Ultrashort Intense Pulse Laser Propagation In Low-Density Plasmas And Air

Ultrashort Intense Laser Pulse Propagation in Low-Density Plasmas and AirXiao Yuan, Doctor of Philosophy, 2002 Dissertation advised by: Lu Baida, Professor of PhysicsThe study of ultrashort intense pulse laser beams in low-density plasmas and air is relevant to a wide range modern application. Often it is desirable that the laser should propagate a long distance at high intensity. This doctoral dissertation covers the research on this subject analytically and the preliminary experiments of a 10 ps KrF laser beam propagation hi air. The main results are as follows:1. The propagation of an ultrashort intense pulse laser beam in low-density plasmas is investigated analytically. Relativitistic pondermotive force and space-charge effects are included, and an equation describing the evolution of the laser spot size is derived. It is shown that self-consistent electron cavitation can lead to self-channeling and thus long distance self-focusing of the laser. The conditions for the long distance propagation are discussed in detail. The results are useful in determining the appropriate size of the channel to be preformed and can guide the future research in ultrashort intense laser propagation.2. The propagation of ultrashort laser pulse in air is analyzed with mechanics analogy by considering Kerr effects and free electron defocusing. The equation describing the evolution of the laser spot is derived. It is shown that long distance focusing requires large initial beam and the propagation distance is very sensitive to the initial divergence angle of the laser beam. The conditions for long distance propagation and self-guiding are given.3. An optical amplification system for 10 ps KrF laser with polarization multiplexing is developed and the energy of 140 ml is extracted. A DFDL model including gain saturation effects is developed and this model is used to study the fluctuation of the pulse width theoretically, which is identical to the experiments. In order for ps pulse amplification, the saturation fluence for KrF at 10 ps and gain of the discharge KrF laser are monitored.4. UV laser self-focusing in air is obviously observed with 10 ps KrF laser. The laser beam spot evolution and electron density in the guiding range are evaluated with the burn pattern and the measured conductivity.The above results will be useful in ultrashort laser pulse amplification and future research in ultrashort intense laser propagation in plasmas and ionizing gases.