  Free-space optics (FSO) is an attractive technology to meet the ever-growing
demand for wireless bandwidth in next generation networks. To increase the
spectral efficiency of FSO links, transmission over spatial division
multiplexing (SDM) can be exploited, where orthogonal light beams have to be
shaped according to suitable amplitude, phase, and polarization profiles. In
this work, we show that a programmable photonic circuits, consisting of a
silicon photonic mesh of tunable Mach-Zehnder Interferometers (MZIs) can be
used as an adaptive multibeam receiver for a FSO communication link. The
circuit can self-configure to simultaneously receive and separate, with
negligible mutual crosstalk, signals carried by orthogonal FSO beams sharing
the same wavelength and polarization. This feature is demonstrated on signal
pairs either arriving at the receiver from orthogonal directions
(direction-diversity) or being shaped according to different orthogonal spatial
modes (mode-diversity), even in the presence of some mixing during propagation.
The performance of programmable mesh as an adaptive multibeam receiver is
assessed by means of data channel transmission at 10 Gbit/s a wavelength of
1550 nm, but the optical bandwidth of the receiver (>40 nm) allows its use at
much higher data rates as well as in wavelength-division multiplexing SDM
communication links.
