In a loose tube, fiber is well shielded from being affected by the surrounding cable elements.
By stranding the tube, it is also possible to design in strain relief from both temperature changes and mechanical forces.
For these reasons, some variations of the loose tube structure have found widespread use.
The loose tube cables are, however, more difficult to terminate and connectorize as compared to the tight buffered fiber cables.
The loose tube can be air filled, but very often a filling gel (compound) is used.
The purpose of the filling compound is to prevent water penetration, and it also restricts fiber movements since the compound is much more viscous than air.
If water is present, it can freeze and cause microbends.
Water has also a detrimental effect on the mechanical strength of the fiber.
The loose tube is normally made of a plastic and has an inner diameter of typically 4-5 times that of the primary coated fiber.
Loose tube materials for loose tube cables need to possess a relatively high modulus and good processability.
During manufacture, optical fiber is fed into the loose tube at a faster rate than it is being produced so that the optical fiber is laid helically within the tube.
To buffer the fibers from each other and from any perturbations on the tube walls, the loose tube is filled with a gel compound similar in composition to the water blocking gels in conventional telecommunication cables.
The loose tube material must therefore be compatible with the gel as in conventional cable insulation.
Polypropylene has been used for loose tube but more recently, polyethylene have become more widely used.