Within the multimode category, another important characteristic is between step index and graded index. Further definition of fibers relates to physical size, optical performance, coatings and strength. I n the case of a multimode fiber, the core diameter is relatively large compared to a wavelength of light. This means that light can propagate through the fiber in many different ray paths, or modes, hence the name multimode.
T wo basic types of multimode fibers exist.
This leads to rays of light being propagated as shown below. W ith all these different ray paths or modes of propagation, different rays travel different distances, and take different amounts of time to transit the length of a fiber. This being the case, if a short pulse of light is injected into a fiber, the various rays emanating from that pulse will arrive at the other end of the fiber at different times, and the output pulse will be of longer duration than the input pulse. This, therefore, limits the bit rate or bandwidth of a multimode fiber. Multimode-graded index fibers therefore have the ability to transport pulses closer together without spreading into each other than do the step index fibers.