Page 179 - Fiber Bragg Gratings

156 Chapter-^ Theory of Fiber Bragg Gratings

Figure 4.12: Total phase change in reflection from a uniform-period grating
as a function of detuning for K a^L = 2 (dashed line) and K a^L = 8 (solid line).
The stronger gratings behaves as a point reflector, since the phase change on
reflection is large and almost constant for the same bandwidth when compared
with the weaker grating.

region outside of the band stop of the grating, the phase of the light
changes according to the unperturbed material refractive index. Into the
band stop, the phase velocity slows down with increasing strength of the
grating.
In Fig. 4.13 is the group delay of the same grating as in Fig. 4.12 in
units of psec/m. This group delay per meter is

Figure 4.13: The normalized delay of a uniform period grating as a function
of normalized de-tuning. For the larger coupling constant, the group delay in the
center of the band is constant, while at the edges it increases rapidly, but is
confined to a small spectrum.