Specialty Optical Fibers Handbook

chemical resistance, the carbon coating plays a vital role. When H

2

gas is present

at the high temperatures encountered in the down-hole environment, the optical

aging of the fibers can be rapid. The temperatures encountered in the application

of these fibers can be comparable to the highest temperatures used in accelerated

aging experiments. At these temperatures, H

2

diffusion becomes rapid and the

dopants in the fiber core become increasing reactive with hydrogen. The losses

due to reacted hydrogen can greatly exceed those due to unreacted molecular H

2

.

The extent to which H

2

molecules react in the fiber core to form lossy OH and

Ge defects is highly dependent on the core glass composition. The use of a

carbon coating with low H

2

permeability can greatly ...