Study Of Intensity Clamping During Femtosecond Laser Filamentation

When a powerful femtosecond laser pulse propagates in transparent media, it canretain high laser intensity over a long distance without diffraction. At the same time,the molecules along the propagating path will be ionized and generating a plasmachannel. This unique nonlinear optical phenomenon is named filamentation. Duringrecent years, femtosecond laser filamentation has attracted a lot of research interest,this is not only because it involves a lot of physical process, but also due to itspotential applications such as lighting control and remote pollutants sensing. Thefundamental physical mechanism of filamentation is mainly attributed to the dynamicbalance between the optical kerr effect induced self-focusing and the plasmadefocusing. This balance also sets an up-limit for the highest light intensity that onecould possibly achieve in the media, this phenomenon is known as intensity clamping.This thesis is devoted to study the intensity clamping during the filamentation. Weconfirmed the intensity clamping effect under TW level femtosecond laser in variousgas media and during the dual beam interference. We suggested a simple method tocharacterize laser peak intensity inside the filament in air. According to numericalsimulations，We find that intensity clamping is the dominant mechanism to inducehigh stability of the fluorescence signal emitted by filaments.At first, this thesis studied the intensity clamping effect both in air and argon.The plasma densities and peak intensities inside the filaments are measured by themethod based on the Stark broadening approach, the result show that although thelaser peak power was increased from0.1TW up to1.5TW, the peak intensity insidethe produced filament is nearly constant. Our work provides a further confirmation onthe universality of intensity clamping effect under TW level femtosecond laser in airand argon. We also studied the intensity clamping effect during the dual beaminterference. We have realized the interference of two ultrashort laser filaments. Theformation of a dynamic plasma grating induced by the interference pattern isobserved. The intensity variation induced by the interaction between two beams has been qualitatively interpreted by studying the corresponding change of the nitrogenfluorescence signal, however, qualitative analysis of the experimental resultsindicates that intensity clamping still governs the intensity evolution during thedual-beam interaction. The expectation of greatly increasing laser intensity by dualbeams superposition might not be realistic.Optical intensiy is a key parameter for studying the physical mechanism ofIntensity clamping. However, measurement of laser intensity inside a femtosecondlaser filament is a challenging task. This thesis suggests a simple way to characterizelaser peak intensity inside the filament in air. It is based on the signal ratiomeasurement of two nitrogen fluorescence lines, namely,391nm and337nm.Because of distinct excitation mechanisms, the signals of the two fluorescence linesincrease with the laser intensity at different orders of nonlinearity. An empiricalformula has been deduced according to which laser peak intensity could be simplydetermined by the fluorescence ratio. This method solves the long-pendingfundmental problem in the field of Filamentation nonlinear optics, and provides anew approach to study intensity calmping.Femtosecond laser filamentation is particularly interesting for remote sensingpollutant in the atmosphere. The pollutant molecules inside the filament could bedetected by analyzing the filament induced fluorescence spectrum. This thesis studiedthe local shot-to-shot stability of the fluorescence of nitrogen in air. It is found thatthe root-mean square fluctuation of the fluorescence signal is at least one order ofmagnitude lower than that of the linear propagation case. The further theoreticalanalysis point out that intensity clamping is the dominant mechanism to induce thishigh stability.At the end, we summarize the main result of our research work, and somesuggestions for the future study are also presented: First, the study of the intensityclamping phenomenon during dual filaments interference at different incidenceangles and higher pulse energies is suggested. Second，when the laser pulse is notgaussian profile in time and space, and the central wavelength of the laser pulse is not800nm, the method suggested in this thesis to measure laser peak intensity inside thefilament need to be further discussed. Third, when high energy femtosecond laser is tightly focused in air, considerable plasma continuum has been observed, andsaturation of ionization associated with the neutral molecular depletion and doubleionization process need to be taken into account. The advanced experimentalapproach （time-gating technique） and more sophisticated theoretical model shouldbe needed.