Control Of Multiple Filaments During Femtosecond Laser Filamentaion

Femtosecond laser filamentation is a nonlinear phenomenon whereby the pulsedlaser beam non-diffractive propagates with an almost unchanged diameter over adistance much longer than the Rayleigh length. It is known that filamentation isresulted from the dynamical balance of the laser beam self-focusing, which is inducedby the optical Kerr effect, and the defocusing effect of the self-generated weakplasmas. There is a series of nonlinear phenomena taking place during filamentation,such as intensity clamping, self phase modulation, supercontinnum generation, etc.Femtosecond laser filamentation has attracted considerable interest because of itswide range of applications, including pollution detection, lightning control andmicrowave-guiding.In practice, when the laser power is much higher than the critical power, thebeam will break up into multiple filaments. The mechanism of multiple filamentformation is due to the non-uniform wave front either because of inherentimperfection of the laser itself intensity distribution or the external perturbation, suchas turbulence. These filaments are not independent to eachother. They will completefor the energy from the background reservoir. The underlying physics of multiplefilament competition is essentially field re-distribution inside the pulse duringpropagation. As a result, multiple filaments are randomly distributed, the number andlocation of the filaments is unpredictable, leading to the unique phenomenon named“optical turbulence”. Obviously, this will decrease the quality of the beam andconstitutes a serious drawback in applications. A challenge for systematic controllingof multiple filamentation thus emerges.There are two main aims to control of multiple filamentation. One is to suppressthe multiple filamentation, avoiding the energy competition among them. This canimprove the robustness and prolongation of multiple filamentation. It could be usefullfor the applications such as pulse compression and long-distance detection. The otherway is to overcome the disorder of multiple filamentation through the nonlinearpropagation leading to a spatial stability. It could benefit the applications including micro-fabrication and microwave-guiding. In this thesis we will study the newapproachs of controlling of multiple filamentation.We first study the suppression of multiple filaments based on a telescope system.By decreasing the beam diameter with telescope, we successfully suppress thegeneration of the multiple filaments. The experiment leads to a longer and stablefilament. Futhermore, the longitudinal distribution of the laser peak intensity insidefilament in air is studied by measuring the signal ratio of two nitrogen fluorescencelines,391nm and337nm. The laser peak intensity initially remains almost constantinside the filament. However, before the end of the filament, surprisingly the laserintensity undergoes dramatic increase. It reveals that the duration of the created shortpulse could be sub-cycle which the paper “Intensity Spikes in Laser Filamentation:diagnostics and application” of Physical Review Letters in2009have been predictedtheoretically.Secondly, we study the suppression of multiple filaments based on an axiconlens. Quasi-Bessel beam could be generated by focusing a Gaussian beam with theaxicon. Since Bessel beam features remarkable non-diffractive central spot, it resultsin a high contrast filament along its long focal depth during nonlinear propagation.The suppression of multiple filaments is experimentally observed. The axicon canelongate the length of a single filament to improve the robostness. Also, theexperimental result indicates the occurence of the pulse compression in ourexperiment.Thirdly, we study the controlling of multiple filaments based on an axicon. Byfocusing a femtosecond laser with an axicon in which the peak power is many timeshigher than the critical power for self-focusing(P>500Pcr), multiple filaments aregenerated. Filaments locate at the core and ring structures of the quasi-Bessel beamcreated by the axicon. We have demonstrated that regularization of multiple filamentscould be realized instead of “optical turbulent” in experiments, meanwhile the lengthof the filaments are much longer than that when no axicon is applied. Correspondingnumerical simulation reveals that cylindrical-symmetry-broken of the initial beamprofile could be responsible for the occurrence of multiple filamentation by using anaxicon as focusing optics. At the end, we theoretically study the controlling of multiple filaments in air byusing axicon array. By focusing a femtosecond laser with an single axicon in air, theextension of background energy reservoir is larger than the case that without axicon.It is hard to control multiple filaments in air with an single axicon. Therefore weproposed an new technique to control multiple filaments in air by using axicon array.We have proved that axicon array can not only elongate the length of filaments butalso realize a deterministic multiple filaments pattern. It could benefit the applicationswhere not only the spatial distributions but also the lengths of the multiple filamentsneed to be concerned. In addition, we have introduced the fabrication technique ofaxicon array by laser direct writing and inductively coupled plasma technology.