Heat-shrinkable tube for protection of optical fiber splices

Abstract: A packaging assembly for protecting splices of optical fibers comprises an inner heat-shrinkable tube (1), an outer thinner heat-shrinkable tube (5) and a reinforcing rod (3). The reinforcing rod has a metal layer (7) on a flat surface thereof. When heating the assembly for applying it tightly to a splice an electrical current is passed through the metal layer. Then heat is developed which causes the tubes to shrink. The metal layer can be applied to have a larger resistance in its central region where it then will be heated first, the shrinking of the tubes then also starting in the central region, as seen in the longitudinal direction of the tubes. The metal layer (7) can be made to act like an electrical fuse, being evaporated when the heat is sufficiently intensive and then automatically interrupting the electrical current.

Description: TECHNICAL FIELD The invention is related to protecting splices of optical fibers, in particular to heating a heat-shrinkable tube in order to make the tube shrink around a splice of optical fibers which is to be protected, and more particularly to an assembly including a heats shrinkable tube intended to be used for protecting splices of optical fibers. BACKGROUND Optical fibers are presently widely used for communicating information such as in large telecommunication systems, primarily owing to their large reliability, their insensitivity to electrical interference and their high capacity. In order to facilitate handling optical fibers, they are often provided as fiber ribbons, in which a plurality of optical fibres are located adjacent to each other, optically isolated from each other and from the environment, but in some manner fixed in parallel to each other, for example by means of a polymer coating. Such a ribbon usually contains 4, 6, 8 or 12 parallel fibers. When splicing two different optical fibers, which is usually performed by welding, the polymer coatings of the fibers must first be removed. Thus, both the individual coating of the optical fibers and the material, which secures the fibers to each other to form a fiber ribbon, must be removed. Thereupon the uncovered or bare fibres are cut in order to thereafter be welded to each other. Heat-shrinkable tubes are often used for protecting such splices of optical single fibers and of optical ribbon fibers, which are used in the telecommunication field. Then such a tube is applied over an end of a single fiber or of an optical fiber cable such as a ribbon fiber, and then the cable is spliced to another cable. Finally the heat-shrinkable tube is moved to cover the bare portions of the optical fibers and then heated making it shrink and be firmly attached around the bare portions and to the splice. When heating the tube it must be carefully observed that no air remains inside the shrinking tube since such remaining air degrades the mechanical supporting and protecting function of the tube. When heating the protection tubes often ovens are used having a U-shape. Also resistance wires applied inside the tube have been proposed. Thus, in U.S. patent 4,509,820 a protective packaging assembly for a spliced portion of optical fibers is disclosed, the assembly incorporating one or more electrical resistance heating elements on the inner side of a heat-shrinkable tube in order to be capable of heating both the tube and a hot-melt adhesive. Electrical heating elements for heat-shrinkable tubes are also disclosed in the Japanese patent applications 58-009111, 58-009112, 58-009113, 60-061705, 7-248425, 58-023008, 54-118254. A similar assembly in which a resistance wire is wound around a splice reinforcing rod is described in Japanese patent application 58-055908. A problem existing in such protective packaging assemblies comprises the selection, when conducting an electrical current through a heating resistance element of a protective assembly for shrinking a protection sleeve, of the time when the heating is sufficient and the electrical current is to be interrupted. SUMMARY OF THE INVENTION It is an object of the invention to provide a protection assembly for optical fiber splices which has a reinforcing member and which can be easily produced. It is another object of the invention to provide a protection assembly for optical fiber splices which can be easily used, not requiring extra control equipment. In particular, a problem solved by the invention is how to reduce the risk of air being trapped inside a sleeve for protecting optical fiber splices and how to produce such a sleeve in a simple and reliable way. Another problem solved by the invention is how to select, when conducting an electrical current through a heating resistance element of a protection assembly for shrinking a protection sleeve, the time when the heating is sufficient and the electrical current is to be interrupted. Thus generally, a packaging assembly for protecting splices of optical fibers comprises as conventional an outer thin heat-shrinkable tube, preferably an inner, thicker heat-shrinkable tube, and a reinforcing rod. The reinforcing rod has an electrically conductive coating such as a metal layer on a flat surface thereof. When heating the assembly for applying it tightly to a splice an electrical current is passed through the metal layer. Then heat is developed which causes the tubes to shrink. The metal layer can be applied to have a larger resistance in its central region where it then will be heated first, the shrinking of the tubes then also starting in the central region, as seen in the longitudinal direction of the tubes. This reduces the risk of forming strapped air bubbles. The metal layer can be made to act like an electrical fuse, being evaporated when the heat is sufficiently intensive and then interrupting the electrical current. This provides an easy and automatic operation of the assembly. This operation like an electrical fuse can obviously also be obtained for other forms of the heating resistance element different from a layer, for example by applying inside the tube or tubes any elongated element, such as a resistance wire, the element having a smaller cross-sectional area at its central portion or by using an elongated element made of a material having a resistance per unit which is larger in the central portion than in end portions thereof. Applying a metal layer, e.g. a metal sheet cut a suitable shape, to a surface of the reinforcing rod, is a very simple operation. Obviously the resistance in the longitudinal direction of such a layer is also very easily controlled. For example the layer can easily be made narrower in its central portion where it will then have a higher resistance and thus be heated first and most intensely. The metal layer can also be printed on a flat surface and thereupon etched what will provide a very accurate control of the resistance of the various parts of the metal layer. Generally thus, in a central portion of such a layer of an electrically conductive material as seen in a longitudinal direction of the layer, the layer will then have a larger resistance per unit length as taken in the longitudinal direction of the layer than in end portions of the layer. Termed in another way, the layer of such an electrically conductive material generally has a substantially uniform resistivity and in order to obtain a more intense heating where required, it can thus have a smaller cross-sectional area in its central portion as seen in a longitudinal direction of the layer than in end portions of the layer. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of a non-limiting embodiment with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a conventional heat-shrinkable tube for protecting fusioned optical fiber splices, Fig. 2 is a perspective view of heat-shrinkable tube having a heating resistance element connected to an electrical power supply, and Fig. 3 is a perspective view of a reinforcing rod having an electrically conducting layer on a flat surface. DETAILED DESCRIPTION A conventional assembly for protecting a splice of two optical fibers is shown in the perspective view of Fig. 1. The assembly comprises an inner heat-shrinkable tube 1 made of a thick artificial resin material, a stiff glass-rod 3 having a D-shaped cross-section and an outer heat-shrinkable tube 5 made of a thin artificial resin material. The outer tube 5 surrounds the two other components and keeps the other components in their places to form the assembly. In some cases no inner tube 1 is used, the outer tube 5 then being made thicker. The flat surface of the rod 3 is in contact with the outer surface of the inner tube 1. When making a splice between two optical fibers, not shown, the assembly is applied around one of the fibers by letting the fiber pass from one side of the assembly through the inner tube 1 to expose its end on the other side of the assembly. A portion of the exposed end is then rid of its protective polymer layer and the bare end portion is fusion-spliced to the end of the other optical fiber, from which also the protective coating has been removed. Thereupon the protective assembly is made to slide along the spliced fibers to cover the bare portions of the fibers. The protective assembly is heated to make the tubes 1, 5 shrink and to apply the inner tube firmly to the splice portion and the bare fiber portions and to hold the reinforcing rod 3 with a suitable force at the splice. The conventional heat-shrinkable assembly of Fig. 1 can be modified to include a resistance heating element as illustrated in the perspective view of Fig. 2. The glass rod 3 has a coating 7 on its flat surface, the coating being made of an electrically conducting material such as a suitable metal. The coating extends to the end surfaces of the rod, where it can be electrically contacted, as also seen in the figure, by electric terminals 9 of a power supply 11. Then an electrical current can pass through the conductive coating 7 and heat it and thereby the glass rod 3, the inner tube 1 and the outer tube 5, in order to make the tubes shrink to seal the splice portion. The conductive coating or layer 7 can have a higher resistance in its central portion, at the centre of the glass rod, as seen in the longitudinal direction thereof. Such a conductive layer is illustrated in Fig. 3. The conductive layer is there shown to be narrower in its central region. This will make the heat developed when connecting the conductive layer to a power supply be larger in the central region, then starting the shrinking operation of the tubes at the central regions thereof, also as seen in the longitudinal directions thereof. This operation will reduce the risk of obtaining air bubbles trapped inside the heat-shrinkable tubes. The conductive layer preferably has a uniform thickness. The thickness and the width in the central region according to Fig. 3 can be selected so that the conductive layer or metal layer is evaporated in the central region when a sufficiently high temperature has been obtained, i.e. generally after a predetermined time for a predetermined electric current being applied to the layer when starting the heating or a predetermined voltage being applied to the layer. Then the heating is automatically interrupted and the shrinking operation is complete.

Claims: An assembly for protecting bare portions of optical fibers and/or splice portions of optical fibers, the assembly comprising an outer heat-shrinkable tube (5), a reinforcing rod (3) made of an electrically isolating material and located inside the outer heat-shrinkable tube and an electrically conductive resistance element also located inside the outer heat-shrinkable tube, characterized in that the reinforcing rod comprises a substantially flat surface and that the resistance element comprises a layer (7) of an electrically conductive material applied to the substantially flat surface of the reinforcing rod. An assembly according to claim 1, characterized in that in a central portion of the layer (7) of an electrically conductive material taken in a longitudinal direction of the layer, the layer has a larger resistance per unit length as taken in the longitudinal direction of the layer than in end portions of the layer. An assembly according to any of claims 1 - 2, characterized in that the layer(7) of an electrically conductive material has a substantially uniform resistivity and has a smaller cross-sectional area in its central portion taken in a longitudinal direction of the layer than in end portions of the layer. An assembly according to any of claims 1 - 3, characterized in that the layer (7) of an electrically conductive material has a substantially uniform resistivity and a substantially uniform thickness and has a smaller width in its central portion taken in a longitudinal direction of the layer than in end portions of the layer. An assembly according to any of claims 1 - 4, characterized in that the layer (7) of an electrically conductive material has such a cross-sectional area that when conducting a predetermined electric current through the layer and thereby heating the layer and adjacent components of the assembly the layer will after a predetermined time period at least partly evaporate to break a path of the electrical current through the layer in order to automatically shut off the electrical current and thereby the heating. An assembly according to any of claims 1 - 5, characterized by an inner tube (1) of a heat-shrinkable material located inside the outer tube (5), the reinforcing rod (3) being located outside the inner tube. A method of producing an assembly for protecting bare portions of optical fibers and/or splice portions of optical fibers, the method comprising the steps of: providing an outer heat-shrinkable tube (5), placing a reinforcing rod (3) made of an electrically isolating material inside the outer heat-shrinkable tube, placing an electrically conductive resistance element also inside the outer heat-shrinkable tube, characterized in that in the step of placing the electrically conductive resistance element a layer (7) of an electrically conductive material is applied to a substantially flat surface of the reinforcing rod to form the electrically conductive resistance element. A method according to claim 7, characterized in that in applying the layer (7) of an electrically conductive material the layer is applied to have, in a central portion of the layer of an electrically conductive material taken in a longitudinal direction of the layer, a larger resistance per unit length as taken in the longitudinal direction of the layer than in end portions of the layer. A method according to any of claims 7 - 8, characterized in that in applying the layer (7) of an electrically conductive material the layer is applied to have a substantially uniform resistivity and a smaller cross-sectional area in its central portion taken in a longitudinal direction of the layer than in end portions of the layer. A method according to any of claims 7 - 9, characterized in that in applying the layer (7) of an electrically conductive material the layer is applied to have a substantially uniform resistivity and a substantially uniform thickness and a smaller width in its central portion taken in a longitudinal direction of the layer than in end portions of the layer. A method according to any of claims 7 - 10, characterized in that in applying the layer (7) of an electrically conductive material the layer is given such a cross-sectional area that when conducting a predetermined electric current through the layer and thereby heating the layer and adjacent components of the assembly the layer will after a predetermined time period at least partly evaporate to break a path of the electrical current through the layer in order to automatically shut off the electrical current and thereby the heating. A method according to any of claims 7 - 11, characterized by the additional step of placing an inner tube (1) of a heat-shrinkable material inside the outer heat-shrinkable tube (5) and outside the reinforcing rod (3).

Inventors: DUMITRIU ION, DUMITRIU, ION
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 2000
Main classification: G02B6
IPC codes: G02B6