Grating Couplers in Silicon-on-Insulator: The Role of Photonic Guided Resonances on Lineshape and Bandwidth

In this work we focus on the problem of the bandwidth of 1D grating couplers in the standard Silicon On Insulator (SOI) platform, consisting of a 220 nm silicon layer on top of an SiO2 cladding (typical thickness around 2 um) on a silicon substrate.. Grating couplers offer a viable solution to the problem of fiber-to-chip light coupling in integrated silicon photonics [1], and understanding the mechanism underlying their operation could give useful hints on increasing their performances. We analyse 1D grating couplers by means of numerical simulations with the 2D FDTD method. We consider different sizes of the incoming light beam, defined by the Mode Field Diameter (MFD), and we propose for the bandwidth a model based on two contributions. The first, of Gaussian lineshape, comes from the k-space Fourier broadening of the incident field and dominates for small MFD (the typical MFD of single mode fibers used in telecommunication is about 10 mu m and falls in this regime). The second, of Lorentzian lineshape, comes from the intrinsic broadening of the photonic mode propagating inside the gratings above the cladding light line [2], and dominates for large MFD (>approximate to 50 mu m). When both contributions are comparable we see that the coupling spectrum results in a Voigt lineshape. This is clearly seen in Fig. 1, where we plot the coupling spectra of the same grating structure for different values of theMFD. The evolution of the spectrum from purely Gaussian to purely Lorentzian, passing through a Voigt, is evident.