Page 40 - Fiber Bragg Gratings

2A. Photosensitization techniques 21

[34]. The sheer numbers of different fibers available worldwide, further
complicates the picture and by the very nature of the limited fiber set
available within the framework of a given study and the complex nature
of glass, comparisons have been extremely difficult to interpret. This is
not a criticism of the research in this field, merely a statement reiterating
the dilemma facing researchers: how to deal with far too many variables!
In order to draw conclusions from the available data, one can simply
suggest a trend for the user to follow. A choice may be made from the set of
commonly available fibers. For a certain set of these fibers (e.g., standard
telecommunications fiber) the method for photosensitization may be sim-
ply hydrogenation, or 193-nm exposure. It is often the availability of the
laser source that dictates the approach.

2.4.1 Germanium-doped silica fibers
Photosensitivity of optical fibers has been correlated with the concentra-
tion of GeO defects in the core [33,34]. The presence of the defect is
indicated by the absorption at 240 nm, first observed by Cohen and Smith
[35] and attributed to the reduced germania state, Ge(II). The number
of these defects generally increases as a function of Ge concentration.
Figure 2.3 shows the absorption at 242 nm in a perform with the germa-

Figure 2.3: Absorption at 242 nm in preform samples before and after col-
lapse as a function of Ge concentration (after Ref. [36]).