Beam Profile Partitioning Based Spatial Modulation And Its Spectral Effect Of Filamentation Induced By Intense Laser

The propagation of ultrashort laser pulse in optical medium brings on lots of nonlinear optical phenomenons, such as the generation of plasma channels, conical emission of white light, spectral broadening and pulse self-compression. The generation of harmonic not only provides us a new method to reach short wavelength range; the harmonic with broadband also excites us a lot in the further compression possibility on pulse duration. However, how to obtain the stable broadband ultraviolet harmonic radiation, there still are many problems to solve. They have been obstacles preventing the potential values of filamentation from being put into practical applications.In recently years, many relevant institutions have found that the filamentations can be well stabilized by inducing spatial modulation. Some new measures were taken to modulate the laser spatial intensity distributing, for instance setting an aperture in beam path, or using masks, astigmatism, elliptic beam et al., and they have got their initial success in controlling filamentations. It has also been proved a good method to modulate the spatial intensity distributing in optical field by inducing the diffractions and interferences. The molecules and atoms in the optical medium respond drastically in such strong electric field of femtosecond laser pulse and their densities redistribute in periods accordingly. This process also affects its nonlinear course subtly.In view of these above, we studied the spatial intensity distribution of two Gaussian beams with constant phase and the same wavelength, and the multi-interference among different parts of the same beam when they propagate synchronously in optical medium. We also discussed the influences on harmonics taken by the modulation in theory and experiment. It includes the broadened spectrum of pump laser and harmonic, their stabilities and so on.The main contents of this thesis are as follows: 1. We studied the basic theories of the generation of plasma channel and third order harmonic, that include their main characteristics, and the physical effects accompanied. It is manifested by the theory that the filamentation occurs in air when the input peak power exceeds a critical threshold for self focusing of 5GW.2. Based on the common point light source model and the traits of femtosecond in practice, we examined the interference between two Gaussian beams and the interference among the inferior beams on a ring shaped input beam. We also demonstrated the spatial modulated intensity distributions pattern by simulation. It is found that the angles between two input beams and the width of a ring shaped input beam count a lot on the spatial modulation effects.3. The periodic structures in space and the nonlinear effects caused by spatial modulation were observed. In the experiment, we divided the profile of an input femtosecond beam with a large beam radius, then focused them. It was easy to find the periodic modulation in optical field around the focus. An exciting result was got when we detected the spectrum of from the plasma channel after modulating: the spectrum of output pump laser got a considerable broadening especially the spectrum third order harmonic generated was drastically broadened to 14.8nm which was 3 times lager than it had been without modulation. By doing that, the third order harmonic became quite clear and highly centralized compared with its former profile. We can see that spatial modulation has successfully promoted the quality and stability of its harmonic generated.