  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
technique - adjoint method - optimizes 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-mode 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.
