<p> In this work, tunable Mid-Infrared (IR) light sources based on quasi-phase matched (QPM) difference frequency generation (DFG) by periodically poled lithium niobate (PPLN) crystals are studied. The theory of DFG and the characteristics of lithium niobate crystals are described and analyzed. Characteristics of the wavelength tuning of QPM DFG by PPLN crystals are studied. In order to analyze in detail, simulation and experimental data of the widely tunable mid-IR laser source around 2 1- μm to 5 1- μm wavelength are presented. The simulations of DFG process by PPLN are conducted based on the nonlinear optics reported. In the experiment, a 1.064 μm Nd:YAG laser and a tunable Ti:sapphire laser are employed as the signal and pump lasers, respectively. Based on the studies of the wavelength tuning characteristics at different temperatures, an optimization procedure to achieve a maximum wavelength tuning range is proposed. The potential applications in gas detection of the mid-IR source are also described briefly. Recommendation for future works and potential applications of the PPLN DFG based mid-IR lasers are discussed. </p> / Thesis / Master of Applied Science (MASc)

¡@¡@We integrated the laser-heated pedestal growth (LHPG) system with accurately controlled electrodes to build up our in situ poling system. The ZnO and MgO doped periodically poled lithium niobate crystal fiber were fabricated with the poling system. This poling system has the advantage of convenience and fast growth, but the ¡§screen effect¡¨ caused by free charges which exist near the molten zone must be eliminated. The micro swing resulted from the electric force is a feasible solution, because it can disarrange the free charges and reduce the ¡§screen effect¡¨. However, without excellently controlled micro swing, the uniformity of the poled domain pitch will loose and the conversion efficiency can not be improved. After analysis of the measured current data, the approximate system current model was presented and the proportional dependence between system current and micro swing was verified. Thus the system current was applied as the micro swing feedback signal, with that the variation of the micro swing was reduced from 25% to 15%. The stability of CO2 laser power is also a dominant factor to determine the quality of poled crystal fiber. The variation of the CO2 laser power was controlled within 1%. All the complicated works and precise control during the crystal fiber growth were accomplished with the LabVIEW program. ¡@¡@A novel and simple self-cascaded SHG + SFG scheme is presented for the generation of tunable blue/green light using ZnO doped periodically poled lithium niobate crystal fiber (PPLNCF) with a single designed pitch. A PPLNCF with a uniform period of 15.45£gm, the maximum conversion efficiency for the second harmonic generation and the cascaded SHG + SFG blue light can reach up to -9.2 dB and -31.9 dB, respectively. The 3 dB bandwidth of the tunable blue light is 3 nm (475-478 nm). In order to expand the tuning bandwidth range, a QPM gradient periodical structure was designed and can provide a 3 dB bandwidth of 65 nm for the tunable blue/green light output by simulation. We have successfully grown a crystal fiber with the domain pitch of 18.9 £gm for the C-band wavelength converter. The crystal length is 1.8 mm, the effective nonlinear coefficient of the lithium niobate crystal fiber is 18.2 pm/V that equals 0.53¡Ñdideal (34.4 pm/V). The conversion efficiency for converting the CW laser in C-band is about -59.3 dB.

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Le premier volet de cette thèse est consacré à la modélisation des phénomènes de transport dans le LN. Partant d'une analyse critique des modèles de bande usuels, nous montrons leur inadéquation dans le cas du LN et nous proposons un modèle de saut basé sur la théorie des petits polarons. Nous étudions d'abord par simulation Monte-Carlo la décroissance d'une population de polarons liés NbLi4+ relaxant vers des pièges profonds FeLi3+. Nous montrons que les pièges FeLi3+ ont des rayons effectifs particulièrement grands, rayons qui augmentent encore à température décroissante, et limitent considérablement les longueurs de diffusion des polarons. Les résultats de simulations sont ensuite confrontés aux résultats expérimentaux obtenus par différentes techniques ; Absorption photo-induite, Raman, Enregistrement holographique et Pompe-sonde. Le deuxième volet de cette thèse est consacré aux applications électro-optiques dans le LN périodiquement polarisé (PPLN). Sous l’effet d’une tension électrique, l’indice de réfraction du PPLN est périodiquement diminué et augmenté, formant ainsi un réseau d’indice activable électriquement. Un premier composant utilisant l’effet électro-optique dans du PPLN a été développé et démontré expérimentalement. Dans ce composant, la lumière est défléchie sous l’effet de la tension électrique par le réseau d’indice. Ce déflecteur de Bragg atteint une efficacité de diffraction proche de 100% avec une faible tension de commande de l’ordre de 5 V. Un deuxième composant a également été proposé, où la lumière se propage perpendiculairement aux parois de domaines du PPLN. Dans cette configuration un réflecteur de Bragg électro-optique peut être réalisé / The first part of this thesis is devoted to the modeling of transport phenomena in the LN. From a critical analysis of the usual band models, we show their inadequacy in the case of LN and we propose a hopping model based on the theory of small polarons. We first study by Monte-Carlo simulation the population decay of bound polarons NbLi4+ in deep traps FeLi3+. We show that the traps (FeLi3+) have particularly large effective radii, which increase further at decreasing temperature, and considerably limit the diffusion lengths of the polarons. The results of simulations are then compared with experimental results obtained by different techniques; Light-induced absorption, Raman, Holographic storage and Pump-Probe. The second part of this thesis is devoted to electro-optical applications in the periodically poled LN (PPLN). Under the effect of an electrical voltage, the refractive index of the PPLN is periodically decreased and increased, thus forming an electrically activatable index grating. A first component using the electro-optical effect in PPLN has been developed and demonstrated experimentally. In this component, the light is deflected under the effect of the electrical voltage by the index grating. This Bragg deflector achieves a diffraction efficiency of close to 100% with a low drive voltage of the order of 5 V. A second component has also been proposed, where light propagates perpendicularly to the domain walls of the PPLN. In this configuration an electro-optic Bragg reflector can be realized