Patent ID: 7079312

Claim:
An optical amplifier comprising an input pump stage and one or more further pump stages, and control means for controlling pump currents supplied to the pump stages to vary the gain whilst limiting noise due to multipath interference (MPI) and amplified spontaneous emission (ASE), wherein, up to a predetermined amplification level, the gain is controlled by varying the pump current to the input pump stage, whilst ensuring that the pump current applied to the input pump stage is maintained at such a level that a finite, non-zero low value, below the pump current to the input pump stage, must at all times be applied to the or each further pump stage to maintain a required output power as the gain is varied up to the predetermined amplification level, and, above the predetermined amplification level, the pump current supplied to the input pump stage is maintained at a desired maximum value, and the pump current supplied to at least one said further pump stage is increased from said low value to control the gain as the gain is increased above the predetermined amplification level.