Research On Linear Polarization Measurement Based On Spectral Modulation

Absorbing aerosols are one of the most important factors affecting global and regional climate.The spectral and polarimetric properties of absorbing aerosols in the ultraviolet-visible band can be used to detect them effectively,but traditional spectral measurement and polarimetric measurement techniques make it difficult to detect the continuous spectral information and polarization information in this band simultaneously.Spectral Modulation(SM)polarization measurement technology combines spectral measurement technology and polarization measurement technology and uses the spectral modulation module to modulate the polarization information of incident light in the spectral dimension,enabling snapshot acquisition of continuous spectral and polarimetric information of the target.The orthogonal dual-channel SM system is an optimized detection scheme of spectral modulation technology,which can overcome the channel crosstalk in the frequency domain and can greatly improve the recovered spectral resolution compared with the single-channel SM system.In order to achieve the effective detection of absorbing aerosol and develop a new generation instrument for aerosol polarization remote sensing detection,this paper applies SM technology to the ultraviolet-visible light band,and conducts in-depth research on the dual-channel linear polarization measurement system based on SM.The specific research contents include the following aspects:1.Firstly,the modulation mechanism of the SM technique was studied.We derived the polarimetric measurement model of the spectral modulation module,analyzed the effect of the spectral sampling rate and spectral spreading effect of the spectrometer on the modulated spectrum in depth.The polarization measurement error model of the spectral modulation module is also derived,and the influence of single and coupled error sources on the polarization is quantified and the error tolerance values of the spectral modulation module are given according to the corresponding polarization measurement accuracy.2.The principal prototype of the Ultraviolet-Visible spectropolarimeter(UVISP)has been developed.The key components of the spectral modulation module were analyzed.The retardance of the Fresnel Rhomb for different angles of incident light and the design method of a“athermal”multiple-order waveplate were investigated.Next,the radiometric model of UVISP is derived according to the optical design of the instrument,laying the foundation for the subsequent determination of the calibration scheme and the demodulation of the polarization information.3.A study of the geometric calibration,spectral calibration,radiometric calibration and polarimetric calibration of UVISP has been completed.We used the geometric and spectral calibrations to achieve the field of view matching and spectral matching of orthogonal modulated spectra.The problem of inconsistent spectral response of the orthogonal channel is solved using the absolute radiometric calibration method.According to the characteristics of the radiometric model of the instrument,a polarimetric calibration method is given.The measurement results of fully and partially polarized light show that the polarimetric calibration method can effectively improve the polarization measurement accuracy of the instrument,and the accuracy can reach 0.01 when the polarization degree of incident light is about 0.2.4.Finally,outdoor sky scanning experiments with UVISP and the sun photometer(CE318N)were carried out.The experimental results show that the deviation of UVISP spectral measurement is within 0.1 nm;the difference of polarization degree measured by the two instruments in the cross-band is less than 0.01;the average radiance difference of the two instruments in the two contrast bands of 440nm and 500nm is 5.3%and 3.6%.The above results show that UVISP can accurately obtain the spectral polarization information of atmospheric scattered light in the ultraviolet-visible light band,which can be applied to the detection of absorbing aerosols.