Sabitu, Rabiu Imam
(2020)
Modelling of novel photonic devices based on micro-structure engineered for space division multiplexing optical communication system.
PhD thesis, University of Nottingham.
Abstract
Standard single mode fibre has almost explored all degree of freedom, making it hard to accommodate considerable further enhancement in data carrying capacity in next generation optical networks. As a repercussion of the posing threat of a future capacity crunch, the consequential global effort to search for a viable alternative has been mobilised in recent years. The idea is to explore new fibre types, new photonic devices, and system, aiming to support much higher capacities by orthogonally combining multiple transmission paths over the same fibre, thereby upgrading or replacing the current optical communication system by the spatial dimension. The hope is to provide means of higher information stream per unit area to enable cost reduction and power saving benefits through the improved optical device integration and system interconnectivity.
This thesis presents a few novel types of researches in the area of optical photonic devices for space division multiplexing system. The research comprises the design of two different mode converter devices; one is based on the two multi-ring core fibre, and the other is based on a tilted fibre assisted by the dual arc waveguides; and modelling of two novel high dispersion fibres using LP-mode matrix method. The thesis also discusses high-speed mode division multiplexing transmission supporting four signal mode channels.
The design of the first converter device involves varying the lateral height and the distance that separates the two multi-ring core fibres, yielding a 94% coupling efficiency. The second converter device uses a multilayer core multimode fibre due to its low bending loss, high precession in mode confinement and ease of fabrication. The converter input was tilted at an optimised angle of 120 in order to align perfectly with the waveguides connected to the horizontal fibre having the same properties. Mode conversions of an LP11 to LP01, LP02 to LP11, and LP21 to LP01 were achieved with the conversion efficiency of 89.6%, 91.2% and 94.34% respectively.
Furthermore, a novel dispersions equalisation technique for multilayer core four-mode fibre using LP mode-matrix method is presented. Each of the core ring radii of the alternating centre-high and centre-low core index profile was optimised to a specific thickness value. The core ring radius was varied in each case to determine the optimised thickness for the dispersion equalisation. The optimised thickness of the centre high index core fibre denoted by R1, R2, and R3 were found to be 4.25 µm, 2.6 µm and 2.3 µm respectively. Similarly, an optimised thickness of the centre low index core fibre, which was denoted by X1, X2, X3 and X4 was found to be 2.0 µm, 3.0 µm, 4.20 µm and 1.60 µm respectively. At 1.55 µm wavelength, these values were used to model the two fibres to achieve an equalised modal dispersion.
Finally, performance analysis of high-speed mode division multiplexing transmission system was carried out. The binary non-return-to-zero and return-to-zero-on-off keying modulation at 10 Gb/s and 20 Gb/s bit rate were adopted using direct detection. The transmission distance was optimised by employing the dual port dual derive Mach Zehnder modulator. Expanding the proposed devices to accommodate more high order modes can enhance the bandwidth of a future spatial mode based-communication system.
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