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Articles The Basics of Fiber Optic Cable

More and more telephone and cable systems are converting to fiber optic cable rather than copper wire. Local Area Networks (LANs), particularly for high-capacity backbones, are also using more fiber than copper.


SPEED: Fiber optic networks operate at speeds up to 2.5 gigabits per second, as opposed to 155 megabits per second for copper.
BANDWIDTH: Substantially larger carrying capacity.
DISTANCE: Signals can be transmitted further without needing to be "refreshed" or strengthened.
RESISTANCE: Greater resistance to electromagnetic noise such as radios, motors or other nearby cables.
MAINTENANCE: Fiber optic cables costs much less to maintain.


There are three types of fiber optic cable: single mode, multimode and plastic optical fiber (POF).

Single Mode cable is a single stand of glass fiber with a diameter of 8.3 to 10 microns. (One micron is 1/250th the width of a human hair.)

Multimode cable is made of multiple strands of glass fibers, with a combined diameter in the 50-to-100 micron range. Each fiber in a multimode cable is capable of carrying a different signal independent from those on the other fibers in the cable bundle. POF is a newer plastic-based cable which promises performance similar to single mode cable, but at a lower cost.

While fiber optic cable itself is cheaper than an equivalent length of copper cable, fiber optic cable connectors and the equipment needed to install them are more expensive than their copper counterparts.

Fiber optic cable functions as a "light guide," guiding the light introduced at one end of the cable through to the other end. The light source can either be a light-emitting diode (LED)) or a laser.

The light source is pulsed on and off, and a light-sensitive receiver on the other end of the cable converts the pulses back into the digital ls and 0s of the original signal.

Even laser light shining through a fiber optic cable is subject to loss of strength, primarily through dispersion and scattering of the light, within the cable itself. The faster the laser fluctuates, the greater the risk of dispersion. Light strengtheners, called repeaters, may be necessary to refresh the signal in certain applications.


Let us review the case of TV signal being delivered via a fiber optic cable to a TV set-top box. An analog TV signal is first converted to a digital signal by a Digital-To-Analog (DAC) converter. A laser is pulsed on and off rapidly in response to the (digital) signal input, converting the electrical signal to a laser light signal. The light travels down the fiber optic cable toward its destination which, if the distance is great enough, is strengthened by a repeater. The process is reversed at the receiving end. The light from the end of the fiber optic cable shines onto a light-sensitive receiver which converts the light back into digital electrical pulses. The digital information is then converted back into an analog TV signal by another DAC before being fed to the TV set.

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