Contents 1. Introduction
In recent years, increasing throughput of optical communication systems has primarily relied on higher symbol rates and advanced modulation formats. Giving the steadily rising demand for communication bandwidth, however, the limits of these approaches are becoming increasingly obvious [1], [2], especially for short-reach intra-datacenter links, where cost- and energy-efficient implementation is key. Parallel transmission via spatially separated channels is seen as an attractive alternative for bandwidth scaling with linearly increasing technical effort [2]. To maintain the associated fiber installations manageable, notable effort has been spent to replace commonly used fiber ribbons by more compact multi-core fibers (MCF) [3]–[8]. However, low-loss coupling of light between MCF and standard linear arrays of vertical-cavity surface-emitting lasers (VCSEL) or photodiodes (PD) still remain challenging. Current solutions either rely on fan-out structures to connect the cores of an MCF to individual fibers by fused fiber couplers [5], [6] or require device arrays in non-standard 2D arrangements that are precisely matched to the cross-section of the respective MCF [4], [7], [8]. These solutions are technologically complex and challenging to scale, in particular when it comes to compact short-reach data-center transceivers that are subject to stringent constraints in footprint and in assembly costs.
