PumP-Induced TemPoral conTrasT degradaTIon In oPTIcal ParameTrIc chIrPed-Pulse amPlIfIcaTIon

LLE Review, Volume 111 135 Introduction Laser–matter interactions in new regimes have occurred due to the generation of high-intensity optical pulses using largescale laser systems.1 The interaction regime of a laser pulse with a target is basically set by the peak intensity of the pulse, which is fundamentally proportional to the ratio of the pulse energy to the duration of the pulse and surface of the focal spot. Intensities of the order of 1022 W/cm2 have been claimed,2 and facilities delivering high-energy, high-intensity laser pulses are under operation or construction.3 The interaction can be detrimentally impacted by light present before the main pulse since absorbed light can lead to physical modification of the target.4 The temporal contrast of a laser pulse is the ratio of the peak power of the main pulse to the power of the light in some predetermined temporal range before the main pulse. The contrast can be reduced significantly during the generation and amplification of laser pulses, and contrast degradation manifests itself as isolated prepulses or as a slowly varying pedestal. Incoherent laser and parametric fluorescence can significantly impact the contrast of laser pulses and can lead to a long-range pedestal on the recompressed pulse.4,5 This contrast degradation is fundamental since fluorescence is always present for classical optical amplifiers. The contrast of optical parametric chirped-pulse amplifiers, however, is also detrimentally impacted by temporal variations of the intensity of the pump pulse that induce spectral variations on the stretched amplified signal via the instantaneous parametric gain. This is a practical limitation that can be eliminated or reduced by proper design of the pump pulse and the optical parametric chirped-pulse amplification (OPCPA) system.