Abstract: A gain-locked dual stage or multi stage optical amplifier. The input signal is injected into an optical waveguide, such as an optical fiber, provided with at least a first and a second amplifying stage. Each amplifying stage is pumped by a corresponding pump source producing a pump signal having an initial pump power value. Means are provided to monitor the initial pump power of the first pump source, the initial pump power of the second pump source and the pump power of the portion of this signal unabsorbed by the first amplifying stage. The initial pump power of the first and second pump signals are controlled together, being set to a same controlled value which is calculated based on the pump power of the unabsorbed signal from the first amplifying stage.