Shrinkback-Induced Attenuation in Loose Tube Cables

摘要

As optical fibers are deployed deeper within telecommunications networks, more frequent mid-span accesses of cables are occurring, and there have been reports of field failures in pedestals and splice closures in which un-cut buffer tubes have been coiled. This work describes an investigation that determined buffer tube shrinkage can cause this type of problem. When the buffer tube is within a cable, the shrinkage is limited by the structure of the cable; however, when a length of the buffer tube is removed from the cable and coiled within a closure, excessive shrinkage may be induced in the tube through thermal cycling. Lengths of polyethylene (PE) and of polypropylene (PP) buffer tubes were measured as the temperature was cycled. Shrinkage was caused by contraction at low temperatures and by shrinkback at high temperatures. The PE buffer tube demonstrated a shrinkback ten times higher than the PP tube.Another experiment temperature cycled buffer tubes with live fibers while the change in attenuation was measured. PE buffer tubes showed elevated attenuation at the first -40℃ cycle due to thermal contraction of the buffer tube. After the buffer tube had been heated to 70℃ and then cooled again, the attenuation was much greater due to shrinkback. The PP buffer tube showed very little change in attenuation. The attenuation measurements correlated well with the length measurements. A new test is recommended to evaluate the potential for buffer tube shrinkage induced failure modes in access network cables.