Filamentation Of Femtosecond Laser Pulses Based On The Spatial Modulations

Since the filamatation in air was observed for the first time in 1995,it has extended the frontiers of nonlinear optics due its numerous nonlinear interactions with air and its valuable potential applications.The self-guided propagation of femtosecond laser pulse in air reveals surprising features on the nature between light and air.The physical origin of the formation of filaments involves group velocity dispersion,diffraction,self-phase modulation,self-focusing,multi-photons ionization,plasma defocusing,self-steeping,multi-photons absorption,etc.The process of its formation can be described by the action of mainly two nonlinear physical effects that filamentation is one of the prominent phenomenon which is explained as a result of dynamic competition between Kerr self-focusing and plasma defocusing.The thesis focuses on how to modulate the initial beam in the spatial domain to realize various novel propagation patterns via changing the initial phase and amplitude and it comprises mainly four sections as following.In order to investigate where we should impose the modulation for the beam,the spatial and temporal features of femtosecond laser filamentation,which is several times higher than the critical power,influenced by strong air turbulence at three typical propagation distances have been studied numerically,namely,a strong turbulence occurring right before focal lens,before the filamentation,right after the lens,respectively.In comparison with these three cases,we have found that the light modulation would impose the strongest influence on the filamentation process,which has acted as a cornerstone for our following works.For the phase modulation section,we have numerically studied femtosecond laser filament arrays generation in air by using two-dimensional acousto-optic modulation(AOM).Similar regular filament distributions have been achieved compared with filamentation by using a microlens array(MLA)in air.The overall length of filamentation with the same onset can be prolonged almost triple as much as that generated in the case of single lens.The modulation frequency,namely,acoustic period,has a great effect on the elongation.The AOM offers a convenient way to control the filamentation process.For the amplitude modulation section,filamentation with the amplitude modulation of Mathieu beam in air are investigated numerically.Non-diffracting characteristic of Mathieu beam is partly maintained in the intense femtosecond laser area even though it is only an amplitude modulation.Then,the spatiotemporal dynamics of femtosecond filamentation are investigated numerically pertaining to different intensity distributions of zero-order Mathieu beams with the ignorance of phase alteration for adjacent lobes.The side filaments are formed due to constructive interferences of the light fields of the side lobes.For the beam with appropriate lobes,the whole length of filament can be extended.The diffraction-free characteristic can still be retained in the femtosecond filamentation area even through ignoring the phase alternation of adjacent lobes.Thus,several high peak intensity spikes behind the continuous clamping segment can be illuminated by the remaining energy of side filaments.For the filamentation utilizing special light field modulations,it comprises three parts,namely,elongating the parabolic filament,constructing arbitrary trajectories of filament,and realizing elliptical filaments as well as solitions or light bullets.In the first part,we have found multiple filaments(MFs)which are formed in the first order accelerating parabolic beam can benefit the accelerating action of filaments and enhance the length of parabolic trajectory.The nonlinear dynamics of multiple filaments in self-accelerating actions by using corrected accelerating parabolic beams(CAPBs)are numerically investigated.By increasing the amount of main lobes,longer displacement of extended the curved filament takes place.In the second part,we have numerically demonstrated the formation and control of novel snake filaments in air.The specific fields of satellite lobes make a great contribution on alternating the accelerating direction of the main lobe.As a result,the filament accelerates in a certain transverse area within two phase boundaries that are called phase wells.Lastly,we have proposed theoretically various kinds of filaments via the Mathieu modulation.Our results indicate curved filaments,in-phase and out-phase solitons and nonlinear light bullets can be formed successfully in air.Through calculated initial Mathieu accelerating beam(MAB),curved filament is able to propagate along a predesigned elliptical trajectory.By transforming the MAB into an axial symmetrical structure with in-phase and out-phase modulations,we obtain two kinds of solitons in air,respectively.The latter case can even propagate in a breathing fashion.With a ring structure of MAB,we successfully form a light bullet in air that generates a chain of intensity peaks over extended distances.These unique filaments can offer significant advantages for numerous potential applications,such as super-continuum enhancement,micro-engineering of materials,THz radiation generation,curved plasma channel based lightning guiding and laser disturbance or damage with obstacle avoidance.