Many-mode grating couplers by avoiding undesired couplings

TL;DR

This paper introduces a principle to design multi-mode grating couplers that avoid undesired mode couplings, enabling high-efficiency coupling of many modes with compact devices, supported by extensive numerical experiments.

Contribution

It identifies scaling laws and design principles to maximize high-efficiency multi-mode couplings while avoiding cross-couplings, demonstrated through inverse-design experiments.

Findings

Achievable mode counts of 5--10 for 2D couplers.

Tens of Fourier components in 3D couplers can enable hundreds to thousands of modes.

Simulations predict ~5% efficiency for 100 modes, a tenfold improvement.

Abstract

To couple many independent modes from free space to on chip, the key challenge is not enhancing the many necessary coupling rates (scattering-matrix elements) between targeted mode pairs. Instead, the key is to avoid additional cross-couplings to undesired modes, due to the presence of multiple simultaneously satisfied phase-matching conditions. With this principle, we identify scaling laws for the maximum number of high-efficiency multi-mode couplings that may be achievable for a given refractive index and design region, which are strongly supported by extensive numerical inverse-design experiments in 2D (one-dimensional coupler patterns, scattering in 2D). For such couplers, typical mode counts of 5--10 appear achievable. Three-dimensional couplers (patterned across two dimensions) can be markedly better, with tens of Fourier components in a single-layer device offering the possibility of high-efficiency coupling of hundreds to thousands of modes in relatively compact form factors. Numerical simulations of such a device, without any parameter optimization, predict efficiencies on the order of 5\% for 100 modes -- a collective order-of-magnitude improvement over previous designs.