Effect of initial chirp on supercontinuum generation in dispersion decreasing fibers

Numerical simulations based on generalized nonlinear Schrödinger equation are used to study the effect of initial chirp on supercontinuum generation in dispersion decreasing fibers. The simulation results show that the effect of initial chirp on SC generation is highly related to the choices of pumping conditions and fiber parameters. When the pumping condition and the fiber parameters are different, the effect of initial chirp on SC generation may follow different patterns. To better discuss the effects of pulse parameters and fiber parameters on SC generation, the three normalized parameters: input soliton order, normalized dispersion slope and normalized effective fiber length are introduced. For a given input soliton order and a given normalized dispersion slope, there exists an optimal normalized effective fiber length for generating a flattest SC spectrum. On the condition of normalized effective fiber length in the vicinity of its optimal value, when DDF has small normalized dispersion slope, proper positive chirps can significantly enhance the SC bandwidth, while negative chirps or too large positive chirps suppresses the SC bandwidth. There is a wide range of positive chirps that can enhance the SC bandwidth, but the range of proper positive chirps become narrower as input soliton order decreases; When DDF has a large normalized dispersion slope and the pump pulse is a lower-order soliton, the enhancement of SC bandwidth by initial chirp is insignificant, and the widest SC spectrum is generated when the initial chirp is close to zero.