Information Technology Reference
In-Depth Information
FIGURE 5.6
Fiber-optic cables.
Top: a tight-buffer single-strand cable. Bottom: a loose-buffer multistrand cable.
nanometers (nm). Operating characteristics of a particular fiber are normally spec-
ified at discrete wavelengths, which correspond to the output wavelengths of com-
mon light-emitting diode (LED) and semiconductor laser emitters/transmitters.
Multimode fiber is normally used at 850 nm or at 1300 nm, while single-mode fiber
is used at 1310 nm or 1550 nm. For any given installation, only one mode and one
wavelength are used. The fiber cable should be characterized for operation at that
mode and wavelength.
Optical fiber is rather like a waveguide for light. While RF waveguides are hol-
low with metal sides, optical fiber is solid. During manufacture, the refractive index
of the glass is made to vary with the diameter of the fiber core. A difference in
refractive index is what makes light actually bend between air and water, for exam-
ple. The light is sent down the fiber and literally bounces off the “walls” formed by
the step in the refractive index. In a single-mode fiber, the transmitting emitter is a
laser diode, chosen so that the light entering the fiber will be coherent and straight
along the axis of the fiber. A multimode fiber, on the other hand, often uses a graded
refractive index. Light entering from an LED (not a laser) enters at all angles, but is
coached down the fiber by the gradient of refraction. Multimode fiber is also avail-
able in stepped index, but the step occurs much nearer the outer diameter than with
single-mode fiber. Single and multimode operations are shown in Fig. 5.7.
Optical fiber can carry very large bandwidths of information at low attenua-
tion. Multimode fiber can be used at distances exceeding 3000 m, although other
constraints, such as transmitted bandwidth, may limit operation to less distance.
