Research On Structure Design And Transmission Performance For Electrodes Of High-speed And High-Power Photodetectors Array In Optical Fiber Communications

In recent years,all kinds of communication services have been developed rapidly in the grand background of cloud computing and big data.In order to meet the growing demand of network bandwidth and transmission capacity,optical fiber communication system is also developing towards the new system with ultra high speed,large capacity and high integration.In the high-speed and long-distance optical fiber transmission system,the coherent reception with different modulation formats becomes the main receiving mode of the system,and since the local oscillator laser and the optical preamplifier are implemented in the optical fiber system,the photodetector which is the key components of the receiver,needs to meet the requirements for high-speed response and high-power photoelectric conversion.In order to solve the limitation of single photodetector in high-power reception,the incident optical signal can be fed into a photodetector array(PDA)consisting of several discrete photodetectors,aiming at receiving high-power input optical signals efficiently within the linear range of each photodetector.It relieves the harsh demands of a single device on material growth,process preparation,etc.and overcomes the trade-off between high power and high speed.Meanwhile,PDA is purposed to reduce the absorption layer thickness of each device,thereby increasing the overall array chip frequency response and output power,and furthermore reducing the heat generation effect of the device and ease the design complexity of single photodetectors.The electrodes of PDA,as an important part of photodetector array,determine the way in which the individual photodetector elements are distributed and integrated in PDA and have a huge impact on the high-speed performance of PDA.The well-designed electrode structure of the PDA reduces the effects of parasitic parameter on the performance of the array,making it possible to achieve higher frequency response and greater output power of the PDA.This thesis focuses on the theoretical and experimental research on structure design and transmission performance for electrodes of high-speed and high-power PDA in optical fiber communications.The detailed research work and innovative achievements are summarized as follows:1.The basic theory of structure design for electrodes of photodetectors and PDA is studied.The research progresses of the PDA and its electrodes structure at home and abroad is analyzed and compared.It provides a guideline for structure design for electrodes of high-speed and high-power PDA.2.The electrodes structure of a single photodetector is designed,and the electrode structure parameters are simulated and optimized by simulation software.The simulation results show that the characteristic impedance of the optimized electrode structure is about 50? at 10MHz-100GHz,and the insertion loss(S21)is-1.23dB at 100GHz.3.The structure of serials four-element photodetector array electrodes is designed and optimized.In particular,the distance between different photodetector electrode units and the parameters of coplanar waveguide(CPW)are optimized.The optimized simulation result shows that the transmission coefficient of serials photodetector array electrodes is reduced by 3dB at 38GHz.As the distance between different electrode units to the signal output ends is different,there is a build-up of time delay as the electrical signal propagates through the electrodes,which will have an impact on the high-speed performance of electrodes.The problem is further analyzed to propose a new structure for electrodes of PDA.4.A novel parallel two-element photodetector array electrodes structure is proposed and designed to overcome the problem cited above.The structure of the photodetector array is simulated and optimized by using simulation software based on characteristic impedance and transmission coefficient.The simulation results show that the characteristic impedance(Z0)of the optimized electrodes structure is kept at about 50?in the range of 10MHz-100GHz,and the insertion loss(S21)is-1.3dB at 100GHz.5.The electrodes structure of centrosymmetric multi-element photodetector array is proposed and designed.The three-element and four-element centrosymmetric photodetector array electrodes structure are optimized and simulated.The simulation results show that the insertion loss of the optimized electrodes structure is-1.1dB(three-element)and-1.7dB(four-element)at 100GHz.6.The series four-element electrodes of PDA are fabricated,and the test platform is built to test the transmission performance of the electrodes structure.The experimental results show that the distance between each photodetector electrode unit and the signal output ends in the series four-element electrodes of PDA has an impact on the overall bandwidth of the electrodes.The fabricated electrodes structure has an insertion loss of-2.8 dB at 40 GHz,which is basically consistent with the simulation results.7.The fabrication of the parallel two-element photodetector array electrodes is completed.As a contrast,the optimized configuration of electrodes for PDA and the structure before optimization are fabricated.The test platform was set up to test the transmission performance of the electrodes structure.The experimental results show that the 3dB bandwidth of the optimized electrodes is above 40GHz,and the transmission coefficient reaches-1.4dB at 40GHz.Compared with the electrodes structure before optimization and the series electrodes of PDA,the transmission loss decreased by 1.1dB and 1.4dB respectively.8.The parallel two-element photodetector array consisting of two uni-traveling-carrier(UTC)photodetectors is fabricated and a test system is constructed to test the ohmic contact,dark current,responsivity and high-speed performance of the UTC-PDA.The experimental results show that the ohmic contact resistance of P electrode and N electrode of UTC-PDA are 14.1? and 7.6? respectively.The dark current of the PDA is 4.15nA at the reverse bias of 1.9V.The responsivity of the array is 0.20A/W,and the 3dB bandwidth is 13.6GHz at the reverse bias of IV.Specifically,the 3dB bandwidth of PDA implementing designed electrodes increased by 6.6GHz comparing to the PDA before optimization.