Page 165 - Fiber Bragg Gratings

142 Chapter 4 Theory of Fiber Bragg Gratings

Figure 4.8: Types of Bragg gratings categorized by action of coupling. The
schematics show various gratings in the core of an optical fiber. All gratings
are shown to be transversely uniform. A nonuniform transverse refractive index
modulation profile enhances coupling to either different mode orders of guided
or to the radiation field. A shows a guided mode reflection grating. B shows a
reflecting guided mode polarization coupler, mode converter, or radiation mode
coupler ("side-tap" grating). C is a polarization coupler for copropagating modes,
also knows as a "rocking filter" [31]. D is the copropagating guided-mode to
radiation-mode coupler, also known as the "long-period grating."

4.3 Coupling of counterpropagating
guided modes

The simplest form of interaction is between a forward-propagating and an
identical backward-propagating mode. However, for a general approach,
dissimilar modes are considered for the counterpropagating (reflected)
mode phase matching with Eq. (4.2.21) rewritten as