Study On Several Key Techniques Of Snapshot White-light Channeled Imaging Polarimeter

Polarization imaging is a technique that detects the polarization state of light from a target scene.Compared with traditional photoelectric imaging techniques,polarization imaging provides not only the information about the optical intensity of the target optical radiation but also the polarization degree,angle and ellipticity of the detected target scene.This technology has many important applications,such as remote sensing and atmospheric observation,target recognition,astronomical and geographic detection,machine vision,precision machining and measurement,biological medicine,and military investigation.Simultaneous multi-polarization imaging can obtain different polarization states in one image.Channel-modulated polarization imaging is a snapshot simultaneous polarization imaging technique that can obtain several or all polarization Stokes vectors of the target under the condition of one-time exposure.Channel-modulated simultaneous polarization imaging has several advantages,such as snapshot,simple structure,and low energy consumption.It also does not need image registration.Thus,channel-modulated full polarization imaging is expected to be a key research direction in the future.On the basis of the structure and theory of snapshot white-light channel modulation multi-polarization imaging,several key problems of this technique are studied in depth to perfect its basic theory,determine the allowable error range of each component,and optimize its structure.Major works and achievements are enumerated below.(1)The influence of the grooving density of grating on imaging results of dispersioncompensated polarization Sagnac interferometer(DCPSI)is determined.The theory of DCPSI requires that the grating period is much larger than the wavelength.Thus,the diffraction angle of grating is approximately proportional to the wavelength.However,the grating period is considerably greater than the wavelength,and a clear boundary that the grating period is larger than the wavelength multiple is absent.The limit condition of the grating line logarithm is presented,a clear index for the selection of grating is provided,and the theory of grating in DCPSI is improved.Based on the detailed study of basic theory of grating diffraction angle in DCPSI,without an approximate calculation,derives the relationship between the diffraction angle and the period of interference fringes of DCPSI,the mathematical expression of the grooving density of grating is obtained,which is consisted of the order number of diffraction,the focal length of imaging lens,spectral bandwidth of incidence beam and the length of the imaging sensor along the direction of interference fringes.Experimental results show that the formula is correct.This conclusion provides a reference for researchers to select grating parameters.(2)The effect of multi-order diffraction of grating on the imaging quality is presented.In DCPSI,two transmission blazed gratings are used to generate multiple diffraction beams.Most of these beams eventually reach the imaging sensor to participate in imaging,causing various interferences in the final imaging and resulting in a lower imaging effect than expected.The energy proportion of each order and its energy distribution along the spectral have been obtained by studying the theory of grating multi-order diffraction.Imaging quality may be improved by eliminating the energy of higher diffraction orders by a double telecentric light path to reduce the influence of multi-order diffraction on the imaging results.These conclusions have been verified by experiments and simulations.Meanwhile,simulation results show that the dual-telecentric light path method can achieve certain effects but cause some new problems,such as limiting the angle of field of view and reducing the light flux.(3)It is indicated that the WGBS substrate significantly decreases the visibility of the obtained interference fringes,and a method to completely compensate the error introduced by the WGBS substrate is presented.Large differences still exist between the experimental and simulation results even with a smaller grating diffraction angle and using bi-telecentric light path to reduce the influence of multistage diffraction.Simulation results show that the error is mainly caused by the WGBS substrate,the actual WGBS that used generally adopts a single surface grid plating process,namely,the direct plating aluminum grids on a glass substrate surface,two arms in structure differs a WGBS glass substrates is caused by this structure form,lead to the "DC"(Dispersion Compensation,Dispersion Compensation)of DCPSI affected.This paper proposes a method to eliminate the error introduced by the WGBS substrate by using a compensation plate.A compensation plate identical to the substrate is introduced in the arm without the WGBS substrate.Relative to the ideal state,the imbalance between the two arms is caused by the fact that only one of the two arms of the interferometer has a glass substrate,and a compensation plate is introduced in the other arm to rebalance the two arms,which can theoretically completely eliminate the errors introduced by the WGBS substrate.Simulation results show that the method can completely eliminate the influence of the WGBS substrate on the ideal structure,and the experimental results also prove the effectiveness of the method.(4)The allowable range of the arm length difference of DCPSI is given.DCPSI requires that both arms have the same length.However,during processing,assembly,and debugging,the arm lengths are not exactly the same.The allowable range of arm length difference is determined by studying its influence on measurement results.Analysis of the influence of the position of the mirrors on the instrument shows that the arm length difference eventually leads to the different periods of interference fringes of different wavelengths,forming a similar effect of “dispersion”.In experiment,when the imaging size of the spatial domain is fixed,after the superposition of sinusoidal fringes with different periods,only when the difference of periods is greater than a certain value can the interference fringes with different wavelengths be indistinguishable in the edge region.Based on this theory,the mathematical expression of limiting arm length difference is given.The correctness of the expression is verified by experiments and simulations.(5)Polarization Sagnac interferometer with reflective grating for white-light channeled imaging polarimeter is proposed.The polarization Sagnac interferometer with reflective grating for white-light channeled imaging polarimeter(RGPSI)can realize the multi-polarization imaging of snapshot white-light channel modulation.The structure consists of two identical reflective flare gratings,a polarization splitter,a polarizer,and an imaging system(camera).Calculation results show that the included angle between the two beams generated by RGPSI is close to 0 under ideal conditions,and the shear distance is approximately proportional to the wavelength.In other words,the interference fringe generated can be analyzed in accordance with the theory of DCPSI,and the polarization information can also be demodulated by the reference light method.Theoretical analysis and experiments show that RGPSI can realize the function of DCPSI.RGPSI has obvious advantages: it only has 6 dof of installation and adjustment(without considering the coplanar element center),eliminates the influence of transmission grating substrate(generally 3 mm thick)on the beam,and only needs to consider the support of three elements,that is,the instrument structure becomes simple and the difficulty of assembly and adjustment is reduced.