Glass fibres of pure and erbium- or neodymium-doped yttria–alumina compositions

Abstract

Optical fibres doped with lanthanide or transition-metal elements can serve as in-line lasers and amplifiers for fibre-optic telecommunications systems. In general, most such fibre lasers use conventional silica-glass fibres doped with erbium or neodymium. But silicon dioxide absorbs strongly in the infrared for wavelengths of greater than 4 µm or so, limiting the infrared range over which such lasers can operate. Some other oxide materials do not absorb significantly until longer wavelengths—the absorption coefficient of crystalline silica at 4 µm is equal to that of yttrium oxide at 7.1 µm and of sapphire (a form of alumina) at 5.1 µm, for example¹. Glass fibres made from these materials would therefore expand the range of fibre lasers into the mid-infrared. But molten oxides that do not contain silica typically have a viscosity too low to support fibre-pulling processes. Here we demonstrate that containerless processing, in which a molten sample is levitated by a flow of inert gas, permits sufficient undercooling of molten yttrium aluminium garnet (YAG:Y₃Al₅O₁₂) to access a viscosity range conducive to fibre-pulling. The process is particularly effective if the molten material of stoichiometric YAG composition is doped with Nd₂O₃ in place of Y₂O₃, or with excess Al₂O₃; and it should also work with other dopants, because molten oxides are good solvents. Fibres could be drawn from a melt doped with Er₂O₃ in the presence of excess Al₂O₃. These fibres have the potential to extend the operating range of oxide glass-fibre lasers.