Research On Femtosecond Laser Filamentation In Different Atmospheric Environments And Its Characteristics Of Heat Deposition Process

As the intense femtosecond laser pulses propagate in atmosphere,due to the competition between the Kerr self-focusing effect and the plasma induced defocusing effect,a dynamic equilibrium will reach,resulting in a filament structure that travels long distance.The transmission of femtosecond laser filamentation in atmosphere is accompanied by a rich of physical effects,including the heat deposition caused by laser pulse energy deposition.The heat deposition plays an important role in triggering local airflow disturbance,promoting the freezing and settling of supercooled water droplets,and clearing and forming high-transmission channels in cloud and fog.Studying the mechanism of related phenomena can improve our understanding of cloud and precipitation processes in specific environments.Therefore,it has important theoretical significance and application background to study the characteristics of atmospheric transmission of femtosecond laser filaments,especially the heat deposition effect of optical filaments.In this paper,based on the existing research and the basic theory of cloud precipitation physics,using the simulation model of femtosecond laser pulses with thermodynamic equations,we firstly study the nonlinear propagation of femtosecond laser in dry and wet environment,cloud and fog environment and its heat deposition effects of the formed filament by using the simulation model of femtosecond laser propagation.The effects of water vapor content,cloud particle concentration and particle spectrum distribution on the heat deposition efficiency of femtosecond laser are clarified.Then,a method was established to analyze the influence of heat deposition process on the freezing efficiency of supercooled water droplets within the scope of cloud chamber,and the mechanism of the influence of heat deposition on the formation of snow crystals with different mass induced by optical filaments is discussed with this method.Finally,we study the method of using temporal Airy beam to prolong the optical filament and enhance the thermal deposition of the optical filament simultaneously by analyzing the simulation results of laser transmission.We summarized the related research contents and innovative achievements as follows:(1)In the nonlinear Schr?dinger equation describing the propagation of femtosecond laser pulses,the term describing ionization of water vapor molecules is added,and the spatiotemporal evolution and heat deposition characteristics of femtosecond laser pulses at 800nm and 248nm band are compared.We found that ionization of water molecules has almost no significant effect on the nonlinear transmission of femtosecond laser pulse with 800 nm wavelength,but can seriously affect the transmission of 248nm laser pulses.For laser pulses at 248nm,water vapor ionization can significantly affect the characteristics of clamped light intensity,electron density,radius and total energy of deposition.With the increase of water vapor content,the clamping light intensity decreases,the electron density increases and the total energy of deposition increases.However,in the higher humidity environment,the change of the maximum values of electron density and deposition energy is smaller and smaller,and tends to be saturated.The heat deposition of optical filaments can be enhanced by increasing the power of the incident pulse and using femtosecond pulse with wider pulse width in the environment of high relative humidity.(2)In the nonlinear Schr?dinger equation describing the propagation of femtosecond laser pulses in the atmosphere,the nonlinear terms describing the atmospheric turbulence and particle scattering are added,and the propagation model of femtosecond laser cloud is constructed,and the method for theoretical analysis of the characteristics of heat deposition of femtosecond laser filamentation in the cloud and fog disturbance environment is established.Based on the model,the effects of particle concentration and particle spectral distribution on the filamentation characteristics and thermal deposition of femtosecond laser are analyzed.The results show that both atmospheric turbulence and particle scattering have obvious effects on femtosecond laser propagation,and the effect of particle concentration on femtosecond laser filamentation is more obvious than that of particle size.The higher the particle concentration,the smaller the clamping light intensity,the smaller the electron density and the smaller the central energy fluence.The higher the particle concentration is,the greater the total amount of deposited energy is,but the peak value of energy deposition rate appears in an earlier position and the value of the peak gradually decreases,so the local temperature and pressure disturbance is also smaller.Comparing the transmission characteristics of femtosecond laser pulse in three kinds of cloud environment,it is found that the same incident pulse produces the highest electron density in cumulus,and the lowest in fog environment.The energy loss of laser pulse is the most in fog environment,but the energy deposition rate of laser filaments in cumulus is the largest.(3)A method for analyzing the effect of filament heat deposition process on the freezing efficiency of supercooled water droplets within the scope of cloud chamber is put forward and the influence of different focusing conditions,different pulse energies and different transmission media on the turbulence velocity and mass of snow crystals is discussed.First of all,the influence of changing the incident pulse energy on the filamentation characteristics of femtosecond laser is studied under the external focusing conditions of f=50 cm and f=30 cm The results show that,when the incident pulse energy increases from E_in=4.5 m J to E_in=7.1 m J for pulse with f=30 cm,the intensity of multifilament tends to be stronger,the number of multifilament tends to be more,and the position of multifilament formation tends to form behind the focus position,the maximum amplitude of local temperature(air pressure)disturbance caused by the filament is also larger,which is conducive to form stronger turbulence of air flow and promote the collisional growth of ice particles.However,when the incident pulse energy increases from E_in=4.5 m J to E_in=7.1 m J for pulse with f=50 cm,the multifilament phenomenon occurs before the focusing position,and the energy competition between the filaments leads to the early dissipation of the pulse energy.The results show that although the pulse energy is increased,the air velocity and snowfall mass increase little or even decrease.Then,we compared the heat deposition characteristics of femtosecond laser pulse in air,argon and helium under the same focusing conditions.The results show that femtosecond laser can form longer filaments in argon,resulting in a larger total deposition energy and a larger range of energy deposition distribution The temperature and pressure disturbance caused by thermal deposition in argon can reach 220 K and 8.0×10⁴ Pa,respectively,which is conducive to the formation of stronger airflow disturbance and more snow crystals.These results show that the increase of laser pulse energy can improve the heat deposition of filaments to a certain extent,but with the increase of pulse energy,the formation of multifilament,especially the position of multifilament,has an effect on the efficiency of thermal deposition,and the distribution range of laser pulse energy deposition also has an impact on the heat deposition of filaments.These results have some reference value for improving the efficiency of femtosecond laser thermal deposition and promoting the application of femtosecond laser in the atmosphere.(4)Based on the numerical simulation results,we compared the transmission processes of temporal Airy laser pulses with positive time delay,negative time delay and two-way time delay with Gaussian pulses.It is found that,when fixing the initial pulse energy,the temporal Airy pulses tend to form a longer filament than the Gaussian pulse.The Airy pulse with negative time delay can form the longest optical filament,which is related to the intense splitting of the back edge of the pulse and the energy flow between the main lobe and the side lobe during the pulse transmission.By increasing the incident intensity of laser pulse,increasing the pulse width,and using airy pulse with a large radius,longer filament can be achieved.By expanding the radius of airy pulse,increasing the pulse delay time and increasing the pulse width,the total deposited energy by filaments can be effectively increased,and it is possible to improve the efficiency of the cloud physical process affected by femtosecond laser heat deposition.