In multi mode fibres the light can take many different routes through the fibre. The lengths of these paths will not be the same, so all the light Will not arrive at the detector at the same time, resulting in pulse broadening. the longer the fibre is, the greater difference there will be in the lengths of the light paths, so the more the pulse will spread. As a result, there is a trade off between length of fibre and the data ranges that can be transmitted- the longer the fibre, the lower the data rates have to be.

Graded index fibres overcome this problem by having a refractive index which changes gradually at the boundary between the core and the cladding.

Multi mode graded fibres have higher bandwidth and can transmit over longer distances than multi mode step-index fibres. Multi mode step-index fibres are rarely used in modern telecommunications.

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