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激光通信终端光学系统是激光通信终端中承担光束发射、接收任务的核心部件,其主要功能为保证发射星上终端发射光束质量,使对面终端可接收到足够的光功率;保证终端接收系统成像质量,使其可准确判断对面终端位置,并可接收到足够的信号光功率。激光通信终端光学系统设计指标如表1所示。
Parameters Value Wavelength/nm 1 550 FOV1/μrad 500 FOV2/mrad 5 Wave aberration 0 field RMS value/nm ≤44.3 0 field PV value/nm ≤387.5 >0.5 field RMS value/nm ≤44.3 >0.50 field PV value/nm ≤387.5 Isolation/dB ≥70 Table 1. Main technical indexes of optical system of laser communication terminal
根据指标要求选取离轴三反无焦望远镜的初始结构进行优化,主、次镜成一次中间像,目镜将一次像转成平行光,限制光线出射角不小于5°,用Code V软件完成了图2中的光学设计,从调制传递函数曲线可看出,最终系统的成像质量优良,在150线对/mm时全视场MTF值>0.5,满足使用要求。
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视场设置如图3所示,将光学圆视场划分为21个,采用米字形方法,其中1#代表中心视场,2#、3#、4#、5#代表0.3视场;6#、7#、8#、9#、16#、14#、16#、18#、20#为0.5视场;10#、11#、12#、13#、15#、17#、19#、21#为边缘1视场。
各表面上光斑分布如图4所示,光线充满入瞳,因主镜离轴量为147 mm,光斑在主镜上光斑分布如图4(a)右半部分所示,次镜和折转镜上光斑能量分布如图4(b)所示,从图4(c)中间像面光斑分布可以看出,中间像面成像质量良好。
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各面上最小入射角、最大入射角和面元距离如表2所示,其中最核心的优化控制条件为最小入射角>4°,可有效抑制镜面散射光对系统隔离度的影响。
Name of the element Minimum incident angle/ (°) Maximum incident angle/ (°) Bin distance Primary 5.1 23.9 221.3 Audition 4.1 25.9 170.6 Turn the mirror 29.8 38.3 218.5 Three mirror 8.4 12.6 286.8 Table 2. Minimum incident angle, maximum incident angle and surface element distance on each face
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激光通信终端光学系统设计的波像差结果如图5所示。
各视场设计结果如表3所示,满足系统技术指标要求。
X /(°) Y /(°) RMS (λ:1550 nm) FOV1 0 0 0.007 5 0 −0.014 324 0.007 3 0 0.014 324 0.008 4 −0.014 324 0 0.007 6 0.014 324 0 0.007 6 FOV2 0 −0.07 0.018 4 0 0.07 0.019 9 −0.07 0 0.014 3 0.07 0 0.014 3 −0.143 24 0 0.040 8 0.143 24 0 0.040 8 Table 3. Design results of each field of view
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综合考虑灵敏度分析、曲率半径和K误差、综合误差分析,以出瞳处折射角控制在±30"以内,小视场波像差RMS控制在0.028λ以内,大视场波像差RMS控制在0.091λ以内,给出了最终的公差和检测精度要求,其中反射镜面形最大公差7 nm,间隔公差最大±6 μm,偏心和倾斜公差最大±10"。
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根据技术指标要求,建立杂散光仿真分析模型,如图6所示。
当光学镜面(主镜、次镜、折转镜和目镜)取不同粗糙度,隔离度分析结果为如表4所示(包含镜面固定结构件和其他机械件)。
Roughness/nm Isolation/dB 0.5 74.2 0.8 71.5 1 69.7 Table 4. Simulation of isolation degree of optical system with different mirror roughness
根据隔离度仿真分析结果,针对光学加工表面粗糙度提出了明确的技术指标要求:各反射镜表面粗糙度Rq<0.5 nm。最终加工镀膜完成后,实测其表面粗糙度,并代入实测表面粗糙度值进行了隔离度仿真分析,仿真结果满足指标要求,具体数据如表5所示。
Roughness/nm Isolation/dB Primary: 0.45 74.9 Audition: 0.47 Turn the mirror: 0.44 Three mirror: 0.33 Table 5. Measuring the isolation degree of optical system with different mirror roughness
Design of optical system of high isolation laser communication terminal
doi: 10.3788/IRLA20200521
- Received Date: 2021-03-10
- Rev Recd Date: 2021-04-16
- Publish Date: 2021-07-25
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Key words:
- photo-communication /
- off-axis three-mirror /
- optical design /
- isolation
Abstract: Laser communication has outstanding advantages such as large information capacity, high optical gain, high anti-interference and anti-interception capabilities. It was an important technical means to solve high-speed communication problems. According to the requirements of transceiver isolation and two-way duplex working mode in laser communication. After simulating and analyzing the isolation of different optical structures, it was proposed to use off-axis three-mirror optical antennas to reduce backscattering to achieve high transceiver isolation. In the optical design, the smallest incident angle on the optical surface was controlled through iterative optimization, and clear index requirements were put forward for the fine processing technology of the optical surface, and finally the isolation was more than 70 dB. The simulation analysis shows that the object field angle of 5 mrad isolation is 73.7 dB, and the measured isolation of Φ150 mm off-axis three-mirror optical antenna can reach 73 dB, which is consistent with the results of simulation analysis. It meets the requirements of satellite laser communication system tracking and communication for light antenna isolation, and can be used for inter-satellite laser communication.