The use of multiple channels in graded index fibre to increase bandwidth capacity in optical fibre communications

Nowadays we are living in a well-developed technological world, supporting new services and networks with vast amounts of data streams that are being pushed through fibre optic communications systems. Thus the use of optical fibres as a transmission medium is being progressively increased for deployment in ever-wider fields. A significant increasing demand of global social information in modern communication is leading to an exponentially increasing demand for high transmission carrying capacity via fibre optical network systems. This drives towards a higher information carrying capacity than the standard systems can handle. To meet the higher bandwidth requirements with a higher capacity per cross sectional area of the fibre for future communications, maximising the density of the channels is seen as an effective solution, accomplished by simultaneously propagating the individual channels within the same fibre. A new multiplexing technique, spatial division multiplexing (SDM) based on a multi-core fibre (MCF) and a multi-mode fibre (MMF), has proved it is possible to overcome the current limitation of the carrying capacity. Our greatest concern in this research is to overcome the limited transmission capacity of current existing optical fibre systems and to progressively increase the bandwidth capacity with a simple and cost effective approach. This can be accomplished by transmitting a multiplicity of channels down a single graded-index fibre (GI-MMF) with a large core diameter. This would allow a significant increase communication bandwidths for a range of short haul communications. The proposed method exploits the phenomenon of self-imaging in the GI fibre due to the interference between the excited modes which leads to the reproduction of the original beam profile periodically along the distance of propagation. This allows the maintainance of crosstalk levels between the plurality of communication channels lower than -25 dB, and also ensures a reduction of optical losses in the perturbed-tolerance fibre for short-reach networks. We observe that crosstalk levels of nearly -30 dB can be achieved for eight spatially independent transmission channels in a GI fibre of 200 μm diameter with a well separated angle of a 45° spacing distance between adjacent channels, and with an optimum distance of 60 μm from the centre of the structure. In addition, the key studies in this work have emphasised the theoretical studies in the perspective of the fractional Fourier transform (FRT), a generalisation of the Fourier transform, and the formation of reproductions of the incident arbitrary beam profiles, defined in term of Gaussian beams with an equivalent beam diameter of 10 μm at their corresponding self-imaging distance. It was found that the launched beams simultaneously propagate and re-arranging themselves periodically at the self-imaging planes along the length of the simulated commercially available fibres and the proposed large core GI-MMF, structured with 200/400 μm (core/cladding) diameters with a numerical aperture of 0.132. The results of self-imaging length intervals were in a good agreement with the analytical predication in both of a single channel transmission and a high-density transmission. Fibre bending and harsh environmental variations are of particular importance to most optical links since they affect the transmission capabilities of the fibre system. We have demonstrated the behaviour of light propagation in both a perfectly straight graded index fibre and as the fibre undergoes perturbations due to bending and temperature changes. A low propagation loss of 0.75 dB/5° bend was found for a single channel transmission, whilst the less sensitive to bending effects of approximately 6.27 dB/5° bend were realised from four spatial channels in our proposed large core-small NA GI-MMF with the curvature radius of 400 mm. More interestingly, bending does introduce a critical issue for the realisation of the re-imaging of the spatial channels at the self-imaging planes. On the other hand, the self-imaging effect of the graded index multimode fibre induced by temperature variations has no significant modification on the transmitted beam due to an extremely small change in refractive index and insignificantly modified transmission distance of the perturbed fibre. Overall, it must be concluded that a high quality self-imaging is restricted by the fibre perturbation to periodically reproduce the spatial input channels as well as the consistency in the formation of the self-imaging distances.