Page 37 - Fiber Bragg Gratings

18 Chapter 2 Photosensitivity and Photosensitization of Optical Fibers

Figure 2.1: A schematic of proposed Ge (or Si) defects of germania-doped
silica. The characteristic absorption of the Ge(l) is —280 nm (4.4 eV) [18] and is
a trapped electron at a Ge (or Si) site; Ge(2) has an absorption at 213 nm (5.8
eV) and is a hole center. The peroxy radical has an absorption at 7.6 eV (163 nm)
and at 325 nm (3.8 eV) [15,16].

band [22-24]. This electron can be retrapped at the original site or at
some other defect site. The removal of this electron, it is believed, causes
a reconfiguration of the shape of the molecule (see Fig. 2.2), possibly also
changing the density of the material, as well as the absorption. It appears
that the Ge(l) center is the equivalent of the germanium defects observed
in a-quartz, known as the Ge(I) and Ge(II), but less well defined [23],
Phosphorus forms a series of defects similar to those of germanium.
However, the photosensitivity is limited at 240 nm and requires shorter
wavelengths, such as 193-nm radiation [24].
Other defects include the nonbridging oxygen hole center (NBOHC),
which is claimed to have absorptions at 260 and 600 nm, and the peroxy