Study On The Fabrication Technology Of High-Precision Optical Surfaces Based On Epoxy Replication

Mirrors with high precision and high stability are one of the core elements in optical systems.Shorter optical processing cycle,lower cost,better performance and more diversified selection of mirror materials are required.Conventional optical fabrication processes,which have some disadvantages,include grinding,coarse polishing,precise polishing and so on.First,there is a direct correlation between fabrication time and mirror size.The fabrication time increases greatly with the mirror size enlarging.Second,there are only a limited number of materials could be polished,such as glass,silicon carbide,beryllium,aluminum alloy,titanium alloy.Most of these materials except glass need extra processing.The hardness of silicon carbide cause hardship for the polishing.There are two phases in reaction-bonded silicon carbide,namely silicon carbide and silicon,which leads to difficulty for the acquiring of super-smooth surface.Surface modification is used to improve the polishing performance of reaction-bonded silicon carbide.Metal mirrors,including beryllium,aluminum alloy and titanium alloy are hard to eliminating the scratches and defects and need to be coated with nickel phosphorus alloys before polishing.Third,there is a direct correlation between fabrication difficulty and mirror shape.It is relatively easy to polish flat and circular mirror.Meanwhile,it is much harder to polish off-axis aspheric or polygonal mirrors.Epoxy replication is an efficient and low-cost way to fabricate optical mirrors.In this replication process,the master,which contains the desired optical surface,is epoxied to the substrate.The distance between the master and the substrate is filled in by the epoxy resin.When the pieces are separated,the epoxy resin layer is transferred to the substrate producing a replicated mirror.However,the surface figure accuracy will deteriorate when epoxy replication method is used to fabricate large mirror due to thermal and solidifying stress.Firstly,this dissertation focuses on the systematic research of epoxy replication and finite element analysis has been used to simulate this replication process.An optimized method has been developed to determine the thickness of the master and predict the surface figure after replication.By selecting proper materials and optimizing fabrication parameters,the lattice-matched and uniform nanolaminates have been fabricated.Parameters about pressure,temperature,inorganic filler,stress relieving and demoulding have been studied comprehensively,and the accuracy of surface figure after replication has been improving.In addition,the nanolaminates and magnetorheological finishing are combined to correct the surface figure aberrations after replication in the mirrors with a diameter larger than 100 mm.Secondly,to meet the need of spaced-based active mirrors,especially active primary mirror,a bimorph active mirror using epoxy replication method has been fabricated for the first time in China.A finite element model has been established to optimize relevant parameters,such as mirror size and electrode pattern.A sphericalΦ160mm prototype mirror has been fabricated and tested,which meets the optical accuracy requirements.Thirdly,epoxy replication method is also used to fabricate rigid mirror.A parabolic replicated mirror with a diameter of Φ180mm and a plane replicated mirror with a diameter of Φ500mm have been fabricated within 5 and 10 days,respectively.The precision shape(RMS<20 nm)and low surface roughness(Rq=0.6nm)are both achieved.There are no reports about the fabrication of optical mirrors with a diameter of 500 mm using replication method.This method has also been used successfully in these mirrors made from hard-to-polish materials or with complex structures.Lastly,the surface figure has been tested constantly affected by many factors,such as high temperature,low temperature,damp heat,irradiation and chemical solvent.The stability of these replicated mirrors has been proved.