First we will discuss about Single Mode Fiber. Well the Single-mode fiber (SMF, sometimes called mono mode) optic cable is most commonly used by telephone companies and in data installations as backbone cable. Single-mode fiber-optic cable is not used as horizontal cable to connect computers to hubs. The light in a single-mode cable travels straight down the fiber (as shown in Figure 1.5) and does not bounce off the surrounding cladding as it travels. Typical single-mode wavelengths are 1,310 and 1,550 nanometers. Before you install single-mode fiber-optic cable, make sure the equipment you are using supports it. The equipment that uses single-mode fiber typically uses lasers to transmit light through the cable because a laser is the only light source capable of inserting light into the very small (8- to 10-micron) core of a single-mode fiber.

Multimode Fiber-Optic Cable

Multimode fiber (MMF) optic cable is usually the fiber-optic cable used with networking applications such as 10Base-FL, 100Base-F, FDDI, ATM, and others that require fiber optics for both horizontal and backbone cable. Multimode cable allows more than one mode of light to propagate through the cable. Typical wavelengths of light used in multimode cable are 850 and 1,300 nanometers. There are two types of multimode fiber-optic cable: step index or graded index. Step-index
multimode fiber-optic cable indicates that the refractive index between the core and the cladding is very distinctive. The graded-index fiber-optic cable is the most common type of multimode fiber.

The core of a graded-index fiber contains many layers of glass; each has a lower index of refraction going outward from the core of the fiber. Both types of multimode fiber permit multiple modes of light to travel through the fiber simultaneously (just see below Figure). Graded-index fiber is preferred because less light is lost as the signal travels around bends in the cable. The typical multimode fiber-optic cable used for horizontal cabling consists of two strands of fiber (duplex); the core is either 50 or 62.5 microns (micrometers) in diameter, and the cladding is 125 microns in diameter (the measurement is often simply referred to as 50/125-micron or 62.5/125-micron).