Hyperspectral Imaging Technology Based On Acousto-optic Modulation Type In The Visible Band

As the new type of spectroscopic element where ultrasound and light waves can interact wit h acousto-optic interactions in anisotropic media,the acousto-optic tunable filter under Bragg modulation could be regarded as both a spectroscopic element and polarizing element.This kind of filter has been widely used in fields of hyperspectral imging technology due to its prominent advantages of large aperture angle,high diffraction effic iency,and fast tuning speed.C urrently,foreign research on hyperspectral imaging technology based on acousto-optic tunable filter is more mature,while the domestic research starts late,which is basically at the stage of basic theory and exploratory experime nts and still needs lots of in-depth theoretical and experimenta l research.In view of this,based on Prague modulation of acousto-optic tunable filter technology,this paper carries out the relevant theoretical analys is and experimenta l research,aiming t o apply the acousto-optic tunable filter technology in t he field of hyperspectral imaging perfectly and play a positive role in the development of hyperspectral imaging technology in China.Theoretically,beginning w ith the optical properties and acoustic properties of TeO₂ single crystal and based on the basic equations of parametric interaction,this paper has established the theory of coupled wave equation of acousto-optic interaction.According to the coupling wave equation and the moment um matching condition,the basic theoretical tuning modes of two kinds of inc ident light with different polarizat ion directions perpendicular to each other are deduced,thus to select the appropriate inc ident angle and the inc ident direction of the ultrasonic shear wave.O ther performance parameters of two acousto-optic tunable filters are calculated for the experimental requirements.K ey issues of the separation method for the 1st order diffracted light outside the medium,diffraction drift caused by chromatic aberration and reduced RF drive power during the process are solved.The influence of the cone-shaped beam on the internal separation angle,external separation angle,spectral bandwidth and diffraction efficiency of the acousto-optic tunable filter has analyzed and stringent requirements on the front collimating optical system has been put forward based on this.In this experiment,the relationships between the inc ident wavelengt h and the ultrasonic driving frequency,the incident angle and the ultrasonic driving frequency of the designed non-collinear acousto-optic tunable filter,as well as the performance parameters of the spectral bandwidth,diffraction efficiency,spatial resolut ion and diffraction drift outside the crystal have been measured in details,thus to optimize the performance according to the measurement results unt il it meets the hyperspectral imaging requirements.The single crystal hyperspectral imaging experiment system has been built based on the design of the acousto-optic tunable filter,the in fluence of the chromatic aberration of the broadband light source within the tuning range on the drift of the diffracted light has been firstly studied via experiments,the relation function between the image drift and the wavelength of the incident light has been given to provide a reliable basis for the design of the rear optical receiving system.The far-fie ld imaging experiment has been carried out with the built hyperspectral imaging system w ithin the spectral region of 419.48⁸65.07 nm and detection range of 100²00 m.Detailed experimenta l demonstration on the feasibilit y of the double-filter ing technique has been carried out for the first time with the double crystal,and the center wavelength interval of the diffracted light is gradually reduced by tuning the ultrasonic driving frequency attached on the two acousto-optic tunable filters,thus to obtain a narrower spectral bandwidth and higher signal-to-noise ratio.The absorption peaks of didymium glass have been measured by the single-filtering method and the double-filter method respective ly under this experimental system and the superiority of the double-filtering method has been detected through comparison.The secondary filtering method is applied to the far field target imaging,the far-fie ld target imaging is performed by tuning the ultrasonic drive frequency interval at different center wavelength intervals and good imaging results are achieved.Through the experimenta l study,this system has been further tested to get a clear image with the pr oposed secondary filter method by ensuring a high spectral resolut ion at the same time,achieving the true ‘integration of image and spectrum'.A method based on the acousto-optic tunable filter under the broadband light source of rapid ly calibrating the p hase retardation of the liquid crystal phase variable retarder and the relationship between the wavelength of the incident light and the driving vo ltage has been put forward.This method could effective ly calibrate the phase retardance in any wavelength in the tuning range at different drive voltages.The hyperspectral polarizat ion imaging system has been built based on two AO TF and two LC VR,the targets with clear polarizat ion direction in t he room have been identified and the intensity relat ionship betwee n the polarization targets at different phase retardance has studied with details.The imaging effect of the system over the entire wavelengt h tuning range has been tested by tuning the driving voltage of the two LC VRs and driving the ultrasonic frequencie s of the two AO TFs.Polarization imaging o f far field targets has been tested in different diffraction wavelengths and different phase retardances condit ions.The imaging has been studied for the four groups of different phase retardances with the central wavelengths of 457.37 nm,556.31 nm,658.54 nm and 755.55 nm,respectively,the image is clear and the far field targets are dist inct after processing.It has been found through the study that artificial targets of buildings and iron fences with smooth and regular surfaces are clearly articulated;natural targets of bushes with rough surface have no obvious contour rules.Imaging polarization can well distinguish artific ial targets from natural scenes with these phys ical differences.In the end,Stokes imaging has been done with the incident light wavelengt h of 755.55 nm and the central wavelength intervals of 0.4 nm,0.8 nm,1.2 nm and 1.6 nm,respectively.This is also the purpose of this paper: more sophist icated spectral information could be detected and the polarizat ion informat ion could also be obtained based on the system.The method combining hyperspectral resolut ion and polarizat ion detection has greatly improved the capacity of extracting the target information under complex background environments.