Calibration of single optical wedge compensation test system error by computer generation hologram
doi: 10.37188/CO.EN.2021-0004
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摘要: 单光楔补偿检测法具有良好的适用性、鲁棒性和灵活性，但是在检测光路中存在多种误差耦合，误差解耦困难，影响了单光楔补偿检测的精度和可信度。针对这一问题，本文提出一种计算全息法(Computer Generation Hologram, CGH)标定单光楔补偿检测光路系统误差的新方法。文中首先分析了单光楔补偿检测法系统误差的来源，并对CGH标定光楔补偿器的可行性进行了分析。结合工程实例，对口径为150 mm的单光楔补偿器设计了CGH，经分析可得CGH的标定精度为1.98 nm RMS，CGH标定后单光楔补偿检测精度为3.43 nm RMS，该精度能够满足大口径凸非球面反射镜的高精度检测要求。结果表明：CGH可以准确标定单光楔补偿器的位姿和检测光路的系统误差，解决了检测光路中误差解耦困难的问题，提高了单光楔补偿检测的准确性和可靠性。使用CGH标定得到Tap#2和Tap#3的检测光路系统误差分别为0.023λRMS和0.011λRMS。Abstract: As a testing method for large convex aspheric surface, the single optical wedge compensation test has good applicability, robustness and flexibility. However, various errors are coupled with one another during the test process and these errors are difficult to decouple. This affects the accuracy and reliability of the tests. To address this, a method is developed to calibrate the system error of single optical wedge test paths using a Computer Generation Hologram (CGH). We first analysed the source of system error in the optical path of a single optical wedge compensation test as well as the feasibility of using CGH for the calibration of an optical wedge compensation test system. In combination with engineering examples, a CGH was designed for optical wedge compensators with a diameter of 150 mm. Based on the analysis results, the calibration accuracy of the CGH was 1.98 nm RMS, and after calibration the test accuracy of single wedge compensation was 3.43 nm RMS, thereby meeting the high-precision test requirements of large convex aspheric mirrors. This shows that CGH can accurately calibrate the pose of single optical wedge compensators and the test system errors of optical paths. Thus we address the problems affecting error decoupling in test optical paths, and improve the accuracy and reliability of the single optical wedge compensation method. Meanwhile, using CGH calibration, the system errors of the test optical paths, Tap#2 and Tap#3, were 0.023 and 0.011 λ RMS, respectively.
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Key words:
- computer generation hologram /
- optical test /
- diffraction /
- optical wedge
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Figure 1. (a) Single optical wedge test optical path. (b) Schematic of the test wavefront. w1, w2, w3, w4 are theoretical incident wavefront, actual incident wavefront, theoretical aspheric surface and actual aspheric surface, respectively.
Figure 2. (a) Surface shape of the optical wedge A. (b) Surface shape of optical wedge B. (c) Transmitted wave aberration. (d) Transmission sphere system error. (e) Tap#1 non-null error. (f) Tap#2 non-null error. (g) Tap#3 non-null error.
Figure 5. Main area of the CGH. (a) Design residual; (b) diffraction order energy level distribution; (c) diffraction pattern; (d) surface phase setting parameters
Figure 7. Alignment area. (a) Design residual; (b) diffraction order energy level distribution; (c) diffraction pattern
Figure 9. Convex spherical stitching test result (D = 197 mm) (a) Actual full-aperture surface; (b) sub-plane; (c) stitching test result; (d) the residual difference between the stitching test result and the full-aperture surface result
Figure 10. (a) Calibration optical path diagram of optical wedge compensator. (b) CGH and interferometer aligned on the optical path. (c) Insertion of the optical wedge compensator
Figure 11. Calibration results for the test path system error. (a) CGH alignment result by Tap#2. (b) Test path system error by Tap#2. (c) CGH alignment result by Tap#3. (d) Test path system error by Tap#3
Table 1. Basic parameters
Item Tap#1 Tap#2 Tap#3 Sub-aperture
planningNumber 1/21 8/21 12/21 Off-axis/mm 0 145 175 Subaperture/mm 118 114 116 Departure/μm 0.74 19.2 28.5 Need compensation × √ √ Optical wedge
structure
parametersDiameter /mm 150 150 Centre thickness /mm 20 20 Tilt/(°) 3.2 0.77 Wedge/(°) 6.3 6.3 Material F_Silica F_Silica 
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Table 2. Adjustment tolerance analysis of the optical wedge compensator
Wedge-interferometer dist./mm Wedge-mirror. dist./mm x-tilt
/(°)y-tilt
/(°)Eccentric eccentricity/mm RMS Test
system0.15 0.15 0.005 0.01 3 9.21×10−3λ Calibration
system0.05 0.05 0.0015 0.001 0.05 8.35×10−3λ
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Table 3. CGH fringe contrast
Diffraction order Amplitude CGH Phase CGH 0 73.05% ±1 93.09% 61.08% ±3 78.92% 99.97% ±5 54.01% 87.29%
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Table 4. CGH error
Error type Value design residual 1.77×10−5λ coding error 9.00×10−4λ substrate error 3.00×10−3λ characterization distortion 1.57×10−4λ RMS 3.10×10−3λ
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Table 5. Single optical wedge compensation test accuracy after CGH calibration
(nm) Error Value Measuring random error 2.5 CGH calibration test optical path system error 1.98 Accuracy of stitching algorithm 1.26 RMS 3.43
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