Simulation analysis of isolation between laser communication ground test equipments
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摘要: 受空间所限，激光通信地面测试平台与被测终端之间的距离远小于实际通信距离，导致测试平台光机等器件产生的后向散射杂光进入被测终端，从而严重影响被测终端的测试性能。从被测终端与测试平台间的光学干扰问题出发，本文研究了被测终端与测试平台间隔离度的关系，分别设计了卡塞格林和离轴三反光学天线，并根据杂散光传输模型，采用杂散光分析软件分析了光学天线结构形式及表面粗糙度两方面对隔离度的影响。分析结果表明，采用离轴三反光学天线时的隔离度明显高于卡塞格林光学天线，且隔离度随着光学表面粗糙度的减小而增大，当光学表面的粗糙度达到0.892 nm时，隔离度可达−86.22 dB。最后，推导了ABg模型与Harvey模型参数间的关系，并根据粗糙度与TIS计算公式，得出粗糙度分别为0.7 nm及0.5 nm的ABg模型参数，它们的终端间隔离度分别为−94.39 dB和−97.3 dB，实现了−90 dB的隔离度指标。Abstract: The distance between a laser communication ground test platform and the terminal under test is far less than the actual communication distance due to space limitations. As a result, the backscattered stray light generated by the test platform optical device will enter the terminal under test, and the signal will seriously affect the performance of the terminal under test. Aiming at this problem, we research the isolation relationship between the tested terminal and the test platform based on the optical interference problem. The Cassegrain and off-axis three-mirror optical antenna are designed respectively. According to astigmatic transmission model, the stray light analysis software is employed to analyze the influence of optical antenna's structure and surface roughness on the isolation. The results of the analysis show that the isolation when applying the off-axis three-mirror optical antenna is significantly higher than that applying the Cassegrain optical antenna, and that this isolation increases with a decrease in the roughness of the optical surface. When the optical surface's roughness reaches 0.892 nm, the isolation is −86.22 dB. Finally, the relationship between the ABg model and the Harvey model parameters is derived. According to calculation formula of the roughness and TIS, the ABg model parameters with roughness of 0.7 nm and 0.5 nm are theoretically obtained. The isolation between the terminals is −94.39 dB and −97.3 dB, achieving an isolation rating of −90 dB.
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Key words:
- laser communication /
- optical antenna /
- isolation /
- surface scattering model /
- stray light
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图 4 离轴三反光学天线二维布局图
Figure 4. Two-dimensional layout of off-axis three-mirror optical antenna
图 7 离轴三反光学天线的杂散光模拟
Figure 7. Stray light simulation of off-axis three-mirror optical antenna
图 9 不同粗糙度镀膜基片的散射数据拟合曲线
Figure 9. Scattering fitting curves of coated substrate with different roughnesses
图 10 采用不同粗糙度反射镜时探测器的光通量图
Figure 10. Detector′s luminous flux maps when applying reflectors with different roughnesses
图 11 低粗糙度下探测器的光通量图
Figure 11. Luminous flux maps of detector when applying low roughness reflector
表 1 光学天线的设计指标
Table 1. Design specifications of optical antenna
指标名称 参数值 测试波长/nm 632.8 望远镜放大倍率 15±0.1 望远镜主口径/mm ≥610 有效视场/mrad ≥±6 像质要求 中心视场：0.033λ（rms）、0.25λ（PV） 边缘视场：0.050λ（rms）、0.33λ（PV） 
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表 2 卡塞格林光学天线波像差
Table 2. Wave aberration of Cassegrain optical antenna
波段（μm） 项目 视场 中心视场 边缘视场 0.632 8($\lambda $) 波像差 P-V 0.033 8$\lambda $ 0.143 7$\lambda $ RMS 0.006 5$\lambda $ 0.032 4$\lambda $ 斯特尔比 0.999 8 0.999 7
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表 3 离轴三反光学天线波像差
Table 3. Wave aberration of off-axis three-mirror optical antenna
波段（μm） 项目 视场 中心视场 边缘视场 0.632 8($\lambda $) 波像差 P-V 0.037$\lambda $ 0.140$\lambda $ RMS 0.009$\lambda $ 0.024$\lambda $ 斯特尔比 0.998 0.987
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表 4 两种光学天线的隔离度对比
Table 4. Comparison of isolation between two optical antennas
光学天线类型 进入探测器的杂光数目 光通量大小(W) 隔离度(dB) 卡塞格林 7 214 0.011 24 −49.49 离轴三反 20 2.94 × 10−5 −75.32
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表 5 具有不同粗糙度镀膜基片的散射数据的ABg模型拟合参数
Table 5. ABg model fitting parameters of scattering data of coated substrate with different roughnesses
粗糙度(nm) A B g RMSE 0.892 2.15$\times10^{-8} $ 7.51$\times10^{-8} $ 4.624 0.008 798 1.297 2.47$\times10^{-8} $ 7.93$\times10^{-8} $ 4.389 0.001 32 1.646 1.34$\times10^{-8} $ 5.37$\times10^{-8} $ 4.986 0.007 629 2.327 1.17$\times10^{-7} $ 4.92$\times10^{-7} $ 4.290 0.005 674 6.479 1.01$\times10^{-7} $ 3.81$\times10^{-7} $ 4.503 0.006 217
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表 6 采用不同粗糙度反射镜对应的隔离度
Table 6. Isolations when applying reflectors with different roughnesses
粗糙度 进入探测器的杂光数目 杂光光通量大小（W） 隔离度(dB) 0.892 2 2.39$\times10^{-6} $ −86.22 1.297 4 3.19$\times10^{-6} $ −84.96 1.646 7 5.11$\times10^{-6} $ −82.92 2.327 11 1.22$\times10^{-5} $ −79.14 6.479 20 2.94$\times10^{-5} $ −75.32
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表 7 采用低粗糙度反射镜时对应的隔离度
Table 7. Isolations when applying low roughness reflector
粗糙度
(nm)进入探测器的
杂光数目杂光光通量
大小（W）隔离度
(dB)0.5 1 1.86$\times10^{-7} $ −97.30 0.7 1 3.64$\times10^{-7} $ −94.39
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