| In ICF (Inertial Confinement Fusion) research, efficient-broadband THG (Third-Harmonic Generation) technology is the key technology to improve the laser-target coupling and the uniformity of illumination on target. Nevertheless, the bandwidth using existed THG technology is still narrow (≤2nm), thus broader-bandwidth higher-efficiency THG technology should be developed to meet the requirement of ICF. We investigate, mainly from the view of group-velocity mismatch, problems in the mixing process of broadband THG, and five advances this thesis achieved are presented as follows:1. Relations among group-velocities of 1ω, 2ωand 3ωwaves in mixing process of broadband THG are analyzed theoretically and the group-velocity matching equation is established for the first time, which gives theoretical guidance for experiment. Obtained result through numerical simulation shows that efficiency and bandwidth of THG can reach maximum values when that equation is satisfied.2. Based on the three-wave group-velocity matching equation, the feasibility of the non-critical phase-matching method in THG process is discussed. Then a specific advice is given to grow the crystal with proper retracing wavelength for THG.3. A new constructed crystal is proposed to compensate the severe group-velocity mismatch between 1ωand 2ωwaves during THG of ultrashort pulses. Simulation results with it to be mixer show that THG conversion efficiency can be increased greatly.4. Performances (efficiency and bandwidth) of THG using CLBO crystals in the condition of phase-modulated pulses are analyzed. Results are contrasted with those using KDP crystal at the same input condition, which imply that CLBO crystal is much superior to KDP crystal in efficiency and bandwidth.5. THG conversion properties of piled chirped pulses are studied elementally. A predigest model is used to optimize the parameters of cascaded crystals, which will be a guidance for the design of frequency converter and a foundation for the future simulation. |
