Silica- and germanate-based rare earth doped glasses for fiber lasers

Since an extension of spectral region from near-infrared to mid-infrared covered by fiber lasers belongs to ambitions of today’s research, germanate-based glasses have been proposed, fabricated and their optical properties studied. Glass samples were prepared by Modified Chemical Vapor Deposition method and by conventional glass melting processes. Fabricated preforms suitable for fiber drawing were optically characterized by refractive index and absorption spectra measurements. A limit of rare earths doping of GeO₂-based glasses without phase separation was found comparable to SiO₂-based glasses (thousands of ppm). Preforms prepared by Modified Chemical Vapor Deposition contained only up to 29 mol. % of GeO₂ in core, although pure GeO₂ glass layers were deposited. This effect caused transparency limitation of such materials to 2-2.5 μm. Implementing of effective drying processes during GeO₂-based glass conventional melting lead to decrease of residual OHcontent to the level comparable with optical silica glass HOQ 310. The shift of infrared transmission edge of bulk GeO₂-based conventionally prepared glasses to longer wavelengths towards to 5 μm was observed. These facts support the idea of potential use of GeO₂-based glass matrices for fiber lasers operating in mid-infrared region