Abstract:
An add/drop node used in an optical network having two fiber paths for carrying light corresponding to a plurality of channels in opposite directions may include a pair of add/drop modules that correspond to one of the plurality of channels. This arrangement advantageously provides a simple approach for connecting the components of an add/drop node.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to and claims benefit of priority to Swedish Patent Application No. 9903521-4, filed Sep. 27, 1999, entitled “CONNECTION OF AN ADD/DROP NODE,” which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical communication system, in particular to an add/drop node of an optical WDM (Wavelength Division Multiplexing) network. 
     2. Discussion of the Background 
     Optical networks using WDM (Wavelength Division Multiplexing) technology are become more and more prevalent. These networks utilize a number of add/drop nodes that are interconnected; as the number of nodes increases, the complexity of the connections also increases. A simple approach is needed to connect these add/drop nodes to the network. In addition, upgrades to the add/drop nodes should likewise be simple. For example, when the capabilities of an existing add/drop node is expanded to operate at another wavelength band, the work that is required to upgrade the add/drop node should be minimized. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an add/drop node for an optical WDM-network that has a simple mechanism to connect the node to the circulating fiber paths of the network. 
     According to one aspect of the invention, an add/drop node used in an optical network having two fiber paths for carrying light corresponding to a plurality of channels in opposite directions comprises a pair of add/drop modules that correspond to one of the plurality of channels. Each of the add/drop modules includes an add device that is configured to add light to one of the fiber paths, and a drop device configured to deflect a portion of the light from another one of the fibers paths. The pair of add/drop modules have identical construction. The above arrangement advantageously provides a simple approach for connecting the components of an add/drop node. 
     According to another aspect of the invention, an optical communication system comprises a plurality of fiber paths that carry optical signals corresponding to a plurality of channels in opposite directions. A plurality of add/drop nodes are coupled to the fiber paths. Each of the add/drop nodes includes a pair of add/drop modules that corresponds to one of the plurality of channels. Further, each of the add/drop modules includes an add device that is configured to add light to one of the fiber paths, and a drop device that is configured to deflect a portion of the light from another one of the fibers paths. The pair of add/drop modules have identical construction. The above approach simplifies scaling of the add/drop node. 
     According to yet another aspect of the invention, an add/drop node used in an optical network having two fiber paths for carrying light corresponding to a plurality of channels in opposite directions comprises a pair of add/drop modules that correspond to one of the plurality of channels. Each of the add/drop modules includes a means for adding light to one of the fiber paths, and a means for deflecting a portion of the light from another one of the fibers paths. The pair of add/drop modules have identical construction. The above arrangement advantageously permits ease of expanding the capabilities of the add/drop node. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram of an optical network with a ring architecture providing connectivity among a hub node and four client nodes, in accordance with an embodiment of the present invention; 
     FIG. 2 is a schematic diagram of the connection lines of an add/drop module used in the add/drop nodes of the network of FIG. 1; 
     FIG. 3 is a view from the side of the inside of a housing of an add/drop module, according to an embodiment of the present invention; 
     FIG. 4 is a front view of a rack with a number of add/drop modules mounted thereon, according to an embodiment of the present invention; 
     FIG. 5 is a block diagram of the connections of the add/drop modules within the add/drop nodes, according to an embodiment of the present invention; 
     FIG. 6 is a diagram of the connections of the add/drop modules using loosely extending fiber within the add/drop nodes, according to an embodiment of the present invention; and 
     FIG. 7 is a perspective view showing the inside of the housing of an add/drop module according to an embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, a block diagram of an optical fiber WDM-network having a ring configuration is shown. Two optical fiber paths  1   e ,  1   w  form the ring configuration, whereby one fiber path  1   e  carries light propagating in the east direction and the other fiber path  1   w  carries light propagating in the west direction. 
     In an exemplary embodiment, network  100  includes a hub node  2  and four client nodes  3  (i.e., Client  1 ,  2 ,  3  and  4 ). The nodes  3  are connected to the fiber paths  1   e ,  1   w  to add and drop light from the fibers  1   e ,  1   w . Although four client nodes  3  are shown, a generalization to n client nodes can be made, where n=1, 2, . . . An nth Client node  3  receives and transmits information in a narrow wavelength band (also referred to as a channel) around a single wavelength λ n . In this example, the hub node  2  can receive and transmit information from all channels—i.e., over all wavelengths λ n , n=1, 2, . . . For each client node  3 , the hub node  2  is connected to an electrical client portion  5 . The electrical client portion  5  includes a transmitter  7  that converts electrical signals to optical signals and a receiver  9  that converts the optical signals to electrical signals. The electrical client portion  5  couples 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  also contains an optical receiver-transmitter combination  13 ,  15  for transmission of light signals. The receiver  13  receives the light signal from the transmitter  7  of the electrical client portion  5 , and outputs to the corresponding transmitter  15 , which provides a well-defined light signal in the narrow wavelength band used for the respective client. In turn, the optical transmitter  15  outputs to an optical fiber via an optical connector; the optical fiber connects to add/drop modules  23   l ,  23   r.    
     Furthermore, the optical client portion  11  includes an optical receiver-transmitter combination  21 ,  17   e , and  17   w ; the combination  21 ,  17   e , and  17   w  employs two optical receivers  17   e ,  17   w  that are connected to receive light from the add/drop modules  23   l ,  23   r  through optical fibers and optical connectors. One receiver  17   e  is used to receive light propagating in the network, in the appropriate fiber, in an east direction. Another receiver  17   w  receives light propagating in the west direction of the ring network. The outputs of the two optical receivers  17   e ,  17   w  are connected to inputs of a combining element  19 , which combines the received signals and provides the composite signal to a transmitter  21 . The output terminal of transmitter  21  is connected to the receiver  9  in the electrical client portion  5  through a respective connector and fiber, as shown. 
     Hub node  2  includes a number of add/drop modules  23   i ,  23   r . One pair of such add/drop modules  23   l ,  23   r  is provided for each client node  3  in the network  100 , in which one add/drop module  23   l  receives and transmits light signals in the leftward direction from the hub node  2 . The other module  23   r  receives and transmits in the rightward direction from the hub node  2 . Each add/drop module  23   l ,  23   r  is connected in the two ring-shaped fiber paths  1   e ,  1   w  of the network. The left add/drop module  23   l  has an add device  25   l  that is connected to the fiber ring path  1   w  and a drop device  27   l  that is connected to the other fiber ring path  1   e.    
     The add device  25   l  is connected to the transmitter  15  in the optical client portion  1  of the corresponding client node, which in this case is client  1 , through fiber pieces, a 50/50 splitting coupler  29  and a respective connector. The drop device  271  is connected to the receiver  17   e  in the optical client portion  11  for the same client node (i.e., Client  1 ) through a fiber and a respective connector. 
     Similarly, the right add/drop module  23   r  in the pair includes an add device  25   r  that is connected to the fiber ring path  1   e  and a drop device  27   r  that is connected to the other fiber ring path  1   w . The add device  25   r  is connected to the transmitter  15  in the optical client portion  11  for the client node (Client  1 ) through fiber pieces, the splitting coupler  29  and the respective connector. The drop device  27   r  is connected to the receiver  17   w  in the optical client portion  11  for the client node through a fiber and the respective connector. 
     The add devices  25   l ,  25   r  contain some coupling or combining element (not shown) and, if required, a notch filter (not shown) that blocks light of the wavelength band or channel for which the add/drop module is designed. This optional notch filter filters out the light of the corresponding wavelength band before the light of the same wavelength band is added in the combining element. Likewise, the drop devices  27   l ,  27   r  possess some splitting and filtering elements for tapping off only the light of the wavelength band or channel for which the add/drop module is designed. 
     Because the add/drop modules  23   i ,  23   r  are similar in design and functionality, they can have identical physical configurations. 
     The two fiber ring paths  1   e ,  1   w  are connected to the left and right sides of hub node  2 . At each such side, monitor modules  31   l ,  31   r  are interposed between the respective add/drop modules  23   i ,  23   r , and the fibers  1   e  and  1   w , thereby permitting monitoring of the signals outputted from the hub node  2 . Each of the monitor modules  31   l ,  31   r  includes an add coupler  33   l ,  33   r  that adds, for example, some control signal and a tap  35   l ,  35   r  for tapping off some small portion of the incoming light power (e.g., 1%). 
     The schematic diagram of FIG. 2 illustrates to connections of the add/drop modules  23   i ,  23   r . That is, the connections of FIG. 2 pertain to add/drop modules  23   i ,  23   r ; however, for explanatory purposes, the components of add/drop module  23   l  are described. The module  23   l  includes a housing  41  which contains the add device  25   l  and the drop device  27   l . Light from fiber  1   e  of the two ring paths  1   e ,  1   w  of the network  100  enters the module  23   l  at a connector  43  that is attached to a fiber piece  45  extending loosely outside the housing  41 . The fiber piece  45  has a thick protective sleeve that continues within the housing up through a weld section  47 , which splices an end of an optical fiber  49  that has a standard thin protective sleeve to the fiber piece  45 . The other end of the optical fiber piece  49  is connected to an input of the add device  25   l . The output of the add device  25   l  is connected to a fiber piece  51 , which in turn is connected on the same ring path  1   e  as the input connector  43 . The other input of the add device  25   l  is connected to a connector  57  that is attached to the housing  41  through a fiber piece  55 . 
     The connector  43  thus receives light from the ring path  1   e ; the light is carried on fiber  45  through the weld section  47 , the fiber piece  55 , the add device  25   l , and the fiber piece  51  to the connector  53 . The output connector  53  enables the light to continue along the ring path  1   e.    
     Light from an optical client portion  11  enters the module  23   l  at the connector  57 , and travels through the fiber to the add device  25   l , which adds the light to the ring path  1   e . In the module  23   i , a set of connected fiber pieces, 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   l  the drop device  27   l  is connected. Accordingly, a connector  63  is connected to the ring path and is attached to an end of a well protected fiber piece  65  that extends partly outside the housing  41 . The other end of the fiber piece  65  is connected to a standard fiber piece  69  through a weld section  67 . Fiber piece  69  is in turn connected to one of the two outputs of the drop device  27   l . The input of the drop device  27   l  is connected to fiber piece  71 , which receives light from a connector  73  that is attached to the housing  41 . The connector  73  is connected in the respective ring path  1   w . The other output of the drop device  27   l  is connected to an end of a fiber piece  75 , which has the other end connected to connector  77 . Connector  77  is attached to the housing  41  and is coupled to a receiver  17   w  in the optical client portion  11  Light from the ring path  1   w  enters the module  23   l  at the connector  73  and travels through fiber  71  to the drop device  27   l , which passes the light to fiber  69  and then the weld  67 . Thereafter, the thick fiber  65  carries the light to the loose connector  63 , which in turn is connected in the ring path  1   w  of the network  100 . Light of a specific wavelength band is tapped off in the drop device  27   l  and continues through the fiber  75  to the connector  77 . The connector  77  then carries the light to the optical client portion  11 . 
     The connections within housing  41  of add/drop module  23   l  are configured in the same manner for the right add/drop module  23   r . Accordingly, for clarity, FIG. 2 shows the add device as  25   l, r  and the drop device  27   l, r  to convey the fact that the configuration of the housing  41  pertains to both add/drop modules  23   i ,  23   r . The physical layout of the interior of an add/drop module housing  41  is shown in FIG.  3 . 
     FIG. 3 shows a side view of the inside of the housing of the add/drop module. The module housing  41  has a substantially flat portion from which various wads stand out. The wads all have the same height and connect to an essentially flat lid (not shown), which is mounted over the housing  41 . The walls form two circular winding cores  81  with sufficiently large diameters, e.g., about 50 mm, to allow the fibers to be wound around them, as to not be subjected to bending radii that are too small (i.e., curvatures that are too large). The two winding cores are placed at some distance of each other to permit the fibers to pass therebetween. 
     By arranging the two cores in this manner, the fiber pieces can be comfortably handled, so that the fibers can be spliced to the devices in the case of fiber breaks or bad splices. In addition, the direction of the fiber pieces at the points where they are connected to the devices can be selected to not have bends that are too small; for instance, the fibers can be placed in a figure eight configuration around the two cores. Inside the wads that form the winding cores, through-holes  83  may be arranged to permit easy handling of the module. The add devices  25   l ,  25   r  and the drop devices  27   l ,  27   r  can be attached between outstanding wads  85  at the top of the housing  41 . 
     Attaching devices are provided at the lower edge of the housing devices to attach the housing  41  to a rack. The attaching devices include 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 to allow fibers that connect the module to the associated optical client portion  11  to be held therein. 
     FIG. 4 shows a front view of the add/drop modules and the monitor modules mounted in a rack. The loosely extending fiber pieces  45 ,  65  (FIG. 2) are not shown in this figure. Comparing FIG. 4 to FIG. 1, it is seen that the connection of all left add/drop modules is as indicated in FIG. 3, in which the extending fiber pieces  45  are inserted in the mating connectors  53 ,  73  in an adjacent module at the left side. This connection is illustrated in the schematic view of FIG. 6 (as well as FIG.  5 ). 
     In FIG. 5, the same basic connection as in FIG. 1 is illustrated; the add and drop devices in the right modules  23   r  are transposed as compared to the left modules  23   l . As a result, a cross coupling exists between the group of left modules  23   l  and the group of right modules  23   r  and between the right group of modules  23   r  and the right monitor module  31   r.    
     The same connection of the modules is illustrated in FIG. 6, in which the connections between the add/drop modules via loosely extending fiber pieces are shown. From FIG. 6, it is clear that all add/drop modules can have an identical construction; only the tapping-off devices and filters are tailored to the channel for which the respective module is designed. The cross coupling resides in the middle of the assembly of add/drop modules, between the left and right modules. This cross connection can be made by connecting the loose fiber pieces  10   l  with optical connectors at each end in a cross configuration; alternatively, a particular cross connecting module can be used for the cross connection. Such a cross connecting module may have the same exterior shape as the other modules. 
     In FIG. 4, a multitude of add/drop modules  23   i ,  23   r  are illustrated. However, only a portion of the add/drop modules  23   i ,  23   r  may be required for a particular network architecture. Consequently, dummy modules with the same exterior layout but without the connectors and the loosely extending fibers are inserted. The dummy modules serve as place holders for expansion. 
     The two monitor modules  31   l ,  31   r  have the same exterior connectors as well as interior devices. However, the interior connections differ because of the orientation. The connections are shown in FIG.  1 . 
     The client nodes  3  have a similar design to that of the hub node  2 ; however, the client nodes  3  are configured to receive and transmit in only one wavelength band. For example, the add/drop modules  23   i ,  23   r , as described above, can be used in the client nodes  3 . It is noted that in other network ring architectures, the client nodes  3  can be designed to receive and transmit in more than one wavelength band. In this case, the client nodes  3  will have the same structure as the hub node  2 . 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.