Page 152 - Fiber Bragg Gratings
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4.2 Coupled-mode theory 129
Figure 4.1: Refractive index modulation in the core of a fiber for different
visibilities of the fringe pattern. Also shown is the average refractive index change
in the core (dashed line). The unperturbed core-to-cladding refractive index differ-
3
ence is 5 X 10~ , while the maximum refractive index modulation for unity
3
visibility is ±2 X 10~ .
of the fiber for different visibilities. Also shown is the effect on the average
core index. Note that the change in the average index in the core is
constant, irrespective of the visibility of the fringes, although it remains
a function of Sn. In the example shown, however, both the average index
and the refractive index modulation 8n increase with UV exposure time.
The perturbed polarization can now be related to refractive index
change shown in Eq. (4.2.19) to give
Including Eq. (4.2.20) in Eq. (4.2.13) results in
