The present invention relates generally to a pressure transporting system, in which an elongated body is transported within an elongated hole by blowing with a pressurized gas. More particularly, the present invention relates to an optical fiber cable construction adapted for laying by blowing with a pressurized gas, in which an optical fiber unit consisting of one or more optical fibers wrapped with a foamed coating layer is transported within a pipe formed of polymer or the like by blowing with a pressurized gas.
While various methods have been employed to lay out optical fiber cables, it has recently been proposed that one or more optical fibers be laid within so-called a pipe cable, i.e. an assembly of polymer pipes, by blowing with a pressurized gas such as air or the like, and the pipe cable being then laid. An example of this method is disclosed in GB patent applications Nos. 8231840 and 8309671. The method of laying optical fiber cables by blowing with pressurized gas has several advantages: no external forces such as tension will be exerted upon the optical fiber so that it will not be damaged during laying; since existing polymer pipes can be used, laying out in complicated routes is possible; fiber replacement and cable extension or connection can readily be achieved; maintenance is therefore easy and installation cost is relatively low.
FIGS. 1A, 1B and 1C show the cross-sectional structures of conventional optical fiber cables designed to be laid by blowing with pressurized gas. FIG. 1A shows an optical fiber unit 20 consisting of an assembly of seven bare or coated optical fibers 21 that are surrounded with a filled layer 24 and a foamed coating layer 22. FIG. 1B shows an optical fiber cable 201 having the optical fiber unit 20 blown into a pipe 23 under pressure. FIG. 1C shows a pipe cable 200 having an assembly of seven optical fiber cables 201 encased in an outer sheath 25.
The foamed coating layer 22 is an outer layer on the optical fiber unit 20 and is typically made of a foamed polymer, particularly, a polyethylene foam, in order to satisfy design considerations for lightweightness and large resistance to wind pressure.
The optical fiber unit 20 is to be inserted into the polymer pipe 23 which is typically made of polymer, for example, polyethylene.
Even if uniformity in the outside diameter of the optical fiber unit, uniformity in the shaping of the pipe, and optimization of the optical fiber unit diameter with respect to the inside diameter of the pipe are attained, the conventional fiber cable laying system has not been completely satisfactory not only in terms of ultimate blowing distance and speed that are achieved when the optical fiber unit is laid by blowing into the pipe but also with respect to the system configuration of a transmission network and the cost expended in cable installation. Further, the conventional optical fiber cable laying system cannot lay a very long optical fiber unit in a pipe, that is, an optical fiber unit has to be connected to the other units in order to lay the fiber cable at a long distance. In order to deal with these problems, the present inventors conducted various studies and found that the coefficient of friction between optical fiber unit and pipe and the static electricity occurring between them are two principal factors that deteriorates the effects mentioned above.
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