Research On Broadband Terahertz Wireless Communication In Plasma

The plasma is produced when the spacecraft enter our atmosphere with the temperature rise sharply because of friction.As the aircraft carrier of communication,microwave has lower inherent frequency than the plasma that led to interruptions of communications.This is the blackout which is a problem urgently to be solved in the aerospace field.Improving the carrier frequency by using broadband terahertz wave as a carrier is an effective way to solve the blackout.At present,system theory combined with practice research of the propagation of terahertz wave in plasma is short.The thesis focus the forgoing problems and the main works are as follows.The GaAs photoconductive antenna radiation spectrum characteristics of THZ wave on the radiation mechanism was studied.Pump laser illuminates photoconductive antenna to electron-hole pair,which can be regards as plasma.According to the theory of collective oscillation of plasma,the process of Langmuir wave converting to electromagnetic wave in non-uniform plasma is investigated and the theoretical model of the laser induce plasma to radiate terahertz waves from GaAs photoconductive antenna was proposed.Based on this model,the spectral bandwidth and the peak frequency of terahertz waves generated by GaAs photoconductive antenna were calculated by statistical method.The theoretical calculation results are consistent with the experimental values.Then,the effect factors of peak frequency of terahertz waves generated by GaAs photoconductive antenna were discussed,which provides theoretical foundation for design and fabricate GaAs photoconductive antennas that can generated high power terahertz waves with the peak frequency in the atmosphere transmission windows.A terahertz wireless communication link based on a terathertz time domain spectroscopy system(THz-TDS system)was built.The effect of the transmission of square waves at different terahertz carrier waves and different modulation frequency was studied,and the results shows that the peak-to-peak value of received square wave is proportional to the bias electrical field of GaAs photocondctive antenna.The effect of sound signal frequency to terahertz communication system with GaAs photoconductive antenna as a carrier wave source was analyzed,and the wireless transmission of whole sound frequencies signal was realized with THz-TDS system at atmospheric environment in lab.The research is mainly focused on the propagation characteristics of broadband THz wave in homogeneous plasma and inhomogeneous plasma.The propagation characteristics of broad band terahertz waves in homogeneous plasma:The homogeneous plasma was generated by DC glow discharge plasma,and its charactieristic parameters were diagnosed by Langmuir probe method and calculated by discharge current theory.The results show that the plasma concentration of homogeneous plasma is ranging from 1017 m-3 to 1019 m-3.The propagation characteristics of broad band terahertz waves in the glow discharge plasma and arc plasma was investigated.The reflectivity,transmissivity and attenuation of terahertz waves in the homogeneous plasma generated by DC glow discharge were simulated,and both the simulation results and experiment results show that terahertz waves can freely transmit in the DC glow discharged homogeneous plasma with the same concentration。The propagation characteristics of broad band terahertz waves in inhomogeneous plasma:The helium jet plasma and neon jet plasma were generated by high voltage radio frequency discharge.The plasma characteristics of both plasmas were diagnosed by emission spectrum diagnosing method and terahertz wave diagnosing method,and both methods show that the plasma concentration is ranging from 1017 m-3 to 1018 m-3.The propagation characteristics of broad band terahertz waves were experimentally investigated in jet plasma with sheath and high collision frequency,in jet plasma with thin sheath and lower collision frequency and in jet plasma with low collision frequency,and the transmissivity and attenuation of terahertz waves in those plasmas were calculated.The results show that terahertz wave with the frequency of 1.9 THz has a large attenuation about 22 dB in the jet plasma with sheath and high collision frequency,and the attenuation is less than 8 dB at other frequencies.The 5 lays structure model of jet plasma was proposed to simulate the propagation characteristics of terahertz waves by SMM method,and the results shows that the terahertz waves were attenuated sharply in sheath with high concentration and high collision frequency,which is consistent with experimental results.In conclusion,broad band terahertz waves have good penetrability in plasma except that some frequencies with attenuation,so the terahertz waves with low attenuation band can be used as carrier wave in communication to slove the "blackout".The thesis provides both experimental and theoretical foundation to terahertz wireless communication for solving the blackout by using terahertz wave as the carrier.