Flat supercontinuum generation pumped by femtosecond pulses in zero-dispersion wavelength of photonic crystal fiber

Two kinds of supercontinuum (SC) sources are successfully generated by propagating 200-fs unamplified and amplified pulses through a 0.85-m long nonlinear photonic crystal fiber (PCF), respectively. The spectra bandwidth of amplified-femtosecond-pumped SC is about 870 nm spanning 480 nm to 1350 nm, which is flat to 1.2dB over 100 nm spreading from 550nm to 650 nm. With the same PCF, SC spectra pumped by unamplified-femtosecond-pulses are narrower and unevener. A detailed simulation is carried out to help us understand the mechanism of supercontinuum evolution. For pump wavelength located at the zero dispersion wavelength of the PCF, spectra are broadened by the interaction between SPM and higher-order dispersion at early stage. With increasing the pump power, the spectra are broadened by fission of higher solitons and parametric four-wave mixing (FWM). When the peak power is up to mega-watt, FWM plays a notable role in flattening and further broadening the supercontinuum spectra in short wavelength side. An effective way to generate a flat SC laser source pumped by femtosecond pulses is also demonstrated.