Study Of Silicon-based Traveling Wave Photodetectors For Microwave Photonics

The emerging field of integrated microwave photonics(MWP)is gaining more and more attentions.High-performance photodetectors are the key devices,and their research is of great significance to the development of integrated MWP.In analog optical communication systems,the insertion loss,link gain,spurious free dynamic range and signal-to-noise ratio of an analog optical link can be significantly improved by increasing the input optical power,and the photodetector needs to increase the saturated output power accordingly.Distributed traveling wave photodetectors(TWPD)are utilized to increase the saturation photocurrents,maintain high bandwidths under the velocity and impedance matching,and they are particularly suitable for silicon photonic integrated circuits.In this paper,the theoretical and experimental studies on silicon-based traveling wave photodetectors for microwave photonics are carried out.The main work includes the following aspects:1.We present the structure of a silicon distributed TWPD with parallel optical feeding.With equivalent circuits of the PD and the CPW electrode,we obtain a complete circuit model of the silicon distributed TWPD,and the frequency response of silicon distributed TWPD is obtained according to the transfer matrix method and the superposition principle.The impedance and velocity matching of silicon distributed TWPD and the capacitive loading effect of PD are analyzed.2.Two different distributed equivalent circuits of CPW electrode are compared.Through the analytical model established,the frequency response of periodically distributed TWPD is calculated,and the design of CPW electrode and optical delay is optimized.An aperiodically distributed TWPD whose CPW electrode is open at the input terminal is proposed based on the capacitive loading effect of PD array and aperiodic loading technique.Its bandwidth(25.7GHz)is slightly lower than the bandwidth(26.5GHz)of periodically distributed TWPD with with a 50 ? matching impedance at the input terminal,but the former doubles the photocurrent on the load in a frequency range from DC to 24.7 GHz.3.The layout of distributed TWPD with different structures is designed,such as different impedance and velocity matching,periodical and aperiodical distribution,different PD numbers,different input optical coupling methods and different optical power splitters.The optical heterodyne method is introduced,and the test framework is given,which is used to measure the RF characteristics of devices.Some devices in the layout are tested:the responsivities of 4-stage and 8-stage periodically distributed TWPDs at the-4V reverse bias are 1.09A/W and 1.23A/W,respectively;and their saturation currents are up to 80.82 mA and 156.64 mA,respectively.However,the maximum saturation currents reported for 4-stage and 8-stage periodically distributed TWPDs only reach 65 mA and 112 mA,respectively.