Microbending

The attenuation of an optical fibre results from physical phenomena either occurring within the fibre or coming from the environment. This is the sum of light lost by scattering in the fibre, absorption by the fibre materials, leakage of light out of the core due to environmental factors (e.g. microbends). Scattering and absorption losses dominate in every fibre. 

Figure 20. Limitation of excess microbending loss of photon transmission through light guides, as a function of relaxation modules of the stabilizing polymer coating.

Microbending—Curvatures of the fiber which involve axial displacements of a few micrometers and spatial wavelengths of a few millimeters. Microbends cause loss of light and consequently increase the attenuation of the fiber. 

J.S. Stolken, A.G. Evans A microbend test method for measuring the plasticity length scale. Acta Mater. 46, 5109 (1998)... 

Figure 5 Relaxation of microbending losses and coating modulus for an optical fiber spooled under tension.

With respect to conventional fiber, the potential benefits of a hollow-core photonic crystal fiber are lower transmission losses (due to both reduced absorption and Raleigh scattering) and lower nonlinearities, since the core is air. In only a few years of research, interest in this area has led to remarkably low-loss fibers (13 dB/km at 1500 mn 100 m length)." With continued advances toward lower-loss photonic crystal fibers, efforts also have begun to address field-level issues, including microbend losses" " and fiber splicing." ... 

Coating Properties and Fiber Performance Fiber Strength Microbending Loss Material Considerations... 

Microbending Loss Relation to Coating Properties Microbending Loss Relation to Fiber Cabling... 

Microbending Loss. As a result of their small diameters, optical fibers bend very readily. This feature is advantageous in that the transmission medium is very flexible and easily routed. However, when the spatial period of the bending becomes small (approx. 1 mm or less), some of the light rays normally guided by the fiber are lost through radiation. Such small period distortions may occur when a fiber is wound on a spool under tension or when it is placed in cable structure. The phenomenon is termed microbending (11), and... 

Figure 13. Excess microbending losses measured in the experiment of Figure 12 for various single mode and multimode fibers as a function of the 30-min, 23 °C tensile relaxation modulus of the coating material. The relaxation modulus-time relationships of the coating materials are depicted in the inset. (Reproduced with permission from Ref. 6. Copyright 1980 IEEE.)...

Average microbending losses as a function of axial compressive strain on fibers in a ribbon for three different fiber coatings. (Reproduced with permission from Ref. 27. Copyright 1982 Society of Plastics Engineers.)... 

Du Font s thermoplastic polyester elastomer (TPE) Hytrel has been used in an underwater fiber optic cable produced by Shiplex Wire and Cable Corporation. The elastomer is used to position the fiber optics and is chosen because of its modulus properties that help prevent microbends in the fiber optics. The thermoplastic polyester elastomer is extruded over a central steel wire that imparts strength to the cable. Six fiber optic strands with a diameter of 5 mils each are then positioned and covered by a second Hytrel layer, which is added as part of a coextrusion with nylon to obtain greater abrasion resistance. Additional steel layers and a longitudinally formed copper tape are added, and the whole structure is then enclosed in a polyethylene jacket. 

Fignre 16.46 Measuring impact response in a composite with fiber optics through change of optical intensity due to (a) fiber microbending and (b) fiber fracture [103]. 

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