The core of a low phonon energy glass optical fibre is doped with trivalent Dysprosium ions for application as an efficient optical fibre amplifier operating around 1.3 .mu.m. The device utilises the transition .sup.6 H.sub.9/2 -.sup.6 F.sub.11/2 to .sup.6 H.sub.15/2. The low phonon energy (high quantum efficiency) glasses include mixed halides, chalcogenides and chalcohalides. Improvements in efficiency can be achieved by co-doping of the fibre core with Eu.sup.3+ and/or Tb.sup.3+ ions.