Page 56 - Fiber Bragg Gratings

2.6 Summary of photosensitive mechanisms in germanosilicate fibers 37

Figure 2.13: AFM scan of a D-fiber surface. Surface etched after grating
inscription to reveal the chemically modified structure (courtesy Marc Douay, Ref.
[88]).

vacancy) after UV exposure [89]. This is formed by the conversion of the
electron-trapped Ge(I) center, which absorbs at ~5 eV. The change in the
population of the GeE' centers cause changes in the UV absorption spec-
tra, which lead to a change in the refractive index directly through the
Kramers-Kronig relationship [90] [Eq. (1.1.4)]. This process is common
to all fibers. The color center model, originally proposed by Hand and
Russell [91], only explains part of the observed refractive index changes
4
of ~2 X 10~ in nonhydrogenated optical fibers [92,93].
The second mechanism is a structural alteration in the mechanical
nature of the glass and was pointed out several years ago by Bernardin
and Lawandy [94]. In the model, a collapse of a higher-order ring structure
was proposed as a possible effect of irradiation, leading to densification.
The densification of silica under UV irradiation is well documented [95].