Mechanical Failure of Bent Optical Fiber Subjected to High Power

As optical fiber penetrates further into the communications infrastructure and comes closer to the home or business, higher optical power levels are expected. Several studies have shown that sharply bent optical fiber will fail prematurely when exposed to high optical power levels. In an extreme case, where the fiber is bent to a maximum bend stress on the order of 2 GPa and subjected to a power level of 1–2 W in the near-infrared wavelength window, optical fiber will fail in minutes. Time to failure decreases with increasing bend stress and optical power. A recent report suggests that power levels in the range of a few hundred milliwatts may be enough to induce delayed failure in bent fiber. This study explores the progression of events leading to failure. Light that escapes the core of bent fiber passes into the coating, where a small amount is absorbed and converted to heat. The coating heats to a stable temperature and visually darkens with time. This is followed by an abrupt rise in temperature, which occurs as the coating transforms to a highly absorptive material, consistent with thermal runaway. The abrupt rise in coating temperature stimulates viscoelastic deformation of the glass. Glass deformation is explained in terms of the ability of highly quenched glass to experience viscous flow at temperatures well below the glass transition range (i.e. sub-Tg aging or relaxation). As the glass portion of the fiber moves toward a “kinked” configuration, it concentrates more power on a smaller region of coating, resulting in further temperature increase. There is no evidence of the fiber fuse effect in the lower viscosity glass core. The final kinked configuration of the glass fiber leads to complete attenuation of the light and failure is complete. Coating decomposition is self-limiting with no visible flame. A coating with a refractive index near or below that of silica was found to virtually eliminate this failure mode.