Page 119 - Fiber Bragg Gratings

98 Chapter 3 Fabrication of Bragg Gratings

hydrogenated sample resulted in a chirped grating with a bandwidth of
6.0 nm [101].
Stretching a fiber prior to writing a grating shifts the Bragg wave-
length in the relaxed state [7]. Byron and Rourke [103] applied the
stretch-write technique to form a chirped grating with a scanned phase
mask. As the UV beam (2 mm long) was stepped across the phase mask
(and the fiber), the prestrained (0.6%) fiber was also relieved of strain in
15 steps of 0.04%. A chirped grating with a bandwidth of 10 nm for a
grating length of 30 mm was demonstrated. Care has to be taken with
this method, since the fiber can easily slip when under tension.
As will be appreciated with the above methods, the bandwidth of the
chirp is generally small; in order to increase the chirp, it is necessary to
write a chirped grating in the first place. Continuously chirped gratings
with larger chirp values can be fabricated with two beams with dissimilar
phase fronts. If one parallel beam is interfered with a second diverging
beam, the resulting interference pattern will have a period that varies
with spatial position in the fringe plane. Figure 3.27 shows the scheme
for writing chirped gratings with two diverging, two converging, or a

Figure 3.27: Nonuniform wave fronts used in the interferometer to produce
chirped gratings [104]. Two cylindrical lenses with focal lengths Fl and F2 create
a chirped interference pattern at the fiber. The third cylindrical lens with focal
length F3 focuses the interfering beams into a stripe at the fiber.