Backscattering design for a focusing grating coupler with fully etched slots for transverse magnetic modes

Jahn Hoffmann,Giampaolo Pitruzzello,K. Marvin Schulz,Alexander Yu. Petrov,Manfred Eich,Lena Simone Fohrmann

doi:10.1038/s41598-018-36082-z

Abstract

Grating couplers are a fundamental building block of integrated optics as they allow light to be coupled from free-space to on-chip components and vice versa. A challenging task in designing any grating coupler is represented by the need for reducing back reflections at the waveguide-grating interface, which introduce additional losses and undesirable interference fringes. Here, we present a design approach for focusing TM grating couplers that minimizes these unwanted reflections by introducing a modified slot that fulfills an anti-reflection condition. We show that this antireflection condition can be met only for the Bloch mode of the grating that concentrates in the dielectric. As a consequence the light is scattered from the grating coupler with a negative angle, referred to as “backscattering design”. Our analytic model shows that the anti-reflection condition is transferrable to grating couplers on different waveguide platforms and that it applies for both TE and TM polarizations. Our experimentally realized focusing grating coupler for TM-modes on the silicon photonics platform has a coupling loss of (3.95 ± 0.15) dB at a wavelength of 1.55 µm. It has feature sizes above 200 nm and fully etched slots. The reflectivity between the grating coupler and the connected waveguide is suppressed to below 0.16%.

Highlights

Silicon wire waveguides can strongly confine light to the nanoscale and exhibit a low propagation loss enabling long-range, on-chip guiding of light
While grating couplers operating with TE-polarized light are well established, only a few examples for TM polarization have been presented in the literature[11,18,19,20]
We experimentally determine the coupling loss of the fabricated grating couplers by a standard cut-back technique which enables one to distinguish between coupling loss of the grating coupler and propagation loss in the waveguide

Summary

Introduction

Silicon wire waveguides can strongly confine light to the nanoscale and exhibit a low propagation loss enabling long-range, on-chip guiding of light. One way to overcome these reflections is to use shallow etched grating coupler designs[11,12,13] In this approach, the mode mismatch is reduced, due to the fact that the grating has a weaker modulation and the Bloch modes have a higher group velocity and less back-propagating fields. The inclusions can be used to adjust the scattering strength of the grating for an optimal overlap with the fiber mode[21] They decrease a reflection by reducing the refractive index contrast in the grating and back propagating field components of the Bloch modes, similar to shallow etched designs. Even for electron-beam lithography, the small feature sizes are challenging to realize, in optimized grating coupler designs employing apodized grating lines that require a high accuracy and only allow for small fabrication tolerances[21,25]

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Results

Conclusion