Abstract:
In an arrangement for producing a branch in a telecommunications cable ( 1 ) containing several stranded elements with optical fibers, at least one stranded element is cut, the optical fibers of the stranded element are spliced to the optical fibers of a branch line ( 2 ), and the branched area is protected with a sleeve ( 3 ). The branch line ( 2 ) comprises a number of optical fibers, each of which is loosely arranged by itself in a small plastic tube ( 10 ). A predetermined length of the tube ( 10 ) is removed from approximately the middle of the optical fiber elements, and each optical fiber ( 13 ) from which the tube ( 10 ) has been removed is stored in a cassette ( 12 ). The cassettes ( 12 ) are stored in a container ( 5 ). The sleeve housing ( 4 ) is stored in a container ( 5 ). The sleeve housing ( 4 ) with the cassettes ( 12 ) and the branch line ( 2 ) with the optical fibers ( 13 ) located in the tube ( 10 ) are a unit which is prefabricated at the factory.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention concerns an arrangement for branching a telecommunications cable containing several stranded elements with optical fibers, where at least one stranded element is cut, the optical fibers of the stranded element are spliced to the optical fibers of a branch line, and the branched area is protected with a sleeve. 
     2. Description of the Prior Art 
     Most of the existing fiber-optic networks of telecommunication companies comprise point-to-point connections insofar as access to the individual optical fibers is not required. However, new offerers in the telecommunications market such as e.g. supply enterprises, railroad companies, municipal utilities etc. need to access the individual optical fibers because at the time the cables are laid it is not known when and where another subscriber must be connected after the cables have been installed. The possibility of obtaining access to an individual optical fiber after the cable installation must therefore be provided, without impairing any other active optical fiber i.e. one via which information is already being transmitted. 
     The architecture of such networks is normally an annular structure which needs not necessarily to be a closed one. This means that the optical fibers are looped at or in the vicinity of the subscriber. 
     The Raychem Company has developed a sleeve for access to the individual optical fibers which can be obtained in the market under the trade name FIST. 
     In a first application of this sleeve, the cable is cut, a predetermined length of the cable jacket and any central strength element, if present, is removed and the optical fibers of both cables as well as the cable ends are inserted through openings in the bottom part of the sleeve which has the shape of a hooded sleeve. The optical fibers are spliced to each other and are individually placed into cassettes. If one or several subscribers must already be connected when the sleeve is being produced, the optical fibers provided for the connection are spliced to the optical fibers of the subscriber cable. The cables that were inserted through the bottom part are permanently sealed inside the openings. 
     According to another solution, a predetermined length of the jacket and the central element are removed and the uncut optical fibers are inserted through an oval opening in the bottom part of the hooded sleeve. This requires bending the optical fibers which can cause a breakage of the sensitive optical fibers. In this case as well, the excess lengths of the individual optical fibers are placed into separate cassettes. 
     Both solutions require that each bared optical fiber must be threaded through a small plastic tube at the installation site to protect it against mechanical damage. This is a time-consuming operation which makes the production of a sleeve at the installation site considerably more expensive. Another disadvantage of these sleeves is that they can only be incorporated during the initial installation, a subsequent incorporation is not possible. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to present an arrangement for producing a branch in an optical telecommunications cable, which can be accomplished in a significantly shorter time, and which can also be installed into an already installed optical telecommunications cable. It is significant in this case that each individual fiber or each individual optical fiber of the branched part of the cable can be accessed in the sleeve. 
     The essence of the invention is that a first sleeve is provided which encloses a branch connection for at least one stranded element. This first sleeve is a conventional sleeve with a number of splice supports for the spliced optical fibers, such as are known e.g. from EP 0 490 133 A (equivalent to U.S. Pat. No. 5,204,938). This sleeve does not need to be reopened and can therefore be laid underground for protection. 
     The second sleeve is a sleeve which is prefabricated at the factory and is designed to be opened at any time. The sleeve encloses a number of easily accessible cassettes. Each cassette contains a predetermined slack length of an optical fiber. On the outside of the cassette, the ends of the optical fibers are inserted into a small plastic tube. Several of such small plastic tubes are bundled together and a number of such bundles pass through a plastic conduit. The cited elements are prefabricated into a unit so that only the individual optical fibers must be connected by means of conventional splicing to the corresponding optical fibers of the branched stranded elements, and 8 to 12 splices are stored in a cassette. The length of the optical fibers with small plastic tubes which protrude from the sleeve is long enough so that the sleeve can be located at a corresponding distance from the cable. The second sleeve, this is a hooded sleeve as a rule, can be stored in a container made of concrete for example. 
     A length of about 2.5 m of the “cable” comprising the plastic conduit with the bundles is stored in the container so that the sleeve can be removed from the container for the purpose of connecting a new subscriber. 
     The invention will be fully understood when reference is made to the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view in partial section of an arrangement for branching a telecommunication cable. 
     FIG. 2 is a side elevational view of a stack of cassettes with a branch cable extending therefrom. 
     FIG. 3 is a side elevational view similar to FIG. 2 but with a larger number of cassettes. 
     FIG. 4 is a top plan view of a cassette with a portion broken away to reveal internal structure. 
     FIG. 5 is a top plan view similar to FIG. 4 but showing a different internal structure. 
     FIG. 6 is a perspective view of the container in the arrangement for branching the telecommunication cable with a portion broken away to reveal internal structure. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a principal cable  1 , usually a standard cable with 144 fibers, from which a portion of the fibers, e.g. 24 or 48, are to be branched off. This branch can either be installed between two cable lengths of two to three kilometers each during the cable laying, or in shorter lengths to an already installed cable. In both of the cases cited first, the unbranched fibers are spliced, and 8 to 12 splices are stored in a cassette. The fibers to be branched off are spliced to the fibers of a branch channel  2 . The branch area is subsequently protected with a sleeve  3 . This sleeve  3  needs not to be reopened as a rule. For that reason a sleeve such as is known e.g. from EP 0 490 133 A can be used. 
     In a case cited in third place, the principal cable  1  is bared, a predetermined length of the sheath is removed, the fibers to be branched off are bared and spliced by means of conventional splicing to the fibers of the branch cable  2 . As mentioned in the cases cited first, the branch area is subsequently protected with the sleeve  3  and buried. 
     The branch cable  2  is a plastic conduit in which several bundles of individual small plastic tubes are stored. Each plastic tube contains an uncut optical fiber. The branch cable  2  is inserted through the bottom part  4   a  of a hooded sleeve  4  with a removable hood  4   b , and is sealed. All the openings in the bottom part  4   a  are circular openings to prevent the problems which can occur when oval openings are sealed. The inside of the hooded sleeve  4  has a number of cassettes (e.g., see FIGS.  2 - 5 ), each of which is separately accessible. 
     The hooded sleeve  4  is separated from the branch sleeve  3  in an underground container  5 , which is equipped with a removable but preferably lockable cover  5   a.    
     One or more windings  2   a  of the branch cable  2  are stored in the bottom of the underground container  5  so that the hooded sleeve  4  can be taken out of the container  5  for the purpose of an installation. 
     The branch cable  2  and the hooded sleeve  4  form a unit which is prefabricated at the factory. For example if two new subscribers are being connected to the principal cable  1  or the network, the optical fibers of the subscriber cables  6   a  and  6   b  are connected to the corresponding optical fibers in the hooded sleeve  4 . Several windings of the subscriber cables  6   a  and  6   b  are also stored as slack lengths  6   c  in the bottom of the container  5 . Since both the branch cable  2 , because of the missing central strength element and other tension relief elements, as well as the subscriber cables  6   a  and  6   b  because of their small diameter, are extremely flexible, the winding diameters of the slack lengths  6   c  and  2   a  are also very small. The result is an extremely low cost of the container  5  and its installation. 
     For example, if two new subscribers are being connected to the network, the cover  5   a  is taken off, the hooded sleeve  4  is removed, the sleeve  4   b  is detached and the desired cassettes become accessible. The subscriber cables  6   a ,  6   b  can be attached to the desired fiber by means of known splicing techniques. The subscriber cables  6   a ,  6   b  which exit through the bottom  4   a  are sealed with respect to the bottom  4   a . In the same way, the cable  2  is sealed with respect to the bottom opening  4   a . The sealing presents no difficulties since all the openings in the bottom  4   a  are circular and the incoming and outgoing cables  2 ,  6   a ,  6   b  are also circular. 
     In a cable  1  for the cited purpose, the number of optical fibers is e.g. 144, in some cases up to 192. Twelve each optical fibers are combined in a stranded element. One stranded element is e.g. a small plastic tube in which the 12 optical fibers are stored. One optical fiber and one cassette are provided in the hooded sleeve  4  for each optical fiber of the element to be branched off. In the hooded sleeve  4 , which is prefabricated at the factory, an optical fiber is encased in a small plastic tube with an outside diameter of e.g. 0.9 mm. Lengths of about 15 m are detached from an element produced in this manner. About 2.5 m of the small plastic tube is removed from the center of such a length. The bared length of the optical fiber is stored in a cassette and the ends of the small plastic tube are attached to the inlet of the cassette. Several, e.g. 2×12, of such cassettes are prepared. The small plastic tubes protruding from the cassettes are formed into two bundles and different color tapes are wrapped around them. The two bundles are inserted into a corrugated plastic conduit which passes through an opening in the bottom part  4   b  of the hooded sleeve  4  and is sealed. The cassettes are stacked and the hood  4   b  is connected to the bottom part  4   a  in a waterproof manner. The space between the bundles and the corrugated plastic conduit is sealed in the usual cable technology manner to prevent lengthwise water propagation. 
     FIG. 2 illustrates the branch cable  2  with its plastic conduit  7 , preferably a corrugated plastic conduit, and bundles  8  and  9 . Each bundle  8 ,  9  comprises several small plastic casings or tubes  10 , which are held together by a thread or a tape  11 . The threads or tapes  11  have different colors for the purpose of identifying the bundles  8  and  9 . Each one of the individual small plastic tubes  10  is inserted and exits from a cassette  12 , i.e. each cassette  12  contains an optical fiber with a predetermined slack length. The cassettes  12  form a stack which is stored in the hooded sleeve  4 . Each cassette  12  can be accessed separately so that a new connection can be established without interfering with other subscribers. 
     FIG. 3 illustrates a stack comprising a larger number of cassettes  12 . Beyond that FIG. 3 shows that the conductors  6   d  of a subscriber cable  6  are connected to the optical fibers. 
     FIG. 4 illustrates a cassette  12  into which the small plastic tube  10  is inserted and from which it exits. The optical fiber  13  lies loosely inside the cassette  12  and touches its inner periphery. The slack length of the optical fiber  13  inside the cassette  12  is about 2.5 m. 
     To produce the unit comprising the cassette  12 , the small plastic tube  10  and the optical fiber  13 , first a length of about 15 m of a small plastic tube  10  containing an optical fiber  13  is cut from a slack length at the factory. In the center of the 15 m length the small plastic tube  10  is removed from a length of 2.5 m and the bared optical fiber  13  is placed into the cassette  12 . 
     The individual incoming and outgoing small plastic tubes  10  are then combined into bundles  8  and  9  at the factory and wrapped with the tapes or threads  11  and inserted into the corrugated plastic tube  7 . The space between the bundles  8  and  9  and the corrugated plastic tube  7  is sealed in accordance with cable technology to prevent lengthwise water propagation. The corrugated plastic tube  7  is then inserted into an opening in the bottom part  4   a  of the hooded sleeve  4  where it is sealed. The hood  4   b  is mounted after the cassettes  12  are stored in the not illustrated frame of the hooded sleeve  4 . In this way, the prefabricated sleeve with cassettes  12 , each of which contains an optical fiber  13 , is ready for use. 
     FIG. 5 illustrates a cassette  12  in which a conductor  6   d  of a subscriber cable  6  with its optical fiber is spliced to the optical fiber  13  by means of a splice reinforcement tube  14 . 
     FIG. 6 illustrates an underground container with a sleeve housing stored therein. 
     The underground container comprises an external housing  15  made of concrete e.g., which is constructed of a tub-shaped bottom part  15   a  and several segments  15   b  and  15   c  placed on the bottom part  15   a . A cover  16  made of cast iron e.g., is placed on the segment  15   c.    
     The incoming and outgoing cables  2  and  6   a ,  6   b , preferably optical fiber cables, are inserted into the bottom part  15   a  through unnumbered bores, and are secured with cable bolts  17  and  18  and sealed. 
     The bottom part  4   a  of the sleeve  4  has a shoulder  4   c  with a bore  4   d  provided therein. 
     Two holders  19  and  20  are attached to the inside wall of the container  15  at opposite sides and different heights. They are placed on and bolted to supports  21 . Each of the holders  19  and  20  has a stem (only stem  20   a  is illustrated). 
     In the normal condition, the sleeve  4  rests on the holder  19 . To that end, the stem  19  passes through the bore  4   d . The sleeve  4  is secured with a rod  22  which has a bushing  22   a  at its lower end, making a tightly fitting connection between the stem  19   a  and the bushing  22   a , e.g. by bolting or by means of a bayonet fastener. The rod  22  has a hexagon  22   b  at its upper end to facilitate the attachment or the removal of the connection. 
     In the event installation work must be provided on the sleeve  4 , the connection between the rod  22  and the holder  19  is loosened by rotating the rod  22 , the sleeve  4  is taken out of the housing  15  by rotating the sleeve  4  counter to the direction of the windings  6   c.    
     The rod  22  with its bushing  22   a  is placed on the higher holder  20  where it is secured. The shoulder  4   c  or the bore  4   d  of the sleeve  4  is placed on the cylindrical end  22   c  of the rod  22  where it is secured in a manner known per se. The sleeve  4  has a handle or a grip  4   e  to facilitate its removal. 
     By loosening the tension strap  4   f  which connects and pressure-proofs the bottom part  4   a  and the hood  4   b , the hood  4   b  can be removed to make the cassettes  12  (FIGS. 2 and 3) therein accessible. 
     After the installation work is completed, the hood  4   b  is placed on the bottom part  4   a  and the tension strap  4   f  is laid around it and tightened. After that the rod  22  is loosened from the holder  20 , the sleeve  4  is raised and lowered into the inside of the housing while it is rotated in the direction of the cable windings  6   c , and placed on the lower holder  19 . Finally, the rod  22  is secured. 
     At the end, the cover  16  is placed on the housing  15 . 
     The preferred embodiment described above admirably achieves the objects of the invention. However, it will be appreciated that departures can be made by those skilled in the art without departing from the spirit and scope of the invention which is limited only by the following claims.