Page 24 - Fiber Bragg Gratings

1.2 Materials for glass fibers 5

of the commonly used core dopants, germanium, belongs to Group IVA,
as does silicon and replaces the silicon atom within the tetrahedron,
coordinated with four oxygen atoms. Pure germania has a band edge at
around 185 nm [24]. Apart from these pure material contributions, which
constitute a fundamental limit to the attenuation characteristics of the
waveguide, there may be significant absorption loss from the presence of
impurities. The OH~ ion has IR absorptions at wavelengths of 1.37, 0.95,
and 0.725 /mi [25], overtones of a stretching-mode vibration at a funda-
mental wavelength of 2.27 /an. Defect states within the ultraviolet and
visible wavelength band of 190-600 nm [26] also contribute to increased
absorption. The properties of some of these defects will be discussed in
Chapter 2.
The presence of phosphorus as P 2O 5 in silica, even in small quantities
(—0.1%), reduces the glass melting point considerably, allowing easier
fabrication of the fiber. Phosphorus is also used in fibers doped with rare
earth compounds such as Yb and Er for fiber amplifiers and lasers. In
high concentration rare earth ions tend to cluster in germanium-doped
silicate glasses. Clustering causes ion-ion interaction, which reduces the
excited state lifetimes [27], Along with aluminum (A1 2O 3 as a codopant
in silica) in the core, clustering is greatly reduced, enabling efficient
amplifiers to be built. Phosphorus is also commonly used in planar silica on
silicon waveguide fabrication, since the reduced processing temperature
reduces the deformation of the substrate [28].
Fluorine and trivalent boron (as B 2O 3) are other dopants commonly
used in germania-doped silica fiber. A major difference between germa-
nium and fluorine/boron is that while the refractive index increases with
increasing concentration of germanium, it decreases with boron/fluorine.
With fluorine, only modest reductions in the refractive index are possible
(—0.1%), whereas with boron large index reductions (>0.02) are possible.
Boron also changes the topology of the glass, being trivalent. Boron and
germanium together allow a low refractive index difference between the
core and cladding to be maintained with large concentrations of both
elements [29]. On the other hand, a depressed cladding fiber can be fabri-
cated by incorporating boron in the cladding to substantially reduce the
refractive index.
The density of the boron-doped glass may be altered considerably by
annealing, by thermally cycling the glass, or by changing the fiber drawing
temperature [30]. Boron-doped preforms exhibit high stress and shatter
easily unless handled with care. The thermal history changes the density