Optical cables are designed to protect the optical fibers from damage due to the rigors of installation and from the demands of the surrounding environment. No single optical cable design is
universally superior in all applications, however. In order to meet application – specific requirements, outside plant (outdoor), indoor outdoor cables, and inside plant (indoor) cables must be designed for their intended installation environment. The consequences of optimizing a cable design for outdoor use can prove counterproductive to meeting the requirements for indoor placement and vice versa. For example, the most popular cable jacket material for outdoor use will not pass flame resistance tests required for placement indoors. In general, optical fiber cables installed in an outdoor environment are exposed to more severe mechanical and environmental conditions than are experienced in the protected, climate-controlled, indoor environment. Outdoor installations (usually lashed aerially, pulled through duct, or directly buried in the ground), are subjected to combinations of ultraviolet (UV) radiation, standing water, cable-gnawing rodents, temperature extremes and other outdoor-specific hazards. Loose tube optical cable designs are optimized for outside plant applications and have demonstrated over 20 years of proven field performance.
Performance at Extreme Temperatures
The major constituents of an optical cable structures are silica glass and polymeric plastics. For a given temperature change, the rate/magnitude of material expansion and contraction will be different because each material possesses a different coefficient of thermal expansion. The loose tube cable establishes a strain-free environment for the optical fiber by mitigating the influences of this effect. Loose tube cable manufacturing processes ensure that the optical fiber to buffer tube length ratio is controlled such that no optical fiber is compressed against the tube wall when the tube expands or contracts with changes in temperature. The strain-free environment established in the loose tube cable design compensates for movement in the cable structure without inducing mechanical forces on the fiber. This characteristic enhances the operating temperature range of the loose tube design.
Protection from Ice Crush Effects (Waterblocking requirement)
Ice crush hazards affect optical cables in locations where standing water and freezing temperatures coexist. In a confined space, significant tensile and compressive forces can be generated from the expansion of water as it transitions to a solid state under cold conditions. In optical fiber cable applications, this effect can occur in water-filled outdoor conduits or within the cable core itself. Water migration inside the cable’s outer jacket can result in the formation of ice crystals within the optical fiber cable core. This ice will impart stresses in close proximity to the optical fibers and may result in an unacceptable increase in attenuation or even fiber breakage. Therefore, it is essential to prevent the intrusion and uncontrolled movement of water inside the cable.
A loose tube fiber optic cable is designed to provide maximum protection against water penetration and water migration by utilizing intrusion preventative measures in both the cable core and the buffer tubes. Water-blocking protection of the cable core is accomplished by surrounding it with a dry water-swellable tape and yarns, or with a gel, to stop the entry and migration of water should the cable’s outer jacket be breached. This protective measure is included primarily to maintain the mechanical integrity of the cable itself (e.g. prevent ice crush from within the cable, fungus growth, or corrosion of metallic cable members when present). The water-blocking protection, water-swellable yarn or gel, is placed in the buffer tubes with the optical fiber during manufacture of the cable.
As stated previously, loose tube cable establishes a strain-free environment for the optical fiber by mitigating the influences of external effects. In addition to the benefit provided at extreme temperatures, this attribute also enhances the performance of the loose tube cable design under a variety of mechanical forces. Installation practices and installed system conditions can subject the cable to tensile, flexure, twisting, crush, impact and bending forces. By isolating the fiber from these external forces, the loose tube design ensures maximum cable life in an outdoor environment.
The optical fiber cable must also be able to withstand direct exposure to ultraviolet sunlight in aerial installations. In the outdoor environment, light, heat and moisture combine to cause optical, mechanical and chemical changes in materials and the first line of protection of any optical fiber cable is the cable outer jacket. Carbon black, which is compounded into the jacket material to provide maximum ultraviolet protection, is the best defense against ultraviolet degradation.
The predominant users of outside plant cable continue to specify loose tube cables in outdoor environments. Loose tube cables must pass rigorous mechanical, environmental and optical
tests in accordance with accepted Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) fiber optic test procedures (FOTPs). Loose tube cables are specifically designed to perform in harsh outdoor environments with minimal performance degradation.
A new standard, ICEA S-104-696, “Standard for Indoor – Outdoor Optical Fiber Cables” has been developed which addresses the need for an interbuilding and intrabuilding cable. These cables can be loose tube or tight buffered for the ease of termination such as premises cable, but must offer the tensile strength, waterblocking protection, and UV protection of an outside plant cable. These inter-/intrabuilding cables are typically used for short runs to connect to another building(s) and some limited premises applications.