Pulse-contrast degradation due to pump phase-modulation in optical parametric chirped-pulse amplification system.

We theoretically study the pulse-contrast degradation of optical parametric chirped pulse amplification (OPCPA) system pumped by a temporally phase-modulated laser. We reveal two physical mechanisms that reduce the contrast in such a parametric process: the effect of phase transfer caused by group-velocity mismatch (GVM) and the effect of FM-to-AM conversion caused by group-velocity dispersion (GVD). Degradation of the contrast is quantified via numerical simulations. To reduce the effect of phase transfer, a pair of OPCPAs seeded by the signal and idler respectively, is studied for effectively improving the contrast in such a case with pump phase modulation. The results presented in this paper are of particular importance for extremely intense laser applications requiring high contrast.