Phosphate glass core and silica cladding laser fiber

The standard silica-based EDFAs have doping level limitations and thus limited gain per unit length. Phosphate glass can contain high concentrations of lanthanide ions. It is also a unique laser host that provides high efficiency of energy transfer from Yb ions to the upper laser level of Er ions and efficient lasing at ∼1.5 μm under pumping into Yb absorption band. For these two reasons phosphate glass could be an ideal candidate for short-length EDFAs. But phosphate glass is mechanically much less stronger than silica one and is liable to corrosion by air moisture. It is also rather difficult to weld the phosphate glass fiber together with a standard silica one because of quite different softening temperatures. The goal of the present work is to develop a composite Yb-Er doped optical fiber that combines the advantages of both materials — phosphate and silica glasses. To the best of our knowledge, the idea to fabricate a laser fiber with rare-earth activated phosphate core and silica cladding was first implemented in [1] where laser action in a multimode fiber with neodymium-doped phosphate glass core and silica cladding was demonstrated.