ASPHERES ADVANCE IMAGING OPTICS Pennsylvania

Imaging chips get bigger, pixels get smaller, cameras are incorporated into more (and more-challenging) applications, and somehow the lenses for those imagers are expected to keep up. Lens makers and optical designers are meeting the increased demands in several ways, most notably by using a type of lens known as an asphere.

An asphere is any shape that is not perfectly flat or spherical. Technically, a donut or a cone could be called a wildly divergent asphere. In practice, however, most aspheres only diverge slightly from the spherical. An aspheric surface on an optical element usually can't be distinguished from a sphere simply by eyeballing the surface.

For hundreds of years, spherical surfaces have dominated optics for a simple reason: there was a reliable method for grinding and polishing spherical surfaces. This method could also grind multiplelenses at once. Spheres are not, however, ideal optical elements because, by definition, they suffer from spherical aberration. Designing an imaging lens is a complicated balancing act, requiring designers to use spherical surfaces of differing radii and materials to minimize aberrations over the entire field of view, while also keeping the number of elements reasonable. When the specs are challenging, the number of surfaces may go up, and therefore the size of the lens, the weight, and the cost also increase.

In the extreme, this can lead to huge, heavy, and incredibly expensive lenses, such as those used for microlithography. But for most applications, that is unacceptable. "It's a bit of an oversimplification," says Paul Dumas, Technology Manager at QED Technologies (Rochester, N.Y.), a company that makes asphere-figuring equipment, "but to get more freedom for lens optimization, you have to either add more surfaces or make the surfaces more complicated."

An asphere is a more-complicated surface. It can provide the same performance with fewer elements—or better performance with the same number of elements. "Aspheres allow you to shorten the optical path," explains Steve Sokach, director of sales at SCHOTT North America (Elmsford, N.Y.), and reduce both the size and weight of lenses. Often, one aspheric component can replace two or more spherical components. Stuart W. Singer, Vice President of Industrial Optics at Schneider Optics (Hauppauge, NY) explains, "it's not just spherical aberrations, you can correct other aberrations and higher-order terms as well."

A major benefit of aspheres is that they can be designed to correct aberrations exactly. An aspheric surface integrated into a fairly standard design form can provide more resolution or reduce distortion, explains Sam Sadoulet, Director of Engineering and R&D at Edmund Optics (Barrington, N.J.). "The applications are there, the need is there," says Sadoulet, "People want aspheres today."

"As recently as 15 years ago," adds Singer, "aspheres were common only for astronomy, optical test equipment, or defense applications" —situations in which spherical lenses just wouldn't do. Aspheres were difficult to make, and thus too costly for other applications. "But in the past 20 years, the manufacturing technology has really evolved, and in the last five years it has skyrocketed."

Twenty years ago, users who needed aspheres in the US tended to go to Tinsley Laboratories, who were known for making repeatable, and reliable aspheres. The company made large mirrors for the Keck telescope, among other projects. Figuring out how to achieve the desired surface was tricky, however. Singer says, "it's unbelievable, the amount of brainpower put into these things."

HOW ASPHERES ARE MADE

Aspheres are still more expensive than spherical elements, by roughly an order of magnitude, according to Dumas. "To break even on price," he says, "that needs to come down to about two or three times as expensive." Aspheres cost more than spherical lenses because they are take longer to manufacture because the working tools must be smaller than the surface: creating an asphere simply takes longer.

Changes in manufacturing are making aspheres more economical today. Aspheres are manufactured by four major methods: diamond-turning, computer numeric controlled (CNC) grinding & polishing, magnetorheological finishing and molding.

Diamond-turning is, basically, cutting the surface using a very precise diamond-tipped lathe. This technique is often used for elements destined for high-end IR imaging system. The materials (such as germanium) used for these IR optics are so costly that if an asphere can reduce the number of parts in the lens, then it is cost-effective to turn. Although diamond-turning leaves machining marks on the surface, the scatter from these marks tends to be negligible at IR. This limits the use of diamond-turning in the visible, however. A number of companies make diamond-turning equipment, including Moore Nanotechnology Systems (Keene, N.H.).

CNC grinding & polishing of aspheres involves the same fabrication steps as for spheres but using computer-controlled subaperture tools. Unlike traditional spherical grinding and polishing machines (which can process as many as dozens of lenses in one batch), these machines process one lens at a time, which limits throughput. They have only been on the market for a few years in industrial settings. A company in Germany, Schneider Opticmachines (not to be confused with Schneider Optics) is a major maker of these machines. Aspheric CNC grinding is a well-developed and reliable technology. Aspheric CNC polishing, however, is not deterministic. This is in direct contrast to the deterministic magnetorheological finishing (MRF) polishing process.

If one needs to produce a few thousand pieces, then MRF works well, explains Singer. But for high-volume consumer applications—like cellphone cameras, for example—precision glass molding is the most cost-effective answer. "You start with a nice spherical preform and put it into a mold base," says Sadoulet. "Then heat the glass to point where it is soft and apply pressure with the molds." This method has been in development since the 1970s, and Kodak's disk camera introduced in the 1980s was one of the first products to incorporate a molded aspheric element. Major optics manufacturers including Kodak, Corning, Toshiba, and SCHOTT have used the technique. Sadoulet explains, "the trick is to get to the volume that allows the tooling to be amortized correctly. Because the tooling is expensive, you need volumes of hundreds of thousands or millions of pieces per month."

There are also materials issues. "It's the glass that makes it special," says SCHOTT's Sokach: "This equipment is set up to run at specific temperatures and profiles. We have a number of specialty glasses designed for this, which has really driven down costs." Typically, one wants a glass that softens at lower temperatures, among other properties. Other materials issues include making molds that are compatible with the process and developing coatings to release the glass gracefully from the mold. The entire process is surrounded by trade secrets.

Edmund Optics, says Sadoulet, "is spending a lot of R&D dollars to develop processes for mid-volume glass molding." He believes that in a few years, molding capability will be necessary. The company is also offering aspheres as catalog optics to give people a chance to experiment with them without having to justify molding-tool prices. He sees people beginning to use aspheres that they can get cheaply. (In this case, "cheap" means under $200 for an inch-diameter asphere.) "It's planting a seed," he explains.

Manufacturing doesn't cover just lens-shaping: Creating uniform coatings on wild aspheres is not an easy task, and assembling a lens with the aspheric surface in the correct position can be an additional challenge. Also, in development are new methods for making aspheres, including molding aspheric polymers to spherical glass lenses and depositing non-uniform coatings onto spherical surfaces.

METROLOGY

After asphere-manufacturing techniques developed, the next limiting factor has been measuring the surfaces. Most methods produce something like a topographic map of the surface either showing the shape or showing the deviation from the desired shape. Aspheres, more than conventional lenses, need metrology during production. One can use lasers to create interference patterns that can indicate how uniform a surface is. The wavefront of the laser beam can be easily made planar or spherical, but each asphere requires a null lens (typically a hologram) made to fit that specific shape.

Also, interferometry can measure the surface to within nanometers (fractions of a laser's wavelength) but combining that sort of precision with a wild asphere, in which the surface diverges as much as millimeters from a spherical shape, has required new techniques. One solution, called stitching interference metrology is fairly recent. As these tools become available, manufacturing aspheres becomes easier.

FUTURE

The experts agree that more aspheres will appear in imaging lenses in the coming years. Singer says, "there will be aspheres in everything. Almost all photography shops (makers of photography lenses) are using three or four aspheric elements now in consumer and professional lenses." Sokach adds, "As imaging gets smaller, you'll see greater use of aspheres and other materials." And Sadoulet says, "Aspheres will be more and more integrated. The next few years will be telling about which technology can adapt to what users want."

One of the governors on the adoption of aspheres now lies with lens designers, says Sadoulet. "It will take a few years for designers to acquire the knowledge and know-how of designing with aspheres." Lens-design programs are capable of handling aspheres, but designers also need to learn about what features are producible, and how best to use the aspheric surfaces.

Because aspheres are so often customized to the specific lens, Sadoulet doubts that they will be commodidized. Although manufacturers are pushing hard to reduce costs, aspheres will remain either as simple solutions in huge consumer markets or in higher-end imaging products. Sadoulet says, "If I just bought a $5,000 sensor, I don't mind spending a couple hundred dollars to make sure that my sensor can see everything it is capable of seeing."

So far, the discussion has focused on lenses in which one element has one aspheric surface. But as metrology and manufacturing tools come online, it may be cost-effective to use multiple mild aspheres in a lens design.

Singer says, "When you used one aspheric surface, you could eliminate one to two elements." But what if both surfaces of an element are aspheric? "Maybe you could eliminate two to four elements?" Singer sounds enthusiastic. "Boy, oh boy, you can do a lot."