Page 41 - Fiber Bragg Gratings

22 Chapter 2 Photosensitivity and Photosensitization of Optical Fibers

nium concentration [36]. The slope in this graph is —28 dB/(mm-mol%)
of Ge before the preform sample is collapsed (dashed line). After collapse,
the number of defects increases, and the corresponding absorption
changes to —36 dB/(mm-mol%) (Fig. 2.3 continuous line).
Increasing the concentration of defects increases the photosensitivity
of the fiber. This can be done by collapsing the fiber in a reducing atmo-
sphere, for example, by replacing oxygen with nitrogen or helium [36] or
with hydrogen [37,49].
The 240-nm absorption peak is due to the oxygen-deficient hole center
defect, (Ge-ODHC) [38] and indicates the intrinsic photosensitivity. It can
be quantified as [39]

g
ne
where c* 242 nm i ^ absorption at 242 nm and C is the molar concentration
of GeO 2. Normally C lies between 10 and 40 dB/(mm-mol% GeO 2). Hot
hydrogenation is performed on fibers or preforms at a temperature of
~650°C for 200 hours is 1 atm hydrogen [40]. The absorption at 240 nm
closely follows the profile of the Ge concentration in the fiber [33], and k
has been estimated to be large, —120 dB/(mm-mol% GeO 2).
The saturated UV-induced index change increases approximately lin-
early with Ge concentration after exposure to UV radiation, from ~3 X
4
5
10~ (3 mol% GeO% 2) for standard fiber to -2.5 X 10~ (-20 mol% GeO 2)
concentration, using a CW laser source operating at 244 nm [49]. However,
the picture is more complex than the observations based simply on the
use of CW lasers. With pulsed laser sources, high-germania-doped fiber
(8%) shows an the initial growth rate of the UV-induced refractive index
change, which is proportional to the energy density of the pulse. For low
germania content, as in standard telecommunications fiber, it is propor-
tional to the square of the energy density. Thus, two-photon absorption
from 193 nm plays a crucial role in inducing maximum refractive index
changes as high as —0.001 in standard optical fibers [41]. Another, more
complex phenomenon occurs in untreated germania fibers with long expo-
sure time, in conjunction with both CW and pulsed radiation, readily
observable in high germania content fibers [47]. In high-germania fiber,
long exposure erases the initial first-order grating completely, while a
second-order grating forms. This erasure of the first-order and the onset
of second-order gratings forms a demarcation between Type I and Type
HA gratings.