A Study On On-line X-ray Wavefront Metrology Techniques

Wavefront is a curved surface composed of equal phase surface when light wave propagates to a certain position.The essence of wavefront sensing is quantitative phase retrieval.Because in the visible light and higher frequency field,the detector can only detect the amplitude of a light wave.Therefore,it is necessary to extract phase information from intensity measurement using various technique.The property of wavefront is determined by the light source and the design of optical path in a certain beamline.The defects and surface errors of optical elements will result in wavefront distortion.For example,the surface error and misalignment of the X-ray mirror will affect its focusing performance,and the wavefront error caused by the combined refractive lens will also affect the intensity distribution of the focusing spot,which has adverse effects on some experiments.Therefore,it is important to get fully knowledge of the properties of X-ray wavefront in order to understand the properties of optical elements in the beamline,guide the processing of optical elements and reduce the influence on the experimental results.X-ray wavefront sensing is also widely used in the area of imaging.X-ray imaging is one of the most important techniques to detect the internal structure of material in the real space.Compared with the traditional X-ray absorption imaging,phase retrieval methods have the advantages of high spatial resolution and low radiation dose to samples.Therefore,the internal structure information of the sample can be better understood by using the quantitative phase recovery technique.In this paper,combined with the actual requirements of the current X-ray wavefront sensing,through the in-depth study of a variety of phase retrieval theories and algorithms,the following research results are obtained1.The current method of solving the transport of intensity equation(TIE)requires an aperture to construct the boundary conditions or requires the size of the phase function to be measured to be smaller than the field of view and located in the center of the field of view.These limitations make it difficult for TIE to be applied in hard X-ray field.To solve this problem,we propose to solve the TIE-based hard X-ray experiments under the homogeneous Neumann boundary condition.Firstly,the method to obtain this boundary condition is provided,and the practicability and universality of the boundary condition in hard X-ray experiments are verified by simulation and experiments based on BL09 B,Shanghai Synchrotron Radiation Facility(SSRF).Using this boundary condition,no extra need to limit size of the phase function,and there is no need to construct the boundary condition with an aperture or use other optical elements to assist imaging.Therefore,TIE can be universally applied to the field of hard X-ray.2.We designed a real-time and on-line hard X-ray monitoring device.Combined with our method of solving TIE,the optical system can not only get the information of light intensity,but also get the real-time changes of incident wavefront.It provides an effective monitoring method for the adjustment of active optical elements in synchrotron radiation beamline and the diagnosis of single pulse properties of free electron laser.3.In order to meet the increasing demand of single pulse property diagnosis of free electron laser,a real-time,on-line method for monitoring spatial properties(wavefront distribution,intensity distribution,coherence,etc.)of beam using a grating is proposed.The simulation and experiment results based on BL19U2(SSRF)show that the first-order diffraction beam and the incident beam are completely consistent in many aspects,which shows the equivalence of the on-line detection of the first-order diffraction beam and the incident beam.This method provides a new method for on-line diagnosis of synchrotron radiation and X-ray free electron laser.4.Meanwhile,we propose a real-time,on-line and high-precision technique by combining the grating spectral properties and near-field speckle tracking method.Compared with the traditional near-field speckle tracking method,this method treats the speckle patterns not only as a function of position,but also as a function of time.By collecting the reference map and sample map at the same time when using a grating,the error caused by the external environment(vibration of light source,vacuum pump and water-cooling unit,etc.)in the phase retrieval process can be significantly reduced,and the accuracy of phase recovery experiment can be greatly improved.5.Since there is no hard X-ray coherent diffraction imaging platform domestically,we have designed and construct the first domestic experimental platform that dedicated to hard X-ray coherent experiments.According to the actual design parameters of BL19U2(SSRF),the optical elements(slit,K-B mirror)in the optical path are adjusted,and a pinhole with radius of micron is used to obtain the fully coherent beam.The experimental platform is built by using nano scanning platform and imaging detector and other elements.And the corresponding algorithm is compiled.The experimental results show that the platform supports single pulse and scanning modes.And the spatial resolution of the configuration is about 50 nm.This platform fills the gap in the field of hard X-ray coherence experiment domestically,and provides effective support for the development and application of hard X-ray coherent-based experiments and methods.