Page 64 - Fiber Bragg Gratings

2.7 Summary of routes to photosentization 43

2. There is a change in the position of the band edge of germania-
doped silica in the deep-UV spectrum, which alters the refractive
index after UV exposure. This effect is difficult to quantify owing
to the problems associated with measurements in the vacuum
ultraviolet and the low transparency of silica below a wavelength
of —190 nm, although some measurements have been made [113],
suggesting that there is no shift in the edge, just an increase in
the absorption in the deep UV.
3. High-temperature hydrogen treatment reduces germania, produc-
ing an enhanced concentration of GeO molecules [114]. Again
bleaching of the absorption at 240 nm partly contributes to the
index change. Other effects as in points 1 and 2 above prevail.
Chemically, the reduction process may occurs as follows:

4. Molecular hydrogen. The suggested reaction is the formation of
GeH and OH ions from a Ge(2) defect. The GeH is responsible for
the change in the refractive index via the Kramers-Kronig rule.
The possible route may be as follows:

It is not clear what the reactions with a pure alumina doped silica
core may be, but it is possible that a similar set of observations
may occur; the addition of phosphorus in germania-doped fibers
reduces the concentration of GeE' centers, increasing with in-
creased fabrication temperatures or reducing conditions [115].

Figure 2.14 compares a set of different fibers exposed to the same
intensity of radiation at 262 nm for different times. The comparison shows
the refractive index growth rate for fiber with different photosensitization
treatments when exposed to UV radiation, per mol% Ge in the fiber,