Abstract:
A multi-degree expandable reconfigurable optical add drop multiplexer (ROADM) based on a wavelength-selective crossconnect (WSXC), and method for upgrading the same. The WSXC generally consists of an outer layer of optical fan-out devices, and an outer layer of optical fan-in devices. At least one inner layer of optical fan-out or fan-in devices, including at least one wavelength switch, is disposed between the outer layer of optical fan-out devices and the outer layer of optical fan-in devices in a cascaded arrangement relative to the outer layers. At least one output port of an optical fan-out device in the outer layer of optical fan-out devices is connected to an input port of an optical device in the at least one inner layer, and at least one output port of an optical device in the at least one inner layer is connected to an input port of an optical fan-in device in the outer layer of optical fan-in devices.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates generally to optical networks, and more particularly, to a methodology and system that facilitates network growth through expandable multi-degree reconfigurable optical add-drop multiplexers (ROADMs). 
       BACKGROUND OF THE INVENTION  
       [0002]    In less than a decade, the state of the art in fiber-optic transport systems has progressed from simple point-to-point chains of optically amplified fiber spans to massive networks with hundreds of optically amplified spans connecting transparent add-drop nodes spread over transcontinental distances. Cost reduction has been the primary driver for this transformation, and the primary enabler has been the emergence of the reconfigurable optical add/drop multiplexer (ROADM) as a network element (NE). 
         [0003]    Exploiting the inherent wavelength granularity of wavelength-division multiplexing (WDM), an optical add/drop multiplexer (OADM) allows some WDM channels (also referred to as wavelengths) to be dropped at a node, while the others traverse the same node without electronic regeneration. Previously, it was necessary to terminate line systems at each node served, and then regenerate the wavelength signals destined for other nodes. The ability to optically add/drop a fraction of a system&#39;s wavelengths at a node was first achieved using fixed OADMs. These were constructed from optical filters, and by enabling wavelengths to optically bypass nodes and eliminate unnecessary regeneration, they provided significant cost savings. However, because traffic growth is inherently unpredictable, it is advantageous for the add-drop capability to be reconfigurable. 
         [0004]    ROADMs provide many advantages beyond the savings achieved by optically bypassing nodes. In the future, multi-degree ROADMs with adequate reconfiguration speeds may enable shared-mesh restoration at the optical layer. Shared mesh restoration significantly reduces the number of wavelength channels that must be installed as redundant protection circuits. ROADMs also provide operational advantages. Because ROADMs can be reconfigured remotely, they enable new wavelength channels to be installed by simply placing transponders at the end points, without needing to visit multiple intermediate sites. In addition to these cost-saving benefits, ROADMs will enable new services. For example, if transponders are preinstalled, then new circuits can be provided on-demand. The rapid network reconfiguration provided by ROADMs could also become an enabler of dynamic network services, such as switched video for IPTV. For all of these reasons, ROADMs will continue to have a significant effect on the design of optical networks. 
         [0005]    Generally, a ROADM is defined as a NE that permits the active selection of add and drop wavelengths within a WDM signal, while allowing the remaining wavelengths to be passed through transparently to other network nodes. Thus, the simplest ROADM will have two line ports (East and West) that connect to other nodes, and one local port (add/drop) that connects to local transceivers. In today&#39;s networks, optical links are typically bidirectional, so each line port represents a pair of fibers. When using conventional local transceivers that can process only a single wavelength at a time, the number of fibers in the add/drop port sets the maximum number of wavelengths that can be added or dropped at a given node. 
         [0006]    A ROADM with only two line ports (East and West) is referred to as a two-degree ROADM. Practical networks also have a need for multi-degree ROADMs that can serve more than two line ports. In addition to providing local add/drop of from each of its line ports, the multi-degree ROADM must be able to interconnect any individual wavelength from one line port to another, in a reconfigurable way. The degree of a multi-degree ROADM is equal to the number of line-side fiber pairs that it supports (it does not include the number of fiber pairs used in the add/drop portion of the ROADM). 
         [0007]    Many designs for multi-degree ROADMs are based on modules known as wavelength selective crossconnects (WSXCs). A WSXC is a module which accepts WDM optical signals into each of its plurality of inputs, then routes each incoming wavelength to one of its plurality of outputs in a selectable and reconfigurable way. As suggested by the word ‘crossconnect’, any incoming wavelength can be individually switched to different output ports as needed. An incoming wavelength may have access to the full set of outputs, or it may be restricted to a plural subset of outputs. Conversely, the output signal at a given wavelength on a given output port may be chosen from different input ports, either the full set of input ports or a plural subset of the input ports.  FIG. 1  shows two ROADM designs of degree three utilizing a WSXC. In  FIG. 1   a,  both the line-side signals and the local add/drop signals pass through the WSXC  100   a.  The local add/drop signals are demultiplexed and multiplexed via multiplexer/demultiplexers (mux/demux)  102   a.  In  FIG. 1   b,  the line-side signals pass through the WSXC  100   b,  but the local add/drop signals are passed around the WSXC  100   b  via power splitter(PS)/combiners(PC)  104   b  coupled to mux/demux  102   b.  Note that for  FIG. 1   b,  each add/drop fiber is assigned to a specific line-side direction (port). In contrast, the design of  FIG. 1   a  allows each wavelength from any add/drop fiber to be ‘steered’ to different line-side ports. 
         [0008]    A full ROADM provides add/drop (de)multiplexing of any arbitrary combination of wavelengths supported by the system with no maximum, minimum, or grouping constraints. A partial ROADM only has access to a subset of the wavelengths, or the choice of the first wavelength introduces constraints on other wavelengths to be dropped. The drop fraction of a ROADM is the maximum number of wavelengths that can be simultaneously dropped, divided by the total number of wavelengths in the WDM signal. If a given add or drop fiber is capable of handling any wavelength, it is said to be colorless. If a given add or drop fiber can be set to address any of the line ports (e.g., east or west for a 2-degree ROADM), it is said to be “steerable.” A NE is characterized as “directionally separable” if there is no single failure that will cause a loss of add/drop service to any two of its line ports. 
         [0009]    An example of a WSXC  200  for connecting three fiber pairs (three bidirectional ports) in the ROADM of type shown in  FIG. 1   b  is depicted in  FIG. 2 . The WSXC  200  includes three power splitters (PSs)  202  and wavelength selective switches (WSSs)  204 . The PSs  202  and WSSs  204  in the WSXC  200  are coupled to PSs  202 , power combiners  203  and amplifiers  206  in the ROADM. Any incoming wavelength can be routed to either of two output fibers. For example, a wavelength coming from the East port can be sent out through the West port or the South port. The design of  FIG. 2  does not allow loopback, the process by which a wavelength entering from the East port can be sent back out on the output fiber of the same (East) port. Some other WSXC designs, do support loopback. 
         [0010]      FIG. 3  depicts a multi-degree ROADM  300  that supports colorless, steerable add/drop functionality, based on a WSXC core. The ROADM  300  comprises a plurality of 1×8 power splitters (PSs)  302  and 8×1 WSSs  304 . A transponder bank  306  is coupled to a tunable multiplexer (MUX)  308  and an input port of a first of power splitters  302 , and a tunable demultiplexer (DMUX)  310 , which is coupled to the output port of a WSS  304 . Similarly, a transponder bank  312  is coupled to a tunable MUX  314  connected to an input port of a second of power splitters  302 , and a tunable DMUX  316  connected to an output port of a WSS  304 . This configuration has a maximum degree M=9 where M=N+1 for a WSS having N×1 ports. The maximum degree M will be reduced by one if the ROADM is required to support loopback from the input fiber of a given degree to the output fiber of the same degree. 
         [0011]      FIG. 4  is a schematic of a ROADM  400  similar to that of  FIG. 3 , where a WSS is substituted for each PS. The ROADM  400  comprises a plurality of 1×8 WSSs  402  and 8×1 WSSs  404 . A transponder bank  406  is coupled to a tunable multiplexer (MUX)  408  and an input port of a first of WSSs  402 , and a tunable demultiplexer (DMUX)  410 , which is coupled to the output port of a first of WSSs  404 . Similarly, a transponder bank  412  is coupled to a tunable MUX  414  connected to an input port of a second of WSSs  402 , and a tunable DMUX  416  connected to an output port of a second of WSSs  404 . As with the ROADM of  FIG. 3 , this configuration has a maximum degree M=9 where M=N+1 for WSS having 1×N at the input ( 402 ) and N×1 ports on the output  404 , assuming that loopback is not required. 
         [0012]    Carriers wish to deploy systems in the most cost-effective manner possible. Today, it is far more cost-effective to initially deploy the minimal amount of equipment that can smoothly evolve to meet future needs, rather than to deploy a fully loaded system configuration from the very beginning. Currently and for the foreseeable future, transponders make up the dominant cost of a fully loaded optical communication system. If a full set of transponders were included in the initial deployment, then a substantial cost would be incurred before the network had sufficient traffic to support the expense. Therefore, systems are routinely designed to permit incremental deployment of transponders on an as-needed basis. Similar considerations also apply to multiplexers, although the economic drivers are not as strong. In general, modular growth will be supported whenever the additional cost and complication of upgrading to higher capacity in the future is small compared to the financial impact of a full equipment deployment at startup. By designing this pay-as-you-grow approach into ROADMs, the network itself can grow in a cost-effective manner. Traditional networks grow by adding and interconnecting stand-alone line systems, incurring substantial cost and complexity. By using ROADMs that allow for modular deployment of additional ports, network growth can benefit from both the equipment and operational efficiencies of integrating line systems as they are needed into a seamless network. Because networks are deployed over the course of years, carriers prefer to be able to grow the nodes of the network from terminals or amplifiers, into 2-degree ROADMs, and eventually into multi-degree ROADMs. This not only allows the expense to be spread out over years, it also enables the network designers to respond to unforeseen traffic growth patterns. 
         [0013]    For ROADMs based on WSXCs, the degree of the ROADM is dependent on the degree of the WSXC module. It would therefore be desirable to provide a new type of WSXC that can be scaled to large degree. It would also be desirable to provide a method of upgrading the WSXC after installation, increasing the number of degrees to accommodate growth in traffic being carried by the fiber optic network. 
       SUMMARY OF THE INVENTION  
       [0014]    In accordance with an aspect of the invention, there is provided a wavelength-selective crossconnect (WSXC), comprising: an outer layer of optical fan-out devices, each of the outer layer of optical fan-out devices having an input port and a plurality of output ports; an outer layer of optical fan-in devices, each of the outer layer of optical fan-in devices having an output port and a plurality of input ports; and at least one inner layer of optical devices disposed between the outer layer of optical fan-out devices and the outer layer of optical fan-in devices, wherein at least one output port of an optical fan-out device in the outer layer of optical fan-out devices is connected to an input port of an optical device in the at least one inner layer, and at least one output port of an optical device in the at least one inner layer is connected to an input port of an optical fan-in device in the outer layer of optical fan-in devices. To achieve the reconfigurable wavelength routing required of the WSXC, the at least one inner layer of optical devices includes at least one WSS. 
         [0015]    The fan-out devices in the outer layer may comprise a power splitter or 1×N wavelength selective switch (WSS), and the fan-in devices in the outer layer may comprise a power combiner or N×1 WSS. The optical devices in the at least one inner layer may comprise 1×N or N×1 WSSs, as either a fan-out or fan-in layer. 
         [0016]    In accordance with one aspect of the invention, the at least one inner layer comprises a first inner layer and a second inner layer of optical devices: the first inner layer of optical devices comprising a plurality of optical fan-out devices arranged in a cascade with respect to the outer layer of optical fan-out devices, each of the fan-out devices in the first inner layer comprising an input port and a plurality output ports; the second inner layer of optical devices comprising a plurality of optical fan-in devices arranged in a cascade with respect to the outer layer of optical fan-in devices, each of the fan-in devices in the second inner layer comprising a plurality of input ports and an output port, each of the optical fan-out devices in the outer-layer being coupled to an optical fan-out device in the first inner layer, an optical fan-in device in the second inner layer, or an optical fan-in device in the outer layer of optical fan-in devices, and each of the optical fan-in devices in the outer layer being coupled to an optical fan-in device in the second inner layer, an optical fan-out device in the first inner layer, or an optical fan-out device in the outer layer of optical fan-out devices. 
         [0017]    In accordance with another aspect of the invention, there is provided a method for upgrading a WSXC comprising an outer layer of optical fan-out devices, each optical fan-out device having an input port and a plurality of output ports, and an outer layer of optical fan-in devices, each optical fan-in device having an output port and a plurality of input ports. The method includes the steps of: adding at least one inner layer of optical devices disposed between the outer layer of optical fan-out devices and the outer layer of optical fan-in devices, and connecting at least one output port of an optical fan-out device in the outer layer of optical fan-out devices to an input port of an optical device in the at least one inner layer, and connecting at least one output port of an optical device in the at least one inner layer to an input port of an optical fan-in device in the outer layer of optical fan-in devices. The at least one inner layer of optical devices includes at least one WSS. 
         [0018]    In one implementation, the method includes arranging a plurality of optical fan-out devices in the at least one inner layer in a cascade with respect to the outer layer of optical fan-out devices, each of the fan-out devices in the at least one inner layer comprising an input port and a plurality output ports. 
         [0019]    In an alternative implementation, the method includes arranging a plurality of optical fan-in devices in the at least one inner layer in a cascade with respect to the outer layer of optical fan-in devices, each of the fan-in devices in the at least one inner layer comprising a plurality of input ports and an output port. 
         [0020]    In another expedient, the at least one inner layer comprises a first inner layer and a second inner layer of optical devices, and the first inner layer of optical devices comprises a plurality of optical fan-out devices arranged in a cascade with respect to the outer layer of optical fan-out devices, each of the fan-out devices in the first inner layer comprising an input port and a plurality output ports; and the second inner layer of optical devices comprises a plurality of optical fan-in devices arranged in a cascade with respect to the outer layer of optical fan-in devices, each of the fan-in devices in the second inner layer comprising a plurality of input ports and an output port. In this case, the method of upgrading the ROADM further comprises: connecting each of the optical fan-out devices in the outer-layer to an optical fan-out device in the first inner layer, an optical fan-in device in the second inner layer, or an optical fan-in device in the outer layer of optical fan-in devices, and connecting each of the optical fan-in devices in the outer layer to an optical fan-in device in the second inner layer, an optical fan-out device in the first inner layer, or an optical fan-out device in the outer layer of optical fan-out devices. 
         [0021]    In accordance with another aspect of the invention, there is provided a method for upgrading a WSXC including at least one layer of optical fan-out devices and a first and second layer of optical fan-in devices, each optical fan-out device connected to a plurality of optical fan-in devices, and each optical fan-in device in the first layer connected to at least one optical fan-out device, and each optical fan-in device in the first layer initially having no open input port, and at least one optical fan-in device in the second layer initially having at least one open input port. The method comprises: establishing at least one new connection from a fan-out device to an input port on the second layer of fan-in devices; shifting signals from an input port on the first layer of fan-in devices to the newly-connected input port on the second layer of fan-in devices, freeing up an input port on a fan-in device in the first layer; removing the old connection to the newly-freed input port; and adding a new fan-in device to the second layer of fan-in devices, the new fan-in device coupled to the newly-freed port of the fan-in device in the first layer. 
         [0022]    In accordance with yet another aspect of the invention, there is provided a method for upgrading a WSXC including at least one layer of optical fan-in devices and a first and second layer of optical fan-out devices, each optical fan-in device connected to a plurality of optical fan-out devices, and each optical fan-out device in the first layer connected to at least one optical fan-in device, and each of the optical fan-out devices in the first layer initially having no open output port, and at least one optical fan-out device in the second layer initially having at least one open output port. The method comprises: establishing at least one new connection from a fan-in device to an output port on the second layer of fan-out devices; shifting signals from an output port on the first layer of fan-out devices to an output port on the second layer of fan-out devices, freeing up an output port on a fan-out device in the first layer; removing the old connection to the newly-freed port; and adding a new fan-out device to the second layer of fan-out devices, the new fan-out device coupled to the newly-freed port of the fan-out device in the first layer. 
         [0023]    These aspects of the invention and further advantages thereof will become apparent to those skilled in the art as the present invention is described with particular reference to the accompanying drawings 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0024]      FIG. 1   a  is a schematic of an exemplary prior art 3-degree ROADM with steerable add/drop, based on a WSXC; 
           [0025]      FIG. 1   b  is a schematic of an exemplary 3-degree ROADM with fixed add/drop, based on a WSXC; 
           [0026]      FIG. 2  is a schematic of an exemplary prior art WSXC in the ROADM of the type shown in  FIG. 1   b;    
           [0027]      FIG. 3  is a schematic of an exemplary prior art multi-degree ROADM utilizing power splitters and WSSs; 
           [0028]      FIG. 4  is a schematic of an exemplary prior art multi-degree ROADM using all WSSs; 
           [0029]      FIG. 5  is a schematic of an illustrative three-layer 4-degree WSXC in an initial deployment accordance with aspects of the invention; 
           [0030]      FIG. 6  is a schematic of a 5-degree WSXC that has been upgraded from the 4-degree ROADM shown in  FIG. 5 ; 
           [0031]      FIG. 7  is a schematic of another illustrative three-layer 4-degree WSXC in an initial deployment in accordance with aspects of the invention; 
           [0032]      FIG. 8  is a schematic of a 5-degree WSXC that has been upgraded from the 4-degree WSXC shown in  FIG. 7 ; 
           [0033]      FIGS. 9-13  are schematics of an exemplary upgrade process in accordance with aspects of the invention for converting a three-layer 4-degree WSXC ( FIG. 9 ) to a 5-degree WSXC ( FIG. 13 ); 
           [0034]      FIGS. 14-16  are schematics of an exemplary upgrade process in accordance with aspects of the invention for converting an initial deployment of a 2-degree WSXC ( FIG. 14 ) to a 3-degree WSXC ( FIG. 15 ), to a 6-degree WSXC ( FIG. 16 ), with two inner layers of optical fan-out and fan-in devices; 
           [0035]      FIG. 17  is a schematic of a WSXC including a plurality of optical amplifiers in one embodiment; and 
           [0036]      FIG. 18  is a schematic of a WSXC including a plurality of optical amplifiers in an another embodiment 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    Embodiments of the invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout to the extent possible. Before embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of the examples set forth in the following description or illustrated in the figures. The invention is capable of other embodiments and of being practiced or carried out in a variety of applications and in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
         [0038]    Referring to  FIG. 5 , there is depicted a schematic of an illustrative WSXC  500  in an initial deployment in accordance with an aspect of the present invention. ROADM  500  comprises an outer layer  502  of 1×M optical “fan-out” devices (i.e., power splitters or wavelength selective switches (WSSs))  504  and an outer layer  506  of P×1 optical “fan-in” devices (i.e., power combiners or WSSs)  508 . An inner layer  510  of N×1 optical fan-in devices, comprising a plurality of WSSs  512 , is arranged in a cascade with respect to the outer layer  506  of optical fan-in devices  508 . In the depicted embodiment, M=6, P=3, and N=3 for clarity, but it will be understood that this arrangement may be implemented with any integers N, P, and M. A WSS is an optical device that routes different spectral components from the desired input port(s) to the desired output port(s) without optical-to-electrical-to-optical conversion. In the embodiment shown in  FIG. 5 , each WSS  512  is arranged with a plurality of input ports  514  and a single output port  516 . Each of the fan-out devices  504  has an input port  518  and a plurality of output ports  520 , with a 1×6-port fan-out device shown. In the initial deployment of the illustrative WSXC  500 , there are three unused ports on each fan-out device  504  as indicated by the dotted lines in the schematic. The output ports  520  of each fan-out device  504  in the outer layer  502  couple the fan-out device  504  to a plurality of WSSs  512  in the inner layer  510  via the respective input ports  514 . Each fan-in device  508  has a plurality of input ports  522  and an output port  524 . One of the input ports  522  of each fan-in device  508  in the outer layer  506  is coupled to an output port  516  of a WSS  512  in the inner layer. In the example shown, there are two unused input ports  522  on each fan-in device  508  in the outer layer  506  for future growth as shown by the dotted lines. Also, it will be understood by those of skill in the art that the fan-out devices  504  in the outer layer  502  may be coupled to either the WSSs  512  in the inner layer or the fan-in devices  508  in the outer layer  506 . Similarly, the fan-in devices  508  in the outer layer  506  may be coupled to either the WSSs  512  in the inner layer  510 , or the fan-out devices  504  in the outer layer  502 . 
         [0039]      FIG. 6  is a schematic of a five-degree WSXC  600 , which has been expanded from the 4-degree ROADM depicted in  FIG. 5 . In this embodiment, an additional fan-out device  604  has been added to the outer layer  602  of fan-out devices  604 , and an additional fan-in device  608  has been added to the outer layer  606  of fan-in devices  608 . Additional WSSs  612  have been added to the inner layer  610  of fan-in devices  612 . The new connections and hardware are shown with dashed lines in  FIG. 6 . Similarly, the open output ports  620  of the fan-out devices  604 , open input ports  614  of the WSSs  612 , and open input ports  622  of the fan-in devices  608  for further growth are shown as the dotted lines in  FIG. 6 . Note, all other connections are similar to those shown in  FIG. 5 , where like numbers represent like elements. 
         [0040]      FIG. 7  is a schematic of a growable WSXC  700  in an initial deployment that is similar to the WSXC  500  of  FIG. 5 . However, in this embodiment an inner layer  710  of 1×N fan-out WSSs  712  are provided in lieu of the fan-in WSSs  512 . Here the plurality of WSSs  712  is arranged in a cascade with respect to the outer layer  702  of fan-out devices  704 . Each WSS  712  is arranged with an input port  714  and a plurality of output ports  716 . Each of the fan-out devices  704  has an input port  718  and a plurality of output ports  720 , with a 1×3-port switch shown. Each fan-in device  708  has a plurality of input ports  722  and an output port  724 . In the initial deployment of the illustrative ROADM  700 , there are three unused ports  722  on each fan-in device  708  as indicated by the dotted lines in the schematic. The output ports  720  of each fan-out device  704  in the outer layer  702  couple the fan-out device  704  to a WSS  712  in the inner layer  710  via the respective input ports  714 . Each of the output ports  716  of WSS  712  is connected to an input port  722  of a fan-in device  708  in the outer layer  706 . In the example shown, there are two unused output ports  720  on each fan-out device  704  in the outer layer  702 , and three unused input ports  722  on each fan-in device  708  in the outer layer  706  for future growth as shown by the dotted lines. 
         [0041]      FIG. 8  is a schematic of a five-degree WSXC  800 , which has been expanded from the 4-degree WSXC  700  depicted in  FIG. 7 . In this embodiment, an additional fan-out device  804  has been added to the outer layer  802  of fan-out devices  804 , an additional fan-in device  808  has been added to the outer layer  806  of fan-in devices  808 . Additional WSSs  812  have been added to the inner layer  810  of fan-out devices. The new connections and hardware are depicted with dashed lines in  FIG. 6 . Similarly, the open output ports  820  of the fan-out devices  804 , open output ports  816  of the WSSs  812 , and open input ports  822  of the fan-in devices  808  to enable further growth, are shown as dotted lines in  FIG. 8 . All other connections are similar to those shown in  FIG. 7 , wherein like numbers represent like elements. 
         [0042]      FIGS. 9-13  are schematics that depict a methodology for upgrading a WSXC of the type shown in  FIG. 5  when all ports of the outer fan-in layer are full. The method involves rolling an existing connection to a new connection between inner layers, then removing the old connection to the outer layer, freeing up a port on the outer layer, and adding a new WSS to the inner layer of fan-in devices.  FIG. 9  illustrates the WSXC  900  in an initial deployment where all input ports  922  of fan-in devices (WSSs)  908  in the outer layer  906  are full. In this example, each WSS  908  is coupled to a fan-in WSS  912  in the inner layer  910  and a fan-out device  904  in the outer layer  902 . Using a similar convention to that employed in  FIG. 5 , each fan-out device  904  in layer  902  has an input port  918  and a plurality of output ports  920 , where one of the output ports  920  is free. Each WSS  912  has a plurality of input ports  914 , one of which is free, and an output port  916 . Each fan-in device  908  has a plurality of input ports  922  and an output port  924 . 
         [0043]    In  FIG. 10 , a first step of the upgrade is illustrated by WSXC  1000 , where a new connection is added between each of the fan-out devices  1004  in the first layer  1002  and the fan-in devices  1012  in the inner layer  1010 . Using a similar convention to  FIG. 9 , each fan-out device  1004  in the outer layer  1002  includes an input port  1018  and a plurality of output ports  1020 , each WSS  1012  in the inner layer  1010  includes a plurality of input ports  1014  and an output port  1016 , and each WSS  1008  in the outer layer  1006  includes a plurality of input ports  1022  and an output port  1024 . The new connection  1026  is added by connecting a previously open output port of each fan-out device  1004  to a previously open input port  1014  of each WSS  1012 . 
         [0044]    In  FIG. 11 , a second step of the upgrade is illustrated by WSXC  1100 , where open ports are created in the outer layer of fan-in devices. Using a similar convention to  FIGS. 9 and 10 , each fan-out device  1104  in the outer layer  1102  includes an input port  1118  and a plurality of output ports  1120 , each WSS  1112  in the inner layer  1110  includes a plurality of input ports  1114  and an output port  1116 , and each WSS  1108  in the outer layer  1106  includes a plurality of input ports  1122  and an output port  1124 . The connection  1126  that was previously added between each fan-out device  1104  and WSS  1112  enables the removal of a connection  1128  (indicated by the dotted lines in  FIG. 11 ) between an output port  1120  of each fan-out device  1104  and an input port  1122  of each WSS  1108  in the outer layer  1116 , thereby freeing up an input port  1122  in each WSS  1108  in the outer layer  1116 . 
         [0045]    In  FIG. 12 , a third step of the upgrade is illustrated by WSXC  1200 , where additional WSSs  1212  are added to the inner layer  1210  of WSSs. Using a similar convention to  FIGS. 9-11 , each fan-out device  1204  in the outer layer  1202  includes an input port  1218  and a plurality of output ports  1220 , each WSS  1212  in the inner layer  1210  includes a plurality of input ports  1214  and an output port  1216 , and each WSS  1208  in the outer layer  1206  includes a plurality of input ports  1222  and an output port  1224 . Additional WSSs  1212  as shown by the dashed lines have been added to the inner layer  1210  by connecting the respective output ports  1216  thereof to the open output ports  1222  on the WSSs  1208  in the outer layer  1206 . 
         [0046]    In  FIG. 13 , a fourth step of the upgrade is illustrated by WSXC  1300 , where a new row of devices are added to increase degrees of the ROADM from four to five. Using a similar convention to  FIGS. 9-12 , each fan-out device  1304  in the outer layer  1302  includes an input port  1318  and a plurality of output ports  1320 , each WSS  1312  in the inner layer  1310  includes a plurality of input ports  1314  and an output port  1316 , and each WSS  1308  in the outer layer  1306  includes a plurality of input ports  1322  and an output port  1324 . An additional fan-out device  1304  as shown by the dashed lines has been added to the outer layer  1302  of fan-out devices, additional WSSs  1312  have been added to the inner layer  1312  of fan-in devices, and an additional WSS  1308  has been added to the outer layer  1306  of fan-in devices. New connections between the fan-out devices  1304  in the first layer  1302  and the WSSs  1312  in the inner layer  1310 , and a new connection between a fan-out device  1304  and a WSS  1308  in the outer layer  1306  are shown by the dashed lines and indicated at  1326 . This exemplary process transforms the 4-degree WSXC  900  shown in  FIG. 9  to the 5-degree WSXC  1300  shown in  FIG. 13 . It will be appreciated by those skilled in the art that a similar methodology may be employed to upgrade a three-layer ROADM comprising an inner layer of optical fan-out devices (instead of fan-in devices) that are arranged in a cascade with respect to the outer layer of fan-out devices as represented by the embodiment of  FIG. 7 . 
         [0047]      FIGS. 14-16  depict an illustrative growth path from a 2-degree WSXC  1400  in  FIG. 14 , to a 3-degree WSXC  1500  in  FIG. 15 , to a 6-degree WSXC  1600  in FIG.  16  in accordance with another exemplary embodiment that utilizes an inner layer of optical fan-out devices and an inner layer of optical fan-in devices. 
         [0048]      FIG. 14  depicts an initial deployment of the 2-degree WSXC 1400 , which comprises a first or “outer” layer  1402  of fan-out devices (WSSs)  1404 , and a second or “outer” layer  1406  of fan-in devices (WSSs)  1408 . Each WSS  1404  comprises an input port  1418  and a plurality of output ports  1420 , and each WSS  1408  comprises a plurality of input ports  1422  and an output port  1424 . In the initial deployment, there are two free output ports  1420  on each WSS  1404  and two free input ports  1422  on each WSS  1408 . 
         [0049]      FIG. 15  illustrates a growth path to a 3-degree WSXC  1500 , which comprises an outer layer  1502  of fan-out devices (WSSs)  1504 , an outer layer  1506  of fan-in devices (WSSs)  1508 , an inner layer  1510  of fan-in devices (WSSs)  1512  arranged in a cascade with respect to outer layer  1506 , and an inner layer  1530  of fan-out devices (WSSs)  1532  arranged in a cascade with respect to outer layer  1502 . The hardware and connections utilized in the upgrade are shown by dashed lines. Each WSS  1504  includes an input port  1518  and a plurality of output ports  1520 , each WSS  1508  includes a plurality of input ports  1522  and an output port  1524 , each WSS  1512  includes a plurality of input ports  1514  and an output port  1516 , and each WSS  1532  includes an input port  1534  and a plurality of output ports  1536 . As shown, the WSSs  1504  in outer layer  1502  couple to either a fan-out WSS  1532  in layer  1530 , a fan-in WSS  1512  in layer  1510 , or a fan-in WSS  1508  in layer  1506 . Similarly, a fan-in WSS  1508  couples to either a fan-in WSS  1512  in layer  1510 , a fan-out WSS  1532  in layer  1530  or a fan-out WSS  1504  in layer  1502 . The free input ports  1514 ,  1522  on the fan-in devices and output ports  1520 ,  1536  on fan-out devices to enable future growth are depicted by the dotted lines in the drawing. 
         [0050]      FIG. 16  depicts the growth path from the 3-degree WSXC  1500  shown in  FIG. 15  to a 6-degree WSXC  1600 . The WSXC  1600  comprises an outer layer  1602  of fan-out devices (WSSs)  1604 , an outer layer  1606  of fan-in devices (WSSs)  1608 , an inner layer  1610  of fan-in devices (WSSs)  1612  arranged in a cascade with respect to outer layer  1606 , and an inner layer  1630  of fan-out devices (WSSs)  1632  arranged in a cascade with respect to outer layer  1602 . Each WSS  1604  includes an input port  1618  and a plurality of output ports  1620 , each WSS  1608  includes a plurality of input ports  1622  and an output port  1624 , each WSS  1612  includes a plurality of input ports  1514  and an output port  1516 , and each WSS  1532  includes an input port  1534  and a plurality of output ports  1536 . In this upgrade, additional fan-out WSSs  1604  have been added to the outer layer  1602 , additional fan-out WSSs  1632  have been added to the inner layer  1630 , additional fan-in WSSs  1612  have been added to the inner layer  1610 , and additional fan-in WSSs  1608  have been added to the outer layer  1606 . The newly added hardware and connections therebetween are again illustrated by dashed lines. The free input ports  1614 ,  1622  on the fan-in devices and output ports  1620 ,  1636  on fan-out devices to enable future growth are depicted by the dotted lines in the drawing. 
         [0051]    Additional layers of fan-in and fan-out devices may be connected to the edge layers indirectly, through intermediate layers. By this iterative process of layering, a ROADM of arbitrarily large degree may be constructed, subject only to practical limitations such as optical loss (even loss can be overcome with optical amplifiers, but these add to the cost, and can degrade the optical signal to noise ratio of the signal, which can have negative system implications). 
         [0052]      FIG. 17  is a schematic of a WSXC  1700  similar to the WSXC  1500  shown in  FIG. 15 , but where a plurality of optical amplifiers  1738  are disposed between an outer layer  1702  of fan-out devices (WSSs)  1704  and an inner layer  1730  of fan-out devices (WSSs)  1732 . The connections between the WSSs  1712  and WSSs  1708  in the layers of fan-in devices are not amplified. 
         [0053]      FIG. 18  is a schematic of a WSXC  1800  similar to the WSXC  1700  shown in  FIG. 17 , but where a plurality of optical amplifiers  1838  are disposed between an inner layer  1810  of fan-in devices  1812  and outer layer  1806  of fan-in devices  1808 . The connections between the WSSs  1804  in the outer layer  1802  of fan-out devices and the WSSs  1832  in the inner layer  1830  of fan-out devices, and the connections between the WSSs  1804  in the outer layer of fan-out devices and WSSs  1808  in the outer layer  1806  of fan-in devices are not amplified. It will be appreciated by those skilled in the art that amplifiers may be placed at either the input side ( FIG. 17 ), the output side ( FIG. 18 ), at locations proximal to both the input and output sides, or at the center of the fabric between either the inner and outer layers or between an outer layer and an inner layer on the opposite side if necessary. 
         [0054]    The above-described WSXC expedients also have the ability to provide for enhanced multicasting. Optical multicasting is the capability to divide the input power on individual wavelengths and to simultaneously deliver those signals to multiple ports. Present WSS have limited multicast capability, with the maximum number of multicast outputs K typically much smaller (2 or 4) than the number of ports N. By providing two layers of WSSs, the number of simultaneous multicast outputs can be increased to K 2 . For three layers, the multicast output count would be even larger. The foregoing detailed description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the description of the invention, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.