On Monday, 19 October 2020 02:06:49 UTC+1, John Rumm wrote:
On 18/10/2020 21:37, Vir Campestris wrote:
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.

You are describing what are known as "graded index" and "step index"
fibres. (Monomode and multimode can be made using both types, although
the graded index cause less modal dispersion).

Monomode tend to use a much thinner core (and are usually step index as
there are not many advantages to using graded index for monomode - but
its more difficult to manufacture), so that they can basically only
accept one incident angle of light entry.

If you bend a fibre some of the light leaks out. A couple of weeks ago I
watched a fibre leased line being installed. The installers had a very
nice tone tracer for fibre. The fibre (encased in its coloured protective
plastic sheath) was pushed into a curved slot which bent it, causing
enough light to leak out that they could monitor traffic and play an
audible tone if a pulsed laser was attached to one end of the fibre.
The bend radius looked like 2 or 3cm.

John