High-Speed Electroabsorption Modulators and p-i-n Photodiodes

Abstract: This thesis presents methods for optimization ofelectroabsorption modulators (EAM) and p-i-n photodiodes (PD)withrespect to high frequency (HF) performance. Differentreactive matching techniques are investigated such as lumpedmatching and integrated transmission line (TML) layouts includingoperation in the traveling-wave (TW) regime. The performance gainoffered by these techniques of matching as well as theirinfluence on the device design for optimum utilization arediscussed.A comprehensive study of microwave properties for anintegrated EAM-TML in the quasi-TEM regime is presented.Conformal mapping techniques are used to derive microwaveparameters from device geometry where non-dissipative elements,resulting in characteristic impedance and propagation index, aregiven in a closed-form expression. Effective numerical methodsfor evaluation of dissipative elements are presented. The studyincludes experimental evaluation of EAM-TML samples withcorrespondence to the numerical model.Integrated EAM-TML devices operating in traveling wave regimeare presented; where a 3dBe bandwidth (BW) increase for EAMs witha factor of 80% over the corresponding RC-limit is demonstrated.Design consideration for the TW implementation is discussed;similarities and differences compared to a traditional lumped EAMdesign are emphasized for full utilization of the TWimplementation.The problem with low TML impedance for EAM-TMLs is addressed,and a novel segmented TML implementation is proposed anddemonstrated. 50W terminated TW-EAM devices operating in theattenuation limit are demonstrated experimentally. EAMs withstate of the art performance with respect to broad bandwidth andexcellent return-loss to a 50W-microwave system in a widefrequency range are presented. The design trade-off between TMLimpedance and attenuation for optimum device performance isdiscussed.The potential performance-gain obtained with inductive tuningof p-i-n photodiodes is addressed. Serial inductive tuning is anattractive implementation to extend the device bandwidth over thelimitations given by the RC-time constant and the transit-timedelay. It can be used both for noise suppression to improve thesignal to noise ratio (SNR) with respect to internal receivernoise that originates from a front-end amplifier, as well as asolution to the technical problem of improving the receiverequalization for increased bitrate without degrading the SNR orintroducing intersymbol interference (ISI). The design impact forthe choice of the absorption layer thickness of a high-speedvertical photodiode (VPD) is indicated.Optical communication, Fiber optic, Electroabsorptionmodulators, p-i-n photodiodes, transmission line, traveling wave,high-speed, microwave photonics