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Reduced mode coupling and nonlinear interaction in optical fibres

Gulistan, A. (2019). Reduced mode coupling and nonlinear interaction in optical fibres. (Unpublished Doctoral thesis, City, University of London)

Abstract

One of the main objectives of this dissertation is to understand the nonlinear effects in optical fibres caused by stimulated Brillouin scattering (SBS). SBS is a nonlinear process, where, a sound wave is generated due to electrostriction effect that creates a travelling Bragg gratings and prevents delivery of high optical power beyond the SBS threshold. An improved full-vectorial numerically efficient Finite Element Method (FEM) based computer code is developed to study complex light-sound interaction in optical waveguides. An improved polar meshing technique is used to efficiently discretised the computational domain consisting of circular boundaries such as optical fibres. The existing structural symmetry of the optical waveguide is also exploited in both optical and acoustic modal solutions and only a half or quarter structure is simulated, as needed. This allows a more efficient element distribution on a smaller region compared to full structure with similar computational resources that result in the improved modal solution accuracy and reduced modal degeneration. The existence of spurious (non-physical) modes in full-vectorial acoustic modal solution and their elimination using the penalty method is also proposed and tested in this thesis. Penalty term consisting of the curl-curl section of the acoustic formulation enforces the acoustic field to suppress the rotational energy of the propagating acoustic wave. As a result of the penalty method, a significant improvement in the solution accuracy and quality of acoustic modes is demonstrated in both low and high index contrast optical waveguides.

A standard single mode Germanium doped Silica fibre is used to study light sound interaction, and overlap of 93 % between fundamental optical and acoustic modes has been calculated. This acoustic-optic overlap is directly related to the calculation of the SBS threshold. A lower SBS overlap results in an increased SBS threshold and allows more power to be transferred in the optical fibre. Through rigorous numerical simulations, the fibre geometry and refractive index profile are modified, and a layer of a high acoustic index is introduced in the cladding such that the acoustic mode propagates in the cladding. The optical refractive indices of core and cladding are kept same while the acoustic index of 2nd layer is increased by doping it with Boron and Germanium (3.394% B2O3 + 2% GeO2). The acoustic mode completely shifts in the 2nd layer and as a result of this technique, an extremely low overlap of 2.5 % is calculated between optical and acoustic modes.

Another objective of this research was to reduce the nonlinear effects in optical fibres is the use of large mode area (LMA) fibres such as few mode and multimode fibres. LMA fibres provide large effective area resulting in less nonlinear effects for a given power compared to single mode fibres. However, the existence of more than one mode may result in the inter-mode mixing and energy may transfer from one mode to its neighbouring propagating mode. Higher order modes of a MMF has twofold advantage as these modes provide a large effective area and also exhibit a weaker coupling with other modes. To mitigate this, we have proposed two novel techniques to increase the modal stability between higher order modes of a multimode step-index fibre. The modal stability is directly related to the effective index difference (Δne f f ) between a given mode of propagation and its neighbouring antisymmetric modes. One of the technique involves the use of strategically located low or high index doped strips along the circumference of MMF such that the modal stability between LP0,n, a higher order mode and its neighbouring antisymmetric LP1,n−1 and LP1,n+1 modes can be increased. We have shown that the modal stability of LP09 mode and its neighbouring antisymmetric LP18 and LP19 modes increases more than 35 % from its original value. In the second technique, we have used an array of strategically located air-holes to increase the modal stability of LP06 mode and its neighbouring antisymmetric LP15 and LP16 modes up to 54 %. Similarly, an air-hole array is also used to increase the modal stability of a few-mode fibre. The Δne f f between first four (LP01, LP11, LP21 and LP02) modes of a four-mode fibre is increased more than 30 %. In both the methods we have shown the effect on the effective index difference due to change in the strips width or central location and holes width or central location, respectively. Moreover, it is also shown that both proposed techniques are scalable and can be used to increase the modal stability of other higher order modes.

Publication Type: Thesis (Doctoral)
Subjects: T Technology
Departments: Doctoral Theses
School of Mathematics, Computer Science & Engineering
URI: https://openaccess.city.ac.uk/id/eprint/23779
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