Abstract: In a gain-controlled erbium-doped optical amplifier, gain control is achieved by clamping the gain of a laser cavity to be equal to the overall cavity loss and by fixing the resonant wavelength of the amplifier to be at a first wavelength. When an optical signal to be amplified having a second wavelength different from the first wavelength passes through the amplifier the gain experienced by the signal depends entirely on the gain of the cavity, and not on the intensity of the signal. If the first wavelength is arranged to be at the peak of the sum of the absorption and emission cross sections of erbium, the amplifier exhibits minimum sensitivity to ambient changes in temperature.