Research On Fabrication Of Large-sized Stitching CGHs For Wavefront Modulation

After more than half a century of development,holographic technology has developed from traditional optical holography to computational holography based on virtual simulation.At present,computational holography has been widely used in many fields,such as 3D holographic imaging,information encryption storage,spatial filtering,optical interferometry,etc.In high precision optical interferometry,computer-generated hologram(CGH)plays a particularly important role as a wavefront modulation element.Due to the advantages of flexible design,high testing accuracy and low production cost,the zerointerference testing method based on a CGH has become a mainstream development direction of modern high-precision measurement technologies.The outstanding advantages of CGH applications are manifested in the high precision testing of aspherical surfaces and the alignment of complex optical systems.With the development of micro-lithography technology,it is possible to fabricate CGHs with high precision,which greatly promotes the applications of CGH in the field of high precision optical testing.In order to meet the increasing detection capability and high resolution requirements of advanced optical systems,the aperture of optical mirrors is getting larger and larger.Accordingly,the size of CGH used for highprecision testing of large aperture aspheric surfaces and alignment of multi-sub-mirror optical systems is also getting larger and larger.However,it is difficult to meet the application requirements of large-sized CGHs above 12 inches with the current lithography capability.In order to meet the development needs of large-sized and high-precision CGHs,this paper proposed a two-step lithography stitching method to realize the fabrication of large-sized CGHs,and analyzed the testing influences caused by manufacturing errors during the process.The main contents of the dissertation are as follows:1.The limitation of scalar diffraction theory in the applications of high line density CGHs and the line width threshold of design are discussed and analyzed.Scalar diffraction theory is the theoretical basis of diffraction elements.With the increase of aspheric aperture and the limitation of CGH processing size,the line densities of designed CGHs become higher and higher,which leads to the limitation of scalar diffraction theory.Based on the mathematical analysis model of a linear grating and the rigorous finite difference time domain method of vector diffraction theory,the simulation and comparative analysis of high line density are carried out to clarify the threshold conditions of CGH fringe linewidth under scalar approximation and the influence of limitations on diffraction efficiency,modulation phase and reconstruction geometric parameters.2.The research on high precision manufacturing process of large-sized stitching CGHs.Based on laser direct writing lithography,a two-step lithography stitching method is proposed to expand the exposure area of the pattern.Through the measurement and calibration of the machine tool motion errors and the overlay errors in stitching process,the fabrication accuracy of the exposure pattern and the two-step stitching method can be improved.An experimental element is designed to verify the feasibility of the proposed stitching method.On the basis of the above experiment,a surper large-sized 300mm×600mm CGH is successfully fabricated for pratical engineering.3.Theoretical analysis of CGH fabrication errors and their influences on the wavefront testing.The fabrication errors of CGH mainly include substrate errors and pattern drawing errors.The influences of substrate errors on testing are analyzed from the aspects of thickness,material refractive index,surface figure and surface parallelism.By establishing error simulation model and evaluation method,the influences of positioning errors,orthogonality errors and overlay errors which lead to pattern distortion are discussed and analyzed in optical testing applications.4.Validation experiments of error theory analysis model and evaluation method.The orthogonal error of 80?rad,transverse overlay error of 1?m perpendicular to the suture and longitudinal overlay error of 400 nm along the suture are introduced into the CGH patterns by human design,respectively.The above components are fabricated by a high-precision lithography equipment,and the wavefront testing platform is built to obtain the experimental results,which are compared with the theoretical simulation results to verify the accuracy of the theoretical model.With the simulation results as reference,the error compensation of the experimental test results is carried out,and the residual errors verify the feasibility of the evaluation method.In this paper,the key techniques involved in the fabrication of stitching largesized CGHs are investigated.By measuring and calibrating machine tool errors and overlay errors,the fabrication accuracy of stitching CGHs is improved.The two-step lithography stitching method is used to fabricate large-sized CGHs.At the same time,the error analysis model and evaluation method are established to further improve the test accuracy of stitching CGHs in the measurement process.The relevant research work in this dissertation is of great significance to the high-precision testing of large aperture aspheric surfaces and the alignment of multi-sub-mirror optical systems.