Metasurfaces for Free-Space Coupling to Multicore Fibers

Abstract

Space-division multiplexing (SDM) with multicore fibers (MCFs) is envisioned to overcome the capacity crunch in optical fiber communications. Within these systems, the coupling optics that connect single-mode fibers (SMFs) to MCFs are key components in achieving high data transfer rates. Designing a compact and scalable coupler with low loss and crosstalk is a continuing challenge. Here, we introduce a metasurface-based free-space coupler that can be designed for any input array of SMFs to a MCF with arbitrary core layout. An inverse design techniqueadjoint methodoptimizes the metasurface phase profiles to maximize the overlap of the output fields to the MCF modes at each core position. As proof-of-concepts, we fabricated two types of 4-core couplers for MCFs with linear and square core arrays. The measured insertion losses were as low as 1.2 dB and the worst-case crosstalk was less than -40.1 dB in the O-band (1260-1360 nm). Owing to its foundry-compatible fabrication, this coupler design could facilitate the widespread deployment of SDM based on MCFs. Index Terms-metasurface, multicore fiber (MCF), spacedivision multiplexing (SDM), coupler, fan-in and fan-out (FIFO) I. INTRODUCTIONPTICAL fibers are the foundation of communication systems today. Single-mode fibers (SMFs) have enabled high data transfer rates thanks to a variety of multiplexing techniques that encode information onto the dimensions of light, namely, its amplitude, phase, polarization, wavelength, and space. The first four dimensions of light are widely utilized in today's SMF-based systems and are expected to be exhausted in the near future. With current data demands increasing by an order of magnitude every four years [1], the impending "capacity crunch" has made space-division multiplexing (SDM), that is using the spatial degree of freedom of light, an active area of research over the past decade [2][3][4][5][6][7]. Multicore fiber (MCF) is a promising candidate for enabling SDM by parallelizing signal transmission through each of its cores [8][9][10][11]. However, backward compatibility with existing SMF-Manuscript received xx.