Investigation Of Propagation Of Femtosecond Laser Pulses In Air

Femtosecond laser technique has been the hot research area since its emergence.The propagation of femtosecond laser in air has also attracted the great attention.During the propagation of femtosecond laser pulse in air, a thin filament will beformed due to the dynamic balance between the self-focusing resulting from Kerreffect and plasma defocusing resulting from ionization, which is the well-knownfilamentation process. After the filamentation of femtosecond laser in air, itspropagation is no longer limited by the Rayleigh length, making the long distancepropagation possible. Quite a few interesting physical phenomena are involved in thefilamentation, such as the Kerr self-focusing, self-phase modulation, plasmadefocusing, supercontinnum generation, third-order harmonic generation and conicalradiation etc. These phenomena can find extensive and promising applications, suchas the long distance detection, laser lightning, remote sensing, rain and snow making,laser induces fusion, laser weapon and so on.As the femtosecond laser pulse propagate freely in the air, the distance where thefilament forms is relatively long, making it difficult to probe and study the filament,to overcome this problem, focusing lenses are often used to shorten the filamentationdistance before the investigation of filamentation. In this thesis, the optical filamentsare chosen as the research object, and by changing the focal length of focusing lensand initial energy of laser pulse the filament length and focusing distance can be wellcontrolled. Analyzing the experimental data, we arrive at the following conculsions:in the case of not changing the focal length, along with the decrease of the initialenergy of laser pulse, the collapse distance deviates more and more with respect to thefocal length of the lens, the intensity luminance and of filament get lower, and lengthof filament becomes shorter, as a result, we can not observe filamentation any moreeventually; In the case of not changing the initial energy of the laser pulse, along withthe decrease of the focal length of lens, the collapse distance and filament length getshorter, while the intensity of filament increases gradually. In addition, we simulate the filamentation by numerically solving the nonlinear Schrodinger equation under thesame conditions, i.e. the wavelength, energy, duration, beam radius of the laser pulsesare same as the experimental conditions. By calculating the plasma density and spatialdistribution of intensity, and comparing with the experimental results, it is found thatthe collapse distances obtained in theoretical calculations are in fairly good agreementwith the experimental results.