Specialty Optical Fibers Handbook

Alternative techniques for making this rod entail creation of the silica particles

in a flame through hydrolysis rather than oxidation. In OVD, the flame traverses

along a mandrel and the silica particles are collected as porous soot. Multiple

passes of the torch build up the soot (and the index structure) radially. After the

mandrel is removed, the soot is purified in a chlorine-containing atmosphere in a

furnace and sintered to clear glass. In vapor-phase axial deposition (VAD), the

chemistry is similar, but the flame is directed at the end of a rotating rod. As the

rod is gradually withdrawn, a long soot boule is grown axially. Multiple torches

are used to form the Ge-doped core and the pure silica cladding. Because porous

boules created by outside ...