http://www.corning.com: Using Corning’s patented outside vapor deposition technology, Corning manufactures ultra-pure telecommunication grade optical fiber. To ensure consistency and reliability, Corning produces all of the glass materials for its fiber.
The fiber making process begins by creating “preforms” of glass on proprietary vapor deposition lathes, with chemistry controlled so precisely that contaminant levels are measured in parts per billion. These preforms are sintered into a solid, dense, transparent glass and subsequently drawn into coated optical fiber the diameter of a human hair, with dimensional precision at the one-micron level. Additional manufacturing steps test the fiber for strength at 100,000 pounds per square inch and fully measure its optical performance before shipment to customers.