Abstract:
This thesis investigates Spectral broadening in index-guiding silica photonic crystal fibers (PCFs) designed for all-anomalous dispersion at a 1.2 μm pump (30 kW). Two hexagonal lattices are compared—a 3-Ring and a 5-Ring cladding—with pitch Λ = 2 μm and air-fill ratios d/Λ= 0.6, 0.7, 0.8. Linear properties—chromatic dispersion D(λ), effective mode area Aeff(λ), nonlinear coefficient γ(λ), and confinement loss—are extracted using full- vector finite-element eigenmode analysis with perfectly matched Rings. These wavelength-dependent parameters feed a Generalized Nonlinear Schrödinger Equation (GNLSE) solver to predict three outputs: spectral power vs wavelength, distance vs temporal delay, and distance vs wavelength evolution. Both fibers place the pump in the anomalous regime, but they differ markedly in long-wave performance. The 3-Ring design exhibits slightly stronger anomalous curvature near 1.2 μm yet suffers orders-of-magnitude higher confinement loss beyond ≈2.5 μm, leading to early decay of red-shifted components. The 5-Ring design delivers flatter anomalous dispersion, smaller Aeff and hence higher γat the pump, and—critically—much lower long-wave leakage, preserving energy transport toward 3 μm. Nonlinear propagation maps confirm smoother soliton dynamics, robust dispersive-wave generation, and broader, more uniform spectra for the 5- Ring fiber. Across both designs, increasing d/Λ reduces Aeff and raises γ; the combination d/Λ ≈ 0.7–0.8 in the 5-Ring geometry is recommended as the optimized design. The 3- Ring fiber is retained as a comparative baseline to highlight improvements in dispersion flattening and long-wavelength guidance.
