Study Of Diffraction Optical Element Fabricated By Polar Coordinate Femtosecond Laser Direct Writing On Curved Surface

Diffractive optics elements(DOEs)has been widely applied in the field of laser technique,aerospace,military field etc,for its strong design ability,small size,light weight,etc.In recent years,with the rapid development of the optical technology,it made a higher request to the DOEs.Currently,the method of preparing DOEs is mainly used single point diamond turning,lithography and laser direct writing.The outline of the components has been restricted by the shape of the diamond cutter in single point diamond turning,so it is difficult to complete the complex structures;Although lithography is widely used but it always has limitations in the field of curved surface processing;Laser direct writing technology has the advantages of no mask,high spatial resolution and wide range of processing materials,which can realize the processing of any complex structure with high precision,so it has been widely applied in the field of mico-nano fabrication.Compared with the conventional laser direct writing technology,the femtosecond laser direct writing technology(Fs LDW)has a unique two-photon absorption characteristics which can break through diffraction limit.Femtosecond laser can penetrate the surface of the material into the material,can achieve the true threedimensional structure of arbitrary shape processing.So in this paper,we use the high precision and true three-dimensional Fs LDW to prepare diffractive optical elements.There are three kinds of traditional Fs LDW: First is the Fs LDW that based on piezoelectric platform,the processing precision of this technology is high,and it can reach nanometer level,but the processing efficiency and the size of the device of the technology are limited by the speed and the travel of the piezoelectric platform,this results in its low processing efficiency and the size of the fabricated device is usually not more than a few hundred microns.;The second is the Fs LDW that based on scanning galvanometer,this technology has high processing efficiency,but there is a contradiction between the scope and the numerical aperture of the focusing lens,the processing accuracy and processing size can not have both.The third is the Fs LDW that based on linear motor linear scanning,the processing range of the technology is large,but in the process of its processing,it is necessary to overcome its own inertia of frequent acceleration and deceleration,this kind of movement causes the low efficiency of the processing system.In order to solve the contradiction between the processing precision,the processing size and the processing efficiency in the process of processing the DOEs with the traditional Fs LDW,we set up a polar coordinate Fs LDW system with high precision,large area and high efficiency.The system consists of two orthogonal linear displacement tables and a high-speed rotating table,the processing range of the system is limited only by the travel of the radial linear displacement table,can achieve the size of the centimeter scale three-dimensional structure of the scan.The position of the system is measured by high precision grating ruler,and it use closed loop negative feedback control,so the repeatability of the system has reached 50 nm,ensured the high precision.The rotation axis of the system can be continuously rotated at high speed during the processing,no need to start and stop frequently,therefore the system has a high processing efficiency.In this paper,the principle and technical parameters of the system are introduced in detail and fabrication of large size DOEs using the system.We first explored the processing conditions of the SU-8 photoresist on the planar substrate,processed a large size three-dimensional ladder structure.Then,the problem of the center alignment and the calibration of the lens curvature is studied,successfully completed the fabrication of large size DOEs on the curved surface.Finally,we discussed the influence of the change of the ambient temperature on the structure,and take measures to reduce the temperature drift in the process,get a more stable processing environment.In this paper,we have solved many difficulties in the fabrication of large area DOEs by the traditional Fs LDW,it provides powerful technical support for the high performance diffractive hybrid optical elements.