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Thesis (PhD)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: The concept of broadband coherent supercontinuum (SC) generation in all-normal dispersion (ANDi) fibers in the near-infrared, visible and ultraviolet (UV) spectral regions is introduced and investigated in detail. In numerical studies, explicit design criteria are established for ANDi photonic crystal fiber (PCF) designs that allow the generation of flat and smooth ultrabroad spectral profiles without significant fine structure and with excellent stability and coherence properties. The key benefit of SC generation in ANDi fibers is the conservation of a single ultrashort pulse in the time domain with smooth and recompressible phase distribution. In the numerical investigation of the SC generation dynamics self-phase modulation and optical wave breaking are identified as the dominant nonlinear effects responsible for the nonlinear spectral broadening. It is further demonstrated that coherence properties, spectral bandwidth and temporal compressibility are independent of input pulse duration for constant peak power. The numerical predictions are in excellent agreement with experimental results obtained in two realizations of ANDi PCF optimized for the near-infrared and visible spectral region. In these experiments, the broadest SC spectrum generated in the normal dispersion regime of an optical fiber to date is achieved. The exceptional temporal properties of the generated SC pulses are verified experimentally and their applicability for the time-resolved study of molecular dynamics in ultrafast transient absorption spectroscopy is demonstrated. In an additional nonlinear pulse compression experiment, the SC pulses obtained in a short piece of ANDi PCF could be temporally recompressed to sub-two cycle durations by linear chirp compensation. Numerical simulations show that even shorter pulse durations with excellent quality can be achieved by full phase compensation. The concept is further extended into the UV spectral regime by considering tapered optical fibers with submicron waist diameter. It is shown that coherent SC spectra with considerable spectral power densities in the usually hard to reach wavelength region below 300 nm can be generated using these freestanding photonic nanowires. Although technological difficulties currently prevent the fabrication of adequate nanofibers, the concept could be experimentally verified by coherent visible octave-spanning SC generation in tapered suspended core fibers with ANDi profile. The work contained in this thesis therefore makes important contributions to the availability and applicability of fiber-based broadband coherent SC sources with numerous high-impact applications in fundamental science and modern technology. / AFRIKAANSE OPSOMMING: Die konsep van breëband koherente superkontinuum (SK) in alles-normaal dispersiewe (ANDi) vesels in die naby-infrarooi, sigbare en ultraviolet (UV) spektrale gebiede word voorgestel en in detail ondersoek. In numeriese studies word eksplisiete ontwerpskriteria vasgestel vir ANDi fotoniese kristal vesel (FKV) ontwerpe wat dit moontlik maak om plat en gladde ultra-breë spektrale profiele te genereer sonder noemenswaardige fynstruktuur en met uitstekende stabiliteit en koherensie eienskappe. Die sleutel voordeel van SK genering in ANDi vesels is die behoud van ’n enkele ultrakort puls in tyd met ’n gladde en saamdrukbare fase distribusie. In die numeriese ondersoek van die SK generering is die dinamika van fase selfmodulering geïdentifiseer as die dominante nie-lineêre effek wat verantwoordelik is vir die nie-lineêre spektrale verbreding. Daar word voorts aangetoon dat die koherensie eienskappe, spektrale bandwydte en saamdrukbaarheid in tyd onafhanklik is van die inset pulsduur vir konstante drywing. Die numeriese voorspellings stem uitstekend ooreen met die eksperimentele resultate wat verkry is met twee ANDi FKVÕs wat optimeer is vir die naby-infrarooi en sigbare spektrale gebied. In hierdie eksperimente is die breedste SK spektrum gegenereer wat tot hede in die normaal dispersiewe regime met ’n optiese vesel behaal is. Die besondere eienskappe van die genereerde SK pulse is eksperimenteel bevestig en die toepasbaarheid vir tyd opgelosde studie van molekulêre dinamika is gedemonstreer. In ’n addisionele nie-lineêre puls kompressie eksperiment is SK pulse verkry in ’n kort stuk ANDi FKV wat in tyd saamgedruk kon word tot sub-twee siklus tydsduur deur liniêre tjirp kompensering. Numeriese simulasies toon aan dat selfs korter pulse met uitstekende kwaliteit behaalbaar is met volledige fase kompensasie. Die konsep is verder uitgebrei na die UV spektrale gebied deur ’n koniese vesel te beskou met sub-mikron diameter. Daar is aangetoon dat koherente SK spektra met noemenswaardige spektrale drywing in die golflengte gebied onder 300 nm, wat gewoonlik as moeilik toeganklik beskou word, bereik kan word deur hierdie vrystaande fotoniese nano-vesels aan te wend. Alhoewel tegnologiese probleme die vervaardiging van voldoende nano-vesels verhinder, kon die konsep eksperimenteel bewys word deur koherente sigbare oktaafspannende SK te genereer in koniese gesuspendeerde kern vesels met ’n ANDi profiel aan te wend. Die werk wat in die tesis vervat is, maak dus belangrike bydraes tot die beskikbaarheid en toepasbaarheid van vesel gebaseerde breëband koherente SK bronne met verskeie hoë impak toepassings in fundamentele wetenskap en moderne tegnologie.

Photonic crystal fibres (PCFs) are special fibres with air holes which run along the whole fibre length. These holes not only determine the fibres' unique properties, but also provide a new degree of freedom for fibre modications. In this thesis, we focus on hole control in PCFs from two perspectives: during their fabrication and after they have been made. We found for the first time that the direct information of viscosity was not necessary for description of the fibre drawing process. This conclusion matched our experimental results without recourse to any adjustable fitting parameters. By post-processing of PCFs, which modifies the cladding and core structure and shape, we have achieved a series of novel devices for both linear and nonlinear applications. We have demonstrated fibre devices with cores resembling Young's double slits that have good performance in terms of compatibility and intensity enhancement for a specific application in fibre optic spectrometers. The bulk of this thesis reports on higher-order modes and their nonlinear applications. We achieved all-fibre, low loss and broadband mode converters in highly nonlinear PCFs (HNPCFs) which converted the fundamental mode (LP01) to a higher-order mode (LP02), which can then be converted back if necessary. This higher-order mode has been used for supercontinuum (SC) generation and four wave mixing (FWM) at wavelengths unobtainable for the fundamental mode. This is achieved by utilising the profound dispersion properties of the higher-order mode. We also demonstrated another kind of mode conversion: from the fundamental mode to a Bessel-like beam or its Fourier transform version, an annular beam. Three different methods were implemented experimentally to achieve this non-diffractive, self-healing beam.

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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Esta tese descreve a demonstração experimental de dispositivos baseados em fibras de cristal fotônico (PCFs), que aproveitam a flexibilidade estrutural oferecida pela matriz de capilares que compõe a seção reta da fibra, de forma a preencher estes capilares com líquidos e materiais nanoestruturados. Para o caso de materiais nanoestruturados, uma vez preenchida a fibra, os materiais nela inseridos interagem eficientemente com a luz guiada. Essa arquitetura diferenciada em relação às fibras ópticas convencionais abre novas perspectivas no desenvolvimento de aplicações como óptica não-linear e sensoriamento. PCFs de núcleo líquido, por outro lado, impõe dificuldades para a implementação de dispositivos práticos, devido às altas taxas de evaporação dos líquidos inseridos. Por esta razão, foi desenvolvida uma nova técnica para vedar seletivamente ambas as faces externas do núcleo líquido de uma PCF, utilizando um polímero curável. Estes tampões poliméricos evitam a evaporação, causando um impacto mínimo no guiamento da luz, tornando o dispositivo usável por semanas. Esta nova técnica de vedação foi empregada em um experimento para a geração de supercontínuo em uma PCF com núcleo de água destilada, proporcionando uma estabilidade de pelo menos 1 hora. Combinando líquidos e materiais nanoestruturados, foi também foi desenvolvido um sensor de temperatura baseado no preenchimento do núcleo de uma PCF por uma amostra coloidal de nanopartículas semicondutoras de CdSe/ZnS, dispersas em óleo mineral. O espectro de luminescência destes pontos quânticos coloidais é fortemente dependente da temperatura e os resultados obtidos mostraram que a grande interação entre a luz e o colóide, aliada a geometria da fibra, proporcionando uma sensibilidade ~5,5 vezes maior que a apresentada por uma rede de Bragg escrita em uma fibra óptica padrão, com boa relação sinal-ruído. / This thesis describes the experimental demonstration of devices based on photonic crystal fibers (PCFs). PCFs are optical fibers whose core is surrounded by a regular matrix of holes, which runs longitudinally across its length. This singular configuration allows the insertion of liquids and nanostructured material into the fiber. Nanostructured materials embedded inside the fiber efficiently interact with the guided light, opening up possibilities of novel applications regarding the fields of non-linear optics, as well as optical sensing. On the other hand, liquid-core PCFs suffer from some disadvantages concerning practical device applications, on account of the high evaporation of the inserted liquids. In order to address this issue, we developed a novel technique to selectively seal the external faces of a liquid-core PCF, by using a polymer plug. These polymer plugs avoid evaporation while causing a minimum impact on the light guiding characteristics of the PCF. This novel sealing technique was employed in a supercontinuum generation experiment, by using a PCF whose core was water-filled. A temporal stability of at least one-hour on the resulting spectrum was achieved. Combining the above techniques, we also developed a temperature sensor based on the core-filling of a PCF by a colloidal ensemble of CdSe/ZnS semiconductor nanoparticles dispersed in mineral oil. Those colloidal quantum-dots display a luminescence spectrum which is strongly dependent on temperature and the experimental results indicated that the greater interaction between the guided light and the colloidal sample, provided by the fiber geometry, allowed a sensitivity which is approximately 5.5 times than possible with a conventional Bragg grating, while keeping a satisfactory signal-to-noise ratio.

Research towards the development of a gas-discharge fibre laser using noble gases, with target emission wavelengths in the mid-IR. Additional and separate work on gas treatment methods for managing the formation of photo-induced defects in silica glass.

Esta tese descreve a demonstração experimental de dispositivos baseados em fibras de cristal fotônico (PCFs), que aproveitam a flexibilidade estrutural oferecida pela matriz de capilares que compõe a seção reta da fibra, de forma a preencher estes capilares com líquidos e materiais nanoestruturados. Para o caso de materiais nanoestruturados, uma vez preenchida a fibra, os materiais nela inseridos interagem eficientemente com a luz guiada. Essa arquitetura diferenciada em relação às fibras ópticas convencionais abre novas perspectivas no desenvolvimento de aplicações como óptica não-linear e sensoriamento. PCFs de núcleo líquido, por outro lado, impõe dificuldades para a implementação de dispositivos práticos, devido às altas taxas de evaporação dos líquidos inseridos. Por esta razão, foi desenvolvida uma nova técnica para vedar seletivamente ambas as faces externas do núcleo líquido de uma PCF, utilizando um polímero curável. Estes tampões poliméricos evitam a evaporação, causando um impacto mínimo no guiamento da luz, tornando o dispositivo usável por semanas. Esta nova técnica de vedação foi empregada em um experimento para a geração de supercontínuo em uma PCF com núcleo de água destilada, proporcionando uma estabilidade de pelo menos 1 hora. Combinando líquidos e materiais nanoestruturados, foi também foi desenvolvido um sensor de temperatura baseado no preenchimento do núcleo de uma PCF por uma amostra coloidal de nanopartículas semicondutoras de CdSe/ZnS, dispersas em óleo mineral. O espectro de luminescência destes pontos quânticos coloidais é fortemente dependente da temperatura e os resultados obtidos mostraram que a grande interação entre a luz e o colóide, aliada a geometria da fibra, proporcionando uma sensibilidade ~5,5 vezes maior que a apresentada por uma rede de Bragg escrita em uma fibra óptica padrão, com boa relação sinal-ruído. / This thesis describes the experimental demonstration of devices based on photonic crystal fibers (PCFs). PCFs are optical fibers whose core is surrounded by a regular matrix of holes, which runs longitudinally across its length. This singular configuration allows the insertion of liquids and nanostructured material into the fiber. Nanostructured materials embedded inside the fiber efficiently interact with the guided light, opening up possibilities of novel applications regarding the fields of non-linear optics, as well as optical sensing. On the other hand, liquid-core PCFs suffer from some disadvantages concerning practical device applications, on account of the high evaporation of the inserted liquids. In order to address this issue, we developed a novel technique to selectively seal the external faces of a liquid-core PCF, by using a polymer plug. These polymer plugs avoid evaporation while causing a minimum impact on the light guiding characteristics of the PCF. This novel sealing technique was employed in a supercontinuum generation experiment, by using a PCF whose core was water-filled. A temporal stability of at least one-hour on the resulting spectrum was achieved. Combining the above techniques, we also developed a temperature sensor based on the core-filling of a PCF by a colloidal ensemble of CdSe/ZnS semiconductor nanoparticles dispersed in mineral oil. Those colloidal quantum-dots display a luminescence spectrum which is strongly dependent on temperature and the experimental results indicated that the greater interaction between the guided light and the colloidal sample, provided by the fiber geometry, allowed a sensitivity which is approximately 5.5 times than possible with a conventional Bragg grating, while keeping a satisfactory signal-to-noise ratio.