me all new high-capacity and long-distance cable systems, including submarine systems, have used optical fibers as a transmis- sion medium. The advantages of optical fibers include these: Core: refraction index Core diameter is higher than that of Ray of light around 8 or 60 mm surrounding material Laser or LED 125 mm transmitter Figure 4.28 Optical fiber. Transmission High transmission capacity: Optical fibers have a very large bandwidth and they are able to carry very high data rates, up to 50 Gbps. Low cost: The cost of the fiber has decreased to the level of a twisted-pair cable; however, the coating and shielding of the cable increase the cost by a factor of two or more. Tolerance against external interference: Electromagnetic disturbances have no influence on the light signal inside the fiber. Small size and low weight: Fiber material weighs little and the fiber diameter is only of the order of a hundred micrometers instead of a millimeter or more for copper wire. Unlimited material resource: Quartz used in glass fibers is one of the most common materials on Earth. Low attenuation: Attenuation in modern fibers is less than half a decibel per kilometer and it is independent of the data rate. One disadvantage of optical fibers is that they are more difficult to install than copper cables. Installation and maintenance, for example, repair of a broken fiber, require special equipment and well-trained personnel. Another disadvantage is that the radiation of light from a broken fiber may cause damage to the human eye. The safety standards set by IEC restrict the allowable maximum optical power that can be used and they also specify if equipment has to be able to switch off the transmitter in the case of a fiber fault. Note that visible light has a shorter wavelength (700-400 nm) than light used in optical systems. Fibers are divided into two main categories, multimode and single-mode fibers. Multimode fibers, with diameters of 125/60 um cladding/core are used in short-haul applications such as optical LANs. They 