How to Save Fiber from 'The Fuse'

Breck Hitz

Fiber fuse is a catastrophic effect that occurs when an imperfection in the fiber precipitates heating and the local temperature approaches 1000 °C. At that elevated temperature, laser radiation propagating through the fiber is strongly absorbed, and the temperature escalates rapidly to ~10⁴ K. A plasma forms, visible to the eye as a bright white spot, and propagates backward at a velocity of several meters per second toward the source of the laser radiation.

Originally considered a curiosity, fiber fuse has become a concern as optical telecom systems approach the intrafiber power levels (~1 W) that can precipitate it. Once the effect has been initiated, it can propagate through -- and destroy -- kilometers of fiber. Recently, scientists at the Fiber Optics Research Center of the Russian Academy of Sciences in Moscow proposed and demonstrated a technique to stop fuse from destroying long lengths of fiber.

The scientists estimated that the pressure inside the fiber core reaches ~10⁴ atmos in the plasma that propagates up the fiber. If the cladding were weakened, they reasoned, the plasma would burst through the cladding, expand rapidly, cool and collapse. Fiber on the other side of the weakened cladding would not be damaged.

Figure 1. The etched waist in the fiber's cladding cannot contain the high-pressure plasma that accompanies the fiber fuse effect, so the plasma bursts through the cladding and collapses. Fiber on the other side of the waist is saved.

To test their theory, they etched the cladding down to a waist over a 2-mm length of fiber (Figure 1), launched several watts of laser radiation into one end of the fiber and placed a light-absorbing material against the other end. The heat generated when the laser light hit the material transferred to the fiber and was sufficient to initiate the fuse effect. They observed that, if the etched waist were less than a critical diameter (d_c in Figure 1), the fuse always stopped when it hit the waist (Figure 2).

Figure 2. (a) The etched waist is clearly visible in this photo of the undamaged fiber. (b and c) A large bubble forms in the fiber where the plasma burst through the weakened cladding. Unlike Figure 1, in this case the laser light propagated from right to left and the fuse propagated from left to right. In (a) and (b), the graduations at the bottom of (b) are 0.1 mm; in (c), the photograph shows 1 mm of fiber.

Thus, the researchers concluded that detrimental effects of fiber fuse in practical telecom systems may be limited by placing waists along the length of deployed fiber. Damage would be limited to the length of fiber between the waists, and that fiber could be readily replaced. They noted that the waists in the cladding would induce no additional fiber loss.