Optical fiber cables have been in use for communications for some time. The cables which are in use have been provided by a number of different manufacturers, each having its own structural arrangement. Because of the use of glass fibers as the communications medium instead of metallic conductors, it becomes necessary to include strength members in the cables.
In some optical fiber cables, the strength members are disposed centrally of the cable cross section. See, for example, German Offenlegungsschrift No. 25 51 210 in which a plurality of individual tubes are arrayed about a central strength member. Disposed in each so-called loose tube is one or more optical fibers. In such a design, it is typical for the length of the optical fiber generally to equal the length of the tube. In another approach, the strength members are included in a sheath system of the cable. As an example of the latter arrangement, see U.S. Pat. No. 4,826,278 which issued on May 2, 1989 in the names of C. H. Gartside, III, A. J. Panuska, and P. D. Patel. Therein, at least one optical fiber bundle is disposed within a tubular member which may be made of polyvinyl chloride (PVC). The tubular member is enclosed in sheath system elements such as a water blocking tape and an outer jacket. Longitudinally extending strength members are disposed in the outer jacket generally adjacent to the tubular member.
It is commonplace to provide optical fiber cables with optical fibers having a length which exceeds the length of the cable. See for example, U.S. Pat. No. 4,723,831 which issued on Feb. 9, 1988 in the names of B. D. Johnson, W. C. Reed and C. G. Wilson. This is done so that when the cable is handled and routed in tortuous paths during installation, undue strains will not be induced in the fibers. Also, as the amount of excess length increases, the tensile strength needs for the cable are reduced with an accompanying reduction in costs. The excess length of fiber generally has been accomplished by applying predetermined forces to the cable before takeup to stretch the cable sheath components elastically allowing excess fiber lengths to become disposed in each increment of length of the cable as the stretching forces are released and the sheath components return to their unstretched length. As should be apparent, the stretching is caused by controlled forces.
In the prior art, there has been reluctance to use a relatively high excess fiber length. The reason for such reluctance seemed to be the concern that too much excess fiber length results in undesired microbending which leads to increased attenuation.
These considerations relating to bending losses also are important because of additional demands which may be placed on optical fiber assemblies. For example, the core arrangement of the above-identified C. H. Gartside, III, et al. patent has been widely accepted as being suitable for a variety of installations. Accordingly, there is a demand by cable manufacturers for the core itself so that a buyer thereof may add its own sheath system to meet particular needs. If such an optical fiber cable core, as it is referred to commonly in the industry, is to be made available as an end product of a manufacturer, additional requirements must be met. Such an optical fiber cable core must not unduly strain the fibers when the cable core, unprotected by a sheath system, is exposed to a relatively wide temperature range. Also, the tubular member must be capable of providing protection for the optical fibers.
Furthermore, there has been a desire to use optical fiber more widely in aerial installations where right of way already exists. Because of the existence of overhead right-of-way for power lines and the presence of ground wire in overhead transmission, the thought of incorporating optical fibers in composite ground wire has evolved. Here, an optical fiber cable core would be provided by an optical fiber cable manufacturer to a manufacturer of composite ground wire which would incorporate the fiber cable core into the design of the composite ground wire. As should be apparent, an aerial cable is subjected to higher forces caused for example by wind and ice loading, particularly in overhead transmission lines where long spans are involved. Because it is subjected to increased forces, the aerial cable must be provided with sufficient excess fiber length to prevent undue strain.
Such desired uses result in the need for enhanced properties for the optical fiber cable core. What is needed and what seemingly is not available in the art is an optical fiber cable core which is suitable for sale in and of itself. Also, it should be capable of incorporation into a cable package for use, for example, in aerial installations of power lines. Desirably, the sought after optical fiber cable core should retain all the advantageous features of the hereinbefore disclosed C. H. Gartside, III, et al. patented cable but have enhanced features to meet the more demanding uses.