2021, Georgieva, Galina, Voigt, Karsten, Seiler, Pascal M., Mai, Christian, Petermann, Klaus, Zimmermann, Lars

We explore scattering effects as the physical origin of cross-polarization and higher-order modes in silicon photonic 2D grating couplers (GCs). A simplified analytical model is used to illustrate that in-plane scattering always takes place, independent of grating geometry and design coupling angle. Experimental investigations show furthermore that grating design parameters are especially related to the modal composition of both the target- and the cross-polarization. Scattering effects and the associated cross-polarization and higher-order modes are indicated as the main reason for the higher 2D GC insertion loss compared to standard 1D GCs. In addition, they can be responsible for a variable 2D GC spectrum shape, bandwidth and polarization dependent loss.

2022, Georgieva, Galina, Sena, Matheus, Seiler, Pascal M., Petermann, Klaus, Fischer, Johannes, Zimmermann, Lars

Silicon photonic two-dimensional grating couplers for C- and O-band dual-polarization coherent transceivers are analyzed with respect to their polarization splitting/combining performance. Due to scattered light in the grating's plane, a linear cross-polarization results. The latter is responsible for a limited polarization split ratio and a polarizations' non-orthogonality. The impact of these two quantities is evaluated by system-level simulations with regard to OSNR penalties in coherent systems. For both C- and O-band, a design modification for reduced penalties is proposed.