Research On The Propagation Properties Of Ultrashort Pulsed Beams

With the rapid development of laser driver technology in inertial confinement fusion (ICF), ultrashort pulsed beams were applied in high power laser system gradually. In ultrashort pulsed beams propagation, generally speaking, small-scale self-focusing was restrained preferably. And the bandwidth of ultrashort pulsed beams leads to many new questions. Therefore the study of ultrashort pulsed beams has been a focus of the scientific research. On the basis of the modified (3+1)-dimensional nonlinear Schrodinger equation , symmetry split Fourier transform method as the method of numerical simulation ,we investigate the propagation of femtosecond pulsed beams in free space and nonlinear dispersive media. The main results achieved in this thesis can be summarized as follows:1. The Near-field Fresnel diffraction modulation of ultrashort pulsed beams in free space is studied in theory by use of the simulation. The Fresnel diffraction pattern of regular shapes screen such as rectangular aperture, serrated circle aperture and Super-Gaussian beam. The diffraction modulation of ultrashort pulsed-laser is less than CW laser. And the transmission features of the ultra-short pulsed-laser passing through the serrated aperture and spatial filter is studied in allusion to clean and uniform intensity distribution in the high-energy ultrashort pulse-laser system, especial diffraction of Super-Gaussian beam. The result show: The axial pulse envelope propagates undistorted when propagating in free space, but the spatiotemporal coupling can affect its intensity.2. The self-focusing properties of femtosecond pulses in a nonlinear dispersive medium is investigated numerically in the regime where the combined effects of diffraction, Space-time coupling, normal dispersion ,and cubic nonlinearity lead to pulse splitting .The main work and achievements as following.a. Space-time coupling and self-focusingThe space-time coupling can halt the critical collapse of the self-focused field . The small-scale self-focusing leads to beam splitting filament, and then convergent. The beam alternates between the self-focusing and beam splitting, but its peak power becomes weaker and weaker .Simultaneously temporal splitting is observed, and its evolvement is similar to beam. The space-time coupling is similar to the nonlinear shock binging to the self-steepening.b. Nonlinear self-steepening, space-time coupling and self-focusingThe numerical results show that the nonlinear self-steepening make the back of the pulse steepen and the peak power decrease greatly, that halt the collapse of pulse beam and asymmetry splitting.c. GVD,TOD, Nonlinear self-steepening, space-time coupling and self-focusingAs implied by the functional form, space-time coupling relates to the dispersion, that make the TOD and GVD produce a time shift. The shift increases with distance and dispersion. Both GVD and TOD can lead to broadening of optical pulse. But TOD leads to asymmetry broadening. The time delay affects the on-axis intensity that decrease significantly. The Near-field transversal intensity distribution of pulsed beam remains nearly unchanged.