Theoretical Analysis Of Waveguide And Device Design For Evanescently Coupled Avalanche Photodiodes

The field of integrated optics is being exploited. Integrated optics technology is still at the early stage compared to micro-electronics technology, but it has a vast development prospect. The accelerating of information transfer and the explosion of data are creating a high speed and high capacity optical communication system(OCS). It is a key technology to integrate different optoelectronic devices in OCS. The photodetector is an important component of OCS. Furthermore, the photodetector integrated with a waveguide has advantage in performance and reliability. Hence the research of integrated photodetectors is increasingly attracting scientists' attention from both the academe and industry. In this paper, we focused on lowering the polarization dependent loss of waveguides and designing a monolithically integrated photodetector with high speed.In this work, the difference between hybrid integration technology and monolithic integration technology has been analyzed. For an optical receiver, the monolithically integrated photodetector has advantages in device dimension and reliability. The optical paths of light propagating from the fiber to the photodetector, such as normal incidence, side illumination and evanescent wave, have also been studied. Evanescently coupled photodetectors(ECPDs), by contrast, can help relieve the trade-off between carrier transit time and quantum efficiency since the optical absorbing direction is perpendicular to that of carrier transport. Moreover, ECPDs have high-power capability due to uniform carrier distribution along the optical path. Owing to their internal carrier multiplication mechanism, avalanche photodetectors(APDs) are widely used in OCS where high sensitivities are needed. We focus on designing an evanescently coupled avalanche photodiode(ECAPD) with high speed and high responsivity.We propose a factor, the sum of squared deviations of indexes ratios(SSDR), to describe the difference of propagation conditions between transverse-electric mode and transverse-magnetic mode. A method based on this factor is used for designing the waveguide with low polarization dependent loss. We propose two methods for ECAPDs to lower their polarization dependent loss. Air slots are introduced into the matching layer of an III-V compound semiconductor-based ECAPD, and stepped structure is applied in a group IV ECAPD. These designed ECAPDs have low polarization dependence and high coupling efficiency with a short coupling length. Simulation results using software demonstrate these methods.On the basis of above investigation, an ECAPD based on germanium-on-insulator is designed. We simulate and analyze the performance of this designed ECAPD. This ECAPD has low polarization and high quantum efficiency. Its responsivity is 0.64 A/W, dark current is 13 nA and gain-bandwidth product is 143 GHz. The fabrication process for it is fully compatible with standard BiCMOS process.Finally, the responsivity and polarization dependence of this ECAPD with broad spectral range(800-1600 nm) are studied. We propose a method for improving the broad spectral characteristics of this ECAPD.