efraction and diffraction may rhyme, but ORNL researchers are more interested in making them work together in a lens than in a poem.
The lenses in eyeglasses refract light. Light rays are bent as they pass into and out of the lens. The focusing effect of the lens is used to improve vision.
The glistening three-dimensional image that shows up on many credit cards when turned toward the light is a hologram, an example of diffraction. It is made possible by a specially etched pattern of fine lines that diffracts light as bands of color.
Some photonics researchers at ORNL and elsewhere are seeking novel applications for and economical ways of making a lens that both refracts and diffracts light. Such lenses, which are now being made using an expensive process called microlithography, are called hybrid optics.
"The idea is to take a light-refracting lens and etch a pattern on it that diffracts light," says David Sitter of ORNL's Instrumentation and Controls Division. "In this way, one lens can do the work of two, reducing the size, weight, and cost of a lens system and eliminating problems of aligning two lenses."
One use for a single-element "hybrid optics" lens might be to replace and improve the quality of lenses used to focus laser light in compact-disc read-only memory (CD-ROM) drives for personal computers. Such lenses could make future CD-ROM drives smaller, lighter, simpler, and less costly.
Sitter has designed, built, and tested a miniature camera that uses a hybrid-optics lens for use as a sensor. This two-lens camera, he says, performed as well as a commercial camera with three refractive lenses.
"The advantage of the hybrid approach," Sitter says, "is that the diffractive effects can compensate for the undesirable effects of refraction. Each optic balances the other because they have opposite dispersive behavior. For example, in refraction, blue light is bent more than red light, but in diffraction, red light is bent more than blue light. In addition, the diffractive element may be designed to compensate for imaging aberrations, leading to sharper images."
Hybrid-optic lenses are expensive to make because of the high cost of microlithography. In this slow tedious process, chemicals or reactive ions are used to etch miniature patterns in a glass lens to make it diffractive as well as refractive.
ORNL researchers are seeking to develop a less costly process of manufacturing hybrid optics. Bruce Bernacki and Curt Maxey, both of the I&C Division, and Art Miller of the Oak Ridge Y-12 Plant Development Division are studying the possible use of single-point diamond turning for fabrication of hybrid-optic lenses. Modern diamond turning is a computer-controlled machining process that stems from work in the late 1960s at DOE laboratories. It was first used to make sophisticated mirrors for high-energy laser systems. It uses a precisely contoured and polished single-crystal diamond cutting edge that is driven by an ultrahigh-resolution machine to cut precisely curved surfaces. The applications of diamond turning to manufacturing optics are being studied at the Ultraprecision Manufacturing Technology Center at the Y-12 Plant.
"Our plan is to use diamond turning to make metal molds for lenses with the diffractive pattern precisely cut into each mold," Miller says. "The plastic placed in the mold will then pick up the diffractive pattern, forming a hybrid optic having an arbitrary shape such as a sphere."
The group is also working with an industrial partner, Geltech, Inc., of Alachua, Florida, to develop hybrid-optic lenses made from silica glass instead of plastic. To mass produce glass lenses, liquid glass will be cast in molds made by diamond turning. Fused silica offers several advantages over plastic for hybrid optics: it is more resistant to corrosive chemicals, does not release gas molecules under vacuum, undergoes fewer undesirable changes in dimensions and optical performance resulting from temperature changes, and is insensitive to ionizing radiation. As a result, such high-performance hybrid optics are expected to find uses in high-radiation or hazardous chemical environments on earth and in space.
The lens of the future will likely be a "lens on a lens," with one being diffractive and the other refractive. Once the cost of making hybrid optics is lowered to a reasonable level, thanks to photonics research, two lenses may be available for the price of one.
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