Theoretical And Experimental Study On The Key Techniques Of Main Amplifier In High Power Master-Oscillator Power-Amplifier Fiber Laser Source

Master-Oscillator Power-Amplifiers (MOPA) have advantages over typical high power fiber laser with linear cavity in conversion efficiency, output beam quality, availability of high power and compactness; besides, they can further improve the output features of fiber laser systems. Thus, it enjoys a good application and academic value. Recent researches on MOPA focus on the control of the nonlinear optical effects in fiber core of power amplifier to scale the output power and the design of large mode area active fiber which can suppress high-order modes. Therefore, in this thesis, theoretical and experimental study on the key techniques of the power-amplifier-stage in MOPA system is presented in an attempt to construct a better MOPA fiber laser system with high power and high beam quality. The main work included as follows:1. Based on the classic theoretical model and stable-state equations of fiber amplifier, the theoretical model of large-mode-area clad-pumped fiber amplifier is built up. The power equations of multimode signal and pump lights are presented by analyzing the transmission; power flowing, amplification and competition of signal transverse modes. The overlap factors, intensity distribution functions and coupling coefficients of modes are introduced, then the launching condition of free beam on the fiber facet and beam quality of the multimode fiber are studied.2. On the basis of the model mentioned above, the output features of multimode large-mode-area fiber amplifier are simulated by computing the gain, loss and coupling effect of transverse modes. Moreover, fiber length, pump configuration, pump power and coupling, which may influence the output features of amplifier, are considered. It is indicated that in large-mode-area fiber amplifier mode competition could result in degeneracy of the output beam quality, which may otherwise be improved by reducing coupling coefficients, shortenning fiber length and optimizing launching condition. Therefore, high power output which is produced in main-amplifier-stage of MOPA system can be obtained by using large core multimode double-clad active fiber, in which the high-order modes are suppressed by using mode control technology.3. The large-mode-area active fibers in different design are studied respectively to find out the most suitable fiber for the high power MOPA fiber laser system. An analytical method of effective mode area of fundamental mode and high-order mode suppressing for large-mode-area active fibers with arbitrary refractive-index profile and dopant distributions is presented. Moreover, the mode bend distortion induced by fiber coiling, which could influence the performace of active fiber, is numerical simulated. It is shown that the fiber with bybrid profiles, which demonstrate better bending resistance and larger mode area than other fibers, is comparatively applicable in the design of active fiber with core diameter of tens of microns, for the confined dopant profile can improve the output beam quality. Based on the research results, the optimizing design of large-mode-area active fiber with effective mode area 1000μm is presented.4. Key parameters of the amplifier system which may influence the output features are analyzed by numerically simulating the large-mode-area fiber amplifier. Design method and key techniques of yetterbium-doped double-clad fiber amplifier are introduced in detail. The reflection of fiber ends is estimated by discussing the techniques of fiber end polishing and lapping so as to explore a way to suppress the amplified spontaneous emission and restrain the self-oscillations. With the help of the finding above, an optimized high power fiber amplifier system is constructed.5. Based on the high power double-clad fiber amplifier system, a series of experiments are conducted to test the single-frequaency, continuous-wave and ns-regime pulse of Yb3+-doped fiber amplifiers. During the experiments, 2.3W single-frequaency output laser with 1083nm is produced, and average power 832mW and peak power 120W are obtained when seeded by a duration<1ns, 73mW-average power and 7kHz repetition rate pulse. It is worth mentioning that no nonlinear effect is observed throughout the experiments. Moreover, additional experiments are carried out to study the output characteristics of multimode fiber amplifier with coiling, thereby constructing a system to examine the beam quality. By using the system to measure the beam quality factors of amplifiers with different coiling radius, it is indicated that the beam quality can be improved by bending the active fiber, which is consistent with the calculation.