WO2005109060A1

WO2005109060A1 - Network of tubes for optical fibres

Info

Publication number: WO2005109060A1
Application number: PCT/GB2005/001696
Authority: WO
Inventors: George Henry Platt Brown
Original assignee: Emtelle Uk Limited
Priority date: 2004-05-11
Filing date: 2005-05-06
Publication date: 2005-11-17

Abstract

A network of tubes (18, 19) for accommodating one or more optical fibres is disclosed. The internal diameters of the tubes are substantially equal, and not all of the external diameters are substantially equal. At least one of the tubes includes a respective layer of material having a different wall thicknesses to that of another tube.

Description

1 NETWORK OF TUBES FOR OPTICAL FIBRES

The present invention relates to a network of tubes for accommodating one or more optical fibres or optical fibre cables. The invention relates particularly, but not exclusively, to a network of such tube's for providing services such as telecommunications, internet and the like, to subscribers.

Optical fibre cables carry data at very high speeds and as the demand for broadband' internet access grows, there is an increasing requirement for optical fibre cables providing high speed connection to be deployed directly into individual homes or business premises.

Increasingly, optical fibre networks are constructed by setting up a network of tubes, and then subsequently installing the optical fibre cables into the tube network, usually by means of a combination of blowing and . pushing. The tubes by means of which the optical fibre network is constructed are generally assembled into bundles so that they can be more easily managed and installed. An example of such a tube bundle 50 containing 24 tubes 52 is shown in Figure 1.

Traditionally, optical fibre cables have been installed into relatively large diameter tubes, typically having external diameters of between 40mm and 50 mm and internal diameters of 33 mm and 42 mm respectively. The optical fibre cables normally contain a relatively high number of individual optical fibres, typically between 96 and 288, and in some cases, even more. Such optical fibre cables are generally used in the trunk network to interconnect cities and also to provide distribution within cities. It is to be noted, however, that the number of optical fibres which needs to be provided to each individual- business premises or home is relatively small. In fact, it is often possible to provide adequate high speed services to an individual home using just one optical fibre, although the more usual option would be to provide two optical fibres. In view of this demand, miniature optical fibre cables have been developed, which contain relatively small numbers' of fibres, and which can be installed into a network of smaller tubes called micro- ducts. Micro-ducts typically have external diameters of between 3mm and 8mm and an internal diameter of 2.1 mm and 6 mm , as opposed to external diameters of between 40mm and 50mm and internal diameters of 33 mm and 42 mm typical of ducts for traditional optical fibre tubes.

Generally small tubes can be manufactured at much higher .line speeds than large tubes. This is because the smaller the tube, the thinner the wall section necessary to develop a given compression strength. The major factor which determines line speed is the wall thickness because this controls the rate at which the tube cools after extrusion. Therefore apart from the cost benefit of using less raw material there is the further benefit of higher line speeds.

In constructing the network, it might be the case that one or more tube bundles containing twenty four tubes as shown in Figure 1 are installed by the service provider. It is usually the case that one or more tubes are branched out at various points along the route, and connected to a branch tube, for example, to feed various premises. A typical branch tube is shown in Figure 2. o Both the tubes 52 of the tube bundle 50 of Figure 1 and the tube 54 of the branch tube 56 have the same dimensions. The tubes are protected in both cases by an outer sheath. The outer sheath is typically constructed from three layers. First there is typically an aluminium foil layer 58. This serves two purposes. First it acts as a water barrier layer and second it protects to some extent the tubes from the heat of the second 60 and third 62 layers. The second 60 and third 62 layers typically comprise a thermoplastic polyethylene applied by passing the tube_, or- tube bundle through a heated die and extruding the polyethylene over the top of the assembly.

This is applied in two thin layers so that the heat • dissipates quickly and does not have chance to distort the tubes. The branch tube 54 of Figure 2 is much more prone to damage in this respect because it is completely surrounded by the polyethylene of the second 60 and third

62 layers whilst in the case of the tubes of the tube bundle of Figure 1 there is only point contact.

At the branch points the tube 52 from the bundle 50 of Figure 1 needs to be cut and connected to a branch tube, to form a tube joint and to facilitate this the outer sheath of the tube bundle and the tube are removed at this point. Each individual tube, is generally ^'connected to a branch tube using a pneumatic tube connector. In order to make this connection, the outer protective sheath of the tube bundle and the branch tube has first to be removed. It is therefore necessary to fit a protective cover around the connection to provide the protection, which has been lost as a result of the removal of the outer protective sheath. A typical arrangement of such a tube joint including a protective cover, forming part of a tube network, is shown in Figure 3. As can be seen from Figure 3, the tube 1 from the bundle 2 is connected to a branch tube 3 using a connector 4. The joint, and in particular those parts of tubes 1 and 2 from which the outer protective sheath has been removed, are protected by means of protective cover 5.

Once a tube network is complete, optical fibres are installed, often by means of blowing down the individual tubes, to provide an optical fibre connection to each of the individual premises.

Tubes for accommodating optical fibres are generally manufactured to a certain specification such that they possess the requisite qualities to enable easy management of the tube network. In particular, the blowing performance of optical fibre cables is affected by the internal diameter of the tubes into which they are inserted. Generally, the larger the internal diameter of the tube, the greater the air flow and the greater the distance over which the optical fibre cable can be installed by blowing. It is traditionally desirable therefore to have the internal diameter of the tubes as large as possible. It is traditionally equally desirable to keep the external diameter of the tubes as small as possible so as to reduce the overall diameter of the tube bundles. In using tubes having a small external diameter, it is easier to install tube networks in already congested tube networks within cities.

In view of the above, tubes are often manufactured having large internal diameters and small external diameters, resulting in tubes tending to have thin walls. A selection of typical tube dimensions currently employed is listed in Table 1 below.

Table 1.

It is unusual for tubes with different outside diameters to be deployed in the same tube network. The occasions when this does happen generally relate to relatively old networks where the network was originally constructed using for example tubes with an outside diameter of 8 mm but as a result of technical developments over the years it is found that a 5mm tube can be successfully used. The smaller outside diameter tube offers cost benefits creating a desire to use the smaller tube to further extend the original network.

Accordingly, when a tube with a large external diameter is connected to a tube having a small external diameter , a connector such as that shown in Figure 4 is generally used in the network. The connector 5 includes an angled section 6 which tapers from the internal diameter of the tube 7 from the tube bundle to the internal diameter of the branch tube 8.

Technical developments improving the blowing performance of fibre optic cables and tubes have created a trend towards increasingly small tubes and in particular tubes with an outside diameter of 3 mm and an internal diameter of 2.1 mm. These tubes having a ratio of internal to external diameter greater than 0.7 perform very satisfactorily when assembled for example into a bundle containing 24 tubes.

However 3 mm branch tubes which connect to 3 mm tube bundles containing for example 24 tubes are difficult to manufacture. The 3 mm tube is much more easily adversely affected by the heat from the process of applying the outer polyethylene protective layers.^'

A second disadvantage with this method of manufacturing branch tubes is that the branch tube is much more flexible than a tube bundle containing 24 tubes. Small tubes for example 3 mm tubes, having relatively thin walls are easily distorted, as the flexible branch tubes are bent around tighter radii than might be possible with a tube bundle containing a larger number of tubes.

It is desirable therefore for the branch tubes to have a larger external diameter than the tubes of tube bundles containing a plurality of tubes . However, in tube networks containing such tubes, at the locations where a tube 7 is connected to a smaller tube, such as is shown in- Figure 4, a pressure drop tends to occur when blowing the optical fibres in a direction from left to right as shown in Figure 4, since the internal diameter of the tube 7 is greater than that of the smaller tube 8. Such a pressure drop can impede the blowing performance, resulting in . less efficient installation of optical fibres into the tube network when blowing techniques are used. Such a pressure drop can also be problematic in that only uni- directional installation of optical fibres by means of blowing is possible, that is, blowing in a direction from a larger tube to a smaller tube. This reduces the flexibility of any installation process.

Preferred embodiments of the present invention seek to overcome the above disadvantages of the pri'or art.

According to the present invention there is provided a network of tubes for accommodating one or more optical fibres, said tubes each having a respective predetermined internal diameter and a respective predetermined external diameter, wherein said internal diameters of said tubes are substantially equal, and not all of said external diameters are substantially equal, wherein at least one said tube includes a respective layer of material having a different wall thicknesses to that of another said tube.

This provides the advantage that pressure drops throughout the network are substantially eliminated or at least reduced. In at least reducing pressure drops within the network, blowing performance is improved when optical fibres are installed in the tube network, in that optical fibres or optical fibre cables can be installed over greater distances.

The further advantage is provided that bi-directional blowing is facilitated, as opposed to uni-directional blowing only, which is the case with known networks which suffer from pressure drops at various locations throughout the network. By providing a branch tube which has a larger outside diameter, for example 5 mm, and the same size internal diameter, for example 2.1 mm, as a (typically) 3 mm tube of the tube bundle, the network can be extended without problems of blowing performance. It has previously been considered by persons skilled in the art that this solution would be unattractive because tubes with thicker wall sections are slower and therefore more expensive to manufacture. However it has been unexpectedly found that such tubes are significantly better in terms of not distorting when outer protective layers are applied by extrusion. In particular a 5 mm outside diameter tube with an internal diameter of 2.1 mm is sufficiently resistant to deformation and it has been found that the external protective coating can be applied in a single layer instead of two layers. The process involves first wrapping the tube with the aluminium foil layer and then applying a single polyethylene outer protective layer. This cost saving benefit reduces the capital investment required and reduces the operating costs required for manufacturing a suitable branch tube.

A further benefit is that the branch tube of the invention is significantly stronger. Looking at the relative dimension of the tube bundle of fig 2 and the b ranch tube of fig 3 it is clear that the branch tube can be easily bent around much tighter radii than the tube bundle. If the wall thickness of the tube of the branch tube is thin then it is much more likely to deform or collapse as the branch tube is bent. This is not the case for the branch tube of the invention.

Such a network therefore provides the advantage that tubes which provide the optimum commercial benefit both in terms of their cost of manufacture and in terms of the amount of space they occupy in the duct network can be used for the main body of the network in tube bundles containing a plurality of tubes and tubes having the same or similar internal diameters but different external diameters may be used for example to provide suitable single branch tubes which are easier and cheaper to manufacture and more resistant to damage.,

By way of example of a typical thick-walled tube, a tube could be provided having an external diameter of 5mm and an internal diameter of 2.1mm. By way of further example of a typical thick-walled tube, a tube could be provided having an external diameter of 8mm and an internal diameter of 3.5mm.

In a preferred embodiment, the network comprises at least one connector for connecting together at least two said tubes, said connector comprising: at least one first aperture for receiving a first respective said tube; and at least one second aperture for receiving a second respective said tube.

At least one said connector may comprise at least one first tubular element defining a respective said first aperture; at least one second tubular element defining a respective said second aperture; and at least one tubular connecting element between the first and second elements for connecting a said first element to a said second . element .

At least one said connecting element may comprise a tubular member having an internal diameter substantially' equal to the internal diameter of the tubes. By providing a connecting element which is tubular and which has an internal diameter substantially the same as the internal diameter of the tubes, no internal stops are created inside the connector upon which optical fibres or optical fibre cables may catch as they are blown into the tubes .

A preferred embodiment of the present invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings in which: -

Figure 1 is a schematic cross sectional view of a known tube assembly;

Figure 2 is a schematic cross sectional view of a known branch tube;

Figure 3 shows a first known tube joint forming part of a known network;

Figure 4 shows a second known tube joint forming part of a known network;

Figure 5 shows a tube joint forming part of the network of the present invention; and

Figure 6 shows a tube network embodying the present invention.

With reference to Figure 5, a tube joint is facilitated by a connector 10. The connector 10 forms part of a tube network 100 as shown in Figure 6. The connector 10 may be a pneumatic connector and comprises a first aperture defined by a first tubular element 11, a second aperture defined by a second tubular element 12, and a tubular connecting element 13 located between the first 11 and second 12 tubular elements. The first tubular element 11 comprises first and second concentric tubular sections 14 and 15 respectively, wherein the first tubular section 14 is of a larger diameter than the second tubular section 15. Similarly, the second tubular element 12_. comprises first and second concentric tubular sections 16 and 17 ^' respectively, wherein the first tubular section 16 is of a larger diameter than the second tubular section 17.

A first, thick-walled tube 18 may be inserted into the first tubular element 11 of the connector 10, and a second, thin-walled tube 19 may be inserted into the second tubular element 12 of the connector 10. In this way, a joint is formed between the two tubes 18 and 19 and a through-hole 20 is formed between the two elements 11 and 12 by way -of the connecting element 13. By way of example, a thin-walled tube may have an internal diameter which is 0.7 times its external diameter, and a thick- walled tube may have an internal diameter which is 0.44 times its external diameter. However, the internal diameters of the tubes 18 and 19 are the same.

As can be seen from Figure 5, the internal dimensions and in particular the internal diameter of the first tubular section 15 are the same as the external diameter of the thick-walled tube 18. Similarly, the internal dimensions and in particular the internal diameter of the first tubular section 17 are the same as the external diameter of the thin-walled tube 19. In this way, the tubes 18 and 19 are snugly held within the connector 10. In order to provide an improved seal between the tubes and the connector 10, four 0-rings (of which only two are shown) 21 are provided inside each of the tubular sections 14 and 16.

In order to ensure that, when optical fibre cable is blown through the tubes, it is presented with a clear through-path, the tubular connecting element 13 has an internal diameter the same or similar to the internal diameters of the tubes 18 and 19, and the tubular connecting element 13^' is concentric with each of the tubular elements 11 and 12 and the interiors of the tubes 18 and 19. In this way, the optical fibre cable (not shown), when blown through the connector 10, follows a clear path, and there are no internal steps created inside the connector, caused by either a part of the connector itself or either tube, upon which the optical fibre cable can become lodged. Moreover, there is no pressure drop created anywhere in the network, since the internal diameters of all of the tubes in the network are substantially the same. By eliminating pressure drops within the network, blowing performance is significantly improved.

In using such a network, one example of which is described above, tubes may be connected together, to form tube joints, with substantially no pressure drops occurring within the network.

It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only, and not in any limitative sense, and that various alternatives and modifications are possible without departing -from the scope of the invention as defined by the appended claims. In particular, it is to be appreciated that the connector may be adapted to facilitate connection between a plurality of tube pairs.

In this case, the connector would include a plurality of pairs of tubular elements and a plurality of connecting elements . It is also to be appreciated that the network may include tubes having large internal diameters, that is, larger optical fibre tubes as opposed to micro-ducts, as long as the internal diameters of each tube within the network is the same. It is further to be appreciated that the present invention is not limited to a network whereby branch tubes are connected to tubes forming part of a bundle, but may also comprise branch tubes connected to other branch tubes, or a tube from a tube bundle connected to another tube from a tube bundle.

Claims

1. A network of tubes for accommodating one or more optical fibres, said tubes each having a respective predetermined internal diameter and a respective predetermined external diameter, wherein said internal diameters of said tubes are substantially equal, and not all of said external diameters are substantially equal, wherein at least one said tube includes a respective layer of material having a different wall thicknesses to that of another said tube.

2. A network as claimed in claim 1, further comprising at least one connector for connecting together at least two said tubes, said connector comprising: at least one first aperture for receiving a first respective said tube; and at least one second aperture for receiving a second respective said tube.

3. A network as claimed in claim 2, wherein at least one said connector comprises at least one first tubular element defining a respective said first aperture; at least one second tubular element defining a respective said second aperture; and at least one tubular connecting element between the first and second elements for connecting a said first element to a said second element.

4. A network as claimed in claim 3, wherein at least one said connecting element comprises a tubular member having an internal diameter substantially equal to the internal diameter of the tubes.

5. A network of tubes for accommodating one or more optical fibres, the network substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.