Page 120 - Fiber Bragg Gratings

3.1 Methods for fiber Bragg grating fabrication 99

combination of interfering beams. The advantage of using lenses as chirp-
adjusting elements is that any chirp bandwidth is possible, limited only
by the photosensitive response of the fiber. Using such an interferometer,
chirp bandwidths of 44 nm have been demonstrated with a reflectivity of
—80%, covering the entire erbium amplifier gain band. The mechanical
and geometrical positioning of the lenses makes the interferometer easy
to use, although the repeatability may be not be so good. A disadvantage
of this method is the strong curvature of the fringe pattern inscribed in
the fiber, which results in coupling of light to the radiation modes on the
blue side of the grating transmission spectrum [104].

3.1.15 Fabrication of step-chirped gratings
Gratings that are chirped in discrete steps are known as step-chirped.
The concept was introduced by the fabrication of phase masks, which
were not continuously chirped [231. Figure 3.28 shows the principle of
this type of grating. The grating of length L g is split into N sections, each
of length $ and uniform period A n (1 < n < N), differing from the previous
one by <5A, with

where AA^ is the total chirp of the grating and L g = SL X N. If the change
in the period and the sections are sufficiently small, then the grating
becomes continuously chirped. The important choice is the number of
sections required to build in the chirp. This has been analyzed [105], and
it was shown that the length SL of the uniform period section should be

Figure 3.28: The step-chirped phase mask. It is important to ensure that
each section of the grating has an integral number of grating periods, so that the
section lengths SL are only nominally identical [23].