A system (10) for controlling gain of an optical amplifier that is disposed in a WDM optical communication network for amplifying an input WDM signal to produce a corresponding amplified output WDM signal is disclosed. The system comprises means (12, 14, 16, 18) for measuring average optical powers of the input WDM signal over first and second wavebands, said first and second wavebands each covering a wavelength range that includes at least one wavelength channel (channels 1-20 and channels 21-40) of the WDM network and means (26, 28, 30, 32) for measuring the average optical powers of the amplified output WDM signal over the first and second wavebands. The system further comprises means 834) for determining gain values (Gainred, Gainblue) over the first and second wavebands from the average input and output powers; and means (34) for adjusting a variable (22) of the optical amplifier to ensure that the difference (ΔGain) between the gain values is maintained to within a pre-determined value thereby ensuring a substantially flat gain slope profile.
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