Study On Surface Plasmon Directional Coupler For Optical Interconnection

The development of optical interconnection urgently requires a series of integrated photonic devices with ultra-compact structure and low loss,and directional coupler is one of these devices.Surface plasmon-based directional couplers meet the requirements of small size for optical interconnect devices,and have potential for improving parameters such as transmission loss.In this paper,on the basis of our previous research work on hybrid waveguide,two kinds of directional coupler structures based on hybrid surface plasmon waveguide are proposed.The finite element method software COMSOL Muitiphysics and finite-difference time-domain software FDTD Solutions are used to conduct a more comprehensive theoretical study.The transmission and coupling characteristics of directional coupler are thoroughly discussed.It is expected that the optimized design parameters of directional couplers suitable for optical interconnection with ultra-compact structure and low loss can be obtained.Firstly,the research background of surface plasmon directional couplers is introduced,and the characteristics of various types of surface plasmon waveguides are compared and analyzed.And then,the hybrid surface plasmon waveguides with excellent comprehensive performance are selected as the waveguide prototype used in this study.Then,a novel directional coupler structure is designed,which is based on elliptical cylindrical silicon nanowire hybrid surface plasmon waveguide proposed by our group.The transmission and coupling characteristics of the directional coupler are simulated including mode field distribution,effective mode refractive index,propagation length,coupling length and transmission loss.Besides,the performance indexes of the directional coupler,such as additional loss,coupling degree and directivity are evaluated.Finally,to simplify the structure and improve the process compatibility,an improved directional coupler based on rectangular silicon nanowire hybrid surface plasmon waveguide is proposed.Following the same research method of elliptical cylindrical structure,the characteristic parameters of the two directional couplers are comprehensively compared and analyzed.Also,the specific structural optimization of rectangular silicon nanowires is carried out,and its influence on the characteristics of single waveguide and the whole structure are discussed in detail.The results show that in the case of weak coupling,the coupling length of the new directional coupler based on elliptical cylindrical silicon nanowire hybrid surface plasmon waveguide is only 1.646?m,and the propagation loss is as low as 0.076dB??m,and the maximum coupling energy can reach to 80%;the coupling length of improved directional coupler based on rectangular silicon nanowire hybrid surface plasmon waveguide is only 1.693?m,and the maximum coupling energy can reach to 83%.A good trade-off between transmission loss and coupling efficiency can be made by properly adjusting the geometric structure of rectangular silicon nanowires.Both of the two directional couplers exhibit super strong optical field constraints,high coupling efficiency and low transmission loss.They have superior comprehensive performance compared to traditional optical directional couplers,and are more suitable for optical interconnection application.