Reaching the nonlinear regime of the Raman amplification of ultrashort laser pulses.

摘要

Raman amplification and compression of ultrashort laser pulses by counterpropagating a long pump pulse in plasma offers an attractive approach to attain ultra-intense lasers. Theoretically, focused output intensities of ∼ 1025 W/cm2 are accessible by this technique---an improvement of 104 ∼ 105 over currently available techniques. This thesis explores the possibilities of the experimental realization of such a scheme. Three ways of generating ultrashort seed pulses including Optical Parametric Generator and Amplifier, Barium Nitrate Raman shifter and multi-stage benzene Raman amplifier were demonstrated. Plasma with lengths of up to 3 mm long with uniform density distribution were generated using laser induced breakdown of gas jet targets. Plasma density and temperature measurements were performed using optical interferometry technique. Guiding of the pump pulse through a 3 mm long plasma was demonstrated. Amplifications of ∼100 were demonstrated using broadband seed pulses from the OPG/OPA system. The observed bandwidth of the amplified pulse increases with the amplification. Pulse duration measurements with a single point autocorrelator show that shorter output pulses are correlated with larger amplification. In the subsequent experiment using a narrow band seed pulse from the Barium Nitrate Raman shifter, an energy amplification of ∼5000 for a week seed and intensity amplification of ∼1000 for a strong seed. More importantly, the nonlinear features such as gain saturation and pulse compression were observed for the first time. The intensity of the amplified seed exceeded that of the pump by more than one order of magnitude. These experiments clearly showed the feasibility and practical importance of such a laser amplifier. The Raman scheme is becoming an important technology for the next generation of compact, high power and ultrashort laser amplifiers