The present invention relates to a system for writing an optical structure in a waveguide. The system including, means for splitting a light beam into two coherent writing beams, an optical circuit for directing the writing beams along substantially the same optical path in opposite directions such that they produce an interference pattern in an interference region substantially within the waveguide to write the optical structure, said optical circuit including at least two acousto-optic modulators (AOMs) configured to enable a controllable phase shift to be applied to a light beam propagating therethrough, the two AOMs being arranged such that, in use, each AOM shifts the phase of only one of the writing beams, to thereby provide a controllable phase difference between the writing beams.