Research On Coherent Diffraction Based Light Field Reconstruction Technology And Its Applications

Optical imaging technology is one of the important ways for human beings to understand and explore the objective world,and it has been widely used in biomedical science,material science,industrial measurement and other fields.In the optical imaging system,the phase information of light waves contains rich object structure information.However,the existing photodetectors can only record the intensity of the optical field,the phase information cannot be measured directly.Therefore,how to recover the phase information according to the distribution of intensity is an important topic in the basic research of modern optics.Coherent diffraction imaging(CDI)is a type of phase retrieval technology that uses the diffraction patterns to reconstruct the wavefront information through the iterative algorithm.CDI has the advantages of no reference light,Simple experimental setup and can reach the diffraction-limited resolution in theory.Therefore,it has great application potential in the fields of biological cell imaging,X-ray and other short wave imaging and wavefront measurement.Among them,the variable parameter CDI technology obtains multiple diffraction images by changing the parameters of the system,and has attracted extensive attention due to its great flexibility and stable convergence.At present,this technology is in the stage of rapid development.The key technologies in this method,such as the design of optical system,the conditions and speed of data acquisition and the optimization of iterative algorithms also need to be further studied and discussed.Therefore,this paper focuses on the coherent diffraction phase retrieval technology based on variable parameters,and studies two kinds of imaging systems: multi-distances and multi-wavelengths.In view of the possible problems in different systems,we proposed the corresponding solutions to further improve the performance of the imaging system,so that it can meet the application needs of various scenes.In addition,this paper also explores the application of the light field recovery technology based on coherent diffraction in the field of optical fiber modal decomposition and optical encryption.The innovations and main research achievements of this paper may be summarized into the following three points:(1)In order to avoid the mechanical movement of the multi-distance CDI system during the image acquisition process,a static data acquisition scheme based on spatial phase modulation is proposed in this paper.In this method,the spatial light modulator(SLM)is placed on the spectrum plane of 4f system as a dynamic diffraction element,and the diffracted images with different distances are collected by loading the Fresnel lens phase functions with different focal lengths onto the SLM.In addition,in order to reduce the influence of coherent background noise on the reconstructed image quality,in this paper,the speckle field at the optput of multi-mode fiber is added to the systems as illumination.Since the interference between the multi-modes is equivalent to the local averaging of the diffraction image,the contrast and signal-to-noise ratio of the reconstructed image are improved,so the resolution of the imaging system is also improved.Through the comparison experiment with plane wave illumination,that is,in the same measurement conditions,speckle illumination can improve the resolution under plane wave illumination from 28.51 lp/mm to 40.32 lp/mm.In addition,we introduce the multi-mode fiber into single-beam phase retrieval technology for the first time,which can provide some new research ideas for the field of fiber imaging.(2)This paper focuses on the key problems in multi-wavelength CDI system.Firstly,in order to eliminate the influence of wavelength dispersion,an adaptive dispersion correction method based on Sellmeier dispersion equation and search criterion function is proposed in this paper.The experimental results show that the resolution can be improved from 20.16 l P/mm to 57.02 lp/mm after chromatic dispersion correction.The wavelength dispersion correction method proposed in this paper is realized by software calculation,which has the advantages of simple optical setup,low cost and no hardware device,and provides a new and effective solution to solve the chromatic aberration problem in imaging system.Next,in order to avoid multiple exposures and enable high-speed data collection,an innovative setup for the single-exposure multi-wavelength diffraction imaging system based on a blazed grating is proposed.Since the blazed angle varies with the wavelength,the diffraction patterns for the individual wavelengths can be separated from each other and recorded in a single measurement at one time.We provide a dynamic imaging video to demonstrate that this system has high temporal and spatial resolution.In order to alleviate the problem of reduced resolution and signal-to-noise ratio(SNR)of reconstructed images caused by the limited dynamic range of existing detectors(such as CCD or CMOS),this paper proposes a high dynamic range(HDR)imaging technology based on multi-exposure is to improve the resolution of CDI.In our method,the HDR image containing diffraction information of the sample is used in an iterative reconstruction algorithm to improve the resolution and SNR of reconstructed image.We have experimentally verified this method on a multi-wavelength CDI system.The experimental results show that the reconstruction resolution based on HDR technology is increased from the 40.32 lp/mm to 71.84 lp/mm,and the SNR of the reconstructed image contrast is increased by 3d B.This method is simple and effective,and provides a reliable and easy method to improve the resolution of imaging system without additional hardware conditions.(3)In terms of application research,this paper explores the application of optical field recovery technology based on coherent diffraction in the field of fiber modal decomposition and optical encryption.Firstly,in the field of fiber modal decomposition,the modal decomposition method based on the complex amplitude information of the optical field is proposed and verified experimentally using a step-index communication fiber.The experimental results show that whether it is based on digital holography or based on coherent diffraction,the correlation coefficients is more than 0.98,the mean square error is kept within 0.040,and the average power error is about 3.53%.This method is simple,reliable and does not need numerical fitting.Therefore,it has the potential to become a technical solution to promote the application and development of optical fiber field.In the field of optical encryption,an optical diffraction encryption system based on multiwavelength and multi-distance is proposed in this paper.The encryption system adds the parameters of wavelength and distance as the key at the same time,which not only expands the secret key space,but also improves the security of the system.The simulation results show that the encryption system has the strong key sensitivity and robustness.