Les aérosols sont une composante essentielle de la pollution urbaine et de la physico-chimie de l'atmosphère. Il est primordial de disposer de cartographies 3D de leur concentration par méthode lidar.<br />Une méthode originale, à une longueur d'onde, a été développée. Elle est basée sur l'impaction et l'étude de filtres permettant d'obtenir des informations complémentaires sur la distribution de taille des aérosols et sur leur composition. Les résultats, obtenus dans l'UV montrent que la distribution a un mode pour les petites tailles (~100 nm) et un mode pour les particules plus grosses (~1 μm). Notre lidar a aussi permis de mesurer la concentration en aérosols, l'évolution et la stratification de la couche limite. Mais cette méthode reste lourde à cause de l'étude des filtres et utilise comme hypothèse forte que l'atmosphère est homogène verticalement.<br />Pour distinguer entre les modes des particules et obtenir un diagnostique « tout-optique », nous avons étendu dans l'IR, plus sensible aux aérosols de la taille du micron, le système lidar UV existant, plus sensible aux petites tailles.<br />Cette extension est basée sur des oscillateurs paramétriques optiques (OPO). Les différents cristaux susceptibles de produire efficacement de l'IR moyen ont été testés. Ces cristaux sont le KTiOPO4, le KTiAsO4 et le KNbO3. Puis, un de ces OPO a été implanté dans notre système lidar. Les mesures lidar préliminaires de concentration dans l'IR, ont été obtenues pour des gouttelettes d'eau lors d'un épisode de brume. Parallèlement aux systèmes lidar linéaires précédents, un projet de lidar non-linéaire (projet Teramobile), a vu le jour. Une source de lumière blanche provenant des filaments, générés dans l'air lors de la propagation d'un faisceau laser térawatt, est utilisée pour faire du lidar aérosols multispectral. Avec cette source allant de l'UV à l'IR moyen, des mesures de concentration en aérosols seront possibles sans aucune hypothèse a priori contrairement aux méthodes précédentes.

Nonlinear optical frequency conversion is one of the most important key techniques in order to obtain lasers with wavelengths targeted for specific applications. In order to realize efficient and tailored lasers, the quasi-phase-matching (QPM) approach using periodically-poled ferroelectric crystals is getting increasingly important. Also understanding of damage mechanisms in nonlinear materials is necessary to be able to design reliable and well working lasers. This is especially true for high power application lasers, which is a rapidly growing field, where the damage problem normally is the ultimate limiting factor. In this thesis work, several promising novel ferroelectric materials have been investigated for nonlinear optical applications and the emphasis has been put on QPM devices consisting of periodically-poled structures. The materials were selected from three different types of ferroelectric materials: 1) MgO-doped stoichiometric LiNbO3 (MgO:SLN) and LiTaO3 (MgO:SLT), and non-doped stoichiometric LiTaO3 (SLT), 2) KTiOPO4 (KTP) and its isomorphs RbTiOPO4 (RTP), and 3) KNbO3 (KN). The focus in our investigations have been put on the spontaneous polarization switching phenomena, optimization of the periodic poling conditions, and the photochromic optical damage properties which were characterized by the help of blue light-induced infrared absorption (BLIIRA) measurements. With electrical studies of the spontaneous polarization switching, we were able to determine quantitatively, and compare, the coercive field values of different materials by applying triangularly shaped electric fields. We found that the values of the coercive fields depended on the increase rate of the applied electric field. The coercive field of KN was the lowest (less than 0.5 kV/mm) followed by the ones of KTP, SLT, and MgO:SLT (1.5 to 2.5 kV/mm). MgO:SLN, and RTP had relatively high coercive fields, approximately 5.0 to 6.0 kV/mm, respectively. Based on the domain switching characteristics we found, we successfully fabricated periodically-poled devices in all of the investigated materials with 30 μm periodicities and sample thickness of 1 mm. Blue light-induced infrared absorption (BLIIRA) has been characterized for unpoled bulk and periodically-poled samples using a high-sensitivity, thermal-lens spectroscopy technique. SLT showed a large photorefraction effect and the BLIIRA signal could not be properly measured because of the large distortion of the probe beam. The rise and relaxation time of BLIIRA, after switching the blue light on and off was in a time span of 10 to 30 sec except for KTP and its isomorphs, which needed minutes to hours in order to saturate at a fixed value. KN and MgO:SLN showed the lowest susceptibility to the induced absorption. Periodic poling slightly increased the susceptibility of KTP, MgO:SLT, and KN. Relatively high thresholds were observed in MgO:SLT and KN. By increasing the peak-power intensity of the blue light, the induced absorption for MgO:SLN, KTP and KN saturated at a constant value while that of MgO:SLT increase in a constant fashion. This trend is critical issue for the device reliability at high-power applications. / QC 20100830