Study Of Spatiotemporal Evolution Of Chirped Pulse In Different Positions During The Small-scale Self-focusing

During the propagation of the untrashort pulse lasers, it will introduce noises which breakup the high power laser beams into small-scale self-focusing. This brings great damage to the optical components and medium. With the development and widely applications of chirped pulse amplification technology, the peak power of laser pulse is very high which can easily lead to the occurrence of small-scale self-focusing. In this paper, we have studied the spatiotemporal character of small-scale self-focusing from experimental and numerical calculation, especially the temporal evolution of different spatial positions, which has important reference value for high power lasers. The main results are presented as follows:Firstly, experimental and numerical investigated the growth process of small-scale self-focusing with spatial modulation, and we found that there were many growth points at different positions. The growth points appeared in the specific locations when at the fastest spatial modulation frequency. These points will grow with the increase of transmission, and split into many filaments eventually. The simulation results are consistent with the experimental results.Secondly, we investigated the temporal evolution at different positions especially the modulation peak and modulation bottom. When the pulse laser propagating in the medium, its wave front becomes non-homogeneous because of the noise. Based on the propagation equation for ultrashort pulse propagating in nonlinear media, using split-step Fourier algorithm we found that the pulse width at modulation peak is decreasing with the increasing of transmission distance and then splitting into two parts in time domain. However, the pulse width at modulation bottom always broadens.Finally, impacts of the pulse chirp on the spatiotemporal character of small-scale self-focusing have been studied. The results indicated that the positive chirp can restrain the occurrence of small-scale self-focusing. When just have group velocity dispersion, pulse chirp has a little effect on the pulse width at modulation peak. When there only have nonlinear effects, pulse chirp has no influence at modulation peak in time domain, but it has affected the spectrum distribution. When there have group velocity dispersion and nonlinear effects simultaneously, negative chirp accelerates the pulse compression of modulation peak, but it inhabits the pulse broadening of modulation bottom. However, the positive chirp is counterproductive.