Fabrication and optical characteristics of thick GeO₂ sol-gel coatings

Thick, crack-free GeO₂ sol-gel coatings were fabricated using tetraethylorthogermanate and polyvinylpyrrolidone as starting raw materials and investigated as the cladding material for hollow waveguide delivery of CO₂ laser radiation. A favorable GeO₂ coating solution was synthesized by adding PVP to a TEOG-ethanol solution under dry nitrogen atmosphere in a glove box. TEM and viscosity analyses showed the PVP chains adsorbed onto GeO₂ particles capped the surface HO-Ge- groups of particles and provided steric hindrance against aggregation of particles resulting in a stable sol with a viscosity value of about 35 mPa s. TG-DTA analyses revealed that the densification of the GeO₂ gel coatings with PVP progressed within a wide temperature range from room temperature to 650 °C leading to the production of crack-free GeO₂ coatings with a thickness of 4 μm. The IR transmission spectra and XRD studies indicated that calcination of the coatings at 700 °C for 60 min in an oxygen atmosphere removed effectively the organic complexes and hydroxy groups and transformed the coatings into hexagonal GeO₂ crystalline phase. The optical constants of the GeO₂ coatings were extracted from the IR reflective spectra and the complex refractive index N of the coatings was determined to be 0.5631-0.8347i at 940 cm^-1. The theoretical transmission loss and the additional loss due to scattering from roughnesses of this material were calculated. The results showed the optic loss at the wavelength of CO₂ laser based on the GeO₂ coatings as the cladding material can be expected as low as 0.0556 dB/m.