Patent ID: 7018745

Claim:
A method of writing an extended grating structure in a photosensitive waveguide comprising: utilising at least two overlapping beams of light to form an interference pattern, moving the waveguide through an overlap region defined by said overlapping beams, simultaneously varying the phase of at least one of the beams relative to at least one of the other beams utilising a phase modulator, thereby variably controlling the velocity of the maxima within said interference pattern so that such velocity is different from the velocity of the photosensitive waveguide within the overlap region for the duration of the writing, excluding any periods of velocity change when the velocity of said maxima momentarily equals the velocity of the waveguide, wherein: the phase modulator comprises a non-mechanical means for effecting the phase modulation, and variation of the velocity of the maxima within the overlap region relative to the photosensitive waveguide is utilised to vary at least one of the phase, amplitude and period properties of at least a portion of the extended grating structure in the photosensitive waveguide.