Integrated silicon photonic two-dimensional grating couplers

Silicon (Si) photonics has proven its importance in the recent years for enabling the integration of photonic components, using available complementary metal oxide semiconductor (CMOS) fabrication flows. The simultaneous realization of electronics and photonics in the same manufacturing platform allows for the accomplishment of high-performance electronic-photonic integrated circuits (EPICs). Their adoption in optical communication systems meets one essential challenge - the optical interfacing to the transmission links, based on single-mode fibers (SMFs). On the one hand, there is a large mode-field mismatch between the tiny on-chip Si waveguide with an area of about 0.1 µm² and the external SMF with a core diameter of around 10 µm. In addition, there is a severe discrepancy between the polarization natures of both optical modes. While the external SMF supports a mode with two orthogonal, degenerated polarization states, the integrated Si waveguides and remaining components are mostly designed for the fundamental transverse electric (TE) polarization. A Si photonic coupling interface is thus required to deliver polarization handling capabilities as well. A favored component for a simple interfacing is the diffraction grating coupler (GC). In its most basic, one-dimensional implementation, a silicon waveguide mode is laterally enlarged by a taper structure, feeding a periodically etched grating. The latter deflects the light under a small angle with respect to the chip surface's normal. The external SMF is tilted under the same angle and placed at a small distance above the grating. The coupling device in this form allows for mode matching, but not for polarization manipulation. For that reason, its modified form - the two-dimensional grating coupler (2D GC) - is necessary. The latter combines two one-dimensional GCs in such a manner, that both orthogonal SMF polarizations are coupled into two separate Si waveguides, supporting the fundamental TE polarization, and vice versa. This time, the SMF is not only tilted with respect to the vertical, but also oriented towards the grating's symmetry axis between the feeding waveguides. Accordingly, the grating should be able not only to diffract both Si modes in a nearly vertical direction, but also to direct them along the symmetry plane. Although 2D GCs have been known for many years, their efficient and polarization independent design remained very challenging. To overcome this problem, a good basic understanding of the fundamental physical effects in such structures is necessary. This work is entirely dedicated to the investigation and systematization of the physical properties of 2D GCs. The gained theoretical base is used for the 2D GCs' optimization in different aspects. Starting with the theoretical description of the diffraction mechanisms in two dimensions, the interplay between the grating's geometry and coupling angles is demonstrated. Furthermore, an in-depth characterization of the polarization behavior of 2D GCs is presented. New aspects, such as the polarizations' conversion, crosstalk and non-orthogonality are analyzed, tracking their origins back to the in-plane scattering, resulting from the finite size of the grating's perturbing elements with respect to the Si mode. The importance of this physical process in 2D GCs has been underrated until now, and is revealed here as the most determining limitation for the optimal performance of 2D GCs. After this conclusion, several methods for the in-plane scattering's suppression are considered. The feasibility of the proposed approaches is investigated by both numerical simulations and wafer-level experiments. In the end, a novel optimization technique is demonstrated, which allows for the design of efficient 2D GCs with a low polarization crosstalk, low non-orthogonality and low polarization-dependent loss.

Author dc:creator

• Georgieva, Galina Doneva

Advisor dc:contributor.advisor

• Petermann, Klaus

Licence dc:rights.uri

• https://creativecommons.org/licenses/by/4.0/

Language dc:language.iso

en

Identifier URI

https://doi.org/10.14279/depositonce-19184

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/20386