Fabrication and characterization of germanium doped silica ridge waveguides using a novel colloidal suspension approach

A novel colloidal suspension process and associated fundamental principles were investigated to produce optical-quality, pure and germanium-doped silica films for integration of planar lightwave circuits (PLC) with silicon optical bench (SiOB) technology. Studies of the rheology of the colloidal suspensions of nanosized fumed silica and the characteristics of the resultant films (e.g. critical thickness) allowed the selection of an optimal colloidal suspension for thick silica films. Crackfree, silica films up to 1.4 mum in thickness per layer could be obtained after consolidation in 1 atm of He-O 2 mixture at 1300°C. Multiple spin coating followed by the consolidation treatment yielded fully dense silica films of thicknesses in the order of 10 mum, sufficient to meet the dimensional requirement for planar optical waveguides.; For pure silica films, their refractive index was 1.4577 +/- 0.0005 at a wavelength of 632.8 nm. This value is comparable to that of thermally grown oxide on silicon substrate, a confirmation of the density of the silica films. Germanium doping of silica to various concentrations was achieved using colloidal silica suspensions mixed with tetramethylammonium germanate aqueous solutions. Indices of refraction of germanium-doped silica films increased linearly with dopant concentration, thus enabling index tuning (maximum refractive index contrast, Deltan = 0.015) while maintaining the amorphous nature as well as the desired optical properties of the films.; Prototype ridge waveguides were fabricated using 3.1 mol % germanium-doped silica layers on 15 mum thick thermally oxidized silicon substrates by conventional photolithography and reactive ion etching. The propagation loss in the waveguides (6.5 mum high, 10 mum wide in the cross-section dimension) was 3.3 +/- 0.5 dB/cm as measured by the cutback method. Further reduction in propagation loss is possible with improved sidewall smoothness of the waveguides and application of overcladding layer.; This research represents the first known attempt to use the colloidal suspension approach for the fabrication of ridge waveguides on silicon substrates. The knowledge established in the chemistry of colloidal suspensions as well as the correlation between processing and properties of the spin coated pure and doped silica films will prove valuable in further developing and implementing a commercially viable sol-gel method for PLC with SiOB technology.