Evanescent Field Coupling Between a Single-Mode Optical Fibre and a Thin Film Waveguide

Bradley, Laurence Daniel (1986) Evanescent Field Coupling Between a Single-Mode Optical Fibre and a Thin Film Waveguide. PhD thesis, University of Glasgow.

Abstract

The work of this thesis is concerned with a new method of coupling light from an optical fibre to a thin film waveguide using the interaction of the evanescent fields of the two guides. In particular the fibre used was standard single mode fibre, in widespread use in telecommunications networks. The fibre and film characteristics are discussed in chapter 2. In this type of fibre light is guided by a central core region which is typically 8-10um in diameter and is surrounded by a cladding which is typically 125um in diameter. The evanescent field of the guided light decays very rapidly away from the core region. The main difficulties in exploiting the fibre evanescent field are its limited extent, its isolation by the cladding layer and the physical dimensions involved, typically microns. It was necessary to employ a device geometry which allowed micron and sub-micron tolerances to be achieved using mechanical tools. This geometry known as the tapered v-groove coupler (TVC) allowed access to the fibre evanescent field by removing a portion of the cladding and it is described in chapter 4. It also incorporated a planar surface which was suitable for planar film deposition. This allowed thin-film waveguides to be formed directly onto the TVC where they were in rigid and intimate contact with the polished fibre. Evanescent field coupling requires (for significant power transfer) that the two waveguides have almost identical propagation constants. The TVC incorporated a tapered region which reduced the fabrication tolerances involved in achieving synchronism between the two guides. Using the TVC, coupling from fibre to film to fibre was demonstrated with an insertion loss of 2dB. The wavelength dependence of the coupler was investigated and is discussed in chapter 5. Coupling to rib and channel guides is discussed in chapter 6. It was found that although planar films could be straightforwardly aligned with the fibre core using this geometry, alignment of rib guides involved meeting tolerances of microns. This work has important applications in the field of coupling between standard fibres and integrated optical waveguides since it complements, rather than competes with the existing technique of butt-coupling. It is particularly suitable for coupling to very thin films (1um thick) which can be deposited on a silica substrate.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Electrical engineering
• Date of Award: 1986
• Depositing User: Enlighten Team
• Unique ID: glathesis:1986-77416
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jan 2020 09:09
• Last Modified: 14 Jan 2020 09:09
• URI: https://theses.gla.ac.uk/id/eprint/77416