Adaptive optics and beam array optimization in high energy and high beam quality laser
Correction results of the laser beams with a 3×1 and a 1×1 array, respectively. (a)-(d) Near-field beam intensity distribution. (e)-(h) Far-field beam intensity distribution.
High-energy diode-pumped solid-state lasers (DPSSL) with high beam quality are important sources for a wide range of applications such as laser–plasma interactions, hard X-ray generation, and inertial fusion energy (IFE). In order to achieve high pump power, a large-size laser diode (LD) stack is necessary for the DPSSL, since the emitting power density of the LD is limited. However, a large-size LD stack is difficult to be manufactured into an integrated stack but it has to be divided into a LD array, since the temperature distribution is inhomogeneous across the integrated stack, which would broaden the emission spectrum and lower the output energy. When an arrayed pump beam is used, the near-field laser beam distributes in the same array and shows bad beam quality. Therefore, it is needed to improve the beam quality of an arrayed near-field laser beam.
Deformable mirrors (DMs), which are the key devices of the adaptive optics, have been effective in improving the beam quality of high-power lasers and high-energy lasers, could be used to correct the wavefront distortion and improve the beam quality. Note that the incident beams on the DMs are integrated beams in the reported literatures, not the arrayed beams. Therefore, it is necessary to investigate the beam quality control of an arrayed beam by a DM and achieve the best beam quality.
The research group led by Lei Huang from Tsinghua University proposed the beam quality control of high-energy laser with arrayed near-field beam by an adaptive optics configuration and beam array optimization. In a 12.2 J 10 Hz Nd:YAG slab amplifier, the near-field beam was in a 3×1 array, with the beam quality of 10.89 times diffraction limited (TDL). By using a 116-actuator DM to correct the wavefront distortion, the beam quality was improved to 5.54 TDL. Further simulation results showed that the correction ability of the DM and the beam quality after correction were closely related to the beam array. An integrated beam could result in diffraction-limited beam quality after correction. In the experiment, by applying a pump-light homogenizer to transform the 3×1 beam array into an integrated beam, the beam quality was greatly improved from 5.54 TDL to 1.57 TDL after being corrected by the 116-acutator DM in a 10.7 J 10 Hz Nd:YAG slab amplifier. To the best of the research group’s knowledge, this is the first investigation on beam quality control of an arrayed near-field beam in the high-energy DPSSL. This work has been published in Chinese Optics Letters, Volume 17, No. 5, 2019 (Licheng Sun et al., 1.57 times diffraction limited high-energy laser based on a Nd:YAG slab amplifier and an adaptive optics system).
"The high beam quality is a goal as well as a challenge for the high-energy DPSSL." says Lei Huang, "The proposed IOL gives a possible way to achieve higher beam quality in higher energy lasers."
Now, the research can achieve a beam quality of 1.57 TDL in a 10 J class DPSSL. The research group expect that the beam quality could exceed 1.50 TDL for a 100 J class DPSSL in the future experiment by further homogenizing the pump intensity and improving the correction ability of the DM.
封面|利用自适应光学和光束阵列优化提高高能激光的光束质量
变形镜对3×1和1×1分布的阵列光束的校正结果。(a)-(d)近场光强分布；(e)-(h)远场光强分布。
高光束质量高能二极管抽运固体激光器（DPSSL）是激光等离子体相互作用、硬X射线产生和惯性聚变能（IFE）等许多应用的重要光源。
由于作为抽运器件的激光二极管（LD）具有有限的发射功率密度，因此在DPSSL中需要使用大尺寸的LD堆栈以获得高抽运功率。然而大尺寸的LD堆栈很难制造成一个集成的堆栈，这是由于集成堆栈内部的温度分布不均匀，导致LD发射光谱展宽和输出激光能量下降，因此人们将其划分为一个LD阵列。当采用阵列光束抽运时，输出的近场激光也呈现为相同的阵列光束，但光束质量较差。因此有必要提高激光阵列光束的光束质量。
变形镜（DM）作为自适应光学中的关键器件可以用来校正波前畸变，对改善高功率/高能激光的光束质量具有重要的作用。但需要注意的是，在目前的文献中，入射到变形镜的激光光束大多是单光束而不是阵列光束。因此，有必要开展利用变形镜来控制阵列光束质量的研究，以获得最佳的输出。
清华大学黄磊研究员带领的课题组提出了采用自适应光学和光束列阵优化的方法对具有阵列近场光束的高能激光器进行光束质量控制。首先，课题组在一个12.2 J、10 Hz Nd:YAG板条放大器中，得到近场光束呈3×1阵列、光束质量为10.89倍衍射极限（TDL）的激光输出。然后利用一台具有116个致动器的变形镜对激光的波前畸变进行校正，使光束质量提高到了5.54倍衍射极限,但还需继续提高。
进一步仿真结果表明，变形镜的校正能力和校正后的光束质量均与光束阵列密切相关。单光束在校正后会得到衍射极限的光束质量。在10.7 J、10 Hz Nd:YAG板条放大器的实验中，采用抽运光匀化器将3×1光束阵列匀化成单光束，再经过具有116个致动器的变形镜校正后，光束质量由匀化前的5.54倍衍射极限提高到了匀化后的1.57倍衍射极限。
据笔者所知，这是首次对高能DPSSL阵列近场光束质量控制的研究。相关成果发表在Chinese Optics Letters2019年第17卷第5期上。
该研究团队的黄磊研究员指出："高光束质量对于高能激光器而言既是一个目标，也是一个挑战，而本研究工作为在更高能量下获得更好的光束质量提供了一种可能的途径。"
目前，课题组在10 J级的激光器中获得了光束质量为1.57倍衍射极限的激光输出。希望在将来的工作中，通过进一步匀化抽运强度和提高变形镜的校正能力，可在100 J级的高能激光器中获得光束质量优于1.50倍衍射极限的激光输出。