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.