Specialty Optical Fibers Handbook

energy values suggested that the carbon layers for these fibers were different in

structure or below a critical thickness. The initial lag times, t

i

, increased as t

f

increased. This was the expected behavior because both t

i

and t

f

increase as the

coating thickness (d) increases and as the diffusivity of H

2

in carbon coating (D

c

)

decreases. (See Eqs. [14.5] and [14.6].)

14.5.5 Effects of Glass Composition on Hermetic

Fiber Behavior

The diffusion analysis in the previous section assumes that H

2

molecules

diffuse inertly through the fiber’s silica cladding. It also assumes that there are

no sources of hydrogen in the fiber’s cladding material. Most of the