Technologies Of Thz Imaging System Front Ends

Terahertz wave,which is also called as submillimeter wave or far infrared,refers to the electromagnetic wave in the frequency range from 0.1 to 10 THz.Its frequency spectrum lies between microwave and light wave,which leads to the advantages including wide bandwidth,low quantum energy,high penetration,etc..THz technology has wide application prospects in the fields of high-speed communication,biomedical research,radio astronomy,space exploration,and security detection.Among these applications,THz imaging has always been one of the frontier topics and research hotspots,and has attracted interests and investigations by governments and institutes all over the world.THz imaging systems can achieve high resolution imaging,detection of small targets and conceals objects,detection in complex environment,and have been used in the applications of battlefield reconnaissance,disease diagnosis,security checks at public area,nondestructive test and so on.The research level of THz imaging front end,including the performances of highstability THz sources and high-sensitivity detectors have always been the key element in the development of THz imaging systems.The researches of this dissertation,which focus on solid state THz technology,include modeling and optimization of THz semiconductors,as well as development and analyses of key circuits in imaging front end(THz mixers and multipliers).The ultimate goal of these researches is breaking the monopoly of foreign technology and realizing total localization of THz imaging system in China by developing home-made THz Schottky diodes,high-performance THz circuits and imaging front end.The main contents of this dissertation are summarized as follows:(1)THz harmonic mixers.Mixer is the most important circuit in THz receivers,which decides the integral performances of the receivers.The researches start with the modeling and analyses of planar Schottky diodes,while modeling technique based on the combination of 3D EM simulation and nonlinear simulation is proposed.In order to improve the performances of Schottky diodes and integral circuits,the influences from parasitics of Schottky diodes to the conversion loss and noise temperature are analyzed,and the designing method of low parasitics planar Schottky diodes is proposed.This method contributes to the optimization and development of home-made THz Schottky diodes in the future.Based on the theories of Schottky diodes,the 220 and 330 GHz hybrid integrated mixers were developed.Measured results show that both mixers reach international advanced level.Apart from that,due to the demand for integrated circuits,the 220 GHz Ga As monolithic integrated mixer was developed based on domestic technology.The Schottky diodes are directly integrated on the circuit substrate to improve the integration level.Meanwhile,the structures and dimensions of the mixer diodes were optimized to reduce parasitics of the diodes,thus decreasing the conversion loss and noise temperature of the mixer.The experimental results are in good agreement with the simulation predictions,which verifies the modeling of mixer diodes and the proposed circuit optimization methods.(2)THz high efficiency multiplier.Nowadays,solid state multiplier chains are most commonly used to realize transmitters in THz radars.In this dissertation,the 220 GHz high efficiency tripler was developed based on domestic Schottky diodes due to the demand from 220 GHz Vi SAR system.In this part,the accurate 3D electromagnetic model of the multiplier diode was firstly established based on its working principles.In order to take the thermal effects of diodes into consideration,the “electro-thermal”analysis was introduced in the researches of THz multipliers.By establishing “electro-thermal”model of Schottky diodes and introducing anode temperature into simulation of the multipliers,the accuracy of the analyses can be improved.This usage of “electro-thermal”model of THz multiplier diodes in this dissertation is the first case that this method has been used in China.Based on the thermal analyses,the Al N substrate with high thermal conductivity was used to realize 220 GHz high efficiency tripler.Experimental results show that the peak output power and the peak efficiency of the proposed 220 GHz tripler are 38.2 m W and 17.8%,respectively.To the author's knowledge,this 220 GHz tripler exhibits highest peak output power and the peak efficiency among triplers in the similar frequency band.Meanwhile,the simulated results based on “electro-thermal”model are more consistent with measured results compared with simulation based on traditional methods,which verifies the “electro-thermal”analysis in this dissertation.(3)THz integrated receiving front end.Miniaturization and integration have become the inevitable trend of THz front end and system.In this dissertation,a 220 GHz wideband receiver front end is proposed.According to the simulated and experimental results,the direct combination of independent mixer and tripler will lead to the deterioration of the performances of mixers and wideband receivers.To overcome this problem,the method of building matching network based on load-pull techniques and harmonic balance simulation between the cascade circuits in wideband submillimeter receivers was proposed.The 220 GHz wideband integrated front end developed based on this method exhibits better performances and half size compared with direct combination of independent 220 GHz mixer and tripler.In this way,the miniaturization and integration of the front end are realized while the performances are guaranteed.(4)System verification for THz imaging front end.Based on the mixer and multiplier circuits in THz imaging front end developed in this dissertation,the experiments of THz imaging system were carried out,while the performances of the imaging front end can be verified by the imaging results of the systems.Firstly,the 220 GHz radiometer and330 GHz radiometer are developed based on the low-noise 220 and 330 GHz mixers.Experimental results show that the brightness temperature of the 220 GHz and 330 GHz radiometers are 0.46 K and 0.33 K,respectively.The two radiometers were utilized to develop planar scanning passive imaging systems.The obvious imaging results indicate the good performances of the front end of passive imaging systems.Apart from THz passive imaging systems,the experiments of 330 GHz ISAR system and 220 GHz Vi SAR system were also introduced.The 220 GHz mixer,330 GHz mixer and 220 GHz tripler play important roles in these two active imaging systems.The good experimental results of these two imaging radars also verified the good performances of the imaging front ends.