We propose and demonstrate silicon photonic integrated circuits (PICs) for free-space spatial-division-multiplexing (SDM) optical transmission with multiplexed orbital angular momentum (OAM) states over a topological charge range of -2 to +2. The silicon PIC fabricated using a CMOS-compatible process exploits tunable-phase arrayed waveguides with vertical grating couplers to achieve space division multiplexing and demultiplexing. The experimental results utilizing two silicon PICs achieve SDM mux/demux bit-error-rate performance for 1‑b/s/Hz, 10-Gb/s binary phase shifted keying (BPSK) data and 2-b/s/Hz, 20-Gb/s quadrature phase shifted keying (QPSK) data for individual and two simultaneous OAM states.

Space-to-ground optical communication systems can benefit from reducing the size, weight, and power profiles of space terminals. One way of reducing the required power-aperture product on a space platform is to implement effective, but costly, single-aperture ground terminals with large collection areas. In contrast, we present a ground terminal receiver architecture in which many small less-expensive apertures are efficiently combined to create a large effective aperture while maintaining excellent receiver sensitivity. This is accomplished via coherent detection behind each aperture followed by digitization. The digitized signals are then combined in a digital signal processing chain. Experimental results demonstrate lossless coherent combining of four lasercom signals, at power levels below 0.1 photons/bit/aperture.

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While flexible bandwidth elastic optical networking is a promising direction for future networks, the spectral fragmentation problem in such a network inevitably raises the blocking probability and significantly degrades network performance. This paper addresses the spectral defragmentation problem using an auxiliary graph based approach, which transforms the problem into a matter of finding the maximum independent set (MIS) in the constructed auxiliary graph. The enabling technologies and defragmentation-capable node architectures, together with heuristic defragmentation algorithms are proposed and evaluated. Simulation results show that the proposed min-cost defragmentation algorithms can significantly reduce the blocking probability of incoming requests in a spectrally fragmented flexible bandwidth optical network, while substantially minimizing the number of disrupted connections.