Discrete Optical Solitons

Morandotti, Roberto (1998) Discrete Optical Solitons. PhD thesis, University of Glasgow.

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Discreteness is a basic property of nature itself. For example energy transport through polarons, excitons or defects depends critically on the internal molecular structure. Also complex macroscopic systems as waveguide arrays or discrete electrical lattices can be understood by an effective discretisation. This allows for the identification of basic principles and effects of discreteness, which are much more general than the model from which they are derived. One of the most interesting features of all these systems is the existence of robust solitary states, or discrete solitons: in this thesis we report the first experimental observation of discrete spatial optical solitons in arrays of waveguides. Light was coupled in the central waveguide. At low power, the propagating light widened its distribution as it coupled to more distant waveguides. When sufficient power was injected, light was localised close to the input waveguides and its distribution was successfully described by the discrete non-linear Schrodinger equation. By using a non-linear waveguide array we also demonstrate the distinct dynamical properties of discrete solitons, which are not found in the corresponding continuous systems. We observed non-linearly induced locking of initially moving solitons as well as the acceleration of resting solitons. These abrupt changes of the propagation angle with respect to the waveguide direction are a result of the lack of momentum conservation in discrete systems. Our work on optical arrays was concluded by the experimental investigation of the linear and non-linear optical response of a non-uniform waveguide array. By reducing the width of a single waveguide we decreased its effective index and induce waveguiding along the defect. Due to the positive non-linearity the index difference was reduced for increasing power levels with the result that the field escaped. Finally, with the aim of fabricating regions with different non-linearities in a same chip, we performed a comparison between the relevant non-resonant non-linear optical properties of an AlGaAs multiple quantum well heterostructure before and after intermixing. In particular, we experimentally investigated the wavelength dependence of the non-linear refractive index, two-and three-photon absorption and the ratio between self-phase modulation and cross phase modulation.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Stewart Aitchison
Keywords: Electrical engineering, Optics
Date of Award: 1998
Depositing User: Enlighten Team
Unique ID: glathesis:1998-75928
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 17:26
Last Modified: 19 Nov 2019 17:26
URI: https://theses.gla.ac.uk/id/eprint/75928

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