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Temporal shape of laser pulses generated by modern high peak power laser systems plays one of the key roles in modern laser-matter interactions. High intensity laser-matter experiments, especially using Petawatt lasers, require a pre-pulse free shape within the dynamic range well above 10^10 and in the time range up to few picoseconds before the main pulse. In CPA systems laser pulses experience numerous transformations during their amplification, starting from being short, then chirped, amplified, and re-compressed. Nonlinear interaction takes place during the pulse amplification, different techniques are being used to optimize the pulse bandwidth, etc. As a result, the laser pulse accumulates distortions of the spectral phase, also caused by impurities in the optical media, nonlinear effects that take place during the amplification and propagation. This leads to the degradation of its temporal profile and appearance of additional pre-/post pulses and pedestals.
Aiming to identify the major issues and reasons of the pulse degradation, in addition to amplified spontaneous emission we analyzed the temporal profile of laser pulses generated by CPA-based Ti:Sapphire laser systems in the time range of few picoseconds around the major pulse. We have found that within this temporal interval, and 10^10 range of intensity the laser pulse has typically features that are coherent with the major pulse and those which are not. The later are associated with scattering on surfaces of optical elements and in optical media of the laser system. The impact of those features can be partly reduced by optimization of the laser system. The coherent with the major pulse features after nonlinear interaction during amplification generate temporal replicas in front and behind the major pulse. Those are hardly to be cancelled, since the gain Ti:Sapphire media itself generates a ragged coherent with the major pulse post-pedestal, that after nonlinear interaction and the pulse re-compression gets transferred to the leading front of the laser pulse. This feature limits the temporal contrast of Ti:Sapphire lasers, since it is unavoidable.
A new method of the temporal contrast filtering based on rotation of polarization ellipse in noble gases combined with differentially pumped hollow core fiber (HCF) will be discussed also. The method can substitute a commonly used generation of crossed-polarized wave (XPW) in double-CPA laser systems, since it allows the conversion efficiency of > 50%, a smooth beam profile and high energy stability. It supports also a similar to XPW contrast improvement of 3-4 orders of magnitude and allows generating laser pulses with pulse duration in a sub 5-fs range. Contrary to a conventional HCF technique the new method supports a very smooth spectrum of converted pulses and hence a better temporal shape.
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