Page 201 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

Optical Fibers and Optical FIber Amplifiers 195

To appreciate the role of water, we can start by examining a spec-
trum of the air we breathe (Fig. 9.2). The transmittance spectrum of
glass is similar to that of air. The fundamental absorption occurs at
2730 nm. This wavelength has a frequency that corresponds exactly
to the molecular vibration frequency of H 2 O. The first overtone, one
octave higher, occurs at 1370 nm. Both of these absorption bands can
be seen in Fig. 9.2. It is easy to detect the band at 1370 nm by taking
a spectrum of a tungsten lightbulb on a moderately humid day. If you
need to make measurements in this region on a humid day, you can
get rid of the absorption by purging the spectrometer with dry nitro-
gen gas. The high-transparency region around 1550 nm is a kind of is-
land in between regions where the water absorption is high. In glass,
this high-transparency region occurs at the same wavelength: 1550
nm.
The wavelength where minimum attenuation occurs is also influ-
enced by scattering due to random fluctuations in the glass itself.
Glass is amorphous. This means that the atoms of silicon and oxygen
are not arranged in a regular periodic pattern. On the local molecular
level each silicon atom is attached to two oxygen atoms, but the over-
all network of SiO 2 molecules is irregular, as shown in Fig. 9.4. The
fluctuations are frozen into the glass fiber during the fiber drawing
process at high temperature. The SiO 2 molecules are about a thou-
sand times smaller than the wavelength of visible light. Scattering or
diffusion of light by objects much smaller than the wavelength is
called Rayleigh scattering. This same phenomenon is responsible for
the blue color of the sky. Rayleigh scattering strength depends in-

{ Optical Wavelength ( m)
Low-loss region
1.530 m < < 1.700 m
Figure 9.2. Transmittance of air as a function of wavelength. Note that most of the ab-
sorption bands can be related to the presence of water and carbon dioxide. Note that
the transmission is close to 100% at wavelengths around 1550 nm.

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