title: Periodic segmented waveguides in Ti:LiNbO3
Creators: name: Ortega Gonzalez, Daniel
Publisher: name: University of Glasgow
Year: 1998
Subjects: subject: Optics.

description: This aim of this thesis is to study the propagation of light in Periodic Segmented Waveguides (PSWs). The work focused on three main areas: Assessment of the accuracy of representing a PSW with an equivalent continuous waveguide, where the cut-off wavelength is a function of the PSW duty cycle. Experimental verification of the accuracy of this statement is reported, for the first time, through the fabrication and testing of Ti:LiNbO3 PSWs. This is achieved by studying the cut-off wavelength of PSWs. Due to asymmetry, Ti:LiNbO3 waveguides experience cut-off. This presents a problem in the fabrication of PSWs, where as a consequence of the segmentation, there is a reduction in the refractive index difference which reduces the cut-off wavelength. Due to the birefringence of the material the cut-off wavelength is different for the ordinary ray and the extraordinary ray. A model has been developed to obtain the cut-off wavelength as a function of the duty-cycle, as an aid to the fabrication of PSWs. o The equivalent continuous waveguide model can describe some of the properties of PSWs, but does not account for the losses associated with the segmentation. For the first time light propagation in a PSW has been demonstrated using a 3D finite difference beam propagation method (3D FD BPM). The software has been specifically developed to study PSWs. This provides the possibility of observing the field distribution as it propagates over one period, and illustrates the segmentation losses in the waveguide. Careful control of the optical field at the edges of the computational window (to minimise errors) allowed an assessment of the radiation loss for different periods and duty-cycles. Previously published work only investigated the segmentation losses for 2D structures, with radiation towards the substrate only. We compared the mode size of PSWs and its equivalent continuous waveguide. The agreement between the two sets of results confirmed that the model can be used to describe PSWs, and indicated that the 3D BPM accurately simulates PSWs. o Examination of the PSWs interaction in phase matching processes. In this thesis, PSWs were fabricated, and using the second order nonlinear properties of LiNbO3, the linear propagation of PSWs were studied. In grating assisted coupling any difference between the propagation constants is compensated by the period. Grating assisted coupling with PSWs is not new, as it has been used previously in SHG, but at present the behaviour of the propagation constant as the PSW quasi-mode propagates is not evident. Some authors propose a change in the propagation constant as the wave progresses through areas with different indices. But no evidence of this has been found in this work or the literature. In this thesis the fabrication of chirped PSWs and the tuning curves for SHG are reported. All the software used to simulate SHG has been developed to take into consideration the dispersion of the material and some of the limitations imposed by fabrication.