Page 240 - Fiber Bragg Gratings

5.2 Basic principles and methodology 217

wavelength for mass replication. Thus, a number of apodized phase masks
may be required, each of a different wavelength and length.
Apodization requires that the refractive index modulation at the
edges of the grating gradually disappear. As described in Section 5.2.1,
a moire grating is composed of two individual gratings, which leads to
apodization. The Bragg wavelength of a grating can be changed by stretch-
ing the fiber prior to writing [23,24]. Therefore, two gratings written at
the same location but differing in wavelength by exactly one period will
be apodized. The problem is, how can the two gratings be overlaid such
that they have the correct relative phase between them? One possibility
is to use symmetric fiber stretching during the inscription of a grating
[25]. This poor man's apodization technique — the symmetric stretch
apodization method (SAM) — is not only simple to operate, but also appli-
cable to any type of grating that needs to be apodized.
Figure 5.15 shows the schematic of the principle of inscription by
symmetric stretching of the fiber. The technique can be understood as
follows: A grating is first written into a fiber in its relaxed state (Fig
5.15b), for example, by scanning a phase mask, although the method of
inscription is unimportant. The fiber is then stretched by straining it in
opposite directions by exactly one period of the grating in the fiber, and
another grating written on top of the first. Since the fiber is stretched,
the inscribed grating is one period longer than the first (Fig. 5.15a) and
also symmetrically overlaid (Fig. 5.15c).
The central part of the grating periods are overlaid in phase, while
farther away from the center they become increasingly out of phase, until
the edges, where they are IT out of phase. The difficulty of ensuring that

Figure 5.15: A schematic of the symmetric stretch scheme for apodizing
gratings. See text for explanations.