Time- and temperature-dependent material behavior and its impact on low-temperature performance of fiber optic cables

A series of service-affecting field failures in cold weather (-40 degrees C to 0 degrees C) initially and in more moderate conditions (up to 15 degrees C) recently have raised concerns about the temperature-dependent transmission performance of loose tube fiber optic cables. The first field failures occurred in 1550-nm aerial transmission lines while more recent failures have affected 1310-nm operations. Field analyses and laboratory temperature-cycling measurements of the affected cables established that the transmission loss resulted from fiber microbending due to random fiber contacts with the buffer tube wails caused by the axial shrinkage of the buffer tubes relative to the cable central member. High resolution thermal expansion/contraction measurements on commonly used PET (Polybutylene Terephthalate) and PP (Polypropylene) buffer tubes indicated that the total axial shrinkage consists of two components: irreversible shrinkage due to the relaxation of processing-induced stresses and reversible thermal contraction/expansion. The magnitude of both components depends strongly on the buffer tube material and the processing conditions used to manufacture buffer tubes. In this report, we review our work to date in developing field installation and cable design solutions, present a comparison of two buffer tube materials, identify key materials issues that determine buffer tube dimensional stability, and discuss the implications of these issues on the field installation and cable design solutions that we have developed to minimize the risk of low-temperature transmission loss in loose tube cables.