High-precision Laser Differential Confocal Ultra-long Focal Length Measurement Method And Techniques

Ultra-long focal length lens(UFL)is widely used in laser fusion systems,space optical systems and high-energy laser weapons.The focal length measurement accuracy of UFL directly affects the image quality and performance of these large systems.However,the accurate focal-length measurement of UFL is still an unsolved technical bottleneck in optical testing field.Thus,the research of high precision ultra-long focal length measurement has important application value for these large optical systems.Supported by the National Instrumentation Program(NIP)of China “Development and application research for laser differential confocal imaging and testing instrument”(No.2011YQ040136),the dissertation carries out the research of high-precision laser differential confocal ultra-long focal length measurement method and technology.The main research contents are as follows.The research of laser differential confocal ultra-long focal length measurement(LDCFM)method with the capability to self-calibrate the reference lens(RL)focal length and the axial space between the UFL and RL is carried out.Using the differential confocal focusing techniques,the LDCFM measures the RL focal length by identifying the focus and last surface of RL,measures the axial space by identifying the last surface of RL and the vertex of UFL last surface,and measures the variation in focus with and without UFL,and then calculates the focal length with the combination focal length equations.The light path and theoretical analysis models of LDCFM are established.Based on the Kirchhoff's diffraction theory,the 3D point spread function focusing models of LDCFM are deduced,which can provide the theoretical foundation for the analyses of key factors in the LDCFM systems.A self-calibration method of the RL focal length and the axial space is proposed.Using the property that the focus of LDCFM system precisely corresponds to the zero point of the differential confocal curve,LDCFM calibrates the RL focal length by identifying the focus and last surface of RL,calibrates the axial space by identifying the last surface of RL and the vertex of UFL last surface.Thus,LDCFM can solve the calibration difficulty of RL focal length and the axial space well.A laser differential confocal ultra-large radii measurement method is proposed to precisely measure the ultra-large radii of UFL.Based on the property that the zero point of differential confocal curve precisely corresponds to the focus points of measuring beam,this method measures the position differences of UFL by precisely identifying the cat's eye and vertex positions on the last optical surface of RL,and then calculates the radii of UFL with ray tracing theory.The proposed method can greatly shorten the measurement light,reduce the measurement length and improve the measurement accuracy.A LDCFM system with aperture of ?610mm and measurement range of 10 m ~ 50 m is developed,and the main error sources in LDCFM system are analyzed in detail.Based on the LDCFM system,the ultra-long focal length measurement experiments are carried out,and the measurement results are evaluated by using the Monte Carlo method.The experiments results show that relative measurement uncertainty are 0.0034%(34ppm)for UFL with focal length of 31.2 m and 0.0044%(44ppm)for UFL with focal length of 12 m,respectively.In conclution,this dissertation proposed a LDCFM method with the capability to self-calibrate the reference lens(RL)focal length and the axial space between the UFL and RL.Based on the LDCFM method,a LDCFM system with large aperture is developed to verify its feasibility.LDCFM can provide a novel approach for the high-precision focal-length measurement of large-aperture UFL.