Patent Publication Number: US-2006013587-A1

Title: Modular wavelength selective switch

Description:
RELATED APPLICATION  
      This application claims the benefit of U.S. Provisional Application No. 60/587,906 filed on Jul. 15, 2004. 
    
    
     FIELD OF THE INVENTION  
      The invention relates to wavelength selective switches.  
     BACKGROUND OF THE INVENTION  
      Wavelength selective switches operate to separate multiple wavelengths contained in an input signal, and to route each of these wavelengths to a selectable port. Typically, such switches have a fixed number of output ports, and are capable of operating on a fixed number of wavelengths. Conventional designs are not scalable meaning that once the port and/or wavelength capacity of a given wavelength selective switch is exhausted, then in order to provide increased capacity the switch will need to be replaced with a larger model.  
     SUMMARY OF THE INVENTION  
      According to one broad aspect, the invention provides an apparatus comprising: at least one first input port each for receiving a respective input multiple wavelength optical signal; for each first input port, an optical signal separator adapted to separate the input optical signal into at least two portions, and to output each portion to a respective first output port; at least one second output port for outputting a respective output optical signal; for each second output port, an optical signal combiner having at least two second input ports, the optical signal combiner adapted to combine optical signals received at the at least two second input ports; at least one reconfigurable wavelength selective device, each wavelength selective device interconnecting wavelength selectively one of the first output ports to at least one of the second input ports.  
      According to another broad aspect, the invention provides an apparatus comprising: a full-band drop device having an input port, a through port and a first plurality of drop ports; a full-band add device having an input port connected to the through port of the full-band device, and having a first plurality of add ports; a reduced-band drop device having a second plurality of drop ports, and having an input port connected to one of said first plurality of drop ports; a reduced-band add device having a second plurality of add ports and having an output port connected to one of said first plurality of add devices.  
      According to another broad aspect, the invention provides an apparatus comprising: a first main optical path comprising a first wavelength adding device having a first plurality of add ports and a first wavelength dropping device having a first plurality of drop ports; a second main optical path comprising a second wavelength adding device having a second plurality of add ports and a second wavelength dropping device having a second plurality of drop ports; for each of pair of drop ports comprising one port of each of said pluralities of drop ports, a respective optical signal combiner combining outputs of the pair of drop ports into a combined drop port signal; for each pair of add ports comprising one port of each of said pluralities of add ports, at least one optical separator separating an input signal to the two add ports.  
      According to another broad aspect, the invention provides an apparatus comprising: a plurality M of port pairs each comprising an input port and an output port; for each input port, an optical signal separator splitting an input optical signal into at least two portions; for each output port, an optical signal combiner for combining optical signals received at inputs to the optical signal combiner; a plurality of interconnections and wavelength selective switches between outputs of optical signal separators and inputs of the optical signal combiners.  
      According to another broad aspect, the invention provides a method comprising: receiving at least one input multiple wavelength optical signal; for each input multiple wavelength optical signal, separating the input optical signal into at least two portions; outputting at least one output optical signal as a combination of at least two optical signals; wavelength selectively switching at least one of the portions to produce at least one of the optical signals to be combined in the output optical signals.  
      In some embodiments, each non-overlapping subset of wavelengths is a contiguous subset of an overall set of wavelengths.  
      In some embodiments, the WSS function is performed for one of said portions.  
      In some embodiments, the WSS function is performed individually for at least two of said portions.  
      In some embodiments, for at least input optical signal, output optical signal pair: the portions comprise non-overlapping sets of wavelengths, when present, in the input multiple wavelength optical signal; wavelength selectively switching comprises performing a wavelength adding function and/or a wavelength dropping function on at least one of the portions; wherein each portion is passed either directly or via said wavelength adding function and/or wavelength dropping function as a respective one of the optical signals to be combined to produce the output optical signal.  
      In some embodiments, for at least input optical signal, output optical signal pair: each non-overlapping set of wavelengths is a contiguous subset of an overall set of wavelengths.  
      In some embodiments, at least two of the portions are passed via respective wavelength adding functions and/or wavelength dropping functions.  
      In some embodiments, the method further comprises: combining an output of the wavelength dropping function of two optical interconnections into a combined drop signal.  
      In some embodiments, the method further comprises: separating an input signal to respective inputs of two of said add functions.  
      In some embodiments, the method further comprises: combining an output of two of the wavelength dropping device of two optical interconnections into a combined drop signal; separating an input signal into inputs of two of said wavelength adding functions.  
      In some embodiments, the method further comprises inputting a tunable laser output as said input signal.  
      In some embodiments, separating comprises optical interleaving, and combining comprises optical de-interleaving.  
      According to another broad aspect, the invention provides a method comprising: defining a plurality M of port pairs each comprising an input port and an output port; for each input port, separating an input optical signal into at least two portions; for each output port, combining signals received for outputting at the output port; interconnecting and wavelength selectively switching the portions to the output ports.  
      In some embodiments, separating comprises band de-multiplexing.  
      In some embodiments, separating comprises signal splitting.  
      In some embodiments, interconnecting and wavelength selectively switching the portions to the output ports comprises: implementing a degree N cross connect in at least one of the portions, where N&lt;=M.  
      In some embodiments, interconnecting and wavelength selectively switching the portions to the output ports comprises: implementing a degree N′ cross connect in another of the portions, where N′&lt;=M. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Preferred embodiments of the invention will now be described with reference to the attached drawings in which:  
       FIG. 1  is a block diagram of a modular wavelength selective switch provided by an embodiment of the invention;  
       FIG. 2A  is a block diagram of a half-band device provided by an embodiment of the invention with a through path and an add/drop path;  
       FIG. 2B  is a block diagram of a half-band device provided by an embodiment of the invention featuring add/drop capability on each of two paths;  
       FIG. 3A  is a block diagram of a half-band device provided by an embodiment of the invention keeping any-to-any connectivity on some ports using optical signal combiners;  
       FIG. 3B  is a block diagram of a half-band device keeping any-to-any connectivity on some ports using band multiplexers;  
       FIG. 4  is a hybrid configuration with add/drop capability on band A and B and all-optical wavelength cross-connect on band B;  
       FIG. 5  is a block diagram of an add/drop arrangement featuring tunable drop ports and passive add ports;  
       FIG. 6  is a block diagram of a reconfigurable add/drop multiplexer featuring additional upgrade ports serviced by half-band devices;  
       FIG. 7  is a block diagram illustrating the use of half-band devices for east/west traffic and full-band tunability on transponders for steerability;  
       FIG. 8A  is a block diagram of an interleaved device provided by an embodiment of the invention;  
       FIG. 8B  is a block diagram of an interleaved device featuring tunable interleavers as provided by an embodiment of the invention;  
       FIGS. 9A and 9B  show the integration of a tunable interleaver on a photonic lightwave circuit (PLC);  
       FIG. 10  is a block diagram of a modular WSS apparatus featuring passive combiners and splitters;  
       FIGS. 11 and 12  are block diagrams of a modular degree  4  wavelength cross connect;  
       FIG. 13  is a block diagram of the degree 4 wavelength cross connect in Band A of  FIG. 11  with added functionality for a degree 3 wavelength cross connect in Band B;  
       FIG. 14  is a block diagram of the wavelength cross connect in Band A of  FIG. 11  with added through paths for Band B;  
       FIG. 15  is a block diagram of another arrangement for connecting outputs and inputs of optical signal separators and optical signal combiners such as the fixed band multiplexers and demultiplexers of  FIG. 14 ; and  
       FIG. 16  is a block diagram of another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A first embodiment of the invention will now be described with reference to  FIG. 1  which shows a modular wavelength selective switch generally indicated at  40 . The switch features an input port  30  and a plurality of output ports  32 ,  34 ,  36 . The illustrated example shows three output ports, but any number of ports can be employed. The input port  30  is connected to a first fixed wavelength selective device  10  that is responsible for routing subsets of wavelengths received through the input port to a set of output ports of the wavelength selective device  10 . In the illustrated example, it is assumed that there are three such output ports that output subsets labelled A, B and C. In some embodiments, the wavelengths of a given subset are contiguous. The wavelengths of Group A then pass through a 1×K wavelength selective switch  12 . WSS  12  routes each wavelength it receives to a selectable output port of K output ports. In this drawing, three such output ports are shown but any other number of ports can be employed. Preferably, WSS  12  has an output port for each output of the modular WSS. More particularly, it has an output  24  associated with output  32 ; an output  26  which is associated with output  34 ; and output  28  which is associated with output  36 . Output  24  of WSS  12  is connected to an input of another fixed wavelength selective device  18 . Device  18  has a number of inputs equal to the number of outputs of device  10 . Device  18  performs a combining function upon the inputs to produce the overall output at  32 . In the absence of connections to WSS  14  and WSS  16 , described below, device  18  only has a single input. Similarly, the second output  26  is connected to a port of fixed wavelength selective device  20  which produces overall output  34  and output  28  is connected to fixed wavelength selective device  22  which produces overall output  36 .  
      In operation, in the absence of wavelength selective switches  14 ,  16  described below, wavelengths of subset A are routed by fixed wavelength selective device  10  from the input port  30  to wavelength selective switch  12 . Wavelength selective switch  12  performs a wavelength switching function switching any one of the input wavelengths to one of the output ports  24 ,  26 ,  28 . In the illustrated example, any wavelength can be routed selectively to any of the three output ports  24 ,  26 ,  28 . Then the fixed wavelength selective device  18  performs a combining function on signals received on its three input ports to produce the output signal at  32 . However, in the absence of WSS  14  and WSS  16 , there would only be the signal from WSS  12 . The wavelengths selectively routed to output  26 ,  28  also appear at outputs  34 ,  36  in a similar manner. In summary, it can be seen that the effect of the arrangement is to enable the routing of any of the wavelengths of subset A from the input port  30  to any selected output port  32 ,  34 ,  36 .  
      The arrangement of  FIG. 1  is now scalable to allow additional wavelength routing. In particular, a second WSS  14  can be installed as shown in  FIG. 1 . Advantageously, in some implementations this may be able to be done without interrupting traffic on wavelengths of subset A. The second WSS  14  is connected to receive the wavelengths of subset B from the input fixed wavelength selective device  10 , and to perform a wavelength selective function upon these wavelengths to route any wavelength of Group B to any output port of device  14 . The output ports of WSS  14  are then connected to respective input ports of the fixed wavelength selective devices  18 ,  20 ,  22 . Now, with the inclusion of wavelength selective switch  14 , any wavelength in subset B that appears at the input  30  is selectively routable to any output port  32 ,  34 ,  36 . In other words, the operable bandwidth of the overall device has increased with the addition of the second WSS  14 . Similarly, WSS  16  can be added to perform wavelength selective switching between any wavelength of subset C in the input to any selected output port  32 ,  34 ,  36 .  
      Input fixed wavelength selective device  10  is any device capable of performing the desired function of dividing the input wavelength set into the appropriate subsets. Examples of appropriate devices include a band demultiplexer or an optical interleaver. The wavelength selective switch in the illustrated example takes a single input and routes wavelengths to any output port of the device. More generally, the switch may have multiple inputs and multiple outputs.  
      The fixed wavelength selective output elements  18 ,  20 ,  22  at the output are any devices capable of performing the required combination of signals on the three input ports to provide the overall output. In some implementations, they are passive combiners. In other implementations they are wavelength selective devices. Examples of appropriate devices include a band multiplexer or optical de-interleaver. In the illustrated example, the first WSS  12  routes any one of the A wavelengths to any one of three output ports. The inclusion of a second WSS enables the routing of any one of the B wavelengths to any one of three output ports. Finally, the further inclusion of WSS  16  enables the routing of any one of the C wavelengths to any one of three outputs, effectively increasing the number of wavelengths that the modular WSS  40  can switch.  
      In the embodiment of  FIG. 1 , there is wavelength selectivity on all three paths containing the wavelengths of Groups A, B and C. In another embodiment, at least one of these paths is simply a through path. For example, it may be that all of the wavelengths of subset B are to be routed to a predetermined output port  32 ,  34  or  36 . In such an implementation, the output B of the fixed input wavelength selective device  10  would simply be connected directly to an appropriate port of one of the output fixed wavelength selective devices  18 ,  20  or  22  such that all of the light in any of the wavelengths of Group B are routed to the appropriate overall output port.  
      In another embodiment, any or all of fixed wavelength selective devices  10 ,  18 ,  20  or  22  are replaced by configurable wavelength selective devices, such as thin film filters and electro mechanical switches or Fiber Bragg grating thermally tuned.  
      In another embodiment, rather than using a wavelength selective switch in each band, various permutations of add/drop multiplexers are employed. Several examples of this will now be described with reference to  FIGS. 2 through 9 .  
      In some embodiments, the WSSs that are used to switch bands A, B and C (or more generally whatever number of bands are present) are cyclical WSS. Cyclical means that the same WSS can be configured to switch {λ 1 , λ 2 , λ 3  . . . }, or {λ n+1 , λ n+2 , λ n+3  . . . }, or {λ 2n+1 , λ 2n+2 , λ 2n+3  . . . } and so on. The same WSS can be used to work on subsets A, B and C if they happen to be cyclical bands (A=1 to n, B-n+1 to 2n, C=2n+1 to 3n)  
      Referring now to  FIG. 2A , shown is an embodiment of the invention featuring two paths  56 ,  58  between an input port  50  and output port  68 . Input wavelength selective device  52  divides an overall band of wavelengths into subsets A and B such that subset A goes on path  56  and subset B goes on path  58 . Output device  54  combines the signals on the two paths to produce the output  68 . In the illustrated example,  56  is a through path meaning that any wavelength in subset A simply passes through the device directly from the input port  50  to the output port  68 . On path  58  there is add/drop functionality. There is a drop device  60  having a plurality of drop ports  64  through which wavelengths of subset B can be dropped. There is also an add device  62  with add ports  66  through which wavelengths of subset B can be added. In this manner, the add device  60  and the drop device  62  can be implemented to only allow adding and dropping on wavelengths belonging to subset B.  
      In a preferred embodiment, subset A and subset B are one half of an overall wavelength band to be processed by the device. Thus, half of the wavelengths go directly through and half of the wavelengths are subject to adding and dropping.  
      In another embodiment, shown in  FIG. 2B , rather than having through path  56 , path  69  between the input wavelength selective device  52  and the output device  54  is provided, and there is an drop device  70  and an add device  72  similar to path  58 . In this case, adding and dropping for wavelengths of subset A can also be performed. However, it can be seen that there is not full flexibility on the adding and dropping ports. In particular, a wavelength of subset A cannot be dropped to the same port as a wavelength of subset B, and a wavelength of subset A cannot be added from the same port as a wavelength from subset B. This is because separate ports are provided for the adding and dropping within these different subsets.  
      In another embodiment, additional paths between the input device  52  and the output device  54  are provided each with their own respective either through capability or add and/or drop capability. This embodiment is modular in the sense that an initial implementation might only include one path with add/drop capability. This is scalable in include the add/drop capability on other paths.  
      Referring now to  FIG. 3A , an embodiment of a half-band device is shown which is similar to that of  FIG. 2B . However, in this case the drop ports of drop devices  70 ,  60  are passively combined for at least one port. In particular, for ports  74 ,  76  these are combined to produce output  78 . Preferably such a combination is done for each pair of ports on the two drop devices  70 ,  60 . In this manner, any wavelength of input band A or B can be routed to any of the combined drop ports. Similarly, on the add port side the add ports of devices  62 ,  72  can also be tied together such that the add ports behave as a single port. In the illustrated example, port  80  is shown connected to both input ports  82 ,  84  of add devices  62 ,  72 . Preferably, such a port splitting is conducted for each of a set of input ports that are then connected to both add devices  72 ,  62 .  
       FIG. 3B  is similar to the embodiment of  FIG. 3A  with the exception of the fact that rather than using passive combiners and splitters, band multiplexers are employed to keep the any-to-any connectivity enabling lower insertion losses than passive combiners/splitters. Thus, in the illustrated example a band multiplexer  92  is shown combining outputs of ports  94 ,  96  of drop devices  70 ,  60 . Similarly, band multiplexer  106  is shown splitting an input port  100  to ports  102 ,  104  of add devices  62 ,  72 .  
       FIG. 4  shows another embodiment of the invention in which wavelengths of subset A can be added or dropped, while wavelengths of subset B can be added, dropped, or all-optically switched.  
       FIG. 5  shows another embodiment of the invention in which input wavelengths received at input  150  are again split into two different subsets A and B by input device  152 . The two bands pass along paths  156 ,  158 . Path  156  is a through path directly to output device  154  which again performs a combining operation on the two paths. Path  158  passes through drop device  160  which allows one or more of the wavelengths of the subset B to be dropped. The output of device  154  is indicated at  162 . Passive adding is then performed by passive combination elements  164  which produce an add signal  165  which is combined with output signal  162  at  166  to produce overall output  168 . While a particular arrangement of add functionality  164  is shown to allow the addition of eight wavelengths, any appropriate passive add circuitry can alternatively be employed to add fewer or a larger number of wavelengths.  
      To increase capacity in the device of  FIG. 5 , a drop device capable of processing wavelengths of subset A is inserted on path  156 . No change is required on add device  164 . Preferably, drop ports from devices on path  156  and  158  are combined using combiners or band multiplexers.  
      Referring now to  FIG. 6 , another embodiment of the invention features the use of half-band devices to provide upgrade ports for full-band devices. In the illustrated example, there is a main input port  170  connected to a full-band drop device  172 . The drop ports of device  172  include ports  173  and  175 . In order to expand the capacity of the device, drop port  175  is shown connected through to an additional half-band device  180  which performs additional wavelength dropping and has additional ports  181 . Thus, the overall drop ports of the combined devices  172 ,  180  are ports  173 ,  181 . Similarly, on the add side full-band device  176  has input add ports  177 ,  179 . However, half-band device  182  is shown connected to input port  179  so that additional input ports  183  are provided. Thus, the arrangement effectively has add ports  183  plus  177 . Wavelengths not dropped by the drop device  172  are passed along  174  to the add device  176  and on to the output  178 . The arrangement of  FIG. 6  does result in some moderate inflexibility of port assignments because drop ports  181  and add ports  183  can only operate on half-band, while drop ports  173  and add ports  177  can operate on the full-band. Preferably, the additional half-band devices cover another set of wavelengths from half-band devices  180 ,  182 . Furthermore, it can be seen that additional half-band devices can be added similar to devices  180 ,  182  to provide additional ports. In the illustrated example, the “full-band” device has 40λ capacity, and the “half-band” device has 20λ capacity. This is simply an example. In fact, the expansion devices  180 ,  182  can have any number of wavelengths as can the main devices  172 ,  176 , and the number of wavelengths of devices  180 ,  182  and devices  172 ,  176  need not be related by the particular 1:2 ratio of the example.  
       FIG. 7  is another system diagram showing the use of half-band devices for east/west traffic and full-band tunability on transponders for steerability. West traffic enters the arrangement at  200  and leaves at  206 , and east traffic enters  208  and leaves at  214 . West traffic passes through drop device  202  and add device  204 . Similarly, east traffic passes through drop device  210  and add device  212 . Wavelengths can be added to either the east traffic or the west traffic through input ports in the add devices  212 ,  204 . Preferably, the ports are connected together. For example, a first input port  230  is shown connected to respective input ports  234 ,  236  on add device  212  and add device  204 . A band multiplexer  232  sends the wavelengths to the appropriate device. Similarly, output port  222  can receive dropped wavelengths from port  216  of drop device  210  or port  218  of drop device  202 . In the illustrated example, west traffic is on the A band while east traffic is on the B band. Preferably each of the drop ports are tied together in a similar manner to that shown for output port  222  and each of the add ports are tied together in a similar manner to that shown for add port  230 . In operation, a tunable transponder such as a laser can be connected to add port  230  and/or drop port  222  to provide for east/west steerability. Tuning the transponder to any wavelength of band A would enable west communication, while tuning to any wavelengths of the band B would then enable east communication. The transponder might be a tunable laser  231  for add ports or a tunable PIN diode  223  for drop ports. It can be seen that the arrangement of  FIG. 7  can be extended to additional bands.  
      Referring now to  FIG. 8A , in another embodiment of the invention, rather than dividing an input signal into two contiguous bands, an interleaver is provided at the input to divide the channels into even and odd channels. In the illustrated example, input port  250  is connected to interleaver  252  which outputs odd channels on through path  254  and outputs even channels on path  255 . Of course the even and odd ports could be switched to allow the even ports to be the through path. Add functionalities are provided with add device  260  for even ports only, and drop functionalities provided with drop device  258  for even ports only. At the output, device  256  combines the even channels and the odd channels to produce overall output signal  262 .  
      In the embodiment of  FIG. 8B , structurally this looks similar to the embodiment of  FIG. 8A , but in this case there is an interleaver  272  capable of switching between routing the even ports to output path  274  and the odd ports to output path  284  and alternatively sending the odd ports to output path  274  and the even ports to output path  284 . For path  274 , there is a drop device  276  which is tunable to allow dropping of odd channels or even channels. There is also an add device  278  which is tunable to allow adding of odd channels or even channels. Finally, the output device  280  is also tunable to perform the appropriate combination of channels received from path  274  and  284  to produce overall output signal  282 . In one embodiment of the invention, device  280  is simply a passive combiner.  
      For the embodiments of  FIGS. 8A and 8B , the channel spacing on the two paths is double that on the input and output signals. Thus, in the illustrated example with a 100 GHz channel spacing on the input port and the output port, the two paths connecting input and output devices  252 ,  256  have channel spacing 200 GHz. Other channel spacings are possible.  
       FIG. 9A and 9B  describe a tunable integrated bi-directional interleaver-WSS. The same device can be used either for a drop configuration ( FIG. 9A ) or an add configuration ( FIG. 9B ). In the case of WSS realized with parts in waveguide technology, the interleaver can advantageously also be realized in waveguide technology and be integrated on the same substrate with parts of the WSS for compactness.  
      Another embodiment provided by the invention is similar to the embodiment described in detail above with reference to  FIG. 1 . However, in this embodiment, passive combiners and splitters are used in place of the band demultiplexers and/or band multiplexers of  FIG. 1 . An example of this is shown in  FIG. 10 . Preferably, each WSS 1×K A, B or C blocks all other wavelengths but the ones that correspond to respective bands A, B or C. It is therefore an integrated WSS and band blocker. If not, multiple copies of the same wavelengths would go through the arrangement. This arrangement scales to any number of inputs, and passive devices can be used in other embodiments as well.  
      Another embodiment of the invention provides a modular degree N WXC (wavelength cross connect) using modular WSS. A particular example is shown in  FIG. 11  which is a degree 4 example. There are four pairs of input and output ports  400 ,  401 ;  402 ,  403 ;  404 ,  405 ; and  406 ,  407 . The details of the first pair  400 ,  401  will be described, the other pairs being similar.  
      The input port  400  is input to a band demultiplexer  410  which separates a signal on the input port into two signals having non-overlapping wavelength subsets, preferably contiguous sets. In the illustrated example, these are referred to as Band A and Band B. Band A is routed to an input 1×3 WSS Band A device  414  which performs wavelength switching on wavelengths in Band A. In the illustrated example, nothing is connected to the Band B output of demultiplexer  410 .  
      Similarly, the output port  401  is connected to a band multiplexer  412  which combines signals received on Bands A and B. In the illustrated example, there is nothing connected to the Band B input of multiplexer  412 . The Band A input to multiplexer  412  is received from an output 1×3 WSS Band A device  416 .  
      The output ports of the input 1×3 WSS Band A device  414  are each connected to a respective input of an output 1×3 WSS Band A device of another pair of ports thereby enabling any wavelength received on input port  400  to be routed to any of the output ports  403 ,  405 ,  407 .  
      Similarly, the input ports of the output 1×3 WSS Band A device  416  are connected to a respective output port of an input 1×3 WSS Band A device of each other input port  402 ,  404 ,  406 . Therefore, a wavelength received on any input port  402 ,  404 ,  406  can be selectively routed to the output port  401 .  
      The functionality shown is only capable of switching wavelengths of Band A. However, the configuration is modular in the sense that 1×3 WSS Band B devices can now be added after the fact, and connected to the Band B inputs and outputs of the band multiplexers and band demultiplexers, and connected to each other, in a similar manner to the Band A functionality described above. After these additions, the full band A+B arrangement would appear as shown in  FIG. 12 . It is to be understood that the arrangement of  FIGS. 11 and 12 , and the embodiments of  FIGS. 13, 14  described below is particular to the 4 degree case and that the concept easily extends to other degrees.  
      In the embodiment of  FIG. 12 , the additional functionality has been added to provide full degree 4 cross connect functionality for Band B. Alternatively, the degrees implemented on the different bands may be different. For example, when the functionality for Band B is built out, a degree 3 cross connect may be implemented. An example of this is shown in  FIG. 13 .  FIG. 13  is similar to  FIG. 12 , but there is no Band B functionality for ports  406 ,  407 . Rather, the cross connect for Band B is between port pairs  400 ,  401 ;  402 ,  403 ; and  404 ,  405 . It can be seen the degree of the Band B functionality does not need to be decided upon until it is time to install the Band B equipment. This is because each port pair is equipped with the demultiplexing and multiplexing hardware. Alternatively, certain port pairs may be implemented without this functionality in which case it will not be possible to expand the functionality of those ports without adding this, For example for the embodiment shown in  FIG. 13 , the band demultiplexer and multiplexer connected to ports  406 ,  407  is not necessary if it is known that these ports will never need to handle Band B.  
      In another embodiment, degree N cross connect functionality is provided on one band, and pass through connections are provided on another band. An example of this is shown in  FIG. 14 . This arrangement is again similar at first to the arrangement of  FIG. 11 . However, in this case a first passthrough connection  450  is provided between ports  400 ,  405  and a second passthrough connection  452  is provided between ports  404 ,  401 . It can be seen that with the arrangement of  FIG. 11 , passthrough connections between any Band B ports may be added.  
       FIG. 15  shows another example of how input and output port pairs might be interconnected. Shown are four input ports  600 ,  608 ,  618 ,  626  and four output ports  602 ,  616 ,  610 ,  624 . Each of the output ports has an associated wavelength selective switch  606 ,  620 ,  614 ,  628  each with an optional set of add ports, one such set being labeled at  636  for switch  606 . For the input ports, each input port has a respective passive splitter  605 ,  623 ,  613 ,  631  that passively splits the input signal into multiple paths. The combination of a passive splitter on the input ports and wavelength selective devices on the output ports enables a unique wavelength routing function to be achieved. Also shown is an optional set of passive drops  640  connected to passive splitter  605 . Such a set of passive drops might be included for any of the input ports. The wavelength selective switches and passive splitters are then interconnected in a manner similar to that described above for  FIG. 14 . The entire arrangement of  FIG. 14  can then be used to interconnect the band “A” inputs and outputs of the band multiplexers and band de-multiplexers such as shown in  FIG. 14 . The same or a different arrangement can then be used to interconnect the band “B” inputs and outputs. In some embodiments, the passive drops  640  can be instead implemented using a fixed de-multiplexer in which case a wavelength selective dropping function is implemented.  
      Referring now to  FIG. 16  shown is a block diagram of another embodiment of the invention. This apparatus features at least one first input port  500 . There is also at least one second output port  510 . Each of the first input ports has a respective optical signal separator  522  that separates the incoming signal into a set of portions at the first output port  504 . The optical signal separator might be a signal splitter in which case the portions are simply fractions of the power across the entire wavelength band of the input signal, or they might be fixed wavelength specific wavelength selective devices such as band de-multiplexers or optical interleavers in which case the signals that are output on the first output ports  504  are non-overlapping sets of wavelengths. At the output side, each output port  510  has a respective optical signal combiner  506  having a set of second input ports  508 . The optical signal combiner  506  might be a passive combiner or a wavelength selective combiner such as a band multiplexer or de-interleaver.  
      Also shown is at least one wavelength selective device  512 . Two are shown in the particular example illustrated. Each wavelength selective device  512  interconnects at least one of the first outputs to at least one of the second inputs in a wavelength selective manner meaning that particular wavelengths from the first output are routed to particular second input ports. Two particular interconnection examples are shown in the diagram. Interconnections  530  show one of the first output ports  504  wavelength selectively routed to a respective second input port on each of two optical signal combiners  506 . In another example, generally indicated at  532  are interconnections for interconnecting a first output port to a single second input port, with the wavelength selected device also having a number of drop ports in that case. Note that the first example  530  is somewhat analogous to the block diagram of  FIG. 1  described previously, and that the second example  532  is somewhat analogous to the example of  FIG. 2A . However it can be easily seen how both of these systems can be implemented using the generic framework of  FIG. 16 , either on their own or simultaneously.  
      One of more of the wavelength selective devices may also feature wavelength adding capability. Furthermore, in some of the interconnections between the first output ports and the second input ports, there may be more than one wavelength selective device connected in series. An example of this can be seen in the  FIG. 14  embodiment.  
      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.