A physical origin of cross-polarization and higher-order modes in two-dimensional (2D) grating couplers and the related device performance limitations

Abstract

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.

Penalties From 2D Grating Coupler Induced Polarization Crosstalk in Silicon Photonic Coherent Transceivers

“…We have shown that in general 2D GCs convert a part of the power of a given input polarization into its orthogonal counterpart -an effect, which we assign as a linear cross-polarization [22]. We have further indicated in-plane scattering as the physical origin of this conversion [23] -an issue, which inevitably occurs due to the finite size of the diffracting elements of 2D GCs with respect to the optical waveguide mode. In our most recent work [24], the impact of the cross-polarization has been investigated on system level.…”

“…Previous analyses state that polarization-related issues can be attributed to a scattering in the grating's plane. The latter is particularly strong, when an array of elements with identical scattering properties is given [23]. This is the case of uniform 2D GCs.…”

Advances in monolithic high-speed SiGe photonic electronic system integration

“…The related polarization crosstalk was observed in experiments [4] and it was shown that it is associated with a loss of polarization orthogonality [5]. To address the issue of cross-polarization, its physical background was investigated [6]. We indicated in-plane scattering at the grating perturbing elements as the limiting process behind cross-polarization.…”

Toward 2D grating coupler enabled O-band coherent links based on SiGe photonic electronic technology

“…While we found 24) that the cross-polarization scales with the grating's perturbation strength (thus, limiting the coupling efficiency), we indicated in-plane scattering as its physical origin. 25) The latter is caused by the finite size of the grating perturbing elements with respect to the incident optical field. The presence of in-plane scattering and thus of cross-polarization is the main reason for PDL in 2D-GRCs.…”