Investigation of continuous-fibre devices based on coupling to high-index planar overlays

In recent years there has been an increased utilization of optical fibre technology in a variety of fields, telecommunications and signal processing in particular. As a result, the demand for sophisticated optical components has seen a corresponding increase, with devices such as wavelength filters, directional couplers, modulators and switches of particular interest. This thesis describes the design, fabrication and experimental characterisation of a number of continuous-fibre devices whose operation relies on evanescent coupling between the fibre and an overlay planar waveguide. The basic device architecture allows the realisation of several different optical functions, including Bandstop and Bandpass Wavelength Filtering, Intensity Modulation and Directional Switching. The devices were characterised primarily in terms of their wavelength-transmission response to variation of the overlay waveguide parameters: refractive index and thickness. Filter channel linewidths and spacing have been shown to be directly related to the refractive index and thickness of the overlay waveguide and also the distance between the fibre core and the overlay waveguide. As such, the filtering devices can be designed to given specifications using a variety of optical materials in the role of the overlay waveguide e.g. Glass, Lithium Niobate. Channel spacings and linewidths ranging from 10nm-200nm and 2.4nm-100nm, respectively, have been demonstrated while rejection ratios of >20dB and insertion loss of <0.5dB were recorded. Extensive tuning (> 200nm) ofthe wavelength-transmission response of the bandstop and bandpass filters was achieved by variation of the overlay waveguide superstrate refractive index. A modulator/switch device which incorporated Lithium Niobate as the overlay was also demonstrated but required unrealistic drive voltages. Application of > 300V was required to induce 60% modulation/switching. However, the feasibility of the device geometry for realisation of an intensity modulator/switch was established. Calculations based on materials with larger electro-optic coefficients (e.g. Barium Strontium Niobate) predict 10V drive voltages for >90% modulation/switching.

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