A device for erasing overhead modulation of an optical signal, wherein said optical signal is injected into an optical device able to modulate the optical signal (for example a semi-conductor optical amplifier driver by a bias control) and for which one an automatic gain control does not need to detect the residual overhead modulation, is characterised in that said bias control means applies to said amplifier a compensation voltage presenting a modulation out of phase with respect to said overhead modulation, and in that modulation index of said compensation voltage is (substantially) proportional to the modulation index of said optical signal.