Giving the etch to superior optics.

Giving the etch to superior optics

Most high-quality lenses are still made the way Isaac Newton made this -- by grinding and polishing glass against glass. But even when automated, the process takes time, and when the lenses must have a specific geometric form for a particular application, the number of steps required to produce them increases dramatically.

For the last decade, researchers at the MIT Lincoln Laboratory in Lexington, Mass., have explored an alternative technology for making high-quality lenses and other optical elements. Using etching techniques borrowed from the integrated-circuit electronics industry, they can modify the surfaces of lenses to improve their optical properties. "Our aim is to make lenses better, cheaper and lighter," says MIT's Wilfrid B. Veldkamp. The new optical etching process is now developed enough that several companies are starting to use it for the commercial production of lenses.

The technique involves etching a lens surface to create a staircase pattern of notches that scatter light in just the right way. A diffraction grating, in which an array of parallel lines ruled on a glass or plastic surface spreads white light into a rainbow of colors, applies the same principle. By carefully controlling the depth, width and shape of such notches, the researchers can scatter light so it travels only in specific directions. Although the notion that a diffraction pattern can change the direction in which light travels goes back many years, the MIT researchers' achievement lies in learning to make the scattering process efficient. Their carefully computed designs ensure that all the scattering takes place in the appropriate directions so that no light is lost.

The etching process can turn a cheap lens into a high-performance optical element. A spherical lens -- the kind of glass lens easiest to grind and polish -- has the disadvantage that it doesn't focus light rays to a single point. The resulting image is somewhat distorted and fuzzy. But etching a carefully computed pattern of rings on the lens' surface corrects the problem. Similar etched patterns also remove the colored fringes often seen around images made by inexpensive lenses. "You can turn a $10 lens into a higher-quality one just by putting a diffraction-grating pattern on one side," Veldkamp says. Moreover, replacing a bulky, heavy, complex assembly of corrective lenses with a single etched lens capable of producing a clear image greatly simplifies the optical systems in telescopes and other instruments.

The MIT researchers have demonstrated their techniques on a variety of lenses suitable for infrared, visible and ultraviolet light. The trick is to match the lens material, which bends the light, with the appropriate etched pattern.

Because this etching process can be done on a microscopic scale, one of its most promising applications is in microoptics, for creating large arrays consisting of thousands of tiny lenses, each as small as a micron in diameter. Such arrays, which look like frosted glass, can be used for steering laser beams. They also act like a fly's eye to give a wide field of view.

Incorporated into sensors consisting of layered structures that combine optics and electronics, they show promise as integrated, automated vision systems that both "see" and evaluate what is seen. Equipping a telescope with such a scanner would enable it to identify promising spots and zoom in on them without having to scan the whole scene to the same level of detail.

"We're right in the middle of an optics revolution," Veldkamp says. "We're using lithography and etching to replace a very labor-intensive fabriction process." He adds: "The ability to mass-produce diffractive relief elements by embossing, forging or molding from a single master element is what gives this technology great potential. This potential is still largely untapped."