Page 46 - Fiber Bragg Gratings

2.4. Photosensitization techniques 27

2.4.2 Germanium-boron codoped silicate fibers

The use of boron in soda lime and silicate glass has been known for a
long time [53]. It has also been established that boron, when added to
germania-doped silicate glass, reduces the refractive index. The transfor-
mational changes that occur depend on the thermal history and processing
of the glass. As such, it is generally used in the cladding of optical fibers,
since the core region must remain at a higher refractive index. Compared
to fluorine, the other commonly used element in the cladding (in conjunc-
tion with phosphorus), the refractive index modification is generally at
least an order of magnitude larger, since more of the element can be
incorporated in the glass. Thus, while the maximum index difference from
3
fluorine can be approximately —10~ with boron, the index change can
be > — 0.01|. This opens up many possibilities for the fabrication of novel
structures, not least as a component to allow the incorporation of even
more germania into glass while keeping a low refractive index difference
between the cladding and core when both are incorporated into the core.
One advantage of such a composition is the fabrication of a fiber that is
outwardly identical in terms of refractive index profile and core-to-clad-
ding refractive index difference with standard single-mode optical fibers,
and yet contains many times the quantity of germania in the core. The
obvious advantage is the increased photosensitivity of such a fiber with
the increased germania. Indeed, this is the case with boron-germanium
(B-Ge) codoped fused silica fiber [54]. The typical profile of a B-Ge preform
is shown Fig. 2.7. The raised refractive index dashed line shows the

Figure 2.7: The refractive index components due to germanium and boron
(dashed and dotted lines) contributing to the resultant preform profile (continuous
line) [54].