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Bending Effects in Optical Fiber and Solutions

Properties Of Optical Fiber:

The ITU-T recommendation G.65 series  optical fiber design which diameter is 125 micrometer. The center of fiber which carries the signal that is called the core. Fiber properties define it's optical performance.

Image result for image of properties of single mode optical fiber
Core and cladding have different refractive index: the exterior part has a smaller refractive index than the inner one, allowing light reflection according to the classical geometrical optics laws.refractive indexes are varied and accurately controlled during the fiber fabrication through dopant incorporation in the silica-based matrix. The most basic function of a fiber is to guide light, i. e., to keep light concentrated over longer propagation distances despite the natural tendency of light beams to diverge, and possibly even under conditions of strong bending.

There are two types of Bending in Optical Fiber:

1. Macro Bending
2. Micro Bending

1.Macro Bending:- If the radius of a bend is relatively large (say 10 cm or so) there will be almost no loss of light. However, if the bend radius is very tight (say 1 cm) then some light will be lost. As the radius of curvature decreases, the loss increases exponentially until at a certain critical radius of curvature loss becomes observable. If the bend radius is made a bit smaller once this threshold point has been reached, the losses suddenly become extremely large.When a fiber is bent, the field tail on the far side of the centre of curvature must move faster to keep up with the field in the core, for the lowest order fiber mode.

Image result for image of bends in single mode optical fiber

Typical bending radius of a fiber optic cable to be between 10 cm (4 inch) and 30 cm (11 inch), depending to a certain degree on the fiber count in the cable. Bending a fiber optic cable tighter than the specified bending radius can cause damage, even break the fiber carried in the cable. It can also cause a dramatic increase in fiber attenuation.

Solutions:  It is recommended that extra intensive care is taken in order to avoid any bending. At the same time, supplementary units like splice trays, or racks and fiber risers can be used to lower the risk of bend losses. Take the all precaution during the laying of optical fiber. Avoid the sharp bends. In manual pulling first make the "8" of the optical fiber cable.Avoid to apply the sudden tension on the Optical fiber cable.Always remember the limit of bending radius.

Image result for image of micro bends in single mode optical fiber

2. Micro Bending:- Micro bends are repetitive small scale fluctuations in the radius of curvature of the fiber axis.They are caused either by non uniformities in the manufacturing of the fiber or by non uniform lateral pressures created during the cabling of the fiber. Simple we can told that manufacturing defects in optical fiber cable or this type of defects mostly created by manufactures. Micro bending losses can be minimized by placing a compressible jacket over the fiber. When external forces are applied to this configuration, the jacket will be deformed but the fiber will tend to stay relatively straight.

This random bending is usually caused by external mechanical stresses against the cable material that compress the fiber. The result is random, high-frequency perturbations to the fiber.Lateral stresses can be caused by pressure induced by manufacturing or installation or by temperature-induced dimensional changes in cabling materials that cause undesirable fiber/fiber or fiber/cable material interactions. These interactions can give rise to random microscopic bends or curvatures of <1-mm radius that create very small displacements of the fiber core from the fiber axis.

Micro bending effects can be seen at all the commonly used wavelengths in single mode fibers (1310, 1550, and 1625 nm), whereas macro bending effects are seen predominantly at 1550 and 1625 nm.

Micro bending measurement

Microbending Sensitivity,” which describes four test methods to characterize the microbending of fibers.
The four test methods referenced are:
  1. Method A Expandable Drum
  2. Method B Fixed Diameter Drum
  3. Method C Flat Plate
  4. Method D Basketweave.

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  6. The normal recommendation for fiber optic cable is the minimum bend radius under tension during pulling is 20 times the diameter of the cable (d). When not under tension (after installation), the minimum recommended long term bend radius is 10 times the cable diameter.