METHOD OF INSTALLING AN OPTICAL FIBRE UNIT IN A TUBE

Abstract: A method is described of installing a lightweight (preferably less than 10 g/m) optical fibre unit into a tube by pulling using a pulling member. The pulling member may be attached to the pulling member by a braided sheath, or the pulling member itself be a braided tube.

Description: Method of installing an optical fibre unit in a tube This invention relateε to a method of installing an optical fibre unit in a tube. The term "optical fibre unit" is used herein to describe an article which comprises a single optical fibre, or a plurality of optical fibres assembled to form a multi-fibre unit. At the present time there are broadly two approaches to the .in situ installation of optical fibre units in tubes. The first, and older, approach is to pull the fibre unit into the tube using an elongate pulling member. Fibre units which are to be installed in this way are heavyweight units/cables, and are provided with a substantial measure of reinforcement, for example by surrounding the fibre or fibre bundles with a rugged sheath, and/or using a steel, Kevlar, or similar strength member. ("Kevlar" is a Registered Trade Mark for a polyarylimide material) . The philosophy behind this approach is that pulling exerts substantial forces on the optical fibre unit, and that such reinforcement is needed to withstand those forces. It must be borne in mind that the installation force within a fibre unit being installed by pulling increases progressively from the trailing end of the unit to the leading end. The theory has been that for the installation force at the trailing end to be sufficient, the force at the leading end must be so high that, in the absence of such reinforcement, the fibre will be damaged by it mechanically, or even if it survives mechanically, its optical properties will be adversely affected. The second approach is to use a lightweight fibre unit and install it using a flow of fluid in a tube, preferably a flow of a gas, and more preferably, a flow of air. Such a 'method is described, for example, in EP-A-108590. Using a fluid has the effect that the propelling force exerted on the fibre unit iε, at least in theory, substantially the same throughout its length, and this force is much less than the maximum force present in a fibre unit which is being installed by pulling. Accordingly, significant reinforcement of the fibre is not required, and indeed would be undesirable since it would increase the weight per unit length of the fibre and hence the fluid drag required to achieve installation. However, although the blowing approach has attractions, and is being successfully used, it has a number of disadvantages. Thus, the distance over which, and speed with which, fibres can be installed iε limited for any given tube diameter by the preεsure drop and flow reεtrictionε imposed thereby. Increasing the tube diameter to reduce these effects means that the tubes or bundles thereof occupy more space in the network and are thus more difficult to manage, particularly when a number of such bundles are present, for example at exchange buildings. Also, it arguably requires a more skilled installer than is needed when pulling is being used, and it requires more sophisticated installation equipment. Furthermore, this equipment is bulky and its use noisy, and it may be unacceptable to a customer to have this equipment brought on to his premises, should the end from which blowing is to take place be on those premises. Indeed, it may be physically impossible to locate the equipment there. Nevertheless, despite these disadvantages, blowing has been used increasingly, at least in the United Kingdom, for optical fibre unit installation. JP-A-63-124005 (Sumitomo Electric Industries, Ltd.) describes a modification of the blowing method of EP-A- 108590 for installing lightweight fibres. In this, in addition to exerting an installation force on the fibres by means of a flow of air, the fibres are also subjected to a pulling force. It is to be noted that the force exerted by the air is evidently subεtantial, since the air pressure quoted is 7 kg/cm 2 , whereas EP-A-108590 stated that a pressure below about 5.6 kg/cm 2 , usually below half that figure, had been found sufficient to install fibre members of up to 3.5 g/m over lengths of 200m. The present invention is based on the discovery that, provided the optical fibre unit is sufficiently lightweight, it can in fact be succesεfully installed substantially only by pulling, even in routes which are long and which include significant bends, because the low weight per unit length has the result that the required pulling force is below that at which the unit iε likely to be damaged, or itε optical properties impaired even at the leading end of the fibre. Accordingly, the fibre unit which is proposed herein to be inεtalled by pulling should not include any reinforcement which would have the effect of εubεtantially increasing the weight of the fibre, though, as explained below, it may have lightweight reinforcement. Thus, according to one aspect of the invention there is provided a method of installing an optical fibre unit in a tube, in which the fibre unit is a lightweight fibre unit, and at least substantially all of the installation force is provided by attaching a pulling member to one end thereof and exerting a pulling force thereon. The use of pulling not only avoids the disadvantages of blowing referred to above, but generally enables a much higher installation εpeed to be achieved than iε achievable under correεponding conditions uεing blowing. The fibre unit preferably has a weight of not more than 10 g/m, and more preferably lesε than 10 g/m, still more preferably not more than 5 g/m, and for a low fibre count of, say, 8 fibres, is preferably not more than 2 g/m. For a 4 fibre unit might be as little as 0.7 to 0.8 g/m. The fibre unit may have a protective sheath thereon, for example of an acrylate curable by ultraviolet light, optionally containing particulate matter as described in EP-A-345968 (incorporated herein by reference) , to which attention is directed for further details. The fibre unit described in EP-A-345968 is particularly advantageous for present purposes, as the surface thereof, which has protuberances therein as a result of the particulate matter, has been found to provide significantly less resilience to pulling than would otherwiεe be the case. Materials other than UV-curable acrylate may, however, be used, for example high density polyethylene. However, the thicknesε of the sheath need be no greater than is required to protect the surface of the optical fibre member and need not be chosen with a view to providing a high tensile strength for the fibre unit. The fibre unit may have no significant reinforcement, but if it does then the reinforcement muεt be such as will not add greatly to the weight per unit length of the fibre unit. One form of reinforcement which may be used is a thread made of Kevlar (Registered Trade Mark) . A εuitable Kevlar thread will preferably not have a weight per unit length of greater than 0.15 g/m, and may have a weight per unit length of aε little aε 0.05 g/m whilεt still providing significant reinforcement. The tube should be one which exerts a low resistance to the fibre unit being pulled through it, and attention is directed to US-A-4892442 for a description of tubes suitable for this purpose, such tubeε having an inner layer which is impregnated with a highly lubricious material. US- A-4892442 (which iε incorporated herein by reference) deεcribes, by way of example, the use for the inner layer of Teflon (Registered Trade Mark for polytetra¬ fluoroethylene) , silicone impregnated polyethylene, and graphite impregnated polyethylene. Where an impregnated polyethylene is used this is preferably a high density polyethylene. It should be noted that rather than using a tube lined with a material impregnated with a lubricious agent, one could use a tube formed of such a material throughout its cross section. In order to insert the lightweight fibre unit into a tube by pulling, it is necessary that the tube should first have a pulling member located therein, and extending along the length of the tube, and that there should be a suitable means for attaching the trailing end of the pulling member to the leading end of the fibre unit. The first of these requirements can be met by manufacturing a tube and then installing the pulling member therein, for example by blowing. However, it is preferred to manufacture the tube with the pulling member therein. Techniques for manufacturing a large diameter tube (typically about 20-25mm internal diameter) with a pulling member therein are already known, and involve sim ltaneously extruding the tube through an annular die and feeding the pulling member through a circular hole whie: the annular die surrounds. Such techniques can read- y be adapted to produce a tube/pulling assembly of a size .-.ore appropriate to the installation of a single light aigh optical fibre unit, say one having an internal diameter of from 1 or 1.5mm to 10mm. It is to be noted, however, that the fibre unit would normally have a diameter of fror 0.5mm to 1.5mm, though somewhat larger for high fibre ccunts (e.g. 19-fibre unitε) , and that the internal diameter of the tube need not exceed the outside diameter of the fibre unit by as large an amount as is needed when blowing is used. In blowing a fibre unit, a relatively large clearance is necessary, at least partly because of the need to ensure that there is an adequate fluid flow path along the whole tube. There are no such constraints when a fibre unit is being pulled. By way of example, an optical fibre unit having an external diameter of 1.1mm should be capable of satisfactory installation by means of the invention in a tube having an internal diameter of 3.5mm or less, say as little as about 2.5mm, over distances of at least 1000m. The second of the above requirements, for a suitable meanε of attaching the pulling member to the fibre unit, can conveniently be met by a braided sheath. Braided sheaths are already known for connecting large diameter cables to pulling members, but such known sheaths are far too large to be suitable for present purposes. However, small diameter braided sheaths are widely available for an entirely unrelated purpoεe, namely leaderε aε in fly fishing, and it has been surpriεingly found that εuch leaderε can be used, without modification, in the method of the present invention. In their simplest form, the material for such leaders is supplied to the user as a continuous reel, and the required length is cut off as required. For present purposes, a length of about 40 to 90mm is εufficient. The ends of the pulling member and optical fibre unit are each inserted in a respective end of the braided sheath, and when tension is exerted in a direction tending to pull the pulling member and optical fibre unit away from one another and out of the sheath, the friction between the pulling member and the εheath and between the fibre unit and the εheath tends to elongate the sheath and thereby cause it to contract and grip the ends. However, friction by itself is not normally sufficient, and additional means, for example a small quantity of adheεive, preferably a rapidly setting adhesive such as a cyanoacrylate, may be used to ensure an adequate grip. Preferably, the length of pulling member inserted in the sheath, and the length of optical fibre unit inserted in the sleeve, is equal to at least 20 times its own diameter. However, fly fiεhing leaderε are alεo available as individual sheaths which have one open end and one end formed into a loop. Such leaders are particularly convenient for the purposes of the present invention in conjunction with fibre units which have one or more threads of Kevlar or other material therein. The pulling member can be inserted into the open end, and the Kevlar can be tied to the loop. This ensureε that εubstantially the whole of the tenεion in the fibre unit iε carried in that thread or threads, thus further reducing the risk of damaging the optical fibre member (or members) which remains εubstantially untensioned. Some optical fibre units are provided with a rip cord, and in such cases the end of the rip cord may be attached to the braided sheath for the purpose of pulling. In the accompanying drawings: Figures 1 and 2 εhow diagrammatically, and not to scale, the method of the present invention aε carried out uεing the two forms of sheath described above, and a monofilament pulling member; and Figure 3 shows the route used in one of the tests, described herein, of the method of the invention. In Figures 1 and 2 the fibre unit is denoted by reference numeral 1, the braided sheath by reference numeral 2, a monofilament pulling member by reference numeral 3, the tube by reference numeral 4, and the Kevlar thread (Figure 2 only) by reference numeral 5. The invention haε been deεcribed above in relation to the pulling of a single fibre unit into a single tube. However, it can also be used to pull simultaneously a plurality of units into a εingle tube, a plurality of units into a corresponding plurality of tubes, or a plurality of units into a leεεer plurality of tubeε. In each of theεe cases, the leading ends of the pulling members of the fibre units can be pulled by a single pulling apparatus (e.g. a drum) , for which purpose the leading ends of the pulling members may be attached to one another. It should also be noted that, as a modification of what is described above, the entire pulling member may be a braided tube, rather than having just a short length of braided tube acting as a connector to some other form of pulling member. Also, a flow of air or other gas might be used to supplement the effect of the pulling member. This could be achieved, for example, using a simple gas bottle or co presεor connected to the tube with a seal around the fibre unit and the tube. A very low volume of gas, e.g. 0.3 l/min would be used, so that the gas and fibre unit were travelling at more or less the same linear speed (say 28 m/min) . In εuch a caεe the gaε flow iε not intended to provide any substantial part of the installation force, unlike the method described in JP-A-63-124005 referred to above. Some examples will now be given of installations carried out by means of the method of the preεent invention. Example 1 A lightweight fibre unit having a weight of about 0.78 g/m, a diameter of about 0.93mm and a structure as illustrated in EP-A-345968 (a εingle buffered fibre including an outer coating of UV-cured acrylate with protuberances therein produced by particles in the form of hollow glass microεphereε) , waε inεtalled in a tube having an internal diameter of 3.5mm. The tube had an internal layer of a highly lubriciouε material, aε described in US- A-4892442. The inner surface of the tube was smooth. US-A- 4892442 describes, as one option, the use of a tube with ribε on the inner εurface, but we prefer not to use such ribε where, aε here, the fibre unit has protuberances on its outer εurface. The tube, which had a pull cord formed therein at the time of manufacture, ran twice around the internal wall of a room 4m x 2m, with bendε 48mm in radius at each corner, i.e. it waε just under 24m in length. The fibre unit was initially contained in a fibre pan, and its free end was connected at one end of the tube to one end of the pull cord. The other end of the pull cord was attached to a pulling device, which pulled the pull cord from the tube, thereby pulling the fibre unit into the tube. This operation was carried out at three different pulling speeds, and the maximum tension occurring during each of the three pulls waε aε follows: Pulling speed Tension 28 m/m 225 g 15 m/m 160 g 5 m/m 100 g All three tension values were well below the breaking tension of the fibre unit, and well below the value at which the tension might begin to adversely affect the optical and other properties of the fibre unit. Example 2 A longer length of the same type fibre unit as that used in Example 1 waε inεtalled in two longer lengths of the same type of tube as that uεed in Example 1. The inεtallation route iε shown diagrammatically in Figure 3 of the drawings. The lines represent the two tubes. They are shown as being in a spiral formation, but that is purely for ease of illuεtration, and the tube runε along each of the four εides of the room in which the tubes were located were in fact vertically above one another. The longer εideε of the room were each 47m in length, and the shorter sides were 20m. The first tube ran from point C to point B,, and the second tube ran from point B 2 , adjacent B,, to point A. At the corners of the room the tubes went through bends having a radius of curvature of 250mm. A length of the fibre unit of at least about 570m was coiled in a pan, and one end thereof was attached at A to the pull cord in the first tube. A pulling device was attached to the other end of the pull cord at B 1r and pulling was effected until the whole of the pull cord had emerged from the first tube followed by about 300m of fibre unit. The pull cord was allowed to fall on the floor, and disconnected from the fibre unit to which it was attached, but the emerging fibre unit was coiled in a second pan. The operation waε carried out at a εpeed of 33 m/m, taking about 8 minuteε. The final pull tension (the tension increases as more fibre enters the tube, so thiε was the maximum tension) was 120 g. The second pan, with its 300m of fibre unit therein was then turned over, to expoεe the leading end of the fibre unit. Thiε waε then attached at point B 2 to the pull cord in the second tube. The pulling device was attached to the other end of the εame pull cord, at point A, and pulling waε carried out until all the fibre unit in the second pan had been pulled into the second tube, with about 30m protruding from the tube at point A. This pulling was carried out at 30 m/m, and the final tension was 120g. On completion there was a continuous length of about 540m of fibre unit in the first and second tubes, running along a route C, B 1f B 2 , A, with the fibre unit extending straight across from B 1 to B 2 . The gap between B 1 and B 2 was then closed with a closure member, so that the fibre unit was completely enclosed along itε whole length. By way of a teεt, the pulling device then began to pull the 540m length of inεtalled fibre unit out of the combined tube lengths at 27 m/m. The initial tension (and therefore the maximum tension) was found to be 400g. Example 3 The same light fibre unit and tube used in Examples 1 and 2 were used in a route which ran a total distance of 1055m. A single tube extended from a pan at one end along a 635m length sloping downwards at 3°, then through a 90° bend having a radius of curvature of 600mm, and finally uphill at 3° for 420m to a pulling device. Pulling was carried out for most of the pull at 28 m/m, and took a total of 43 minutes. The following table gives the pulling speed and tension for various distances of the leading end of the fibre unit down the tube. DIST (m) SPEED m/m Tension g 300 28 980 600 27 1183 900 26 1250 1000 26 1260 1030 15 916 • 1055 7/0 716 The notation 7/0 againεt 1055m denoteε the fact that over the laεt metre the εpeed fell from 7 m/m to zero.

Claims: CLAIMS : 1. A method of installing an optical fibre unit in a tube, in which the fibre unit is a lightweight fibre unit and at least substantially all of the installation force is provided by attaching a pulling member to one end thereof and exerting a pulling force thereon. 2. A method according to claim 1, wherein the fibre unit has a weight of not more than 10 g/m. 3. A method according to claim 1, wherein the fibre unit has a weight of not more than 5 g/m. 4. A method according to claim 1, wherein the fibre unit has a weight of not more than 2 g/m. 5. A method according to any preceding claim, wherein the fibre unit is not reinforced. 6. A method according to any one of claims 1 to 4, wherein the fibre unit iε provided with reinforcement which doeε not contribute more than 0.15 g/m to the weight of the unit. 7. A method according to any preceding claim, wherein the tube haε the pulling member pre-inεtalled, by forming it therein as the tube iε manufactured. 8. A method according to any preceding claim, wherein the leading end of the fibre unit is attached to the trailing end of the pulling member by a braided sheath. 9. A method according to claim 8, as dependent on claim 6, wherein the reinforcement is provided by a reinforcement member which is used aε the meanε for attaching the fibre unit to the braided sheath. 10. A method according to any one of claims 1 to 7, wherein the pulling member is a braided tube. 11. A method according to any preceding claim, wherein air is caused to flow along the tube during installation. 12. A method according to claim 11, wherein the linear velocities of the air and fibre unit are substantially the same. 13. An optical fibre transmission path made by installing an optical fibre unit in a tube by the method of any preceding claim.

Inventors: BROWN GEORGE HENRY PLATT, TANSEY JOHN, BROWN, GEORGE, HENRY, PLATT, TANSEY, JOHN
Applicants: BICC PLC, BROWN GEORGE HENRY PLATT, CORNING LTD, MAINETTI UK LTD, TANSEY JOHN, BROWN, GEORGE, HENRY, PLATT, CORNING LIMITED, MAINETTI (UK) LIMITED, TANSEY, JOHN
Application year: 1996
Main classification: G02B6
IPC codes: G02B6