In this dissertation, high-power ytterbium-doped fiber amplifiers designed with advanced waveguide concepts are characterized and power scaled. Fiber waveguides utilizing cladding microstructures to achieve wave guidance via the photonic bandgap (PBG) effect and a combination of PBG and modified total internal reflection (MTIR) have been proposed as viable single-mode waveguides. Such novel structures allow larger core diameters (>35 μm diameters) than conventional step-index fibers while still maintaining near-diffraction limited beam quality. These microstructured fibers are demonstrated as robust single-mode waveguides at low powers and are power scaled to realize the thermal power limits of the structure. Here above a certain power threshold, these coiled few-mode fibers have been shown to be limited by modal instability (MI); where energy is dynamically transferred between the fundamental mode and higher-order modes. Nonlinear effects such as stimulated Brillouin scattering (SBS) are also studied in these fiber waveguides as part of this dissertation. Suppressing SBS is critical towards achieving narrow optical bandwidths (linewidths) necessary for efficient fiber amplifier beam combining. Towards that end, new effects that favorably reduce acoustic wave dispersion to increase the SBS threshold are discovered and reported. The first advanced waveguide examined is a Yb-doped 50/400 µm diameter core/clad PBGF. The PBGF is power scaled with a single-frequency 1064 nm seed to an MI-limited 410 W with 79% optical-to-optical efficiency and near-diffraction limited beam quality (M-Squared < 1.25) before MI onset. To this author's knowledge, this represents 2.4x improvement in power output from a PBGF amplifier without consideration for linewidth and a 16x improvement in single-frequency power output from a PBGF amplifier. During power scaling of the PBGF, a remarkably low Brillouin response was elicited from the fiber even when the ultra large diameter 50 µm core is accounted for in the SBS threshold equation. Subsequent interrogation of the Brillouin response in a pump probe Brillouin gain spectrum diagnostic estimated a Brillouin gain coefficient, gB, of 0.62E-11 m/W; which is 4x reduced from standard silica-based fiber. A finite element numerical model that solves the inhomogenous Helmholtz equation that governs the acoustic and optical coupling in SBS is utilized to verify experimental results with an estimated gB = 0.68E-11 m/W. Consequently, a novel SBS-suppression mechanism based on inclusion of sub-optical wavelength acoustic features in the core is proposed. The second advanced waveguide analyzed is a 35/350 µm diameter core/clad fiber that achieved wave guidance via both PBG and MTIR, and is referred to as a hybrid fiber. The waveguide benefits mutually from the amenable properties of PBG and MTIR wave guidance because robust single-mode propagation with minimal confinement loss is assured due to MTIR effects, and the waveguide spectrally filters unwanted wavelengths via the PBG effect. The waveguide employs annular Yb-doped gain tailoring to reduce thermal effects and mitigate MI. Moreover, it is designed to suppress Raman processes for a 1064 nm signal by attenuating wavelengths > 1110 nm via the PBG effect. When seeded with a 1064 nm signal deterministically broadened to ~1 GHz, the hybrid fiber was power scaled to a MI-limited 820 W with 78% optical-to-optical efficiency and near diffraction limited beam quality of M_Squared ~1.2 before MI onset. This represents a 14x improvement in power output from a hybrid fiber, and demonstrates that this type of fiber amplifier is a quality candidate for further power scaling for beam combining.

Hybrid microstructured fibers, utilizing both air holes and high index cladding structures, provide important advantages over conventional fiber including robust fundamental mode operation with large core diameters (>30 mu m) and spectral filtering (i.e. amplified spontaneous emission and Raman suppression). This work investigates the capabilities of a hybrid fiber designed to suppress stimulated Brillouin scattering (SBS) and modal instability (MI) by characterizing these effects in a counter-pumped amplifier configuration as well as interrogating SBS using a pump-probe Brillouin gain spectrum (BGS) diagnostic suite. The fiber has a 35 mu m annularly gain tailored core, the center doped with Yb and the second annulus comprised of un-doped fused silica, designed to optimize gain in the fundamental mode while limiting gain to higher order modes. A narrow-linewidth seed was amplified to an MI-limited 820 W, with near-diffraction-limited beam quality, an effective linewidth similar to 1 GHz, and a pump conversion efficiency of 78%. Via a BGS pump-probe measurement system a high resolution spectra and corresponding gain coefficient were obtained. The primary gain peak, corresponding to the Yb doped region of the core, occurred at 15.9 GHz and had a gain coefficient of 1.92x10(-11) m/W. A much weaker BGS response, due to the pure silica annulus, occurred at 16.3 GHz. This result demonstrates the feasibility of power scaling hybrid microstructured fiber amplifiers

This thesis focuses on the investigation and development of an integrated optical biosensor based on enhanced Raman techniques that will provide label-free detection of biomolecules. This is achieved by using hollow core photonic crystal fibers (HC-PCF), nanoparticles, or both. HC-PCF is a unique type of optical fiber, with continuous ‘channels’ of air (typically) running the entire length. The channels serve to confine electromagnetic waves in the core of the fiber, and tailor its transmission properties. Using HC-PCF as a biosensor requires development of a robust technique to fill hollow-core photonic crystal fibers. Though several groups have reported selective filling of HC-PCF’s core, the processes are cumbersome and limit the choice of liquid to avoid multimode behavior. In my Master’s thesis, I presented a simple technique to non-selectively fill all the HC-PCF channels with samples. The non-selective filling preserves the photonic bandgap property of the fiber, and yields an extremely strong interaction of light and the sample that produces considerable enhancement of the Raman signal from the analyte. Up to now, non-selective filling was accomplished through capillary action and it delivered a Raman signal enhancement of approximately 30-fold, which is not sensitive enough to detect biomolecules at the clinical level. Moreover, there were issues of reliability and reproducibility, due to evaporation, filling and coupling light into the fiber. The objective of this PhD research was to overcome these problems by developing a robust optical fiber platform based on Raman spectroscopy that can be used in a clinical setting. I initially focused on heparin, an important blood anti-coagulant that requires precise monitoring and control in patients undergoing cardiac surgery or dialysis. Since the Raman spectra of heparin-serum mixtures exhibits Raman peaks of heparin with poor signal-to-noise ratios, I concentrated on enhancing the heparin Raman signal and filtering out the spectral background of the serum to improve detection sensitivity. Reaching maximum enhancement of the Raman signal required a strong interaction of light and analyte, which can be achieved by using hollow core photonic crystal fiber as I had used in my Master’s research. Using a small piece of HC-PCF I was able to reach an enhancement in the heparin Raman signal of greater than 90-fold. With this degree of enhancement, I was able to successfully detect and monitor heparin in serum at clinical levels, something that had never been accomplished previously. After developing HC-PCF as a Raman signal enhancer, I focused on making the HC-PCF sensor robust, reliable and reusable. This was achieved by integrating the HC-PCF with a differential pressure system that allowed effective filling, draining and refilling of the samples in an HC-PCF, under identical optical conditions. To demonstrate the device’s detection capabilities, various concentrations of aqueous ethanol and isopropanol, followed by different concentrations of heparin and adenosine in serum, were successfully monitored. To further improve the sensitivity of the HC-PCF based Raman sensor, I incorporated surface enhanced Raman scattering (SERS), by introducing nanoparticles into the HC-PCF fibers. The research focused on determining the optimal volume and size of silver nanoparticles to achieve maximum enhancement of the Raman signal in the HC-PCF. The HC-PCF enhanced the Raman signal of Rhodamine 6G (R6G) approximately 90-fold. In addition, the optimal size and volume of AgNP enhanced the Raman signal of R6G approximately 40-fold, leading to a total enhancement of approximately 4,000 in HC-PCF. This was then used to demonstrate the application of a SERS based HC-PCF sensing platform in monitoring adenosine (a clinically important molecule), as well as malignant cells such as leukemia. Finally, I used hollow core crystal fibers to significantly enhance the efficiency of two-photon photochemistry. Although two-photon photochemical reactions are difficult to achieve with a small volume, I accomplished it by using a novel platform of HC-PCF to efficiently execute the two-photon induced photodecarbonylation reaction of cyclopropenone 1, and its conversion to the corresponding acetylene. The simple optical design configuration involved coupling an 800-nm tsunami laser to a short piece of HC-PCF filled with the sample. This allowed me to increase the efficiency of two-photon induced photochemistry by 80-fold, compared to a conventional spectrophotometer cuvette. Thus, this work leads to the use of HC-PCFs to more effectively study two-photon induced photochemistry processes, which was limited due to the difficulty of detecting photochemical events with a small excitation volume.

Dans ce manuscrit, nous rapportons les développements, théoriques et expérimentaux, en cours à TRT ainsi qu’à TAV et au LAC, visant la réalisation d’un gyromètre résonant passif en fibre optique à cœur creux atteignant des performances permettant la navigation inertielle. Nous y décrivons mathématiquement l’effet Sagnac, effet relativiste à la base des mesures optiques dans les gyromètres. Ensuite, nous exposons en détail les méthodes utilisées à ce jour pour mesurer des rotations avec des gyromètres passifs par les différentes équipes de recherches. Nous explicitons les limitations de ces méthodes, et en quoi la fibre optique à cœur creux semble être la solution la plus prometteuse pour pallier les défauts des gyromètres passifs résonants actuels. Une partie de cette thèse est alors consacrée à l’étude des propriétés physiques des fibres à cœur creux (Kagomé et bande interdite photonique), telles que leur atténuation, leur capacité à maintenir la polarisation, et leur rétrodiffusion. Nous présentons la première mesure de zone aveugle (plage de faibles vitesses de rotations non mesurables par un gyromètre) dans un gyromètre résonant passif en fibre à cœur creux. Un modèle mathématique est posé pour expliquer le lien entre cette zone aveugle et la rétrodiffusion au sein de la cavité résonante. Nous décrivons ensuite un protocole expérimental permettant de s’affranchir de cette limitation dans notre gyromètre. Nous détaillons enfin la mise en œuvre de ce protocole et caractérisons les performances ainsi atteintes par notre gyromètre / In this manuscript, we report the theoretical and experimental developments at TRT, TAV and LAC, aiming the realization of a hollow-core passive resonant fiber optical gyroscope that can achieve navigation grade performances. We mathematically describe the Sagnac effect, which is a relativistic effect used to optically probe mechanical rotations. Then, we detail the state of the art in passive resonant fiber optical gyroscope development. We identify their limitations, and explain why the hollow core fiber seems to be the best solution to cope with the actual limitations of such gyroscopes. We then focus on two different types of hollow core fibers: Kagome and photonic bandgap. We evaluate their performances in terms of transmission, polarization holding and backscattering. We describe the first measurement of a lock in region in a hollow core fiber passive optical gyroscope, i.e the range of rotation rates that cannot be measured because of backscattering. A mathematical model is propounded to link the lock in to the backscattering of the cavity. We then discuss the experimental protocol that we implemented to circumvent this limitation. Finally, we characterize the performances of our gyroscope based on these features