WO2003040763A2

WO2003040763A2 - Installing optical cable systems

Info

Publication number: WO2003040763A2
Application number: PCT/EP2002/012405
Authority: WO
Inventors: Martin Mccourt; Alan Eyres
Original assignee: Ccs Technology, Inc.
Priority date: 2001-11-09
Filing date: 2002-11-06
Publication date: 2003-05-15
Also published as: GB0126942D0; AU2002358486A1; WO2003040763A3

Abstract

A process for installing an optical cable system with optical performance management, for example, in a dispersion management system, involves cable duct sections each extending between a respective two access points. The length of each installed cable duct section is measured from within it, and appropriate optical cables for achieving the required performance installed and connected. In an important application, some of the cables have positive and some negative chromatic dispersion characteristics and the ratio of their lengths is chosen to obtain effective dispersion management. A number of ways of measuring the duct length are described, some using various graduated or measured members extending through the duct, others measuring pigs passed through the duct.

Description

Installing Optical Cable Systems This invention relates to a process for installing optical cable systems for communications purposes. Cable length can affect optical performance characterisation, for example, chromatic dispersion or optical attenuation are length dependent optical performance characteristics. A concept of the present invention is to match passageway length data with optical cable characteristics for the purpose of attaining desired optical performance results. In other words, length dependent optical performance characteristics are accounted for in creating high performance, installed cable systems. The invention therefore contemplates length measurement of passageways to enable effective cable matching. Passageways include, for example, pipes, sewers, ducts in overhead lines, trunks, chambers, channels, tubes, shafts, conduits, duct sections, and/or other suitable passageways for cable installations .

The present inventive concepts can be applied to various optical performance characteristics; however, an exemplary embodiment of the present invention involves chromatic dispersion management. It is known that one of the factors that may limit the bandwidth of an optical cable system is optical pulse broadening due to chromatic dispersion in the fibre . It is also known that optical fibres can be made with either positive or negative dispersion, from which it follows that in principle the effect of dispersion can be controlled, or ideally eliminated, by using appropriate lengths and distributions of the two fibre types in each optical path. This principle is not easily applied, as the length of cable sections is seldom known with the required degree of accuracy: in extreme cases, the true length of a cable duct has been found to be as much as 40% longer than the nominal length of its route, owing to the combined effect of unmapped rise and fall in the ground, variations in depth below the ground surface, diversions around trees, around street furniture, or around other utilities' installations and the like that were unknown to the installation designer or were not in the expected position (sometimes because they encountered and were diverted around another obstacle) . The concepts of the present invention include measuring a passageway, sizing or matching the optical cables using the measurement data and length dependent optical performance characteristics of the cables, then installing the cables.

In accordance the exemplary embodiment of the invention, a process for installing an optical cable system with dispersion management comprises:

(a) installing cable passageways, for example, duct sections, each extending between a respective two access points; (b) measuring from the length of each installed cable duct section;

(c) installing optical cables at least partly into the said cable duct sections; and

(d) connecting the fibres of those cables so as to obtain the desired optical results.

In^' the exemplary context of dispersion, step (c) preferably includes installing cables with positive or negative dispersion characteristics so that dispersion over the system is controlled or managed. Preferably the duct sections are installed and measured before the cables to be installed are manufactured, so that they can be made in the requisite lengths related to the actual lengths of the sections, so as to minimise offcuts . Where possible, the system should be designed so that cable lengths are substantially equal to the lengths of the duct sections in which they are to be installed, plus maintenance loops, and cable to cable connections are made in a simple manner at the access points. However, this may sometimes be incompatible with the need for dispersion management, and when this problem arises it will normally be more economical to install an extra, dispersion balancing, length of one of the cables at one of the access points rather than provide an additional access point that was not originally planned. An access pit may be enlarged, if necessary, to accommodate the required length of coiled cable. Other options may present themselves in installations where a significant length of cable is laid elsewhere than in underground ducts (for instance inside a large building or a tunnel) .

The lengths of the cable passageways, for example, duct sections, can be measured in a number of different ways:

(i) A graduated flexible member may be installed into a duct section temporarily (or permanently, if it is small enough not to interfere with subsequent cable installation) and the graduations read at the two access points. (ii) A flexible member without preformed graduations may be installed into each duct section, marked to indicate the positions of the access points, and withdrawn for the separation of the markings to be measured in an accessible and convenient place by conventional techniques. (iii) Provided a sufficient grip can be achieved, a carriage (commonly called a"pig") provided with an odometer "wheel" (which may be an endless belt or other rolling body) can be passed through the duct sections and the rotation of the wheel counted to determine the duct length. (iv) A ^Λpig" fitted with a miniature gyroscopic dead- reckoning navigator may be passed through the duct sections and its successive co-ordinates integrated to determine the length of its path between access points, (v) Where it is not desired or not possible to measure the duct before cable manufacture, then length graduations may be marked on the cable itself, or its installed length measured by some other technique. A particularly preferred technique in this case would be by reflectometry on the fibre, or one or more of the fibres, contained in the cable: this would have the significant advantage, from a dispersion management viewpoint, of measuring the actual fibre length and not relying on the sometimes unsafe assumption that the excess length of fibre in the installed cable is close to its factory-measured or design value.

The flexible member used in methods (i) and (ii) could be a conventional measuring tape, but in some installations this may not be accurate enough as it will not follow the same path as the cable around bends in the duct: a graduated rod, rope or flexible tube may be preferred.

The flexible member or pig, when required, may be inserted into and/or passed through the duct section in various ways. In all cases they could be pulled through by rods or ropes, or in some cases a stiff flexible member might be pushed into the duct, as in conventional cable installation practice. Flexible members can also be installed by "blowing" using a stream of air or other suitable fluid (with or without fitting a piston or parachute to the leading end of the flexible member so that the member is advanced predominately by a pressure differential or by viscous drag respectively) , using one of the techniques that has long been known for installation of cords for subsequently pulling in cables and in recent years has sometimes been used to install optical fibres and cables directly, without the need to apply heavy tension to them. If the flexible member is sufficiently small as well as sufficiently flexible, a coil or other package of it may be blown through the duct while its tail is held at the launch end, so that it unrolls as it travels. Pigs can also be advanced by flow of a fluid, but in this case a piston is likely to be needed, or they could be actively driven using a motor and wheels or the like. A motor might be powered by energy stored in the pig (for instance electricity from a fuel cell or battery, pneumatic power from a compressed air cylinder, or fuel gas or liquid from a suitable tank) .

Different measuring techniques may be preferred for different installations, or possibly for different duct sections in the same installation, and not all the methods described may be practicable for some installations (in particular, advances in miniaturisation may be needed before it is practicable to use inertial navigators in pigs to fit small cable ducts, and odometer techniques will always ^'be difficult if the duct wall is slippery) .

The invention will be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a simple optical fibre installation; and each of

Figures 2-6 illustrates a different way of measuring the length of a duct section from within in using the process of the invention.

Figure 1 represents a very simple optical fibre installation simply conveying digital optical signals from a source 1 to a single remote receiver 2. The source and receiver are connected by cables predominately in any suitable cable passageway, for example, ducts, and they proceed by way of a series of access points 3 which will may be up to 2 km apart, or a little more, if a cable blowing technique is used to make the installation under favourable conditions, but much less if rodding is to be used. There are thus likely to be at least tens or hundreds of access points in a practical installation, only a few of which have been shown. Access points may take the form of underground pits (often called "manholes") , or they could be cabinets or buildings above ground.

The access points 3 are interconnected by ducts (often comprising an outer duct accommodating a number of smaller "subducts" and within these optical cables are laid. At some of the access points, the cables will be jointed; usually there will be some access points which the cable simply passes through. The present inventive concepts apply to measuring a cable passageway and using the measurement data to size the cables for obtaining a desired optical result, for example, controlling optical dispersion or other length dependent optical characteristic. The inventive concepts are applied, in this exemplary installation, where some of the cables 5 comprise fibres with positive dispersion and other cables 6 comprise fibres with negative dispersion, and these need to be interconnected and distributed in the system so that the aggregate dispersion from the source to the receiver is controlled. For example, the dispersion can be minimised, and the aggregate dispersion at every intermediate point is sufficiently small that the optical signal pulses remain clearly spaced from their neighbours. This is achieved by ensuring that the ratio of the total lengths of positive- and negative-dispersion cables is inversely proportional to the numerical values of their respective dispersions per unit length and that the continuous length of either dispersion type is nowhere long enough to threaten the pulse spacing. In the exemplary embodiment of the present invention, the ducts joining access points are installed first, and then their lengths measured from within. It will be understood that where a cable length is to pass through one or more access points without jointing, it is a matter of indifference whether the individual duct lengths are measured, or only the aggregate length of the ducts accommodating that cable length.

Figure 2 illustrates a first measuring technique, and shows a duct 7 made up of many sections of plastics tube, typically of PVC or HD polyethylene. The length "a" is required. A conventional sectional rod, as used to insert a rope for conventional cable pulling is inserted into the duct from one of the access points 3 until it is observed to emerge at the entry to the next access point, and the number of rod .sections 8, 9 used is counted, their length being known. Manual counting may be difficult under site conditions, so it is preferable to use an instrumental 'method to count the entry of rod sections into the duct end: typically the rod sections will have metal end-fittings which may be detected using conventional electrical circuitry to observe ferromagnetic or electrically conductive material; or visible markings could be applied and observed photo- electrically; in either case, conventional counters are readily available. If precision of measurement better than ± one section length (say ± 1.5m) is needed, the rod sections, or at least one of them (9) , could be provided with markings (say every 100 mm) , but this is not necessary as it is equally possible to measure the distance "b" with an ordinary measuring tape and so deduce the fractional length of rod within the duct. The rod may be immediately withdrawn and re-used to measure another duct section; preferably a pulling rope is drawn into the duct section in removing the rod, for subsequent use in pulling in the cable. Figure 3 shows a second measuring technique in which an ordinary graduated fabric measuring tape 9 is inserted into the duct section from a coil 10 contained in a gas-tight chamber 11 by blowing with compressed air from a cylinder 12. When it reaches the second (right-hand) access point, it is caught by an appropriate device (e.g. a braid tube closed at one end) and, once stationary, its end positioned by an operator in relation to the duct opening 13; the operator at the insertion end can then gently pull the tape taut and directly read the section length from the tape. The tape may be strong enough to use to draw in a small pulling rope, but the technique may be more appropriate for use when the cable is to be installed by one of the blowing techniques.

Figure 4 illustrates a very similar technique, the difference being that what is blown into the duct section is a plain length of yarn 14 (preferably a glass yarn or a synthetic polymeric yarn for good dimensional stability and strength) ; when in place and tensioned, marks are applied to it at 15 and 16 (say using ink or a spray can of quick-drying paint) ; the yarn is then withdrawn and its length between the two marks measured by any conventional technique.

Figure 5 shows a "pig" 17 for use in a significantly different form of the invention. It is generally cylindrical and a clearance fit in the size of duct it is designed for. A motor 18, supplied by an energy store 19 (such as a battery or a gas bottle) drives a shaft 20 which operates a worm- gear 21. This engages a number of externally-toothed belts 22 to couple and drive them; to simplify the sketch, it has been drawn as if there were four of these distributed at 90° intervals around the axis, though in practice three at 120° would usually be preferred. The toothed belts are mounted on spring-biased idler rolls 23 which urge it into contact with the worm-gear on the one hand and the wall 24 of the duct on the other. Idler wheels 25 are used to support the end of the pig remote from the belts (or two sets of belts might be employed, at opposite ends of the pig) . At least one of the toothed belts is coupled to an odometer 31 by any convenient mechanism 32, and thus records the distance travelled. Odometer readings just need to be taken before and after the pig drives itself through the duct.

The alternative design of pig shown in Figure 6 comprises a closed tubular shell 26 designed to be propelled through a duct by pressure of fluid 27 (either air or water is suitable) acting on a flexible piston 28 that engages the wall of the duct. It encloses a suitably miniaturised inertial navigation system 29, its power unit 30 and a memory device 31 (all represented diagrammatically) for storing co-ordinates generated by it at suitable time intervals. After the pig has passed through the duct section to be measured, the stored data can be transferred to a computer programmed to integrate and deduce at least the distance travelled. If desired, it is possible to analyse the stored data further to get useful information about the sinuosity and/or gradient of the route to assist in assessing the suitability of the duct for installation of the cable by a particular blowing or other technique. Once the lengths of the duct sections are known, cables can be matched to the installed conditions, taking optical characteristics into account i the required lengths to build an installation with joints at some of the access points so that a desired optical result is achieved in the installed system. In the exemplary embodiment, the installed system includes the required distribution of positive- and negative- dispersion fibres to achieve the required quality of dispersion management. If necessary (returning to figure 1) a length of either type of cable that is necessary for dispersion management (unless an access point were to be moved or added) but not for reaching the end of the route may be coiled at one of the access points, 4.

A further group of alternative measuring techniques (not illustrated because they will not usually be preferred in view of the amount of scrap cable they are likely to generate) use the installed cable itself as a measuring instrument :

(i) length graduations are applied to at least the first few metres of the cable wound on the supply reel (the total length of the cable being predetermined) ; on installation, the excess length of cable remaining at the starting end is graduated and with the start end appropriately positioned at the exit end of the duct, the length of the duct can be read off on the cable; (ii) the length of the cable is measured or otherwise known before installation; once installed, the length exposed at each of the ends of the duct is measured and subtracted from the length of the cable to arrive at the length of the duct; (iii) after the cable has been installed, and with or without first cutting off any excess length, the length of at least one fibre of the cable is measured by an optical diffractometry technique; this can be used, (after allowing for any length to be trimmed but either with or without allowing for the excess length of fibre used in cable making to protect the fibre from tensile stress) as the duct length. In fact it is a better measurement for use in dispersion management as the excess length may vary somewhat and it is the length of fibre that determines the extent of pulse broadening due to its dispersion.

Any discussion of the background to the invention herein is included to explain the context of the invention . Where any document or information is referred to as "known", it is admitted only that it was known to at least one member of the public somewhere prior to the date of this application . Unless the content of the reference otherwise clearly indicates , no admission is made that such knowledge was available to the public or to experts in the art to which the invention relates in any particular country (whether a member-state of the PCT or not) , nor that it was known or disclosed before the invention was made or prior to any claimed date . Further, no admission is made that any document or information forms part of the common general knowledge of the art either on a world-wide basis or in any country and it is not believed that any of it does so.

Claims

CLAIMS :

1 A process for installing an optical cable system for achieving a desired optical performance comprising:

(a) providing cable passageways, each passageway extending between at least two access points;

(b) measuring from within the length of each cable passageway;

(c) installing optical cables having length dependent optical characteristics, at least partly into the said cable duct sections; and

(d) inter-connecting the fibres of those cables so as to obtain a desired optical performance of the system.

2 A process as in claim 1 in which said length dependent characteristics comprise positive and negative dispersion characteristics.

3 A process as claimed in claim 1 in which the cables to be installed are manufactured after the duct sections have been installed and measured and in the requisite lengths related to the actual lengths of the sections. 4 A process as claimed in claim 1 or claim 2 in which at least some cable lengths are substantially equal to the lengths of the duct sections in which they are installed.

5 A process as claimed in any one of claims 1-3 in which an extra, dispersion balancing, length of one of the cables is provided at one of the access points.

6 A process as claimed in any one of claims 1-5 in which the lengths of the duct sections are measured by a graduated flexible member installed into a duct section whose graduations are read at the two access points. 7 A process as claimed in any one of claims 1-5 in which the lengths of the duct sections are measured by installing a flexible member without preformed graduations into each duct section, marking it to indicate the positions of the access points, withdrawing it and measuring the separation of the markings.

8 A process as claimed in any one of claims 1-5 in which the lengths of the duct sections are measured by a "pig" . provided with an odometer wheel which is passed through the duct sections and the rotation of the wheel counted to determine the duct length. 5 9 A process as claimed in any one of claims 1-5 in which the lengths of the duct sections are measured by a "pig" fitted with a miniature gyroscopic dead-reckoning navigator which is passed through the duct sections and its successive co-ordinates integrated to determine the length of its path 10 between access points.

10 A process as claimed in any one of claims 1-5 in which the lengths of the duct sections are measured by graduations marked on the cable itself.

11 A process as claimed in any one of claims 1-5 in which 15 the lengths of the duct sections are obtained by measuring the installed length of the cable.

12 A process as claimed in claim 9 comprising the step of measuring by reflectometry the length of the fibre, or one or more of the fibres, contained in the cable.

20 13 A process as in claim 1 in which said length dependent optical characteristics comprise optical attenuation. 14 A process for installing an optical cable system with dispersion management substantially as described with reference to Figure 1.

25 15 ■ A process for installing an optical cable system with dispersion management substantially as described with reference to Figure 1 and any one of Figures 2-6.^'