Forming Fine Lines on a Curved Surface

Daniel C. McCarthy

There are many ways to fabricate diffractive optical elements, including diamond milling, soft lithography, laser direct writing and thermally selective oxidation. Few methods, however, apply to concave or convex lenses, and the list thins further if the element requires precise alignment and submicron resolution. Applications that require such precision could benefit from recent experiments.

Laser direct-writing and lithography techniques combined to form precisely aligned 100-mm concentric circular gratings on concave lenses. The gratings' linewidths ranged from 0.7 to 10 µm. Courtesy of the Chinese Academy of Sciences.

Combining laser direct writing and lithography techniques, researchers at Changchun Institute of Optics, Fine Mechanics and Physics, part of the Chinese Academy of Sciences in Changchun, have fabricated a 100-mm concentric circular grating with linewidths from 0.7 to 10 µm on a concave lens surface.

The group first spin-coated photoresist to a 0.9-µm thickness onto a 110-mm-diameter concave substrate with a 504-mm radius of curvature. To align the substrate with the air-bearing spindle, they directed a 632-nm HeNe laser beam through a binary phase grating, which served as a beamsplitter. The beam passed through the grating, reflected off the lens surface and recombined to generate interference fringes that remained motionless only when the lens rotated around its axis of symmetry. This aligned the lens with submicron accuracy and helped position the direct writing beam -- a 150-mW HeCd laser operating at 442 nm -- at the proper radial location on the substrate. Atomic force microscopy confirmed that the diffractive optical element produced had 10-µm periodicity.

The precise alignment and line resolution could benefit applications including ultraviolet spectroscopic instruments and measurement of convex secondary mirrors.

Yongjun Xie, who engineered the system, said that, although thermally selective oxidation can generate optical elements on curved surfaces, it cannot produce comparable line profiles with continuous surface relief. Other methods, such as diamond milling, are feasible only for relatively soft materials such as aluminum or copper.

The technique, however, is not without drawbacks, Xie said. It cannot produce nanometer patterns, nor is it ready for mass production. Hence, the researchers plan to produce a curved gray-scale mask, which lends itself better to mass production. They also plan to eliminate the etching step by scaling the patterns onto sol-gel film.