The outer sheath of an optical cable primarily provides mechanical protection, environmental resistance, and fire safety, rather than carrying optical data itself.

Function of the Outer Sheath

The outer sheath, also called the jacket or outer jacket, protects the internal optical fibers from physical damage, moisture, chemicals, and environmental hazards such as UV exposure or abrasion during installation and operation . It also prevents water ingress, which is critical for outdoor or undersea cables, often using water-repellent gels, powders, or solid barriers . In addition, the sheath can absorb mechanical stress and distribute tension along the cable, especially when combined with strength members like aramid yarn, fiberglass, or steel wires .

Materials and Properties

Outer sheaths are made from various materials depending on the application:

• PVC (Polyvinyl Chloride): Common for indoor cables, offering flexibility and basic protection .
• PE (Polyethylene): Suitable for outdoor use due to UV and moisture resistance .
• LSZH (Low Smoke Zero Halogen): Used in fire-sensitive environments to reduce toxic smoke and halogen emissions .
• Plenum and Riser-rated materials: Designed for building installations with specific fire performance requirements . These materials are selected not only for mechanical and environmental protection but also for fire resistance, with ratings such as OFNP/OFCP (highest flame retardant grade) and OFNR/OFCR according to the National Electrical Code .

Additional Features

Some outer sheaths include ripcords for easy jacket removal, Kevlar layers for tensile strength, or armoring for protection against rodents, construction damage, or underwater hazards . In multi-fiber cables, the sheath may also include light-absorbing layers to prevent crosstalk between fibers .

Summary

The outer sheath of an optical cable does not carry optical signals but is crucial for protecting the fibers, ensuring durability, and meeting safety standards. It provides mechanical strength, moisture and chemical resistance, fire protection, and environmental shielding, with material choice tailored to the cable's intended application .