Chen Shanqiu

Recent progress of adaptive beam cleanup of solid-state slab lasers in Institute of Optics and Electronics, Chinese Academy of Sciences

The beam qualities of high power solid-state slab lasers are severely limited by many factors such as thermal effects of the gain medium. Simultaneously achieving high beam quality and high average power remains a fundamental problem in the development of high power lasers. Adaptive optics systems are able to significantly improve beam qualities by compensating for the static and dynamic phase distortions of the laser beams. In recent years, Institute of Optics and Electronics, Chinese Academy of Sciences has developed low-order aberration compensators, weighted least-square wavefront reconstruction algorithms, and generic real-time wavefront processors for solid-state slab lasers. Based on these key components, over two dozens of adaptive optics systems are delivered to a variety of solid-state slab laser systems in China for beam cleanup. With effective operations of these adaptive optics systems, the beam qualities of the laser systems have all been well improved.高功率固体板条激光器的光束质量严重受限于增益介质热效应等多种因素。如何同时获得高平均功率和高光束质量是激光发展过程中面临的一个基本物理问题。自适应光学技术能够有效补偿固体板条激光系统输出光束的静态和动态像差，是改善光束质量的有效手段。近年来中国科学院光电技术研究所掌握了低阶像差补偿器、加权优化波前复原方法、通用波前处理机等关键技术，为国内多个固体板条激光系统研制了二十余套自适应光学光束净化系统，显著改善了光束质量，保障了上述激光系统的有效运用。

Active beam cleanup of pulsed slab laser based on 59-unit adaptive optics system

Beam quality is one of the key technical targets in the high-power laser system, and it can be dramatically improved by using adaptive optics. In this paper, a research on the near field distribution of the pulsed slab laser has been carried out. A Shack Hartmann wavefront sensor was used to measure the aberrations, and the wavefront reconstruction was accomplished afterwards. Because of the special near field distribution, the diffraction limited factor β has been revised for better describing beam quality. The theory model of the correction of aberrations with deformable mirror (DM) was introduced. Based on the theory model, the numerical analysis of the correction ability of DM with 59 actuators and correction of measured wavefront have been carried out, the result showed that the peak value of far field was improved 3.2 times, and the beam quality was increased 3 times. An experimental setup for beam cleanup of pulsed laser has been established. A demonstrated experiment was carried out on this platform. The result of the experiment indicated that the output beam quality of the pulsed slab laser can be validly improved by beam cleanup using adaptive optics system with 59-actuator DM. Specifically, the peak value of far field has been raised by 3 times, and the factor β has been increased by 3 times as well, which was mostly in accordance with the numerical analysis. The conclusion of this paper offered reference for the achievement of near diffraction limited beam quality in the future.

Synchronous model-based approach for wavefront sensorless adaptive optics system

In this paper, an aberration correction algorithm for wavefront sensorless adaptive optics system is presented, which is based on the approximately linear relation between the mean square of the aberration gradients and the second moment of far-field intensity distribution. To demonstrate algorithms performance, an experiment system of aberration corrections for Fresnel zone plates imaging is set up. The correction results show the excellent performance of this method in correction speed, and correction capability. Comparing with conventional stochastic parallel gradient descent (SPGD) algorithm, the correction speed with this method is improved about four times for the similar correction effect.

Comparison between iterative wavefront control algorithm and direct gradient wavefront control algorithm for adaptive optics system

Among all kinds of wavefront control algorithms in adaptive optics systems, the direct gradient wavefront control algorithm is the most widespread and common method. This control algorithm obtains the actuator voltages directly from wavefront slopes through pre-measuring the relational matrix between deformable mirror actuators and Hartmann wavefront sensor with perfect real-time characteristic and stability. However, with increasing the number of sub-apertures in wavefront sensor and deformable mirror actuators of adaptive optics systems, the matrix operation in direct gradient algorithm takes too much time, which becomes a major factor influencing control effect of adaptive optics systems. In this paper we apply an iterative wavefront control algorithm to high-resolution adaptive optics systems, in which the voltages of each actuator are obtained through iteration arithmetic, which gains great advantage in calculation and storage. For AO system with thousands of actuators, the computational complexity estimate is about O(n(2)) similar to O(n(3)) in direct gradient wavefront control algorithm, while the computational complexity estimate in iterative wavefront control algorithm is about O(n) similar to (O(n)(3/2)), in which n is the number of actuators of AO system. And the more the numbers of sub-apertures and deformable mirror actuators, the more significant advantage the iterative wavefront control algorithm exhibits.