In this PhD thesis an experimental study of modal characterization methods on large mode area photonic crystal fibers (PCFs) is performed and the development of a new ytterbium-doped photonic bandgap PCF rod fiber is presented. The first focus of this work is to use modal characterization methods which can accurately resolve propagation modes and their weights in optical waveguides to examine guiding properties and single-mode (SM) operation of different PCFs. A spatially and spectrally resolved (S2) imaging setup is developed to evaluate the SM properties of flexible PCF with a 40 μm core diameter. The limitations of the method are discussed and another modal characterization method is introduced. Cross correlated (C2) imaging, which can resolve modes in fibers with very large cores, is described. C2 imaging is then successfully employed to verify the resonant coupling condition between core modes and the cladding band as the underlying mechanics to ensure SM operation of the new rod fiber design (85 μm core diameter), which was developed during this thesis work. The second focus of this work is the study of the new ytterbiumdoped rod fiber design under active operation. Performance of the rod fiber is evaluated in high power laser and laser amplifier configurations. The high power rod amplifier setup including the seed source is developed and characterized. Results obtained from the rod fiber showed simultaneously SM, near diffraction limited output beam quality with high average power and pulse energy generation using both laser and laser amplifier configurations. Modal instabilities (MIs) in high power fiber amplifiers are discussed, and a memory effect of the MI threshold level together with a recovery method and evidence of improved performance while suppressing MIs are reported. Thermally induced refractive index changes in the core under high power operation is discussed and quantified as a shift of the photonic bandgap in the rod fiber design.
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Broeng, Jes, Lægsgaard, Jesper, Alkeskjold, Thomas Tanggaard