Research Of Hybird-Coplanar Traveling-Wave Photodiode

Photodiode is a key component in the optical fiber communication system,and its performance determines the transmission distance and transmission capacity of the entire system.Especially in optical heterodyne millimeter wave system,it is necessary that high-speed and high-power photodiodes have been used to convert the optical signal into millimeter-wave signal.The response bandwidth of photodiode,so far,has been reported to be up to terahertz frequency orders,but the microwave signal power output from the photodiode is only on the order of milliwatts,and the transmission distance is only a few meters.Therefore,in wireless communication system,transimpedance power amplifier has to connect to the end of the photodiode to amplify the power and increase the transmission distance,but the amplifier will reduce the response bandwidth of the system and introduce noise.A hybrid-coplanar traveling-wave photodiode was designed in this thesis,in which a directional coupling optical waveguide structure has been used to improve the nonuniform distribution of photocurrent,a uni-traveling carrier structure was introduced,meanwhile,the waveguide and electrode was designed to realize the traveling wave structure to improve the response bandwidth and output power of the photodiode.The research mainly focuses on the hybrid-coplanar traveling-wave photodiode in this thesis,and the following results were obtained:1.The modes in the active optical waveguide(active waveguide and active directional coupling waveguide)were analyzed by the effective refractive index method and the commercial software Rsoft.Especially,the thickness of the active layer was0.12?m(E^y_mn Mode)or 0.14?m(E^x_mn Mode)at the highest light absorption efficiency.The optical power distribution of the active directional coupling waveguide with the coupling layer thickness of 0.08-0.10?m was the most uniform.2.Two mode size converters were proposed,namely the double taper mode size converter and the taper mode converter.Their structure and coupling efficiency were determined by the commercial software Rsoft.The mode coupling efficiency of the double taper mode size converter with the upper tapered waveguide length of 1250?m and the lower tapered waveguide length of 1000?m was 83%,the mode coupling efficiency of the taper mode size converter with the length of 800?m was 90%.3.The epitaxial layer structure of the hybrid-coplanar traveling-wave photodiode was designed according to the theory of uni-traveling carriers.The hybrid-coplanar traveling-wave photodiode was analyzed using transmission line theory,and 4 sets of intrinsic layer thickness and waveguide ridge width satisfying the impedance matching conditions were obtained.Meanwhile,the electric phase velocity and attenuation constant were calculated,and the characteristics impedance was simulated by the commercial software CST.Then,the semiconductor software Silvaco ATLAS was used to simulate the photoelectric characteristics of the hybrid-coplanar traveling-wave photodiode,in the photodiode,the relationship between the bandwidth limited by the drift time of photogenerated electrons and the thickness of the intrinsic layer is found.An output photocurrent of 332 m A,a photoelectric responsivity of 1.12 A/W,and an external quantum efficiency of 88.7%were expected.4.The velocity mismatch impulse response and frequency response of the hybrid-coplanar traveling-wave photodiode were studied,and the total bandwidth of the photodiode was ideally about 83.205 GHz by the bandwidth of the transit time limit and the bandwidth of the velocity mismatch limit.Finally,the small signal model of the hybrid-coplanar traveling-wave photodiode was analyzed by the optical heterodyne method.