METHOD AND APPARATUS FOR BLOWING AN OPTICAL FIBRE MEMBER

Abstract: An apparatus is provided for introducing at least one optical fibre member (6) into a tubular passageway. The apparatus comprises a pressure vessel (1) for receiving the optical fibre member (6), the vessel (1) either containing compressed air or other gas or being provided with an inlet (2) by means of which compressed gas can be introduced. The vessel is connected to the passageway via a fibre-introducing device, e.g. a venturi (11) or a mechanical device, which creates slack in the optical fibre member.

Description: This invention relates to a method and apparatus for blowing an optical fibre member. A method is known from EP-A-108590 for installing an optical fibre member in a tubular passageway using a flow of air which travels along the tubular passageway in the desired direction of travel of the optical fibre member. For this purpose the optical fibre member comprises an outer sheath surrounding one or more optical fibres, the sheath being of low density so as to facilitate its being carried by the flow of air. One feature of the method and apparatus described in EP-A-108590 is that the optical fibre member is introduced into the tubular passageway through a pressurised blowing head via a seal, which must allow the fibre member to pass through freely and must minimize the escape of air. To introduce the optical fibre member it passes between a pair of drive wheels located within the blowing head. The drive wheels engage the optical fibre member in the nip between the wheels and impart a longitudinal force to the optical fibre member in the direction of introduction. An object of the present invention is to provide a method and apparatus for introducing one or more optical fibre members into a tubular passageway which avoids the need for a seal around the fibre member and eliminates rearward escape of air and reverse viscous drag on the fibre downstream of the wheels. This is particularly advantageous in the case of optical fibre members which have only a single optical fibre in a sheath, since such members are so lacking in rigidity that applying an effective longitudinal force to them by means of drive wheels is difficult, e.g. optical fibre members in which the product of Young's modulus and the second moment of area, a product normally denoted as EI, is of the order of 10⁻⁶ Nm². According to the present invention there is provided an apparatus for installing at least one optical fibre member into a tubular passageway, comprising a pressure vessel for containing a supply of the optical fibre member, the vessel either containing a gas under super-atmospheric pressure or being provided with an inlet by means of which such gas can be introduced; means defining a path for the optical fibre member and gas to travel from the vessel into the tubular passageway; and a venturi for exerting viscous drag force on the optical fibre member to create slack in the optical fibre member as it travels along the said path. Two embodiments of the invention are shown in the accompanying drawings, in which: Figures 1 and 2 are diagrammatic views of the embodiments The embodiment shown in Figure 1 comprises a vessel 1 capable of withstanding a considerable pressure of pressurised air or other gas therewithin. Typically, the pressure in the vessel might be in the region of 150 psig. An inlet 2 is provided for introducing compressed air of low moisture content into the vessel. The inlet is provided with a valve (not shown) to enable it to be closed. The vessel 1 has an outlet tube 3 which is formed Integrally with the remainder of the vessel and which is connected via suitable connectors 4 to a flexible armoured hose 5. The hose 5 is connected via connectors 10 to a venturi tube 11 having an internally convergent portion 11a, a constant diameter portion 11b, and a divergent portion 11c. The venturi tube 11 is connected via connectors 12 to a further flexible armoured hose 13 which leads via a valve 7 and connectors 8 to the end of a tubular passageway 9 into which the optical fibre member is to be introduced. The hoses 5 and 13 have the same internal diameter as the tubular passageway 9. The vessel 1 contains the entire length of optical fibre member 6 which is to be introduced into the passageway 9. For simplicity of illustration only a small portion of the optical fibre member is shown, but it is to be understood that in practice the proportion of the volume of the vessel filled with optical fibre member is likely to be considerably greater than is shown. In Figure 1 the optical fibre member is shown as being randomly deposited in the vessel. Alternatively, however, the optical fibre member could be held on a reel, as is illustrated in the case of the embodiment shown in Figure 2, or in alternative formats to control tension and torsion during payout. In one method of operation of the embodiment shown in Figure 1 the leading end of the optical fibre member 6 is threaded manually through the tube 3, connectors 4, hose 5, connectors 10, venturi 11, connectors 12 and hose 13 as far as the valve 7, with the connectors 10 and 12 at this stage being disconnected to allow this to occur. The connectors 10 and 12 are then connected in air-tight manner and compressed air is introduced into the vessel 1 through the inlet 2. Valve 7 can be either closed or open, and while compressed air is being initially introduced this valve is kept shut so as to allow the pressure within the vessel to increase. When a sufficient pressure has been achieved within the pressure vessel the valve 7 is opened and air can pass at a relatively high initial speed from the vessel 1 into and through the passageway 9. The drag force exerted thereby on the optical fibre member propels it along the passageway, and the process continues until all except the trailing end portion of the optical fibre member 6 has passed into and along the passageway 9. During this process the compressed air supply can be maintained so as to keep the pressure within the vessel at a level sufficient to cause the optical fibre member to move at the required speed and for the required distance. However, provided there is sufficient air at sufficient pressure within the vessel 1 it may not be necessary to maintain the compressed air supply, since as long as the pressure within the vessel is significantly above atmospheric the optical fibre member will continue to be propelled along the passageway 9. The use of the flexible hoses 5 and 13 provides flexibility in locating the vessel 1 while the apparatus is being prepared for blowing. The effect of the venturi is to exert a greater fluid dynamic force on the optical fibre member 6 as it passes through the venturi than it experiences once it enters the passageway 9. This has the result that there is slack in the optical fibre member upstream of the passageway, which is advantageous in terms of conveying the fibre. It is to be understood that this slack is continuously provided as the optical fibre member enters the passageway, not just provided to the leading end portion thereof. The embodiment shown in Figure 2 comprises a vessel 21 which, like the vessel 1 in Figure 1, is designed to withstand a high internal cyclic pressure. As in Figure 1, a flow of air from the vessel 21 is used to propel an optical fibre member 6 into and along a passageway 9. In Figure 2 the optical fibre is shown as being held on a reel 22, though it could be randomly deposited as in Figure 1. The pressure vessel 21 is connected to the passageway 9 via a side-arm tube 23, to which a supply of compressed air is connected, and a venturi arrangement 24. Because the passageway 9 is of small cross-section, and therefore resists flow of air from the blowing head 10 into its upstream end, when the compressed air supply is turned on the pressure in the vessel 11 rises to substantially the pressure of the supply and stays at that pressure. The arrangement 24 comprises a venturi 25, similar to the venturi 11 in Figure 1, and a venting device 26 by means of which a substantial part of the air entering the tube 23 is vented rather than passing down the passageway. The illustrated venting device 26 comprises an air-permeable tube 27 and an air outlet 28. Further details of venturi devices can be found in our European patent specification EP-A-0318280. Using any of the illustrated embodiments more than one optical fibre member may be blown simultaneously from a single vessel in a twisted or untwisted (layless) arrangement. Alternatively a plurality of optical fibre members may be blown simultaneously into a passageway from a corresponding plurality of vessels, by introducing them into the passageway through a manifold. An option is to use one long length stored within the vessel to complete more than one installation, each in a different passageway. A further option entails pulsing the air flow, for example by switching the air supply or by venting, in order to assist in moving the fibre member. Yet a further option is to provide mechanical vibration of the upstream end of the passageway 9, or fibre member, to further assist in moving the fibre member. The invention as described above has a number of advantages over the blowing technique of EP-A-108590 in addition to those already indicated. Thus, the need for an electrical power supply to the blowing head is eliminated, making it more suitable for work in hazardous areas, e.g. flammable atmospheres. The vessels which contain the fibre member provide protection for it, provided the pressurised air supply is clean, from mechanical and environmental hazards during transport and installation. Also the installation method can be relatively quiet. Further, there need be no moving parts. Other advantages derive from the fact that the devices used for introducing the fibre member into the passageway derive force from the viscous drag of air over the entire surface of the fibre member. Because of this the force is low and uniform and is maintained regardless of speed of the member. Thus the introducing device is tolerant of changes of speed and only operates at low stress in either tension or compression. There is, of course, a limit to the length of fibre member which can be blown by the apparatus described above. This limit will vary depending on the precise characteristics of the apparatus and of the fibre member being blown. For convenience of description let us suppose that the limit in particular circumstances is 1 km. If it is desired to blow a longer length of fibre member this can be achieved by having an appropriate number of blowing apparatuses spaced at intervals along the intended path of the fibre member at 1 km intervals. Thus, for example, to install a fibre member 5 km in length one would have five such apparatuses, one at the beginning and the other four spaced at 1 km intervals therefrom. In addition, four fibre receiving units are required, for example in the form of fibre coiling machines, one associated with each of the four blowing apparatuses excluding the one at the beginning. The way in which the above mentioned 5 km length of fibre member is blown is as follows. First, the blowing apparatus at the beginning is used to blow the fibre member into and through the first 1 km section of passageway extending from the first blowing apparatus to the location where the second blowing apparatus is situated, so that 4 km of fibre emerges from the downstream end of this section of passageway and is taken up as it emerges on the coiling machine situated there. This 4 km length of fibre member is then placed in the vessel of the second blowing apparatus, which not only has a fibre outlet (corresponding to the outlet tube 3 of Figure 1) but also a fibre inlet. At the inlet there is a seal to ensure that the required pressure can be maintained in the vessel, and the free end of fibre member is introduced into the fibre outlet. The 4 km length of fibre member is then blown from the vessel of the second blowing apparatus so that 3 km of fibre member arrives at the location of the third blowing apparatus. This process is repeated with 2 km of fibre member reaching the fourth blowing apparatus, 1 km reaching the fifth blowing apparatus, and the leading end of the fibre member reaching the end of the 5 km path. It will be appreciated that the process just described will result in the fibre member being threaded through the vessels of all the blowing apparatuses except that at the beginning. To enable these apparatuses to be removed for reuse, each vessel is preferably split in a suitable manner. For example, each vessel can be formed of a container and a removable fitted lid, with the fibre inlet and fibre outlet both being formed in the lid, and the lid can be split into two parts along a plane passing through the fibre inlet and fibre outlet.

Claims: An apparatus for installing at least one optical fibre member (6) into a tubular passageway (9), comprising a pressure vessel (1, 21) for containing a supply of the optical fibre member, the vessel either containing a gas under super-atmospheric pressure or being provided with an inlet (2) by means of which such gas can be introduced; means (3, 4, 5, 10, 12, 13) defining a path for the optical fibre member and gas to travel from the vessel into the tubular passageway; and a venturi (11, 24) for exerting viscous drag force on the optical fibre member to create slack in the optical fibre member as it travels along the said path. An apparatus according to claim 1, comprising a reel (22) disposed within the pressure vessel, on which reel the optical fibre member is held. An apparatus according to either preceding claim, wherein the said gas is air. An apparatus according to any preceding claim, comprising a reel disposed within the pressure vessel, on which reel the optical fibre member is held. An apparatus according to any preceding claim, wherein the path-defining means comprises at least one flexible member (5, 13).

Inventors: DITCHFIELD, NEVILLE JAMES, HANDLEY, GRAHAM ROBERT, RADCLIFFE, RONALD BYROM
Applicants: BICC PUBLIC LIMITED COMPANY, CORNING LIMITED
Application year: 1988
Main classification: G02B6
IPC codes: G02B6