ation of any applied stress. within the tube by overfeeding, strain relief of 0.2 to 0.4% can Polyethylene sheath Optical fibres Aluminium polyethylene laminate Paper tape Central strength member Polyethylene bedding layer Figure 15.5 Tightly buffered optical fibre trunk cable Cable design 15/13 After Before Wavelength in nm Figure 15.6 Single mode fibre before and after cabling readily be obtained. However, there is clearly some uncertainty on polymer such as nylon 12 to an overall diameter of between 0.5 and the positional stability of the fibre, and contacts between the fibre 1.0mm. This can be applied in such a manner to apply a longitudinal and inside wall of the tube (which is relatively rough) can give rise compression to the fibre and obtain a strain relief of 0.2% or more. to microbending loss. Multifibre encapsulation in a gel filled loose tube for high fibre Long term movement and water blocking problems can be over- count cables is also attractive e.g. up to 12 fibres can be inserted in come by introducing a gel into the tube. Thixotropic gel compounds a tube of 2mm inside diameter and gel filled. have been especially developed for optical fibre cables (Bury, 1985) with a wide temperature range and a fine granular structure. The gel is normally highly viscous and supportive of the fibre, but changes Cable design to low viscosity under stress and allows rapid movement of the fibre during cable elongation of compression. With this arrangement With the essential building block of well buffered and encapsulated good protection of a fibre with a single hard acrylic coating is fibres, optical cables can be manufactured in a similar way to obtained. conventional cables. A common alternative to loose tube encapsulation is tight buffer- A design for installation in long underground ducts is shown in ing. In this the coated fibre has extruded an additional jacket of Figure 15.5. Tight jacketed buffered fibres are laid helically around Copper pair Polyethylene sheath Aluminium/plastic laminate Central strength m