In the argot of fiber optics engineers, a "fiber fuse" occurs when a fiber, overloaded with optical power, fails catastrophically. Several mechanisms have been suggested to explain the phenomenon, but now a team from OFS Research Laboratories in Murray Hill, N.J., has shown that fiber fuse is caused by a classic Rayleigh instability resulting from capillary effects in the molten silica that surrounds the vaporized fiber core.In a fiber fuse, a brilliant, highly visible intrafiber burn propagates backward from the original damage site toward the optical source at speeds that can reach several meters per second. Measurement of the burn's color temperature indicates that the fiber core reaches temperatures as high as 5400 K, sufficient to vaporize the glass. The threshold power density to ignite the fuse is in the range of several megawatts per square centimeter -- although in the absence of a fiber imperfection or other perturbation, fibers can carry much higher power densities without catastrophic failure.The OFS scientists analyzed the bubbles and voids left behind when a fiber experienced the effect and concluded that capillary instabilities in the molten glass are responsible. They noted that the physics of an electrical fuse, in which a gaseous layer of air surrounds a cylinder of molten metal, and the fiber fuse, in which a liquid cylinder of molten silica surrounds a gaseous void, are similar.Thus the term "fiber fuse," which was coined colloquially, is apt from a technical perspective.