This invention relates to optical fiber units for use in an optical fiber cable.
An optical fiber cable usually comprises a plurality of elongated optical fiber units wound spirally within an outer sheath. Each unit comprises an optical fiber telescoped within and spaced inwardly from an outer tubular jacket so that an annular space is defined between the fiber and the jacket.
In some applications, the cable is exposed to high-pressure hazardous gases which must be blocked against progressing through the interior of the cable. For example, United States Department of Energy specifications for certain optical fiber cables require that the cable be capable of blocking flow at end pressures as high as 125 p.s.i. In part, gas blockage is achieved by filling the annular space between the fiber and jacket of each optical fiber unit with a material which establishes a pressure-tight seal between the fiber and the jacket.
Previous materials which have been used as fillers include non-silicone materials such as polyurethane jelly, photo-curable polymers, semi-liquid organic polymers, and a material sold under the trade designator "INDOPOL". Difficulty has been encountered with those materials in that prior materials which are sufficiently viscous to block flow at the specified gas pressure excessively restrict free movement of the fiber in the jacket and thus cause undesirable changes in certain optical properties of the fiber when tension is applied to the cable. On the other hand, those prior materials with a relatively low viscosity enabling substantially free movement of the fiber are not capable of resisting high gas pressures when such materials are used to fill an annular space of sufficiently large cross-sectional area to impart desired properties to the fiber unit. Moreover, the viscosity of the previously used materials changes significantly over the wide temperature ranges to which the fiber unit may be exposed, such materials typically becoming excessively stiff at low temperatures and excessively fluid at high temperatures.