3.2.9 This page explains some factors that reduce signal strength in optical media.
Fiber-optic cable is not affected by the sources of external noise that cause problems on copper media because external light cannot enter the fiber except at the transmitter end. The cladding is covered by a buffer and an outer jacket that stops light from entering or leaving the cable.
Furthermore, the transmission of light on one fiber in a cable does not generate interference that disturbs transmission on any other fiber. This means that fiber does not have the problem with crosstalk that copper media does. In fact, the quality of fiber-optic links is so good that the recent standards for gigabit and ten gigabit Ethernet specify transmission distances that far exceed the traditional two-kilometer reach of the original Ethernet. Fiber-optic transmission allows the Ethernet protocol to be used on metropolitan-area networks (MANs) and wide-area networks (WANs).
Although fiber is the best of all the transmission media at carrying large amounts of data over long distances, fiber is not without problems. When light travels through fiber, some of the light energy is lost. The farther a light signal travels through a fiber, the more the signal loses strength. This attenuation of the signal is due to several factors involving the nature of fiber itself. The most important factor is scattering. The scattering of light in a fiber is caused by microscopic non-uniformity (distortions) in the fiber that reflects and scatters some of the light energy.
Absorption is another cause of light energy loss. When a light ray strikes some types of chemical impurities in a fiber, the impurities absorb part of the energy. This light energy is converted to a small amount of heat energy. Absorption makes the light signal a little dimmer.
Another factor that causes attenuation of the light signal is manufacturing irregularities or roughness in the core-to-cladding boundary. Power is lost from the light signal because of the less than perfect total internal reflection in that rough area of the fiber. Any microscopic imperfections in the thickness or symmetry of the fiber will cut down on total internal reflection and the cladding will absorb some light energy.
Dispersion of a light flash also limits transmission distances on a fiber. Dispersion is the technical term for the spreading of pulses of light as they travel down the fiber.
Graded index multimode fiber is designed to compensate for the different distances the various modes of light have to travel in the large diameter core. Single-mode fiber does not have the problem of multiple paths that the light signal can follow. However, chromatic dispersion is a characteristic of both multimode and single-mode fiber. When wavelengths of light travel at slightly different speeds through glass than do other wavelengths, chromatic dispersion is caused. That is why a prism separates the wavelengths of light. Ideally, an LED or Laser light source would emit light of just one frequency. Then chromatic dispersion would not be a problem.
Unfortunately, lasers, and especially LEDs generate a range of wavelengths so chromatic dispersion limits the distance that can be transmitted on a fiber. If a signal is transmitted too far, what started as a bright pulse of light energy will be spread out, separated, and dim when it reaches the receiver. The receiver will not be able to distinguish a one from a zero.
The next page will discuss the installation, care, and testing of optical fiber.
3.2.10 This page will teach students how to troubleshoot optical fiber.
A major cause of too much attenuation in fiber-optic cable is improper installation. If the fiber is stretched or curved too tightly, it can cause tiny cracks in the core that will scatter the light rays. Bending the fiber in too tight a curve can change the incident angle of light rays striking the core-to-cladding boundary. Then the incident angle of the ray will become less than the critical angle for total internal reflection. Instead of reflecting around the bend, some light rays will refract into the cladding and be lost.
To prevent fiber bends that are too sharp, fiber is usually pulled through a type of installed pipe called interducting. The interducting is much stiffer than fiber and cannot be bent so sharply that the fiber inside the interducting has too tight a curve. The interducting protects the fiber, makes it easier to pull the fiber, and ensures that the bending radius (curve limit) of the fiber is not exceeded.
When the fiber has been pulled, the ends of the fiber must be cleaved (cut) and properly polished to ensure that the ends are smooth. A microscope or test instrument with a built in magnifier is used to examine the end of the fiber and verify that it is properly polished and shaped. Then the connector is carefully attached to the fiber end. Improperly installed connectors, improper splices, or the splicing of two cables with different core sizes will dramatically reduce the strength of a light signal.
Once the fiber-optic cable and connectors have been installed, the connectors and the ends of the fibers must be kept spotlessly clean. The ends of the fibers should be covered with protective covers to prevent damage to the fiber ends. When these covers are removed prior to connecting the fiber to a port on a switch or a router, the fiber ends must be cleaned. Clean the fiber ends with lint free lens tissue moistened with pure isopropyl alcohol. The fiber ports on a switch or router should also be kept covered when not in use and cleaned with lens tissue and isopropyl alcohol before a connection is made. Dirty ends on a fiber will cause a big drop in the amount of light that reaches the receiver.
Scattering, absorption, dispersion, improper installation, and dirty fiber ends diminish the strength of the light signal and are referred to as fiber noise. Before using a fiber-optic cable, it must be tested to ensure that enough light actually reaches the receiver for it to detect the zeros and ones in the signal.
When a fiber-optic link is being planned, the amount of signal power loss that can be tolerated must be calculated. This is referred to as the optical link loss budget. Imagine a monthly financial budget. After all of the expenses are subtracted from initial income, enough money must be left to get through the month.
The decibel (dB) is the unit used to measure the amount of power loss. It tells what percent of the power that leaves the transmitter actually enters the receiver.
Testing fiber links is extremely important and records of the results of these tests must be kept. Several types of fiber-optic test equipment are used. Two of the most important instruments are Optical Loss Meters and Optical Time Domain Reflectometers (OTDRs).
These meters both test optical cable to ensure that the cable meets the TIA standards for fiber. They also test to verify that the link power loss does not fall below the optical link loss budget. OTDRs can provide much additional detailed diagnostic information about a fiber link. They can be used to trouble shoot a link when problems occur.
This page concludes this lesson. The next lesson will discuss wireless media. The first page will discuss Wireless LAN organizations and standards.
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