Icf Laser Drive Front-end System Pulse Shaping

High power solid laser facility is one of the perfect drivers among the driving technology of Inertia Confinement Fusion. The ability of pulse shaping is one of the key factors in the all-fiber front-end system of the facility. The temporal and frequency characteristics and propagation properties of shaping pulse impact greatly on the output pulse. The laser pulse with appropriate parameter, such as pulse width, shape, energy and spectrum width should have high signal-to-noise ratio and beam quality in the front-end system. During the propagation of the shaping pulse in the fiber, polarization, dispersion and self-phase modulation would influence the characteristics of it. The article studied the two pulse-shaping technologies, the capabilities of the key subassemblies and the propagation of the shaping pulse. The main contents can be summarized as follows:1. The pulse characteristics in the modulated front-end system were studied. In allusion to the fiber lasers integration for the front-end of LMJ, the phase-modulated pulse was affected by the effects, such as gain narrowing in the fiber amplifier and dispersion in the fiber. The influence was quantified according to define the FM-AM factor. The propagation of the phase-modulated pulse in the polarization-maintaining fiber (PMF) was analyzed.The results have shown that the FM-AM factor was affected by polarization extinction ratio (PER) degradation of fiber connectors, modulation index, modulation frequency and differential group delay between both axes of the PMF. To decrease the intensity modulation would be realized by optimizing these parameters in system design.2. Based on the pulse-shaping system of all-fiber pulse stacking, the work principle and the performance of the important assemblies was studied. The pulse-spreading induced by dispersion of chirped fiber Bragg grating (CFBG) was analyzed by using the transfer matrix. The temporal and frequency characteristics of chirped stacking pulse were discussed.The results have shown that the spreading quantity varied with the chirped factor of the fiber grating, and the temporal profile and power spectrum of the stacking pulse were modulated, and the modulation degree was determined by the chirp parameter and the delay between the sub-pulses. The temporal modulation period of the stacking pulse would be smaller than that of the target physics requirement if the two parameters were controlled carefully.3. The influence of linear dispersion and self-phase modulation on the chirped stacking pulse was studied by using the nonlinear Schrodinger equation. The results have shown that the initial chirp parameter, peak power and delay between the sub-pulses would impact the temporal and frequency characteristics of stacking pulse during propagation in the fiber.The theoretical methods and results would be useful for the optimal design of the front-end system of high power solid laser facility.