Page 203 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

Optical Fibers and Optical FIber Amplifiers 197

versely on the fourth power of the wavelength. This means that light
having a wavelength of 750 nm will suffer 16 times more attenuation
than light having a wavelength of 1500 nm. You can verify from Fig.
9.3 that the Rayleigh law is at work. As we have already seen, attenu-
ation at longer wavelengths is limited by residual water molecules in
the glass. The best compromise in today’s technology occurs at 1550
nm. Glass engineers continue to experiment with ways to lower the
attenuation further. One approach is to introduce impurities that re-
duce the equilibrium water vapor content. Another is to fabricate
glass compositions that can be drawn at lower temperatures, reducing
the amplitude of structural fluctuations in the glass. However, atten-
uation is only part of the story that explains why glass optical fibers
are a commercial success.

9.3 Optical Fiber Engineering
In the previous section, we discussed the importance of low optical at-
tenuation. This is certainly the feature that made optical fibers look
attractive to telecommunications engineers. But it is a long way from
a piece of glass with low absorption to an optical fiber product that
can be made to the same specifications day after day and sold as a
product.
A commonly used process to make optical fiber starts with a hollow
tube of high-purity fused silica. A soot of silica, doped with germani-
um is deposited by chemical vapor deposition on the inside of the
tube. This is called inside vapor deposition or IVD. The tube and soot
are heated so that the soot turns to glass. The tube is pulled at high
temperature like taffy along its long axis until the hollow region in
the center disappears, creating a preform. The germanium dopant
gives the core region an index of refraction n 1 that is higher than that
of the cladding n 2 . This assures that the fiber will act as a waveguide.
The index difference between n 1 and n 2 is controlled carefully. If there
is too much germanium in the core, the fiber will still act as a wave-
guide, but the difference in thermal expansion between the core and
the cladding will result in stress that will cause cracks that will lead
to mechanical failure of the fiber.
The preform is heated again in a fiber drawing tower and the fiber
is pulled from the preform. The outer diameter of the fiber is about
125 microns and the core diameter is about 9 microns. The core and
the diameters are very carefully controlled. As we will show present-
ly, the core diameter is determined by the index difference. Careful
control of the core diameter and its position inside the fiber are cru-
cial for obtaining low-loss splicing of one fiber to another.
Careful control of the cladding diameter is required to present a

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