In this book, the novel designs of photonic crystal fibers with optimized optical properties such as index guiding, highly birefringence, small walk-off, near zero dispersion, normal dispersion, anomalous dispersion, highly nonlinear, single mode in silica and non-silica materials is proposed. In these photonic crystal fibers, the physical dynamics behind the new frequency generation is explored when an ultra-short pulse (femtosecond pulse) propagates through these fibers. The nonlinear optical phenomena which are responsible for the generation of supercontinuum is investigated.