Design and analysis of double-layer trapezoidal groove of polarization-independent beam-combination gratings with high diffraction efficiency
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摘要:
为了满足强激光系统中合束光栅的宽带、高衍射效率及偏振无关的需求，本文提出了一种双层梯形结构的偏振无关合束光栅。首先，基于严格耦合波理论，建立了一种以粒子群优化算法为核心的偏振无关合束光栅设计模型，通过随机生成特征波长实现效率特性寻优。然后，详细分析了单层梯形和双层梯形结构光栅的槽深、占宽比、侧壁倾角等结构参数对光栅衍射效率及带宽的影响。最后，对两种结构光栅的电场增强特性进行分析讨论。结果表明，双层梯形结构偏振无关合束光栅在51 nm（
1038 nm−1089 nm）带宽范围内实现99%以上的理论衍射效率，相比传统单层梯形结构具有更大的工艺容差，容差范围内均满足30 nm带宽和98%的高衍射效率，同时具有更低的光栅近场增强，可以拥有更强的抗激光损伤能力。本文提出的宽带高衍射效率双层梯形结构光栅可以提高激光系统的输出功率，在激光合束领域具有重大的应用价值。Abstract:In order to meet the needs of broad band, high diffraction efficiency and polarization independent, a double-layer trapezoidal polarization independent beam grating is proposed in this paper. Firstly, based on the strict coupled wave theory, a design model of polarimetric independent combined beam grating based on particle swarm optimization algorithm is established, and the efficiency characteristics are optimized by randomly generating characteristic wavelengths. Then, the effects of slot depth, width ratio, side Angle and other structural parameters on the diffraction efficiency and bandwidth of single-layer and double-layer trapezoidal grating are analyzed in detail. Finally, the electric field enhancement characteristics of the two structures are analyzed and discussed. The results show that the double-layer trapezoidal polarimetric beam independent grating achieves a theoretical diffraction efficiency of more than 99% in the bandwidth range of 51 nm (
1038 nm−1089 nm), and has a larger process tolerance than the traditional single-layer trapezoidal structure, which meets the bandwidth of 30 nm and the high diffraction efficiency of 98% in the tolerance range, and has lower near-field enhancement of the grating. Can have a stronger resistance to laser damage. The proposed double-layer trapezoidal grating with wide band and high diffraction efficiency can improve the output power of laser system, and has great application value in the field of laser beam combination.-
Key words:
- Diffraction grating /
- Polarization independent /
- Process tolerance /
- Electric field
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图 1 实时衍射效率和评价函数值以及寻优结构参数示意图。(a)寻优时随机波长点的TE、TM以及平均衍射效率；（b）寻优迭代时评价函数值的变化情况；(c) 寻优参数的粒子实时位置
Figure 1. Real time diffraction efficiency and evaluation function values and optimization structure parameters. (a) TE, TM and average diffraction efficiency of random wavelength points during optimization; (b) The change of evaluation function value during optimization iteration; (c) Optimize the particle real-time position of the parameter
图 2 单层梯形结构偏振无关合束光栅
Figure 2. Single-layer trapezoidal structure polarization-independent combined beam grating
图 3 单层梯形光栅在中心波长反射-1级自准直入射时的理论衍射效率
Figure 3. Theoretical diffraction efficiency of a single-layer trapezoidal grating at the center wavelength reflected -1 order autocollimation incident
图 4 槽深（Depth）及占宽比（Duty cycle）变化对TE和TM平均衍射效率的影响
Figure 4. Effect of slot Depth and Duty cycle on the average diffraction efficiency of TE and TM
图 5 双层梯形结构偏振无关合束光栅
Figure 5. Double trapezoidal structure polarization-independent combined beam grating
图 6 双层梯形光栅在中心波长反射-1级自准直入射时的理论衍射效率
Figure 6. Theoretical diffraction efficiency of double-layer trapezoidal grating at center wavelength reflectation-1 self-collimation incident
图 7 槽深（Depth）及占宽比（Duty cycle）变化对TE和TM平均衍射效率的影响
Figure 7. Effect of groove Depth and Duty cycle on the average diffraction efficiency of TE and TM
图 8 侧壁倾角变化对TE和TM平均衍射效率的影响。（a）侧壁倾角容差，（b）76°刻蚀深度和占宽比容差，（c）78°刻蚀深度和占宽比容差，（d）82°刻蚀深度和占宽比容差
Figure 8. Effect of sidewall Angle variation on the average diffraction efficiency of TE and TM. (a) sidewall dip tolerance, (b) 76° etch depth and specific width tolerance, (c) 78° etch depth and specific width tolerance, (d) 82° etch depth and specific width tolerance
图 10 单层梯形结构Ey分量的振幅值。(a)TM；(b)TE
Figure 10. Amplitude value of Ey component of single-layer trapezoidal structure.(a)TM；(b)TE
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