Talking of fibre

PeterC · 2020-10-14T15:16:03+00:00

Small estate on edge of a village, van and 3 chaps stringing a cable alongthe telegraph poles. Cable was about 6mm dia., black with a yellow stripealong it.Any idea what it is? I'm assuming frome the diameter and low flxibility(there were a couple of loose coils taking up a lot of the footway and thebit being handled wasn't floppy like flex) that it's possibly FTTP. The vandidn't have any markings on it.

Dave Liquorice 5 years ago

Good point, well made. Of course having the split further away means you have combined all 32 (or WHY) customer fibres into one.

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.

There's about half a dozen standard wavelengths used across multimode and single mode fibre. Around 1550 nm is one of wavelengths used by singlemode. IR starts just below visible red at 700 ish nm and goes to about 1 mm.

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Vir Campestris 5 years ago

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

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Tim Streater 5 years ago

I think it's also thinner.

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John Rumm 5 years ago

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.

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jrwalliker 5 years ago

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

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Andy Burns 5 years ago

When I hired a fluke fibre tester, it came with a small "mandrel" about that diameter which you wrapped a few turns of the fibre around for testing, I think it was to attenuate the light that had taken a highly reflected path through the fibre and only measure the more direct paths?

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