Engineering the temporal shape of a chirped pulse compressed in a pressure-gradient-based cascaded hollow-core photonic crystal fiber

A theoretical model is discussed to investigate the impact of a gas-filled hollow-core photonic crystal fiber (HC-PCF) on the temporal shape and width of a picosecond input pulse. For this purpose the nonlinear Schrödinger equation is numerically solved to simulate the evolution of a slowly varying approximated soliton propagating along the waveguide. To obtain a highly desirable pulse shape with the shortest width at the output, four fiber schemes proposed as case-I to case-IV HC-PCF are modelled in which the pressure style of the filler gas and the chirp of the input pulse are purposefully engineered. The well-known distance parameter and the Q factor are used to evaluate the temporal feature of the ultimate pulse. The final study on the compressed pulse characteristics is performed using a standard merit function (SMF) that is introduced to obtain the most optimum output pulse shape and width. It is found that the application of a positive gradient for the gas pressure and an adequate chirp value for the input makes it possible to reach the lowest width and the highest temporal quality. The final results suggest that the cascaded case-IV fiber is capable of generating the most optimal pulse width of 18.1 fs with the highest SMF value of 12, indicating the highest temporal quality and the lowest distance parameter of 46.3% and 194, respectively. The most interesting feature of the results is the fact that by the proper use of a gradient from for the gas pressure the effect of chirp can be directed in a way that the highest temporal quality and the most desired width of compressed pulse obtains while the input power is kept low.