Ultrabroadband Continuum Spectrum Generated By Femtosecond Laser Pulses In Ar Gas

Since supercontinuum from 400nm to 700nm was measured by R. R. Alfano and S. L. Shapiro, the ultrabroadband continuum has been a hot subject. Especially in recent years, the improvement in the intensity of the laser pulse and the development of the ultrashort laser technique, more and more mediums can be used to obtain supercontinuum and the spectra become more and more broad. Now, supercontinuum from IR to UV has been obtained in laboratory. The applications of lidar, optical communication, spectroscopy, biomedicine, being source to obtain shorter pulse and so on have been developed.Propagation of an intense laser pulse in Kerr medium results in self-focusing because of the nonlinear index of refraction. The self-focusing makes the radius of the beam decrease and the intensity increase to the ionization threshold, and then the plasma is generated. Then the pulse propagated long distance with the same intensity and the same configuration, as a filament. The physics mechanisms of the form of the filament are: self-focusing model, moving focus model and Dynamic spatial replenishment model. In the progress, the supercontinuum because of the self-phase modulation, four-wave mixing and the plasma is formed.In order to obtain stable spectra with broader bandwidth, we need take account of many conditions such as original chirp, pulse energy, focus length, pressure and so on. This is our main purpose on this paper. Our work includes:1,Build and protect the Ti: sapphire CPA system. The pulse from the system is at 800nm, 25fs, 10Hz, pulae energy of 25mJ. Then we study the difference of the different rare gases and choose Ar.2,We obtain broadband continuum by the focused pulse from the Ti: sapphire CPA system propagating in the Ar sampler. By studying the supercontinuum in different conditions, we know the influence of original chirp, pulse energy, focus length, pressure of Ar. We obtained stable and 10~3THz supercontinuum with the pulse energy of 20mJ, the original chirp of 2.9×10~4fs~2, the concave mirror with f=500mm, the length of the Ar sampler of 1.5m and the pressure of Ar of 2.3atm. 3,We study the supercontinuum also, and find that the supercontinuum in air is not evident, but better with longer focus.4,We measured the far-field beam style in the best condition, and the divergence was 3.1 mrad.