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Fiber-Optic Cables

While plastic wires are used in some specialized, low bandwidth, short haul applications (e.g., automobiles and airplanes), glass predominates. Generally speaking, fiber cables contain a large number of pairs of glass fibers, as the additional cost of redundancy is relatively low. Oftentimes, only a few of the fibers are active, with others being left dark for backup or future use. While current technology, although more expensive, allows two-way transmission over a single fiber, two fibers generally are used, with one transmitting in each direction.

The mass production of glass fiber employs several techniques, all of which take place in a vacuum environment. First, silica is heated to the point that it vaporizes. The ultra-pure glass vapor is then deposited on a designated surface to create a glass cylinder. That cylinder is then reheated and collapsed into a preform cylinder. The preform cylinder is reheated and drawn, in a process known as broomsticking, into fibers which can be as long as 10 km in length.

The light pulse travels down the center core of the glass fiber, which is especially pure. Surrounding the inner core is a layer of glass cladding, with a slightly different refractive index. The cladding serves to reflect the light waves back into the inner core. Surrounding the cladding is a layer of protective coating, such as Kevlar, which seals the cable and provides mechanical protection (Figure 3.8). Typically, multiple fibers are housed in a single sheath, which may be heavily armored. Glass optical fibers are of two basic types, multimode and monomode (or singlemode).


Figure 3.8  Glass fiber optic cable, side view, and cross section.

Multimode fiber is less expensive to produce, but performs less well, as the inner core is larger in diameter. As the light rays travel down the fiber, they spread out due to a phenomenon known as modal dispersion. Although reflected back into the inner core by the cladding, they travel different distances and, therefore, arrive at different times. As the distance of the circuit increases and the speed of transmission increases, the pulses of light tend to overrun each other in a phenomenon know as pulse dispersion. At that point, the light detector is unable to distinguish between the individual pulses. As a result, multimode fiber is relegated to applications involving relatively short distances and lower speeds of transmission (e.g., LANs and campus environments).

Monomode fiber has a thinner inner core; therefore, it performs better than does multimode fiber over longer distances at higher transmission rates. Although more costly, monomode fiber is used to advantage in long-haul, and especially in high bandwidth, applications [3-14].

Light Detectors

These are of several basic types, with the most common being PhotoINtrinsic diodes (PINs) and Avalanche PhotoDiodes (APDs). The light detectors serve to reverse the process accomplished by the light sources, converting optical energy back into electrical energy. APDs, although more expensive, are preferable, as they use a strong electric field to accelerate the electrons flowing in the semiconductor. This results in an avalanche of electrons. Therefore, a very weak incoming light pulse will create a much stronger electrical effect. Although more sensitive, APDs require more power and are more sensitive to ambient temperatures.

Analog or Digital?

Fiber optic systems can be either analog or digital in nature, although digital is much more common. Analog systems simply vary the intensity of the lightwave. Digital systems pulse on and off to represent 1s and 0s; differences in the length of the pulses can represent multiple 1s and 0s. As digital systems offer significant advantages, all long haul fiber systems used in carrier networks are digital. While significant amounts of analog fiber are being deployed by CATV providers for purposes of cable TV delivery, those systems are intended to be upgraded to digital technology when practical.

Bandwidth

Fiber offers by far the greatest bandwidth of any transmission system, often in excess of 2 Gbps in long haul carrier networks. Systems of 10 Gbps and 20 Gbps have been deployed, and 40 Gbps and 50 Gbps systems have been tested successfully on numerous occasions in laboratory environments. AT&T Bell Laboratories recently successfully tested 40 Gbps, error-free transmission at distances of 900 miles. The theoretical capacity of fiber is in the Terabit (Tbps) range, with current monomode fiber capacity being expandable to that level with the application of subsequent generations of electronics.

Error Performance

As fiber is dielectric (a nonconductor of direct electric current), it is not susceptible to EMI/RFI; neither does it emit EMI/RFI; The light signal will suffer from attenuation, although less so than other media. Such optical attenuation can be caused by scattering of the optical signal, bending in the fiber cable, translation of light energy to heat, and splices in the cable system. Error performance, depending on the compression scheme utilized, ranges between 10-9 and 10-14, one errored bit in every 100 trillion [3-16].

Distance

Monomode fiber optic systems routinely are capable of transmitting unrepeatered signals over distances in excess of 200 miles (322 km.). As a result, relatively few optical repeaters are required in a long-haul system, thereby reducing costs, and eliminating points of potential failure or performance degradation. In fact, tests have indicated that unrepeatered signals can travel distances of up to 8,000 miles (12,900 km.) Such systems employ a process of chemical amplification, achieved by chemically doping a section of the cable with erbium, a metallic rare earth element. When the laser pulse strikes that section of the cable, the chemical is excited and, in turn, amplifies the light pulse.


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