Study of the synthesis of SiO2–TiO2–GeO2 gel glass for hollow waveguide application in CO2 laser delivery

Abstract

The SiO₂–TiO₂–GeO₂ sol–gel glass system was firstly ascertained to be the material to use in the fabrication of a hollow waveguide for application in CO₂ laser delivery. SiO₂–TiO₂–GeO₂ sols of various compositions were prepared by using Si(OC₂H₅)₄, Ti(OC₄H₉)₄ and Cl₃GeCH₂CH₂COOH as the precursors. It was found by viscosity and TEM analysis that the stability of the sol was weakened greatly when the content of germanium dioxide reached 35% because of self-polycondensation of hydrolyzed Cl₃GeCH₂CH₂COOH. The sol with a composition of 40SiO₂·30TiO₂·30GeO₂ was chosen as the coating solution because of its relatively high content of Ti and Ge atoms and its long lifetime of 30 days. It was confirmed by XRD analysis that no crystal TiO₂ and GeO₂ phases separated from SiO₂ and that the material remained glassy. Compared with the IR reflectivity peaks of silica glass, the IR reflectivity peaks of the SiO₂–TiO₂–GeO₂ gel glass are greatly broadened due to the formation of various mixed bonds between Ti, Si and Ge through bridging O atoms. The wavelength of the CO₂ laser is included in this peak and a refractive index below one is obtained. The use of this material in a hollow waveguide structure for the delivery CO₂ laser is discussed on the basis of the calculated complex refractive index and transmission loss. The results show that a SiO₂–TiO₂–GeO₂ sol with a composition of 40SiO₂·30TiO₂·30GeO₂ is a good candidate material for this application.