7/31/2019 Intramodal Dispersion is Pulse Spreading That Occurs Within a Single Mode

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Modal dispersionIt is important to examine the nature and effects of modal transmission. Afiber that has a high NA and/or diameter will have a large number of modes(rays of lights) operating along the length of that fiber. An omnidirectional

light source (i.e. one that effectively radiates light rays equally in alldirections) such as an LED will emit several thousand rays of light in asingle pulse. Because the light source injects a broad angle of beam into thecore, each mode of light travelling at a different angle as it propagtes downthe fiber will therefore that it will take different length of time for each rayto travel from one end of the fiber to the other. The light transmitter willlaunch all modes into the fiber exactly at the same time, and the signal willappear at the the beginning of the fiber as a short sharp pulse. By the time

the light reaches the end of the fiber, it will have spread out and will appearas an elongated pulse. This is referred to as a modal dispersion .

The light ray that travels down the center axis of the fiber is referred to asthe fundamental mode and is the lowest order mode possible. The light raysthat travel the shorter distances down the length of the fiber are the lowerorder modes and the rays that travel the longer distances down the fiber arethe higher order modes.

If the input light pulses are too close together, then the output pulses willoverlap on each other, causing inter-symbol interference at the receiver. Theeffect of modal dispersion would rely on this development. This situationwill make it difficult for the receiver to distinguish between pulses and willintroduce errors into the data. This is the major factor in the fiber opticcables (multimode types) that limit transmission speed. This is illustrated infigure below.

It can be seen from this diagram that it will be difficult for the receiver todistinguish between the output pulses as they overlap on each other as theyexit the fiber core (inter-symbol interference).Modal dispersion is measured in nanoseconds and is given by the followingformula:

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Where,

D = total dispersion of a pulseD0 = pulse width at the output of the fiber in nanosecondsD i = pulse width at the input of the fiber in nanoseconds

Modal dispersion increases with increasing NA, and therefore, thebandwidth of the fiber decreses with an increse in NA. The same rule appliesto the increasing diameter of the core.