The move to 1310 nm was prompted by the lower loss at the longer wavelengths. Thus systems could go further betweem repeaters with 1310 sources. But since the fiber supported longer lengths at 1300 nm, the bandwidth of the fiber was maximized at that wavelength. GbE brought new interest in this fiber, since it required lasers to reach the modulation frequencies required. But it brought up new interest and concern over running lasers in multimode fiber, a known problem from the early days.
Lets try to explain the problem.
The concept for multimode fiber is based on the fact that light travels at different speeds in different materials. By making the core of multimode fiber of many layers of materials with a lower index of refraction as you go away from the center of the core, we can manipulate the way the modes, or rays of light, travel in the core of the fiber. If the variation in n is properly chosen, the light on the outer part of the core will travel as fast as the light in the center of the core, reducing modal dispersion. As with most things in the real world, the actual fiber does not exactly compensate for the different path lengths in the core, but it does a pretty darned good job. But the bandwidth of multimode fiber has been tested for the last 15 years under conditions that duplicate the use with LEDs, not lasers.