Research On The Beam Shaping Of Multi-mode Fiber Laser

In the research of inertial confinement fusion, in order to achieve real targeting demands, we need to achieve high uniformity of the actual irradiation of the target surface. Because the laser beam output by a variety of high-power laser devices can not directly meet the requirements. Therefore, in order to make laser distribution uniform on the target surface, we need to study the smoothing of the light field output by a variety of high-power laser devices. However, a key factor of the laser fusion power generation is the efficiency of the laser driver. High efficiency drive will greatly reduce the required pump energy of drives, which contributes to make the system more compact and the costs lower. Waveguide lasers, such as Yb fiber laser of diode-pumped, their laser conversion efficiency is close to70%, and can be operated with high repetition frequency, which will provide potential opportunities in the development of laser fusion power generation. Based on the considerations above, G.A.Mourou, based on fiber amplification network, has proposed laser driver to realize the idea of inertial confinement fusion in2008in use of direct drive and fast ignition to target. Direct drive demands high irradiation of the target surface of fiber laser, which means an optimization design of a large number of units and the distribution characteristics of the fiber laser beam, so as to achieve the requirement of uniformity of the irradiation of the target surface. Therefore, this paper focuses on the analysis of smoothing properties of irradiation of the multimode fiber laser target surface. We analyzed the effects, by means of lens array smoothing, on the light intensity of multimode fiber laser beam. And we researched various factors that lead to the change of smoothing characteristics of multimode fiber laser beam.The main research works and conclusions are as follows:1. Experimental study on the smoothing properties of target surface of the fiber laser. We used a diffracted lens array system for the smoothing experiment of light field output by multimode fiber laser, and adopted different array of lenses and Fourier lens as the smoothing components to demonstrate that the sub-cell size parameter of the array lens has seriously affected its smoothing, and its smoothing would not have been improved with decreasing size. Meanwhile, we can explore other variable parameters in the experiments so as to draw a series of comparative experiments, and discusses the factors and control methods that influence the smoothing properties of target surface of multimode fiber laser.2. By means of computer simulation and experimental data, we took a deep analysis of actual effects of different parameters on the smoothing of final target surface. Then we found that in the experiments, the detailed effect law, to its smoothing, of the focal distance from the target surface, the intensity of fiber laser, lens array parameters, laser beam experiments and other experimental parameters, and the trend of reduction of target surface unevenness. Based on fiber amplification network, the results have been contrasted with the theoretical results, in order to study the smoothing of the laser-drive in inertial confinement fusion in the future.3. The use of appropriate lens array parameters in the corresponding experiment, can lead to obtain the minimum focused spot at the target surface with the percentage of less than5%of a root mean square on the top of the light field on the target surface. The top of the light field is steep, and the low-frequency modulation is relatively less, which demonstrated that it is feasible to use multimode fiber laser beam for its smoothing, so as to achieve the irradiation of target surface of laser inertial confinement fusion.Highlights of the dissertation are as follows:1. We studied the smoothing methods of multimode fiber laser beam on basis of the fiber amplification network. In use of array of optical fiber source and diffractive lens array, we obtained a relatively better smoothing in the experiments.2. The experiments verified the detailed effects of array of number of optical fiber source and focal distance from the target surface on the smoothing of the target surface, demonstrated the feasibility of the design, and pointed out that increasing the number of fiber laser beam can effectively improve the smoothing.