Raman characterization of planar waveguides fabricated by electron-beam irradiation of germanium-doped flame hydrolysis deposited silica

The structural changes responsible for the refractive index variations induced in Ge-doped flame-hydrolysis deposited (FHD) silica by electron-beam irradiation have been studied using confocal micro-Raman spectroscopy. Two structural mechanisms, an increase in the concentration of 4- and 3-membered rings in the silica structure and a reduction of the oxygen bridging bond angle between silica tetrahedra have been observed. The formation of small membered rings occurs along the penetration depth of the electrons in the layer, decreasing with depth. The reduction of the angle between silica tetrahedra only occurs on the surface of the material. The results are consistent with previous measurements of the compaction of the material and will determine the shape of the refractive index profile of the waveguides fabricated using this technique.