Optical Communications, 2nd Edition

1.5 THE FIBER OPTICAL CHANNEL

An optical fiber is a thin pipe of glass, through which one can insert light and have it shine out at the output end. Hence a fiber can transmit a modulated light field from its input to its output. The basic construction of a fiber is shown in Figure 1.16, and corresponds to a central glass core encased in a plastic shielding. The core is made of high-quality glass through which the light can propagate. Glass, or a similar silicon compound, is used, because it offers the least attenuation for optical transmission. The core is supported by the shielding, or cladding, which provides mechanical protection, isolates the core from external radiation, and aids in confining the light to only the glass core. The cladding is opaque, and basically reflects any escaping light back into the core. Typically the core diameter is on the order of micrometers, whereas the fiber cross-section (core plus cladding) is on the order of several millimeters. Thus the fiber is literally a thread of transmission path over which the modulated light field can be propagated. This ability to pack large amounts of modulated data over an extremely small spatial area is an overwhelming advantage for optical fiber communications [8, 9].

The key parameters describing the propagation properties of the fiber light field are the refractive index of the core and cladding. The cladding is usually designed to have an index a few percent smaller than that of the core. (Glass typically has ...