Design and Simulation of Vertical Grating Coupler for Photonic Integrated System-in-Package

Abstract

As we approach the limits in operating speed of IC’s predicted by Moore’s Law, some alternative way of increasing velocities while reducing dimensions has to be found. In this context, optical interconnects appear to be an optimal solution that avoids the main limitations that electrical interconnects have. To implement these optical interconnects in the already well developed CMOS integrated environment, the System-in-Package (SiP) and the Silicon on Insulator (SOI) technologies arise as a good solution to achieve these goals. In this Thesis optical single mode strip nano waveguides and a grating coupler for vertical coupling to optical fibres are presented, designed and simulated. Their design parameters are optimized, and their performance analyzed and discussed. Firstly, the characteristics of large single mode rib waveguides and their direct butt coupling to fibres is studied. Their performance is found to be not good enough when the dimensions were reduced to under 1 μm, with losses as high as 14.5 dB. Therefore, nano strip waveguides are studied and simulated. They provide high light confinement and good propagation characteristics. Some performance characteristics (confinement factor, single mode region, effective index) are studied, for core widths in the range 100 – 600 nm. The vertical coupling of these nano strip waveguides to optical fibres by means of a grating coupler is simulated with the FDTD technique. The main design parameters of this grating coupler are optimized, obtaining losses of 7 dB for TE polarisation and 8.5 dB for TM. The misalignment tolerances are also analysed. The crosstalk effects between this grating and an hypothetical underlying waveguide are studied, showing that no light couples to the lower waveguide.