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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.'"
Similarly, the bends can prove catastrophic for nautical divers.
When I was a student worker I was "volunteered" to pull cable. We were running fiber from the Science Center to the Health Professions building. At the time I had no idea what fiber was even made of, I just knew to take it from point A to point B. It was a pretty fun day though, I still remember swinging on the fiber like it was vine. Good times.
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... the "new" thing being reported is the microbends fail by going opaque when higher optical powers are being transmitted in the fibers. For modern systems in most inter-city networks, the number of channels (40, 80, ...) is going up, as is the power per channel. This is a combination not seen earlier in installations where most fibers (bent or not) carried fairly low power signals.
Interestingly enough, microscopic dust particles are equally hazardous to the system's health at these high power levels. Dust particles caught in unclean connectors has been known to scatter enough power to fuse/weld (its those friggin laser beams) together the connector parts together.
yawn.
yes, 42.
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.
Don't bend your fiber!
Don't polish your helmet!
I'm sick of these technical articles making moral judgements on my hobbies.
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Optical fibres can be used as couplers between two lasers (the second laser amplifies the signal from the first). We use such a system in our lab, where average powers of 40-100W can be sent down a single fibre (multimode in our case). If the surface of the end of the fibre gets scratched, or if dust lands on it, the tip can explode. With each pulse (it's a 25kHz pulsed laser) another piece of fibre is destoyed, and it acts like a fuse. If you don't turn the laser off quickly you can soon lose kilometres of fibre. All that's left is a ringing in your ears and a few bits of scorched plastic.
I design and build fiber-coupled semiconductor lasers as a day job, and some of the stuff in our R&D lab has a significantly higher power than what is currently used in most systems out there. A fiber bend radius that leaks/absobs x% of the power at 10mW with no difficulty becomes dangerous when you put a 5W laser in the system - the amount of leaked power becomes enough to fry fiber claddings (especially if the fiber was metalized for soldering to a package) and make a crunchy black line where a perfectly good bit of cable had been moments before.
The take-home message of all of this is that as optical powers go up to increase bandwidth, some existing fiber installation methods may need to be re-thought. That said, I'd doubt that this will have much of an impact on many systems outside of long-haul lines since local systems don't need to have powers of this type to get the bits across town or around an office building.
In grad student (chemistry) days I ran an apparatus for stimulated, mass-selective Raman spectroscopy of molecular clusters. The Raman pump beam was two colors, generally tuned for power (a watt or two each, depending on tuning). The molecular clusters were formed in a vacuum chamber and we had a quartz window to let the laser light in. If there was a speck of dust on the window at the point the beam entered, the absorption was sufficient to start drilling a hole in the window. The noise was our cue to cut the laser beam before the window was breached (there were dedicated electronic circuits to protect the vacuum chamber's diffusion pumps, but we didn't want to take the risk of failure).
Of course, we spent a lot of time cleaning that window, and for that matter all the optics.
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