Specialty Optical Fibers Handbook

containing an array of mandrel elements is assembled. The mandrels are indi-

vidually tensioned to ensure uniformity along the length of the mold. The mold is

subsequently filled with colloidal silica dispersed at high pH with an average

particle size of 40 nm.

The pH is lowered by the addition of an organic ester, causing the sol to gel.

At the wet gel stage, the mandrel elements are removed, leaving air columns

within the gel body. The gel body is then dried, purified thermochemically to

remove organic and transition metal contaminants, and sintered into vitreous

silica. Because contaminants are removed in the dried gel body, the sol-gel

process is reasonably insensitive to contaminants introduced during mold as-

sembly, unlike stack-and-draw, or ...