Slashdot Mirror
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.'"
I don't know how the average fibre installer works today, but I know the few times I played with it, we always installed with corners were gentle enough that a full loop would be about 30 cm. This included ensuring no significant load on the fibre at the attachment points, so no 90 degree bends at the switch or server.
I'm only talking about the last few feet, not the 'last mile' of course, but if I upped the power and had a fibre failure, I'd be saying very rude things to the rep of the company that did the installation (if they survived the .bomb, of course).
... 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.
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.
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.
The clearance system sounds logical. It is not. It is completely arbitrary. -- John Bolton
Nah. The maximum radius doesn't depend whatsoever on the intensity of light passing through the fiber. If the radius is too tight, light will leak out, regardless of how weak. Light is already leaking out of these over-bent fibers. The problem is when the power gets too high, and the amount of leaked light becomes so great that it actually starts heating up the cladding.
BTW, the maximum curvature radius you can use depends on both the material the fiber itself is made of, and the material the cladding is made of. You want the two materials to have dissimilar indices of refraction -- the more dissimilar, the tighter you can bend the fiber without light leaking out of it. To some degree it also depends on the frequency of light you are using. But it does not depend on the intensity of the light.