A method and apparatus for modulating an RF signal. The RF signal is supplied to a grating modulator having a grating, and light of at least a first wavelength is supplied as a first optical carrier to the grating modulator. The first carrier light is modulated by the grating modulator, wherein the first wavelength of the first carrier coincides with a null in the third derivative of a transmittance spectrum of the grating. Optionally, light of at an additional, second wavelength as a second optical carrier, is supplied to the grating modulator which modulates the second carrier light. The second wavelength of the second carrier coincides with another null, but different than the first mentioned null, in the third derivative of a transmittance spectrum of the grating.