Feature: Communications
Programmable photonic processors for free-space optic communications
By a research team at Politecnico di Milano, Italy T
he concepts of space diversity and space multiplexing are well established in communications systems and widely used in
microwave wireless systems to implement high-capacity multiple-input, multiple- output (MIMO) links. In the optical domain, space-division multiplexing (SDM) has been used for a long time, but only recently has it been seriously considered as a strategy to tackle the capacity limitations of optical fibres.
Fibre-optic SDM systems Fibre-optic SDM systems exploit multicore or few-mode fibres 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 digital signal processing. A DSP should run at the bit rate to recover the signal integrity by undoing the mode mixing during fibre
propagation. To reduce the DSP’s power consumption and speed limitation, several solutions have been proposed to perform all-optical demultiplexing and unmixing of optical guided modes at the receiver. Te same evolution seen in fibre-optic
communications is now happening in free-space optics (FSO). Tis type of communication is attracting growing interest, seen as a potential solution to meet the increasing demand for wireless bandwidth and the low latency requirements of the Internet of Tings (IoT). As with fibre-optic communications, SDM in FSO requires orthogonal sets of beams (or modes), and several pioneering demonstrations have used orbital angular momentum modes, Bessel beams and Laguerre-Gauss modes. To generate several orthogonal beam
configurations and demultiplex them at the receiver, the light beams must be shaped according to amplitude, phase and polarisation profiles. Traditionally these operations are performed using
18 July/August 2023
www.electronicsworld.co.uk
bulk optics, such as classical lenses and diffractive elements. Spatial light modulators offer more flexibility and reconfigurability in beam manipulation; however, they have limitations, such as low speed (a few hundred Hz), the possibility of modifying only the phase of light, and require computationally-heavy calibration techniques. A powerful alternative technology for
manipulation of FSO beams is offered by programmable photonic processors. Tese are general-purpose photonic ICs, made of meshes of tuneable, integrated interferometers that can implement arbitrary linear transformations. Because of their flexibility, they are already found in many different applications, including reconfigurable filters, the unmixing of guided modes, vector-matrix multiplication and computing, quantum information processing and neural networks. By connecting the input/output ports of these architectures to the optical antennas, these processors can be used
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