Fabricating non-uniform diffraction gratings

A device comprising:

• a diffractive optical element (DOE) having one or more layers on at least partially in a substrate, each layer including an orthogonal pupil expansion (OPE) diffractive element and an exit pupil expansion (EPE) diffractive element,
• wherein the OPE diffractive element comprises a first non-uniform grating configured to deflect part of input light beams propagating in the substrate into the EPE diffractive element in the substrate,
• wherein the EPE diffractive element comprises a second non-uniform grating configured to deflect part of deflected light beams from the OPE diffractive element out of the substrate, and
• wherein the first non-uniform grating has a first characteristic that varies along a first direction, and the first non-uniform grating has increasing diffraction efficiencies along the first direction associated with the first characteristic that corresponds to linearly varying densities of ions implanted in a medium of the substrate along the first direction.