Bragg grating and method of producing a Bragg grating using an ultrafast laser

A method for inducing a spatially modulated refractive index pattern in an at least partially light transmissive or absorbing material, comprising the steps of:

• providing the at least partially light transmissive or absorbing material;
• disposing a mask to be used as an interferometer, adjacent to the partially light transmissive or absorbing material;
• and,
• irradiating surface of the mask with electromagnetic radiation having a predetermined wavelength range and having a pulse duration of less than or equal to 100 picoseconds, wherein the mask is disposed to permit a portion of the electromagnetic radiation to interact with the mask and be incident on the at least partially light transmissive or absorbing material, the interaction of the electromagnetic radiation with the mask for producing a plurality of diffracted orders greater than two, wherein at least one of the diffracted orders is a pair, the interaction of the electromagnetic radiation with the mask for producing a spatial intensity modulation pattern within the at least partially light transmissive or absorbing material, the electromagnetic radiation incident on the at least partially light transmissive or absorbing material being sufficiently intense to cause a change in an index of refraction of the at least partially light transmissive or absorbing material, wherein electromagnetic radiation interacting with the surface of the mask has a sufficiently low intensity to not significantly alter produced spatial intensity modulation properties of the mask, wherein the step of disposing a mask to be used as an interferometer includes the steps of disposing and orienting the mask adjacent to the at least partially transmissive material at at least a distance “d” such that group velocity walk-off results in pure two-beam interference within the at least partially transmissive or absorbing material wherein the distance “d” is chosen such that the difference in times of arrival of the order pairs due to group velocity walk-off results in the pure two-beam interference pattern of sub-beams of said pulse of light that have passed through or reflected off of the mask.