The variation of group velocity creates the mode partition noise.
2.
Over a long length of fiber, the cumulative effect is to create jitter, " i . e . " mode partition noise.
3.
This behaviour, present mainly in high power SLEDs, is similar to what is observed with multimode Fabry-Perot laser diodes where filtering makes evident the presence of mode partition noise ( mostly at low RF frequencies ) due to competition among several lasing modes.