Page 147 - Materials Chemistry, Second Edition

134 2 Solid-State Chemistry

difference in index of refraction between the core and cladding materials. In a
graded-index fiber, the refractive index in the core decreases continuously between
the axis and the cladding. This causes the light rays to bend smoothly as they
approach the cladding, rather than reflect abruptly from the core-cladding boundary.
Light may be lost by attenuation due to absorption by impurities and scattering from
microscopic density variations in the glass. In order to achieve sufficient transpar-

ency, the concentration of impurities such as iron and hydroxyl ions (OH ) must be
reduced to less than 1 and 10 ppb, respectively.
A fiber optic system typically consists of a transmitting device that generates
the light signal, a fiber cable that transmits the light, and a receiver. The
information (voice, data, or video) is encoded into electrical signals. At the
light source, these electrical signals are converted into either digital or analog
light signals. Once the signals are converted to light, they travel down the fiber
until they reach a detector, which changes the light signals back into electrical
signals. Finally, the electrical signals are decoded into the original voice, data,
and/or video information.
The most common method to make optical fibers is heating a rod (preform), of the

desired refractive index, to temperatures of ca. 2,000 C. The preform is made from

the high-temperature (2,000–2,300 C) reaction of SiCl 4 in the presence of dopant
[81]
gases such as BCl 3 , GeCl 4 . Once the tip of the preform is melted, it falls by
gravity to form a thin strand. This wire is threaded through a coating reel, and then
pulled into an optical fiber of the desired diameter. The draw towers used for this
process are impressive buildings, often 8–10 stories in height. The speed of the
1
pulling process (typically 10–20 m s ) governs the ultimate diameter of the fibers.
For subsequent applications, the fiber is spooled onto shipping reels and cut to the
desired length.
Another interesting application for glasses is for light control, referred to as
“smart glass.” We are all familiar with movie scenes where a top-secret meeting
takes place, and a flip of the switch instantly darkens the windows. More routinely, it
is now commonplace to have self-dimming mirrors that react to trailing vehicle
headlights. Three main technologies are responsible for these intriguing materials

applications: photochromic glasses, electrochromic devices (ECDs) and suspended-
particle devices (SPDs).
Photochromic glasses exhibit a darkening effect upon exposure to particular
wavelengths (usually in the UV regime) of light, and date back to the work of Corning
[82]
that have appeared in television commercials
TM
in the 1960s. Transitions Lenses
use this technology, effectively protecting eyes from harmful UV irradiation. The
darkening effect results from redox reactions (e.g., Eq. 47) involving microcrys-
talline metal halides (e.g., AgCl, [83] CuCl [84] ) that are present within the glass.
As one would expect, the size of these dopants must be controlled to prevent
reduced transmittance due to scattering before photochromic darkening may take
place. However, it has been proposed that the photo-induced formation of
nanoparticles (see Chapter 6) may also contribute to the observable darkening
effect.