Design and performance analysis of integrated focusing grating couplers for the transverse-magnetic TM00 mode in a photonic BiCMOS technology

Galina Georgieva,Karsten Voigt,Lars Zimmermann,Christian Mai,Anna Peczek

doi:10.1186/s41476-020-00129-4

Abstract

Focusing grating couplers for the excitation of the fundamental transverse-magnetic (TM) mode in integrated silicon photonic waveguides are designed and characterized under the boundary conditions of a photonic BiCMOS foundry. Two types of waveguide geometries are considered – a nanowire and a rib waveguide. Wafer-scale experimental results for nanowire TM grating couplers are in excellent agreement with numerical investigations and demonstrate a robust behavior on the wafer. The mean coupling loss and the 3σ interval are -3.9 ± 0.3 dB. The on wafer variation is three times lower than for the fundamental transverse-electric (TE) polarization. Similarly, the coupling in rib waveguides is examined as well. The results indicate that the rib waveguides require a modified geometry when designed for TM. In general, the nanowire waveguide type is more suitable for TM coupling, showing a stable and repeatable performance.

Highlights

Over the recent years, integrated optical communication systems are gaining in importance and are receiving a strong interest from both industry and research
[22], we described the numerical design procedure for wire TM Focusing grating coupler (FGC) in a photonic BiCMOS technology and verified its reliability by manual on-chip measurements
We demonstrate highly robust wire TM FGC designs, which are less sensitive to fabrication variations in comparison to their TE counterparts

Summary

Introduction

Over the recent years, integrated optical communication systems are gaining in importance and are receiving a strong interest from both industry and research. For their accomplishment, silicon photonics offers the possibility of the integration of optics in parallel with electronics on existing CMOS or BiCMOS platforms [1, 2]. The most important property of silicon photonic waveguides is the high index contrast between Si as a core and SiO2 as a cladding. Light can be guided in waveguides with very small dimensions, which allows for a high-density integration and a large-scale fabrication. The high index contrast inevitably leads to a strong birefringence – the fundamental. Signal regeneration enabled by four-wave-mixing (FWM) involving the TM mode was demonstrated in a single-mode regime [9]

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