Optical waveguide fabrication in silica using flame hydrolysis
This thesis is concerned with the fabrication, assessment and application of doped silica waveguides using Flame Hydrolysis Deposition. Deposition apparatus has been designed and constructed. This equipment consists of a gas supply assembly, a bubbler cabinet, a deposition box and a chemical scrubber. An optimum sintering regime for the low density silica soot has been established consisting of 60 minutes at 1250oC. This regime is dependent on the levels of P2O5, GeO2 and/or TiO2 doping in the silica host. Independent control of layer thickness and index is achieved. Refractive index can be varied by changing the doping levels, and thickness, by increasing the number of traversals of the hydrolysing flame. Film homogeneity in terms of layer thickness was found to be difficult to control and up to 30% variation in thickness was obtained in some samples. Thermophoretic effects were shown to play an important part in the deposition process. Titanium and phosphorus doped films were found to suffer film degradation and aging. This led to the formation of titanium rich crystals and crystal agglomerates which produced Rayleigh/ Mie scatter in the sintered films. No such problem was found with Germanium, and all films were subsequently fabricated using Germanium and Phosphorus. No out-of-plane scatter was observed in the sintered films. Ridge waveguides were fabricated using lithography and Reactive Ion Etching in a CHF3 plasma, giving waveguides with smooth side walls. Loss assessment was carried out using two techniques, both of which were non-destructive. One involved a video camera to scan across the waveguide and detect the out-of-plane scatter. This technique was sensitive to scattering centres and was less accurate for short lengths of guide. Loss figure varied from 0.2 to over 5 dB/cm depending on the scan length. The second technique involved turning the waveguide into a Fabry-Perot resonator, by coating the end faces of the waveguide. Temperature induced cycles in the output intensity could be used to obtain the waveguide attenuation. This technique was sensitive to the facet angles of the guides which could contribute significantly to the loss measured. Such resonances were obtained in only one sample and gave a loss figure of over 9 dB/cm which is considered unreliable. Films were doped with both Nd3+ and Er3+ using a solution doping technique and fluorescence spectra were obtained for both. Increased scatter was observed in the planar films with such doping. Holographic gratings were fabricated on planar waveguides with a view to their incorporation as feedback elements in a laser structure. Second Harmonic Generation was observed for the first time in planar rib waveguide structures doped with Phosphorus and Germanium. In a `seeding' experiment where 1064nm radiation was launched at the same time as a second harmonic `seed', a two-hundred fold increase was observed in generated second harmonic signal over the background level.