Theoretical and optimized design of low-repetition-rate high-energy pulse amplification in multi-stage MOPA systems

Based on the modeling of a fiber amplification system, a closed form rate and power evolution equation for low-repetition-rate pulse amplification in a multi-stage fiber master-oscillator-power-amplifier (MOPA) is constructed. In order to ensure a high-energy conversing efficiency at the lowest cost, a novel multi-stage optimization method is proposed for the optimization of the multi-stage MOPA system. Differently from the individual optimization method for single fiber amplifiers, every stage amplifier is considered as a part of the whole system and optimized by analyzing the final output pulse energy. As an example, the system parameters of a two stage ytterbium-doped fiber MOPA system including the fiber length, pump power, and peak power of the seed source are optimized by using the proposed method. Under the condition of obtaining almost the same output pulse energy, the fiber length and pump power of the first stage optimized by the new method are, respectively, shorter and lower, 1.6 m and 57.3 W, than those optimized by the individual optimization method. Meanwhile, the optimization results also show that increasing the input signal energy is not an effective way to increase the output energy. The proposed theoretical optimization method can provide an important guide for the design and optimization of the low-repetition-rate pulse amplification in a multi-stage MOPA system