From this, the calculated concentration using the PDM is found to differ by 9 % from the concentration value obtained by direct absorption, providing an alternative method of calibration for when direct absorption measurements are not possible. The use of FMS in the NIR is also demonstrated as a potential alternative to CEAS for monitoring O<sub>2</sub> at 760 nm. FMS detection is performed on atmospherically broadened O<sub>2</sub> and a time-normalised α<sub>min</sub>(t) of 2.45 ×10<sup>−6</sup> cm<sup>−1</sup> s<sup>1/2</sup> is obtained, which is two orders of magnitude less sensitive than the value of α<sub>min</sub>(t) = 2.35 ×10<sup>−8</sup> cm<sup>−1</sup> s<sup>1/2</sup> obtained with CEAS. This combined with the experimental requirements of an FMS system, make its use for detection of O<sub>2</sub> a less practicable option compared to CEAS for real-time breath analysis. The latter work in this thesis involves a change in focus to detection of trace gases in breath in the mid-infrared (MIR). The move of spectroscopic detection to the MIR exploits the larger absorption cross-sections available in this region, and to achieve this, a relatively new form of semiconductor laser, the quantum cascade laser (QCL) is used. The design of a continuous wave QCL spectrometer at 8 μm and its operating characteristics are demon- strated and improvements in its performances are also discussed. This QCL system is then utilised to demonstrate the potential of monitoring species in breath, namely the narrow- band absorber methane and the broadband absorber acetone, taking into consideration the potential interference from other absorbing species in breath and the different spectroscopic characteristics exhibited by these molecules. Finally, the potential to further improve the sensitive detection of trace gases in breath in the MIR is also investigated with studies on the use of CEAS and multipass cells. In this work, the molecule of interest is the biomarker OCS, using transitions of the 2ν<sub>2</sub> band at 1031 cm<sup>−1</sup> , that are probed using a 10 μm QCL. The application of CEAS in the MIR is not as well developed as in the NIR, and the experimental consequences of using optical cavities at these wavelengths, where equipment tends to be more limited, are investigated and sensitivities discussed in the context of other literature. The experimental procedure of optimising a cavity for CEAS using the off-axis alignment method is also studied in detail, as well as the addition of WMS to further improve the signal quality. An effective absorption pathlength of ∼ 100 m was achieved in the cavity, with a bandwidth reduced α<sub>min</sub>(BW) of 1.7 ×10<sup>−7</sup> cm<sup>−1</sup> Hz<sup>−1/2</sup> using WMS CEAS achieved. With the poorer quality optics and limitations in equipment in the MIR for CEAS experiments, the use of a multipass cell, a 238 m Herriott cell, is also investigated as an alternative to the use of an optical cavity at 10 μm. Detection of OCS using direct absorption and WMS is demonstrated in the Herriott cell, achieving α<sub>min</sub>(BW) = 2.03×10<sup>−8</sup> cm<sup>−1</sup> Hz<sup>−1/2</sup> using WMS. This shows an improvement in sensitivity compared to WMS CEAS, and also shows the potential for future work on biomarker detection, as it approaches the ∼ ppb levels required for breath analysis.
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Meyers, Jason. "Tunable diode laser absorption spectroscopy characterization of impulse hypervelocity CO2 flows." Doctoral thesis, Universite Libre de Bruxelles, 2009. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/210279.
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Tunable diode laser absorption spectroscopy using an external cavity diode laser operating in the infra-red has been developed to monitor CO2 in the freestream of the Longshot hypervelocity facility at the Von Karman Institute for Fluid Dynamics. The Longshot facility offers a unique European facility for ground testing and numerical validation applications, however, some of the traditional data rebuilding aspects are in question. A non-intrusive absorption<p>sensor could significantly aid in improving the knowledge of freestream static values thereby improving the models used in data rebuilding and numerical simulation. The design of such a sensor also expands the spectroscopic capabilities of the Von Karman Institute.<p><p>The absorption sensor is designed around the single P12 (00001)-(30013) rovibrational transition near 1.6µm (6218.09cm-1 specifically) which yields relatively weak direct absorption levels at about 3.5% per meter for typical Longshot freestream conditions. However, when handled carefully, adequate signal-to-noise can be acquired to exploit significant flow information. By being able to operate in this range, total sensor cost can be easily an a factor of two or more cheaper than sensors designed for the deeper infrared. All sensor elements were mounted to a compact portable optics bench utilizing single-mode optical fibers to allow for quick installation at different facilities by eliminating tedious optical realigning. Scans at 600Hz were performed over 20ms of the 40ms test time to extract core static temperature, pressure and velocity.<p><p>These results are compared with the current state of the Longshot data rebuild method. The non-uniform flow properties of the shear layer and test cabin rested gas accumulation was of an initial concern. The temperature and density gradients along with significant radial velocity components could result in DLAS temperature, pressure and velocity that are significantly different than that of the target freestream inviscid core values. Fortunately, with the proper selection of the P12 rotational number, this effect could be more or less ignored as the higher temperature and lower density gas of this region is relatively transparent.<p><p>Ultimately, acquired temperature and density were moderately accurate when compared to Longshot rebuilt results owing primarily to the baseline extraction which poses issues for such low absorption signals. However, the extracted velocity data are quite accurate. This is a definite puls for the sensor as the freestream enthalpy of cold hypersonic facilities is dictated primarily by the kinetic energy contribution. Being able to compare velocity gives insight to the level of vibration non-equilibrium in the flow. The velocity of the DLAS and the Longshot rebuild are quite close. This adds more weight to the argument that vibrational excitation is very low (if present at all) in the free stream and that the van de derWaals equation of state usage and constant specific heat assumption might be an adequate model for the data rebuild after all.<br>Doctorat en Sciences de l'ingénieur<br>info:eu-repo/semantics/nonPublished
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Thompson, Alexander W. J. "Multimode absorption spectroscopy of CO and CO₂ gas mixtures." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:ebcb2031-1165-414e-88d8-69465bdaf291.
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The development of multimode absorption spectroscopy (MUMAS) for multi-species detec- tion and its potential for process control or environmental monitoring is reported. The simultaneous detection of CO and CO2 is demonstrated in a proof-of-principle experiment for applications in industrially relevant gas species monitoring. The technique of MUMAS is extended to the near infrared in order to detect these and other industrially relevant species. A laser was designed and constructed to emit a multimode spectrum in the region of 1.57um to take advantage of the spectral overlap of the second vibrational overtone of CO and the combination band 3ν1 + ν3 of CO2. The laser consisted of a semi-confocal cavity employing an Er:Yb glass chip as the gain medium. The laser was pumped by a 1W laser diode at 980nm and emitted up to 30mW in a bandwidth of 180GHz. The laser emitted between 6-10 modes depending upon the selective cavity length. Mode spacings varied between 18GHz to 33GHz with an individual mode linewidth of less than 8MHz. The laser modes were simultaneously scanned using a piezo-electric transducer (PZT) in order to modulate the cavity length at frequencies between 1Hz and 10Hz. A system for linearizing the MUMAS spectra with respect to frequency was devised based on a transmission spectra of a confocal Fabry-Perot etalon. Refinements to the MUMAS fitting code were developed to improve the computational efficiency. An initial demonstration of MUMAS on a known gas mixture of CO and CO2 was per- formed. The ratio of CO:CO2 concentrations in the gas mixture was measured with an accuracy of 0.4% which was within the supplier’s quoted uncertainty. MUMAS is then applied to the detection of CO and CO2 concentrations in exhaust gas produced by a 1.3 litre 4-cylinder turbo-charged spark ignition engine. Relative and absolute concentrations were derived from MUMAS signals and values compared to measurements using a 4-gas analyser. Concentrations of CO and CO2 were measured using MUMAS to a precision of 0.17% and 0.23% respectively compared to less than 0.1% for the 4-gas analyser. Ratios of CO and CO2 were determined with a precision of 0.28 using MUMAS compared to 0.11 with the 4-gas analyser. The detection limit of CO was found to be 1486ppm in these circumstances. Finally a discussion is presented of potential improvements arising from wavelength mod- ulation spectroscopy and cavity enhancement techniques.
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Burkart, Johannes. "Optical feedback frequency-stabilized cavity ring-down spectroscopy - Highly coherent near-infrared laser sources and metrological applications in molecular absorption spectroscopy." Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GREAY045/document.
Full textAbstract:
La spectroscopie d'absorption moléculaire est un outil incontournable non seulement pour la physique fondamentale et la métrolgie mais aussi pour des domaines aussi divers que les sciences environnementales, la planétologie ou l'astrophysique. Ces dernières années, des techniques spectroscopiques qui exploitent l'amplification résonnante d'interaction entre lumière laser et molécules dans une cavité optique ont fourni des détectivités exceptionnelles sur l'axe d'absorption, tandis que l'axe de fréquence des spectromètres n'atteignait généralement pas le même niveau de précision.Dans cette thèse, nous avons répondu à ce défi en développant la spectroscopie en cavité par temps de déclin stabilisée en fréquence par rétroaction optique (OFFS-CRDS en anglais). Cette nouvelle technique présente une combinaison unique de stabilité et résolution fréquentielles sub-kHz, d'un niveau d'intensité lumineuse intra-cavité de l'ordre du kW/cm^2, d'une detectivite de 2 x 10^(−13) cm^(−1)Hz^(-1/2) limitée par le bruit de photons, et d'une limite de détection de 8.4 x 10^(−14) cm^(−1) sur une plage spectrale étroite. Ces performances inédites sont dues à l'asservissement de la cavité spectroscopique à un laser balayé en fréquence par modulation à bande latérale unique et stabilisé par rétroaction optique avec une cavité en V de réference ultrastable. Pour transférer la cohérence de ce laser sub-kHz à des lasers plus bruiteux dans d'autres gammes spectrales à travers un peigne de fréquence optique, nous avons exploré une nouvelle méthode de clonage de phase par une correction anticipative à large bande passante et démontré une erreur résiduelle de phase de 113 mrad. En appliquant l'OFFS-CRDS à la spectroscopie du CO2 à 1.6 μm, nous avons obtenu un spectre large bande avec une dynamique de 8 x 10^5, et nous avons déterminé douze fréquences de transition absolues avec une exactitude de l'ordre du kHz en mesurant des Lamb dips sub-Doppler en absorption saturée avec un dispositif équipé d'un peigne de fréquence. Par ailleurs, nous avons procédé à une analyse détaillée des sources d'erreurs systematiques en CRDS et nous avons déduit une formule analytique pour le déclin de cavité non-exponentiel dans un régime faiblement saturé qui est susceptible de contribuer à de futures mesures de moments de transition dipolaire indépendantes de la concentration. Nos résultats ouvrent des perspectives prometteuses pour des applications métrologiques de l'OFFS-CRDS, comme par exemple l'étude de profils de raie poussés, la mesures de rapports isotopiques et la spectroscopie d'absorption saturée extensive dans le proche infrarouge<br>High-precision molecular absorption spectroscopy is a powerful tool for fundamental physics and metrology, as well as for a broad range of applications in fields such as environmental sciences, planetology and astrophysics. In recent years, spectroscopic techniques based on the enhanced interaction of laser light with molecular samples in high-finesse optical cavities have provided outstanding detection sensitivities on the absorption axis, while the spectrometer frequency axis rarely met as high precision standards.In this thesis, we addressed this challenge by the development of Optical Feedback Frequency-Stabilized Cavity Ring-Down Spectroscopy (OFFS-CRDS). This novel technique features a unique combination of sub-kHz frequency resolution and stability, kW/cm^2-level intracavity light intensity, a shot-noise limited absorption detectivity down to 2 x 10^(−13) cm^(−1)Hz^(-1/2), as well as a detection limit of 8.4 x 10^(−14) cm^(−1) on a narrow spectral interval. This unprecedented performance is based on the tight Pound-Drever-Hall lock of the ring-down cavity to a single-sideband-tuned distributed-feedback diode laser which is optical-feedback-stabilized to a highly stable V-shaped reference cavity. To transfer the coherence of this sub-kHz laser source to noisier lasers in other spectral regions through an optical frequency comb, we have explored a novel high-bandwidth feed-forward phase cloning scheme and demonstrated a residual phase error as low as 113 mrad. Applying OFFS-CRDS to the spectroscopy of CO_2 near 1.6 μm, we obtained a broadband spectrum with a dynamic range of 8 x 10^5 and retrieved twelve absolute transition frequencies with kHz-accuracy by measuring sub-Doppler saturated absorption Lamb dips with a comb-assisted setup. Furthermore, we have performed a comprehensive analysis of systematic error sources in CRDS and derived an analytic formula for the non-exponential ring-down signal in a weakly saturated regime, which may contribute towards future concentration-independent transition dipole moment measurements. Our results open up promising perspectives for metrological applications of OFFS-CRDS, such as advanced absorption lineshape studies, isotopic ratio measurements and extensive saturated absorption spectroscopy in the near infrared
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Golebiowski, Dariusz. "High resolution FTIR spectroscopy using a femto-OPO laser source and cavity enhanced absorption." Doctoral thesis, Universite Libre de Bruxelles, 2015. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/221150.
Full textAbstract:
Dans cette thèse, nous améliorons et utilisons le montage expérimental développé au laboratoire nommé femto/OPO-FT-CEAS. Ce montage combine une source laser femto/OPO, une cavité optique haute finesse et un interféromètre à transformée de Fourier. Il permet d'enregistrer des spectres sur un intervalle de 150 cm-1, avec un coefficient d'absorption minimal de 3x10-9 cm-1, à une résolution de 2x10-2 cm-1 et un temps d’acquisition de 2 heures. Un chemin d'absorption de 20 km a été obtenu dans une cellule de 145 cm. Différents miroirs à hauts indices de réflexion permettent d'accéder à deux gamme spectrales dans le domaine de l'infrarouge proche :6200-6700 cm-1 et 7700-8300 cm-1.Le montage femto/OPO-FT-CEAS a été utilisé afin d'enregistrer des spectres à température ambiante. La molécule OCS a été étudiée dans les gammes spectrales de 6200 à 6700 cm-1 et 7700 à 8300 cm-1. Les nouvelles données rovibrationnelles ont été intégrées au modèle global développé par le Prof. Fayt de l'université catholique de Louvain. Un échantillon de CO2 enrichi en oxygène 17 a également été étudié dans la gamme spectrale de 7700 à 8300 cm-1. Les données ont été traitées avec l'aide du Dr. Lyulin l'institut d'optique atmosphérique de Tomsk, Russie.Le montage femto/OPO-FT-CEAS a également été modifié pour enregistrer des spectres de molécules refroidies au sein d'un jet supersonique. Les molécules de N2O, C2H4 et H12C13CH en abondance isotopique naturelle ont été étudiées. Le montage permet de refroidir les molécules étudiées jusqu'à 10 K et un coefficient de 5x10-8 cm-1 a été obtenu. Ce montage a également permis d'enregistrer des spectres CEAS et CRDS de NH3 à des températures de 17 et 14 K respectivement. L'analyse des spectres aété réalisée avec l'aide des Profs. Fusina et Di Lonardo de l'Université de Bologne, Italie.Une cellule de 145 cm pouvant être refroidie à l'aide de réfrigérants liquides a également été développée en vue de remplacer une cellule à température ambiante de 77 cm utilisée dans le montage femto/OPO-FT-CEAS.Enfin, les montages FANTASIO+ et femto/OPO-FT-CEAS ont été utilisés afin afin d'enregistrer des spectres de HCOOH à température ambiante et à 10 K. Les données ont été traitées avec l'aide du Dr. Perrin de l'Université Paris-Créteil, France.<br>Doctorat en Sciences<br>info:eu-repo/semantics/nonPublished
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Shrestha, Jyoti Chandra [Verfasser]. "Theoretical and experimental study of intracavity laser absorption spectroscopy / Jyoti Chandra Shrestha." Kassel : Universitätsbibliothek Kassel, 2015. http://d-nb.info/1069569550/34.
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Dang, Dung Do. "Tuneable Diode Laser Absorption Spectroscopy with Optical Fiber : Noise Analysis and Noise Reduction." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for elektronikk og telekommunikasjon, 2006. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-13089.
Full textAbstract:
A prototype of a fiber based absorptionspectroscopy instrument is built. A single mode fiber pigtailDistributed Feedback laser (DFB) is used to scan a known$mbox{NH}_3$ absorption line near $1512nm$ ($6614cm^{-1}$). $2mm$diameter InGaAs PIN photodiodes are used to convert the light signalto electrical signals by use of special designed transimpedanceamplifiers. Laser modulation, coherent detection at twice themodulation frequency (second harmonic detection), other electronics,and digital signal processing are provided by Norsk Elektrooptikk.bigskip The DFB laser from NEL (NLK1S5EAAA) delivers 10mW fiberoutput and has maximum output around 30mW at maximum injectioncurrent of 200mA. This laser was found to be very quiet at dcoperation. However, in wavelength modulation spectroscopy operationit exhibit large modulation noise that was found to stem from thefiber coupling inside the laser module. A detection limit of$6times10^{-5}$ to $1times10^{-4}$ in absorbance unit with 0.712moptical path length and an effective measurement bandwidth of 52Hzwas achieved by this laser. This is significantly above thedetection limit with free air lasers which achieve $3times10^{-6}$relative absorbance (0.15ppm $mbox{NH}_3$ at room temperature andatmospheric pressure). The acquisition time for each concentrationmeasurement takes 3.2s. Hence, the response time is very fast whichis a common property of wavelength modulation spectroscopy.bigskip The modulation noise problem was attacked by an electronicnoise cancellation scheme. The idea was that since the noise sourceis inside the laser module and no external optical component can beused to reduce it. With great linearity and reliability propertiesof fused biconical taper (FBT) couplers the laser beam is split intotwo identical beams with nearly equal amount of the same noise. Onebeam, signal beam, is used to induce second harmonic signals in thegaseous path. The other beam goes directly to an identicalphotodetector and is used as a reference signal. Two specialdesigned noise canceller circuits were built to remove commonsignals. Both the circuits, based on current subtraction (notoptimized) or voltage subtraction, give comparable results. Themodulation noise was reduced by more than 12dB. A detection limit of$5times10^{-6}$ was achieved with this prototype. Even aftertransmitting the laser beam through a 3200 meter single mode fiber asignal to noise ratio (relative to 100ppm signal) of $180times$.Hence, a sensitivity is $1.1times10^{-5}$ in absorbance unit wasachieved with the basic noise canceller (not optimized). Thiscorresponds to a detection limit of 0.55ppm ammonia (12.5ppm withoutcancellation). The optimized noise canceller could reduce this by afactor of 3 or more.bigskipThe basic noise canceller based on current domain subtraction wasinvestigated in detail and optimized. This new circuit suppressedthe intensity modulation signal by more than 70dB. The 2f noise wasreduced to approximately 50ppb or $1.1times10^{-6}$ in absorbanceunit with an effective noise bandwidth of 52.3Hz and 1m optical pathlength. This is a record sensitivity with such simple electronics.The sensitivity of our fiber based prototype is limited by thefluctuating etalon fringing in the signal beam. Under normaloperation the prototype achieves approximately $pm20$ppb zero pointdrift in the concentration with an acquisition time of 48s (3.5Hznoise bandwidth). By our knowledge these results are the bestavailable and are competitive with the much more complex frequencymodulation spectroscopy technique.bigskipThese balanced transimpedance amplifiers also have the ability tomeasure the absorption spectra directly without modulation.Determining the absorption linewidth and its shape is now a straightforward task. By use of dual beam configuration and noisecancellation thermal effects that caused background fluctuationswere greatly suppressed. The laser delivers high intensity light,$sim22mW$. The dominating noise is the photocurrent shot noise andthe etalon fringing from optical components in the signal path.Remote sensing, in-situ, noninvasive measurements can be performedwith this instrument. In some cases it's flexibility, and remotesensing possibility are of great attractiveness for the industry.The sensitivity is also improved by a factor of two with the fiberbased tuneable diode laser absorption spectroscopy instrument.bigskipIn the end of this thesis we demonstrate the ability to measuremultiple absorption lines simultaneously. The number of measurementpaths is limited by the laser output power.
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Hopper, David J. "Investigation of laser frequency stabilisation using modulation transfer spectroscopy." Thesis, Queensland University of Technology, 2008. https://eprints.qut.edu.au/16667/1/David_John_Hopper_Thesis.pdf.
Full textAbstract:
Optical frequency standards are necessary tools for accurate measurement of time and length. In practice these standards are stabilised laser systems locked to a known frequency reference. These references are typically the resonant frequencies of the atoms of an absorption medium that have been theoretically calculated to a high degree of accuracy. This thesis describes a combination of experimental and theoretical research performed on modulation transfer spectroscopy (MTS)--a technique used to frequency stabilise a laser in order to produce an accurate frequency reference--with emphasis placed on developing techniques and procedures to overcome the limitations found in existing MTS stabilised laser systems. The focus of the thesis is to generate a highly accurate frequency reference by researching the system parameters that will increase the signal to noise ratio and improve the accuracy of the reference through refinement of the signal structure.
The early theoretical interpretation of MTS was effectively a low absorption approximation that occurs at low pressures. This approximation ignores the depletion of beam energy through absorption and is a distinct limitation of the theoretical model in its ability to accurately predict the influence of a range of system parameters on signal strength and structure. To overcome this limitation a 3-D (or volumetric) analysis was developed and is presented here for the first time. This volumetric model is a measure of two depleted beams interacting collinearly in an absorbing medium of iodine and is described to accurately predict the signal maximum as a function of pressure for all wavelengths. This model was found to be more accurate in predicting the influence of system parameters on the signal strength and structure, including that of pump beam intensity, pressure, saturation parameter, cell length and modulation parameters.
The volumetric model is a novel approach to MTS theory but is more complex computationally than the traditional low pressure model and therefore more difficult to implement in many situations. To overcome this problem a hybrid model was developed as a combination of the low pressure and volumetric models.
The comparison between the rigorous volume model and the hybrid model indicate that there is a deviation in the signal strength at high pressures. However, the agreement was very good in the pressure regimes that are commonly used to realise actual frequency references. Comparison of the hybrid model to experimental data was performed over a range of different wavelengths (532 nm, 543.5 nm, 612 nm and 633 nm) and found to be in close agreement. This gives confidence in the model to accurately predict signal strength and structure in any situation.
Three mechanisms have been identified that limit the accuracy of frequency references due to the creation of residual amplitude modulation (RAM) where it shifts the frequency of the reference. The influence of RAM is included in the hybrid model as a ratio of the amplitude modulated and frequency modulated components of the saturating beam. These RAM production mechanisms result from the modulation of the saturating beam, the overlap of the beams in the medium, and the differential absorption of the sidebands in the medium. While the first mechanism has been previously reported the latter two are discussed here in detail for the first time. RAM generated by the modulators used (acousto-optic or electro-optic modulators) was typically of the order of 10% to 12%, depending on the excursion of the created sidebands. RAM generated by an asymmetric beam overlap with the modulators used was found to be as large as 30%. A combination of these two independent mechanisms can be used to provide a "RAM-free" state of the system by using one to cancel the effects of the other. The third RAM generation process--medium induced RAM--is difficult to remove but through a careful combination of absorption related parameters--namely, pump intensity, cell length, pressure and detector phase--the effects of RAM can be removed, leading to a distortion free MTS signal.
Further investigation into the predictions provided by the hybrid model shows that there is a complex relationship between cell length and the optimum pressure required for maximum signal strength, such that longer cell lengths will not necessarily improve the signal strength. This is contrary to conventional thinking and is important in the MTS design process to reduce unnecessary costs and improve the signal to noise ratio and frequency accuracy. Optimisation of frequency stabilised laser systems using MTS are generally performed using trial and error. Comparison of these optimum parameter values to those predicted by the hybrid model show that for popular wavelengths such as 532 nm they are similar.
In addition, the hybrid model is able to predict the frequency shifts that arise within the system parameters used and has shown that existing systems being used at 532 nm, 633 nm and 778 nm could improve their signal to noise ratio and accuracy through a variation in the parameters. A methodology based on the hybrid model is presented that can be used to calculate the optimum parameters for maximum signal strength and a "RAM-free" state for any wavelength. This systematic approach can therefore be used to guide the design of actual frequency stabilised laser systems prior to and during the design process.
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Hopper, David J. "Investigation of laser frequency stabilisation using modulation transfer spectroscopy." Queensland University of Technology, 2008. http://eprints.qut.edu.au/16667/.
Full textAbstract:
Optical frequency standards are necessary tools for accurate measurement of time and length. In practice these standards are stabilised laser systems locked to a known frequency reference. These references are typically the resonant frequencies of the atoms of an absorption medium that have been theoretically calculated to a high degree of accuracy. This thesis describes a combination of experimental and theoretical research performed on modulation transfer spectroscopy (MTS)--a technique used to frequency stabilise a laser in order to produce an accurate frequency reference--with emphasis placed on developing techniques and procedures to overcome the limitations found in existing MTS stabilised laser systems. The focus of the thesis is to generate a highly accurate frequency reference by researching the system parameters that will increase the signal to noise ratio and improve the accuracy of the reference through refinement of the signal structure. The early theoretical interpretation of MTS was effectively a low absorption approximation that occurs at low pressures. This approximation ignores the depletion of beam energy through absorption and is a distinct limitation of the theoretical model in its ability to accurately predict the influence of a range of system parameters on signal strength and structure. To overcome this limitation a 3-D (or volumetric) analysis was developed and is presented here for the first time. This volumetric model is a measure of two depleted beams interacting collinearly in an absorbing medium of iodine and is described to accurately predict the signal maximum as a function of pressure for all wavelengths. This model was found to be more accurate in predicting the influence of system parameters on the signal strength and structure, including that of pump beam intensity, pressure, saturation parameter, cell length and modulation parameters. The volumetric model is a novel approach to MTS theory but is more complex computationally than the traditional low pressure model and therefore more difficult to implement in many situations. To overcome this problem a hybrid model was developed as a combination of the low pressure and volumetric models. The comparison between the rigorous volume model and the hybrid model indicate that there is a deviation in the signal strength at high pressures. However, the agreement was very good in the pressure regimes that are commonly used to realise actual frequency references. Comparison of the hybrid model to experimental data was performed over a range of different wavelengths (532 nm, 543.5 nm, 612 nm and 633 nm) and found to be in close agreement. This gives confidence in the model to accurately predict signal strength and structure in any situation. Three mechanisms have been identified that limit the accuracy of frequency references due to the creation of residual amplitude modulation (RAM) where it shifts the frequency of the reference. The influence of RAM is included in the hybrid model as a ratio of the amplitude modulated and frequency modulated components of the saturating beam. These RAM production mechanisms result from the modulation of the saturating beam, the overlap of the beams in the medium, and the differential absorption of the sidebands in the medium. While the first mechanism has been previously reported the latter two are discussed here in detail for the first time. RAM generated by the modulators used (acousto-optic or electro-optic modulators) was typically of the order of 10% to 12%, depending on the excursion of the created sidebands. RAM generated by an asymmetric beam overlap with the modulators used was found to be as large as 30%. A combination of these two independent mechanisms can be used to provide a "RAM-free" state of the system by using one to cancel the effects of the other. The third RAM generation process--medium induced RAM--is difficult to remove but through a careful combination of absorption related parameters--namely, pump intensity, cell length, pressure and detector phase--the effects of RAM can be removed, leading to a distortion free MTS signal. Further investigation into the predictions provided by the hybrid model shows that there is a complex relationship between cell length and the optimum pressure required for maximum signal strength, such that longer cell lengths will not necessarily improve the signal strength. This is contrary to conventional thinking and is important in the MTS design process to reduce unnecessary costs and improve the signal to noise ratio and frequency accuracy. Optimisation of frequency stabilised laser systems using MTS are generally performed using trial and error. Comparison of these optimum parameter values to those predicted by the hybrid model show that for popular wavelengths such as 532 nm they are similar. In addition, the hybrid model is able to predict the frequency shifts that arise within the system parameters used and has shown that existing systems being used at 532 nm, 633 nm and 778 nm could improve their signal to noise ratio and accuracy through a variation in the parameters. A methodology based on the hybrid model is presented that can be used to calculate the optimum parameters for maximum signal strength and a "RAM-free" state for any wavelength. This systematic approach can therefore be used to guide the design of actual frequency stabilised laser systems prior to and during the design process.
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Rushworth, Cathy M. "Analysis of small volume liquid samples using cavity enhanced absorption spectroscopies." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:df00e411-4be9-4816-ae46-a1fe948053b6.
Full textAbstract:
Cavity enhanced absorption spectroscopies have earned themselves a place as one of the methods of choice for sensitive absorption measurements on gas-phase samples, but their application to liquid samples has so far been more limited. Sensitive short pathlength analysis of liquid samples is required for online analysis of microfluidic samples, which are processed in channels with dimensions of tens to hundreds of micrometres. Microfluidics is important for a range of applications including drug discovery and environmental sensing. This thesis explores the application of cavity enhanced absorption spectroscopies to short pathlength (0.010 mm to 2 mm) analysis of sub-microlitre volumes of liquids. Three experimental set-ups have been been examined. Firstly, a single-wavelength cavity ringdown (CRD) spectrometer operating at 532 nm was assembled using two 99.8% reflectivity mirrors. High optical quality flow cells with short pathlengths ranging from 0.1 mm to 2 mm were inserted into this cavity at Brewster’s angle. The detection limit of the set-up with each inserted flow cell was established using a concentration series of aqueous potassium permanganate (KMnO₄) solutions. For the 1 mm flow cell, a detection limit of 29 nM KMnO₄ or 1.4 x 10⁻⁴ cm⁻¹ was established. Several different types of microfluidic devices were also inserted into the cavity, and it was found that the losses arising from the inserted chip were highly dependent on the method of chip manufacture. The CRD set-up with inserted 1 mm flow cell was applied to the detection of two important species, nitrite and iron(II), via analyte-specific colourimetric reactions. Detection limits of 1.9 nM nitrite and 3.8 nM iron(II) were established. The second experimental set-up utilised broadband, supercontinuum light generated in a 20 m length of nonlinear photonic crystal fibre. Broadband mirrors with around 99% reflectivity over the wavelength range from 400 to 800 nm were used to form the cavity, and a miniature spectrometer was used to wavelength-resolve the time-integrated cavity output. Flow cells and microfluidic chips were inserted into the cavity either at normal incidence or at Brewster’s angle. This set-up was employed for reaction analysis of an iron complexation reaction with bathophenanthroline, and for a model organic reaction, the Diels-Alder reaction between anthracene and 4-phenyl-1,2,4-triazoline-3,5-dione. The same broadband set-up was also used for pH measurements using bromocresol green indicator solution. Using dual-wavelength CRD spectroscopy, the pH sensitivity was established to be around a few milli pH units. Finally, an alternative type of cavity, formed from a loop of optical fibre has been investigated. A novel light-coupler was designed and fabricated in 365 μm core diameter multimode optical fibre. Sample designs employing both direct and evanescent wave absorption were investigated in small-core and large-core optical fibres, and the lowest detection limit of 0.11 cm⁻¹ was determined in direct absorption measurements, with a pathlength of 180 μm, using our novel light coupler in 365 μm core diameter optical fibre.
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Anderson, Thomas Nathan. "The development and application of a diode-laser-based ultraviolet absorption sensor for nitric oxide." Thesis, Texas A&M University, 2003. http://hdl.handle.net/1969/112.
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Wyngaard, Adrian Leigh. "Saturated absorption spectroscopy of rubidium and feedback control of LASER frequency for Doppler cooling." Thesis, Cape Peninsula University of Technology, 2018. http://hdl.handle.net/20.500.11838/2621.
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Thesis (MTech (Electrical Engineering))--Cape Peninsula University of Technology, 2018.<br>This research investigates the absorption spectra of rubidium and the feedback
control of an external cavity diode laser. This research is a necessary
prerequisite for laser (Doppler) cooling and trapping of rubidium atoms.
Cooling rubidium atoms down to such low temperatures can be achieved
using the Doppler cooling technique. Here a laser is tuned to remain resonant
with a speci c atomic transition. To do this, the absorption spectra of
rubidium must therefore be observed. All of the above require a reasonable
knowledge about topics such as atomic physics, laser cooling and trapping,
feedback control systems, and absorption spectroscopy. A discussion of these
topics is provided.
We have utilised an experimental setup which allowed for measurements
of the Doppler broadened and Doppler free absorption spectra of rubidium,
as well the analysis of the Zeeman e ect on the Doppler free spectra. The
setup consisted of a saturated absorption spectrometer for high resolution
spectroscopy and a Michelson interferometer for calibrating our measurements.
In analysing the Zeeman e ect we added a set of Helmholtz coils to
the saturated absorption spectroscopy arrangement to measure the splitting
of the hyper ne energy levels.<br>French South African Institute of Technology (F'SATI)
National Research Foundation
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Norén, Edvin. "Simultaneous detection of potassium, water vapor and temperature with tunable diode laser absorption spectroscopy." Thesis, Umeå universitet, Institutionen för fysik, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-105950.
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Existing tunable diode laser absorption spectroscopy (TDLAS) sensors for potassium (K) and for water vapor (H2O) and temperature were combined to enable simultaneous measurements in combustion and gasification processes. In-situ real-time detection of the above mentioned combustion parameters will improve the understanding of ash-formation during thermochemical conversion of biomass. Simultaneous measurements facilitate the experimental procedure and decrease the methodological uncertainty introduced by the heterogeneous nature of the pellets. The K sensor is based on direct absorption spectroscopy (DAS), whereas the H2O system employs wavelength modulation spectroscopy (WMS) together with two-line thermometry for temperature assessment. Two methods for combining the laser beams were evaluated, the first involving dichroic elements, the second using available fiber optic combiners. The latter method was considered advantageous. An existing LabVIEW program was modified to allow for simultaneous signal generation and data acquisition for both sensors. The sensors were then tested separately in a low pressure K cell and in ambient air. The combined sensor was applied to simultaneous measurements above various pelletized biofuels during combustion in a single pellet reactor (SPR). Significant difference in absolute concentration and time histories were observed between fuels, in particular for K and temperature. Significant K concentrations were only observed during the devolatilization phase. The combined sensor will be useful in fundamental combustion research.
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Hendricks, Adam Gerald. "Determination of Flame Dynamics for Unsteady Combustion Systems using Tunable Diode Laser Absorption Spectroscopy." Thesis, Virginia Tech, 2003. http://hdl.handle.net/10919/36293.
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Lean, premixed combustion has enjoyed increased application due to the need to reduce pollutant emissions. Unfortunately, operating the flame at lean conditions increases susceptibility to thermoacoustic (TA) instability. Self-excited TA instabilities are a result of the coupling of the unsteady heat release rate of the flame with the acoustics of the combustion chamber. The result is large pressure oscillations that degrade performance and durability of combustion systems.
Industry currently has no reliable tool to predict instabilities a priori. CFD simulations of full-scale, turbulent, reacting flows remain unrealizable. The work in this paper is part of a study that focuses on developing compact models of TA instabilities, i.e. acoustics and flame dynamics. Flame dynamics are defined as the response in heat release to acoustic perturbations. Models of flame dynamics can be coupled with models of combustor enclosure acoustics to predict TA instabilities. In addition, algorithms to actively control instabilities can be based on these compact models of flame dynamics and acoustics.
The work outlined in this thesis aims at determining the flame dynamics model experimentally. Velocity perturbations are imparted on laminar and turbulent flames via a loudspeaker upstream of the flame. The response of the flame is observed through two measurements. Hydroxyl radical (OH*) chemiluminescence indicates the response in chemical reaction rate. Tunable Diode Laser Absorption Spectroscopy (TDLAS), centered over two water absorption features, allows a dynamic measurement of the product gas temperature. The response in product gas temperature directly relates to the enthalpy fluctuations that couple to the acoustics.
Experimental frequency response functions of a laminar, flat-flame burner and a turbulent, swirl-stabilized combustor will be presented as well as empirical low-order models of flame dynamics.<br>Master of Science
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Li, Hejie. "Near-infrared diode laser absorption spectroscopy with applications to reactive systems and combustion control /." May be available electronically:, 2007. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.
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de, Ghellinck d'Elseghem Vaernewijck Xavier. "Instrumental contribution to FTIR spectroscopy using a femto-OPO laser source and cavity enhanced absorption." Doctoral thesis, Universite Libre de Bruxelles, 2012. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/209629.
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L'expérience nommée femto-FT-CEAS a été mise en place durant ce travail. Elle<p>combine une source d'absorption laser femto-opo, une cavité optique haute finesse<p>et un interféromètre à transformée de Fourier. Ce montage instrumental a permis<p>d'enregistrer des spectres sur un domaine de 100 cm−1 avec un coefficient d'absorption<p>minimal détectable de 5 10−10cm−1 pour des paramètres de résolution spectrale de<p>2 10−2cm−1 et un temps d'enregistrement d'une heure. Le coefficient d'absorption<p>equivalent par moyenne quadratique par élément spectral correspondant est de 1.2<p>10−12cm−1 Hz−1/2. Un chemin optique équivalent de 13 km a été obtenu dans une<p>cellule de 77 cm de long. La gamme spectrale accessible est de 6000 − 6700 cm−1 et<p>7800 − 8300 cm−1.<p>Les spectres enregistrés à l'aide du montage femto-FT-CEAS ont permis d'attribuer<p>plus de 1000 transitions rovibrationelles d'échantillons de CO2 enrichis en 17O,<p>d'OCS, CS2, H12C13CD, et CH3CCH. Les bandes suivantes ont été enrégistrées :<p>pour 12C17O2, les bandes 30013e, 30012e, 30011e, 31113e, 31113f, 31112e, 31112f,<p>et 31111e; pour 17O12C18O, les bandes 30013e, 30012e, et 30011e; pour 16O12C32S,<p>les bandes 540, 0, 501, 0, 511, 1e, 511, 1f, 601, 0, 460, 0, 421, 0, 431, 1e, 431, 1f,<p>521, 0, 341, 0, 351, 1e, 351, 1f, 302, 0, 312, 1e, 312, 1f, 402, 0, 322, 2e, et 322, 2f;<p>pour 16O12C34S, les bandes 501, 0 et 302, 0; pour 16O13C32S, la bande 302, 0 ;pour<p>16O12C33S, la bande 302, 0; pour 12C32S2, les bandes 0403, 0114, 3003 et 2203; pour<p>32S12C34S, la bande 3003. Les conventions de notation vibrationelles utilisées ici sont<p>pour CO2, ν1 ν2 l ν3 n où les bandes vibrationelles perturbées par la résonance de<p>Fermi ν1 2ν2 sont ordonnées selon n par ordre décroissant d'énergie. Pour l'OCS,<p>les conventions de notation sont ν1 ν2 ν3, l et pour le CS2, ν1 νl2 ν3. Leur analyse<p>spectrale a été réalisée, aboutissant à de nouvelles constantes rovibrationelles ou des<p>constantes améliorées par rapport à celles de la littérature. Les raies rovibrationnelles<p>attribuées sont données. L'analyse des spectres de H12C13CD et de CH3CCH a étéréalisée par l'équipe du Pr. L. Fusina (Université de Bologne, Italie).<p>L'expérience a aussi été installée autour d'un jet supersonique, enregistrant les<p>structures rotationnelles des bandes vibrationelles ν5 + ν9 de C2H4, 3ν3 de N2O et<p>ν1 + ν3 de H12C13CH en abondance naturelle, toutes avec une température rotationelle<p>inférieure à 35 K.<p>Finalement, un programme de simulation de ligne de base a été écrit.<br>Doctorat en Sciences de l'ingénieur<br>info:eu-repo/semantics/nonPublished
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Matindi, Tresor. "Development of VUV tunable laser spectroscopy techniques for characterizing calcium fluoride." Thesis, Stellenbosch : Stellenbosch University, 2014. http://hdl.handle.net/10019.1/95804.
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Thesis (MSc)--Stellenbosch University, 2014.<br>ENGLISH ABSTRACT: The large band gap (approximately 11.5-12.1 eV) and high transmission of calcium fluoride
(CaF2) crystal in the ultraviolet (UV) and vacuum ultraviolet (VUV) region makes it
an important material for optics for laser applications in UV. However, CaF2 degrades
during long exposure to UV irradiation due to defect generation. The formation of selftrapped
excitons (STE) is considered the first step in defect generation. In this project
the possibility of observing STE states in CaF2 using a narrow bandwidth tunable VUV
laser source is investigated. This is the first spectroscopy study of an alkaline earth fluoride
using VUV tunable laser radiation instead of a fixed wavelength laser. The use of a VUV
tunable laser source has potential for determining the energies of the STE states, which are
unknown. Our main objective is addressed by developing techniques to measure absorption
spectra of pure and doped CaF2 samples, using a VUV scanning monochromator and a
tunable VUV laser, and by doing a literature study. The results obtained with the scanning
monochromator show absorption features in 126-180 nm range of all our samples. These
vary for different samples and correlate with information from the supplier on the samples’
fluorescence spectra. Total absorption of the VUV light by CaF2 in the 115-126 nm range is
observed. With the narrow bandwidth tunable laser light, absorption spectra were obtained
in the range of 143-146.7 nm of all our CaF2 samples. No significance peaks which can be
related to the STE states in CaF2 were observed in the VUV laser absorption spectra, but
the results are valuable to improve the technique. The conclusion is that either a different
spectral range or fluorescence detection can be investigated in future.<br>AFRIKAANSE OPSOMMING: Die groot bandgaping (ongeveer 11.5-12.1 eV) en hoë transmissie van kalsiumfluoried
(CaF2) kristal in die ultraviolet (UV) en die vakuum ultraviolet (VUV) gebied maak dit
’n belangrike materiaal vir optika vir laser toepassings in die UV. CaF2 degradeer egter
gedurende langdurige blootstelling aan UV lig as gevolg van die generering van defekte.
Die vorming van ’n elektron-holte paar wat deur die kristalstuktuur gestabiliseer word teen
rekombinasie (self-trapped excitons, afgekort STE) word beskou as die eerste stap in defek
generering. In hierdie projek word die moontlikheid ondersoek om STE toestande in CaF2
waar te neem deur die gebruik van ’n afstembare VUV laserbron met emissie in ’n smal
spektrale band. Dit is die eerste spektroskopiese studie van ’n aardalkali-fluoried deur die
gebruik van afstembare VUV laserlig in plaas van ’n vaste golflengte laser. Die gebruik
van ’n afstembare VUV laserbron het potensiaal vir die bepaling van die energieë van die
STE teostande, wat onbekend is. Ons hoofdoel word aangespreek deur die ontwikkeling
van tegnieke vir die meet van absorpsie spektra van suiwer en gedoteerde CaF2 monsters
met behulp van ’n VUV skanderende monochromator en ’n afstembare VUV laser, en
deur ’n literatuurstudie. Die resultate wat behaal is met die skanderende monochromator
toon die absorpsieprofiele van al ons monsters in die 126-180 nm spektrale gebied. Die
absopsieprofiele varieer vir die verskillende monsters en korreleer met die inligting van die
verskaffer oor die fluoressensie spektra van die monsters. Totale absorpsie van die VUV lig
deur CaF2 in die 115-126 nm gebied is waargeneem. Met die smalband afstembare laserlig
is absorpsie spektra in die 143-146.7 nm gebied vir al ons CaF2 monsters verkry. Geen
beduidende pieke wat verband hou met die STE toestande in CaF2 is waargeneem in die
VUV laser absorpsie spektra nie, maar die resultate is waardevol vir die verbetering van
die tegniek. Die gevolgtrekking is dat of ’n ander spektraalgebied of fluoressensiedeteksie
in die toekoms ondersoek kan word.
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Pal, Avishekh. "Laser Remote Sensing of Trace Chemical Species Using 10.6 μm CO2 Laser Enhanced Breakdown Spectroscopy and Differential Absorption Lidar". Scholar Commons, 2008. https://scholarcommons.usf.edu/etd/441.
Full textAbstract:
Several different laser remote sensing techniques related to the detection of trace chemical species were studied. In particular, a Differential-Absorption lidar (DIAL), a Laser-Induced-Breakdown Spectroscopy (LIBS) lidar, and a Raman lidar were studied. Several of the laser spectroscopic techniques that were used were common throughout these different studies. More precisely, 10.6 μm CO2 laser related spectroscopy was common for the DIAL and LIBS studies, and 266 nm Nd:YAG laser related spectroscopy was used for the LIBS and Raman studies.
In the first system studied a tunable CO2 DIAL system was developed for the first time to our knowledge for the potential detection of the explosive Triacetone Triperoxide (TATP) gas clouds. The system has been used to measure gas samples of SF6, and has shown initial absorption measurements of samples of TATP contained within an enclosed optical absorption cell. DIAL/Lidar returns from a remote retroreflector target array were used for the DIAL measurements after passage through a laboratory cell containing the TATP gas. DIAL measured concentrations agreed well with those obtained using a calibrated Ion Mobility Spectrometer. DIAL detection sensitivity of the TATP gas concentration in the cell was about 0.5 ng/μl for a 0.3 m path-length. However, the concentration of TATP was found to be unstable over long periods of time possibly due to re-absorption and crystallization of the TATP vapors on the absorption cell windows. A heated cell partially mitigated these effects.
In the second set of studies, a Deep UV LIBS system was developed and studied for the remote detection of solid targets, and potentially chemical, biological, and explosive substances. A 4th harmonic Q-Switched Nd:YAG laser operating at 266 nm was used for excitation of the LIBS plasma at standoff ranges up to 50 m . The LIBS plasma emission covering the range of 240 – 800 nm was enhanced by use of a nearly simultaneous 10.6 μm CO2 laser that increased the LIBS plasma emission by several orders of magnitude. The emission spectrum was used to detect and identify the species of interest. Plasma temperatures on various solid substrates were measured. An increase in the plasma temperature of about 5000 K was measured and analyzed, for the first to our knowledge, due to the addition of the CO2 laser pulse to the LIBS plasma generated by the Nd:YAG laser. An optimum temporal overlap of the two laser pulses was found to be important for the enhancement.
Finally, in a third related lidar system, initial 266 nm Raman lidar studies were conducted at detection ranges of 15 m. However, significant spectroscopic background interferences were observed at these wavelengths and additional optical filtering is required.
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Hoarty, David John. "Measurement of ion and electron populations in laser produced plasmas by x-ray absorption spectroscopy." Thesis, Imperial College London, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.300085.
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Dyroff, Christoph. "Tunable diode laser absorption spectroscopy for trace gas measurements with high sensitivity and low drift." Karlsruhe Univ.-Verl. Karlsruhe, 2008. http://d-nb.info/992791588/04.
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Ueno, Keisuke. "A study of microho1low cathode discharge plasmas by laser absorption spectroscopy of excited helium atoms." Kyoto University, 2019. http://hdl.handle.net/2433/242494.
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Botha, G. N. "Development of an external cavity diode laser for application to spectroscopy and laser cooling and trapping of rubidium." Thesis, Stellenbosch : University of Stellenbosch, 2009. http://hdl.handle.net/10019.1/2307.
Full textAbstract:
Thesis (MSc (Physics))--University of Stellenbosch, 2009.<br>In the presented study a diode laser was characterised and used for spec-
troscopy, measuring the resonance lines of atomic rubidium. The characteristics
of diode lasers and external cavity diode lasers (ECDL) for the purposes of ab-
sorption spectroscopy were investigated and an experimental setup for tunable
diode laser spectroscopy using an ECDL was developed. In external cavity diode
lasers, the advantages of low cost, small size and e ciency of a diode laser is
combined with tunability and a narrow frequency bandwidth. The ECDL was
applied in experimental setups for absorption spectroscopy and saturated ab-
sorption spectroscopy. Measurement of the absorption of atomic rubidium's D2
line near 780 nm is discussed. The Doppler broadened, as well as the Doppler
free spectrum of the ne and hyper ne structure of the D2 line were measured
and is discussed. Finer control of the ECDL's stability and frequency, using a
servo circuit, were investigated and tested. An overview is given of laser cool-
ing and trapping of neutral rubidium atoms, which is the main application the
ECDL were developed for.
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Zhao, Zhizhong. "Laboratory studies of the atmospheric oxidation of dimethyl sulfide using laser flash photolysis coupled with tunable diode laser absorption spectroscopy." Diss., Georgia Institute of Technology, 1995. http://hdl.handle.net/1853/25672.
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Reiss, Boris. "Development of a two-tone frequency modulation tunable diode laser absorption spectrometer (TTFM-TDLAS) for measuring methane fluxes." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape3/PQDD_0016/MQ56199.pdf.
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Carleton, Wesley. "Rocket Motor Diagnostics using Tunable Diode Laser Spectroscopy for Chemically Non-Reacting Air/Water Vapor Mixture in Internal Flow." ScholarWorks@UNO, 2013. http://scholarworks.uno.edu/td/1729.
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This research is for the implementation of non-intrusive measurement techniques in the study of high temperature pipe flow. A low pressure, laboratory scale hybrid rocket motor simulator was built to achieve high temperatures with various gases. A quartz test section was designed, built, and implemented into the existing test setup to accommodate the laser beam of the existing Tunable Diode Laser Absorption Spectrometer (TDLAS) system which was designed to observe water vapor. A super-heated water vapor injector was designed to obtain the desired water vapor concentrations. Flow characteristics were simultaneously recorded using the existing TDLAS system and the DAQ system for temperatures for later comparison. A numerical study using a commercial CFD package was used to predict the flow characteristics at certain locations for experimental comparison. Based on this study, it is concluded that the TDLAS can be used to make real time temperature measurements of heated internal gas flows.
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Makita, Hiroki. "Time Resolved Absorption Spectroscopy for the Study of Electron Transfer Processes in Photosynthetic Systems." Digital Archive @ GSU, 2012. http://digitalarchive.gsu.edu/phy_astr_theses/16.
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Transient absorption spectroscopy was used to study light induced electron transfer processes in Type 1 photosynthetic reaction centers. Flash induced absorption changes were probed at 800, 703 and 487 nm, and on multiple timescales from nanoseconds to tens of milliseconds. Both wild type and menB mutant photosystem I reaction centers from the cyanobacterium Synechocystis sp. PCC 6803 were studied. Photosystem I reaction centers from the green algae Chlamydomonas reinhardtii, and the newly discovered chlorophyll-d containing organism Acaryochloris marina, were also studied.
The flash induced absorption changes obtained for menB mutant photosystem I reaction centers are distinguishable from wild type at 800 nm. MenB mutant photosystem I reaction centers displays a large amplitude decay phase with lifetime of ~50 ns which is absent in wild type photosystem I reaction centers. It is hypothesized that this ~50 ns phase is due to the formation of the triplet state of primary electron donor.
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Twigg, Marsailidh M. "Application of tunable diode laser absorption spectroscopy for the investigation of surface-atmosphere exchange of ammonia." Thesis, University of Ulster, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.445036.
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Bloch, Jonathan C. (Jonathan Craig). "Extending frequency modulation spectroscopy : sensitive and selective high resolution laser absorption in the visible and ultraviolet." Thesis, Massachusetts Institute of Technology, 1996. http://hdl.handle.net/1721.1/10861.
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Wu, Shun. "Direct fiber laser frequency comb stabilization via single tooth saturated absorption spectroscopy in hollow-core fiber." Diss., Kansas State University, 2014. http://hdl.handle.net/2097/18373.
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Doctor of Philosophy<br>Department of Physics<br>Kristan L. Corwin<br>Portable frequency references are crucial for many practical on-site applications, for example, the Global Position System (GPS) navigation, optical communications, and remote sensing. Fiber laser optical frequency combs are a strong candidate for portable reference systems. However, the conventional way of locking the comb repetition rate, frep, to an RF reference leads to large multiplied RF instabilities in the optical frequency domain. By stabilizing a comb directly to an optical reference, the comb stability can potentially be enhanced by four orders of magnitude. The main goal of this thesis is to develop techniques for directly referencing optical frequency combs to optical references toward an all-fiber geometry.
A big challenge for direct fiber comb spectroscopy is the low comb power. With an 89 MHz fiber ring laser, we are able to optically amplify a single comb tooth from nW to mW (by a factor of 10^6) by building multiple filtering and amplification stages, while preserving the comb signal-to-noise ratio. This amplified comb tooth is directly stabilized to an optical transition of acetylene at ~ 1539.4 nm via a saturated absorption technique, while the carrier-envelope offset frequency, f0, is locked to an RF reference.
The comb stability is studied by comparing to a single wavelength (or CW) reference at 1532.8 nm. Our result shows a short term instability of 6 x10^(-12) at 100 ms gate time, which is over an order of magnitude better than that of a GPS-disciplined Rb clock. This implies that our optically-referenced comb is a suitable candidate for a high precision portable reference. In addition, the direct comb spectroscopy technique we have developed opens many new possibilities in precision spectroscopy for low power, low repetition rate fiber lasers.
For single tooth isolation, a novel cross-VIPA (cross-virtually imaged phase array) spectrometer is proposed, with a high spectral resolution of 730 MHz based on our simulations.
In addition, the noise dynamics for a free space Cr:forsterite-laser-based frequency comb are explored, to explain the significant f0 linewidth narrowing with knife insertion into the intracavity beam. A theoretical model is used to interpret this f0 narrowing phenomenon, but some unanswered questions still remain.
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Liao, Chen-Ting, and Arvinder Sandhu. "XUV Transient Absorption Spectroscopy: Probing Laser-Perturbed Dipole Polarization in Single Atom, Macroscopic, and Molecular Regimes." MDPI AG, 2017. http://hdl.handle.net/10150/624358.
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We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is monitored in dilute helium, dense helium, and sulfur hexafluoride (SF6) molecules. We observe and quantify the time-dependence of various transient phenomena in helium atoms, including laser-induced phase (LIP), time-varying (AC) Stark shift, quantum path interference, and laser-induced continuum structure. In the case of dense helium targets, we discuss nonlinear macroscopic propagation effects pertaining to LIP and resonant pulse propagation, which account for the appearance of new spectral features in transient lineshapes. We then use tunable NIR photons to demonstrate the wavelength dependence of the transient laser induced effects. In the case of molecular polarization experiment in SF6, we show suppression of XUV photoabsorption corresponding to inter-valence transitions in the presence of a strong NIR field. In each case, the temporal evolution of transient absorption spectra allows us to observe and understand the transient laser induced modifications of the electronic structure of atoms and molecules.
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CAVALCANTI, FABIO. "Desenvolvimento de um laser pulsado com emissão em 1053 nm para utilização na técnica de "Cavity Ring-Down Spectroscopy." reponame:Repositório Institucional do IPEN, 2014. http://repositorio.ipen.br:8080/xmlui/handle/123456789/11790.
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Submitted by Claudinei Pracidelli (cpracide@ipen.br) on 2014-11-10T10:46:11Z
No. of bitstreams: 0<br>Made available in DSpace on 2014-11-10T10:46:11Z (GMT). No. of bitstreams: 0<br>Dissertação (Mestrado em Tecnologia Nuclear)<br>IPEN/D<br>Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP
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Doyle, Liam A. "Density mapping of species in low temperature laser-produced plasmas." Thesis, Queen's University Belfast, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.287439.
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