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 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. 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. 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. 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.

This thesis use laser diode, lens, grating, and mirrors to composite the external cavity system to retrieve the thickness of the liquid crystal and its characteristics. This way is different to the traditional way of measure. It has good accuracy on the vertical resolution and the vertical resolution can reach to 0.3£gm. At the same time, it can develop the high quality of horizontal resolution. On the basis of the correlation between the cavity length and the wavelength of the semiconductor laser, the system is capable of developing high horizontal resolution of accurate liquid crystal measurements. The horizontal resolution can reach to 40£gm. Furthermore, by adding bias on liquid crystal through this system, it expanded more understandings on the influences of bias and induced electric field of the electrodes to the tilting angle of the liquid crystal.

Mid-infrared lasers in the 3-5 µm range are important for wide variety of applications including trace gas sensing, infrared counter measures, free space optical communications, etc. GaSb-based type-I quantum well (QW) diode lasers are an attractive choice due to their relatively simple design and growth tolerances, as compared with quantum cascade lasers and interband cascade lasers. Excellent diode lasers have been demonstrated for wavelengths up to ~3.0 µm, employing GaInAsSb/AlGaAsSb QW active regions. But, device performance tends to degrade at longer wavelengths, due to Auger recombination and decreasing QW valence band offsets. In this work we look into the feasibility of using highly strained GaInAsSb/GaSb QWs as active regions for diode lasers operating at wavelengths beyond 3.0 µm. Heavy strain in the QW can improve valence band offset and also increase the splitting between the heavy and light hole bands which can help minimize Auger recombination. Through optimized molecular beam epitaxy (MBE) growth conditions we were able to incorporate up to 2.45 % compressive strain in these QWs enabling laser operation up to 3.4 µm at room temperature. An alternate path to extend the emission wavelength is to incorporate dilute quantities of nitrogen into the QW. Incorporating dilute quantities of substitutional nitrogen into traditional III-V’s strongly reduces the bandgap of the alloy. The advantage for the case of GaSb based dilute-nitrides is that the bandgap reduction is almost exclusively due to the lowering of the conduction band leaving the valence band offsets unaffected; thus providing a path to mitigating hole leakage while extending the emission wavelength. Although GaSb-based dilute-nitrides are a potentially elegant solution for extending the operating wavelength of GaSb-based type-I QW diode lasers, the luminescence efficiency of this material system has been relatively poor. This is most likely due to the presence of a high concentration of point defects, like nitrogen substitutional clusters. Through careful optimization of MBE growth conditions and post growth annealing, we demonstrate improved luminescence efficiency. With further optimization this material system can potentially extend the emission wavelength of GaSb-based type-I QW diode lasers even further into the mid-infrared spectrum. / text

No description available.

<p> A detailed experimental and theoretical study has been performed on several different photochemical systems. Lead-salt tunable diode lasers operating in the infrared region have been used as diagnostic tools to probe the molecules in these gaseous systems. Knowledge of these systems is expected to be useful in evaluating future schemes of laser isotope separation.</p> <p> In the course of this work a computerized digital technique has been developed that allows molecular parameters such as linestrength and linewidth to be obtained by measuring the infrared absorption on vibrational-rotational transitions of the molecule. Molecular concentrations can then be determined enabling one to gain valuable insight into the chemical processes occurring in the system.</p> <p> The digital technique was first tested on CO2 gas in a multi-pass White cell to verify the validity of the measurements. Subsequently, measurements were performed on mixtures of NH3/N2, NH3/Ar, HTO/H2O, and HTO/air. Those NH3 measurements that could be compared to previous measurements were found to be very accurate. For the HTO system, no previous measurements on linestrength and linewidth for the transitions examined have been performed.</p> <p> Described next is the application of the tunable diode laser diagnostic system to the investigation of infrared multiphoton dissociation of deuterated chloroform immersed in a chloroform bath. The sensitivity of the technique allowed for the measurement of the few parts per million of DCl formed by the photolysis of natural abundance CDCl3 in CHCl3.</p><p> In addition, the feasibility of transient detection with tunable diode lasers was examined. High fluence CO2 laser pulses were used to dissociate C3F6 or C2F3Cl and create CF2 radicals. Current-modulation of the tunable diode laser made it possible to monitor the transient CF2 radicals as they were formed, and as they subsequently decayed. The sensitivity of the transient detection technique was found to be limited by detector noise.</p> / Thesis / Master of Science (MSc)

No description available.

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. / Master of Science

The detection and measurement of gas properties has become essential to meet rigorous criteria of environmental unfriendly emissions and to increase the energy production efficiency. Although low cost devices such as pellistors, semiconductor gas sensors or electrochemical gas sensors can be used for these applications, they offer a very limited lifetime and suffer from cross-response and drift. On the contrary, gas sensors based on optical absorption offer fast response, zero drift, and high sensitivity with zero cross response to other gases. Hence, over the last forty years, diode laser spectroscopy (DLS) has become an established method for non-intrusive measurement of gas properties in scientific as well as industrial applications. Wavelength modulation spectroscopy (WMS) is derivative form of DLS that has been increasingly applied for making self-calibrated measurements in harsh environments due to its improved sensitivity and noise rejection capability compared to direct absorption detection. But, the complexity in signal processing and higher scope of error (when certain restrictions on operating conditions are not met), have inhibited the widespread use of the technique. This dissertation presents a simple and novel strategy for practical implementation of WMS with commercial diode lasers. It eliminates the need for pre-characterization of laser intensity parameters or making any design changes to the conventional WMS system. Consequently, sensitivity and signal strength remain the same as that obtained from traditional WMS setup at low modulation amplitude. Like previously proposed calibration-free approaches, this new method also yields absolute gas absorption line shape or absorbance function. Residual Amplitude Modulation (RAM) contributions present in the first and second harmonic signals of WMS are recovered by exploiting their even or odd symmetric nature. These isolated RAM signals are then used to estimate the absolute line shape function and thus removing the impact of optical intensity fluctuations on measurement. Uncertainties and noises associated with the estimated absolute line shape function, and the applicability of this new method for detecting several important gases in the near infrared region are also discussed. Absorbance measurements from 1% and 8% methane-air mixtures in 60 to 100 kPa pressure range are used to demonstrate simultaneous recovery of gas concentration and pressure. The system is also proved to be self-calibrated by measuring the gas absorbance for 1% methane-air mixture while optical transmission loss changes by 12 dB. In addition to this, a novel method for diode laser absorption spectroscopy has been proposed to accomplish spatially distributed monitoring of gases. Emission frequency chirp exhibited by semiconductor diode lasers operating in pulsed current mode, is exploited to capture full absorption response spectrum from a target gas. This new technique is referred to as frequency chirped diode laser spectroscopy (FC-DLS). By applying an injection current pulse of nanosecond duration to the diode laser, both spectroscopic properties of the gas and spatial location of sensing probe can be recovered following traditional Optical Time Domain Reflectometry (OTDR) approach. Based on FC-DLS principle, calibration-free measurement of gas absorbance is experimentally demonstrated for two separate sets of gas mixtures of approximately 5% to 20% methane-air and 0.5% to 20% acetylene-air. Finally, distributed gas monitoring is shown by measuring acetylene absorbance from two sensor probes connected in series along a single mode fiber. Optical pulse width being 10 nanosecond or smaller in the sensing optical fiber, a spatial resolution better than 1 meter has been realized by this technique. These demonstrations prove that accurate, non-intrusive, single point, and spatially distributed measurements can be made in harsh environments using the diode laser spectroscopy technology. Consequently, it opens the door to practical implementation of optical gas sensors in a variety of new environments that were previously too difficult. / Ph. D.

Résumé : Ce travail avait pour but de contribuer à la détermination précise des paramètres de raies du disulfure de carbone en vue d'applications atmosphériques. Il s'agissait de déterminer théoriquement et expérimentalement les coefficients d'élargissement collisionnel de raies de la bande gamma 3 du disulfure de carbone perturbé par l'air atmosphérique ainsi que leur dépendance en température. Pour cela, nous avons déterminé les coefficients d'élargissement collisionnel du disulfure de carbone perturbé par l'azote, l'oxygène et enfin par l'argon, principaux composants de l'air atmosphérique, aussi bien que leur dépendance en température. En outre, nous avons déterminé théoriquement et expérimentalement les coefficients d'auto élargissement collisionnel de raies de la bande gamma 3 - gamma 1 du disulfure de carbone, à température ambiante. La dépendance vibrationnelle des largeurs collisionnelles étant négligeable, les résultats sont valables quel que soit le niveau supérieur de la transition. Enfin, nous avons déterminé les intensités absolues de quelques raies de la bande gamma 3 - gamma 1 du disulfure de carbone qui nous ont permis de mettre en exergue une des applications atmosphériques des paramètres de raie mesurés en laboratoire. Pour la partie expérimentale, un spectromètre diode-laser haute résolution (5x10-4cm-1) a été utilisé pour enregistrer les différents spectres. De ces derniers, nous avons déterminé les différents paramètres de raie d'absorption en ajustant aux profils expérimentaux des modèles de profils théoriques. Du point de vue théorique, les différents coefficients d'élargissement collisionnel ont été calculés sur base du formalisme semi-classique d'Anderson-Tsao-Curnutte amélioré par J. Bonamy et D. Robert, moyennant quelques modifications pour accorder les valeurs à celles mesurées en laboratoire. Ceci a permis de valider les différents potentiels d'interaction pour les différents systèmes moléculaires considérés. Abstract The purpose of this thesis was to contribute to the accurate determination of line parameters of carbon disulfide for atmospheric applications. We have determined experimentally as well as theoretically the collisional broadening coefficients of lines in the gamma 3 band of carbon disulfide perturbed by the atmospheric air and their temperature dependence. To this end, we determined collisional broadening coefficients, as well as their temperature dependence, of carbon disulfide diluted in nitrogen, oxygen and argon; the main components of the atmospheric air. In addition, we determined the self broadening coefficients of lines in the gamma 3 - gamma 1 band of carbon disulfide at room temperature. As the vibrational dependence of collisional widths is negligible, our results are valid whatever the higher level of transition. Finally, we determined the absolute intensities of a few lines in the gamma 3 - gamma 1 band of carbon disulfide, which have enabled us to highlight one of the atmospheric applications of line parameters measured in the laboratory. For the experimental part, a high resolution diode-laser spectrometer (5x10-4cm-1) was used to record the different spectra. Of these, we determined the parameters of absorption line by adjusting theoretical lineshape models to experimental profiles. From a theoretical point of view, different collisional broadening coefficients were calculated on the basis of semi classical formalism of Anderson-Tsao-Curnutte improved by J. Bonamy and D. Robert, with some modifications to make the calculated values more consistent with those measured. This enabled us to validate the different potentials of interactions for the different molecular systems considered.

Thesis (MSc (Physics))--University of Stellenbosch, 2006. / External cavity diode lasers are used increasingly as sources of light in applications ranging from industrial photonic systems to basic laboratory research on the interaction of light and atoms. External cavity diode lasers offer more stable output frequency and narrow spectral bandwidth than the typical free-running diode lasers. These characteristics are achieved by exploiting the sensitivity of diode lasers to external optical feedback. In this study the design and development of an external cavity diode laser system for future applications in spectroscopy and laser cooling of rubidium atoms is presented. The external cavity diode laser including mechanical components and control electronics of the system is developed from basic components. The system uses frequency selective optical feedback from a diffraction grating in a Littrow configuration to provide collimated, narrow-band, frequency tunable light near 780 nm. The external cavity diode laser is designed to increase the mode-hop-free frequency tuning range, and allow accurate frequency tuning and stabilisation. A low-noise current source and a temperature controller for thermal stability were developed as part of the system since the output frequency changes with temperature and current. The temperature controller is optimised experimentally for the thermal characteristics of the external cavity. An electronic sidelock servo circuit for frequency locking of the external cavity diode laser to an external reference for long term frequency stabilisation is proposed and discussed. The servo circuit electrically controls the grating tilt and the current through the diode laser in order to lock the frequency of the diode laser. The external cavity diode laser is optimised and characterised near 780 nm. Results obtained in this study indicate that the external cavity diode laser is suitable for future applications in spectroscopy and laser cooling of neutral rubidium atoms.