Optical waveguides – Miscellaneous
Reexamination Certificate
1997-05-08
2001-01-09
Ngo, Hung N. (Department: 2874)
Optical waveguides
Miscellaneous
C254S134400, C254S13430R
Reexamination Certificate
active
06173107
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to optical fibre and other lightweight and flexible transmission lines. More particularly, the invention relates to a method and apparatus for installing such communications media.
BACKGROUND OF THE INVENTION
Optical fibre cables carrying optical fibre transmission lines have heretofore been installed by the same methods as conventional metal conductor cables. Such methods usually involve pulling the cable with a pulling rope through a previously laid cable duct. Frequently the cable duct already contains one or more conventional cables at the time of installing the optical fibre cable.
Unlike the metal conductors of a conventional cable, the optical fibres are easily damaged by tensile stress. Such stress may, for example, propagate micro-cracks, leading to fibre breakage in the long term. It is, therefore, standard practice to reinforce optical fibre cables by providing a central strength member, usually one or more steel tension wires, about which the optical fibres are disposed. The strength member takes up, and thus increases the ability of the cable to withstand, tensile stresses accompanying installation of the cable.
Unfortunately, the central strength member usually provides insufficient protection against local stresses caused by pulling a further cable through the same duct. The conventional approach of installing at the outset optical fibre cables containing sufficiently large numbers of optical fibres to satisfy foreseeable future traffic demands is a way of overcoming this problem. In consequence, first time installation of optical fibre cables containing dozens or even hundreds of optical fibres are currently envisaged despite the fact that to begin with a small fraction of the installed fibres would provide ample traffic carrying capacity. A further reason for installing optical fibre cables of comparatively large dimension is that the smaller the cross section of the cable the more prone the cable becomes to wedging in between those cables already present in the duct.
The first time installation of large diameter optical fibre cables with high numbers of optical fibres, is, however, undesirable for a variety of reasons. Firstly, there are problems of a technical nature inherent in such cables, such as, for example, the difficulty of forming joints and of achieving the required high strength-to-weight ratios. Secondly, there are clear economical drawbacks in committing large resources to install initially unused fibre capacity, particularly in view of the comparatively recent origins of optical fibre technology which lead one to expect continued substantial reductions in the price and improvement in the quality of optical fibres. Thirdly, there is the serious risk of damaging in a single incident very large numbers of expensive optical fibres. Finally, there is an appreciable loss in flexibility when routing high density optical fibre transmission lines.
A method of installing optical fibres with pulling ropes and pull chords is described in “Sub-ducts: The Answer to Honolulu's Growing Pains”, Herman S L Hu and Ronald T. Miyahara, Telephony, Apr. 7, 1980, pp 23 to 35. The installation method described there proceeds as follows: a section of existing 4-inch (100 mm) duct is rodded and thereafter between one and three individual 1-inch (25 mm) polyethylene tubes are inserted into the duct using pulling ropes. The polyethylene tubes form subducts into which an optical fibre cable can be pulled with the aid of a nylon pull chord which has previously been inserted into the subduct by means of a parachute attached to its leading end and pushed through the subduct with compressed air.
The method just referred to does deal with some of the problems discussed above, but only to a very limited extend. Thus, it enables fibre capacity to be increased in up to three stages, and separates the optical fibre cables from those cables already in the duct, thereby greatly reducing the likelihood of jamming, and hence overstressing, of the optical fibre cable.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to overcome, or at least appreciably mitigate the majority of the aforementioned problems of installing optical fibre transmission lines.
It is another object to provide a method of installing optical fibre transmission lines which is comparatively simple and yet flexible and economical. Moreover, it is an object of the present invention to utilize this same method to install other lightweight and flexible wire transmission lines as well. The transmission lines may comprise one or more of the following, in any combination: optical fibres, wires or other electrical conducting media, other dielectric transmission media, or any other medium capable of carrying data. The only other requirement is that the transmission medium should be sufficiently lightweight and flexible for installation by the method of the invention. The term “transmission line” as used hereinafter in this specification and claims shall be construed to mean transmission lines as defined above.
According to the present invention, a method of advancing a lightweight and flexible transmission line along a tubular pathway comprises inserting the free end of such a line into a previously installed pathway, and propelling the line along the pathway by fluid drag of a gaseous medium passed through the pathway in the desired direction of advance.
It will be appreciated that to generate sufficient fluid drag to propel the transmission line, the gaseous medium has to be passed through the pathway with a flow velocity much higher than the desired rate of advance.
The terms “lightweight and flexible” with respect to the transmission line are to be understood as meaning “sufficiently lightweight and flexible” for the transmission line to be propelled by the fluid drag.
Whether the transmission line is sufficiently lightweight and flexible and the flow velocity sufficiently high is readily determinable by a simple trial and error experiment, guided, if necessary, by the theoretical model discussed below.
The flow velocity of the gaseous medium may be steady or may be suitably varied, for example either between a first velocity producing no, or insufficient, fluid drag to propel the fibre or wire member, and a second velocity producing sufficient fluid drag to propel the fibre or wire member, or between a first and second velocity both producing sufficient fluid drag for propelling the fibre or wire member. Conveniently the variations in velocity take the form of repeated abrupt changes between the first and second velocity.
The aforementioned variations in flow velocity may include periods during which the flow is reversed with respect to the desired direction of advance of the transmission line.
It is to be understood that more than one transmission line may be propelled along the same tubular pathway.
A transmission line may, for example, comprise a single optical fibre or wire, protected by at least a primary coating but preferably contained within an outer envelope. Alternatively, a fibre or wire member may comprise a plurality of optical fibres or wires contained within a common envelope. The envelope may loosely or tightly surround the fibre (wire), or fibres (wires).
The method may be used for insertion of an optical fibre or wire member into, or its withdrawal from, the pathway.
The gaseous medium is chosen to be compatible with the environment in which the invention is performed, and in ordinary environments will be a non-hazardous gas or gas mixture. With the proviso about compatibility with the environment, the gaseous medium is preferably air or nitrogen.
The tubular pathways and/or the fibre or wire members are conveniently but not necessarily of circular cross-section, and the fibre or wire member is always smaller than the pathway.
In practice, when installing an optical fibre member, the pathway internal diameter will generally be greater, and frequently much greater than 1 mm, and the external diameter of the fibre mem
Cassidy Stephen A.
Reeve Michael H.
British Telecommunications public limited company
Ngo Hung N.
Nixon & Vanderhye PC
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