Laser micromachining optical elements in a substrate

A method of forming optical elements in a substrate, the method comprising:

• providing a variable aperture;
• providing a pattern mask set comprising pattern masks, each of the pattern masks comprising shape-defining elements, the pattern masks differing from one another in density of the shape-defining elements by first density increments;
• juxtaposing one of the pattern masks with the variable aperture to form a combined mask for forming the optical elements in a defined averaging area of the substrate, the averaging region having an averaging region area that is a small fraction of a substrate area;
• setting a size of the variable aperture;
• aligning the averaging region of the substrate and the combined mask with one another;
• directing a light beam through the combined mask such that the variable aperture limits the light beam in size and the selected pattern mask spatially modulates the light beam in intensity;
• focusing the light from the combined mask onto the averaging region of the substrate to form optical elements in an active sub-region of the averaging region, the optical elements having an average density within the averaging region determined by the density of the shape-defining elements in the one of the pattern masks and the size of the variable aperture;
• repeating the juxtaposing, the setting, the aligning, the directing and the focusing with different combinations of the size of the variable aperture and the one of the pattern masks with respective averaging regions of the substrate to form optical elements in the substrate, wherein the optical elements have average densities that vary among the averaging regions in increments that are small compared to the first density increments.