Specialty Optical Fibers Handbook

concentration resulting from the complete oxidation of SiCl

4

shifts the equilib-

rium further toward GeCl

4

by the law of mass action. Low oxygen partial

pressure has the same effect.

3.6.2 Purification from Hydroxyl Contamination

A second important aspect of MCVD chemistry is the incorporation of the

impurity OH



[30] because reduction of OH



in optical fibers to ppb levels is

essential for realization of low attenuation in the 1.3- to 1:55 mm region. Hydro-

gen species originate from three sources: diffusion of OH



from the substrate

tube during processing, impurities in the starting reagents and carrier oxygen

gas, and contamination from leaks in the chemical delivery system.

The OH



level in the fiber is controlled by the reaction

H

2

O þ Cl

2

! 2HCl þ

1

=

2