Fiber fire

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The fiber Brand (engl .: fiber fuse ) is the destruction within an optical fiber at high temperatures, which by high local energy densities are caused. The destruction spreads along the optical waveguide at a speed of approx. 0.3-3 m / s in the direction of the laser source.

Basics

With the introduction of wavelength division multiplexing (WDM) with the help of powerful EDFAs , stronger (external) pump lasers and the use of Raman amplifiers , the optical performance has increased significantly at some points in optical communication networks in recent years. The increased optical performance leads to the following effects:

  • Non-linear fiber effects (self-phase modulation, cross-phase modulation, stimulated Brillouin scattering or the four-wave mixing ), which generally lead to a reduction in the transmission quality, but are fully reversible when the power is reduced. The non-linear effects do not lead to permanent damage to the glass fiber.
  • Irreversible destruction of the optical waveguide or the optical fiber connector due to excessive optical (total) power, the so-called fiber fire.

Occurring problems

The following irreversible damage is possible:

  1. Possible damage to fiber connection surfaces if they are dirty
  2. Damage to the fiber core over a long distance due to spontaneous combustion, the so-called " fiber fuse effect "
  3. Damage to the fiber core due to bending or excessive heating: This can lead to increased power absorption and, in the worst case, a fiber fire, also known as a " fiber fuse ", can begin.
  4. Dangers to the safety of the human body through possible damage to the optical fiber network (escaping light power)

The irreversible destruction of the fiber core by the so-called " fiber fuse effect " got its name from the analogy with a fuse, the burning end of which moves towards the beam source (usually a laser).

This phenomenon was discovered for the first time in 1987 by Kashyap et al., Who at that time determined an extremely rapid increase in the absorption coefficient with relatively low optical powers of approx. 1 watt above a critical temperature threshold Tkrit of 1323 K for glass fibers. The absorption coefficient is below the temperature limit Tcrit at approx. 0.01 dB / km and increases at a temperature of Tcrit + 50 K to values ​​of approx. 1900 dB / km. This has been demonstrated for a wavelength of 1064 nm. The power densities occurring in this case were approx. 1-3 MW / cm 2 , which is far below the destruction threshold for silicon oxide of approx. 10 GW / cm 2 .

Explanations for the occurrence of the phenomenon

Over time, three scientifically founded theories have been put forward, which could result in the rapid increase in the absorption coefficient. These are the following assumptions:

  1. The formation of point (Frenkel) defects at the germanium centers of a doped fiber: However, according to current knowledge, the germanium atoms are not solely responsible for the occurrence of the fiber fuse effect, as this also occurs in fibers that are not with germanium but with other dopants are added.
  2. An electronic conductivity of the fiber due to thermal ionization of the germanium-doped core: The course of the absorption coefficient at temperatures below 1873 K could be explained well by the Arrhenius equation , but not the behavior of the glass fiber at temperatures around 2293 K, because from this temperature a strongly different one Behavior is present. This assumption only applies to fibers that are mixed with germanium. The Arrhenius equation describes the chemical reaction rate as a function of temperature.
  3. A thermochemical SiO production in silicon glass: When evaporated silicon oxide is deposited on the fiber again and cools, it can take on various colors from brown to black. Strong absorption takes place on these. This agrees very well with experimental observations on the fiber fuse effect.

The fiber fuse effect is initiated in the fiber core and affects the cladding of the fiber during the effect. The fiber core is melted by high temperatures, turns into a plasma-like state when it evaporates, leaving a cavity behind. This process is repeated many times in the direction of the laser source and each time it emits a white-blue light that moves at a speed of approx. 0.3-3 m / s, depending on the type of fiber, optical power and wavelength.

The resulting bubbles (voids) are believed to be the result of a Rayleigh-Taylor instability caused by hairline cracks in the molten silicon surrounding the core. This instability can occur when two different densities collide at an interface. If the connecting surface is ideally smooth, there is an unstable state of equilibrium when the layers change into the liquid or gaseous state of aggregation . If a disturbance occurs at this boundary layer, it grows exponentially, the state of equilibrium is no longer present and the media mix. This is z. B. triggered by thermal effects.

Furthermore, it was determined by Raman spectroscopy that oxygen is released when bubbles are formed during the fiber fuse process.

Backscatter measurements have shown that the light source produced here has a temperature of approx. 5400 K and is comparable to a black body with a similar temperature due to the radiation characteristics.

The fiber fuse effect is according to Dianov et al. an optical discharge, i.e. an extremely dense plasma. In the effect, the plasma-like fiber core is destroyed by high temperatures and high pressure (10000 K and 10000 atm) due to the formation of bubbles.

For the process to begin, an external trigger is required as the triggering moment. This can be initiated by the following effects:

  1. sharp kink in the fiber
  2. Contact of the fiber end face with absorbent materials
  3. Dirt and dust in fiber connectors - couplers
  4. electrical discharge process over the fiber (breakthrough in field strength)

Depending on the wavelength and dopants, each fiber has a power threshold above which the fiber fuse effect can occur.

The corresponding mode field diameter also influences the occurrence of the fiber fuse phenomenon. The smaller this is, the greater the light intensities there are in the core. Consequently, lower pump powers are required to initiate the effect.

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  1. ↑ K. Seo, N. Nishimura, M. Shiino: Evaluation of High-power Endurance in Optical Fiber Links , Furukawa Review, No. 24, pp. 17-22, 2003
  2. ↑ a b c R. Kashyap, KJ Blow: Observation of catastrophic self-propelled self-focussing in optical fibers , Electron Letters, Vol. 24 No. 1, pp. 47-49, 1988
  3. ↑ Y. Shuto, S. Yanagi, S. Asakawa: Simulation of Fiber Fuse Phenomenon in Single-Mode Optical Fibers , Journal of Lightwave Technology, Vol. 21, No. 11, pp. 2511-2517, 2003
  4. ↑ Y. Shuto, S. Yanagi, S. Asakawa, M. Kobayashi, R. Nagase: Evaluation of High-Temperature Absorption Coefficients of Optical Fibers , IEEE Photonics Technology Letters, Vol. 16, No. 4, pp. 1008-1010, 2004
  5. ↑ Steffen Brinkmann: MHD instabilities in accretion disks , Ruprecht-Karls-Universität, Heidelberg, Faculty of Physics and Astronomy, diploma thesis, March 2004
  6. ↑ a b D. P. Hand, P. St. Russell: Solitary thermal shock waves and optical damage in optical fibers: the fiber fuse , Optics Letters, Vol. 13, No. 9, pp. 767-769, 1988
  7. ↑ EM Dianov, IA Bufetov, AA Frolov: Destruction of silica fiber cladding by the fuse effect , Optics Letters, Vol. 29, no. 16, pp. 1852-1854, 2004