Rare-earth-aluminum oxide-composition glass fibers 5-50 μm in diameter and containing up to 50 mole % rare-earth oxide were drawn from undercooled liquids 550-650 K below the equilibrium melting point. The fibers have tensile strengths of ∼6 GPa, glass transition temperatures of ∼1150 K, and infrared transmission up to ∼5500 nm. The optical properties of erbium-doped fibers containing up to 12.5 mole % Er2O3 were investigated. The 1/e lifetime of the 4I13/2 excited state was 0.8-7 ms, decreasing with increasing Er concentration. Amplified spontaneous emission measurements indicate extremely broadband spectra, up to 135 nm (3-dB width) in 0.5 mole % fibers. Although this result is encouraging, the gain bandwidth, which has not been measured, is likely narrower. Glass fibers were crystallized by heat treatment under tension at temperatures of 1300-1900 K to form flexible, creep resistant polycrystalline monofilaments with tensile strengths up to 2.4 GPa.