The higher-order modes, the bouncing rays, tend to leak into the cladding as they propagate down the fiber-optic cable. They lose some of their energy into heat. This results in an attenuated output signal. The input pulse is split among the different rays that travel down the fiber-optic cable. The bouncing rays and the lowest-order mode, traveling down the center axis, are all traversing paths of different lengths from input to output. Consequently, they do not all reach the right end of the fiber-optic cable at the same time. When the output pulse is constructed from these separate ray components, the result is chromatic dispersion.
Fiber-optic cable that exhibits multimode propagation with a step index profile is thereby characterized as having higher attenuation and more time dispersion than the other propagation candidates. However, it is also the least costly and is widely used in the premises environment. It is especially attractive for link lengths up to 5 kilometers. It can be fabricated either from glass, plastic, or PCS. Usually, MMF core diameters are 50 or 62.5 m.
Typically, 50-m MMF propagates only 300 modes as compared to 1100 modes for 62.5-m fiber. The 50-m MMF supports 1 Gbps at 850-nm wavelengths for distances up to 1 kilometer versus 275 meters for 62.5-m MMF. Furthermore, 50-m MMF supports 10 Gbps at 850-nm wavelengths for distances up to 300 meters versus 33 meters for 62.5-m MMF. This makes 50-m MMF the fiber of choice for low-cost, high-bandwidth campus and multitenant unit (MTU) applications.
August 25, 2011