Fiber cable shrinkage
Fiber optic cables are designed in such a way that excess length of the optical fiber compensates for cable expansion, which occurs as a result of bending stress, tension or thermal expansion. Depending on the cable structure, the degree of compensation can be 0.5 to 1.5%.
All materials expand and contract under the influence of temperature. The thermal expansion coefficient of the outer sheath of a fiber cable is 10 times higher than that of its core. During thermal cycling tests, the cable jacket will shorten to a far higher degree than the fiber inside at low temperatures.
However, the disadvantage of designed-in fiber overlength is the fact that it becomes more pronounced with increasing shrinkage of the outer cable. The additional fiber overlength is compensated by a spiral winding inside the cable jacket and this may result in micro bending. This effect can be verified by the increasing IL-value.
A closer look at reversible and irreversible cable shrinkage
We call cable shrinkage due to cooling ‘reversible’ because the original condition is restored when the cable is heated. The dimension of the temperature-related change in length is defined by the material-specific material expansion coefficient. The material expansion coefficients of plastics – the outer material – are about 10 times higher than those of glass – the cable core. When cooling from +20° C to -5° C, for example, the cable experiences a temperature-related shrinkage of approximately 0.5 %. As a rule, this is not noticeable, since the additional excess length of the fiber core will still fit into the cable.
Irreversible shrinkage, as the name implies, cannot be undone. Plastics are so-called amorphous materials in which the building blocks (molecules) do not follow an order. During cable extrusion, the hot plastic is pressed through a die. This produces high shear forces, which force the molecules to align in the longitudinal direction of the cable. Before the liquefied plastic can change back to its amorphous state, it is quenched in the extrusion line’s water bath. The orientation of the molecules in the longitudinal direction of the cable is frozen.
As long as the plastic is not heated above the so-called glass transition temperature, the structure remains ‘frozen’. At temperatures above the glass transition temperature, the restructuring into the amorphous state begins. This causes the cable sheath to shrink – irreversibly. For High-Density Polyethylene (HDPE) the glass transition temperature is approx. 100° C, for Fire Retardant Low Smoke and Low Halogen (FRLSZH) it is between 50° C and 60° C.
The shrinkage rate mainly depends on the material, jacket cross-section and extrusion parameters. Cable shrinkage is particularly observed in cables with loose cable constructions such as pigtails, (mini)breakout cables, simplex/duplex cables and Flextube cables. The irreversible shrinkage is between 1-5 %.
Temperature-induced reversible cable shrinkage and irreversible structural cable shrinkage add up. Excessive irreversible shrinkage leads to fiber stress (bending radius, crushing effect) and can no longer be compensated by the cable construction.
If attenuation in the low temperature range increases from cycle to cycle, cables will display a high degree of irreversible shrinkage.
Low shrinkage fiber optic cables
Low shrinkage cables display low irreversible shrinkage at increased temperatures. During a temperature cycling test, the attenuation deviations of low shrinkage cables are stable over the entire duration. A cable jacket with low-shrink characteristics maintains optical performance during temperature variations. Controlling and reducing cable shrinking directly improves optical cables’ mechanical and optical performance.
Download the Brochure ‘Shrinking Behaviour’ under Downloads: Cable knowledge to find out more, or get in touch with our experts!