
23Optical Fibers: Geometrical and Guiding Properties
2.3 Optical Fiber: General Properties
2.3.1 Geometrical Structures and Index Profile
An optical ber consists of two concentric dielectric cylinders. The inner cylinder, or core,
has a refractive index of n(r) and radius a. The outer cylinder, or cladding, has index n
2
with
n(r) > n
2
and a larger outer radius. Core of about 4–9 μm and a cladding diameter of 125 μm
are the typical values for silica-based single-mode optical ber. A schematic diagram of an
optical ber is shown in Figure 2.4.
The refractive index n of an optical waveguide is usually changed with radius r from the
ber axis (