High power top-hat pulses for efficient OPA pumping

Optical parametric chirped pulse amplification (OPCPA) is rapidly gaining ground as an approach to generate intense few-cycle pulses and reach IR wavelength ranges. However, the high OPA gain has its negative side: for a bell-shaped intensity profile of the pump pulse/beam the unsaturated gain varies exponentially with intensity, resulting in an effective temporal/spatial filter. Here we demonstrate shaping of high-energy broadband Yb amplifier pulses for the generation of a top-hat temporal profile that provides efficient pumping of an optical parametric amplifier. Phase-only modulation is applied by an acousto-optic programmable dispersion filter (AOPDF). Whereas the described technique requires minimal changes of the pump laser source, it brings significant advantages: 1) a substantial increase in the OPA conversion efficiency, 2) avoidance of OPCPA gain roll-off in the pump pulse wings, i.e. the temporal equivalent of spectral gain narrowing, 3) increase of the OPA signal and idler pulse contrast as compared to the standard use of bell-shaped pump pulses whereby the chirped seed pulse is purposely overlapped only with a narrow temporal section of the pump around the pulse peak and the temporal wings of the pump pulse are responsible for raising the ASE (superfluorescence) background outside the temporal overlap with the seed pulse.