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Natural Science Forum / Physics / Optics / July 2008High 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 > Hello, > [quoted text clipped - 14 lines] > > Dennis 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. > > Hello, > [quoted text clipped - 21 lines] > > - Zitierten Text anzeigen - It is a fiber with 50 µm core! > Hello, > [quoted text clipped - 14 lines] > > 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 > > Hello, > [quoted text clipped - 22 lines] > > Arnie 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 > > > Hello, > [quoted text clipped - 35 lines] > > - Tekst uit oorspronkelijk bericht weergeven - 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 > Hello, > [quoted text clipped - 3 lines] > fibers is less then 0.6 dB/km @ 1300nm and less then 2.4 dB/km @ 850 > nm. Are these the losses obtained from teh OTDR traces How long are the fibre > It's no problem with hydrogen. I've checked it. Just curious but how do you check this? > 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? 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. > Is it possible to measure it or to make it "visible". 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 > BTW, there are 4 fibers in a metallic tube with a diameter of 1.42 mm. > > Greeting, > > Dennis > > Hello, > [quoted text clipped - 5 lines] > > Are these the losses obtained from teh OTDR traces Yes! > How long are the fibre Between 400m and 700m. > > It's no problem with hydrogen. I've checked it. > > Just curious but how do you check this? 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. > > On my OTDR are no events displayed. The curve is normal. > > In my opinion, it looks for micro- or macrobending. Does anyone has [quoted text clipped - 5 lines] > with the computed length from OTDR measurements. OTDR measrurmetns will > of course only be as good as teh known effective index of the fibre. 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. > > Is it possible to measure it or to make it "visible". > > 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 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? > cheers > [quoted text clipped - 5 lines] > > > Dennis >>> Hello, >>> I have a problem with multimode fibers. The attenuation of the fibers [quoted text clipped - 9 lines] > > Between 400m and 700m. 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. >>> It's no problem with hydrogen. I've checked it. >> Just curious but how do you check this? [quoted text clipped - 3 lines] > wavelength. In my opinion, the were checking at 1385nm the amount of > the hydrogen peak and compared it to fibers with good attenuation. OK that was just a minor confusion, I'd consider this an OH peak. >>> On my OTDR are no events displayed. The curve is normal. >>> In my opinion, it looks for micro- or macrobending. Does anyone has [quoted text clipped - 13 lines] > when we're using > the wrong index, nonserious. 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 >>> Is it possible to measure it or to make it "visible". >> Do they measure the same loss with a laser - power meter test set? [quoted text clipped - 7 lines] > microbending > or not? What kind of results would I expect? 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 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  SignatureAlbert Badnitz P.O. Box 8 Doonside 4135 SOUTH AFRICA > Hello, > [quoted text clipped - 14 lines] > > Dennis |
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