Thesis consists of introduction, followed by five main chapters and conclusions. Introduction is devoted to the explanation of filamentation process. The mechanisms responsible for filament generation and evolution are explained first and various models of the filamentation are presented as well. The second chapter is devoted to the study on supercontinuum spectrum dependence on the initial pulse diameter. The chapter 4 is devoted to the study of the filamentation in scattering medium, and covers the performed experiment, explanation of new developed numerical scheme, and finally comparison of the numerical and experimental results. The chapter 5 describes the multifilamentation with elliptical beam. The role of the four wave mixing to the filaments periodicity is briefly explained. The periodicity dependence on the wave of intensity as well the evolution of multifilamentation is shown. Three dimensional study of the modulation instability during multifilamentation process is also presented in chapter 5. The last chapter 6 is devoted to the origin of the rogue wave statistics in supercontinuum generation, and comparison of the numeric results with experimental. At the end (chapter 7) the main conclusions of the current work are presented. / Disertaciją sudaro įvadas, penki pagrindiniai skyriai ir išvados. Įvade aiškinamas sudėtingas šviesos gijų formavimosi procesas. Paaiškinami fizikiniai reiškiniai, lemiantys šviesos gijų generacijos bei evoliucijos ypatybes, supažindinama su įvairiais šviesos gijų susidarymo ir sklidimo modeliais. Antrajame skyriuje nagrinėjama pradinio pluošto diametro įtaka superkontinuumo spektrui. Trečias skyrius, skirtas šviesos gijų generacijos sklaidančioje terpėje tyrimui, apima atlikto eksperimento bei sukurtos naujos skaitmeninio modeliavimo schemos pristatymą, ir, galiausiai, skaitmeninio modeliavimo ir eksperimentinių rezultatų palyginimą. Ketvirtame skyriuje nagrinėjama daugelio gijų generacija elipsiniais pluoštais. Trumpai paaiškinta kaskadinio keturbangio maišymo įtaka šviesos gijų periodiškumui. Ištirta šviesos gijų periodo priklausomybė nuo bangos intensyvumo, ir atskleista daugelio gijų generacijos dinamika. Moduliacinio nestabilumo analizė ir jos įtaka taip pat pateikiama ketvirtame skyriuje. Penktajame skyriuje nagrinėjama ekstremalių įvykių tūrinėje Kerro terpėje atsiradimas. Palyginami plataus spektro generacijos kompiuterinio modeliavimo ir eksperimento metu surinkti statistiniai duomenys. Pabaigoje pateikti pagrindiniai darbo rezultatai ir išvados.

Disertaciją sudaro įvadas, penki pagrindiniai skyriai ir išvados. Įvade aiškinamas sudėtingas šviesos gijų formavimosi procesas. Paaiškinami fizikiniai reiškiniai, lemiantys šviesos gijų generacijos bei evoliucijos ypatybes, supažindinama su įvairiais šviesos gijų susidarymo ir sklidimo modeliais. Antrajame skyriuje nagrinėjama pradinio pluošto diametro įtaka superkontinuumo spektrui. Trečias skyrius, skirtas šviesos gijų generacijos sklaidančioje terpėje tyrimui, apima atlikto eksperimento bei sukurtos naujos skaitmeninio modeliavimo schemos pristatymą, ir, galiausiai, skaitmeninio modeliavimo ir eksperimentinių rezultatų palyginimą. Ketvirtame skyriuje nagrinėjama daugelio gijų generacija elipsiniais pluoštais. Trumpai paaiškinta kaskadinio keturbangio maišymo įtaka šviesos gijų periodiškumui. Ištirta šviesos gijų periodo priklausomybė nuo bangos intensyvumo, ir atskleista daugelio gijų generacijos dinamika. Moduliacinio nestabilumo analizė ir jos įtaka taip pat pateikiama ketvirtame skyriuje. Penktajame skyriuje nagrinėjama ekstremalių įvykių tūrinėje Kerro terpėje atsiradimas. Palyginami plataus spektro generacijos kompiuterinio modeliavimo ir eksperimento metu surinkti statistiniai duomenys. Pabaigoje pateikti pagrindiniai darbo rezultatai ir išvados. / Thesis consists of introduction, followed by five main chapters and conclusions. Introduction is devoted to the explanation of filamentation process. The mechanisms responsible for filament generation and evolution are explained first and various models of the filamentation are presented as well. The second chapter is devoted to the study on supercontinuum spectrum dependence on the initial pulse diameter. The chapter 4 is devoted to the study of the filamentation in scattering medium, and covers the performed experiment, explanation of new developed numerical scheme, and finally comparison of the numerical and experimental results. The chapter 5 describes the multifilamentation with elliptical beam. The role of the four wave mixing to the filaments periodicity is briefly explained. The periodicity dependence on the wave of intensity as well the evolution of multifilamentation is shown. Three dimensional study of the modulation instability during multifilamentation process is also presented in chapter 5. The last chapter 6 is devoted to the origin of the rogue wave statistics in supercontinuum generation, and comparison of the numeric results with experimental. At the end (chapter 7) the main conclusions of the current work are presented.

In 1995 Braun and co-workers reported the observation of self-channeling of femtosecond laser pulses over 20 m in air. This first observation of femtosecond filament triggered a series of studies, which discovered an exciting physics beyond the interaction of ultrashort laser pulses with transparent dielectric media. This dissertation aims at comprehensive study of transient spatiotemporal phenomena (space-time transformations, pulse splitting and compression, filament propagation dynamics, supercontinuum generation and multiple filamentation) that take place during self-focusing of intense femtosecond laser pulses in various self-action regimes in transparent dielectric media with instantaneous Kerr nonlinearity. Various nonlinear optics diagnostic methods (three dimensional mapping, auto- and cross-correlation measurements, frequency resolved optical gating measurements), spatially resolved frequency spectra measurements, statistical analysis of spectral intensities and energies, numerical simulations were employed. The dissertation addresses important issues regarding complete physical understanding of the evolution cycle of femtosecond filaments in normally dispersive media, physical nature of spatiotemporal light bullets generated by filamentation in the anomalous group velocity regime, detailed statistical aspects of supercontinuum generation, and spatiotemporal characterization of multiple filaments. Analysed factors are useful for practical applications when... [to full text] / Ultratrumpiesiems šviesos impulsams sklindant skaidriose terpėse dėl jų saviveikos gali formuotis ypatingi šviesos dariniai – šviesos gijos, kurios pirmą kartą stebėtos 1995 m. Braun ir kt. Pirmuosius šviesos gijų stebėjimus sekė eilė tyrimų, kurie atskleidė įdomius femtosekundinių lazerinių impulsų saviveikos reiškinius, kartais apimančius platesnes netiesinės optikos tyrimų sritis. Šios disertacijos tikslas – išsamiai ištirti dinaminius erdvėlaikinius reiškinius (šviesos gijų sklidimą, erdvėlaikines transformacijas, impulsų skilimą ir spūdą, superkontinuumo generaciją ir daugelio šviesos gijų atsiradimą) įvairiuose intensyvių femtosekundinių lazerio impulsų saviveikos režimuose skaidriose dielektrinėse terpėse su momentiniu Kero netiesiškumu. Tyrimams naudoti įvairūs netiesinės optikos metodai (trimatė lazerinė tomografija, auto- ir kryžminės koreliacijos matavimai, dažninės skyros optinės sklendės autokoreliaciniai matavimai), erdvinės-dažninės skyros spektrų matavimas, statistinė spektrinių, energijos matavimų analizė, skaitmeninis modeliavimas. Atliktų tyrimų visuma išplečia esamas žinias apie femtosekundinių šviesos impulsų saviveiką ir netiesinių optinių sistemų dinamiką normalios ir anomalios grupinių greičių dispersijos atvejais, vedančią prie nestabilumo ir ekstremalių optinių įvykių žadinimo. Kita vertus, ištirti veiksniai naudingi praktiniuose taikymuose, kai siekiama formuoti tvarkias erdvines-laikines struktūras, žadinti ekstremalias optines bangas arba... [toliau žr. visą tekstą]

Ultratrumpiesiems šviesos impulsams sklindant skaidriose terpėse dėl jų saviveikos gali formuotis ypatingi šviesos dariniai – šviesos gijos, kurios pirmą kartą stebėtos 1995 m. Braun ir kt. Pirmuosius šviesos gijų stebėjimus sekė eilė tyrimų, kurie atskleidė įdomius femtosekundinių lazerinių impulsų saviveikos reiškinius, kartais apimančius platesnes netiesinės optikos tyrimų sritis. Šios disertacijos tikslas – išsamiai ištirti dinaminius erdvėlaikinius reiškinius (šviesos gijų sklidimą, erdvėlaikines transformacijas, impulsų skilimą ir spūdą, superkontinuumo generaciją ir daugelio šviesos gijų atsiradimą) įvairiuose intensyvių femtosekundinių lazerio impulsų saviveikos režimuose skaidriose dielektrinėse terpėse su momentiniu Kero netiesiškumu. Tyrimams naudoti įvairūs netiesinės optikos metodai (trimatė lazerinė tomografija, auto- ir kryžminės koreliacijos matavimai, dažninės skyros optinės sklendės autokoreliaciniai matavimai), erdvinės-dažninės skyros spektrų matavimas, statistinė spektrinių, energijos matavimų analizė, skaitmeninis modeliavimas. Atliktų tyrimų visuma išplečia esamas žinias apie femtosekundinių šviesos impulsų saviveiką ir netiesinių optinių sistemų dinamiką normalios ir anomalios grupinių greičių dispersijos atvejais, vedančią prie nestabilumo ir ekstremalių optinių įvykių žadinimo. Kita vertus, ištirti veiksniai naudingi praktiniuose taikymuose, kai siekiama formuoti tvarkias erdvines-laikines struktūras, žadinti ekstremalias optines bangas arba... [toliau žr. visą tekstą] / In 1995 Braun and co-workers reported the observation of self-channeling of femtosecond laser pulses over 20 m in air. This first observation of femtosecond filament triggered a series of studies, which discovered an exciting physics beyond the interaction of ultrashort laser pulses with transparent dielectric media. This dissertation aims at comprehensive study of transient spatiotemporal phenomena (space-time transformations, pulse splitting and compression, filament propagation dynamics, supercontinuum generation and multiple filamentation) that take place during self-focusing of intense femtosecond laser pulses in various self-action regimes in transparent dielectric media with instantaneous Kerr nonlinearity. Various nonlinear optics diagnostic methods (three dimensional mapping, auto- and cross-correlation measurements, frequency resolved optical gating measurements), spatially resolved frequency spectra measurements, statistical analysis of spectral intensities and energies, numerical simulations were employed. The dissertation addresses important issues regarding complete physical understanding of the evolution cycle of femtosecond filaments in normally dispersive media, physical nature of spatiotemporal light bullets generated by filamentation in the anomalous group velocity regime, detailed statistical aspects of supercontinuum generation, and spatiotemporal characterization of multiple filaments. Analysed factors are useful for practical applications when... [to full text]

Le travail de cette thèse a porté sur l'étude de la propagation d'ondes dans une fibre optique à biréfringence forte. Nous avons procédé à une caractérisation générale de l'instabilité modulationelle en fonction des biréfringences linéaire et non linéaire, en dispersion anormale et surtout en dispersion normale. L'étude a été consacrée à une fibre optique non linéaire en régime "pulsé", où l'excitation n'est pas étendue mais plutôt localisée. Dans ce cas, l'analyse de stabilité linéaire standard ne parvient pas à décrire l'évolution linéaire de cette perturbation. Ainsi, nous avons eu recours à un problème à valeur initiale menant aux deux régimes convectif et absolu. Nons avons ensuite mis en évidence, pour la première fois, une transition du régime absolu vers le régime convectif, en caractérisant chacun des deux régimes en fonction des biréfringences. Les résultats numériques sont en excellent accord avec nos prédictions analytiques. Nous avons évalué l'impact de la brisure de symétrie sur la génération des supercontinuums. Ces derniers jouent un rôle primordial dans la formation des ondes scélérates. Nous avons opté pour une analyse statistique basée sur la fonction de densité de probabilité des pics les plus intenses. Nous avons analysé l'impact du "walk off" et de la dispersion d'ordre trois sur l'émergence de ces ondes. Nos simulations numériques montrent que les ondes scélérates peuvent être contrôlées par la biréfringence linéaire et elles sont encore plus prononcées en présence de la dispersion d'ordre trois. Finalement, une optimisation de la génération des ondes scélérates nous a permis de trouver une onde géante, reproduite par le soliton de Peregrine. / The work of this thesis has focused on the study of wave propagation in a high birefringent fiber. We have carried out a general characterization of the modulational instability as a function of the linear and nonlinear birefringences, in abnormal dispersion and especially in normal dispersion. The study was devoted to a non-linear optical fiber in a "pulsed" regime, where the excitation is not extended but rather localized in time. In this case, standard linear stability analysis fails to describe the linear evolution of this type of perturbations. Thus, we reformulate the problem as an initial value problem leading to convective and absolute instabilities. Then, we evidenced, for the first time, a transition from the absolute to the convective regime and we characterized each of them by linear and nonlinear birefringences. Numerical results are in excellent agreement with our analytical predictions. We evaluated the impact of the symmetry breaking on the generation of supercontinuums that play a crucial role in the formation of rogue waves. We performed a statistical analysis based on the probability density function of the most intense peaks. We analyzed the impact of the "walk off" and the third order dispersion on the emergence of these waves in abnormal and normal dispersion. The results of the numerical integration of the governing equations show that the rogue waves can be controlled by linear birefringence and are even more pronounced in presence of the third order dispersion. Finally, an optimization of the generation of extreme waves has allowed us to find a giant wave, reproduced by Pergerine soliton.

Geração de luz supercontínua é um efeito caracterizado por um forte alargamento espectral induzido por efeitos não-lineares no meio óptico, e que encontra uma vasta gama de aplicações, como tomografia por coerência óptica, espectroscopia e metrologia de freqüência. No presente trabalho, os fenômenos físicos responsáveis pela geração de luz supercontínua são estudados teoricamente, utilizando a ferramenta numérica desenvolvida para dar suporte à teoria. Os efeitos de polarização relacionados à geração de luz supercontínua também são analisados e, neste contexto, a utilização de uma fibra de cristal fotônico com formato tipo-D e alta birrefringência é proposta para servir como meio óptico. As características do supercontínuo gerado nesta fibra são analisadas teoricamente, novamente com o apoio da ferramenta numérica. A análise conclui que a fibra proposta apresenta consideráveis vantagens em relação às fibras convencionais, principalmente por ser capaz de manter o estado de polarização da luz supercontínua estável. / Supercontinuum generation is an effect characterized by a strong spectral broadening induced by nonlinear effects in the optical medium. There is a wide range of applications for this effect, such as optical coherence tomography, spectroscopy and frequency metrology. In this work, the physical phenomena responsible for supercontinuum generation are theoretically studied relying on the developed numerical tool to support the theory. The polarization effects related to supercontinuum generation are also assessed and, in this context, a highly birefringent D-shaped photonic crystal fiber is proposed as a medium for supercontinuum generation. The characteristics of supercontinuum generation in the proposed fiber are theoretically analyzed. The analysis concludes that the proposed fiber shows considerable advantages over conventional fibers, mainly due to the capability to keep the state of polarization of the supercontinuum stable.

Les milieux Kerr soumis à des faisceaux optiques contrapropageants provenant, soit d'une cavité Pérot-Fabry plan-plan ou d'une boucle de rétro-action, ont été très étudiés pour la génération de structures transverses. De nombreux développements théoriques dans la configuration de la cavité ont prédit l'existence de structures localisées (solitons). Cependant aucune mise en évidence expérimentale de ces structures n'a été réalisée à ce jour. Dans la configuration de la boucle de rétro-action optique, les régimes fortement non linéaires présentent des structures rares et intenses, très localisées aussi bien spatialement que temporellement. C'est dans ce cadre que nous nous intéressons expérimentalement à la structuration localisée de la lumière dans ces deux systèmes. La première partie de cette thèse est consacrée à la mise en évidence expérimentale de solitons spatiaux dans une cavité Pérot-Fabry plan-plan Kerr à diffraction positive. Ensuite, nous étudions cette cavité quand elle est soumise à de la diffraction négative. Des parois de domaines propagatives sont alors observées. Celles-ci sont bloquées par le forçage spatial lié à l'inhomogénéité du profil de pompage optique gaussien qui donne lieu à une localisation de la lumière. Nous étudions succinctement le cas limite d'une cavité sans diffraction dans laquelle les parois de domaines survivent. Pour finir nous effectuons une étude du régime très fortement non linéaire dans le dispositif de rétro-action optique grâce à une approche statistique. Nous observons l'apparition de structures localisées scélérates associées à l'émission d'un supercontinuum spectral spatial. / Kerr media subject to counterpropagating optical beams in a plan-plan Pérot-Fabry cavity or optical feedback have been deeply studied for tranverse pattern generation. Many theoretical developments in the cavity configuration predict the existence of localized structures (solitons). However, no experimental evidence of these structures have been giben up to now. In the feedback configuration, the highly nonlinear regimes present rare and intense localized structures in space and in time. In This this framework, we are interested in studying experimentaly the localization of light in the both previous systems. The first part of this thesis is devoted to the experimental evidence of spatial solitons in a Pérot-Fabry Kerr plan-plan cavity with positive diffraction. Then we study this cavity submitted to negative diffraction. Propagative domain walls are observed. They are pinned by the spatial forcing coming from the inhomogeneous gaussian optical pumping profile that induces light localization. We study briefly the case of cavity without diffraction in which domain walls persist. We findly perform a study of highly nonlinear regimes in the optical feedback device using a statistical approach. We observe the emergence of rogue localized structures associated with the emission of a spatial spectral supercontinuum.

Raman biosensors are appealing for many biomedical applications, due to their accuracy and speed. In addition, Raman microscopy is a non-labeled imaging technique that offers chemical contrast based on Raman vibrational frequencies. However, the weak Raman signal represents a significant obstacle to using Raman in biological applications. The objective of my PhD research, presented in this thesis, is to enhance the Raman signal, thereby enabling it to be used in a wide variety of biomedical applications. More specifically, the research focuses on two different Raman signal enhancement techniques. The first is to improve the Raman signal using hollow-core photonic crystal fibers; this enhanced the Raman signal of ethanol 40 times. The second approach is by generating a coherent anti-Stokes Raman scattering (CARS) signal. We demonstrated CARS microscopy of myelin (lipid-rich) structures using a single femtosecond Ti:sapphire laser, and a photonic crystal fiber (PCF) with two closely lying zero dispersion wavelengths (ZDWs). Generating low noise supercontinuum (Stokes beam) out of two closely lying ZDW PCFs, enabled us to perform fast data acquisition (84 μs per pixel) CARS imaging using a homebuilt microscope. However, the application of this fiber is often limited to CARS imaging of molecular species with vibrations at wavenumbers ≥ 2000 cm−1 Raman shift. In addition, as it is not a polarization maintaining fiber, it cannot be used for polarization CARS microscopy. A polarization-maintaining PCF with two far-lying zero dispersion wavelengths offers important advantages for polarization CARS microscopy, and for CARS imaging in the fingerprint region. This PCF, though commercially available, has had limited use for CARS microscopy in the C-H bond region. The main problem is that the supercontinuum from this fiber is typically noisier than that from a standard PCF with two closely-lying zero dispersion wavelengths. To overcome this, we determined the optimum operating conditions for generating a low-noise supercontinuum out of a PCF with two far-lying zero dispersion wavelengths, in terms of the input parameters of the excitation pulse. We measured the relative intensity noise (RIN) of the Stokes and the corresponding CARS signal, as a function of the input laser parameters in this fiber. We demonstrated that the results of CARS imaging using this alternate fiber are comparable to those achieved using the standard fiber for input laser pulse conditions of low average power, narrow pulse width with a slightly positive chirp, and polarization direction parallel to the slow axis of the selected fiber. Finally, we demonstrated a novel fiber-delivered, portable, multimodal CARS exoscope, for minimally invasive in-vivo imaging of tissues. The device was based on a micro-electromechanical system-scanning mirror and miniaturized optics, and light delivery by photonic crystal fibre. A single Ti:sapphire femtosecond laser approach is used to produce CARS and two photon excitation fluorescent and second harmonic generation images of different samples using the new setup. The high resolution and distortion-free images achieved with various samples, particularly in the reverse direction (epi), successfully demonstrate proof of concept, and paves the way to minimally or non-invasive in vivo imaging. Moreover, combining this novel endoscope with a portable femtosecond fiber laser will accelerate delivering multimodal nonlinear imaging endoscopy/microscopy to clinical bed-side applications.

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