Design of a free-space optical link based on multipole-phase division multiplexing.

In recent decades, control of the spatial structure of electromagnetic waves has boosted the research of new modulation formats for telecommunications and quantum applications. In particular, the exploitation of orthogonal spatial modes as distinct information carriers in the so-called spatial-division multiplexing has been dominated so far by beams carrying orbital angular momentum (OAM). However, practical solutions based on OAM-mode multiplexing still suffer from critical issues related, for instance, to efficient generation and long-distance transmissions in free space. In a more general approach to wavefront propagation, we have recently introduced an innovative framework based on beams with harmonic phases characterized by multipole structures devoid of phase singularities, referred to as multipole-phase beams. This new paradigm offers efficient multiplexing and sorting in a full-optical and compact architecture based on conformal transformations, promising, moreover, to overcome the limits of previous solutions in free-space transmissions. The generalization to high orders of multipole phase is considered here, investigating a possible layout for a free space optical link. Numerical simulations are performed and discussed to validate the theory and show the potentialities of this new framework for telecommunications, both at the classical and single photon regimes.