Laser Beam Shaping System Based On Adaptive Optics

Because of its good properties, the laser has been widely used since its invention.However, by the fast development of Science and technology, the Gaussian laser beam has been unable to meet the requirements of beam shape and energy distribution in practical applications. This requires the laser beam shaping technique to obtain the laser beam which satisfies the application requirement. Compared to other techniques,the laser beam shaping technique based on adaptive optics has the best potential and future. Because of modulating the complex amplitude of incident light by controlling the deformable mirror surface, this technique has the advantages of flexible control,good adaptability, diversification of surface, high light transmission efficiency, high damage threshold. But, the actual research lacks not only laser beam shaping system analysis, but also the effect of deformable mirror on system performance. In order to deeply understand and give full play to the performance of the laser beam shaping system based on adaptive optics, a series of in-depth and detailed analysis is carried out in this paper.Firstly, in order to make quantitative analysis on the performance of laser beam shaping system, the metric function system of system performance, based on near field modulation phase and far field shaping intensity, is established, including the PV and RMS of modulation phase, the correlation coefficient and mean square deviation of plastic intensity.Secondly, near field phase modulation is an important research object of the laser beam shaping system. For square or circular uniform target intensity, due to its mathematical properties, their approximate solution of near field modulation phase could be obtained by integral asymptotic approximation. Owing to the limit of integral asymptotic approximation, a stochastic parallel gradient descent algorithm based on Zernike mode is proposed to obtain near field modulation phase.Thirdly, the target intensity size, which is an important parameter of the laser beam shaping system, not only has a direct impact on the near field modulation phase,but also affects the far field plastic intensity. By using the mathematical method such as Lagrange multiplier, the influence to the near field modulation phase and far field shaping intensity is analyzed.Fourthly, pupil truncation directly influences the incident field and the near field modulation phase, and indirectly affects the far-field plastic intensity. The rules about the effect of pupil radius on the system performance are studied at first. Then, based on the rules, the mathematical models of far field shaping intensity metric functions,about pupil radius and target intensity size parameters are proposed. Based on the mathematical models, further research, about how to select optimal pupil radius when designing laser beam shaping system is carried out.Fifthly, the deformable mirror is the key phase modulation element in laser beam shaping system based on adaptive optics. The surface of deformable mirror is determined by the influence function and drive control voltage. The influence of the parameters in the mathematical model of influence function, including drive space,coupling coefficient and Gauss index, on the performance of laser beam shaping system, is studied. And the regularity provides evidence to deformable mirror parameters selection.Sixthly, the purpose of laser beam shaping system is to obtain the far field intensity which is approximate to the target intensity as far as possible. The least square method is usually used to fit the near field modulation phase by deformable mirror, which could obtain the least square fitting error. The relationship between the least square fitting error and the best system performance is studied. The research indicates that the system performance isn’t the best when the square fitting error is the least. On this basis, a stochastic parallel gradient descent optimization algorithm is proposed to fit the modulation phase. Compared to the least square method, the performance of the laser beam shaping system is significantly improved.Seventhly, the properties of modulation phase are researched to choose a suitable deformable mirror. Then, a sensor-less laser beam shaping system based on Bimorph is set. The stochastic parallel gradient descent control algorithm is adapted to achieve the goal of shaping square uniform intensity.Eighthly, in order to make up the disadvantages of laser beam shaping system based on Bimorph, a new system, based on the bimorph mirror, 3mm spacing PZT deformation of the deformable mirror and Shack-Hartmann wave-front sensor, is built.The direct slope method is adapted to achieve far-field square and circular uniform target intensity shaping by close loop. To validate the influence of fitting method, the stochastic parallel gradient descent control algorithm is used to optimize the voltages with the voltages of the direct slope method as initial value. Compared to the direct slope method, the capability of shaping system is improved.