A phosphate glass originally developed at ORNL to isolate radioactive waste shows promise as a novel optical material for collecting, delivering, focusing, or amplifying light. One possible use for phosphate glass may be in improved optical fibers for laser surgery.

A phosphate glass containing lead and indium, whose composition is based on that of a high-durability glass, was developed and patented in the late 1980s by Lynn Boatner and Brian Sales in ORNL's Solid State Division. More recently, Boatner and Sales in collaboration with Steve Allison of ORNL's Engineering Technology Division have fabricated optical fibers and lenses from this phosphate glass--a good example of how interdivisional cooperation can advance research and development opportunities in photonics.

"Our phosphate glass exhibits a unique combination of properties," Boatner says. "It has a high index of refraction and the ability to bend light at high angles. It is also chemically stable and durable. In addition, it has a low melting point and is transparent over a wide range of wavelengths. Perhaps most important, it readily dissolves rare-earth elements, so it can be used in developing new optically active devices such as fiber-optic amplifiers and new lasers."

The ORNL researchers recently tested optical fibers made at Virginia Polytechnic Institute and State University from the phosphate glass covered with a conventional silicone-polymer cladding. They found that the fibers exhibited a very high numerical apertur--a measure of the largest angle at which incoming light can enter a fiber and pass through it efficiently.

"For normal communication-grade fiber, this input angle may be only 12 degrees from the normal of the surface," Allison says. "High-numerical-aperture fiber made from our phosphate glass can transmit light at incident angles of almost 70 degrees. Thus, this fiber should be much better at collecting light from diffuse sources."

The ORNL researchers say the phosphate glass has great potential for use in specialized fiber sensors for strain, temperature, and electric-field measurements and in light-delivery systems, including optical displays and laser surgery. "Fluoride fibers used to carry infrared laser light to patients tend to be easily damaged by water vapor," Allison says. "We think the ORNL glass fiber may be better suited for this type of laser surgery, whose advantages over conventional surgery include reduced pain and faster healing."

The phosphate glass is also a promising material for lenses. "Because of the material's high index of refraction, less curvature is required for a lens of a given power," Allison says. "The lower the curvature, the smaller the lens aberrations that can distort and limit a lens' usefulness."

Thanks to its unique set of characteristics, ORNL's new material may have a wide variety of applications, making it one of the most interesting new optical glasses.

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