Generic selectors
Exact matches only
Search in title
Search in content
Search in posts
Search in pages

Fiber cable shrinkage

Causes, Effects and a Solution


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!

5GFiber OpticsLocal Area NetworksSmart Cities

Contact Us

Subscribe to our Newsletter!

Stay up-to-date with the latest information from R&M.

Subscribe now

Leave a comment

Your email address will not be published. Required fields are marked *

This might also interest you

How 5G is shaking up the network infrastructure market

The unprecedented fast growing need for bandwidth and low latency driven by 5G brings service providers great opportunities to create value.



New perspectives and the need for an end-to-end approach


Cable fire behavior: an overview and considerations

Working with fire performance testing methods such as IEC and understanding their output.


Twenty Years of Cat. 6

R&M produced one hundred million Cat. 6 modules in 20 years


Last-mile cabling

Which deployment type is best for your network?


Choosing chemical resistant structured cabling systems the smart way

The conditions under which structured cabling systems are deployed aren’t always ideal.



The biggest threat to fiber access network cabling?


More cable know-how from the specialist

Parties that are largely new to the cable industry are in need of advice.


Recently Read

Send this to a friend