Design Of Optical Components For Terahertz Detecting Imaging Array

The Terahertz(THz)detection imaging technology is such a research hotspot in the field of Terahertz Science and Technology.The THz detection imaging technology based on the micro bridge structure has characteristics of room temperature operation,real time array imaging,compatibility with the silicon CMOS technology and high detecting performance and so on.It has broad market prospects in the field of the non-destructive inspection,the security and protection,the biomedical imaging etc.In this paper,three parts of the THz optical assembly for the THz detection imaging system are studied including the THz beam homogenizer design,the THz imaging lens design,and the new type THz window design.A THz beam homogenizer based on a micro-spherical mirror array is presented to flatten the THz beam's energy distribution and expand the THz beam simultaneously.Combined with the TracePro software,the modeling and ray tracing simulation are accomplished,and factors influencing the effect of beam expansion as well as homogenization are analyzed.The configuration parameters are finally optimized where the cell pitch of the micro-spherical mirror is 600 ?m with a focal length of 2.26 ?m.At the meanwhile,the number of cells in the array is 31*31 and the angle between it and the incident THz light is 45 degree.A flattened THz beam spot with an 8-times expanding in the horizontal direction and a 10-times expanding in the vertical direction respectively are achieved at a distance of 200 mm away from the THz source which meets the design requirements.A THz imaging lens that contains three pieces of glasses is designed.The proposed lens is made of high-resistance silicon(HRFZ-Si)with a layer of 18?m thick parylene-C thin film coated on the front surface of each single lens as anti-reflection coating.The optimized effective focal length of the THz imaging lens is 36.4 mm,and the effective relative aperture value is 0.81,while the effective length of the designed system is 8.99 mm.The field-angle of the designed THz imaging lens is 20 degree.The evaluation results show the lens can distinguish the minimum size of 3.23 mm 1 meter away.The ray fan curve indicates that the imaging lens designed has good correction to the primary spherical aberration.In addition,the MTF curve shows that the overall image of the lens is pretty good,and the resolution of the details can meet the needs of terahertz detection imaging array.At the same time,the parameters that influence the Fresnel lens design are studied and a large relative aperture Fresnel lens with parameters F=0.33 and f =38 mm is presented which achieves good focusing property in the 2.52 THz waveband.The permeation characteristics of the HRFZ-Si coated with different materials are investigated in the THz band.The anti-reflection coating of PI thin film is prepared by spin coating on the surface of HRFZ-Si.Measurement results show that the THz transmittance increases by 11% with a 19 ?m thick PI thin film coated on the front surface of HRFZ-Si,while a double coated 15 ?m thick PI thin film increases by 35 %.Based on the simulation optimization analysis,the periodic sub-wavelength relief structure with periodic pitch of 27 ?m,duty cycle of 0.75 and relief depth of 13 ?m is designed to reduce the reflectivity of HRFZ-Si to 3.8% in 2.52 THz waveband.Besides,in order to reduce the influence of 7~14 ?m background infrared radiation to THz detector,a patch-type square metal mesh array based on Frequency Selective Surface(FSS)theory is designed.The influence of different parameters on the filtering performance is analyzed by CST software.FSS structures with different sizes are prepared respectively.And then,the transmittance curve in middle infrared band and THz band are measured respectively.The measurement results show that the FSS structure of the infrared light transmittance attenuations to 20 % while the transmittance THz light is above 97 %,which achieves the designed goals.