Bragg gratings in optical waveguides, glasses and thin oxide films induced by excimer laser radiation

Abstract

The purpose of this thesis is the fabrication and characterisation of sub-micron period (~500nm) Bragg relief grating reflectors in optical waveguide structures using interferometric excimer laser ablation. Furthermore, the work presented in this thesis describes investigation of the feasibility of the application of excimer laser grating micromachining for the fabrication of waveguide gratings which may be used as wavelength filters in the 1.5µm band for optical communications applications. For this purpose the following studies were carried out: (i) the ablation of sub-micron relief gratings on Tl+ ion-exchanged channel waveguides in Er/Yb-codoped B1664 glass, using 193nm excimer laser radiation and (ii) the ablation of sub-micron relief gratings in InOx and Ta2O5 thin oxide films overlaid on K+ ion-exchanged channel waveguides in BK-7 glass, using 248nm excimer laser radiation. For the waveguide gratings in Tl+ ion-exchanged Er/Yb-codoped B1664 glass reflectivities greater than 28dB were obtained from a 6mm long grating. Similarly, reflectivities greater than 4.7dB and 18dB were produced by 16mm long gratings in K+ ion-exchanged channel waveguides in BK-7 glass overlaid with InOx and Ta2O5 thin oxide films, respectively. In parallel, the ablation behavior of borosilicate glass using 193nm radiation and that of InOx and Ta2O5 thin oxide films using 248nm radiation were studied experimentally. Waveguide grating theory based on the study of the dispersion properties of multi-layer step-index planar and graded-index channel waveguides and on the scattering behaviour of grating structures in waveguides, has been developed and analysed. This theoretical background is employed to design the waveguide gratings used and to explain the experimental results obtained. Finally, the photorefractive properties of InOx and Ta2O5 thin oxide films under UV radiation are experimentally investigated