Numerical studies of supercontinuum generation based on quasi-rectangle wave pumping

Spectral broadening and the generation of supercontinuum are inherent features of nonlinear optics, and have been studied intensively for many years. Supercontinuum generation has been found numerous applications in such various fields as spectroscopy, pulse compression, and the design of tunable ultra-short femtosecond laser sources. Generally, supercontinuum is mainly generated by hyperbolic-secant soliton pulse. It is rarely investigated about the spectral broadening when pumping with pulse of a rectangle or quasi-rectangle shape, which is an interesting direction of supercontinuum generation. In this paper, supercontinuum generation based on soliton, quasi-rectangle, and rectangle pulses are investigated numerically. Firstly, we introduced the general nonlinear Schrödinger equation (GNLSE) to deduce the temporal and spectral evolution of pulse broadening in the photonic crystal fiber. Secondly, with different types of pulses, including hyperbolic-secant, ultra-Gaussian and rectangle pulses, we investigated the generation of new frequency and spectral broadening by numerical methods. Comparisons of characteristics of spectral and temporal evolution with different pulses are made in the following parts of paper. It is found that the generation of dispersive waves and Raman effect play main roles in the spectral broadening and generation of supercontinuum, as the broadband wave is generated by the coupling of Raman soliton and dispersive waves through cross-phase modulation. Apparent side-bands can be observed in our simulation under the condition of quasi-rectangle and rectangle pulses pumping, which affect the smooth of spectrum. Thus, to our acknowledge, a flat supercontinuum can be generated by changing the parameter of pulse to make a better coupling of the side-bands.