| Master- Oscillator Power Amplifier fiber laser system has application value for its high power, good beam quality, high reliability and flexible light road design. In recent years, the research work mainly focused on improving the peak power and beam quality of MOPA system, and the amplified spontaneous emissions(ASE) nonlinear effect and multimode beam coupling are main restricted factors of improving the peak power and beam quality. In this paper, MW peak power level MOPA fiber laser system was studied in detail, and aim to explore the methods to suppress ASE, nonlinear effects, multimode beam coupling and improve the peak power and beam quality of the system:1. The transient rate equation is built based on the energy band, and the ASE effect, nonlinear effect, mode coupling and pulse amplification distortion are studied in details, and the relationship between ASE and signal power and the influence of pulse width and pump power onsignal distortion. Then the principles several kinds of nonlinear effects were analyzed to explain the mutual restriction between stimulated Raman scattering and four waveguide mixing and the influence of pulse width to the stimulated Brillouin scattering threshold. Then the gain fiber model is built to simulate the model distribution in different fiber parameters and provide the theories of large mode area and less mode gain fiber.2. Mainly focus on the pulse semiconductor laser seed source of the MOPA fiber laser system. The influence of relaxation oscillation and self-pulsing to the physical characteristics and light pulse were analyzed. Inhibit relaxation oscillation and generate stable relaxation oscillation can be achieve through drive current modulation, and the stable ps optical pulse can be obtained by experiment, and on the basis of the narrow pulse seed light power was developed, the minimum pulse width of 835 ps and repetition frequency of 25 kHz to 200 kHz stable continuous tunable optical pulse output was obtained.3. The studies focus on the restriction of ASE, nonlinear multimode coupling and self-oscillation effects to the peak power and beam quality.Through the optimization design on signal power, the gain fiber, the pump power and facet technology, we explore the methods to suppress the ASE, nonlinear, and self-oscillation. According to the calculationresults: the signal power, gain fiber length and pump methods are related to ASE light power and wavelength.When we amplify the weak signal like μW level, the forward pump and cascade amplifier structure can be used to improve the magnification times, and the compromising consider the ASE and slope efficiency to determine the gain fiber length; the pulse amplitude of signal, gain fiber length and pump methods have an important influence on nonlinear effects, the maximum peak power can be obtained when the gain fiber reach gain saturation, and combine the backward pump, the nonlinear threshold can be improved. Through strip the cladding power and coreless endcap, the power density can be decreased and the reflection can be decreased by angle of facet andcoat the anti-reflective coating, these technologies can suppress the self-oscillation and thermal effect.4. The mode field distribution and mode gain were studied through theoretical research and numerical calculation, explore the methods how increase the mode area and beam quality. The modes distribution and gain in the six refractive index and doped ions distribution simulated; the effect of fiber bending on mode distribution and modes loss also studies. Based on the conclusions, we put forward the gaussian hybrid fiber, which has large mode area and resist to bending distortion of modes distribution, the mode area can be achieved as 1.17 x 103 um2, and the fundamental mode accounts for 94.67% of the total power.5. All fiber and space coupling structures MW peak power fiber laser system are built. In all fiber structure, the gaussian hybrid gain fiber was used in the main fiber amplification, the optical pulse width and peak power were 1.2 ns and 1.56 MW respectively at the condition of 25 kHz, the output characteristics of multimode fiber amplifiers with different coiling diameter and got nearly signal mode output beam, = 1.52, = 1.56. In the space coupling system, the gaussian hybrid gain fiber was replace byrod-like photonic crystal fiber, the pulse width and peak power were 1.12 ns and 1.12 MW respectivelyat the condition of 25 kHz, = 1.23, = 1.27. Finally, the experiment carried out to study acousto-optic modulation optical pulse cutting, it can expand the repetition rate of the system and made the repetition rate from 1 kHz to 200 kHz and continuously adjustable.
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