Die Messung ultrakurzer Laserpulse ist ein Eckpfeiler der ultraschnellen Laserphysik, da die Gültigkeit eines Experiments von der Glaubwürdigkeit seiner Messtechnik abhängt. Etablierte Puls-Charakterisierungstechniken beruhen jedoch häufig auf einer Mittelung über viele Pulse. Daher können sie falsche Informationen liefern, wenn die zeitliche Form von Puls zu Puls variiert. Diese Dissertation bietet Strategien zum sicheren Erfassen und Messen einer Degradierung der Puls-Kohärenz mit Hilfe von frequenzaufgelöstem optischem Gating (FROG), spektraler Phaseninterferometrie für die direkte Rekonstruktion elektrischer Felder (SPIDER) und Dispersionsscan (D-scan). Zu diesem Zweck werden Verbesserungen der Charakterisierungstechniken entwickelt. Die in dieser Arbeit entwickelten neuen Werkzeuge eröffnen nun einen Weg zur Untersuchung der Degradierung der Inter-Puls-Kohärenz, was eine zuverlässige Ultrakurzpulsmetrologie ermöglicht und das zuvor nicht nachweisbare Problem der Pulsfolgeninstabilität löst. / The measurement of ultrashort laser pulses is a cornerstone of ultrafast laser physics, as the validity of any experiment depends on the credibility of its measurement technique. However, established pulse characterization techniques often rely on averaging over many pulses. Therefore, they can return incorrect information if the temporal shape varies from pulse to pulse. This thesis provides strategies to safely detect and measure interpulse coherence degradation, using frequency-resolved optical gating (FROG), spectral phase interferometry for direct electric-field reconstruction (SPIDER), and dispersion scan (d-scan). To this end, improvements of the characterization techniques themselves are devised. The set of new tools developed in this thesis now opens up an avenue for the investigation of interpulse coherence degradation, leading to a more reliable ultrashort pulse metrology and solving the previously undetectable problem of pulse train instability.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / Grades de Bragg nÃo-lineares tÃm sido consideradas desde o final do sÃculo passado para aplicaÃÃes em sistemas de comunicaÃÃes Ãpticas e sensoriamento. O estudo de pulsos ultra-curtos em grades de Bragg lineares, entretanto, sà tem sido considerado nos Ãltimos anos, devido ao desenvolvimento de tÃcnicas numÃricas especÃficas para se resolver o problema. Neste trabalho, foi realizado um estudo analÃtico-numÃrico das caracterÃsticas de transmissÃo e reflexÃo das grades de Bragg nÃo-lineares. Pela primeira vez, foram consideradas variaÃÃes periÃdicas da nÃo-linearidade no dispositivo operando no regime de onda continua, levando a uma nova classe de grades nÃo-uniformes. CaracterÃsticas dos estados bi- e multi-estÃveis foram extensamente investigados nas grades de Bragg nÃo-lineares. TambÃm, pela primeira vez, foi realizado o estudo numÃrico de pulsos ultracurtos ($sim$1 ps) incidindo em grades nÃo-lineares. O enfoque foi dado para a dependÃncia da intensidade de um pulso ultracurto ao passar por tal grade. Foram estudadas, ainda, as dependÃncias na forma temporal da profundidade de modulaÃÃo da grade e do Ãndice nÃo-linear. Grades apodizadas foram consideradas, jà que estas sÃo de importÃncia fundamental nos sitemas de comunicaÃÃes modernos. / Nonlinear fiber Bragg gratings has been considered since the end of last century for applications in optical communications and sensor techniques. The investigation of ultrashort pulses in linear Bragg gratings, however has been considered in the last few years due the development of specifical numerical techniques to solve this problem. In the present work an analytical and numerical study of the reflection and transmission characteristics of nonlinear Bragg gratings was done. For the first time, it has been considered periodic variations of the nonlinearity in that devices operating in the continuous wave regime, leading to a new class of nonuniform gratings. It was extensively investigated the bi- and multistable characteristics in these nonlinear fiber Bragg gratings. Also, for the first time, the numerical study of ultrahsort pulses ($sim$1 ps) incident in nonlinear gratings was done. The focus was the input pulse intensity dependence on that gratings. Also, the depedences in the time shapes of grating index modulation depth and nonlinear index were studied. Apodized gratings were considered since they are of fundamental importance in modern communications systems.

Dans cette thèse, nous proposons l'utilisation d'un nouveau type de fibre à cristal photonique, la fibre Kagomé à coeur creux, pour la livraison d'impulsions ultra-courtes en endoscopie non linéaire. Ces fibres permettent la livraison d'impulsions sans distorsion sur une large bande spectrale, avec un faible bruit de fond, grâce à la propagation dans le cœur creux. Nous avons résolu le problème de la résolution spatiale, à l'aide d'une microbille en silice, insérée dans le cœur de la fibre Kagomé. Nous avons développé un système d'imagerie compacte, qui utilise un tube piézo-électrique pour le balayage du faisceau, un système achromatiques de microlentilles et une fibre Kagomé double gaine, spécialement conçue pour l'endoscopie. Avec ce système, nous avons réussi à imager des tissus biologiques, à l'extrémité distale de la fibre (endoscopie), en utilisant des différentes techniques tels que TPEF, SHG et CARS, un résultat qui ne trouve pas d'égal dans la littérature actuelle. L'intégration dans une sonde portable (4,2 mm de diamètre) montre le potentiel de ce système pour de futures applications en endoscopie multimodale in-vivo. / In this thesis, we propose the use of a novel type of photonic crystal fiber, the Kagomé lattice hollow core fiber, for the delivery of ultra-short pulses in nonlinear endoscopy. These fibers allow undistorted pulse delivery, over a broad transmission window, with minimum background signal generated in the fiber, thanks to the propagation in a hollow-core. We solved the problem of spatial resolution, by means of a silica micro-bead inserted in the Kagomé fiber large core. We have developed a miniature imaging system, based on a piezo-electric tube scanner, an achromatic micro-lenses assembly and a specifically designed Kagomé double-clad fiber. With this system we were able to image biological tissues, in endoscope modality, activating different contrasts such as TPEF, SHG and CARS, at the distal end of the fiber, a result which finds no equal in current literature. The integration in a portable probe (4.2 mm in diameter) shows the potential of this system for future in-vivo multimodal endoscopy.

Fiber based ultrashort pulse laser sources are desirable for many applications; however generating high peak powers in fiber lasers is primarily limited by the onset of nonlinear effects such as self-phase modulation, stimulated Raman scattering, and self-focusing. Increasing the fiber core diameter mitigates the onset of these nonlinear effects, but also allows unwanted higher-order transverse spatial modes to propagate. Both large core diameters and single-mode propagation can be simultaneously attained using photonic crystal fibers. Thulium-doped fiber lasers are attractive for high peak power ultrashort pulse systems. They offer a broad gain bandwidth, capable of amplifying sub-100 femtosecond pulses. The longer center wavelength at 2 ?m theoretically enables higher peak powers relative to 1 [micro]m systems since nonlinear effects inversely scale with wavelength. Also, the 2 [micro]m emission is desirable to support applications reaching further into the mid-IR. This work evaluates the performance of a novel all-fiber pump combiner that incorporates a thulium-doped photonic crystal fiber. This fully integrated amplifier is characterized and possesses a large gain bandwidth, essentially single-mode propagation, and high degree of polarization. This innovative all-fiber, thulium-doped photonic crystal fiber amplifier has great potential for enabling high peak powers in 2 [micro]m fiber systems; however the current optical-to-optical efficiency is low relative to similar free-space amplifiers. Further development and device optimization will lead to higher efficiencies and improved performance.

Lasers find a wide range of applications in many areas including photo-biology, photo-chemistry, materials processing, imaging and telecommunications. However, the practical use of such sources is often limited by the bulky nature of existing systems. By fabricating channel waveguides in solid-state laser-gain materials more compact laser systems can be designed and fabricated, providing user-friendly sources. Other advantages inherent in the use of waveguide gain media include the maintenance of high intensities over extended interaction lengths, reducing laser thresholds. This thesis presents the development of Yb:tungstate lasers operating around 1μm in waveguide geometries. An Yb:KY(WO₄)₂ planar waveguide laser grown by liquid phase epitaxy is demonstrated with output powers up to 190 mW and 76 % slope efficiency. This is similar to the performance from bulk lasers but in a very compact design. Excellent thresholds of only 40 mW absorbed pump power are realised. The propagation loss is found to be less than 0.1 dBcm⁻¹ and Q-switched operation is also demonstrated. Channel waveguides are fabricated in Yb:KGd(WO₄)₂ and Yb:KY(WO₄)₂ using ultrafast laser inscription. Several of these waveguides lase in compact monolithic cavities. A maximum output power of 18.6 mW is observed, with a propagation loss of ~2 dBcm⁻¹. By using a variety of writing conditions the optimum writing pulse energy is identified. Micro-spectroscopy experiments are performed to enable a fuller understanding of the induced crystal modification. Observations include frequency shifts of Raman lines which are attributed to densification of WO₂W bonds in the crystal. Yb:tungstate lasers can generate ultrashort pulses and some preliminary work is done to investigate the use of quantum dot devices as saturable absorbers. These are shown to have reduced saturation fluence compared to quantum well devices, making them particularly suitable for future integration with Yb:tungstate waveguides for the creation of ultrafast, compact and high repetition rate lasers.