A net gain of 20 dB at 2722 nm has been obtained in an Er3+-Pr3+ co-do
ped fluoride fibre pumped at 800 nm. Co-doping the fluoride fibre with
Er and Pr instead of doping with Er only improved the net gain of the
optical amplifier made with this fibre and reduced significantly the
pump power requirement. These improvements can be explained by the emp
tying of the I-4(3/2) level of Er3+ towards the (3F3, 3F4) levels of P
r3+.