Theoretical and Experimental Study on Optical Devices Using Highly Nonlinear Photonic Crystal Fibers

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The strong waveguide dispersion in photonic crystal fibers (PCFs) provides unique opportunities for nonlinear optics with a zero-dispersion wavelength far below the limit of~1.3μ m set by the material dispersion of silica. By tuning the air hole diameter, the pitch, and the number of rings of air holes, the strong waveguide dispersion can in principle be used to extend the zero-dispersion wavelength well into the visible, albeit to some extent at the cost of multi-mode operation. We developed an analysis software based on a full-vectorial finite element method and studied in detail the interplay of the zero-dispersion wavelength, the cutoff wavelength, and the leakage loss in the parameter space spanned by the hole diameter, the pitch, and the number of rings of air holes. As a particular result, we identified the values of hole diameter and pitch which facilitate the shortest possible zero-dispersion wavelength, while the fiber is still single-mode for longer wavelengths. In addition, the genetic algorithm integrated with the full-vectorial finite element method was used to optimize the structural parameters of dispersion compensating PCF (DCPCF) Raman amplifiers. The optimized DCPCF provided negative dispersion coefficient, negative dispersion slope, and broadband dispersion compensation over C-band or S-band with gain-flattened Raman characteristics. Furthermore, the Raman gain efficiency and chromatic dispersion of a hole-assisted fiber with and without minimum allowable bending radius were measured. Numerical predictions from the theory were shown to be in good agreement with the experimental results.