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Glowing optical fiber for identification

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bitwelder:
Some time ago I saw that during installation of an optical fiber that was passing by our side of the office building the serviceman must have put on the remote end of the fiber a testing/detection device that was making the whole fiber glow.

How is that accomplished? I thought that the regular optical fiber for networking is only capable to sending light axially.
Is is only about shooting light at the right angle into the fiber (outside the core?), or does also the installed fiber itself need to have some kind of side-glowing cladding capability?

coppice:
Inject light into the core of the fibre and you will see nothing, as the loss is extremely low. Inject the light into the outer layer of glass, and you will see some glow.

Marck:
If you have access to the cores you can connect a visual fault locator at one end that injects visible light.  Then all you need to do is look for the glow at the end of the fibre.  You can also put a bend in the fibre and visible light will then show up through the sides of the core. 

Fibre cores are not as fragile as people might have you believe in our training we tied a knot in the core which showed huge losses but once you released the knot results returned to normal.  The problem is if they are mistreated and you break a core its a massive pain to terminate or splice all the cores at that point to fix it. 

Keep in mind that these VFL’s have an output up around 20mw so looking directly at the ends of the cores is not recommended.

Its been 10 years since i did my training so things may have changed a bit since then.

M

TERRA Operative:

--- Quote from: Marck on December 12, 2019, 02:24:54 am ---Fibre cores are not as fragile as people might have you believe in our training we tied a knot in the core which showed huge losses but once you released the knot results returned to normal.

--- End quote ---

This is actually a workable way to make a poor man's attenuator.

Grab a pen or pencil and stick a light tester on the fiber then wrap it around until the light level drops to the correct level then tape it in place (usually done so you don't blow out the Rx sensor with a too powerful signal).

Also, you can get light testers that will look for the small amount of light leakage through the walls of the fiber so you can get a go/no-go confirmation if a fiber is dark or not.

Marck:
Throwing a multimode fibre in the single mode port is another trick you can use to get enough attenuation if you want to run gear up on the bench and don’t have 5 km of fibre laying around.

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