Fiber optic cable is normally covered with a substantial outer plastic sheath in order to reduce abrasion and to provide the cable with extra protection against external mechanical effects such as crushing.
The cable sheath is said to contain the cable core and may vary in complexity from a single extruded plastic jacket to a multilayer structure comprising two or more jackets with intermediate armoring.
However, the plastic sheath material tends to give very limited protection against the penetration of water into the cable. Hence an additional water barrier is usually incorporated. This may take the form of an axially laid aluminum foil/polyethylene laminated film immediately inside the sheath as used by British Telecom.
Alternatively the ingress of water may be prevented by filling the spaces in the cable with moisture-resistant compounds. Specially formulated silicone rubber or petroleum-based compounds are often used which do not cause difficulties in identification and handling of individual optical fibers within the cable form.
These filling compounds are also easily removed from the cable and provide protection from corrosion for any metallic strength members within the fiber. Also the filling compounds must not cause degradation of the other materials within the cable and must remain stable under pressure and temperature variation.
Water Blocking Filling Compounds
Filling compounds are used in fiber optic cables to prevent the ingress of water into the cables. Moisture around the fiber can cause existing microcracks to propagate which can cause degradation or even failure of the system over time.
Filling materials are generally used in two different places in the cable. The first location is in the loose tubes or fiber enveloping areas. The second is in the interstices of the cable which includes all the areas not in direct contact with the fiber.
The filling material in the loose tubes can be either a gel or powder compound. The filling material is used to block water from entering and to prevent wicking of the water along the fiber. The filling material used in the interstices of the cable is a water blocking material which is a very thick gel.
Depending on the application of the cable, filling material may be used in both the buffer tubes and the interstices or only in one of the locations.
For direct buried, aerial or duct installations, cables are generally manufactured with filling materials in both locations. More recently, most cable designs have employed filling materials in both locations.
The chemicals that the filling materials are made of are very important as they may affect the optical parameters. There have been numerous studies in the past few years on hydrogen migration into the core of the fiber which causes an increase in attenuation. Silicon has proven to be one material which has experienced problems with hydrogen migration.