Ultrafat Laser Filament Array Generation

Ultrafast laser filamentation occurs when high power femtosecond laser pulses propagate in optical medium. During fialmentation, ultrafast laser pulses can propagate long distances without significantly losing peak intensity. The major physical mechanism of filamentation is a dynamic balance between the optical Kerr effect induced self-focusing and the de-focusing effect caused by either plasma diffraction or high-order-Kerr-effect. Fruitful nonlinear processes are involved during filamentation, inculding self-focusing, photonization, intensity clamping, self-phase modulation, self-steepening, and space-time focusing, etc. Due to the bright prospects in the wide range of applications, such as remote sensing, lightning control and pulse compression, ultrafast laser filamenatation has attracted considerable interest.When the laser power is higher than the critical power for self-focusing, multi-filamentation can be frequently observed in practice due to the perturbation in the intensity distribution of the initial beam pattern or the refractive index perturbation of the optical media. Depending on the phase differences, crossing angles or distances among them, multiple filaments will interact with each other, manifesting as repelling, attraction, fusion or energy exchange etc. As a consequence, multiple filaments are normally distributed disorderly in space. However, in some specific applications, such as white light array, filament assisted microwave guiding and massive micro-fabrication using filament arrays, spatial regularization of multiple filaments are demanded. Therefore control of multiple filamentation has become a hot research in related field recently. In this dissertation, we focus on the filament array generation mechanism.Firstly, this dissertation studied the filament array generation dynamics by focusing ultrafast laser pulses with axicon in methanol. The experimental results demonstrate multiple filaments are located on the central spot and ring structures of the quasi-Bessel beam created by the axicon. The outcome of simulation suggests the cylindrical symmetry breaking in the initial beam profile is the main reason for the filament array generation when focusing ultrafast laser pulses with the axicon.Secondly, we study the filament arrays generation mechanism in air by using three kinds of step phase plates with π phase lag, namely, the semicircular phase plate, the quarter-circle phase plate, and eight-octant phase plate. Experimental results and simulations show that the spatial arrangement of the filament array is determined by the geometrical shapes of the phase plates. The separation distances and the length of the filmanet array can be controlled by different focal lenses. The study results further indicate that by using an axicon, filament array in the form of ring shape could be realized. The separation distances between filaments are almost independent of the propagation distance, while the lengths of the filaments could be significantly elongated at the same time. Our research has provided a new technical approach to produce a filament array potentially possessing photonic crystal structure and characteristics.At the end, we study the self-guided propagation mechanism of multiple light channels without ionization at the post-filamentation stage. The experimental results show that after the filament was ended, the laser beam was divided into multiple distinguished millimeter-scale spots with larger low intensity energy background surrounded. These spots propagated with low divergence which is even significantly lower than that given by a nonlinear propagation of a laser beam with similar diameter (FWHM) and power. The corresponding numerical simulation reveals that the low intensity energy background is the main mechanism to support this nonlinear propagation process.