When a transmitter injects light from a singlemode laser into the center of a multimode optical fiber, it may generate multiple signals due to a phenomenon called Differential Mode Delay (DMD).
The solution is to use a fiber optic Mode Conditioning Patch Cable. A Mode Conditioning Patch Cord is a duplex multimode fiber optic cable that has a small length of singlemode optical fiber at the start of the transmission leg.

This type of fiber optic patch cord launches the laser light into a short segment singlemode fiber that is aligned with a precise offset from the center of the core in the "mode conditioning" area of the optical fiber. As a result, the launch condition more closely resembles a standard LED launch, yet retains the speed advantages of using a laser.