Specialty Optical Fibers Handbook

high. For instance, 1 ppm

2

of OH will cause about 15 dB/km of loss increase at

1390 nm. By lowering the core dopant levels (e.g., Ge and P), it is sometimes

possible to lower the fiber’s reactivity, but such dopant changes are not always

practical. For instance, fibers with undoped silica cores are less prone to hydro-

gen reactions, but the variety of fiber designs that can be achieved without Ge

and/or P doping is limited. As with molecular H

2

losses, it is possible to eliminate

or at least decrease the losses due to hydrogen reactions by using a hydrogen-

blocking carbon coating.

14.5.2 Hydrogen Losses in Carbon-Coated Hermetic

Fibers

At room temperature, ...