Study On Measuring Large Aperture Aspheric Surface Technique Based On Ultra-precise Rotary Scanning

With continuous development of optical fabrication and measurement technology, the large aperture deep aspheric optical component has been one of the key parts which make important role in following regions: astronomical optics, space optics, detection and identification of foundation space target, atmospheric transmission of laser, inertial confinement fusion etc.. The key factor that confines machining level of large aperture aspheric is decided by measurement method and instrument which are adaptive to machining requirements. For traditional contact measuring method, the fatal flaw is its easy damaging surface of optical elements. Among non-contact measuring methods, both whole aperture interference and sub-aperture stitching interference methods can not meet machining requirements of large aperture aspherics because of difficult fabrication of compensating lens and error accumulation in process of stitching. Therefore, it is necessary to develop a kind of high precise and non-contact measuring system of large aperture aspherics.The subject"Study on Technique of measuring large aperture aspheric surface based on ultra-precise rotary scanning"is to develop a high precise and non-contact three dimensional profile rotary scanning measuring technique of hither plane large aspheric surface which is based on measuring technique of large aspheric surface based on f-θlens system. The diffractive collimated beam generated by two-dimensional phase plate is used as measuring basis beam and dark line of diffractive spot is looked as location basis. The precise location of scanning beam is realized by ultra-precise linear air floating rail and the rotary motion of large aperture aspherics is realized by ultra-precise air floating turntable. The subject develops a effective technique for solving three-dimensional profile measurement of hither plane large aperture aspheric surface and provides theoretical basis and preparation work for developing measurement technique of large aperture deep aspheric surface. The application of this technique in measuring synchrotron radiation aspherics, cylindrical surface, thin and long optical surface is prospective.According to operation principle of diffractive long trace profiler (LTP), the basic characters of f-θlens system based on diffractive collimated technique are analyzed. Considering factual structure of measuring equipment, every factor affecting measuring accuracy of slope is also analyzed. Therefore, the firm theoretical basis for introducing diffractive LTP to measuring technique of large aperture aspheric surface is built.In order to measuring three-dimensional profile of large aperture aspheric surface, a kind of measuring technique of large aperture aspheric surface based on ultra-precise rotary scanning is presented. The rotary scanning measuring of surface under test is realized by combining ultra-precise linear motion and ultra-precise rotary motion, then measuring error of slope caused by motive structure is greatly reduced because of high precise rotary base motion.Aiming at measuring requirement of three-dimensional profile, a kind of two-dimensional slope optical scanning head is presented. It consists of two sub scanning heads which respectively measure slopes along radial and tangential directions, furthermore, the two-dimensional slope of measured spot can be detected simultaneously. The error compensation optical paths in the two sub scanning heads can real-time and dynamically compensate slope error caused by angle shift of laser beam, then the three-dimensional profile measuring error caused by angle shift of laser beam can also be reduced greatly.Considering structure of measuring system of large aperture aspheric surface based on ultra-precise rotary scanning, the effects of every error factor on measuring two-dimensional slope are analyzed based on matrix optics and law of refraction and reflection in vector form. In addition, weights of every error factor are also compared, then theoretical basis for machining and selecting of every part in process of development is provided.In order to reduce measuring error of slope caused by filtering diffractive spot, a filtering method based on curve window is studied and compared with traditional filtering methods. The analyzing results show that the filtering method based on curve window can effectively filter image noise without changing distribution of optical intensity around dark line of diffractive spot, so sub-pixel location error of measurement and reference spots may be reduced greatly. At the same time, in order to select high precise numeric reconstructing method, an assessment method which compares different numerical reconstructing methods in frequency domain is presented, which provides theoretical basis for selecting high precise and strong ability of suppressing noise numerical reconstruction method. Furthermore, measuring accuracy of profile is improved.At last, according to research target, the prototype of large aperture aspheric surface measurement system based on ultra-precise rotary scanning is developed and is tested experimentally. The flat mirror whose diameter is 50mm is used as measured object, and the measuring result is compared with AK100 fizeau interferometer whose measuring level is higher in the world. The experimental results show that PV value and RMS value between the two kinds of method are respectively 0.1λand 0.05λ, and then the measuring method of large aperture aspheric surface based on ultra-precise rotary scanning is demonstrated to be feasible.