Reprinted from Optics and Photonics News, January, 1997 Senior Staff Writer, Erik Kreifeldt

DARPA has identified a need for high-performance optics of arbitrary shapes, particularly shapes more aerodynamic than the spheres, flats, and mild aspheres that make up today's optical arsenal. This fall, the agency put up $24.6 million to fund two four-year programs to develop "conformal optics," which conform to design specifications of aircraft and missiles — instead of the other way around. When the technology trickles down to non-defense applications, conformal optics have the potential to change optics as we know it, according to Pat Trotta, T. I. Fellow at Texas Instruments.

One $12.3 million program supports Texas Instruments, the Boeing Co.'s Defense and Space Group, the Center for Optics Manufacturing at the University of Rochester, Sinclair Optics, and Rochester Photonics Corp. A parallel $12.3 billion effort has been arranged with the Optical Sciences Center at the University of Arizona, Hughes Electro-Optical Systems, Hughes Missile Systems, and McDonnell Douglas Aircraft. Optical Research Associates and Morton International are also involved with the second consortium. "This is an enormous task," says John Greivenkamp, associate professor at the Optical Sciences Center. He says the separate approaches are complimentary. "They're not duplicate efforts."

Windows in the body of aircraft and missiles are necessary for optical systems on-board, such as guidance, tracking and reconnaissance. Conformal optics would offer aerospace engineers a new tool for improving aerospace designs, allowing improvements in field of view, aerodynamics, detectability, and cost. Reducing drag is the name of the game in the aerospace business. Spherical and flat shapes introduce drag, but they are the only shapes now available to missile and aircraft designers from the optics community. Shaping windows on aircraft and missiles that conform to the ideal design would significantly reduce drag.

In addition to reducing drag, conformal optics would allow designs that are more difficult to detect than traditional shapes. Sharp edges, flat planes, and included angles (corners) are more detectable than other shapes, because they are more reflective. Conformal optics would help aerospace engineers optimize aircraft shapes to minimize reflection from radar beams and other detection methods.

Conformal optics that replace spherical windows would also offer a wide field of view. To get that wide field, however, extra aberrations introduced by the non-symmetric shapes need to be accounted for. Non-symmetrical shapes introduce more aberrations than traditional optics, which is one of the primary challenges for designing useful conformal optics.

"There's not anything that's going to be easy on this project." Trotta predicts. They have to reestablish the design, testing, and manufacturing of optics to pull this off. "We're going back to ground zero," Trotta says. They have to come up with design algorithms, manufacturing technology, and all the enabling technology to make conformal optics a viable technology for platform producers, such as Boeing, Texas Instruments, Hughes, McDonnell Douglas, Lockheed-Martin, and so on. "It really is a fresh start," Trotta asserts.

"There's very little in existence to support this technology," says Trotta. Today's optical technology requires defined shapes and is based on lathe, grinder, etc., which is totally inadequate for conformal optics. The researchers have to look at the way optics are designed, fabricated, and applied, without typical rotational symmetries and geometries that simplify optical design and fabrication. On a continuum of complexity, conformal optics are more complex than aspheric optics, which are more complex than traditional spherical optics. Today's aspheres, however, rely on some degree of symmetry, and are still based on lathe-like processing.

Rochester Photonics Corp. will develop diffractive optics for aberration correction. "With optics of this unusual shape, we need more than classical techniques for aberration correction. We need as many degrees' of freedom as we can get — diffractive optics give that," Trotta says

"We have to be more clever with how we correct for aberration," says Greivenkamp. With conformal shapes, there are more aberrations than with spherical optics and the aberrations change at different parts of the window. The Arizona group plans to use non-rotationally symmetric refractive and reflective elements to correct aberrations.

The Center for Optics Manufacturing, the Optical Sciences Center, and Hughes Electro-Optical Systems will develop multiple-axis machines for fabricating conformal optics. Sinclair and Optical Research Associates will work on design codes. Hughes Missile Systems, McDonnell Douglas, Texas Instruments, and Boeing will look at the impact and design issues of conformal optics for aircraft and missile platforms.

Conformal optics open up all these opportunities for aerospace engineers, but the optics need to be affordable as well as high-performance. Reasonable cost will open the door to commercial applications of conformal optics. In addition to aerospace applications, conform optics could improve semiconductor, lithography and imaging systems.

The benefits identified and initial design algorithms developed are very encouraging, according to Trotta. He thinks the technology will change optics as we know it if they can show not only that it's feasible, but also that it can be done economically. It could become economical enough to have a significant financial impact beyond defense applications in a decade or so, Trotta predicts. Applications with plastic conformal optics will probably appear first, and then migrate to glass.

The concepts and requirement for conformal optics go several years back. DoD has been involved from the beginning. It didn't all happen at once — there have been a few attempts to break away from the constraints of classical optics over the years. There are conformal shapes in lighting applications, such as automobile headlights, but aerospace applications are demanding more precision than lighting.

What enabled the development of conformal optics was that DoD identified a significant need for conformal optics and has made a commitment to develop them. Greivenkamp says the growth in accessible computing power allows the massive calculations necessary to go beyond symmetrical optics shapes for both design and control of fabrication processes.