Research On Fiber Optic Amplification Technology Of ICF Front End System

Fiber amplifier was found widespread application in scientific research, industry and military areas, due to its high compactness, high efficiency, high signal to noise ratio, good beam quality. In particular, the ICF front-end system should provide the high-power seed pulse at 1053nm with good beam quality, high signal to noise ratio, and which must satisfy the certain temporal shape, spectral characteristic and polarization characteristic. The output energy, beam quality and signal to noise ratio of the ICF system were determined by ytterbium doped fiber amplifier to a certain degree, and the gain design of amplifier is the key component in the energy distribution of the whole system. However, the ASE noise, nonlinear effect and saturation distortion in the fiber amplifier had badly effect on output ability of the amplifier. Therefore, in order to ensure the output index of the ICF system, such as energy, spectral width, pulse duration and the stabilitily of the ICF system etc, it is very important to design the work point of the amplifier, and figure out the factor which had badly effect on the amplifier such as ASE noise, nonlinear effect and saturation distortion.In this paper, the amplification and transmission characteristics of all fiber ytterbium-doped fiber amplifiers, used in the ICF system, were investigated theoretically and experimentally. The theory of the ytterbium doped fiber amplifier was studied,specially at the distribution of the pump laser, the pump threshold value, the gain of the fiber amplifiers, the storage energy of the Yb³⁺-doped fiber etc. The ASE noise power characteristic and the saturation amplification characteristic of ytterbium fiber amplifier were numerical simulated, based on the practically used parameters, the results show how to reduce the ASE noise and avoid the saturation distortion by selecting the gain fiber and designing the gain of the fiber amplifier respectively. The nonlinear transmission characteristics of the fiber amplifier were theoretically investigated, such as SBS, SRS, SPM, FWM etc. The threshold values of SBS, SRS,SPM and FWM were numerical calculated, the results direct us in the foundation of fiber amplifiers and the ICF front-end system. At the end part of the paper, the amplifiers used in the front-end, such as the pass-through fiber amplifier, the narrow-band double pass fiber amplifier, the wide-band fiber amplifier, the double-clad fiber amplifier etc,were experimented combining with the requirements of the front-end system. The relations between the gain and the signal to noise ratio against the pump power and the gain against signal power were studied, the results verified the simulation. The experiment results were analyzed, and showed the way to improve.