Generation of light in optical fibers made of glasses formed from rare-earth ultraphosphate crystals

Free-oscillation lasing was observed at λ = 1.055 μ in a fiber with a core made of glasses formed from mixed ultraphosphate crystals of compositions Nd_xR_1–xP₅O₁₄ (R = La, Gd, Y, Bi). The core diameter was 60 μ and the fiber length was 3.7 m. The best lasing characteristics were obtained for a fiber with a core of glass produced from an Nd_xBi_1–xP₅O₁₄ crystal: the lasing threshold was 7–10 J and the output energy was 500–600 mJ when the pump energy was 98 J. The efficiency of lasing in ultraphosphate fibers was twice as high as in GLS-24 phosphate glass.