Improving contrast ratio of compressed pulses in OPCPA system by using long duration pump pulses

The fourth order Runge-algorithm was used in the numerical simulation methods to study the influences of the profiles and durations of pump pulse on the compressed pulses. The LBO crystal was used for the nonlinear gain medium in the numerical simulation. When the narrower duration pump pulses were used, a higher contrast ratio of femtosecond was obtained by adopting the more smooth profiles of the pumps (namely the lower order parameters of pump pulses). The contrast ratio of the femtosecond can be improved by longer duration pump pulse. But the contrast ratio can not be further improved by improving the duration pump pulses further, while much lower conversion coefficient in OPCPA system occurs.