High-precision In-situ Measurement of Complex Optical Surfaces
With the continuous improvement of the opto-electronic performances of major equipment such as laser radars and astronomical telescopes, the size of the key optical components has increased to a level of meters, and the requirement on the form quality has achieved a sub-micron or even ten-nanometer level, consequently making a great challenge to the precision optical manufacturing and measurement. Currently the optical surfaces are mainly adopts off-situ measurement method, which is sensitive to environmental interference. The reciprocating transportation and adjustment of the workpieces between the manufacturing machine and the measuring instrument takes more than 70% time of the whole manufacturing process, which severly limits the fabricating efficiency and reliability. The final verification tests of optical components are usually conducted by interferometers, the measuring range of which is generally no more than 10 μm. The compensators or CGH need to be specially designed to measure complex surfaces like aspherics and off-axis paraboloids. In addition, the measuring system is difficult to be aligned, thus it cannot be applied in grinding or rough polishing. Therefore, it is urgent to develop an in-line/in-situ measuring method compatible with the fabricating machines to improve the automation and intelligence level of precision optical manufacturing.
Prof. Xiangchao Zhang et al from Fudan University has developed an in-situ deflectometric measuring method for the ultra-precision optical manufacturing of aspheric and freeform optical surfaces, as shown in Fig. 1. Deflectometry determines the normal vectors of the measured surface based on fringe coding and imaging methods, whichis is similar to the Hartmann test, possesses advantages on efficiency, stability, capability of anti-disturbance and large dynamic range, thus deflectometry has great potential in in-situ measurement. The method analyzes the ray tracing and measures the influence of position deviation of each components in the measuring system on the tracing deviations, through additional constraints supplied by an auxiliary calibration mirror and the moving system embedded in the fabricating machine, the calibrating accuracy of the position of each component in the measuring system can be improved by an order of magnitude, as illustrated in Fig. 2.
Fig 1. In-situ deflectometric measuring system Fig. 2. Coordinate systems in geometrical self-calibration
The pose error and form deviation of the workpiece can be separated based on the statistical properties of deviations in the reverse ray tracing. The nominal shape of the measured surface can be effectively utilized, the traditional one-way position-form mapping can be converted into a two-way mapping, and the height sub-problem and form sub-problem are respectively optimized and solved, as depicted in Figs. 3 and 4. Therefore, the use of extra detection scuh as a third-party instrument can be avoided, which has significantly improved the measuring flexibility and efficiency.
Fig. 3. Height sub-problem Fig. 4. Form sub-problem
Experiments using off-axis paraboloid and other optical compnents shows that the accuracy of proposed deflection measurement method is better than 150 nm RMS. Henceforth it can solve the measuring problem of complex surfaces during the rough fabrication, with its form deviation beyond the measuring range of interferometry. The results were published in Opto-electronic Engineering 2020 vol 7 titled “In-situ deflectometric measurement of optical surfaces for precision manufacturing”.
About The Group
Shanghai Engineering Research Centre of Ultra-Precision Optical Manufacturing, Fudan University has long devoted to the precision fabrication, measurement and integration technologies of high-performance optical components, especially those containing freeform surfaces and micro-nano structures. It has overcome key problems of the cubic mirror in the lithographic machine and the dual-mode dome etc., and has achieved a series of novel results in the measurement and characterization of freeform surfaces and fabrication-measurement integration of precision optical manufacturing. More than 200 papers have been published in Optics Letters, Light: Science and Applications, Optics Express etc. A technological progress prize has been awarded by the Ministry of Education.
Zhang Xiangchao, Xu Min. In-situ deflectometric measurement of optical surfaces for precision manufacturing[J] Opto-Electronic Engineering, 2020, 47(8): 190581.