Silicon Based Grating Coupler For Fiber-chip Coupling

With the fast development of big data,Internet of things and artificial intellegence,a large number of data centers and super computers have come into service.In the data centers and super computers,optical interconnects are playing the role of main artery for enormous data exchange.For lower costs and power consumption,device integration has become more and more important.Among various material platforms,silicon based integrated optical chip has attracted lots of attention,attributing to the combination of high integration,mature fabrication processes and low costs.As research continues,a mass of silicon based passive and active devices have been developed with high performance.Besides these,a practical integrated optical chip also needs excellent coupler for fiber-chip coupling.The biggest challenge for fiber-chip coupling is the serious cross-section mismatch between fiber and silicon waveguide,which will introduce tremendous coupling loss if they are directly connected.Grating coupler(GC)is one of the most promising approaches to overcome this dilemma due to great convenience of easy fabrication,flexible position,large alignment tolerance,wafer-scale test and high dense multichannel coupling.This thesis is focused on how to improve the performance of GC and the major research contents are summarized as follows:(1)The basic principles,performance indexes,design processes,fabrication processes and measurement methods of GC are investigated.(2)How to compact the footprint of GC is explored.A focusing GC is analyzed and optimized to realize a compact GC.For demonstration,the focusing GC is fabricated and a coupling efficiency of-3.9 dB is obtained,which is as high as a conventional GC with six-fold size.Besides,a short and efficient mode size converter is designed by segmented-stepwise method and the conversion loss is decreased from 5 dB to 0.62 dB with a length of 20 ?m.(3)How to improve the coupling efficiency of GC is investigated.A chirped GC is designed to lower the mode mismatch between fiber and GC,and a coupling efficiency of-2.9 dB is acquired with 0.9 dB improvement compared with uniform GC.After that,a suspended GC is proposed to further increase the coupling efficiency to-2.1 dB without complicating the fabrication processes.A silicon based SiNx GC is also theoretically presented with a coupling efficiency of-0.9 dB,which can be applied in multilayer nitride-on-silicon and silicon platforms.(4)The two dimensional GC(2D GC)is studied to overcome the polarization dependence of GC.Combining 2D GC with MMR,a polarization insensitive all-optical clock recovery is successfully realized.A modified vertically coupled 2D GC is demonstrated and the coupling efficiency is improved from-7.0 dB to-5.0 dB with the help of distributed Bragg reflectors(DBRs).The off-normal coupled 2D GC is also investigated.A 2D GC with rhombus array of etching holes is proposed to realize orthogonally outputs.A focusing 2D GC and a suspended 2D GCs are following designed to compact the footprint and improve the coupling efficiency,respectively.With an etching shape consisting of five holes,the polarization dependent loss of the off-normal coupled 2D GC is decreased from 1.5 dB to 0.25 dB.(5)How to simplify the fabrication processes of GC is explored.Using subwavelength structure with the lag effect,a single step etched 1D GC is presented,which can be manufactured with the same processes of strip waveguide.With a chirped structure,a coupling efficiency of-2.8 dB is achieved.Then,a single step etched 2D GC is also demonstrated with similar method and coupling efficiencies of-5.0 dB and-5.3 dB are measured for P and S polarized input light,respectively.