![]() ![]() Such a DSP should run at the bit rate to recover the signal integrity by undoing the mode mixing occurred during fiber propagation. Fiber optic SDM systems exploit multicore or few-mode fibers to increase the spectral efficiency (in terms of bit/Hz/s) of the transmitted signal, but the price to be paid at the receiver is the need for coherent detection assisted by electronic digital signal processing (DSP). In the optical domain, space-division multiplexing (SDM) has been known for several decades 1, but only recently it has started to be seriously considered as a strategy to face the capacity crunch of optical fibers 2. The concepts of space diversity and space multiplexing are well established in communications systems and are widely employed in microwave wireless systems to implement high-capacity multiple input - multiple output (MIMO) links. With the circuit’s optical bandwidth of more than 40 nm, this approach offers an enabling technology for the evolution of FSO from single-beam to multibeam space-division multiplexed systems in a perturbed environment, which has been a game-changing transition in fiber-optic systems. We demonstrate separation of beam pairs arriving from different directions, with overlapping spatial modes in the same direction, and even with mixing between the beams deliberately added in the path. The kernel of the adaptive multibeam receiver presented in this work is a programmable integrated photonic processor that is coupled to free-space beams through a two-dimensional array of optical antennas. ![]() ![]() We show that such beams can be still separated in the optical domain, and simultaneously detected with negligible cross-talk, even if they share the same wavelength and polarization, and even with unknown arriving beam shapes. The different optical beams will, however, generally be overlapping at the receiver, and, especially with atmospheric turbulence or other scattering or aberrations, the arriving beam shapes may not even be known in advance. Free-space optics naturally offers multiple-channel communications and sensing exploitable in many applications. ![]()
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