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

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.