Study Of Ptychography Without Imaging Lenses For Detecting Phase Defect

The phase defect in the optical element is an important factor that influences the beam fine adjustment and imaging resolution of the optical system.The mechanism of the phase defect is complex,and phase defects may occur in various process,including melting,growth,processing,cleaning,use,and handling.The phase defects within an optical element will disturb the wavefront of the incident light beam and induce damages of the optical element itself.In the large-scale and ultraprecise optical engineering installations,urgent requirements have been put forward for defect detection.It is necessary to inspect the defect for full-aperture of the optical element with micron,sub-micron,nanoscale phase accuracy.Ptychography without imaging lenses is adopted to detect phase defects in optical components,and in-depth research has been conducted on several problems that occur during the detection of phase defects using ptychography without imaging lenses.Based on the scalar diffraction theory,the modulation model of phase defect on the incident light is established.It indicates that phase defect can change the intensity distribution of the recording plane,which provides a theoretical basis for phase defect detection using ptychography without imaging lenses.At the same time,a mathematical model for phase defect detection by ptychography without imaging lenses is established.It indicates that the detection accuracy can be affected by noise,scanning position error and axial distance error.Divergent lighting ptychography without imaging lenses is proposed to detect the micro-morphology of the binary optical element.A pupil scanning modulation ptychography without imaging lenses is proposed that simplifies the systematic structure of the conventional two-dimensional mechanical scanning ptychography.The pupil scanning modulation is based on changing the extent of illumination function using an adaptive aperture.These apertures are fixed on the same border or point of intersection that ensures the location of aperture.We sequentially increase the size of the aperture that guarantees the necessary overlap between adjacent object fields.An improved algorithm including the adaptive raised-power estimation constraint and gradient-descent step is proposed to accelerate convergence and avoid stagnation during iterations.These two methods of ptychography without imaging lenses are applied to retrieve the phase-type computer-generated hologram,and their phase detection accuracy is 3.61%and 1.37%,respectively.The scanning position error and axial distance error have serious impacts on the detection accuracy.The correction method for scanning position error based on simulated annealing is studied,which can realize sub-pixel high accuracy correction for scanning position error.The methods of iterative auto focus and image information entropy are proposed to correct the axial distance error by establishing the relationship between the retrieval quality_evaluation parameters and the axial distance.The optimal axial distance can be obtained to reduce the influence of axial distance error on detection accuracy.In the conventional three-dimensional extend ptychography,the optical element with phase defect must be separated into a mass of axial sections that increases the calculation consumption.A fractal dimension positioning defect three-dimensional extend ptychography is proposed to evaluate and reconstruct phase defects.This method is firstly used to locate the defects in the vertical plane,and the complex amplitude of the phase defect is inversely used to locate its axial position.The axial positions of phase defects are calculated,and only several axial sections with defects are located at the calculated positions,which can reduce the number of axial sections,decrease the calculation time and improve the detection efficiency.At the same time,the phase,size,and spatial position of the phase defect can be obtained.The axial position error is better than 1.9%.Compared with the recovery time of 3 defects at different axial position by the conventional three-dimensional ptychography with 10 slices,the recovery time of our proposed method has been reduced by 77%.In order to avoid the shortcomings of time-consuming and scanning position error,multi?wavelength scanning ptychography and single shot ptychography based on RGB single exposure are proposed.The former uses a wavelength tunable laser to produce a series of beams with different wavelengths to illuminate the object instead of a two-dimensional mechanical scanning.The latter uses a trichromatic RGB illumination source that can avoid coherent noise to record three diffi-action patterns simultaneous in a single color CCD.The RGB diffraction pattern can be separated into three elementary RGB channels that will provide three independent RGB diffraction patterns.A method of initial object spectrum synthesis is introduced to accelerate convergence and suppress noise.The data acquisition time for single exposure is only related to the CCD.A single exposure can be used to detect the phase defect in real time,and obtain its phase and size.