Even though KTiOPO4 (KTP) is considered to be one of the best nonlinear materials for quasi phase matched (QPM) frequency conversion in the visible and the near-infrared spectral regions, its use is often limited by poor material homogeneity, high ionic conductivity, a considerable linear absorption and photochromatic damage. On the other hand, the improved material homogeneity and the lower ionic conductivity of bulk Rb-doped KTP (RKTP) make this material an ideal alternative for fabrication of fine-pitch QPM gratings, while the arsenate isomorph KTiOAsO4 (KTA) promises a better performance in the green spectral region and adds the advantage of a wider transparency window in the infrared. Unfortunately, the available studies on these materials are limited and unable to answer the question whether RKTP and KTA are feasible alternatives to KTP in terms of periodic poling and optical performance. The optical performance of the QPM devices depends on the periodic poling quality, therefore, a detailed comprehension of domain-grating formation in the KTP isomorphs is highly desired. The goals of this thesis were to gain a better understanding of the periodic poling process in the KTP isomorphs, in order to study the specifics of ferroelectric domain engineering in KTA and RKTP, and to evaluate the optical performance of these isomorphs. Fine-pitch periodically poled structures were engineered both in KTA and RKTP crystals. It was demonstrated that QPM gratings with excellent quality and with periods as short as 8.49 μm can be fabricated in KTA crystals. Comparative transmission studies have shown that periodically poled KTA (PPKTA) crystals can be superior to KTP for QPM second harmonic generation in the visible spectral region due to lower linear absorption. It was also demonstrated that RKTP is a superior alternative to KTP for high-quality QPM grating fabrication. A consistent room-temperature periodic poling of 5 mm thick RKTP crystals with a period of 38.86 μm has been achieved. The obtained large aperture periodically poled RKTP (PPRKTP) crystals showed an outstanding QPM grating uniformity and excellent optical performance in optical parametric oscillator (OPO) applications. Moreover, it was shown that RKTP is less susceptible to blue-induced infrared absorption than KTP. Finally, a novel and a relatively simple method for self-assembling quasi-periodic sub-μm scale ferroelectric domain structure in RKTP crystals has been presented. It was shown that, after treatment in aqueous KOH/KNO3 solution, periodic poling of RKTP with planar electrodes resulted in one-dimensional ferroelectric domain structure with an average periodicity of 650±200 nm, extending over the whole 1 mm thick crystal. Such self-assembled structures in RKTP were used to demonstrate 5th order non-collinear QPM backward second harmonic generation. / <p>QC 20140114</p>

No description available.

Denna avhandling går ut på att undersöka verkningsgradens temperaturberoende för SHG (second harmonic generation) från 1030nm till 515nm, i en kvasifasmatchad KTP-kristall, för både pulsad och CW (continuous wave)-laser. Detta undersöktes då det har tidigare noteras att verkningsgraden hos en KTP-kristall avtar när effekten på lasern ökar. Det är på grund av ökad värmeutveckling från absorption (se Figur 1). I denna rapport undersöktes detta genom att variera lasereffekten, kyla KTP-kristallen med en temperaturgradient och mäta dess effekter på verkningsgraden. För att undersöka fallet med CW skapades ett simuleringsprogram i Python, då det inte fanns en tillräckligt stark CW-laser tillgänglig. En laser med femtosekundspulser användes för att mäta verkningsgraden för pulsad laser. Resultaten visade att CW-lasern som förväntat var känslig för temperaturförändringar och att det orsakade att verkningsgraden minskade med effekten. Dessutom visade simuleringen att en temperaturgradient var ett effektivt sätt att höja verkningsgraden om den applicerades korrekt. Den pulsade lasern uppvisade ett annat beteende. Den var mycket stabil för stora temperaturspan, 7% skiljde mellan högsta och lägst effekt på ett temperaturspan på 70°C.

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.

In this study, we have developed a high performance multi-photon microscopic system to perform second- harmonic (SH) imaging on a tooth and a KTP crystal . The high sensitivity of the system allows acquisition rate of 300 seconds/frame with resolution at 512¡Ñ512 pixels. The surface SH signal generated from the tooth and the KTP crystal is also carefully verified through micro-spectroscopy, polarization rotation and wavelength tuning. In this way, we can ensure the authenticity of the signal. KTP crystal and the enamel that encapsulates the dentine is known to possess highly ordered structures. The anisotropy of the structure is revealed in the microscopic SH images of the tooth and the KTP crystal samples.

In this study, we have successfully developed a high performance transmission mode Laser scanning for SHG imaging. This setup is capable of acquiring images of size 512¡Ñ512 pixels at a rate of 5.4 seconds/frame. The of samples can thus be imaged, which reflects the samples¡¦ structure and symmetry.

L'émergence de lasers de plus en plus puissants et compacts et le développement de leurs applications engendrent des problèmes de tenue au flux des composants optiques et tout particulièrement des cristaux non linéaires. Dans ce contexte, cette thèse s'attache à l'étude de l'endommagement laser en régime nanoseconde dans les cristaux non linéaires utilisés principalement pour la conversion de fréquence (KTP, KDP, LBO) et les applications électro-optiques (RTP, KDP). Les effets spécifiques aux cristaux liés à leur anisotropie et leurs non linéarités ont imposé le développement d'une métrologie de l'endommagement laser adaptée. Cette métrologie a ensuite été appliquée à l'étude détaillée des cristaux isomorphes de KTP et RTP, et du cristal de LBO. L'influence sur leur tenue au flux laser de multiples paramètres, tels que la longueur d'onde d'irradiation, la polarisation, la direction de coupe du cristal, a pu être étudiée permettant de mieux comprendre les mécanismes responsables de l'endommagement laser dans ces cristaux. L'importance de la caractérisation de la tenue au flux laser de ces cristaux sous irradiation multiple et dans leurs conditions fonctionnelles d'utilisation a été mise en évidence. Par exemple, la coexistence de plusieurs longueurs d'onde au sein du matériau modifie de façon importante la tenue au flux laser du composant

This thesis deals with the fabrication and the characterization of periodically-poled crystals for use in lasers to generate visible and UV radiation by second-harmonic generation (SHG) through quasi-phasematching (QPM). Such lasers are of practical importance in many applications like high-density optical storage, biomedical instrumentation, colour printing, and for laser displays. The main goals of this work were: (1) to develop effective monitoring methods for poling of crystals from the KTiOPO4 (KTP) family, (2) to develop useful non-destructive domain characterization techniques, (3) to try to find alternative crystals to KTP for easier, periodic poling, (4) to investigate the physical mechanisms responsible for optical damage in KTP. The work shows that the in-situ SHG technique used together with electro-optic monitoring, makes it possible to obtain reliable, real-time information regarding the poling quality over the whole crystal aperture during the electric-field poling process. Using this combined monitoring method, both KTP and Rb-doped KTP (RKTP) crystals were successfully poled. By comparing these two crystals, we found that a low-doped KTP has a substantially reduced ionic conductivity and, thus, a high-quality periodic poling can be obtained without otherwise affecting the properties of the crystal. RKTP is a good alternative candidate to KTP for poling purpose. We have also shown that Atomic Force Microscopy (AFM) is an informative tool for investigating domain nucleation, growth, and merging. Furthermore, we have demonstrated a simple technique for 3D characterization of QPM samples. It utilizes a group-velocity mismatched, type-II SHG of femtosecond pulses for layer-by-layer monitoring of the effective nonlinearity along the propagation direction of the beam. The quality of these crystals was finally reflected in a number of SHG experiments with a variety of laser sources. High energies and high efficiencies were thus demonstrated using CW, mode-locked and Q-switched lasers. Gratings with pitches smaller than 3 µm, were demonstrated for first-order UV generation. Type-II QPM SHG was demonstrated as a technique for reducing the fabrication constraints. High intensity light in the visible and the UV leads to modification of the material properties and, eventually, to optical damage. In KTP and its isomorphs, the first sign of material change is an optically-induced absorption. We have used thermal-lens spectroscopy with a common-path interferometer for high-sensitivity measurements of green light-induced infrared absorption dynamics in single-domain and periodically-poled KTP (PPKTP). The saturated, green light-induced absorption has been shown to be consistently higher in periodically-poled crystals, and is attributed to the creation of stoichiometric and interstitial defects in the crystals during the poling process. Finally, irreversible bulk damage thresholds in PPKTP have been determined for pulsed frequency converters. As the characteristics of optical damage are closely related to the material quality, this investigation can provide useful information for crystal manufactures and will help to optimise the crystal growth conditions.

I den här avhandlingen visas hur lasrar och optiska parametriska oscillatorer (OPO:er) kan styras spektralt med hjälp av volymbraggitter. Volymbraggitter utgörs av ett periodiskt varierande brytningsindex som skrivits i ett fototermorefraktivt glas. Gittret reflekterar därmed en specifik våglängd som bestäms av perioden hos modulationen, och kan tillverkas med smal bandbredd och hög reflektans beroende på modulationens längd och styrka. En teoretisk modell har utvecklats för reflektiva volymbraggitters egenskaper om den infallande strålen har en större vinkelspridning än gittrets vinkeltolerans. Detta kan bl.a. inträffa i en laserkavitet där gittret används vid snett infall, och en teoretisk beskrivning är därför ett viktigt redskap för att kunna designa sådana lasrar. Spektral kontroll av ett antal fasta tillståndslasrar med hjälp av volymbraggitter har i försök påvisats, och lasern har därvid både kunnat avstämmas spektralt samtidigt som en avsmalnad spektral bandbredd erhållits. Lasern kan göras väldigt enkel genom att byta ut en av kavitetsspeglarna mot gittret. Tack vare gittrets goda spektrala urvalsmekanism kan lasern låsas var som helst i förstärkningsspektrumet. De tekniker och lasrar som demonstrerats experimentellt är följande: Lasring i en enda longitudinell mod erhölls både för en diodpumpad ErYb:glas-laser vid 1553 nm med ca 10 mW:s effekt och 90 kHz linjebredd samt för en diodpumpad Nd:GdVO4-laser vid 1066 nm med 0.85 W:s effekt. Lasrarnas våglängd kunde avstämmas över större delen av gittrets bandbredd på ca. 30 GHz. Genom att bygga Nd:GdVO4-lasern med en monolitisk kavitet kunde även en spektralt synnerligen stabil laser erhållas med under 40 MHz bandbredd. Tillämpningar för dessa lasrar finns både inom spektroskopi samt som källor för intrakavitetsfördubbling till synliga våglängder. Genom att använda gittret som inkopplingsspegel går det även att framställa lasrar med en väldigt låg kvantdefekt, som därför får minskad värmeutveckling i lasermediet. Detta medger i sin tur att lasrar med höga medeleffekter kan konstrueras, som kan användas bl.a. för olika former av materialbearbetning. I detta arbete har lasrar med låg kvantdefekt byggts med Yb:KYW som laserkristall; både en laser vid 998 nm på 3.6 W som diodpumpades vid 982 nm och med en bandbredd på 10 GHz, samt en laser vid 990 nm på 70 mW som pumpades av en Ti:safir-laser vid 980 nm. Om volymbraggittret används vid snett infall kan den reflekterade våglängden avstämmas genom att gittret roteras. Denna princip användes i en diodpumpad Yb:KYW-laser till att erhålla en brett avstämbar laservåglängd mellan 996 nm och 1048 nm med en maximal effekt på 3 W och med 10 GHz bandbredd. Genom att placera gittret i en retroreflektor kunde avstämningen göras utan att kaviteten behövde linjeras om. En laser som denna kan exempelvis användas för olika typer av materialkarakterisering och spektroskopi. Med optiska parametriska oscillatorer (OPO:er) kan laserljus omvandlas till nya våglängder. Därmed kan OPO:er användas som koherenta ljuskällor där inga effektiva lasrar existerar. OPO-processen kan göras effektiv om en pulsad pump används, och den genererade våglängden kan enkelt styras med hjälp av periodiskt polade (PP) icke-linjära kristaller, såsom PP-KTiOPO4, som användes i detta arbete. En nackdel med OPO:er är att i allmänhet är den genererade signalen tämligen spektralt bredbandig. Signalens bandbredd kan dock avsmalnas betydligt om ett spektralt filter såsom ett volymbraggitter används. Genom att byta ut en av speglarna i OPO-kaviteten mot gittret kan utformningen av OPO:n göras väldigt enkel. I en OPO med en signalvåglängd på 975 nm kunde en avsmalning av bandbredden till 50 GHz påvisas med hjälp av ett braggitter. Detta motsvarar 20 gångers minskning jämfört med om en konventionell spegel används. Som mest erhölls en pulsenergi på 0.34 mJ i signalen. Genom att rotera gittret kunde våglängden avstämmas 21 THz. För att förenkla avstämningen konstruerades även en OPO med gittret i en retroreflektor, samtidigt som kaviteten var av ringtyp. I denna OPO vid en våglängd på 760 nm och med en pulsenergi i signalen på upp till 0.42 mJ erhölls en bandbredd på 130 GHz och ett avstämningsområde på 2.6 THz. Slutligen har en OPO vid 1 µm konstruerats med ett gitter med en transversellt varierande period, s.k. chirp. Därigenom kan våglängden avstämmas väldigt enkelt genom att bara flytta gittret transversellt. En tillämpning av dessa OPO:er är såsom ljuskällor i olika typer av laserbaserade sensorer, i vilka en specifik och stabil våglängd erfordras. Dessutom kan de smalbandiga OPO:erna användas som första steg i ickelinjära processer i flera steg. Smal bandbredd är då viktig för effektiviteten i den påföljande ickelinjära omvandlingen i nästa steg. / The object of this thesis is to explore the usage of reflective volume Bragg gratings in photo-thermo-refractive glass for spectral control of solid-state lasers and optical parametric oscillators, to build tunable and narrowband coherent light-sources. In order to provide a design tool for use of reflective volume Bragg gratings in laser cavities, a theory was developed that describes the performance of the gratings if the incident beam has finite width with an angular spectrum that is comparable to the grating's angular acceptance bandwidth. Spectral control was demonstrated in a number of cw solid-state lasers, in terms of narrow bandwidth and tunable wavelength, by use of a volume Bragg grating. The design could be made very simple by replacing one of the cavity mirrors with the grating. Thanks to the grating's strong spectral selectivity, the lasers could be locked anywhere in the gain spectrum, while the laser bandwidth was substantially narrowed. In particular, the following lasers were demonstrated: Single-longitudinal-mode lasing in ErYb:glass at 1553 nm with 90 kHz linewidth and in Nd:GdVO4 at 1066 nm with a linewidth below 40 MHz. Very low quantum defect in Yb:KYW lasers, diode-pumped at 982 nm and lasing at 998 nm with 10 GHz bandwidth, as well as Ti:sapphire-pumped at 980 nm and lasing at 990 nm. An Yb:KYW laser that was widely tunable from 996 nm to 1048 nm with 10 GHz bandwidth. In nanosecond pulsed optical parametric oscillators (OPOs) based on periodically poled KTiOPO4, narrowband operation and a tunable wavelength were demonstrated with a volume Bragg grating as a cavity mirror. At a signal wavelength of 975 nm, the bandwidth was 50 GHz, a reduction by 20 times compared to using a conventional mirror. A tuning range of 21 THz was also demonstrated. In another OPO at a signal wavelength of 760 nm, a ring-cavity design was demonstrated to provide convenient tuning. A tuning range of 2.6 THz and a bandwidth of 130 GHz was shown. Also, narrowband operation and tuning in an OPO around 1 µm was demonstrated by use of a transversely chirped Bragg grating. / QC 20100813

<p>This thesis deals with the fabrication and the characterization of periodically-poled crystals for use in lasers to generate visible and UV radiation by second-harmonic generation (SHG) through quasi-phasematching (QPM). Such lasers are of practical importance in many applications like high-density optical storage, biomedical instrumentation, colour printing, and for laser displays.</p><p>The main goals of this work were: (1) to develop effective monitoring methods for poling of crystals from the KTiOPO₄ (KTP) family, (2) to develop useful non-destructive domain characterization techniques, (3) to try to find alternative crystals to KTP for easier, periodic poling, (4) to investigate the physical mechanisms responsible for optical damage in KTP. The work shows that the in-situ SHG technique used together with electro-optic monitoring, makes it possible to obtain reliable, real-time information regarding the poling quality over the whole crystal aperture during the electric-field poling process. Using this combined monitoring method, both KTP and Rb-doped KTP (RKTP) crystals were successfully poled. By comparing these two crystals, we found that a low-doped KTP has a substantially reduced ionic conductivity and, thus, a high-quality periodic poling can be obtained without otherwise affecting the properties of the crystal. RKTP is a good alternative candidate to KTP for poling purpose. We have also shown that Atomic Force Microscopy (AFM) is an informative tool for investigating domain nucleation, growth, and merging. Furthermore, we have demonstrated a simple technique for 3D characterization of QPM samples. It utilizes a group-velocity mismatched, type-II SHG of femtosecond pulses for layer-by-layer monitoring of the effective nonlinearity along the propagation direction of the beam. The quality of these crystals was finally reflected in a number of SHG experiments with a variety of laser sources. High energies and high efficiencies were thus demonstrated using CW, mode-locked and Q-switched lasers. Gratings with pitches smaller than 3 µm, were demonstrated for first-order UV generation. Type-II QPM SHG was demonstrated as a technique for reducing the fabrication constraints.</p><p>High intensity light in the visible and the UV leads to modification of the material properties and, eventually, to optical damage. In KTP and its isomorphs, the first sign of material change is an optically-induced absorption. We have used thermal-lens spectroscopy with a common-path interferometer for high-sensitivity measurements of green light-induced infrared absorption dynamics in single-domain and periodically-poled KTP (PPKTP). The saturated, green light-induced absorption has been shown to be consistently higher in periodically-poled crystals, and is attributed to the creation of stoichiometric and interstitial defects in the crystals during the poling process. Finally, irreversible bulk damage thresholds in PPKTP have been determined for pulsed frequency converters. As the characteristics of optical damage are closely related to the material quality, this investigation can provide useful information for crystal manufactures and will help to optimise the crystal growth conditions.</p>