Electron-beam fabrication and focused ion beam inspection of submicron structured diffractive optical elements

Abstract

Diffractive optical elements, designed by computer optimization techniques, can provide high coupling efficiencies between components in optical communications systems, and can be reproduced accurately in large numbers by the same technology employed to make microcircuits. This applications-led development was for optimized f/0.48 Fresnel lenses to couple light from arrays of semiconductor lasers into monomode optical fibers. The lenses were fabricated as four phase level structures in fused quartz using electron-beam lithography and reactive ion etching. The low f number needed to match between the laser and fiber numerical apertures required 0.2 μm feature sizes in the outer region of the lenses etched 2.1 μm deep. Effective use was made of focused ion beam etching and imaging to obtain cross sections of these high aspect ratio structures during process development. Differing lens designs have achieved 34% and 50% coupling efficiencies between 30° full width half peak 1.55 μm lasers and cleaved, monomode system fibers. In the former case, lens to fiber alignment tolerance was ±6 μm, making passive assembly of the lens fiber arrays feasible.