Abstract:
An add/drop node of an optical WDN-network which has two fiber paths for light of a plurality of channels propagating in opposite directions comprises two add/drop modules ( 231, 23   r ) for each of the channels. All the modules are identically constructed. Each module comprises an add device ( 251, 25   r ) for adding light to one of the paths and a drop device ( 27   r,    271 ) for deflecting a portion of light from a second one of the paths. A module comprises a house ( 41 ) enclosing the add device and the drop device. A first fixed connector ( 53, 73 ) is attached to the house for connection in the first path and to a an optical fiber ( 45, 65 ) which extends freely from the house and has a first free connector ( 43, 63 ) at its free end to be attached to the fixed connector of a neighboring add/drop module for continuing the first path through the considered add/drop module to the neighboring module. In the same way a second fixed connector is attached to the house for connection in the second path and to a second optical fiber which extends freely from the house and has a second free connector at its free end to be attached to the fixed second connector of a neighboring add/drop module for continuing the second path through the considered add/drop module to the neighboring module.

Description:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/SE00/01877 which has an International filing date of Sep. 27, 2000, which designated the United States of America. 
     TECHNICAL FIELD 
     The present invention relates to an add/drop node of an optical WDM-network, in particular to the connection of an add/drop node to two paths of the network carrying light in opposite directions, and to a network including such an add/drop node. 
     BACKGROUND 
     Optical networks using WDM (Wavelength Division Multiplexing) are now proposed to be built more and more. In such networks a plurality of add/drop nodes are connected and simple devices should be provided for connecting the nodes to the network. For example, when an existing node is expanded to be capable of receiving and transmitting in another wavelength band the manual work required therefor should be minimized. An optical network having add/drop nodes is for example disclosed in U.S. Pat. No. 5,754,545. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an add/drop node for an optical WDM-network having a simple way of connecting the node to circulating fiber paths of the network. 
     It is another object of the invention to provide an optical WDM-network having an add/drop node built to allow a simple way of connecting the node to parallel fiber paths of the network. 
     Thus generally, an add/drop node is intended to be connected in an optical WDM-network. The network has two parallel fiber paths allowing light of a plurality of wavelength channels to propagate in opposite directions. The add/drop node comprises two add/drop modules for each of the channels. All the modules are identically constructed and most of their connections are very similar to each other allowing a simple mounting and connection of the components of the node and also a simple rearrangement for changing wavelength channels and for adding/deleting wavelength channels used in the network. Each module comprises an add device for adding light to a first one of the paths and a drop device for deflecting a portion of light from a second one of the paths. The add device and the drop device of a module are enclosed by a housing, the housing of the modules being placed in a single row, at the sides of each other and for instance mounted in a rack. A first fixed connector is attached to the housing of a module for connection in the first path and to an optical fiber which extends freely from the housing and has a first free connector at its free end to be attached to the fixed connector of a neighbouring add/drop module for continuing the first path through the considered add/drop module to the neighbouring module. In the same way a second fixed connector is attached to the housing for connection in the second path and to a second optical fiber which extends freely from the house and has a second free connector at its free end to be attached to the fixed second connector of a neighbouring add/drop module for continuing the second path through the considered add/drop module to the neighbouring module. 
     More particularly, in the add/drop node two add/drop modules are provided for each of the channels of light propagating in the network. Each add/drop module has an add device or light combiner for adding light to one of the two optical fiber paths and it has also a drop device for deflecting a portion of light from the other optical fiber path. Furthermore, all the add/drop modules have the same construction. The add/drop modules can then be arranged in two sets, so that the add/drop modules of a first set have their add devices connected in one fiber path and their drop devices connected in the other fiber path. Also, the add/drop modules of the second set then have their add devices connected in the other fiber path and their drop devices connected in said one fiber path. 
     In at least each of the two sets the add/drop modules are placed at the sides of each other and then inner modules and two end modules are obtained in each set. For two adjacent or neighbouring add/drop modules of a set an output of the add device in a first one of the two adjacent add/drop modules can be connected to an input of the add device in a second one of the two adjacent add/drop modules. In the same way, for two adjacent add/drop modules an output of the drop device in a first one of the two adjacent add/drop modules can be connected to an input of the drop device in a second one of the two adjacent add/drop modules. For one of the end add/drop modules comprised in a first set its drop device can have an output connected to an input of the add device of one of the two end add/drop modules in the other, second set. Similarly, for said one end add/drop module of the second set its drop device can have an output connected to an input of the add device of said one end add/drop module in the first set. 
    
    
     
       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 block diagram of an optical network having a ring architecture and a hub node and four client nodes, 
         FIG. 2  is a schematic picture of the connection lines of an add/drop module used in the nodes of the network, 
         FIG. 3  is a view from the side of the inside of a housing of an add/drop module, 
         FIG. 4  is a front view of a plurality of add/drop modules mounted in a rack, 
         FIG. 5  is a block diagram schematically showing the connections of add/drop modules of the nodes, 
         FIG. 6  is a block diagram showing in a somewhat more realistic way the connections of add/drop modules of the nodes, and 
         FIG. 7  is a perspective view showing the inside of a housing of an add/drop module. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1  a block diagram of an optical fiber WDM-network having a ring configuration is shown. Thus two optical fiber paths  1   e ,  1   w  pass in a basically uninterrupted way all around the network, one fiber path  1   e  carrying light propagating in the east direction and the other fiber path  1   w  carrying light propagating in the west direction. 
     The network includes a hub node  2  and in the embodiment shown four client nodes  3 , called Client  1 ,  2 ,  3  and  4 , the nodes being connected to the two basic fiber paths  1   e ,  1   w  for adding and dropping light from the fibers. A client node n receives and transmits information in a narrow wavelength band, also called channel, around a single wavelength λ n , n=1, 2, . . . . The hub node  2  can receive and transmit information in all channels, i.e., on all wavelengths λ n , n=1, 2, . . . . For each client node  3 , the hub node  2  is connected to an electrical client portion  5 . Such an electrical client portion  5  comprises an electrooptic converter or optical transmitter  7  converting electrical signals to optical signals and an optoelectric converter or optical receiver  9  for receiving optical signals converting the received signals to electrical signals. The electrical client portion  5  is through optical fibers connected to an optical client portion  11  in the hub node  2 . The optical client portion  11  has optical connectors for receiving the optical fibers extending from the respective electrical client portion. 
     The optical client portion  11  comprises an optical receiver-transmitter combination  13 ,  15  for transmission, the receiver  13  of the combination receiving the light signal from the transmitter  7  of the electrical client portion  5  providing its output signal to the transmitter  15  of the pair, which provides a well-defined light signal in the narrow wavelength band used for the respective client. The optical transmitter  15  is coupled to an optical connector for providing its output signal on an optical fiber to add/drop modules as will be described hereinafter. 
     The optical client portion  11  also comprises an optical receiver-transmitter combination for receiving, the combination comprising two optical receivers  17   e ,  17   w  connected to receive light from the add/drop modules through optical fibers and optical connectors, one receiver  17   e  being used for receiving light propagating in the network, in the appropriate fiber, in an east direction and another receiver  17   w  being used for receiving light propagating in the west direction in the ring network. The outputs of the two optical receivers  17   e ,  17   w  are connected to inputs of a combining element or optical multiplexer  19  which combines the received signals to provide them to a transmitter  21 , the output terminal of which is through the respective connector and a fiber length connected to the receiver  9  in the electrical client portion  5 . 
     Furthermore the hub node  2  includes a plurality of add/drop modules  231 ,  23   r  one pair of such add/drop modules being provided for each client node  3  in the network. In such a pair one module  231  is adapted to transmit in a left direction from the hub node and to receive from the same left direction. The other module  23   r  of a pair is adapted to transmit in and to receive from the right direction from the hub node. Each add/drop module is connected in the two ring-shaped fiber paths  1   e ,  1   w  of the network. The left add/drop module  231  comprises an add device  251  connected in the fiber ring path  1   w  and an drop device  271  connected in the other fiber ring path  1   e . The add device  251  is, through fiber sections, a 50/50 splitting coupler  29  (only one is shown in the drawing) and the respective connector connected to the transmitter  15  in the optical client portion  11  for the appropriate client node. The drop device  271 , is through a fiber and the respective connector, connected to the receiver  17   e  in the optical client portion  11  for the same client node. 
     In the same way, the right add/drop module  23   r  in the pair comprises an add device  25   r  connected in the fiber ring path  1   e  and an drop device  27   r  connected in the other fiber ring path  1   w . The add device  25   r  is, through fiber sections, a respective splitting coupler  29  and the respective connector, connected to the transmitter  15  in the optical client portion  11  for the client node. The drop device  27   r  is, through a fiber and the respective connector, connected to the receiver  17   w  in the optical client portion  11  for the client node. 
     The add devices  251 ,  25   r  contain some coupling or combining element and, if required, a notch filter blocking light of the wavelength band or channel for which the add/drop module is designed. The optional filter will then stop only light of the wavelength band propagating in the respective fiber ring path  1   w ,  1   e  before light of the same wavelength band is added in the combining element. The drop devices  271 ,  27   r  contain in the same way some splitting and filtering element for tapping off only light of the wavelength band or channel for which the add/drop module is designed. 
     All add/drop nodes  231 ,  23   r  have the same basic design and functions and can thus all be given the same physical shape as discussed hereinafter. 
     The two fiber ring paths  1   e ,  1   w  are connected to the hub node  2  on a left side of the hub node and on a right side of the node. On each such side a monitor module  311 ,  31   r  can be arranged which is thus connected in the two ring paths. The monitor module  311 ,  31   r  comprises an add coupler  331 ,  33   r  for adding, e.g., some control signal and a tap  351 ,  35   r  for tapping off some small portion of the incoming light power (e.g., 1%.). 
     The schematic diagram of  FIG. 2  illustrates the connections of the add/drop modules  231 ,  23   r . Each module comprises a housing indicated at  41 . Light from one ( 1   e ) of two ring paths of the network enters the module at a connector  43  attached to a fiber section  45  extending loosely outside the housing  41 . The fiber section  45  has a thick protective sleeve which extends inside the housing to a weld section  47  which splices an end of an optical fiber  49  having a standard thin protective sleeve to the fiber section  45 . The optical fiber section  49  is, at its opposite ends connected to one of the two inputs of the add device  251 ,  25   r . The output of the add device is connected to a fiber section  51 , which in turn is connected to a connector  53  attached to the housing  41 . The connector  53  should be connected in the same ring path  1   e  as the input connector  43 . The other input of the add device is, through a fiber section  55 , connected to a connector  57  which is attached to the housing  41 . The connector  43  thus receives light from the ring path  1   e  to the fiber  45 , through the weld  47 , the fiber section  49 , the add device  251 ,  25   r , the fiber section  51  and to the connector  53 . The output connector  53  lets the light continue along the ring path  1   e . Light from an optical client portion  11  enters the module at the connector  57 , continues through the fiber  55  to the add device  251 ,  25   r , in which the light is added to that propagating along the ring path  1   e.    
     Furthermore, in the module  231 ,  23   r  there is a set of connected fiber sections, which is parallel to that described and is connected in the other ring path  1   w  of the network, but in which instead of the add device  25   r ,  251 , the drop device  271 ,  27   r  is connected. Thus a connector  63  is to be connected to the other ring path and is attached to an end of a well-protected fiber section  65  extending partly outside the housing  41 ; the other end being the fiber section  65  connected through a weld section  67  to a standard fiber section piece  69 . This fiber section is in turn connected to one of the two outputs of the drop device  271 ,  27   r . The input of the drop device is connected to a fiber section  71  which receives light from a connector  73  attached to the housing  41 . The connector  73  should be connected in the respective ring path  1   w . The other output of the drop device  271 ,  27   r  is connected to an end of a fiber section  75  which has its other end connected to a connector  77  attached to the housing  41 . This connector  77  is, through a fiber, attached to a respective receiver  17   w  ( 17   e ) in the optical client portion  11 . Light from the ring path  1   w  enters the module at the connector  73 , continues through the fiber  71 , the drop device  271 ,  27   r , the fiber  69 , the weld section  67 , the thick fiber  65  to the loose connector  63 , which in turn is connected in the respective ring path  1   w  of the network. Some light of a specific wavelength band is tapped off in the drop device  271 ,  27   r  and continues through the fiber  75  to the connector  77  and therefrom to the respective optical client portion  11 . 
     The physical layout of the interior of an add/drop module housing  41  is shown in the side view of  FIG. 3 . The module housing  41  comprises a substantially flat portion from which various walls stand out. The walls all have the same height and connect to a basically flat lid (not shown) which is mounted over the housing  41 . The walls form two circular winding cores  81  having a sufficiently large diameter, e.g. about 50 mm, allowing that fibers can be wound around them and not being subjected to too small bending radii (too severe curvatures). The two winding cores  81  are placed at some distance from each other to allow the fibers to pass therebetween. By arranging two such cores the fiber sections used can be allowed to have some extra length allowing them to be comfortably handled and to again be spliced to the devices in the case of fiber breaks or bad splices. Additionally, the direction of the fiber sections at the places where they are connected to the devices can be selected to avoid bends which are too small, by placing the fibers, for instance, in a configuration similar to the figure eight around the two cores  81 . Inside the walls forming the winding cores through-holes  83  may be arranged for an easy handling of the module housing. The add devices  251 ,  25   r  and the drop devices  271 ,  27   r  can be attached between outstanding walls  85  at the top of the housing. 
     At the lower edge of the housing  41  devices are provided for attaching the housing to a rack, the attaching devices comprising a notch  87  at the rear side and a snap device  89  at the front side. A channel  91  is formed at the front top side of the house  41  to allow fibers connecting the module to the associated optical client portion to be held therein. 
       FIG. 4  is a front view of the add/drop modules  231 ,  23   r  and the monitor modules  311 ,  31   r  mounted in a rack, the loosely extending fiber pieces  45 ,  65  not being visible in this figure. By comparing  FIG. 4  to  FIG. 1  it is seen that the connection of all left add/drop modules  231  is as indicated in  FIG. 3 , the extending fiber sections  45 ,  65  being inserted in the mating connectors  53 ,  73  in the adjacent module at the left side of the respective module. This connection is illustrated in the schematic view of  FIG. 6 ; see also  FIG. 5 . Thus, in  FIG. 5  the same basic connection as in  FIG. 1  is illustrated, where, in the right modules  23   r  the add devices and the drop devices have changed places with each other. Then a cross coupling must be made between the group of left modules  231  and the group of right modules  23   r  and between the right group and the right monitor module  31   r  as compared to the straight schematic connection of modules shown in  FIG. 1 . 
     The same connection of the modules is illustrated in  FIG. 6  in which the connections between the add/drop modules using loosely extending fiber sections  45 ,  65  is shown. From this figure it appears clearly that all add/drop modules  231 ,  23   r  can have an identical construction, only the tapping-off devices and filters being specific to the channel for which the respective module is designed. The cross coupling is made in the middle of the assembly of add/drop modules, between the left and right modules. Such a cross connection can be made by connecting the loose fiber sections  101  having optical connectors at each end in a cross configuration or a particular cross connecting module can be used. Such a module has the same exterior design as the other modules but has inside just the optical fibers connected cross-wise. 
     In  FIG. 4  a multitude of add/drop modules  231 ,  23   r  are illustrated. However, only a portion of the add/drop modules may be active ones, constructed as described above. At the side of the active modules dummy modules are inserted having the same exterior layout but without the connectors and the loosely extending fibers. The dummy modules are used for just filling up the space between the active modules and the monitor modules. 
     The two monitor modules  311 ,  31   r  both have the same exterior connectors and interior devices as each other but have not the same interior connection lines. The necessary connections appear clearly from  FIG. 1 . 
     The client nodes  3  in the network have the same basic design as the hub node  2  but are designed to receive and transmit in only one wavelength band. The same kind of add/drop modules as described above can for example be used. 
     In other network ring architectures the client nodes can receive and transmit in more than one wavelength band. Then the client nodes can have the same structure as the hub node  2 .