Optimization Research On Output Characteristic For High-power Fiber Laser

As a new generation of solid state laser, high power fiber lasers have been widely used in optical communications, medicine, industry and martial because of its low threshold, high efficiency, good beam quality and compactness. At present, the output power of continuous wave high-power fiber laser has been increased to the ten thousand watts level, and that of pulse fiber laser has reached the megawatt level. However, with the rising output power, how to optimize the overall fiber laser system is an important issue for laser system design. In this thesis, based on the previous works, we have systematically studied the optimization problems of high power continuous wave and mode-locked pulse fiber lasers, and have provided design criterion. The innovative achievements are summarized as follows:Firstly, an improved shooting algorithm is proposed for continuous wave double-clad fiber lasers. The propagation model of continuous wave double-clad fiber laser, which only knows the pump power and the relation of the laser powers at the two ends, belongs to binary differential equation, so the shooting method cannot be directly used. Based on the relation of the laser power values at the two ends, the original equations can be transferred into boundary value problem of the known two-end values, and can be resolved by the shooting algorithm. The improved shooting algorithm can be fairly simple, quick and efficient.Secondly, a shooting algorithm based on the new linear iteration process is proposed for solving the propagation equation of continuous wave double-clad fiber laser. In general, the initial laser power is proportional to the terminal laser power for continuous double-clad fiber laser. And when the guessed initial laser power does not satisfy the boundary condition, the next guessed initial laser power can be directly corrected based on the relation between the initial and terminal laser power until the conditions are satisfied. The new iteration process of the shooting algorithm is quite simple. Under the same precision, its operational speed can improved2to4times compared with the classical or improved shooting method.Thirdly, a new criterion of the fiber optimum length for continuous fiber laser is proposed, and the effects of cavity parameters, such as laser scattering loss and ion concentration on the fiber optimum length and output characteristics is analyzed, which provides the theoretical basis for improving the performace of high-power fiber laser. In order to improve the output performance, the system should be work under the optimum fiber length. In the previous work, the optimum fiber length is analyzed without regard to laser scattering loss and the pump reflector in the output-end. In our work, a more extended model for the optimum fiber length based on its physical concept is built up, which is applicable with laser scattering loss and pump reflector.Finally, the effects of the cavity parameters on the soliton pulse characteristics in negative dispersion region have been analyzed theoretically and experimentally which provides the basis for improving the output power and energy of the pulse fiber laser based on nonlinear polarization rotation passively mode-locked fiber laser. The experiment results show that in the long-cavity negative dispersion mode-locked fiber laser, increasing the pump power not only can change the pulse output state, such as two-pulse and three-pulse, but also can got the fundamental soliton of wave-breaking-free in which the pulse energy and peak power are the maximum and are increased with the increasing pump power under the certain state of polarization controller. Increasing cavity length of the fiber laser, the pulse can be got with the lower fundamental repetition rate, the larger pulse width, the increasing pulse average power and pulse energy. With increasing the overall group velocity dispersion, the pulse width is decreased, and pulse average power, energy and peak power can be greatly improved. Another, the changes in polarization state and cavity length causes the shift of its central wavelength.