Types of Fibre Optic Cables

Fibre optic cables can be split in to two key types, Loose Tube and Tight Buffered.

Loose Tube Cables

Loose Tube fibre optic cables are the type that has traditionally been used in telecommunications networks where they are predominantly installed in external environments. The 250 micron primary coated fibres are contained within a plastic tube which may or may not contain a water blocking gel. One or more tubes are then surrounded by some form of strength member before an outer sheath is applied. The outer sheath may be PVC, LSZH or PE, depending on the environment in to which the cable will be installed. The construction of a typical single loose tube cable is shown below.

Loose Tube FO Cable Construction

The strength elements may be Aramid Yarn or E-glass yarn, both of which provide tensile strength and a degree of crush resistance. E-glass yarn has the additional benefit that it provides a level of resistance to damage by rodents. Cables designed for direct burial will usually have a metallic element (either corrugated steel tape (CST) or braided steel wire) to provide extra crush resistance.

An example of a CST armoured loose tube cable

CST Armoured LT Cable

Each loose tube will typically contain 12 fibres, although some constructions have up to 24 fibres in each tube. The use of multiple tubes in a single cable sheath mean that cables with several hundred fibres can be produced.

Due to the fragile nature of the 250 micron primary coated fibres they are normally terminated using a splicing technique where the cable fibre is fused to a pigtail that is factory terminated with the required connector type.


Tight Buffered Cables

A limitation of the loose tube fibre optic cable construction is its deployment in vertical cable runs. The 250 micron primary coated fibres are not supported in the loose tube which means that there can be significant stress on the fibres when installed vertically. One answer to this is to install loops of cable every 10m or so but the more common modern solution is to use a tight buffered construction.

In a tight buffered cable the 250 micron primary coated fibres have an additional secondary coating (buffer) that has an overall diameter of 900 micron (0.9mm). The 900 micron buffered fibres are surrounded with a strength element and an overall sheath is then applied.

Tight Buffered Fibre Optic Cable Construction

The strength elements may be Aramid Yarn or E-glass yarn and the sheath is typically a Universal Low Smoke Zero Halogen (ULSZH) material, making it suitable for use in both internal and external (when installed in a dry duct) environments. Tight buffered cables tend to have a maximum of 24 fibres but are suitable for both horizontal and vertical cable runs.

Tight buffered fibres may be terminated through the splicing on of pigtails but the addition size and strength of the 900 micron fibres also permits the direct termination of connectors.