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Exploiting a particular wave property for a particular application necessitates components capable of discriminating in the basis of that property. While spectral or polarisation decomposition can be straightforward, spatial decomposition is inherently more difficult and few options exist regardless of wave type. Fourier decomposition by a lens is a rare simple example of a spatial decomposition of great practical importance and practical simplicity; a two-dimensional decomposition of a beam into its linear momentum components. Yet this is often not the most appropriate spatial basis. Previously, no device existed capable of a two-dimensional decomposition into orbital angular momentum components, or indeed any discrete basis, despite it being a fundamental property in many wave phenomena. We demonstrate an optical device capable of decomposing a beam into a Cartesian grid of identical Gaussian spots each containing a single Laguerre-Gaussian component, using just a spatial light modulator and mirror.

Mode-division multiplexing over 33-km few-mode fiber is investigated. It is shown that 6×6 MIMO processing can be used to almost completely compensate for crosstalk and intersymbol interference due to mode coupling in a system that transmits uncorrelated 28-GBaud QPSK signals on the six spatial and polarization modes supported by a novel few-mode fiber.

We demonstrate a 3x1 fiber-based photonic lantern spatial-multiplexer with mode-selectivity greater than 6 dB and transmission loss of less than 0.3 dB. The total insertion loss of the mode-selective multiplexers when coupled to a graded-index few-mode fiber was < 2 dB. These mode multiplexers showed mode-dependent loss below 0.5 dB. To our knowledge these are the lowest insertion and mode-dependent loss devices, which are also fully compatible with conventional few-mode fiber technology and broadband operation.

We investigate the use of "photonic lanterns" as adiabatic mode converters for space-division multiplexing (SDM) systems to interface multiple single-mode fibers to a multi-mode fiber. In a SDM system, minimizing the coupling loss and mode-dependent loss best utilizes all spatial modes of the fiber which increases the capacity, the transmission distance, and minimizes the outage probability. We use modal analysis, the beam propagation method, and a transfer matrix technique to analyze the lanterns throughput along with its mode dependent loss and show that unitary coupling between single-mode fibers and a multi-mode fiber is only possible by optimizing the arrangements of the cores. Results include simulations for three, 12, 15, and 51 core lanterns to couple to six, 24, 30, and 102 spatial and polarization modes, respectively.

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