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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

How to run a semiconductor diode laser in a stable way

Arnesson, Fredrik January 2012 (has links)
Interferometry and holography are two well-known methods for measuring distances, positions, vibrations, index of refraction etc. In these methods a coherent light source is used to create interference between different parts of the light. Since the wavelength of the light is used as reference it is possible to achieve very good accuracy in the measurements. The need of small and cheap light sources for these applications is large and an interesting alternative would be to use ordinary semiconductor diode lasers. These are unfortunately not designed to give sufficiently good coherence. In this Master Thesis work investigations of how the coherence of semiconductor diode lasers is affected by changes in temperature, injection current and between different individuals are performed. A Michelson interferometer is used to create an interference pattern where the contrast then can be analyzed. The contrast is related to the coherence of the laser, i.e., good coherence will give high contrast. The results show that in order to drive the laser in a stable way it is better to hold the temperature constant and varying the injection current until the wanted output power is achieved instead of doing the opposite. The results also indicate that the best coherence is achieved for low temperatures (around 10 OC) and high injection currents (around 80 mA). During these conditions a contrast of 70 % -80 % is achieved. The result of this Master Thesis work gives a hint on how to run a semiconductor diode laser in a stable way.
2

Stress Dependent Behaviour of InGaAsP Semiconductor Diode Lasers

Adams, Charles 08 1900 (has links)
The effects of tension and compression applied to unbonded InGaAsP semiconductor diode lasers have been studied. A theoretical calculation of the stress distribution within the laser and an analysis of the effect of strain on optical gain in semiconductors is presented. The observed dependence of threshold, wavelength, and polarization of the laser output on the applied stress is explained in terms of the strain dependence of the valence-band wavefunctions. The polarization behaviour is found to be related to thermal stress and the structure of the device. A technique has been developed to measure the thermal stress induced by current heating at the 105 dynes/cm2 level. The effect of stress on the below threshold behaviour of the lasers was investigated. The results are consistent with the strain dependence of the TE and TM mode gains. / Thesis / Master of Engineering (ME)
3

Rocket Motor Diagnostics using Tunable Diode Laser Spectroscopy for Chemically Non-Reacting Air/Water Vapor Mixture in Internal Flow

Carleton, Wesley 20 December 2013 (has links)
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.
4

An investigation of the response mechanism of the nitrogen phosphorus detector

Schofield, Paul Anthony January 1999 (has links)
The Nitrogen Phosphorus Detector is a sensitive, selective device used in gas chromatography. It responds selectively towards nitrogen and phosphorus containing organic compounds with detection limits in the picogram range. The detector is of great importance for the measurement of trace levels of drugs, pesticides and herbicides in biological matrices and the environment. There is, however, some dispute in the literature regarding the detector's response mechanism. The detector is based on a hydrogen-air diffusion flame. Two electrodes polarise the flame with a potential difference of about 200 V and the current through the flame is measured using an electrometer amplifier. The selectivity of the system relies on the presence of an alkali metal source, usually rubidium. In the presence of nitrogen- and phosphorus-containing organics, C~ and PO· anions are formed, yielding a current which is the measured response. It has been suggested that this selective response arises from a charge transfer reaction between the rubidium excited states and ~ or PO· and P02• radicals. Using an AlGaAs diode laser, the rubidium excited state population can be modulated and the influence on detector current monitored. Rubidium resonance-enhanced ionisation, laser-induced fluorescence and emission spectroscopy have all been used to further probe the response mechanism of the detector. Results have demonstrated that during response the C~ radical concentration increases. In addition the diode laser can modulate the excited state rubidium concentration altering it by a factor of 2. However despite more that doubling the Rubidium excited state concentration no increase in detector response is observed. From these observations it has been concluded that the above mentioned charge transfer reaction plays little if any role in detector response.
5

Theory of Operating Characteristics of Quantum Dot Lasers with Asymmetric Barrier Layers

Hammack, Cody Wade 27 June 2023 (has links)
In this work, the operating characteristics of quantum dot (QD) lasers with asymmetric barrier layers (ABLs) are studied. Several different cases are examined, in particular: 1) Effect of excited states on static and dynamic operating characteristics Within QDs, in addition to the lasing ground state, carriers can be captured into excited states, where they then decay into the ground state. This excited-state-mediated capture impacts the operating characteristics, limiting the maximum output power and modulation bandwidth. Three separate cases are considered: only indirect capture with electron-hole symmetry, both direct and indirect capture with electron-hole symmetry, and both direct and indirect capture of electrons but only indirect capture of holes. The impact of different parameters on the operating characteristics is studied, with values for maximizing the output power and modulation bandwidth being found. In addition, it is found that parasitic recombination in the active region in the space between QDs causes the output power to saturate at high injection currents for the cases of indirect capture for both electrons and holes and indirect capture for holes but direct and indirect capture for electrons, although the presence of the ABLs causes it to reach saturation at much lower currents. 2) QD laser with only a single ABL To be effective, the materials for ABLs must be carefully chosen to ensure that the band edges properly align to allow one carrier to enter the active region while preventing the other from overshooting it. Due to this requirement, it may arise that a suitable material only exists for one ABL but not the other. The performance of a QD laser with only a single ABL is considered and compared to a conventional QD laser. Specifically, the output power and characteristic temperature are calculated. While the single ABL laser only offers a negligible increase in output power compared to the conventional laser, it offers a considerable increase in characteristic temperature. 3) Analytical derivation of alpha factor in QD lasers with and without ABLs The alpha factor of a semiconductor laser describes the spectral linewidth broadening that occurs in semiconductor lasers due to changes in the refractive index due to the carrier density. While it has been studied experimentally, there has been little work done on deriving the alpha factor of QD lasers analytically. An expression for the alpha factor is found in this work using the real and imaginary parts of the complex susceptibility. For QD lasers with no inhomogeneous broadening, as well as ones with equilibrium filling of QDs with narrow line of QD size distribution, the alpha factor is independent of carrier density, and is therefore the same for any QD lasers, with or without ABLs. For QD lasers with equilibrium filling without a narrow line of QD size distribution, the alpha factor depends on carrier density, allowing for a potential difference between conventional and ABL QD lasers, however the difference between the two will be lessened. / Doctor of Philosophy / Semiconductor lasers are the most widely used laser, due in part to their ability to be controlled using electricity. Semiconductor lasers are used in a wide variety of consumer electronics, such as optical drives, as well as being used in fiber optic communications, where data is transmitted using the laser's light. Fiber optic communications transmit data by controlling the laser's output, where a high output (brighter light) represents a digital one, and a low output (dimmer light) represents a digital zero. Because semiconductor lasers can be directly controlled by changing the amount of current they receive, their output can easily be changed, allowing fast transfer of data. Despite their benefits, semiconductor lasers suffer from a drawback known as parasitic recombination. Parasitic recombination is a process that makes a significant portion of the current injected to generate useful light go to waste, which negatively impacts the laser's performance. One solution to parasitic recombination is the addition of asymmetric barrier layers (ABLs). By adding ABLs, parasitic recombination can be completely removed. In this work, several different cases of semiconductor quantum dot (QD) laser with ABLs are examined. Starting from a set of equations, the operating characteristics of the lasers in the different cases are found. First, the case of excited states is examined. The presence of excited states in semiconductor lasers impacts the rate that current can be converted to light, lowering their performance. By solving the starting rate equations, which describe the way different values change over time, the performance of the laser can be calculated. Specifically, the impact of several tunable parameters on the output power and modulation bandwidth are examined. The modulation bandwidth is how fast the laser output can be changed, which is equivalent to how fast data can be transmitted. Optimum values for the DC injection current, QD surface density (number of QDs per area), and laser cavity length are found.
6

Semiconductor Diode Laser Dynamics / PART A: ON-CAMPUS PROJECT

Park, Randall January 1981 (has links)
Part A of 2 parts. / <p> This report is a study of the dynamic properties of semiconductor laser diodes. The measurement of some important laser diode parameters necessary for dynamic behaviour prediction is described. The relaxation oscillation behaviour for laser diodes pumped with nanosecond time scale current pulses is predicted using both an approximate analytic solution and computer simulations. This predicted behaviour is compared with experimental results. Dynamic experiments with an external cavity for extra optical feedback and a regenerative loop for optoelectronic feedback are also described and discussed. Details of the experimental setups and techniques used are given. </p> / Thesis / Master of Engineering (MEngr)
7

OBSERVATION OF EIT IN RUBIDIUM VAPOR USING THE HANLE EFFECT

Zhang, Yuhong 03 August 2007 (has links)
No description available.
8

Broad-Band Antireflection Coatings for Improved Grating-External-Cavity Diode Laser Performance

Guo, Liqiang 08 1900 (has links)
In this thesis, strong optical feedback is utilized to realize broad-band wavelength tuning and to stabilize the frequency of a semiconductor diode laser in a grating-external-cavity (GEC) configuration. To reach the regime of strong optical feedback, the laser facet through which the feedback occurs has to be antireflection (AR) coated. Multi-layer AR coatings were designed using SiO2, Si3N4, SiOxNy, and a:Si for specific laser waveguide structures, and were fabricated by an electron cyclotron resonance, plasma enhanced, chemical vapor deposition (ECR-PECVD) system. The film thickness and refractive index were monitored by in situ ellipsometry during the deposition. This scheme permitted very low reflectivities, in the order of 5 x 10-4, to be readily and reproducibly obtained. The diode laser thus obtained was used in a strong feedback configuration. Light emitted from the coated facet was collimated and fed back onto the laser cavity after being reflected off a diffraction grating. The diffraction grating provides frequency selectivity, which is a desirable feature for obtaining a stable single longitudinal mode laser. The laser in this configuration oscillated in a single mode with a greater than 30 dB side mode suppression ratio and a wide tuning range. / Thesis / Master of Applied Science (MASc)
9

The Diode Laser Source and the Spatial Light Modulator's Driver Electronics for Miniaturized Holographic 3D Imaging

Subramani, Dinesh 22 December 1998 (has links)
The purpose of this thesis is to develop a low-cost, high power laser diode/fiber illumination system and to design the driver electronics of the spatial light modulator (SLM) for holographic, three dimensional (3D) imaging. A miniaturized laser diode/fiber/polarizing illumination system capable of 15mW of output at a wavelength of 690nm is designed, fabricated, and tested. The size limitations of various commercially available SLM drivers are described and the design to overcome them is suggested. The design describes in detail the timing considerations of the hardware interface and the psuedocode of the software interface between the host computer and the SLM. Experiments carried out to study the spatial uniformity of the SLM and the distortion due to the beam splitter on the structured output from the LIM are explained. / Master of Science
10

Microanastomoses vasculaires assistées par laser diode 1950 nm en chirurgie plastique réparatrice : étude expérimentale et clinique / 1950 nm diode laser-assisted vascular microanastomoses in reconstructive plastic surgery : experimental and clinical study

Leclère, Franck 13 December 2011 (has links)
En chirurgie plastique, le succès des lambeaux libres et des replantations dépend largement de la qualité des microanastomoses vasculaires. Ce travail complète les bases scientifiques de la technique de microanastomoses assistées par laser diode 1950 nm par la mise au point d’une méthode standardisée et l’étude post-opératoire du débit sanguin sur des séries animales. Il pose les jalons des premières séries cliniques. Dans une première série animale (S1), 30 anastomoses terminoterminales carotidiennes et 30 anastomoses jugulaires sont effectuées par la technique laser diode 1950 nm chez des rats Wistar. Les paramètres lasers suivants sont utilisés: taille du spot 400 µm, temps 1s, puissance délivrée de 100 mW à 150 mW (carotides) et de 90 mW à 140 mW (veines jugulaires). Elles sont comparées à J0 puis après 1, 4 et 12 semaines postopératoires, à un groupe contrôle de 30 anastomoses artérielles et de 30 anastomoses veineuses dans les mêmes proportions réalisées à l’aide de la technique conventionnelle par fils. Deux autres séries animales S2 et S3 sont ensuite entreprises à l’aide des paramètres lasers mis en évidence dans l’étude S1. Une série S2 regroupant vingt microanastomoses terminoterminales carotidiennes de rats Wistar est réalisée par la technique laser (n=10, P=120mW, spot 400 &#956;m, durée du spot 1s, 5 spots par paroi, fluence 95 J/cm²) et la technique conventionnelle par fils 10/0 (groupe contrôle, n=10). Pour les deux groupes, la carotide controlatérale non opérée sert de référence pour le calcul du débit sanguin par IRM de flux : Une séquence de positionnement, une séquence anatomique, une séquence angiographique et une séquence de &#64258;ux sont réalisées un jour après l’opération puis après une, quatre et huit semaines. Une série S3 regroupant vingt microanastomoses terminoterminales jugulaires de rats Wistar est réalisée par la technique laser (n=10, P=110mW, spot 400 &#956;m, durée du spot 1s, 4 spots par paroi, fluence 90 J/cm²) et la technique conventionnelle (groupe contrôle, n=10). De même la jugulaire controlatérale sert de référence dans les diverses séquences IRM. Au total, 40 actes microchirurgicaux comprenant 38 lambeaux libres et 2 replantations digitales sont réalisés à l’aide du laser diode 1950 nm. Les microanastomoses artérielles sont terminoterminales dans 36 cas et terminolatérales pour 4 autres. Les microanastomoses veineuses sont toutes terminoterminales. Les microanastomoses sont réalisées par un laser diode 1,95 µm après mise en place de 2-5 points de rapprochement. Les paramètres suivants sont utilisés : taille du spot 400µm, puissance 125mW, 4 à 8 spots sur chaque face, temps d’application (0,7-2s) suivant le diamètre des vaisseaux : la fluence varie de 70 à 200 J/cm². Les études du débit sanguin par les nouvelles techniques d’IRM de flux avec les paramètres maintenant standardisés mettent en évidence l’excellente perméabilité des anastomoses laser sur un modèle animal. Le taux de succès de cette première grande série clinique apparait excellent lorsque comparé à celui des grandes séries de la littérature. Les innovations technologiques devraient conduire à une utilisation plus large de cet outil au bloc opératoire. / In the field of plastic surgery, the most important factor for successful free flap transfer and replantations is a well executed vascular microanastomosis. The aim of these studies is to complement the scientific basis of the 1950 nm diode laser assisted microanastomosis (LAMA) by standardising the technique and studying the postoperative blood flow in animal series. This work introduces the first clinical series. In the first animal series (S1), 30 end-to-end microanastomoses of the carotidis and 30 end-to-end microanastomoses of the external jugular were performed with a 1950 nm diode LAMA technique in Wistar rats. The following laser parameters were used: spot size=400µm, spot duration=1s, Power varying between 100 to 150 mW for arterial microanastomoses and between 90 to 140 mW for venous microanastomoses. They were compared at D0 and at 1, 4 and 12 weeks with 30 conventional arterial microanastomoses and 30 conventional venous microanastomoses.Two other animal series (S2 and S3) were performed using the standardised parameters of the initial studies. In the S2 series, LAMA was performed on a group of 10 carotidis on Wistar rats. Two 10/0 stay sutures and a standard laser tissue welding technique (P=120mW, spot size=400 &#956;m, t=1s, 5 spots for each wall, fluence=95 J/cm²) were used (LAMA group). They were compared with a group of 10 conventional arterial anastomosis (CSMA group). A MRI-positioning sequence (Magnetic Resonance Imaging), an anatomical sequence, an angiographic sequence and a &#64258;ow sequence were performed 1 day after operation and then after 1, 4 and 8 weeks. Similarly, the series S3 included 20 microanastomoses of the external jugular performed with LAMA (n=10, P=110mW, spot size=400 &#956;m, spot duration= 1s, 4 spots for each wall, fluence 90 J/cm²) or the conventional technique (n=10). For the two groups, contralateral non-operated external jugular were used as control.In total, 40 clinical procedures, including 38 free flaps and 2 finger replantations have been performed with the LAMA technique. End-to-end arterial microanastomoses were performed in 36 cases and end-to-side in 4 cases. All venous microanastomoses were performed end-to-end. LAMA was performed with a 1950 nm diode laser after placement of 2-5 stitches. The following laser parameters were used: spot size=400 &#956;m, spot duration=0.7-2s, 4-8 spots for each wall, power=125 mW, &#64258;uence=70-200 J/cm². The animal series S1 demonstrates excellent bloodless patency at &#64258;uences ranging from 90 to 100 J/cm² (Power between 110-130mW) for arterial LAMA, and between 80 and 95 J/cm² (Power between 100-120mW) for venous LAMA. The flow-MRI further demonstrates that 1950 nm diode laser-assisted microanastomoses performed with our standardised parameters is a consistent, reliable and reproducible technique. The success rate of the clinical series appears promising. Technical innovation will most likely lead to greater ease of use of the laser handpiece in the operating room.

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