Feature: Communications
Figure 1: Multibeam FSO receiver a. Schematic of a 9 × 2 diagonal photonic processor comprising two rows of tunable MZIs and implementing a two-beam FSO receiver; b. Microscopic picture of the fabricated silicon chip; c. Detail of the 2D optical antenna array made of 3 × 3 grating couplers in a square configuration; d. Measured far-field pattern radiated by the 2D optical antenna array when all the grating couplers are excited with the same amplitude and phase; e. Detail of a thermally-tunable beam coupler with a transparent monitor detector integrated at the output ports; f. The photonic chip assembled on a PCB integrating the CMOS electronic ASIC for the read-out of on-chip detectors
for on-chip manipulation of FSO beams. For instance, we used a silicon photonic mesh of Mach-Zehnder interferometers (MZIs) to control the complex field radiated by an array of optical antennas, demonstrating several functionalities, like yielding perfectly-shaped FSO beams with non-perfect optical antennas, and imaging through a diffusive medium. In this article we show that a
programmable photonic processor can separate overlapped, orthogonal, arbitrary FSO beams directly in the optical domain. Te circuit implements an adaptive multibeam receiver for FSO systems, which recover the information carried by the received spatially-overlapped FSO beams, with negligible mutual crosstalk. We will provide several examples, including pairs of beams arriving from orthogonal directions (“direction diversity”), as well as beams arriving from the same direction but
shaped according to different orthogonal spatial modes (“mode diversity”), and even when the orthogonal beams have undergone some mixing during propagation.
Multi-beam FSO receiver Te integrated photonic processor we used in our work consists of a mesh of tunable beam splitters using balanced MZIs. Te topology of the circuit is shown in Figure 1a, which includes N = 2 rows of cascaded MZIs. On the leſt side of the mesh, a 2D array
made of M optical antennas (here, M = 9) is used as input/output interface between FSO beams and the guided modes of an array of single-mode optical waveguides. Te optical antennas have standard grating couplers, typically used to couple the light with optical fibres; however, the presented results can be extended to
integrated photonic processors terminated with arbitrary optical antennas, so that each radiation diagram can be optimised for specific applications. On the right side of the mesh, two of
the output waveguides, WGn (n = 1, 2), are used as output ports; the remaining seven are available for monitoring and control purposes. Such a processor can be used to separate,
essentially losslessly, any two “orthogonal” input beams at these two output ports, and this can be accomplished by a progressive self-configuring algorithm based on single- parameter power minimisation feedback loops, without calculations. Formally, by the “orthogonality” of two
beams here we mean two beams that lead to orthogonal complex vectors of amplitudes in the M input waveguides inside the processor. To the extent that two different (and possibly overlapping) input beams lead
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