Research On Measurement Error And Calibration Of Intensity Modulated Spectropolarimetric Imaging System

In recent years,with the development of optical technology and the increase in the demand for information of optical remote sensing in various fields,a new type of optical detection technology,i.e.,spectropolarimetric imaging technology,has been developed.It combines polarization imaging technology with spectral imaging technology,and obtains information such as spatial intensity distribution and physical and chemical characteristics of the target.The amount of information acquired by this technology is greatly increased compared with that of the conventional optical detection technology.Therefore,the spectropolarimetric imaging technology has been gradually applied in various fields.As an advanced technique in the field of spectropolarimetric imaging field,polarimetric spectral intensity modulation(PSIM)technique can realize the static,snapshot and full-Stokes spectral measurement of the target light,which is of great importance for guaranteeing the real-time detection and the accurate information acquisition of a moving target.However,the PSIM technique has not been widely used in engineering since it was proposed,mainly because the measurement error analysis and compensation system for this technique is not complete,and the mechanism of action of the error source is not sufficiently studied.Furthermore,the study of the system polarimetric calibration is less.As a result,the system's measurement error and calibration researches are the keys to promote the development of the PSIM technique.This dissertation is focused on the measurement error and calibration of the intensity modulated spectropolarimetric imaging system.The research is as follows:The modulation process and the Fourier transform method of reconstructing the Stokes spectrum of the PSIM technique are studied.By considering factors such as the channel separation conditions,the spectral resolution of the reconstructed spectrum,and the resolving power of the spectrometer,the formula of designing the thickness of the high-order retarder is given,and its correctness is verified through the simulation.On this basis,an optical scheme of the intensity modulated spectropolarimetric imaging system including fore-optics,PSIM module,imaging optics and Offner imaging spectrometer is proposed.According to this scheme,an airborne intensity modulated spectropolarimetric imaging system for remote sensing is designed,which has the advantages of simple and compact structure,and the scheme can realize the design of a large field intensity modulated spectropolarimetric imaging system.When analyzing the system's measurement error of the polarization spectrum,this dissertation studies the strong polarization devices and the weak polarization devices in the system respectively.The strong polarization devices mainly include the high-order retarder and the linear polarizer in the PSIM module,and the weak polarization devices refer to the optical system composed of the lens interface,the mirror interface and the diffraction grating.The strong and weak polarization devices will both change the polarization state of the incident light,which affects the system's measurement results of the polarization spectrum,but the interactions with light between the two types of polarization devices are different,thus the description and analysis methods taken are also different.For the strong polarization devices,the influences of the polarization errors of the PSIM module,including the orientation errors of the polarization elements and the phase retardance errors of the high-order retarders,on the measurement result are studied.Monte Carlo method is used to analyze the quantitative relationship between the polarization errors of the PSIM module and the measurement result of the system.It is found that the orientation errors of the polarization elements and the phase retardance errors of the high-order retarders will reduce the reconstruction accuracy of the Stokes spectrum.In order to suppress the influences of the polarization errors of the PSIM module,an analytical model of the PSIM module with the polarization errors is derived firstly.According to this model,a new method for determining the orientation errors of the polarization elements based on an auxiliary high-order retarder is proposed.This method can determine the orientation errors of the polarization elements in the PSIM module through a single measurement,and the auxiliary high-order retarder does not affect the normal use of the PSIM module.After the research,it is found that the orientation errors of the polarization elements will also affect the calibration results of phase retardance errors of the high-order retarders.For this reason,the calibration method using a 22.5° linear polarization reference beam is improved.On the basis of determining the polarization errors of the PSIM module,the reconstruction model of the Stokes spectrum is derived and a compensation algorithm of the polarization errors of the PSIM module is presented.Through the simulation analysis and the confirmatory experiment,it can be concluded that the calibration method presented in this dissertation can accurately and efficiently determine the polarization errors of PSIM module.The compensation algorithm can reduce the influence of the polarization errors of the PSIM module on the measurement result effectively.However,the compensation algorithm is only appropriate for the system that the polarization effects of the optical system can be ignored.In order to make the research more universal,the influences of the polarization effects of the weak polarization devices are further studied.The influences of the diattenuation and phase retadance of the optical system on the measurement error are analyzed.On this basis,the polarization effects of the lens interface,the mirror interface and the diffraction grating in the optical system are analyzed interface-by-interface.Through calculating the polarization effects of the lens interface and mirror interface by the method of polarization ray tracing,it can be found that the existence of the polarization-sensitive interface increases the polarization sensitivity of the whole system.The influences of the grating period,groove depth,and incident angle of the light on the polarization characteristics of the grating are studied by the finite difference time domain(FDTD)method.Based on the research results of the polarization effects of optical system,a method of polarization-maintaining optimization design for optical system is proposed.By optimizing the self-designed airborne intensity modulated spectropolarimetric imaging system,it indicates that the method can reduce the polarization effects of the optical system,which is an effective approach to reduce the system's measurement error of the polarization spectrum during the design phase.Based on the researches of the polarization errors of the PSIM module and the polarization effects of the optical system,the coupling effects of various error sources and their combined influences are studied from the perspective of the whole system.Through the detailed derivation,the feasibility of the modularization modeling method for the system is demonstrated.The Mueller matrix of the PSIM module is also studied emphatically for a oblique incident beam.Based on this,a system-wide polarization radiation transfer model is established.According to this model,a polarimetric calibration method for the whole system is proposed.The method uses three linearly polarized light with different polarization states and unpolarized light as the reference light respectively,and uses a high-order retarder as an auxiliary device to determine the polarization effects of the optical system,the polarization radiation transfer coefficient of the system,the orientation errors of the polarization elements and the phase retardances of the high-order retarders.When determining a certain parameter,the influences of other parameters are fully suppressed to ensure the determination accuracy.At the same time,the reconstruction model and method of the Stokes spectrum are improved,and the determination results of the polarization parameters are substituted into the calculation of the Stokes spectrum to compensate the reconstruction errors introduced by the various factors.Finally,the simulation and confirmatory experiment for the polarimetric calibration method are conducted.The results show that the polarimetric calibration method presented in this dissertation reduces the reconstruction error of the Stokes spectrum in the whole field of view by one order of magnitude.The method can ensure the system's measurement accuracy of the polarization spectrum.The mathematical model and the methods of analysis,polarization-maintaining optimization,and calibration,presented in this dissertation,are of great significance to the practical engineering application of the intensity modulated spectropolarimetric imaging system.