|  | 3.2 |
| Optical Media
|  | 3.2.10 |
| Installation, care, and testing of 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 can not 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.
Web Links
A New Fiber-Optic Installation Standard http://www.ecmweb.com/ar/electric_ new_fiberoptic_ installation/