Page 38 - Fiber Bragg Gratings

2.3. Detection of defects 19

Figure 2.2: The GeO defect of germania-doped silica, in which the atom adja-
cent to germanium is either a silicon or another germanium. It can absorb a
photon to form a GeE' defect. The Ge(0) or Ge(3) are a GeE' center [20]. The
GeE' defect shows the extra electron (associated with the Ge atom), which may
be free to move within the glass matrix until it is retrapped at the original defect
site, at another GeE' hole site, or at any one of the Ge(n) defect centers.

radical (P-OHC) [25], believed to absorb at 260 nm. Both are shown in
Fig. 2.1.

2.3 Detection of defects

A considerable amount of work has been done in understanding defects
in glass. Detection of defects may be broadly categorized into four groups:
optically active defects can be observed because of their excitation spec-
trum or excitation and luminescence/fluorescence spectrum while opti-
cally inactive defects are detectable by their electron spin resonance
signature, or ESR spectrum, together with optical emission spectrum.
The model of the defects as shown in Fig. 2.2 suggests the liberation
of electrons on absorption of UV radiation. It should therefore be possible
to detect liberated charges experimentally; since silica has a high volume
resistivity, it is necessary to choose a geometry that can directly enable
the measurement of electric currents. Photosensitivity has been explored