Page 42 - Fiber Bragg Gratings

2.4. Photosensitization techniques 23

Increasing the energy density damages the fiber core, forming Type
II gratings [31]. The thermal history of the fiber is also of great importance,
as is the mechanical strain during the time of grating inscription. Signifi-
cantly, even strains as low as 0.2% can increase the peak refractive index
modulation of the Type IIA grating in high germanium content fiber
[42,43]. High-germania-doped (30%Ge) fibers drawn under high pulling
tension show the opposite behavior [44], indicating the influence of elastic
stress during drawing rather than the effect of drawing-induced defects
[45]. Annealing the fiber at 1100°C for 1 hour and then cooling over 2
days reduces the time for the erasure of the Type I grating, as well as
increasing the maximum refractive index modulation achievable in the
Type IIA regime. With tin as a codopant in high-germanium fiber, the
general overall picture changes slightly, but the dynamics are similar,
except for reduced index change under strained inscription [46]. Thus,
absolute comparison is difficult, and one may use the germania content
as an indicator, bearing in mind the complex nature of the dynamics of
grating formation in germania-doped silica fiber. Typical results for a
high-germania fiber are shown in Fig. 2.4. The growth of the refractive
index modulation as a function of time stops in the case of all three fibers
shown, dropping to zero before increasing once again to form Type IIA
gratings.
Photosensitivity of fiber fabricated under reduced conditions as a
function Ge concentration also increases, but it is not sufficient to interpret
the data by the maximum index change. The reason for this is the induc-
tion of Type IIA gratings [47] in relatively low concentration of Ge. Mea-
surements performed under pulsed conditions reveal that the onset of the
Type IIA grating is almost certainly always possible in any concentration
of Ge; only the time of observation increases with low concentrations,
although for practical purposes this time may be too long to be of concern.
Figure 2.5 shows data from the growth of the average index on UV expo-
sure as a function of Ge concentration in fibers, which have been reduced.
The maximum index should change monotonically; however, above a cer-
tain concentration, the onset of Type IIA forces the observed maximum
index change for point B (20 mol% Ge), since the grating being written
slowly disappears before growing again. While the maximum reflectivity
should increase to higher levels, within the time frame of the measure-
ments this fiber appears to be less sensitive. A better indicator is the
initial growth rate of the index change, since Type IIA grating is not
observed for some time into the measurements. Figure 2.5 shows an