Development And Application Of Partially Coherent Light Propagation Model Of Synchrotron Radiation

For the application of more cutting-edge experimental methods,a new round of free electron laser and diffraction limit ring light source construction and upgrades have been carried out internationally,and more than 20 new light sources with high coherence are under construction or planning.High coherence is an important feature of free electron lasers and diffraction limit cycles.Using the high coherence of X-rays,the performance of many experimental methods has been greatly improved,and people's ability to explore the microscopic world has been improved.The wide application of coherent X-rays has promoted the construction of high-coherence beamline stations,and has also put forward higher requirements for beamline design.Quantitatively study the variation of coherence performance in X-ray propagation,and optimize the beamline design accordingly.The design and construction of the beam line is very important.The earliest developed simulation model in the world is SHADOW based on geometric optics.Its main characteristics are early start,relatively complete development of the propagation model,and fast calculation speed,but it cannot meet the beam line of the new generation of light sources in terms of coherent light processing.Design requirements.There are other models such as SRW,XRT,HYBRID,COMSYL and PHASE.They have their own advantages in the design and simulation requirements of high-coherence light sources,but they also have different defects,which cannot meet the needs of high-coherence beamline design simulation.The MOI(Mutual Optical Intensity)model is based on statistical optics and uses mutual intensity to describe the beam line propagation model of partially coherent light transmission.Using the method of finite element analysis,the wavefront is divided into enough small elements,the mutual optic intensity propagation of partial coherent light in each small element is calculated according to the Fresnel integral,and then the optical effects of each small element are accumulated and superimposed.Finally,the propagation model of partially coherent light in free space is obtained.In the process of processing the small element,it is considered that the inside of the small element is fully coherent,equal in intensity,and equal in phase.However,since the wavelength of synchrotron radiation light is generally on the order of nanometers,the conditions of full coherence and equal intensity are easily met.To meet the equal phase conditions,too many small elements are required to be divided.This undoubtedly requires huge computing power and cannot meet the requirements of efficient design during engineering.Here,we introduce the physical quantity of inplane wave vector to further develop the MOI model.Only less element segmentation is required to obtain more accurate calculation results,realize the needs of efficient and fast simulation,and make the MOI model have higher calculation efficiency and accuracy.Comparing the model before and after the development,it simulates the transmission of part of the coherent light in the free space and in the optical system of the ellipsoidal cylindrical mirror with the surface shape.The excellent characteristics of the developed MOI model are verified,and the new model can give accurate in-plane wave vector propagation results.The mutual optical intensity propagation of partially coherent light through a beamline is calculated for different aperture sizes and positions.The coherence,intensity and phase distribution can be extracted from the mutual optical intensity.The phase distribution depends on the aperture size and position.The results show that the widest flat phase distribution is obtained at the optimized aperture size and position.The aperture plays a more important role for partially coherent light than for incoherent light.The influence of the aperture size and position on the intensity and spot size at the focal plane is also analyzed.Away to obtain a balance between the flat phase distribution area,spot size and intensity for partially coherent light in the beamline is demonstrated.The MOI model is further developed and extended to analyze the mutual optical intensity propagation through the zone plate.It is first time to quantitatively analyze the influence of synchrotron radiation coherence property on the spatial resolution of zone plate.To increase the source coherence length is beneficial to improve the spatial resolution of zone plate,however,apparent diffraction peaks are observed on the focal plane.To reduce the sizes of central stop and order selecting aperture(OSA)can improve the signal-to-noise ratio at cost of the photon flux of the focus spot.The MOI model is used to analyze the coherence propagation through the STXM beamline at Shanghai Synchrotron Radiation Facility.The coherence property increases and the photon flux decreases with the exit slit size decreasing,which is beneficial to improve the spatial resolution of the zone plate.Therefore,the high spatial resolution of STXM technology can be achieved by balancing the coherence property and photon flux.