Optical fibers with higher light transmission and longer life spans offer interesting new solutions for meeting the growing demand for higher quality lighting in medicine. New glass optical fibers offer longer lifetimes and higher light transmission, and experience only minimal changes in color and dispersion, and these characteristics enable them to answer the call for higher quality medical lighting. What’s more, a newly developed environmentally friendly process allows these high-purity multicomponent glass fibers to be manufactured without using lead, arsenic or antimony. In medicine in particular, glass optical fibers have conquered a wide range of applications, mostly in endoscopy and surgical microscopy, but fibers also are used in dental treatment and light therapy. Improved high-purity, multimode step index fibers will pave the way for new ideas that extend well beyond existing applications. This can be attributed to the progress that has been made in the composition and preparation of the special glass from which these fibers are drawn at high temperatures. Multicomponent glass also offers an outstanding price-to-performance ratio compared with quartz glass. Compared to existing fiber products, this means significant performance improvements that are certain to offer major benefits in actual applications. For example, it is now possible to increase the transmission of white light by up to 10 percent (Figure 1). High-purity raw materials for manufacturing the glass are instrumental in achieving this goal. Of course, many years of experience in melting and manufacturing glass – in the area of refining, for instance (driving bubbles out of molten glass) – reveal other ways to avoid glass defects. Figure 1. Puravis multicomponent glass fibers transmit wavelengths in the visible range better than conventional Schott glass optical fibers. Images courtesy of Schott AG. The fibers’ high purity limits shifts in color caused mainly by impurities in the glass. Thus, the illuminated objects retain their natural colors even when longer lightguides are used. Applications such as endoscopy and surgical microscopy are certain to benefit. Ever-thinner optical lightguides, brighter light and smoother true-color illumination are needed to distinguish between certain types of tissue. Comparative measurements on fibers used in the past clearly show this (Figure 2). When used in the standardized light source A, which equates to halogen light that has a color temperature of 2856 K and that is emitted directly, Puravis GOF85 glass fibers from Schott AG exhibit hardly any deviation over a distance of 10 m. A conventional glass fiber (A2) experiences shifts of close to 200 K that can be seen even with the naked eye as the distance increases. This difference becomes even clearer in comparison to normal light (6500 K) from a xenon lamp D65, where a color shift of around 800 K occurs at GOF85, which is significantly lower than that of A2. The effect of dispersion that causes distorted colors along the light field edges in existing fibers (Figure 3) is also less significant when the new fibers are used. Figure 2. The new glass fibers (shown here are the data for the GOF85 product, as opposed to A2) minimize shifts in color, even when long lightguides are used. This can be important for medical and other applications. Lightguides made from the new fibers can be of great benefit here, thanks to their improved numerical aperture that allows them to capture more light. Their low attenuation in the visible range results in even higher light output at the end of the fiber bundle – for instance, 28 percent more light at 400 nm using a 2-m lightguide. Smaller bundle diameters that put out the same amount of light are another alternative, making it easier to construct or install these in thinner endoscopes that support wound healing after minimally invasive surgery. Improved chemical properties These optical properties are not achieved at the expense of chemical stability – in fact, the opposite is true. Puravis fibers have achieved the highest rankings in all four chemical resistance glasses based on the respective ISO standards. This means that they are resistant to acids (resistance class SR 1.0 as described by ISO 8424), alkalis (AR 1.0 as described by ISO 10629), climatic influences (CR 1.0 as described by ISO/CD13384) and staining (FR 0). Optical glasses obviously react extremely sensitively to these types of effects because their optical quality declines or is completely lost when chemical damages occur, or the surface of the glass or the glass material changes. Figure 3. Lighting with Puravis fibers reduces dispersion effects (left), whereas existing glass fibers produce distorted color along the edges of the light field (right). While developing the new fiber, Schott improved its chemical stability – the proprietary raw materials used yield better chemical stability and optical properties than traditional materials, increasing the life span of the fibers. This is particularly important when it comes to reprocessing and sterilizing medical instruments by cleaning or autoclaving. Tests have shown that transmission losses after 100 autoclaving cycles can be reduced by up to 70 percent in the new fibers as compared with other fibers. The new fibers therefore also support the growing hygiene requirements in medicine. Optical durability is also ensured by minimizing possible solarization effects. This was achieved by developing a suitable glass formula that does without the heavy metal lead. The new and environmentally friendly manufacturing process also avoids using arsenic or antimony as refining agents. Thus, any product equipped with these fibers already complies with the European Union directives RoHS and REACH, and is therefore already capable of meeting future environmental requirements. Trendsetting applications Thanks to their improved light transmission in the near-UV range between 350 and 400 nm (Figure 4), these high-tech glass fibers are certain to open up interesting new fields. In this respect, the fibers exhibit significantly higher transmission than conventional products: Because the fibers are lead-free, Schott had to reduce the solarization effects. The solarization effects of the new fiber are minimized in comparison with the standard leaded Schott B3 fiber, so the overall absolute transmission level after solarization is two times higher than that of GOF85 and three times higher (GOF70) than that of the B3 fiber. This paves the way for innovative fluorescence applications in clinical diagnostics involving tissue or tooth decay, and in fluorescence microscopy. Figure 4. Investigation of solarization at 365 nm. GOF70 and GOF80 show a very fast initial solarization at 365 nm, which stabilizes at a transmission level higher than that of the B3 fiber. The B3 fiber shows solarization at this wavelength as well. The effect is very slow and gradual over a long period of time, stabilizing at a low level. Note: The lightguides investigated were fairly long (3 m). Shorter lightguides would result in higher transmission levels. The glass fibers also can be used for industrial purposes such as UV curing of adhesives. They are also suited for use as lightguides for lighting and image transmission solutions in industrial image processing – for instance, in the area of microscopy or to monitor and perform quality assurance on manufacturing processes. Security-oriented industries like aviation and vehicle construction also stand to benefit. Glass fibers are chemically inert and offer extremely high thermal stability. In contrast to conductive (metal) cables, they transmit light without causing any sparks. Fire safety is therefore not an issue. This, coupled with ease of maintenance and a long service life, is but one of the compelling reasons why this fiber is equally suited for medical applications and for illuminating aircraft cabins and vehicle interiors. Figure 5. High-purity Puravis glass fibers are manufactured using an environmentally friendly process that does not use lead, arsenic or antimony. The new multimode-stage index glass fibers are made of extremely pure, select raw materials and are processed in an environmentally compatible manner. This contributes significantly to increased performance with respect to light yield, transmission, color shifting, attenuation and dispersion. It is now possible to realize more sophisticated and innovative new lighting applications in medicine, and the glass fibers support the trend toward miniaturization of medical technology, as in endoscopy. Meet the author Karen Holst has been the product manager responsible for fibers for the Lighting and Imaging Business Unit of Schott AG in Mainz, Germany, since 2007; email: karen.holst@schott.com.