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294 Chapter 6

mode at the wavelengths around 1550 nm (optical C-band, 1530 nm – 1565 nm, bandwidth 4.5
THz = 4500 GHz) where the attenuation in silica reaches the minimum. The modal dispersion
is completely eliminated while the so-called chromatic dispersion stays, but it is low. The main
source of the latter occurrence is the frequency dependence of wave mode propagation constant.
Remind that we discussed the same phenomenon in strip lines. An optical fiber as many open
lines is filled up with two mediums (core and cladding) of dissimilar dielectric constants.
Taking into account that the energy division between mediums is wavelength dependable we
can expect the imminent frequency variation of the mode propagation constant. Theoretically,
a single mode fiber has the maximum possible bandwidth about 1 petabit (1000 terabit) per sec.
Such capacity is equivalent to sending 5000 HDTV videos of two hours in a second. The serious
disadvantage of single mode cables is relatively high cost of production and maintenance.
Besides, the equipment supporting single mode cable is generally more expensive too. Table
8
6.3 demonstrates some advantages of optical cables relative to coaxial ones.
Table 6.3

Note that for some particular applications like sensors, fiber directional couplers, and several
more specialized purposes Subwavelength-Diameter Optical Fibers (SDOF) were developed.
The diameter (1 to 10 nm) of such fibers are fewer than the wavelength of propagating single
mode light wave. If so, SDOF carries the significant portion of their guided energy outside the
core that makes it sensitive to the external environment, i.e. to be a sensor of high sensitivity.

6.3.3 Hollow-Core Photonic Crystal Fiber

The modal and chromatic dispersion in any fiber is the main effect limiting the data
transmission rate of long distance optical cables. Evidently, the chromatic dispersion can be
minimized if we could shift the propagating light beam from silicon core to hollow air core
simultaneously preventing it from radiation by one way or another. It sounds may be weird, but
in 2013 a research team at the University of Southampton in England reported that they had
built a fiber cable that is capable of carrying data at 99.7 percent of the vacuum-speed of light
using the new production technology of so-called Photonic Band Gap (PBG) 2D crystal. Figure
6.3.6 illustrates the cross-section of this type of crystal core. The core lattice consists of “Swiss

8 Public Domain Image, source: http://www.fibersystems.com/pdf/whitepapers/Basics-of-Fiber-
Optics.pdf