| In recent years, the master-oscillator power-amplifier (MOPA) scheme becomes themain choice of high energy, high peak power pulsed laser systems due to its manyadvantages, such as high slope efficiency and easily manageable output pulsecharacteristics via the control of the seed source.In this thesis, high power MOPAs based on ytterbium-doped fibers are studiedboth theoretically and experimentally with an emphasis focused on low repetition,high power pulsed MOPA systems. The main work and findings are summarizedbellow:1. Ultra high power pumped continuous-wave and pulsed ytterbium-doped fiberamplifiers (YDFAs) under a pump power in the range100~1000W are systematicallystudied via numerical simulation for the first time, to the best of my knowledge. Theinfluences of the peak power, pulse width, and repetition rate of the signal pulse onthe performance of the amplifier under different pump-power levels are studied. Theresults show that the accumulation speed of the amplified spontaneous emission (ASE)power between two adjacent pulses increases drastically with the increasing of thepump power. When the repetition rate of the signal decreased to several tens of kHz,the ASE power surpasses the signal’s average power, which usually can not beobserved in conventional high power pumped YDFAs(<100W).2. The evolution of the ASE power distribution in the gain fiber along time isanalyzed for the first time. The feature that the peak of the forward ASE power movesto the fiber’s output end with the increasing of the time is noticed, and is used toexplain why the forward ASE power accumulates more slowly than the backwardASE power. A concept named “ASE power building time” is proposed.3. For pulse-pumped amplifiers, the influence of the width of the pump pulse onthe performance of the amplifier is studied for the first time, to my knowledge.Simulation results reveal that the amplification of the signal-pulse does not alwaysincrease with the width of the pump-pulse, and there exists an optimum pump-pulsewidth for a given signal-pulse width, at which the gain for the signal is the highest.The optimum pump-pulse widths under different pump power are analyzed, and thepump energy consumption, ASE power, output signal peak power under different pump-pulse widths are investigated.4. For the first time, the influence of the timing between the pump-pulse andsignal-pulse on the performance of the amplifier is analyzed. It has been found thatthe signal-pulse gets the maximum gain when its peak is placed to the end ofpump-pulse, and the gain will decrease dramatically if the signal pulse is delayed alittle bit. Farther research reveals that the higher the pump power, the more thedecrease of the gain for the signal-pulse with the same time delay from the optimumtiming.5. An Yb-doped fiber laser, the master-oscillator, and an Yb-doped pulselypumped fiber amplifier, the power-amplifier, are set up and experimentally studied.By optimizing the width of the pump-pulse, the ASE power is successfully reduced to1/8of that of the amplifier under continuous-wave pumping, while the peak power ofthe amplified signal-pulse is improved a little bit. The experimental results show thatthe pulsed-pump method is superior to the conventional continuous-pump method insuppressing the inter-pulse ASE power, which agrees well with our theoretialprediction. |
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