Page 183 - Fiber Bragg Gratings

160 Chapter 4 Theory of Fiber Bragg Gratings

In Fig. 4.16 is shown the practical case of side illumination of optical
fiber with UV irradiation. The absorption in the fiber causes the refractive
index to be highly asymmetric [48]. This asymmetry is like a blaze, since
one side of a propagating mode experiences more of a perturbation than
the other. Consequently, such an asymmetric grating breaks the symmetry
to allow coupling to odd, / = 1, order modes i.e., LP lm. This is also true
of blazed gratings, which are uniform across the core with the same effect
on the guided mode.
A few points should be noted about scattering from a blazed grating.
It is known that scattering of light from bulk blazed gratings [49] is
directional and the phase-matching conditions easily derived for scatter-
ing, in thin and thick holograms [32]. The general approach taken in the
next section is similar in so far as the scattering element is considered
nonlocal and all the scattering events summed to arrive at the final
unbounded coupling to the radiation field.

Theoretical model for coupling to the radiation field
The STG is a useful device for filter applications when used to couple the
guided mode to the radiation field, rather than a mode. It forms a narrow
band stop, whose spectral width is not dependent on the length of the
grating in the same way as a Bragg grating. The wavelength and band-
width is easily adjusted by choice of fiber, and the properties of the grating
are as robust as those of the Bragg grating in terms of temperature

Figure 4.16: Effect of side illumination of a fiber core with UV radiation,
giving rise to a tilted grating