Tunable supercontinuum generation in a high-index-step photonic-crystal fiber with a comma-shaped core

A fused silica high-index-step photonic-crystal fiber with a comma-shaped core is shown to upport two different types of guided modes with bell-shaped intensity profiles, efficiently transforming unamplified 30-fs Ti: sapphire laser pulses into supercontinuum emission through two different physical mechanisms. The modes of the first type provide broadly spanning supercontinuum emission with a smooth spectrum stretching from 450 to 1400 nm. The initial stage of supercontinuum generation in these modes involves four-wave mixing around the wavelength of zero group-velocity dispersion, leading to the depletion of the pump field. The modes of the second type generate supercontinuum with an enhanced short-wavelength wing, dominated by intense spectral lines centered at 400--450 nm. The two regimes of supercontinuum generation and the two types of output spectra are switched by displacing the input end of the fiber with respect to the laser beam in the transverse direction.