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.'"

Comments

The cause of the failure

[groove10]

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.

Re:The cause of the failure

[pclminion]

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.

The problem actually is, when you bend a fiber beyond a certain point, the pulses end up striking the outside wall at an angle steeper than the critical angle. Total internal reflection no longer occurs, and some of the pulse energy escapes the fiber and heats up the coating. The problem isn't that the coating needs to be tougher -- the problem is, the fiber shouldn't be bent that much.

Now, it seems counterintuitive, but the narrower a fiber is, the more sharply you can bend it without a loss of TIR. This is because a narrower fiber causes the pulse to reflect more rapidly as it goes around the corner, so the total bending angle is distributed over more reflections. This keeps the light in the fiber.

I see four ways to solve this: 1) replace the fibers with narrower fibers, 2) replace the cladding with cladding that can take the heat dissipation, 3) use a lower transmission power, 4) have someone go out and assess each place where the fiber bends, and make it bend at a shallower angle if necessary.

Option 3 is pretty much impossible, since you need higher power to get a higher data rate (this is, after all, why the powers keep increasing). I think option 4 is pretty much the best shot.

Looks like some people forgot basic optics when they were laying the fiber...

Re:For everyone too lazy to read....

[Nintendork]

No doubt. You don't even want to bend solid CAT5 cable too much when you use it to wire a building. Typical rule is a 1" radius. Optical cables are much more sensitive to bending with a 2" radius limit.

-Lucas

Re:Huh?

[680x0]

You cant bend fibres, or light will just come shooting out.

Actually, you can bend fibers to a certain minimum radius. The light is reflected off the inside wall of the fiber. Long-haul connections use something called "single-mode" fibers, which I believe is made from glass fibers. Shorter connections use a plastic fiber, called "multi-mode", which can bend more.

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.

Re:In related news...

[Anonymous Coward]

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

Fiber and connectors

[RevMike]

I helped do a job installing fiber in a Manhattan office tower almost fifteen years ago. It was being used to interconnect the datacom closets on each floor with a central datacom room. I haven't had reason to use it since.

Is it still has tedious to put the connectors on the ends?

When I was doing it, IIRC, the process ran something like this:

1. Strip the sheath form the fiber.
2. Epoxy the fiber into a connector, with plenty sticking out from the "business end".
3. Use a special knife to score the fiber flush with the connector.
4. Break off the excess fiber.
5. Attach the connector to a flat round disk which would hold it perfectly perpendicular to a flat surface.
6. Using increasingly fine grits of "sand paper", polish the end if the fiber perfectly smooth and flush to the connector by rubbing it (and the disk) in a figure-8 pattern.
7. Inspect the termination with a microscope.

Multimode vs. single-mode

[Andy Dodd]

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.