  Multicore optical fibers and ribbons based on fiber arrays allow for
massively parallel transmission of signals via spatially separated channels,
thereby offering attractive bandwidth scaling with linearly increasing
technical effort. However, low-loss coupling of light between fiber arrays or
multicore fibers and standard linear arrays of vertical-cavity surface-emitting
lasers (VCSEL) or photodiodes (PD) still represents a challenge. In this paper,
we demonstrate that 3D-printed facet-attached microlenses (FaML) offer an
attractive path for connecting multimode fiber arrays as well as individual
cores of multimode multicore fibers to standard arrays of VCSEL or PD. The
freeform coupling elements are printed in situ with high precision on the
device and fiber facets by high-resolution multi-photon lithography. We
demonstrate coupling losses down to 0.35 dB along with lateral 1 dB alignment
tolerances in excess of 10 $\mu$m, allowing to leverage fast passive assembly
techniques that rely on industry-standard machine vision. To the best of our
knowledge, our experiments represent the first demonstration of a coupling
interface that connects individual cores of a multicore fiber to VCSEL or PD
arranged in a standard linear array without the need for additional fiber-based
or waveguide-based fan-out structures. Using this approach, we build a 3 x 25
Gbit/s transceiver assembly which fits into a small form-factor pluggable
module and which fulfills many performance metrics specified in the IEEE 802.3
standard.
