The tiny filament of glass at the core of a length of optical fibre starts out as two tubes. These tubes are made from fused quartz glass, which is mainly silica and gives it flexible properties. First the glass tubes are dipped in corrosive hydrofluoric acid to remove any oily residues, they are then placed in a pair of lathes that spin and heat both tubes with a hydrogen and oxygen flame. When the tubes turn white they are nearing peak temperature and at 2,000”C the tubes melt together to form one longer tube.
This longer-tube is placed in another lathe where it is turned and heated by a burner before being injected with chemical gases containing liquid forms of silicon and germanium. The heat and gases cause a chemical reaction that leaves a fine white soot inside the tube. As the burner travels up and down the length of the tube the soot fuses to create a solid glass core. The outer glass tube will form the cladding around the core.
Continued heating softens the tube and the new glass inside until the tube collapses in on itself. What you now have is a solid rod called a preform. To thin the preform, it is placed vertically in a drawing tower.
This device heats one end of the rod to 2,000”C until the glass softens and becomes a honey-like consistency. As the glass melts it stretches under its own weight and becomes a very tall, thin glass fibre.
Pulleys and lasers are used to measure the precise tension and diameter of the fibre, which should be just 125 micrometres thick. The fibre is then passed under an ultraviolet lamp to bake on a protective outer jacket. The finished optical fibre is then rolled onto massive drums.