Design and analysis of a double-layer trapezoidal groove of polarization-independent beam-combining gratings with high diffraction efficiency
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摘要:
为了满足强激光系统对于合束光栅的宽带、高衍射效率及偏振无关的需求，本文提出了一种具有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~ 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 independence, 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 polarization-independent combined beam grating based on a 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 polarization independent beam combining grating with double-layer trapezoidal groove 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 grating enhancement. It has 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 systems, and has significant 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, evaluation function values and optimization structure parameters. (a) TE, TM, and average diffraction efficiency at random wavelength points during optimization; (b) the change of evaluation function value during optimization iterations; (c) real-time particle position during optimization
图 2 单层梯形结构偏振无关合束光栅
Figure 2. Polarization-independent combined beam grating with single-layer trapezoidal structure
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 槽深及占宽比变化对单层梯形光栅结构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 for single-layer trapezoidal grating
图 5 双层梯形结构偏振无关合束光栅
Figure 5. Polarization-independent combined beam grating with double-layer trapezoidal structure
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 the double-layer trapezoidal grating at center wavelength reflection -1 auto-collimation incident
图 7 槽深及占宽比变化对双层梯形光栅结构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 for double-layer trapezoidal grating
图 8 侧壁倾角变化对光栅全波段TE和TM偏振平均衍射效率的影响以及不同侧壁倾角下光栅的占宽比和槽深容差。(a)不同侧壁倾角的全波段平均衍射效率，(b) 76°，(c) 78°，(c) 82°的刻蚀深度和占宽比容差
Figure 8. The influence of sidewall angle variation on the average diffraction efficiency of TE and TM polarizations across the entire spectral range for gratings, as well as the width-to-period ratio and groove depth tolerances of gratings at different sidewall angles. (a) The average diffraction efficiency across the entire spectral range for different sidewall angles; (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 grating with single-layer trapezoidal structure. (a) TM；(b) TE
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