Page 73 - Optical Communications Essentials

Optical Fibers

Optical Fibers 63

Figure 4.12. End-face patterns of two possible
holey fiber structures.

Holey fiber. A holey or photonic crystal fiber typically consists of a silica mate-
rial which contains numerous air-filled microscopic holes. Figure 4.12 shows
the end-face patterns of two possible holey fiber structures. The tubular holes
run along the entire length of the fiber parallel to the fiber axis. The size, posi-
tion, and number of holes provide the fiber with specific waveguide properties.
This technology is under development. Potential applications of holey fibers in
telecommunications include dispersion compensation, wavelength conversion,
optical switching, and high-power optical amplification.

4.9. Summary
An optical fiber is nominally a cylindrical dielectric waveguide that confines and
guides light waves along its axis. Basically all fibers used for telecommunication
purposes have the same physical structure, which consists of a cylindrical glass
core surrounded by a glass cladding. The difference in the core and cladding
indices determines how light signals travel along a fiber.
An important physical concept is that only a finite set of light rays that
impinge on the core walls at specific angles may propagate along a fiber. These
angles are related to a set of electromagnetic wave patterns called modes. For a
single-mode fiber, the core diameter is around 8 to 10µm (several wavelengths),
and only the fundamental ray is allowed to propagate. Multimode fibers have
larger core diameters (e.g., around 50µm) and support many modes. The ray
theory can explain a number of fiber performance characteristics, but other
attributes require the wave theory. The power distribution of modes is not con-
fined completely to the core, but extends partially into the cladding. This con-
cept is important when we examine concepts such as optical power coupling.
Light traveling in a fiber loses power over distance, mainly because of absorp-
tion and scattering mechanisms in the fiber. This attenuation is an important
property of an optical fiber because, together with signal distortion mecha-
nisms, it determines the maximum transmission distance possible. The degree

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