Analysis of parametric fluorescence amplified in a noncollinear optical parametric amplifier pumped by the second harmonic of a femtosecond yb:kgw laser

One of the most efficient and widely spread method for generation of ultra-short and high peak power optical pulses is optical parametric chirped pulse amplification (OPCPA). Generated pulses can be used in high field physics experiments, for example, in high harmonic generation in a gas target. However, in order to be suitable for these highly nonlinear experiments generated pulses in addition to having high peak powers must also have high temporal contrast. In optical parametric amplifiers amplification of parametric fluorescence deteriorates generated pulse temporal contrast. It was shown that superfluorescence is generated mostly in the first stage of the amplifier. The aim of this work was to investigate the properties of amplified parametric fluorescence (APF) in a high-gain BBO-based femtosecond noncollinear optical parametric amplifier and examine the differences of APF levels in two different amplification geometries. In this paper, spatial dependance of superfluorescence cone due to nonlinear crystal configuration was analysed. It was shown that the main reason behind the asymmetry of the profile of the superfluoresence cone is better spatial overlap of pump and idler beams due to the pump beam walk-off. Also, an original experimental method was used to determine superfluorescence levels when the signal is being amplified. Amplified parametric fluorescence suppression up to 7,5 times was measured when pump-to-signal energy conversion reached 11.5 \%. Finally, superfluorescence energy levels were measured in both nonlinear crystal orientations and it was shown, that in order to get better energy contrast, PVWC configuration should be used, in which contrast is 100 times better, than in TPM configuration