Aerial optical cables are measured both mechanically and optically to ensure proper installation, performance, and durability under environmental conditions.

Mechanical Measurements

Span and Sag: The distance between poles (span) and the sag of the cable are critical parameters. Reducing the sag increases tension, while longer spans require careful consideration of environmental loads such as wind and ice accumulation . Typical aerial self-supporting cables are designed to handle tensile loads up to 300 lb and short spans of around 150 feet for drop cables . The sag and tension must be calculated to prevent overstressing the cable and to maintain fiber integrity. Cable Diameter and Strength: The cable diameter, including any messenger wire, affects tension and sag. Figure-8 cables often include a steel or dielectric messenger to support the fiber and control shrinkage. The mechanical design ensures that fibers remain in a low-stress condition during temperature changes, ice, or wind . ADSS cables are designed for spans up to 100 meters and are rated for long-term outdoor exposure . Environmental Effects: Temperature variations, ice, and wind can cause contraction, expansion, or bending of the cable. These factors are considered in the design and installation, and testing may include temperature cycling to ensure the fiber remains low-stress and avoids macrobending or microbending losses .

Optical Measurements

Attenuation and Loss: Optical fibers are tested for attenuation using methods such as the cutback method (TIA 455-78) in manufacturing or back reflection methods in the field. The attenuation coefficient, expressed in dB/km, characterizes fiber loss per unit length . OTDR Testing: An Optical Time Domain Reflectometer (OTDR) is used for pre- and post-construction measurements. A lead-in fiber is often employed to eliminate dead zones at the OTDR interface, allowing accurate measurement of fiber length, total end-to-end loss, and splice loss . OTDR traces show reflective and non-reflective splices, helping locate faults and verify installation quality. Splice and Connector Loss: Splices, whether fusion or mechanical, introduce optical loss. OTDR traces help identify these losses and ensure they are within acceptable limits. Reflective splices appear as spikes, while non-reflective splices appear as sudden drops in power .

Summary

Measuring aerial optical cables involves mechanical assessment (span, sag, tension, environmental stress) and optical testing (attenuation, OTDR, splice loss). Proper measurement ensures the cable performs reliably over its lifetime, maintains low-stress conditions for the fibers, and meets design specifications for both installation and environmental resilience .