Thus, in general, multimode fiber is used for the transmission of light over relatively short distances, such as in a LAN environment. STEP-INDEX FIBER The flow of light shown at the top of Figure 3.8 rep- resents the use of one of two types of multimode fiber. In this example the flow of light is shown with respect to the use of a step-index multi- mode fiber. This type of optical fiber has a large core, usually up to 100 um in diameter. As shown in Figure 3.8, some of the rays that make up a light pulse can travel a direct route from end to end, which is denoted as a direct ray. In contrast, other rays bounce off the cladding and arrive at different times at the end of the fiber. Because this action spreads the pulse, another problem resulting from the use of step-index multimode fiber is that the pulses must be spaced sufficiently far apart at the trans- mitter to prevent them from overlapping at the receiver, enabling the receiver to discriminate between pulses. This need for a time delay between pulses limits the pulse rate, which, in turn, limits the amount of information per unit of time that can be transmitted. Although this a. Multimode step-index fiber. Figure 3.8 Higher Comparison between Order Modes CLADDING light flow in multi- mode step-index and graded fibers. Direct CLADDING b. Multimode graded-index fiber. CLADDING CLADDING Chapter Three is a constraint, multimode fiber is suitable for use with an LED light source, which is less costly than a laser. GRADED-INDEX FIBER A second type of multimode fiber contains a core in which the refractive index gradually diminishes from the center of the core outward toward the cladding. Thus, this type of fiber can be considered to have a variable refractive index. The higher refractive index at the center of the core results in the transmission of light rays along the axis occurring more slowly than the transmission of rays near the cladding. In addition, because of the graded index, light in the core follows a helix-shaped curve instead of bouncing off the cl