The impact of water on fibre optic cable
Fiber optic cables are resilient, but still need protection from damp and humidity, writes Asem Shahid of Datwyler
The state of the art relating to standard single-mode fibres
We are in the early stages of what may be the greatest transformation of networking yet, driven by mobility, video consumption and cloud.
For years most fibre and cable manufacturers have been producing their single-mode cables using fibre quality OS2, G.652.D or EN 60793-2-50:2008, B1.3. The fact that there are several standards for single-mode fibres is due to the international standardisation bodies involved. The ITU is the International Telecommunication Union, a specialised agency of the United Nations, which among other things specifies standards for global telecommunications. The IEC (International Electrotechnical Commission) and its European counterpart CENELEC (Comité Européen de Normalisation Électrotechnique) draw up standards for private applications.
Standard G.652 was drawn up by the ITU. IEC/CENELEC incorporated these types of fibre as “OS1” and “OS2” in the standards for application-neutral, generic cabling, ISO/IEC 11801 and DIN EN 51073 – the attenuation values here being higher than in IEC and CENELEC (60793-2-50) documents referred to in this standard. The parameters of IEC 60793-2-50 are similar to those of standard ITU G.652.D.
Standards for water impact
There are two standards dealing with the impact of water on optical fibres. IEC 60793-1-50 covers the measurement method and the “damp heat” test procedure. IEC 60793-1-53 covers “immersion in water”. In the first test the fibres are exposed to the action of damp heat (+85°C, 85% relative atmospheric humidity). In the second they are immersed in temperature-controlled water (+23°C). Both tests last for 30 days, during which period a defined attenuation increase must not be exceeded in either. The repellent effect of the primary protection – i.e. the efficacy of the fibre coating against exposure to water – is also tested.
Fibres at risk
If a fibre optic (FO) cable is damaged or stored without end caps, it is important that water should not be able to “creep” through the cable without restriction. Water penetration into a FO cable is not inherently dramatic because the bundles, which consist mostly of polyethylene (PE), are themselves very water-tight and because the filled bundles, together with the primary protection and the fibre sheath, afford the fibre core a high degree of protection. The transmission characteristics of an optical fibre are therefore relatively well safeguarded.
If, however, the water is able to spread to a closure or splice box, at this point the fibres only have their primary protection as well as splicing protection. Damage to the fibres cannot be ruled out, particularly if the fibre coating is not of the quality stipulated by the standards mentioned above. An irreversible increase in attenuation takes place if water diffuses into the fibre optic material (Si2O). Water molecules also move into micro-cracks in the fibres, enlarging them and dramatically reducing fibre life.
In view of the possible impact which moisture can have on the fibres it is hardly surprising that longitudinal watertightness is one of the most important environmental parameters for FO cables. In accordance with IEC 60794-1-2 F5 a cable is considered to be longitudinally watertight if penetrating water is only able to spread along a defined length in the cable core.