Large-aperture differential confocal interferometric optical element measurement method

Large-aperture interferometers are important in the manufacturing and measurement of large-aperture optical elements, which play critical roles in the fields of optical precision manufacturing and measurement, astronomical telescopes, semiconductor-wafer inspection systems, and inertial-confinement fusion devices. The manufacturing accuracy of optical components, such as the large-aperture surface profile, ultra-large curvature radius, and ultra-long focal length of optical elements, depends on the measuring accuracy during production. Therefore, accurate common reference measurements of multiple parameters for optical elements are crucial for improving the performance of high-end precision instruments. 

 

In a new paper published in Light: Advanced Manufacturing, a team of scientists, led by Professor Weiqian Zhao from MIIT Key Laboratory of Complex-field Intelligent Exploration, Beijing Institute of Technology, China, have proposed a high-precision large-aperture laser differential confocal-interferometric measurement method for measuring multi-parameters of optical elements.

 

This method addressed large-aperture reference mirrors with ton-weights to achieve nanometer precision spatial translational phase shift by implementing air-floating support for the gravity unloading of a heavy-loaded reference lens, combined with in-situ spatial monitoring using capacitive sensors. This strategy enables large-aperture mechanical phase shift interference measurements. And addressed the limitations of long diffraction focal depth and susceptibility to environmental disturbances were addressed by the laser differential confocal measurement method. By integrating laser confocal and interferometric techniques, this method overcomes the bottleneck of comprehensive measurements of multiple parameters with a common reference. This newly developed high-precision measurement method based on a large-aperture laser confocal interferometer allows for the first time, comprehensive measurements of multiple parameters for optical elements with large, medium, and small apertures.

 

These scientists highlight the key features of their method: ‘We proposed a high-precision large-aperture laser differential confocal-interferometric measurement method with three main advantages: (1) It combines the principles of laser differential confocal microscopy and interferometry, which allows for high-precision common baseline measurements for multiple parameters; (2) It employs mechanical phase-shift technology for large-aperture and heavy-loaded reference lenses to overcome the theoretical shortcomings of the existing large-aperture wavelength-tuning phase-shift technique; (3) It leverages the laser differential confocal method, known for its anti-scattering and anti-interference properties, enabling high-precision common baseline measurements for the multiple parameters of optical elements such as ultra-long focal lengths and ultra-large curvature radii'.

 

‘For the large-aperture surface profile measurements, the mean PV is 46.0nm. For the ultra-long focal length, the relative standard deviation is 0.019%, while for the ultra-large radius of curvature, the relative standard deviation is 0.0036%," they added.’

 

‘This method enables high-precision, high-stability, and high-efficiency common baseline measurements for multiple parameters of large, medium, and small-aperture optical elements, providing an effective approach for improving the detection and processing accuracy of large-aperture optical elements,’ the scientists predicted.

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