When using multimode 62.5/125µm optical fibre for transmitting Gigabit data over distances greater than 300 metres, a Mode Conditioning patchcord must be used at each transmitter/receiver. This is because when a single mode laser is launched into the centre of a multimode fibre it may cause differential mode delay (DMD) effects that generate multiple signals.
Mode Conditioning patchcords overcome this problem by launching the laser light from the transmitter into a single mode fibre which is aligned with a precise offset from the centre of the core of a 62.5/125µm fibre in the "mode conditioning" part of the patchcord. The output from the patchcord is then compliant with the IEEE 802.3z standard for 1000Base-LX.
Mode Conditioning patchcords are not recommended for short transmission distances - a few tens of metres - as bit errors may occur/increase.