Page 48 - Fiber Bragg Gratings

2A. Photosensitization techniques 29

study of the decay of gratings written in B-Ge may be found in Ref. [62].
The thermal annealing of gratings is discussed in Chapter 9.
Boron causes additional loss in the 1550-nm window, of the order of
—0.1 dB/m, which may not be desirable. For short gratings, this need not
be of concern.

2.4.3 Tin-germanium codoped fibers
Fabrication of Sn codoped Ge is by the MCVD process used for silica
fiber by incorporating SnCl 4 vapor. SnO 2 increases the refractive index
of optical fibers and, used in conjunction with GeO 2, cannot be used as
B 2O 3 to match the cladding refractive index, or to enhance the quantity
of germanium in the core affecting the waveguide properties. However,
it has three advantages over B-Ge fiber: The gratings survive a higher
temperature, do not cause additional loss in the 1500-nm window, have
a slightly increased UV-induced refractive index change, reported to be
3 times larger than that of B-Ge fibers. Compared with B-Ge, Sn-Ge fibers
lose half the UV-induced refractive index change at ~600°C, similarly to
standard fibers [63].

2.4.4 Cold, high-pressure hydrogenation
The presence of molecular hydrogen has been shown to increase the ab-
sorption loss in optical fibers over a period of time [64]. The field was
studied extensively [65], and it is known that the hydrogen reacts with
oxygen to form hydroxyl ions. The increase in the absorption at the first
overtone of the OH vibration at a wavelength of 1.27 /am was clearly
manifest by the broadband increase in loss in both the 1300-nm and, to
a lesser extent, in the 1500-nm windows. Another effect of hydrogen is
the reaction with the Ge ion to form GeH, considerably changing the band
structure in the UV region. These changes, in turn, influence the local
refractive index as per the Kramers-Kronig model. The reaction rates
have been shown to be strongly temperature dependent [65]. It has been
suggested that the chemical reactions are different on heat treatment
and cause the formation of a different species compared to illumination
with UV radiation. However, no noticeable increase in the 240-nm band
is observed with the presence of interstitial molecular hydrogen in Ge-
doped silica. The highest refractive index change induced by UV radiation
is undoubtedly in cold hydrogen soaked germania fibers. As has been