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Radio beams that carry non-zero Orbital Angular Momentum (OAM) are analyzed from the viewpoint of a Multiple-Input-Multiple-Output (MIMO) communication system. Often, the natural OAM-beam orthogonality cannot be fully exploited because of spatial constraints on the receiving antenna size. Therefore, we investigate how far OAM-induced phase variations can be exploited in spatial de-multiplexing based on conventional (linear momentum) receivers. Performances are investigated vs. position and size of the transmitting and receiving devices. The use of OAM-mode coherent superpositions is also considered, in view of recent work by Edfors et al. [1]. Our final goal is to assess the merits of an OAM-based MIMO system, in comparison with a conventional one.

Abstract:The equations for Four-Wave-Mixing in a Photonic Crystal waveguide are derived accurately. The dispersive nature of slow-light enhancement, the impact of Bloch mode reshaping in the nonlinear overlap integrals and the tensor nature of the third order polarization are therefore taken into account. Numerical calculations reveal substantial differences with simpler models, which increase with decreasing group velocity. We predict that the gain for a 1.3 mm long, unoptimized GaInP waveguide will exceed 10 dB if the pump power exceeds 1 W.

We report highly efficient four wave mixing in a GaInP photonic crystal waveguide. Owing to its large bandgap, the ultrafast Kerr nonlinearity of GaInP is not diminished by two photon absorption and related carrier effects for photons in the 1550 nm range. A four-wave-mixing efficiency of -49 dB was demonstrated for cw pump and probe signals in the milliwatt range, while for pulsed pumps with a peak power of 25 mW the conversion efficiency increased to -36 dB. Measured conversion efficiency dependencies on pump probe detuning and on pump power are in excellent agreement with a simple analytical model from which the nonlinear parameter gamma is extracted. Gamma scales approximately with the square of the slow down factor and varies from 800 W(-1) m(-1) at a pump wavelength lambda(p)=1532 nm to 2900 W(-1) m(-1) at lambda(p)=1550 nm. These values are consistent with those obtained from self phase modulation experiments in similar devices.