Control Of Filamentation And Supercontinuum Of Shaped Femtosecond Laser Pulses

In recent years,the study of filamentation by femtosecond laser pulses and various related nonlinear effects have become one of new and hot research fields.Among them,the supercontinuum(SC)generated from femtosecond laser filamentation has a wide spectral range which can span from ultraviolet to mid-infrared.Because of this spectral characteristic,the SC provides a unique light source for many research and application fields such as the remote sensing and air pollution detection,few-cycle laser pulses generation and biomedical imaging.Conversely,these research and application fields also put forward different requirements for the femtosecond laser filamentation and SC.Therefore,the optimal control of filamentation is critical to the application research.The nonlinear effects during femtosecond filamentation such as self-focusing and multiphoton ionization,and the physical mechanism of SC inculding the self-phase modulation and self-steepening effect result in that the filamentation and SC is strongly dependent on the intensity and phase distribution of the initial laser pulse in the time and space.Therefore,the laser pulse shaping will significantly change the nonlinear process in filamentation and SC.In this paper,combining the laser pulse shaping technology based on liquid crystal spatial light modulator(LC-SLM)and the spatial modulation of microlens array,we modulated the femtosecond laser in time and space,and study the feedback optimal control of filamentation and SC generation in fused silica.The evolutions of filamentation and SC generation under the condition of complex pulse profiles are revealed,and the key scientific issues including the improving the conversion efficiency of SC and the modulation of spectrum intensity is studied.First of all,we study the feedback control of filamentation of shaped femtosecond laser pulses in fused silica.Using the feedback control program based on the genetic algorithm,we study the onset control of femtosecond laser filament in fused silica.The experimental results demonstrated that the filament can be generated at a designated position in fused silica.In addition,a long-distance controllable displacement of the filamentation onset in fused silica is obtained,with a maximum displacement of 5.4 mm.The numerical simulation on filamentation of shaped pulse in fused silica is performed based on the nonlinear Schr?dinger equation.The results are consistent with those of experiments.It is demonstrated that the onset of filamentation depends on the peak intensity and envelope of shaped femtosecond pulse.The more complex of the envelope is,the central peak smaller is and the further position of filament is.By contrast,when the pulse waveform is relatively simple and the principal peak is stronger,the onset of filament becomes earlier.Both in experiment and theory,we further study the intensity modulation of the filaments by shaping femtosecond laser pulse.The results indicate that filamentation intensity can be controlled by modulating the peak power of the shaped pulse.The intensity control of the filament method is meaningful for the controllable change of the refractive index in the specified area of the medium.Then we study the optimal control of the SC generation by the shaped femtosecond laser pulses filamentation in fused silica.We use the feedback optimization of the shaped femtosecond laser pulses to control the spectrum intensity of the SC,and realize the intensity modulation of the SC,forming a distinct spectral peak at preset wavelength position.Furthermore,combining the laser pulse shaping technology and the spatial modulation of microlens array,we investigate the control of the SC generation from the filamentation of the spatiotemporally modulated femtosecond laser pulses in fused silica.The experimental results show that the maximum intensity variation of SC is more than three times,and the blue-side extension control of SC spectrum is also realized.It can be concluded that the peak intensity and envelope distribution of time domain pulse are the main factors affecting the spectral intensity and broadening of SC,that is to say,the steep tailing pulse edges is beneficial to the broadening and enhancement of SC in the blue-side.These results will provide a new method for the technologies of high-power femtosecond SC source.Finally,the effect of the modulation of femtosecond laser pulses on the filamentation and SC generation by Bessel beams in fused silica is studied.The elongation of the filamentation with relative uniform intensity distribution and the optimal control of broadening and conversion efficiency of SC are realized by using temporally chirped femtosecond Bessel beams in fused silica.The experiments show that there exists a negative chirp range where the filamentation length reaches a maximum.We achieve the further broadening and the enhancement of conversion efficiency of the SC generation from the Bessel beams filamentation.The spectral broadening range increases more than 100 nm compared with the initial value,and the maximum conversion efficiency is more than 50%.In addition,the control of the intensity of Bessel beams filamentation and the conversion efficiency of SC is realized by adjusting the delay of double-pulse.