Photorefractive gratings in optical fibres

Abstract

This thesis is an investigation of the fabrication and applications of Bragg gratings produced by the transverse illumination of optical fibres with periodic patterns of ultraviolet light. A general analysis of their optical properties is first presented which considers both the three-dimensional nature of fibre gratings as well as the nonlinear response of the glass to ultraviolet irradiation. The problem of cladding-mode coupling is treated in detail. A review of the phenomena associated with the photosensitivity of germanosilicate fibres is presented, which includes original measurements of photoinduced birefringence. To explain their dynamics, a new model is proposed, based on the photo-thermal ionisation of germania-related oxygen-deficient centres. The effect of core dopants and known sensitisation techniques on the photoinduced index change are evaluated in over twenty different fibres. The fabrication of fibre Bragg gratings using a line narrowed KrF excimer laser and a three-mirror interferometer is described in detail. Narrow, broadband and high reflectivity gratings are obtained in a variety of fibres, including several germania-free fibres. The dynamics of grating formation are measured and are accurately described by theoretical calculations. The writing process is found to be more effective at higher UV intensities, as predicted by the photo-thermal ionisation model. In an investigation of the writing of gratings using single excimer laser pulses, a new regime of grating formation is identified, which occurs above a sharply defined pulse fluence. This mechanism is used to demonstrate -100% reflectivity single-pulse gratings. Both low and high reflectivity single pulse gratings are produced for the first time in a fibre drawing tower. The interaction of KrF excimer laser pulses with germanosilicate fibres is investigated on a nanosecond timescale. This experiment reveals the existence of broadband transient gain and absorption in the 1 ^im region of the spectrum, which provides supportive evidence for a proposed energy-level diagram of oxygen-deficient centres. A new type of channel-dropping filter, the grating-frustrated coupler, is analysed and demonstrated. The performance of a first prototype - 0.7 nm bandwidth, 13 dB isolation, 70% transmission - shows great promise for wavelength multiplexing and line filtering applications. Several other applications are also described, including fibre lasers, dispersion compensators and sensors.

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