Research On Terahertz Imaging Method Based On Spatial Modulation Technology

Because terahertz has many special properties,it is concerned by researchers all over the world.As an important research direction of terahertz,terahertz imaging has very important application value in the fields of human security,medical diagnosis and defect detection.There are two types of terahertz imaging,one is focal plane array terahertz imaging,and the other is single detector terahertz imaging.Both imaging methods have their own contradictory advantages and disadvantages,which restricts the development of terahertz imaging.The technology of terahertz coding technology can solve the problem just mentioned.The key device to implement this technology is a space terahertz modulator,in which a light-controlled spatial terahertz modulator based on silicon material has great potential for exploration.However,due to its own refractive index,silicon has a high reflectivity for both terahertz waves and lasers,which seriously affects its modulation performance and imaging effects.The laboratory proposes a black silicon-based spatial terahertz modulator.The micro-pyramid structure obtained by chemical etching on the silicon surface causes the incident laser to form multiple reflections between the micro-pyramid structures.Thereby increasing the absorption efficiency of silicon for incident laser light.Through experimental tests,it was found that the micro-pyramid structure can reduce the reflectivity of the silicon material to the pump laser from 40% to 13%.And through the THz-TDS system test,it is found that black silicon has better modulation effect than ordinary silicon in terms of modulation performance of terahertz wave,and the modulation depth of terahertz wave at 0.8 THz can reach 95.59%.In order to study the effect of black silicon modulators in actual terahertz imaging,an actual terahertz imaging system based on spatial modulation techniques is needed for experiments.Therefore,this thesis studies the imaging principle of terahertz imaging system based on spatial modulation technology,establishes a mathematical model,and builds a terahertz imaging system.Black silicon and ordinary silicon were used as spatial terahertz modulators for imaging operations and comparative analysis.It is found that the image obtained by black silicon has better definition than silicon,the outline of the terahertz wave is clearer,the imaging precision is higher,and the image has fewer blind spots.We further analyze the influence of terahertz beam distribution characteristics and Gaussian background on imaging results,and propose an effective optimization method.Finally,as imaging scales expand,the use of traditional imaging methods such as point-by-point scanning can dramatically increase imaging time.In order to slow down the sharp increase in imaging time,this thesis proposes to introduce compression sensing technology,and to study its principle and potential applications in terahertz imaging.Using the orthogonal matching pursuit(OMP)algorithm,the simulation experiment of compressed sensing recovery and reconstruction of one-dimensional signal and twodimensional image signal is realized in MATLAB simulation environment.In simulation,the compression ratio can be as low as 21.9%.