Experimental Research On Self-focusing Of High-power Broad Bandwidth Laser Pulses

"Broad bandwidth"and"nonlinearity"are the two key technologies in the field of laser science and technology. The self-focusing is one of the most important nonlinear effects. It was found to be the major factor for the degradation of beam quality and the limitation of laser output power. Broadband laser pulses are expected to break through the power limited bottleneck in high-power solid laser driver, which mainly resulted from the nonlinear effect. It recently has becomes an important direction of development in the field of laser technology. Furthermore, the self-focusing is one of fundamental physical problems in Nonlinear Optics. For the past few years, with the technological advances in ultrashort pulses, the nonlinear propagation of broadband laser pulses and their interaction with matter are to become a siginificant frontier in the field of Nonlinear Optics. Therefore, the investigation of self-focusing of high-power broadband laser pulses is of very great significance both in theory and practical applications. In this dissertation, taking into account the actual requirement of new generation high-power laser driver in our country and the development of the self-focusing theory, we investigated both experimentally and theoretically the self-focusing characteristics and laws of broadband laser pulses during the nonlinear propagation, and obtained the main results as follows:Firstly, we experimentally studied the process of the self-focusing of broadband laser pulses with small-scale spatial modulation in nonlinear medium, and obtained the evolution regularity of the small-scale spatial modulation growth as well as the interaction of modulation growth.The actual optical system device always has a certain size, when broadband laser pulse through it would be diffraction modulation. The diffraction intensity level of different wavelengths of broadband pulse spectral is not the same. The broadband laser pulse with diffraction modulation pass through nonlinear medium, it will occur modulation increase at the particular spatial position and induce pulse beam breakup into filaments. We have focused on the investigation of nonlinear propagation of broadband laser pulse with circular aperture diffraction, observed the process of the formation of small-scale filaments. In particular, we studied in detail the evolution of a single filament and the process of interaction between the filaments. The intensity of single filament strength to a certain value, will release excess energy, and induce the formation of new filaments; the close proximity filaments interact through the interference, making the formation of new filaments in the neighborhood.Secondly, we investigated the evolutionary process of the spatiotemporal noise of the broadband chirped laser pulses in nonlinear medium, and disclosed the interaction between spatial and temporal noises, and the law of the influence of pulse chirp on the evolution of spatiotemporal noises.Based on the nonlinear Schr?dinger equation, the spatiotemporal instability of broadband chirped laser pulse is studied theoretically. We find the pulse chirp does not affect the instability gain spectrum, and the expression of instability gain spectrum is the same as the one of broadband chirped-free laser pulse. At the same time, we measured experimentally the evolutionary process of the spatiotemporal noise of the broadband chirped laser pulses in nonlinear medium, and obtained the gain spectra in the condition of different pulse width (i.e. different chirp) and in the presence of temporal modulation. It is a good agreement with the theoretical results. The experimental results show that the bandwidth can postphone,to some extent, the occurrence of small-scale self-focusing. Finally, the modulation instability of chirped laser pulse and its influence on the spatial self-focusing are investigate by means of numerical simulatons.Lastly, we analyzed both theoretically and experimentally the impact of relaxation effect on the spatiotemporal instability of broadband laser pulses. These results are of very significance for the propagation of broadband laser pulses and the physic of modulation instability.The expression for the modulation instability gain spectrum in non-instantaneous Kerr medium is obtained, which clearly reveals the influence of the finite time of the nonlinear response (relaxation effect) on the spatiotemporal modulation instability. It is shown that, due to the relaxation effect, the spatiotemporal modulation instability can appear for the case of anomalous dispersion and defocusing nonlinearity, while it cannot appear in instantaneous Kerr medium for the same case, and a new modulation instability gain spectrum appears, adding to the conventional modulation instability gain spectrum similar to that in an instantaneous Ker medium. In addition, the bandwidth of modulation instability gain spectrum is extended and the maximum modulation instability gain is reduced with increasing of the relaxation time. We have performed an experiment of modulation instability in carbon disulfide, a typical non-instantaneous Kerr medium, and obtained the gain spectra in the conditions of different pulse width which shows quantitative agreement with the theoretical results.