Surface profiling method for large aspheres: diffraction image-forming approach

摘要

Investigation of surface profiling method for large aspheres becomes more and more important and imperative with the great development of the synchrotron radiation facility (SRF), since the latter puts greater demands on surface quality, shape and figure parameters of the optical elements used in itself. Meanwhile, things became more difficult because of the unique characteristics of the optics used in SRF. As a result, novel surface measurement methods and system have to be developed to cope with such problems. The diffraction alignment technique is introduced into the basic configuration of the angle-position transforming system. By theoretical analysis, it could be found that the novel f-theta system has several outstanding advantages. A novel principle of surface profiling technique based on the above system is proposed, thereafter a novel long trace profiler (NLTP) had been built. The optical system of the NLTP is rather concise, consequently its potential error sources are greatly reduced. In the whole scanning range, the optical path of the measurement and reference beams keep constant, which ensures that the measurement and reference patterns on the back focal plane of the FT lens remain stable and thereby the potential error in data processing could be avoided. The common path configuration of reference and measurement beams is introduced, as a result, such errors introduced by light source, thermal drift, atmospheric turbulence, and so on, can be fully or partly compensated. The NLTP inherits thoroughly the advantages of the sequential scanning method, and can be competent for figure measurement of large or super -large aspherical optics, especially of those optical elements used in synchrotron radiation. In addition, a configuration for three-dimensional surface profiling based on the new approach has been proposed. A prototype of the NLTP has been developed whose scanning range is 370mm and slope resolution is better than 0.25μrad. The experimental results show that the system is very stable and its stability is 0.58 μrad even under an opening operation circumstance without temperature control and air conditioning. An accuracy of 0.7μrad has been achieved within whole scanning range.