Continuous-Wave Terahertz Radiation System By Photomixing

With the high resolution terahertz spectroscopy and imaging techniques inbiomedical, environmental monitoring, anti-drug counter-terrorism applicationbecoming more and more important, the research and development of narrowline-width continuous-wave terahertz radiation source has been the focus of attention.The photoconductive antenna based on photomixing occupies an important place inthe THz radiation source, which can not only transform incident laser intocontinuous-wave terahertz, but also realize the radiation of terahertz wave effectively,moreover it has the advantages of the narrow line-width, working at roomtemperature, simple structure, wide tunable range and low price, etc. However, atpresent there are no experimental reports of achieving photon mixing continuousTHz radiation and detection using photoconductive antenna, owing to the influencesof photoconductive antenna radiation efficiency and photon mixing energyconversion efficiency and other factors.Some researches on the generation of cw-THz radiation by photomixing havebeen done in this dissertation, including the design, fabrication, and characterizationof THz polarization beam splitter, the experimental platform of cw-THz radiation byusing photoconductive antennas as THz radiation source and detector, which isdebugged and optimized. The main works of the dissertation are listed as follows:(1) The basic characteristics of photoconductive antenna were introduced, theprinciples and conditions of photomixing were described, and dynamic characteristicof the semiconductor laser output mode was also introduced.(2) The signal-to-noise ratio of the THz continuous wave radiation system canbe improved by Terahertz polarizer. The performance of polarization beam splitterwas numerically simulated by the commercial simulation tool COMSOL in this paper,the performance of commonly used45°polarizing beam splitter by varying theperiod of the wire-grid, the fill factor, and the thickness of the copper werenumerically analyzed. Because it was not affected by the F-P interference effect, the TM polarized transmittance and the reflectance extinction ratios of polarization beamsplitter were improved, when the incidence angle was changed to Brewster angle.Finally, according to the existing laboratory preparation system, a THz polarizationbeam splitter, the period of which is20μm and fill factor is0.55was designed andfabricated.(3) A transmission/reflection characteristic test system of polarization beamsplitter was built, which is based on Backward Wave Oscillator. Test results showthat: when the angle was Brewster angle incident, the TM polarization transmissionrate was a smooth curve and the reflection of extinction ratio in0.34THz was14.5dBhigher than45°. At present, there are still no reports about Brewster anglepolarization reflection analysis and characterization of THz polarization beam splitter.Terahertz Time-Domain Spectroscopy system was used to test the broadbandtransmission characteristic. The experimentally results show that when thetransmittance of TM-polarized wave of the fabricated polarizer beam splitter is over80%, the extinction ratio is better than20dB between0.2-1.5THz. Experimentalresults are in agreement with simulation.(4) The experimental platform of cw-THz radiation by photomixing was builtusing photoconductive antennas as THz radiation source and detector, andsuccessfully produced and detected terahertz continuous signal by this method indomestic. The coincidence degree of the beat frequency laser beams in the space wasadjusted and the photoconductive antenna was optimized by adjusting the stability ofthe optimal two semiconductor laser output laser mode, the signal-to-noise ratio is40dB, the frequency of cw-THz radiation signal is0.41THz, which was successfullyobtained. Analysis of the experimental results show that the stability of laser outputmode is the key to enhance the signal-to-noise ratio.