Theoretical And Experimental Research On The Self-focusing Suppression By Laser Bandwidth

Self-focusing is an intriguing optical phenomenon, when the High-power laser transmission in the Kerr nonlinear medium. Unlike the common lens focusing, self-focusing is a kind of beam "self-acting" effect, which is closely related with the transverse spatial intensity distribution of the beam itself, for example, Gaussian beam tend to collapse into a point due to the whole self-focusing, while the super-Gaussian beams, characterized by a flat-top, it is more likely to collapse into a ring-shaped profile. So we can see that self-focusing can occur only if the power carried by the beam is exceed to the critical power for self-focusing, and the focused beam profile is determined by the spatial distribution of the beam itself. The self-focusing effect can usually be divided into the whole beam self-focusing (also known as the total beam collapse) and small-scale self-focusing (SSSF). Whole beam self-focusing means the beam as a whole collapse into a focus with a very high strength; while SSSF is refers to the high power laser spontaneous split to form many filaments in the self-focusing medium, and the beam profile is also rely on the local intensity or phase modulation distribution.For high-power solid-state laser systems, SSSF is the more common phenomenon. Because, in high-power solid-state lasers, the peak power of the beam is much greater than the self-focusing critical power, and thus lead to a beam collapse into a focus point, before the beam split to numerous filaments. Since the1970s, a large number of researchers became interested in SSSF. Because, the laser output with high-energy, high-efficiency and high beam quality is necessary for the inertial confinement fusion (ICF) high power solid-state laser driver. But, SSSF often leads to damage in optical materials, and is the major limiting factor in the design of high-power laser systems. Therefore, in a sense, the process of the development of high-power laser systems, which is the process to solve the self-focusing, especially SSSF.To break the restrictions of the load capacity and the output power of the high-power solid-state laser system due to the serious SSSF, investigation on find and propose effective suppression method has been one of the focuses in high-power laser technology fields. Owing to the source of SSSF is the non-uniform distribution of the amplitude or the phase, so improve the near-field beam quality, enhance the uniformity of the beam has been the major method to suppress SSSF. Common techniques include:spatial filter, soft-edged apertures, improves the preparation precision of the optical elements.This dissertation focuses on the SSSF formation mechanism and power conditions. On that basis, propose new scheme to improve the beam uniformity and suppress SSSF in the high power solid-state laser system. We obtained the following results:Firstly, we investigated theoretically and numerically to find the critical power of SSSF for the Gaussian beam. We found that the critical power for SSSF decrease with increasing the modulation amplitude, it means that the critical power for Gaussian beam switch from whole beam self-focusing to SSSF, which is relevant to the initial modulation amplitude.Secondly, we investigated theoretically and numerically the Fresnel diffraction of a phase modulated and spectrally dispersed (PM-SD) beam after passing a hard-edged aperture and SSSF. The theoretical analysis shows that, in comparison with the monochromatic SG beam, the diffraction field of the PM-SD beam is more uniform in the Fresnel domain. The results suggested that the PM-SD beam can smooth out the diffraction intensity spikes. The numerical simulation results demonstrated that the PM-SD beam has a slower nonlinear growth than that of the monochromatic SG beam.Thirdly, we investigated the condition for suppression of SSSF of high-power laser beams by spectral bandwidth. And we found that broadband laser can be used to relax the restriction of nonlinear effect and obviously improve the output power of the high-power solid laser systems, because of the spectral bandwidth of such kind of broadband laser can delay the onset of SSSF. We proved that the degree of self-focusing suppression is closely related to the ratio of the bandwidth and pulse duration, in the same pulse width the big bandwidth the more obvious restrain effect, and in the same bandwidth the restrain effect of bandwidth will become weaker with increasing the pulse duration.Lastly, the growth of interference fringes and random noise caused by nonlinear propagation instabilities, which has been experimentally investigated by using a femtosecond pulsed laser. The results of experiment have proved that broadband laser can be used to relax the restriction of nonlinear effect, the whole gain curve moved to the direction of higher frequency, and relative to the narrow-band laser, the growth curve of the time-integrated spatial contrast with B-integral has a longer flat region of slow growth in the broad band condition, and cause the fast growth turning point of the curves significant moving to large B-integral.