High attenuation with multimode fibers

Hello,

I have a problem with multimode fibers. The attenuation of the fibers is about 1.5 db/km @ 1300nm and about 3.2 db/km @ 850 nm. I haven't had such a problem before. Normally, the attenuation of the fibers is less then 0.6 dB/km @ 1300nm and less then 2.4 dB/km @ 850 nm. It's no problem with hydrogen. I've checked it. On my OTDR are no events displayed. The curve is normal. In my opinion, it looks for micro- or macrobending. Does anyone has experience with this topic? Is it possible to measure it or to make it "visible".

BTW, there are 4 fibers in a metallic tube with a diameter of 1.42 mm.

Greeting,

Dennis

Reply to
Dennis
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What is the diameter of the fiber core? Could it be that you're measuring a fiber with a larger core diameter than you typically encounter? A larger fiber core region (62.5 microns rather than 50 microns, for example) would display higher attenuation characteristics.

Reply to
Walter Gryder

itierten Text ausblenden -

It is a fiber with 50 =B5m core!

Reply to
Dennis

The excess length of the fibres in the metallic tube could be too high. This would result in microbending problems. To check this you can cur a piece of tube of 5 to 10 meters, measure it exactly and then measure the exact length of the fibre in the tube. Normally the fibres should be about 0.1 to 0.4 % longer than the tube.

Arnie

Reply to
Arnie Berkers

Are these the losses obtained from teh OTDR traces

How long are the fibre

Just curious but how do you check this?

It would be unusual to have consistent micro/marco bending along the whole length of a long fibre however the fibre could be spiralled around each other. How does the physical length of the cable compare with the computed length from OTDR measurements. OTDR measrurmetns will of course only be as good as teh known effective index of the fibre.

Do they measure the same loss with a laser - power meter test set? If you use a single mode laser and under fill the fibre then the loss would be less sensitive to microbending

cheers

David

Reply to
DM

The supplier send us test certificates of every single tube, where the excess length is specified between 0.09% to 0.15%. I have 2 FIMT laying in the same way with nearly the same excess length. But you're right, we have had a problem with the excess length before and we have to check the excess length again.

Dennis

Reply to
Dennis

Yes!

Between 400m and 700m.

A company met us with a new measuring system. They were able to check, if the hydrogen peak becomes greater and widen at a specific wavelength. In my opinion, the were checking at 1385nm the amount of the hydrogen peak and compared it to fibers with good attenuation.

There are two tubes laying the same way with the same length. Both tubes are having the nearly the same excess length. They were delivered by the same supplier. I guess you're right with the index. But in my opinion is the failure, when we're using the wrong index, nonserious.

I was not able to measure th loss with a power meter. Do you mean, that when I'm measuring the MM-fibers with a single mode laser at 1330 nm and 1550 nm, I would get a general statement if it's microbending or not? What kind of results would I expect?

Reply to
Dennis

Did you process or treat these tubes in a certain way? The excess length is measured locally and is normally a good indication for the complete length, except when something went wrong. Did you contact your supplier about this? Is it the big one D* or the small one B*? Arnie

Reply to
Arnie Berkers

Fairly short lengths - is teh attentuation an automatic or manual measurement I take it the OTDR is using a short pulse length and the measurment isn't geting corrupted by near end dead zone, ... or tail end of a Fresnel reflection from connector uniting.

OK that was just a minor confusion, I'd consider this an OH peak.

You supplier will quote the measureed cable length, not the measured fibre length- at least that's what I spec., and get when I order cable, although generally this is single mode fibre.

Do you have a good similar cabled fibre? If so test it, get a decent figure for effetive index assuming fibre L = cable L, Now use this value of eff. index with your poor cable. is the fibre length significantly differnt from cabled length

if it is micro bending I'd expect higher loss when the modes are all excited- higher order modes will see higher loss.

If you use a single mode laser and only excite low order modes then they should not suffer the same micro bending losses.

I'd expect fairly large differences for microbending with single mode fibres at 1310 and 1550nm, but not necessarily with multi mode fibres. However as above if it;s micro bending then I'd expect differnces when launched only low order modes, and exciting all modes. e.g. measuring loss with a laser and with an LED.

Another point is that most cable manufacturers make on line measurements of cabled fibre losses with there own OTDR, do they have anytthing to say on this front?

cheers

David

Reply to
DM

Hi Dennis,

We have had "problems" with fibres in metallic tubes before, but I suggest you find out what specification your supplier use, because those attenuation figures are within spec for "regular" fibre as opposed to "premium" fibre

Reply to
Badnitz

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