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Bent Fibers Put Networks At Risk
opticsorg writes "The combination of moderate optical powers and tight bends can prove catastrophic for optical fibers, according to research carried out by BT Exact in the UK. Although the effect is unlikely to cause problems in current networks, it means that designers may need to think carefully before scaling up the power in their systems or deploying Raman amplifiers with pump powers of several hundred milliwatts or more. In the July 10th issue of Electronics Letters, Ed Sikora and his colleagues report that powers as low as 500 mW can induce permanent damage in singlemode fiber that is bent (13 mm bend diameter or less). 'These bends could be found in exchange racks or splice trays, for example, especially if a fiber is tugged or pulled,' Sikora told Optics.org. The BT researchers carried out tests on four types of fiber subjected to a range of bend diameters (5 to 15 mm) and optical powers of up to a few watts. In all cases the fibers fail within 53 hours. 'What was unexpected was that the catastrophic failure can occur in 90 bends at fairly low powers of less than 1 W or so,' said Sikora. 'It's important to understand that we're not saying that networks are going to fall over tomorrow, but as powers go up you have to aware this effect could occur under certain circumstances.'"
Being a network engineer I deal with fibers in a 19" rack. You simply have to bend the fibers in order to keep a clean tidy rack which does not look like a spaghetti. But as long as it's just a simple patchcable which is broken and not a fiber burried somewhere deep, It's just a simple case of shit happens. Just make sure you have your cabletester nearby :)
I'm not a complete idiot... Some parts are missing.
According to the article the cause of failure is an increase in temperature in the fibers when bent.
"the damage is caused by an increase in temperature that occurs when the power leaks out of the fiber at a bend and is absorbed by its coating. This either causes the fiber coating to burn off leaving the silica beneath exposed or if the temperature is high enough (around 1100C) the fiber itself deforms giving rise to a large permanent optical loss."
It would seem that research needs to be done in the optical fiber coatings and their heat transfer properties as the fibers can handle the increased temperature, but the coatings can't. Either that or we are seeing the limits of fiber systems and the amount of load they can carry. Anyone know what the current coatings are made of, or any alternatives to these coatings that would alleviate these problems?
Perhaps this is a good stock tip... When you hear of a company that has created a new fiber optic coating that increases the amount of heat trasnferred away from fibers, jump on their stock.
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Folks, a 5mm bend is darned tight. 5mm is almost 1/5th of an inch. Even a 15mm bend is pretty tight - just over half an inch.
I'd take this "study" with a large block of saly, personally. I never bent myheliax abtebba cable this tight, and I doubt that any sane technician would try to bend glass optical cable this tight, either.
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And don't pull hard on fibre cables, that tends to pull the heads away from the rubber coating, making the cables even more exposed to damage. Or to cause a kink that violates the bending contraints.
While this isn't a life or death situation, even in a production environment ( which should have redundant paths and whatnot built in ), it's probably a big pain in the ass for long runs.
Having said that, how is this anything new?
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At the car parts store is ugly tubing in a closed "C" profile called wire loom. It is somewhat inflexible. It works. But that gets too much important stuff accomplished without enough consultancy firms and PHB's employed. Sheesh. The next thing you know people will be selling bottled water and canned air! (Shaking head.)
But, I guess what the article is saying is that the minimum radius (i.e. how "sharp" the bend is) is larger for higher power signals, and as carriers increase the power (for more bandwidth) they may discover some of the existing bends in their fiber infrastructure suddenly become too sharp.
To understand the radius/diameter of a bend, imagine the fiber following the outside of a circle with the given radius or diameter. If you need a 90-degree turn, you follow around 1/4 of the circumference of the circle.
I agree this is nothing new and should be common sense but it isn't. Bending a wire of any kind will subject one side to compression and the other side to tension, and what kind of failure occurs depends on the material properties involved (i.e. some materials fair better under compression, and others better under tension).
A fairly small percentage of the power is absorbed but as it is absorbed it changes the structure of the coating causing some more absorption until there is a run away effect," said Sikora. "Depending on the input power the temperature can easily go up to 1000C or more."
Thermal run away can have catastrophic consequences. Take copper wiring in a aircraft for example. Place plastic coated copper wire with a excessively small bend radius and over time the plastic will start to crack on the outside of the bend.
If the wire is located in a non pressurized area of the plane, the wire can be subject to extreme levels of condensation. This condensation will come in contact with the exposed wire creating a carbon residue on the outside of the wire. Over time this residue builds and as electricity is run through the residue it is heated, melting more of the plastic cover and exposing more wire. If this occurs on/inside a wire bundle which can contain dozens and dozens of wires you can get anything from a system short(s) to the entire bundle starting on fire.
AC
Don't bend your fiber. I could of swore not bending optial equipment was a given.....
Obviously you have to allow for some amount of bending, or it would be a useless technology. The issue here is that radii originally thought to be "safe", might not be if you pump a strong enough signal through. So the current standards are fine as long as you keep your power at a certain level. If you do need to increase signal strength, then things may start failing.
Is it still has tedious to put the connectors on the ends?
When I was doing it, IIRC, the process ran something like this:
It's perfectly possible for multimode fiber to be glass and single-mode to be plastic. The difference is the diameter of the waveguide itself. Single-mode fibers (At least the waveguide portion, the total fiber is usually similar in thickness for structural reasons) are much thinner than multi-mode fibers, only allowing one waveguide "mode" to exist. (Hence single-mode). Each mode in a waveguide travels at slightly different velocities (Actually, in reality the light travels in the same speed, but certain modes travel longer distances due to the way they bounce within the waveguide), so multimode fiber suffers from pulse spreading since not all of the light travels the same distance.
Glass vs. plastic - Glass is always more transparent. As a result, singlemode fibers ARE usually made from glass since there's not much point in reducing pulse spreading if your attenuation is not reduced.
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I'm having a hard time saying this is surprising.
I agree. It may not be so intuitive for copper, but c'mon, fiber is glass. Of course if you bend it too much there will be problems. When I used to install fiber cabling, we always used a larger bend radius than the standards required -- it just made sense. Not to mention the fact that if you include a service loop in the walls, every time you pull some more of the extra fiber out of the wall, you decrease the bend radius of the service loop.
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This isn't as ridiculous as some make out. It may not be physically possible to install anything into a DWDM system, but it's certainly possible to control powers.
The system I worked on had 160 individual channels essentially multiplexed down to one. Their individual power was controlled using a OOB signal which spoke to the nodes on the system.
At each intermediate node, the EDFAs and raman pumps were also controlled using the OOB signalling. Algorithms were used to keep the system working as best as possible.
However, it wouldn't be outside of a hacker's capabilities to take control of the system, and say, pump of the power of the raman pumps (being easily the most powerful individual laser in the system). Also, increasing the powers of the individual channels (bearing in mind there are 160 here), and the EDFA pumps... well, you could cause some damage.
Of course, inside all the hardware, the fibre is carefully carried on plastic trays to keep well defined radii of turns, and at install, all the external fibres have connectors and trays to stop tight bends, but when some unskilled maintains the system, it could happen...
And I'm not kidding. Using dBm = 10 log10[ P / 1 mW], you get 27 dBm.
Most lasers in the telecommunications world run between -10 dBm and 5 dBm. Over a good fiber link, you can reach over 100km with a couple dBm.
EDFAs and Raman amplifiers may be up in the 20 or 30 dBm range, but they are not widely used, nor will they ever be. You only need that much power for very long runs - like between remote cities in the mid-West US.
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13mm is tremendously below the minimum bend radius spec. As per the Data Communications Cabling FAQ:
"According to EIA SP-2840A (a draft version of EIA-568-x) the minimum bend radius for UTP is 4 x cable outside diameter, about one inch. For multipair cables the minimum bending radius is 10 x outside diameter.
SP-2840A gives minimum bend radii for Type 1A Shielded Twisted Pair (100 Mb/s STP) of 7.5 cm (3-in) for non-plenum cable, 15 cm (6-in) for the stiffer plenum-rated kind.
For fiber optic cables not in tension, the minimum bend radius is 10 x diameter; cables loaded in tension may not be bent at less than 20 x diameter. SP-2840A states that no f/o cable will be bent on a radius less than 3.0 cm (1.18-in).
...
Some manufacturers recommendations differ from the above, so it is worth checking the spec sheet for the cable you plan to use."
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The heat may be the problem being presented here, but the issue could be completely avoided if people spec out their cable installations properly to begin with, and use a little common sense. I don't care how much heat is generated, at 13mm, there's a good chance that the cable will shatter eventually anyway.
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Having worked at JDS Uniphase (world's biggest fiber component manufacturer) for several years, the minimum bend radius is not a suggestion, it is a requirement, especially when you crank the laser power. READ THE LABELS AND FOLLOW THEM!!!
:)
But, when you try make something idiot-proof, the world will make a better idiot.
Long-term, I think the only solution is change the plastic cladding, so that it can't be bent beyond the minimum radius of the fiber.
(and no, I don't work at JDS anymore. But it was fun and I made a pile on the stock
I spent many years working in fiber optics. Bending-induced failure modes have been well known for decades.
To spell it out for those that have no optics knowledge. Brewster's angle (which depends on the relative index of refraction of the two materials involved) is the minimun amgle at which all of the light will be reflected. If the light hits the coating at an angle smaller than this (from the perpendicular), it'll go through into the plastic coating. This means not only signal loss, but heating. Heat and stress tend to change the indices of refraction, thereby changing the Brewster's angle...which in turn can cause more signal loss, and so on. THAT is why bends in optical cables is so bad.
With non-optical cables it's not as critical, but a minimum bend radius will prevent breaking/wear/impedence, a totally different problem with the same end result.