Modal-Dispersion and Limit on Step-Index Multimode Fibers’ Bandwidth

Take a look at the illustration for a step-index multimode fiber. Rays of light enter the fiber with different angles to the fiber axis, up to the fiber's acceptance angle (numerical aperture). Rays that enter with a shallower angle travel by a more direct path, and arrive sooner than those enter at steeper angles (which reflect many more times off the core/cladding boundaries as they travel the length of the fiber). The arrival of different modes of the light at different times is called Modal Dispersion.

Modal Dispersion is also called modal distortion, multimode dispersion, intermodal distortion, intermodal dispersion, and intermodal delay distortion.

Digital communications use light pulse to transmit signal down the length of the fiber, as we explain in the fiber optic network tutorial. Modal dispersion causes pulses to spread out as they travel along the fiber, the more modes the fiber transmits, the more pulses spread out. This significantly limits the bandwidth of step-index multimode fibers.

For example, a typical step-index multimode fiber with a 50 μm core would be limited to approximately 20 MHz for a one kilometer length, in other words, a bandwidth of 20 MHz·km.

September 5, 2011

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