Abstract: The present invention relates to an apparatus for generating a control signal for at least one periodic filter arranged to filter the optical profile of a signal, at least one of the phase and the amplitude of the filter being tunable, the apparatus comprising an optical waveguide for receiving a portion of said signal, optical filtering means connected to said waveguide for filtering the received portion, and arranged to act as a first and second periodic filter, each having substantially the same period as the filter to be controlled, the phase between the periods of the first and second filters being a non-integer of &pgr;, and optical power measuring means arranged to measure the optical power of the signal transmitted by said optical filtering means. A method is also described for determining such a control signal. Such periodic filters can be used to flatten the gain profile of an optical amplifier.