Computing Light Factory A rare look inside the world’s biggest optical-fiber plant. by Katherine Bourzac October 27, 2010 Sponsored by Optical fibers are drawn from large “blanks” like the ones shown here being lifted from holding ovens. The blanks have two layers, both made from silicon dioxide (glass), which will form the basis for corresponding layers in the fiber: a core that transmits light very efficiently and an outer cladding that keeps the light from leaking out. Both the core and the cladding are formed when gas jets inside a high-pressure furnace (preceding image) deposit silicon and small amounts of other elements that influence the optical properties of the glass. Inside rows of furnaces like the ones shown here, the glass blanks are heated to begin stretching them downward. When they descend to a certain point, they’re cut into shorter pieces. The photo on this page was taken inside one of the furnaces just after the cut, leaving behind a tip that glows in the heat. This part of the factory is an older one, where the company makes specialty products; the higher-volume processes are more automated. The shortened glass blanks are placed in tall furnaces and heated once more to begin the fiber-drawing process. The first section of glass that comes down is teardrop-shaped (above) and can’t be drawn into fiber. A worker quickly cuts it off and lets it fall into a bin like the one shown here; this is called “dropping the gob.” (In the most up-to-date part of the factory, this step and the rest of the process are automated inside four-story-high furnaces.) After the gob drops, the glass is pulled and stretched by machines that monitor tension on the material to ensure a fiber of consistent quality. The final product is a fiber just a few micrometers in diameter. The ratio of core to cladding remains the same as it was in the glass blank, though each layer is much thinner. The glass fiber is threaded through tubes (above) and irradiated with ultraviolet light to harden a protective polymer coating on the surface. The finished fiber is packaged on spools (above), tested for its mechanical and optical properties, and shipped out all over the world.