| Terahertz technology has been widely used in wireless communications,imaging and biomedical fields.However,the lack of wide-band and compact terahertz radiation sources limites the development of terahertz technology.The idea of using optical heterodyne to generate terahertz waves has been favored because of its advantages of integration,room temperature operation and broadband tuning.This method requires the use of ultra-high-speed photodiode as optical mixer,and the capability of the entire system is decided by the high-frequency response and output power of the photodiode.So far,High speed and high power of the photodiode still remains challenging.Uni-traveling-carrier photodiodes are considered to be one of the best choices for optical mixers due to their high speed,while the distributed traveling wave electrode structures can eliminate the bandwidth limitation of the RC time constant of lumped electrodes.This thesis addresses the design of a large section Uni-traveling-carrier Traveling-Wave Photodiode（LSUTC-TWPD）,and carries out the following researches:1.Through the commercial software Silvaco Atlas modeling and simulation of the photodiode,the internal electric field and band structure energy of the device under the condition of light injection are simulated,and the eigenresponse bandwidth determined by the carrier crossing time of the device is calculated.The response bandwidth of the device is improved by optimizing the depletion absorber,cliff,and collection layers.With a reverse bias voltage of 2V and high-power optical injection of 6×105 W/μm2,the intrinsic 3d B response bandwidth of the device reaches 309 GHz.2.For the purpose of improving the absorption efficiency of the absorbing layer for input light,the method of oblique light incidence is proposed.The maximum diameter of the input spot allowed by the detector is 9μm,facilitating direct coupling with the output spot of single mode fiber.Rsoft software was used to analyze the light field at different inclination angles and the light absorption efficiency at different waveguide lengths,and when the inclination angle was 9°,the active region could absorb up to 78%of the optical power,and the generated photocurrent was evenly distributed along the waveguide length.3.The equivalent circuit model of LSUTC-TWPD is established through the transmission line theory,and then the characteristic impedance and propagation length of the detector are studied.Finally,the frequency response of the detector under the velocity mismatch is analyzed.The results indicate that when the device input is matched,the bandwidth is obviously improved compared with the open circuit,but it will cause the overall output power decrease.Finally,the total response bandwidth under the matching condition is about 273 GHz by crossing the bandwidth of the time limit and the bandwidth of the speed mismatch limit,and the response in the terahertz band is realized.4.The integrated antennas are used to output terahertz waves to realize apparatus integration,and a complementary butterfly antenna with tuned stubs is designed by CST software,which reaches-40 d B at S11 at 300 GHz,the maximum gain reaches 9.72 d Bi,and the relative bandwidth reaches 26%.The CPW transmission line is used to carry out the conversion of the impedance so as to achieve the impedance match between the antenna and the detector.And finally an integrated scheme is given. |
