Error Analysis And Characterization For Holographic Null Test Of Optical Surfaces

Aspheric surface plays an important role in the field of modern optical systems.Using computer generated hologram(CGH),it can achieve accurate null test of aspheric surface and the accuracy is related to the accuracy of CGH.Based on microscopy for diffractive structures,the fabrication error of CGH is measured and characterized.And the wavefront error induced by the fabrication error is computed and analyzed by combining the scalar and vector diffraction theory.The paper falls into 3 aspects as following:1.The principle of null test with CGH is elaborated.The diffractive ray tracing and nonlinear distortion correction are performed for the null test of an aspheric surface with CGH.the method for forward and reverse design of CGH null test are introduced.It presents the analysis of influence of various fabrication error sources such as CGH encoding,etching depth,duty cycle,pattern distortion,substrate error and steepness,on the diffracted wavefront error.The misalignment induced error,design of frequency carrier and alignment pattern are also discussed.2.A precision two-dimensional translational stage is incorporated in the scanning white-light interferometer.It enables fast evaluation of the diffractive structure parameters for the CGH and realizes measurement of the CGH fabrication errors such as duty cycle,etching depth variation,pattern distortion etc.The fabrication error parameter are characterized in the full aperture.Along with the scalar and vector diffraction theory,wavefront error contributed by the fabrication error is computed and analyzed.At the same time,the established CGH model with fabrication error included is imported into lens design software and the wavefront aberration is simulated.3.A CGH for null test of spherical surface is designed and fabricated.Several methods of calibrating CGH substrate error are discussed and separation of substrate error is completed.The fabrication error is measured and a parametric model of CGH wavefront error is built.By analyzing the influence of misalignment,substrate transmitted wavefront,duty cycle error,pattern distortion and etching depth error,uncertainty of the null test system for optical surfaces is estimated.On the other hand,the test sphere is also calibrated by a high-precision transmission sphere,which verifies the CGH null test result.