Page 47 - Fiber Bragg Gratings

28 Chapter 2 Photosensitivity and Photosensitization of Optical Fibers

contribution due to the germanium concentration, while the negative
refractive contribution is due to the boron, resulting in the continuous
line positive refractive index profile.
It should be noted that with boron and germanium, it is possible to
selectively place a photosensitive region anywhere in the fiber, without
altering the wave guiding properties. Other types of profiles possible are
boron with highly doped germanium in a cladding matched to silica for
liquid cored fibers [55], in-cladding gratings for lasers [56], and special
fiber for side-tap filters and long-period gratings [57,58],
B-Ge codoped fiber is fabricated using MCVD techniques and a stan-
dard phosphorus-fluorine cladding matched silica tube with normal oxi-
dizing conditions. The reactive precursor vapors are SiCl 4, BC1 3, and
GeCl 4, with oxygen as a carrier for the core deposition. For a composition
equivalent to —16 mol% germanium, the photosensitivity in comparison
with 20 mol% unreduced germanium fiber shows an improvement >3-
fold in the UV-induced refractive index modulation as well as an order of
magnitude reduction in the writing time. With respect to 10 mol% reduced
germanium fiber, the improvement in the maximum refractive index mod-
ulation is ~40% with a X6 reduction in the writing time. The maximum
3
refractive index change is close to 10~ for this fiber induced with a CW
laser operating at 244 nm [54].
A point worth noting with B-Ge fibers is the increased stress, and
consequently, increased induced birefringence [59]. The preforms are diffi-
cult to handle because of the high stress. However, the real advantages
with B-Ge fibers are the shortened writing time, the larger UV-induced
refractive index change, and, potentially, fibers that are compatible with
any required profile, for small-core large NA fiber amplifiers, to standard
fibers.
B-Ge fibers form Type IIA gratings [60] with a CW 244-nm laser, as
is the case with the data shown in Fig. 2.4. This suggests that there is
probably little difference due to the presence of boron; only the high
germanium content is responsible for this type of grating. There is a
possibility that stress is a contributing factor to the formation of Type
IIA [61]; recent work does partially indicate this but for germanium-doped
fibers [44].
Typically, gratings written with CW lasers in B-Ge fiber decay more
rapidly than low germanium doped (5 mol%) fibers when exposed to heat.
Gratings lose half their index modulation when annealed at ~400°C (B-
Ge: 22:6.3 mol%) and ~650°C (Ge 5 mol%) [46] for 30 minutes. A detailed