Vertical SOI Grating Couplers Based On Sub-wavelength Structures

Silicon-on-insulator(SOI) has been recently considered as a very promising platform for large-scale and high-density integration of photonic integrated circuits(PICs). One of the big challenges is the coupling of light between a nanophotonic waveguide and a single mode fiber(SMF) due to the large mode size mismatch of them. An efficient approach is to use surface grating couplers as the interface between the nanophotonic waveguide and the outside world. This thesis focuses on the design, fabrication and test of the single etched surface grating couplers based on the standard SOI platform, the detailed works are as follows:(1) Two dimensional models of the apodized grating couplers are built for both transverse electric(TE) and transverse magnetic(TM) polarization. The whole designing procedure of apodizing the grating is given in detail, in which a feasible and programmable method is developed to precisely manipulate the coupling strength of each grating period. A perfect Gaussian output beam can be synthesized for TE mode grating couplers, achieving a field overlap up to 98.3% with the fiber mode;(2) Two types of apodized fully-etched grating couplers are designed according to the effective medium theory(EMT): sub-wavelength grating(SWG) grating couplers and photonic crystal(Ph C) grating couplers. The simulated coupling efficiencies of the designed TE mode SWG grating couplers, TM mode SWG grating couplers and TE mode Ph C grating couplers are 27.2%, 43.3% and 34.7% respectively;(3) The designed TE and TM mode SWG grating couplers are fabricated mainly using electron beam lithography(EBL) and inductively coupled plasma reactive ion etching(ICP). The measured peak coupling efficiency is-7.6d B for TE mode SWG grating couplers with a 1 d B bandwidth of 45 nm, and-6.1d B for TM mode SWG grating couplers with a 1 d B bandwidth of 34 nm, then we propose the optimization of the grating design according to the SEM and measurement results.