Advances in aberration-correcting gratings using electron beam fabrication techniques

We have developed the capability to optimize a diffraction grating with arbitrary groove density and direction as a function of location. The added degrees of freedom allow additional correction of optical aberrations beyond what is available to holographic recordings. Since the groove direction and density can be independent but continuous for all points on the grating, we are not constrained by the limitations of ensuring that the grooves follow a single parametrized function. By fabricating a grating with an e-beam in silicon, we are able to produce a coherent, continuous grating across a silicon substrate. Silicon substrates have a number of advantages for optical designers, with ready availability. Additional advances in fabrication are providing improved grating efficiency. The key advance we report here is the adaptation of existing semiconductor fabrication technology to create a grating with grooves that are functionally independent across the entire grating. By ensuring that the grooves are continuous and coherent, we are able to fabricate a grating with unprecedented optical performance at low cost. Work to date includes fabricated test pieces, testing of the pieces, and refinement of the modeling of the optical performance.