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An all-optical feedback loop is demonstrated which effectively suppresses slow, signal-induced gain fluctuations in Er-doped fibre amplifiers (EDFAs). Gain control is achieved by operating the amplifier in a ring-laser configuration. The working principle is shown theoretically and experimentally by coupling a selected wavelength of the ASE at the output of the EDFA back to the input. Such a feedback loop, which can be fabricated out of all-fibre passive components, could be easily implemented in any system using EDFAs.