Raman amplification of ultra-short laser pulses in plasma

• Main Author: Yang, Xue
• Published: University of Strathclyde 2013
• Subjects: 530
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.605952

This thesis presents the experimental investigation of Raman amplification of ultrashort laser pulses by a counter propagating chirped pump pulse through resonant excitation of a plasma density wave. Since plasma is a gain medium able to withstand very high intensities, this scheme is proposed as the basis of a new generation of laser amplifiers capable of replacing Chirped Pulse Amplification (CPA) systems, which are limited by the damage threshold of solid state gain media. In this experiment, both the pump and the probe beams are generated from a 30 TW Ti:Sapphire CPA laser system. The chirped pump pulse is 250 ps long and centered at 805 nm. The probe pulse, compressed to minimum 70 fs and then frequency downshifted before the interaction with the pump pulse in the plasma, contains maximum 1 mJ of energy on target. The plasma is generated by striking a high voltage discharge across a hydrogen gas filled capillary waveguide with the electron plasma density controlled around 1 x 10p18scmp-3s. In addition to a CCD camera and a spectrometer, a self-built Frequency-Resolved Optical Gating (FROG) system is employed as a real time detector to fully reconstruct the probe pulse amplitude and phase. The experiment is performed in two stages, firstly with low pump energies, about 400 mJ, allowing the verification of chirped pulse Raman amplification mechanisms in the linear regime. In these conditions the energy gain grows exponentially with the pump energy and increases with backing pressure. The theoretical predictions showing that the probe pulse duration should be preserved when using a chirped pump pulse are confirmed. Maximum energy gain is 450%. The second stage is implemented with pump energies up to 950 mJ. Initially the Raman process develops in the linear regime. With the amplification of the probe pulse, the amplification mechanism can transit to a nonlinear process. The highest energy gain is 650%. Obvious bandwidth broadening mainly on the high frequency side and gain saturation with either increasing pump or probe energy are observed. Assisted by 1D aPIC simulations, the transition from the Raman linear regime to the nonlinear regime is analysed.