The Pulse Characterization During Femtosecond Laser Filamentaion In Air

Femtosecond pulse filamentation in air has attracted considerable interest because of its complex physical mechanisms and wide range of applications. The femtosecond pulse undergoes strong transformation during filamentation, which also carries key information of the nonlinear optical processes. Therefore, the pulse characterization during femtosecond laser filamentation in air is crucial for both theoretical and experimental investigations, providing a deeper and more thorough understanding of the complex underlying physical processes inside the filament. This dissertation is devoted to study the femtosecond pulse characterization technique during the filamentation in air. Three measurement techniques have been discussed.The first technique studied is based on the two-photon fluorescence measurement. It retrieves the pulse duration, chirp rate and beam radius by measuring the intensity distributions of the two-photon fluorescence induced by the femtosecond pulse. The experimental results show that the pulse duration, chirp rate and beam radius could be simultaneously quantitatively retrieved and this simple technique would be useful in practice to trace the underlying dynamics of filamentation in air.The second technique studied is named two-color-field frequency-resolved optical gating method (TC-FROG). It retrieves the pulse intensity and phase by measuring the spectral distributions of the generated third harmonic pumped by two-color laser field. First, the physical mechanisms of the third harmonic generation in air by a two-color laser field are studied. It is found that the third harmonic spectrum varies with the temporal delay, chirp rate and the self-phase modulation of the fundamental and the second harmonic wave. Then, an amplitude-phase reconstruction algorithm is designed with the adaptation of Genetic Algorithm. Finally, the theoretical simulation is performed for the electric field reconstruction of Gassian pulse, and the uncertainty between the fitting data and the measurement data is discussed.The third technique studied is based on the nitrogen fluorescence measurement. It retrieves the pulse duration by measuring the intensity of the nitrogen fluorescence in the overlapped zone of the pump pulse and probe pulse, which varies as a function of the temporal delay between the two pulses. First, the distribution model of the nitrogen fluorescence intensity according to the temporal delay is derivated. Then, the pulse duration measurement of the femtosecond laser pulse in the overlapped zone is carried out.In the end, the future work of the thesis is discussed: first, by improving the spatial resolution of the laser beam's cross-section, a 3D measurement of the femtosecond pulse could be realized; second, TC-FROG technique needs further practical confirmation; third, the remote-measurement of the laser pulse duration during the femtosecond laser filamentation in air could be performed by using the technique based on the nitrogen fluorescence measurement.