The Laser Coupling And Beam Evolution Of The Fiber-Based High-Energy High-Power Pulsed Laser System

The demand in high energy density physics research, including the laser-driven in-ertial confinement fusion and high energy particle collider based on the laser plasma ac-celeration, is a major power to push the development of the high-energy high-power laserdriver forward. Because of the ultra-low energy conversion efficiency of the laser driverbased on the Xenon lamp pumped large-aperture Nd-doped glass bulk medium, it can’tmeet the efficient, repetition rate operation requirements for the future laser drivers ofthe inertial fusion energy and high-luminosity high-energy particle collider facilities. Thehigh-energyhigh-powerpulsedfiberlasersystembasedonthefiberamplificationnetworkconcept inherits the features and advantages of the fiber laser which make it a promisingscheme for the future laser driver.In this thesis, the integrated configuration and basic concepts of the high-energyhigh-power pulsed fiber laser system are analyzed and three major constraints, i.e. lasercoupling requirement constraint, single fiber output capability constraint and fiber beamcombining tolerance constraint, are proposed. The key scientific and technological is-sues in this system are briefly summarized. A theoretical model is proposed to analyzethe laser coupling of numerous fiber outputs. The requirements for the mode field diam-eter and output energy from a single-core fiber are given for the mega-joule class laserdriver. Intensity distribution on the target under coherent and incoherent beam combiningconditions is numerically researched. For exploring the ultimate limitation of the outputpeak power from a single-core fiber, a nonlinear beam propagation numerical model isbuilt to model the self-focusing effect in ultra-large-mode-area highly-multimode fibers.Variation of the self-focusing length of the fundamental mode, higher order mode andthe summation of numerous modes with the core diameter and peak power are revealed.The results show that, requirements of the mode field diameter and output energy of thesingle-core fiber according to the laser coupling constraint are one order of magnitudelarger than that can be achieved with the state-of-the-art technology. Therefore, novelultra-large-mode-area waveguide structure and methods to suppress the self-focusing ef-fect requires further research.For describing the pulse evolution process in the single fiber propagation and am- plification chain which is the basic element of high-energy high-power pulsed fiber lasersystem, a combined model which couples the dynamic laser rate equation and Ginzburg-Landau equation is proposed. The incoherent pump process and coherent temporal-spectral evolution of the signal pulse can be described via this model simultaneouslywhichenlargesthedescriptionscopeoforiginaltwomodels. Asapreliminarydemonstra-tion of the single fiber propagation and amplification chain, an all-fiber pulsed amplifiersystem employing the large-mode-area photonic crystal fiber is designed and experimen-tally demonstrated based on the general design principles of the pulsed fiber amplifiersystem. The cleaving and splicing technologies of the large-mode-area photonic crystalfiber are explored. Near milli-joule energy is obtained for10ns square seed pulse whilethe fundamental mode operation is achieved.