面向激光相干合束的亚波长偏振衍射光栅的研究

Coherent laser beam combining has attracted a considerable amount of attention in the development of next-generation laser technologies. Coherent beam combining circumvents physical constraints imposed on traditional laser systems and coherently combines the output from multiple gain elements to obtain a tremendous increase in laser power and maintains a high beam quality. Increasing the splitting efficiency of beam splitters and realizing equipower and arbitrary number of beam splitting is one of the major issues to be addressed in developing coherent beam combining technologies. This project originally proposes a scheme to utilize polarization diffraction gratings for beam splitting with a theoretical efficiency above 99%. The physical realization of such gratings is based on the fabrication of subwavelength structures. The project is meaningful for the advantages of high splitting efficiency, great splitting uniformity, arbitrary splitting number, polarization control of split beams and relatively simple fabrication process. The project delves into the theory, device and system research of subwavelength polarization diffraction gratings and their application in coherent beam combining. It is promising to greatly increase the combining efficiency and output power of coupled laser resonators, eliminates the impact of random phase errors to the system and boosts the rapid development of large-scale and highly efficient coherent beam combining technologies.

激光相干合束技术是下一代激光技术中的研究热点，能规避传统激光器的物理瓶颈，将许多个增益单元的输出相干合成，极大提升各种类型激光器的输出功率并保持较高的光束质量。提高激光分束器件的分束效率并实现等能量和任意数目的分束是激光相干合束技术发展亟待解决的一个问题。本项目创新性的提出基于偏振衍射光栅实现理论分束效率大于99%的分束方案和基于亚波长栅条结构的工艺路线，具有重要的研究价值。该方案具有分束效率高、分束均匀度高、任意分束数目、子光束偏振态可控和制备工艺较简单的优点。本项目从理论、器件和系统三个层面对面向激光相干合束的亚波长偏振衍射光栅进行全面和深入的研究，可望较大提升耦合谐振腔相干合束的效率和输出功率，并消除随机相位波动对合束系统的影响，推动大规模和高效率激光合束技术的发展。

提升激光器系统的输出功率不仅可以增加激光器应用的深度和广度，也可以开拓新的激光应用领域。传统激光器系统随着输出功率的增加会遇到热效应、非线性效应和物理损伤等物理瓶颈以至无法继续提高输出功率。激光相干合束技术不着眼于提升单一激光器的物理极限，而是通过耦合谐振腔或者MOPA结构(Master Oscillator Power Amplifier)将一定数目的激光器单元的输出进行相干合成。由于每个合束单元的运行功率在其物理极限以内，所以不会遇到上述瓶颈问题，而所有合束单元的输出之和则带来了合束系统总输出功率的巨大提升。合束单元光束的高相干性又保证了较高的光束质量。当新型谐振腔结构带来的功率提升逐渐接近理论极限时，激光相干合束技术成为了下一代激光器技术的研究热点之一，并有望将各种类型的激光器功率提升到新的量级。.本项目研究在理论上突破了激光耦合谐振腔的分束/合束器件的效率瓶颈。研究团队从多自由度偏振衍射光栅和几何相衍射光栅的两个不同方向实现了高效率分束/合束器件的理论创新。基于全局优化算法，完成了包含三个自由度的矢量偏振分束衍射光栅的理论设计，相较于传统相位分束光栅，该偏振光栅较大提高分束效率，并可独立控制每一束分束光的偏振态；基于模拟退火算法、严格耦合波理论和几何相原理，完成了一种新型高效率几何相分束衍射光栅的设计，通过二元的亚波长结构实现了多阶相位光栅的分束效率和均匀度。.在器件上完成了高效率高损伤阈值和较低成本的分束光栅的制备和测试。研究团队使用基于洛埃镜的深紫外干涉光刻的工艺，在硅片上制备了亚波长几何相分束衍射光栅，该工艺较EBL和光控取向液晶成本更低且激光损伤阈值更高，可应用于高功率激光合束，实测分束效率高达86%，较达曼光栅提高了15%。.本项目的研究在基于亚波长结构的偏振衍射光栅的高效率分束研究方向取得了创新性的研究成果，对更高效更大规模的激光相干合束技术以及激光并行加工的发展起到推动作用。