THE EFFECT OF PUMP AND SIGNAL FIELD OVERLAP WITH THE ION DISTRIBUTION ON THE EFFICIENCY OF AN ER3+-DOPED FIBER AMPLIFIER

We report the use of a far-field scanning system to measure the pump and signal fields within erbium doped fibre amplifiers. Measurements were carried out on fibres with both uniform dopant throughout the core and dopant confined to the central region of the core. We have demonstrated that the improvement in amplifier performance associated with ion confinement is consistent with the change in overlap between pump and signal fields and the radial ion distribution. It is shown that most of the improvement available through the process of confinement can be obtained through restricting the dopant to the inner 40% of the pump mode field radius. The effects of radial ion confinement on the small-signal gain performance of amplifiers exhibiting pump excited states absorption are discussed and compared to the effects on amplifiers free from pump ESA. We conclude that the process of confinement is only of benefit in amplifiers free from the parasitic effects of pump excited state absorption.