Abstract:
A branching unit for an optical transmission system, an optical transmission system and a method of operation of said branching unit ( 30 ) is disclosed having a first branch ( 38 ) and a second branch ( 40 ), each connected to a main branch ( 42 ). First directional current flow means ( 54 ) and second directional current flow means ( 58 ) are operable to allow electrical current to flow substantially in one direction only along the first branch ( 38 ) and the second branch ( 40 ) respectively such that electrical current flow from the first branch to the second branch and from the second branch to the first branch is substantially prevented and a simultaneous conduction of current between the first branch and the main branch and between the second branch and the main branch is prevented.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a branching unit for an optical transmission system. More particularly, the invention relates to apparatus for supplying such a system and branching unit with electrical power and a method of operation for that apparatus. 
   Optical transmission systems are utilised for sending data from one location to another. Typically, they utilise optical fibres for carrying a modulated light signal, the modulation being controlled according to the data being transmitted. 
   Such transmission systems can have high capacity and so are attractive for high volume communications systems. For this reason, amongst others, they are used for long distance communication systems, for example in submarine communications systems. Periodic amplification of the light signals is usually carried out using repeaters which require an electrical power feed. 
   In addition to repeaters, there are other devices in the optical transmission system which require electrical power. These include detectors for indicating the status of different parts of the system, such as different lengths of transmission line and branching units. 
   To supply power to the system, it is necessary to have an electrical conducting path along with the optical fibres. Usually, the conducting path is provided as a metallic sheath conductor around the bundle of optical fibres. 
   In order to provide a power supply of the desired magnitude, typically at a terminal of the transmission line, a power feed apparatus is used to hold the terminal at a particular voltage to feed power to the conductor. Typically, at the other terminal, a voltage of opposite polarity is applied. 
   In order to increase the reliability of a submarine optical transmission system, branching units are sometimes used. These allow a main, e.g. intercontinental, transmission cable to be connected to two landing transmission cables. In shallow waters and nearer land, damage to the transmission cables from trawlers, etc, is more likely than in deep water. Therefore two landing cables are used to increase the redundancy of the system. A failure or fault in one landing cable can be repaired whilst the data signals are routed along the other cable to and from the main cable. 
   Electrical current for powering electrical devices associated with the optical transmission system is conductable between a first branch of the branching unit and a main branch and between a second branch and the main branch. 
   In order to avoid overloading the conductor of the main cable, power is fed to the main cable from only one of the landing cables. However, in the event that a fault develops in that landing cable, it is necessary to de-power the system in order to repair the fault due to the high voltage nature of the power feed. This can result in a lengthy period of time in which the system is not operative, which is not desirable. In order to use the second landing cable to power the system, it is necessary to disconnect the first landing cable (for example at the branching unit) in order to avoid the power feed from the second landing cable flowing to e.g. an earth fault in the first landing cable. Such disconnection is clearly time-consuming and difficult if the branching unit is under water. 
   SUMMARY OF THE INVENTION 
   Accordingly, in a first aspect, the present invention provides a branching unit for an optical transmission system, the branching unit having a first branch and a second branch, each connected to a main branch, 
   first directional current flow means operable to allow electrical current to flow substantially in one direction only along the first branch, 
   second directional current flow means operable to allow electrical current to flow substantially in one direction only along the second branch, 
   whereby, in use, electrical current flow from the first branch to the second branch and from the second branch to the first branch is substantially prevented. In this way, the first and second directional current flow means may operate as electrical valves so that, in the event of a fault in a landing cable connected to the branching unit, electrical power may be supplied to the main branch from the alternative branch without the current proceeding back along the other branch, away from the main branch. 
   A diode is an example of a directional current flow means. More generally, such a means may, for example, be a device which allows substantial electrical current flow through it in one direction but allows substantially no electrical current flow through it in the reverse direction. 
   Preferably, electrical power may be supplied to or from the main branch to or from the first branch or to or from the second branch, as appropriate. 
   Preferably, the first branch is connectable to a first optical transmission cable at a first branch connection point and the second branch is preferably connectable to a second optical transmission cable at a second branch connection point. 
   Preferably, the first directional current flow means is located between the main branch and the first branch connection point. Similarly, the second directional current flow means is preferably located between the main branch and the second branch connection point. 
   Preferably, a first earth connection point is provided to the first branch between the first branch connection point and the first directional current flow means. More preferably, the first earth connection point is connected to the first branch via a directional current flow means, directed so as to allow, in use, a current flow between the first branch and earth substantially in one direction only. 
   Preferably, a second earth connection point is provided to the second branch between the second branch connection point and the second directional current flow means. More preferably, the second earth connection point is connected to the second branch via a directional current flow means, directed so as to allow, in use, a current flow between the second branch and earth substantially in one direction only. 
   Typically, the branching unit includes detection means for detecting a fault in the branching unit. Additionally or alternatively, the branching unit includes detection means for detecting a fault in a cable attached to the branching unit. 
   Typically, the branching unit is a submarine branching unit. 
   Preferably, one or more of the directional current flow means is a solid state device. More preferably, one or more of the directional current flow means is a diode. Even more preferably, one or more of the directional current flow means has a reverse bias breakdown voltage of more than 1 kV. The breakdown voltage could be higher, perhaps as high as 20 KV. 
   In a second aspect, the present invention provides an optical transmission system having a branching unit according to the first aspect and further including a first optical transmission cable connected to the first branch, a second optical transmission cable connected to the second branch and a main optical transmission cable connected to the main branch, and power feed apparatus connectable to the cables for powering electrical devices attached to the cables. 
   Preferably, the power feed apparatus connectable to the second optical transmission cable is switchable from one voltage to another in response to the detection of a fault in the first optical transmission cable, in order to supply the main cable with electrical power. 
   In a third aspect, the present invention provides a method of operation of a branching unit according to the first aspect, the branching unit having a normal mode of operation and a fault mode of operation, wherein 
   in the normal mode: 
   the first branch is held at a first voltage, a current flows through the first directional current flow means due to a potential difference between the first branch and the main branch, 
   and in the fault mode: 
   the second branch has a second voltage applied to it so that there is a potential difference between the second branch and the main branch, the potential difference having a direction such that a current flows through the second directional current flow means; and 
   due to the potential applied to the second branch, the potential difference across the first directional current flow means is in the opposite direction to that required for substantial current to flow through the first directional current flow means so that substantially no current flows between the main branch and the first branch, 
   whereby in the fault mode substantially all of the current in the main branch flows along the second branch and not along the first branch. 
   Thus, the branching unit may be operable to allow the power feed for the main branch to be supplied via the second branch in the event that a fault develops in the first branch. Of course, if in normal operation the power is fed to the main branch via the second branch, then the branching unit could allow power to be fed to the main branch from the first branch in the event that a fault develops in the second branch. 
   Preferably, in the normal mode of operation, the second branch is held at a voltage such that an electrical current flows between the second branch and an earth connection at the branching unit, thereby powering electrical devices connected to the second branch. 
   Preferably the method includes the step of detecting an occurrence of a fault in the first or second branch. Typically, the fault is an earth fault, but the invention is applicable to other types of fault, e.g. cable breaks. 
   Preferably, the method further includes the step of removing the earth fault from the first branch and subsequently applying a voltage to the first branch such that an electrical current flows between the first branch and an earth connection at the branching unit, thereby powering electrical devices connected to the first branch. 
   Preferably optical data signals are routed through the branching unit such that the data signals travel along the first branch or the second branch, depending on which is powered. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  shows a schematic representation of a submarine optical transmission system and its land-based terminal. 
       FIG. 2  shows a schematic representation of a branching unit according to an embodiment of the invention in normal operation in an optical transmission system. 
       FIG. 3  shows a schematic representation of the branching unit of  FIG. 2  in the event of an earthing fault in the first branch cable. 
       FIG. 4  shows a schematic representation of the branching unit of  FIG. 2  when the power feed is reversed. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a schematic representation of a submarine optical transmission system and its land-based terminal. Terminal  10  provides optical signal input and power feed for optical transmission cable  12 . Terminal  10  is based on land  14 . Cable  12  is laid from the land to the sea  16 , lying on the sea floor  18 . Cable  12  has repeaters  20  for repeating the optical signal down the cable. Repeaters  20  require a power feed, provided by connecting a conducting sheath of the cable to the power feed at the terminal  10 . 
   Damage to the cable in deep water is not likely. However, the cable can be damaged in shallow water  22 , for example by trawlers or anchors. For this reason, dual landing cables are often used for connecting a main optical transmission cable (in deep water) to two landing cables. A branching unit is used to make the connection. Since it is unlikely that both landing (branch) cables will be damaged or develop a fault at the same time, this provision provides the system with some redundancy. 
   An embodiment of the invention is illustrated in  FIG. 2 . In  FIG. 2 , the branching unit  30  has three main electrical terminals, or connection points. These are the first branch terminal  32  (first branch connection point), the second branch terminal  34  (second branch connection point) and the main branch terminal  36  (Main branch connection point). In use, these connect the branching unit  30  to the first branch cable (or first landing cable)  38 , the second branch cable (or second landing cable)  40  and the main cable  42 , respectively. The branching unit also has a first earth terminal  44  and second earth terminal  46 . The dotted lines in the cables represent distance. Typically, the branching unit  30  will be closer (by a factor of around 10, typically) to the terminals  48 ,  50  than to the main terminal  52 , which may be on a different continent to terminals  48 ,  50 . In normal operation, first terminal  48  will supply a positive DC voltage of around 8 kv, in this example. Main terminal  52  will supply a negative DC voltage of around −8 kV, giving a potential difference between terminals  48  and  50  of around 16 kV. Such large voltages are necessary due to the resistance of the cable conductor and due to the voltage dropped across each device supplied with power (e.g. the repeaters spaced around 50 kilometres apart). Of course, the invention may still operate if different voltages are used. 
   The power feed apparatus supplies around 1 A at this voltage, the onus being on maintaining a constant current as far as possible due to the constant current requirements of the repeaters  51  and other devices  53  along the cables for optimum system performance. 
   Due to the sheer length of the main cable  42  and assuming an approximately linear drop in voltage along the system with distance from terminal  42 , the part of the system at zero volts (0V) will lie somewhere along cable  42 . Therefore, during normal operation, terminal  36  will have a positive voltage, typically of several kV. 
   The branching unit  30  has four diodes  54 ,  56 ,  58 ,  60  Alternatively, any switching devices could be used, preferably ones which operate automatically, e.g. require no separate control signal. One is the first branch diode  54 . In normal operation, this is forward biased and allows current to pass from first branch cable  38  to the main branch terminal  36  and onto the main cable  42 . First earthing diode  56  is reverse biased and so no current flows to or from earth here. Thus, power is supplied to the main cable  42  via terminal  48 . 
   A lower limit for their reverse bias breakdown voltage might be 1 kV, but preferably 10 kV. 
   In normal operation, terminal  50  is held at −8 kV. Second branch diode  58  is therefore reverse biased and so no current flows through this diode. Therefore, no current is supplied to main cable  42  via diode  58 . This helps to avoid overloading cable  42 . Since it is desirable to supply power to repeaters and other devices located on second branch cable  40 , current is allowed to flow from earth, through forward biased earthing diode  60  to the power feed terminal  50 . This does not affect the power feed of the main cable due to the isolation effect of reverse biased diode  58 . 
     FIG. 3  shows a schematic representation of the same branching unit and optical transmission system as shown in  FIG. 2 , but this time with an earthing fault  62  developed in first branch cable  38 . This fault (e.g. due to an exposed conductor in the cable) can create molecular hydrogen which is damaging to optical fibres. Accordingly, it is important to prevent current flow across and from the fault as soon as possible. Once the fault is detected, the voltage of terminal  48  must be reduced to zero to stop the current flow. Of course, the invention can address faults other than earth faults, but an earth fault is used as an example here. 
   Due to the reduction in potential of terminal  48 , main branch terminal  36  is now slightly negative, by virtue of main terminal  52 . Therefore, diodes  54  and  56  are forward biased and so some current will flow from earth at earth terminal  44  along main cable  42 . Second branch diode  58  is still reverse biased, and so substantially no current flows through this diode. 
   In order to restore a full power feed to main cable  42 , the voltage of terminal  50  must be reversed from negative to positive. The effects of this are indicated in  FIG. 4 . 
   In  FIG. 4 , power feed  50  is now at a voltage of around +8 kV. Therefore, second earthing diode  60  is now reverse biased and substantially no current flows through diode  60 . 
   Second branch diode  58  is now forward biased and, due to the relatively small voltage drop across it, main branch terminal  36  is now positive. Current flows along second branch cable  40 , through second branch diode  58  and along main cable  42  to main terminal  52 . 
   Since main branch terminal  36  is now positive, diodes  54  and  56  are now reverse biased. Therefore substantially no current flows through these diodes. Substantially all the power feed for main cable  42  comes from power feed  50 . 
   The data signals can be routed along main cable  42  and second branch cable  40 . The fault in cable  38  can be repaired without interfering with the data flow along the optical transmission system. 
   A diode is an example of a directional current flow means. More generally, such a means may, for example, be a device which allows substantial electrical current flow through it in one direction but allows substantially no electrical current flow through it in the reverse direction. 
   The advantage of using diodes instead of, for example, switches such as relays is that diode switching in a high voltage system results in much smaller electrical stress in the system than relay switching. In addition, the switching of the power routing is in a sense automatic using diodes once the plurality of the second power feed  50  is reversed. In contrast, relay switching requires a switch command to be sent to each relay to be switched. Such a system is more complex than the diode system explained here, with more hardware to maintain and/or risk of failure. 
   Due to its length, main cable  42  has a “virtual earth” located somewhere along its length. This provides redundancy in the event that power feed terminal  52  shuts down or when a shunt fault occurs along the main cable  42 . 
   Of course, there is no bar to the use of a similar branching unit at the other end of main cable  42 . In that case, the main cable  42  would branch into two branch cables via a similar branching unit to branching unit  30 . In that branching unit, the diodes would need to be reversed for the unit to function normally, with a positive earth electrode at that branching unit. 
   Embodiments of the present invention have been described by way of example only. Modifications of the embodiments described, further embodiments and modifications thereof will be obvious to the person skilled in the art and as such are within the scope of this invention.