Thulium- and holmium-doped fiber lasers (TDF and HDF) emitting at 2 micron offer unique benefits and applications compared to common ytterbium-doped 1 micron lasers. This dissertation details the concepts, limitations, design, and performance of four 2 micron fiber laser systems. While these lasers were developed for various end-uses, they also provide further insight into two major power scaling limitations. The first limitation is optical nonlinearities: specifically stimulated Brillouin scattering (SBS) and modulation instability (MI). The second limitation is thermal failure due to inefficient pump conversion. First, a 21.5 W single-frequency, single-mode laser with adjustable output from continuous-wave to nanosecond pulses is developed. Measuring the SBS threshold versus pulse duration enables the Brillouin gain coefficient and gain bandwidth to be determined at 2 micron. Second, a 23 W spectrally-broadband, nanosecond pulsed laser is constructed for materials processing applications. The temporally incoherent multi-kW peak power pulses can also efficiently produce MI and supercontinuum generation by adjusting the input spectral linewidth. Third, the measured performance of in-band pumped TDF and HDF lasers are compared with simulations. HDF displays low efficiencies, which is explained by including ion clustering in the simulations. The TDF operates with impressive > 90% slope efficiencies. Based on this result, a system design for > 1 kW average power TDF amplifier is described. The designed final amplifier will be in-band pumped to enable high efficiency and low thermal load. The amplifier efficiency, operating bandwidth, thermal load, and nonlinear limits are modeled and analyzed to provide a framework for execution. Overall, this dissertation provides further insight and understanding on the various processes that limit power scaling of 2 micron fiber lasers.

Thulium fiber lasers have the broadest emission wavelength bandwidth out of any rare-earth doped fiber lasers. The emission wavelength starts from 1.75μm and ends at around 2.15μm, covering a vast swath of the eye safe wavelength region and intersecting with a large portion of mid-infrared atmospheric transmission window. Also, thulium fiber lasers provide the highest average output power of any other rare-earth doped fiber lasers in these wavelength regimes, making them uniquely suited for applications such as remote sensing. At the moment, high power beam propagation of continuous wave laser through the atmosphere in the mid-infrared range is yet to be investigated anywhere. In particular, the effects of atmospheric water vapors on the thulium fiber laser propagation are unknown and are of great research interest. This dissertation identifies the stringent requirements in constructing a high power, single frequency, wavelength tunable, continuous wave thulium fiber laser with the aim of using it to study various atmospheric transmission effects. A fine spectral control scheme using diffraction gratings is explored and improvements are made. Moreover, a fiber numerical simulation model is presented and is used for designing and implementing the thulium fiber laser system. The current limitations of the implemented system are discussed and an improved system is proposed. This will lay the foundation for the future high power atmospheric propagation studies.

The statistics of large currents (tensions) in large random resistor (elastic) networks of two finite components are studied. Each bond in the network is occupied either by a large conductor (strong spring) or a small conductor (weak spring). The magnitude of the current (tension) in a bond is determined by the configuration of the composite which surrounds it. The large bond currents (tensions) are most often found in "critical defects", i.e., those configurations which can generate the largest current (tension) in a bond with the smallest number of determined bonds surrounding it. This dissertation addresses the question: Given a random resistor (elastic) network, what is the probability density for large currents (tensions) occurring in a bond? The probability density is obtained by studying the distribution of the critical defects with the aid of numerical simulation and continuum mechanics. The probability density is found to be controlled by the smallest eigenvalue of the Laplace's (biharmonic) equation prescribed by appropriate boundary conditions.

Transparent ceramics are an important class of optical materials with applications in high-strength windows, radiation detectors and high-power lasers. Despite the many successful developments of the past decades, their challenging fabrication still needs to be perfected to achieve a better consistency in optical quality. In particular, ternary phase materials such as Yttrium Aluminum Garnet (YAG, Y3Al5O12), a long standing high-power laser host, require a precise control of stoichiometry, often beyond the precision of current analytical techniques, in order to reduce scattering losses and the presence of deleterious point defects. This work explores the potential of Laser-Induced Breakdown Spectroscopy (LIBS) for the quantitative analysis of ceramic compositions near stoichiometry. We have designed a compact and automated LIBS system to determine the plasma composition of sintered mixtures of Y2O3-Al2O3 near the garnet composition. The performance of our setup is evaluated and compared to conventional techniques. Optimized conditions for the acquisition of plasma emission spectra are discussed and the intensity ratios of Y+ and Al in the 300 to 400nm spectral range are analyzed using simple plasma models. The results show that, for the selected parameters of our experiments, the fluctuations in plasma temperature are minimal, and the stability of the plasma is improved. Current results show that ceramic compositions can be resolved within 1 at% in oxide and several suggestions are proposed to further increase the accuracy and precision of the method.

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