On 17/10/2020 16:13, Dave Liquorice wrote:
Yes, the light travels straight down the core of fibre rather than
being reflected of the sides which causes different path lengths for
individual photons and thus what starts as tight pulse of photons
ends up spread out over time at a given point some distance away.

If I didn't know I'd be confused by that...

Fibres are made of two different types of glass. On multimode the
boundary between them acts as a mirror; on single mode they are blended
so the boundary acts as a lens and gently steers the beam even when
there is a bend in the cable.

Or something like that. I'm no physicist.

Andy