To obtain seed pulses of large energy, high contrast and broad spectral bandwidth for the high power laser system, a new cross-polarized wave (XPW) generator based on the gadolinium gallium garnet (GGG) crystal is designed. For the injected fundamental wave (FW) pulses exceeding tens of millijoule, the output XPW characteristics of conversion efficiency, spectral broadening and contrast improvement are theoretically analyzed. We also investigated the influence of spatial distribution characteristics of the injected chirped FW pulses on the output XPW. With incident FW pulses of 8 order super gaussian spatial profile for instance, conversion efficiency of ~73%, spectral broadening ratio (SBR) of ~2.7 (from 40nm to 108nm) and contrast boost by >7 orders of magnitude can be obtained. The impact of the FW spatial modulation on XPW was sensitive to the modulation depth, which is significant for spatial quality control of the laser beam in high power laser systems.
Plasma mirror is an effective approach to improve the temporal contrast of high power ultra-short laser system, while it might deteriorate the focal spot, which is reported in some experiments using plasma mirror. In order to investigate such far-field degradation by plasma mirror, we established a spatiotemporal multi-step focusing propagation algorithm based on the formula of plasma expansion and wave-front modulation model. The influence of plasma expansion time, amplitude and spatial frequency of wave-front error on focal spot degradation are quantitatively analyzed. The simulation results reveal that the far-field focal spot degradation by plasma mirror is caused by the non-uniform plasma expansion due to the wave-front error and the wave-front error with higher amplitude and lower spatial frequency has relatively greater effect on the focusing ability. From the perspective of high-contrast ultra-intense output capability, the requirement on the spatiotemporal quality of the pulse is put forward to avoid the far-field focal spot degradation when using plasma mirror in high power ultra-short laser system.
Pulse time delay (PTD) and defocus are mainly introduced by transmitted-based large-aperture beam expander systems in ultrashort high power laser systems due to chromatic aberration, which can significantly reduce the focal-spot intensity by spatially enlarge the spot size as well as temporally distort the pulse profile. In this paper we investigate the chromatic aberration and measure how it deteriorate the focal spot size in SG-II 5PW ultrashort laser system. In addition, we propose and design a simple chromatic aberration pre-compensation scheme based on combination of aspherical lens and spherical mirrors. The simulation results indicate that both PTD and defocus dispersion can nearly be fully compensated by applying this compensation scheme with proper alignment in the system without introducing other kinds of wave-front aberrations.
A model is presented to analyze the effect of chromatic aberration of the spatial filter lenses on the compressed pulse duration. The parameters of the SHENGUANG (SG) II 5 PW laser system are demonstrated as examples. The numerical simulation results show that the compressed pulse duration with full aperture is increased by 3 times compared with the ideal compressed pulse duration in the case of chromatic aberration of the lenses. The result of full-aperture measurement at the SG II 5 PW laser system is in good agreement with the theoretical calculation. In the case of full-aperture measurement, the shortest compressed pulse duration is obtained by adjusting the compressor to compensate for the phase aberration introduced by the chromatic aberration of the spatial filter lenses at the SG II 5 PW laser facility. These results are helpful in finding an optimized pulse duration, considering the chromatic aberration of lenses in the femtosecond laser facility, and in obtaining a deep understanding of the effect of chromatic aberration of lenses on the pulse contrast ratio and pulse duration in ultrashort laser systems.