Page 58 - Fiber Bragg Gratings

2.7 Summary of routes to photosentization 39

radiation around 240 nm. However, the situation changes with radiation
at 193 nm, as has been demonstrated [46].
Silica optical fibers with cores doped with germania, phosphorus, or
alumina all exhibit increased photosensitivity when fabricated in a reduc-
ing atmosphere of hot hydrogen [105,106]. The negative effect of this type
of treatment is the increase in the absorption due to the presence of OH~
ions and also an increase in the refractive index of the core. Flame-brushing,
i.e., heating optical fiber or planar silica waveguides using a flame in a hy-
drogen-rich atmosphere, has also been used for photosensitization [107].
Hydrogen is able to diffuse into the fiber rapidly at elevated temperatures.
Germanosilicate optical fiber codoped with boron has been shown to
be highly photosensitive [40], Another advantage of the presence of boron
is the large reduction in the background refractive index, allowing more
germania to be added in the core for a given core-cladding index difference.
It is believed that with boron physiochemical changes are responsible for
the UV-induced index changes (approximately a few thousandths). Boron-
codoped fiber gratings decay with temperature more rapidly than pure-
germania doped fibers, although there are way to enhance their stability
by using burn-in (see Chapter 9). The presence of boron also increases
1
the absorption loss in the 1500-nm window by ~0.1 dB m~ . An alternative
scheme to circumvent the problems associated with boron is to use Sn
codoping with germania [63], This combination is more difficult to fabri-
cate, but has virtually no additional absorption in the 1500-nm window,
has a similar photosensitivity to the B-Ge system, and exhibits better
temperature stability. There is no reduction in the refractive index with
Sn doping.
Finally, the system that has demonstrated the largest photosensitive
response in germanosilicate optical fibers is high-pressure cold hydrogen
soaking [66]. It has been demonstrated that nearly every germanium ion
is a potential candidate for conversion from the Ge-O to the Ge-H state
[108], causing index changes as large as 0.01, although the ultimate magni-
tude of the index change is not known. Once hydrogenated, these fibers need
to be stored at low temperatures to maintain their photosensitivity, since
molecular hydrogen diffuses out just as readily as it can be introduced. A
major disadvantage of writing gratings into cold-hydrogen-sensitized fiber
gratings is the high loss of several decibels per meter at 1320 nm (OH~
absorption). Deuteration eliminates the absorption at 1320 nm, while
maintaining the photosensitivity [68]. A summary of the all currently
known techniques of photosensitizing fibers is listed in Table 2.2.