Abstract:
An optical cross connect switch includes a programmable demultiplexer placed on every input transmission port, a programmable multiplexer placed on every output transmission port, and a linking fiber between every programmable demultiplexer and multiplexer in the system. The programmable demultiplexers and multiplexers handle internally all the optical communication channels, and can route any specified optical channel from an input port to the desired output port. Advantageously, the arrangement can send several input channels to the same output port, and can efficiently handle a large optical communication channel count, offer a scalable cost-effective solution for expanding switching capacity, and reduce the fiber interconnection count between switch modules. When a new optical communication system is added to the node, it requires only that a programmable multiplexer and demultiplexer be placed on its output and input fibers, respectively, and fibers connected to its neighboring programmable multiplexers and demultiplexers. The arrangement thus scales proportionally with the number of input line systems to the switch, regardless of the number of optical channels, and thus provides a pay as you grow solution. The cross connect can be modified to provide add-drop functionality, and some of the programmable multiplexers/demultiplexers can be replaced with passive or conventional devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Provisional Application Serial No. 60/300,272 which was filed on Jun. 22, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to optical communications, and more particularly to an optical cross connect switch using programmable multiplexers and demultiplexers to switch optical wavelength division multiplexed (WDM) channels among multiple inputs and outputs in an optical communication system. 
     BACKGROUND OF THE INVENTION 
     The transmission capacity of fiber-optic communication systems has increased significantly with the wavelength division multiplexing technique. In a WDM system, multiple channels, where each channel is differentiated by the use of a different wavelength of light, carry modulated optical signals in a single optical fiber. Optical multiplexers are used at the transmitter to combine all the optical channels into the fiber for transmission, and optical demultiplexers are used at the receiver to separate the optical channels for detection. 
     In an optical network, network traffic can be routed from a source to a destination via one or more intermediate nodes, each of which is connected to a plurality of neighboring nodes. Accordingly, each intermediate node requires some switching or cross connection capability to select an appropriate neighboring node in order to route the traffic towards the desired destination. An intermediate node can operate electronically, by (a) terminating wavelength channels at a receiver endpoint, (b) switching the traffic with electronic means, and (c) originating the wavelength channels at transmitter points. Alternatively, the switching nodes can operate transparently, routing the individual optical channels without opto-electronic conversion. Regardless, intermediate nodes are required to switch incoming wavelength channels from an input port to a desired output port, so that a channel originating at an upstream node can pass through the intermediate node enroute toward its downstream node destination. 
     FIG. 1 illustrates the topology of a prior art optical communications system network node with conventional switching functionality. The network node has several input transmission ports  110 ,  120  and  130  carrying multiplexed optical channels and several output transmission ports  154 ,  164  and  174  carrying the outbound multiplexed optical channels. Each input multiplexed channel, e.g. the channel on input transmission port  110 , is demultiplexed by a demultiplexer  112 , separating the optical channels on distinct demultiplexer ports such that optical channel with wavelength λ 1  appears on demultiplexer port  114 - 1 , the optical channel with wavelength λ 2  appears on demultiplexer port  114 - 2 , and so on, through optical channel with wavelength λ N  which appears on demultiplexer port  114 -N. The demultiplexed optical channels from all the input ports  110 ,  120  and  130 , having been demultiplexed by demultiplexers  112 ,  122  or  132 , are introduced to the input ports  180 - 1  through  180 - 3 N of a cross connect switch  140 , which independently routes traffic at each input port towards any one of the cross connect switch output ports  190 - 1  through  190 - 3 N. These switch output ports are connected to the multiplexer ports  150 - 1  through  150 -N,  160 - 1  through  160 -N and  170 - 1  through  170 -N of multiplexers  152 ,  162 , and  172 , respectively, which combine or multiplex each of the channels onto one of the output transmission ports  154 ,  164  or  174 . In order to connect each input channel to the appropriate one of the output transmission ports  154 ,  164  or  174 , cross connect  140  must be arranged to interconnect each of its input ports  180 - 1  through  180 - 3 N with the appropriate one of its output ports  190 - 1  through  190 - 3 N. 
     Cross-connect switch  140  can be implemented by either electronic or optical switching fabrics. The number of cross-connect switch ports has to be as large as the number of input transmission ports times the number of optical channels received via each such port, which typically requires the number of cross-connect switch ports to be in the hundreds or higher. 
     Instead of the arrangement of FIG. 1, the alternative prior art architecture shown in FIG. 2 may be used. All the input optical channels arriving at input transmission ports  210 ,  220  and  230  are demultiplexed in demultiplexers  212 ,  222  and  232  in a similar fashion to the arrangement of FIG.  1 . However, switching is performed by a cluster of small cross connect switches  240 - 1 ,  240 - 2 ,  240 -N, each switch handling a single optical wavelength channel. For example, all of the demultiplexer ports  214 - 1 ,  224 - 1  and  234 - 1  carrying channels with wavelength λ 1  are switched by cross connect switch  240 - 1  to the proper multiplexer port  250 - 1 ,  260 - 1 , or  270 - 1  of multiplexers  252 ,  262  and  272 , respectively. Likewise, for a different wavelength  2 , demultiplexer ports  214 - 2 ,  224 - 2  and  234 - 2  carry channels with that wavelength to cross connect switch  240 - 2  and thence to the proper multiplexer ports  250 - 2 ,  260 - 2 , or  270 - 2 . This arrangement requires an individual cross connect for each of the optical channels. The port count of each cross connect switch is determined by the number of input transmission ports. Since the trend in optical communication systems is to increase the number of optical channels from a few hundred today to over a thousand in the future, the arrangement of FIG. 2 does not scale well and will undesirably require a massive fiber interconnect patch panel. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an optical cross-connect switch is based on and uses the programmable optical multiplexer/demultiplexer as described in co-pending application Ser. No. 09/944,800 filed on Sep. 31, 2001 and assigned to the same assignee as the present application. As described in the aforementioned co-pending application, a programmable optical demultiplexer is arranged to receive a multiplexed optical signal containing a plurality of separate channels, each with an associated wavelength, and independently assign each input optical channel to a desired output port. Likewise, a programmable optical multiplexer is arranged to receive a plurality of separate optical channels, each with an associated wavelength, and combine the different wavelengths into a single multiplexed optical signal that is made available at the multiplexer output port. 
     In accordance with one embodiment of the present invention, an optical cross connect switch includes a programmable demultiplexer placed on every input transmission port, a programmable multiplexer placed on every output transmission port, and a linking fiber between every programmable demultiplexer and multiplexer in the system. The programmable demultiplexers and multiplexers handle internally all the optical communication channels, and can route any specified optical channel from an input port to the desired output port. Advantageously, the arrangement can send several input channels to the same output port. 
     In accordance with other embodiments of the present invention, an optical cross connect switch is arranged to both add and drop WDM channels. The added and/or dropped channels are coupled to/from the previously mentioned programmable multiplexers/demultiplexers via additional linking fibers, through the use of additional multiplexers and demultiplexers, which may be conventional or programmable. 
     In accordance with yet other embodiments of the present invention, either the programmable multiplexers or programmable demultiplexers are replaced by passive combiners or splitters, and concentrators and/or distributors are incorporated in order to reduce the number of transmitters and/or receivers required in the cross connect implementation, and to thereby allow the transmitters and/or receivers that are used to be shared. 
     The cross connect switch arrangement of the present invention efficiently handles a large optical communication channel count, offers a scalable cost-effective solution for expanding switching capacity, and reduces the fiber interconnection count between switch modules. When a new optical communication system is added to the node, it requires only that a programmable multiplexer and demultiplexer be placed on its output and input fibers, respectively, and fibers connected to its neighboring programmable multiplexers and demultiplexers. The arrangement thus scales proportionally with the number of input line systems to the switch, regardless of the number of optical channels, and provides a pay as you grow solution. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be more fully appreciated by consideration of the following detailed description, which should be read in light of the drawing in which: 
     FIG. 1 is an illustration of a network node with a conventional cross connect; 
     FIG. 2 is an illustration of a network node with a conventional wavelength level cross connect; 
     FIGS.  3 ( a ) and  3 ( b ) are illustrations of a programmable multiplexer and demultiplexer, respectively, in accordance with applicants&#39; co-pending application, that are the building blocks of the present invention; 
     FIG. 4 is an illustration of an embodiment of an optical cross connect switch arranged in accordance with the present invention to use programmable multiplexers and demultiplexers; 
     FIG. 5 is an illustration of an embodiment of a cross connect switch in accordance with the present invention, having channel add and drop capability; 
     FIG. 6 is an illustration of a cross connect switch with channel add and drop capability and sharing of local receivers and transmitters; 
     FIG. 7 is an illustration of an embodiment of an optical cross connect switch arranged in accordance with the present invention to use programmable demultiplexers and passive combiners; and 
     FIG. 8 is an illustration of an embodiment of an optical cross connect switch arranged in accordance with the present invention to use programmable multiplexers and passive splitters. 
    
    
     DETAILED DESCRIPTION 
     The present invention describes new architectures for implementing an optical cross-connect that advantageously makes use of the programmable optical multiplexer/demultiplexer described in applicant&#39;s co-pending application identified above. For the purposes of completeness, the functionality of those elements is described in connection with FIGS.  3 ( a ) and  3 ( b ) herein. As illustrated in FIG.  3 ( a ), a programmable optical multiplexer  320  has K multiplexer input ports  310 - 1  through  310 -K and a single multiplexer output port  330 . Each of the multiplexer input ports can receive an optical signal containing one or more optical channels, λ- 1  through λ-N, provided the wavelengths of the channels are different. The optical signals are combined in the multiplexer, and emerge as a composite signal at multiplexer output port  330 . Operationally, multiplexer  320  establishes a unique pathway for each optical channel between any one of the multiplexer input ports  310 - 1  through  310 -K and the multiplexer output port  330 , as prescribed by a control signal  340 , physically preventing the detrimental possibility of combining two optical channels operating on the same wavelength from two different multiplexer input ports. 
     The programmable multiplexer of FIG.  3 ( a ) can also be operated in the reverse direction and function as a programmable demultiplexer, as shown in FIG.  3 ( b ). A single demultiplexer input port  350  receives a multiplexed optical signal containing a plurality of wavelengths or channels, and separates the signal so that one or more of the channels appears at each of the demultiplexer output ports  360 - 1  through  360 -M. The assignment of specific channels to demultiplexer output ports is independent, and is determined by a control signal on input  370 . 
     Referring now to FIG. 4, there is shown an embodiment of an optical cross connect switch arranged in accordance with the present invention. The optical cross connect&#39;s input ports  410 ,  420  and  430  each carry a plurality of multiplexed optical channels at wavelengths λ 1  through λ N  that are each to be independently assigned to a desired output port  454 ,  464  or  474 , as prescribed by a control signal  401 . Note that while FIG. 4 shows three input/output ports, this is for illustrative purposes only, and the number of ports can be fewer or greater, and the number of input ports does not have to be equal to the number of outputs ports. Each input port  410 ,  420  and  430  is connected to a respective programmable demultiplexer  412 ,  422  and  432 , and each output port is preceded by a respective programmable multiplexer  452 ,  462  and  472 . Programmable demultiplexers  412 ,  422  and  432  each have a single programmable demultiplexer input port  410 ,  420  and  430  (which correspond to the optical input ports of the cross connect) and K programmable demultiplexer output ports  414 - 1  through  414 -K,  424 - 1  through  424 -K, and  434 - 1  through  434 -K, where K is an integer greater than one and typically less than the number of wavelengths N, while programmable multiplexers  452 ,  462  and  472  have K programmable multiplexer input ports  450 - 1  through  450 -K, and  460 - 1  through  460 -K, and  470 - 1  through  470 -K, and a single programmable multiplexer output port  454 ,  464  and  474  (which correspond to the optical output ports of the cross connect). As indicated previously, programmable demultiplexers  412 ,  422  and  432  and programmable multiplexer  454 ,  464  and  474  are each arranged in accordance with applicant&#39;s copending application identified above. One programmable demultiplexer output port ( 414 - 1  through  414 -K,  424 - 1  through  424 -K, and  434 - 1  through  434 -K) of each programmable demultiplexer  412 ,  422  and  432 , respectively, is connected by an optical fiber link to an available programmable multiplexer input port ( 450 - 1  through  450 -K,  460 - 1  through  460 -K, and  470 - 1  through  470 -K) of the different programmable multiplexers  454 ,  464  and  474 . The arrangement of the linking fibers is such that there is thus “complete” interconnection between the demultiplexers  412 ,  422  and  432  and the multiplexers  454 ,  464  and  474 . With this interconnection, at least one output from every demultiplexer can be routed to at least one input of each multiplexer via an individual optical fiber link. Stated differently, this means that each of the demultiplexers can be connected by the cross connect switch to any of the multiplexers via a single hop. 
     Still referring to FIG. 4, the connection or routing of each specific channel from cross connect input ports  410 ,  420  and  430  to one of desired cross connect output ports  454 ,  464  and  474  is prescribed by control signal  401 , which is input to a controller  400 . That controller in turn applies individual control signals  411 ,  421  and  431  to programmable demultiplexers  412 ,  422  and  432 , and individual control signals  451 ,  461  and  471  to programmable multiplexers  452 ,  462  and  472 , respectively, so that the cross connect establishes desired optical pathways for each input wavelength λ via a programmable demultiplexer arranged to receive the input wavelength λ on one of its input ports, an optical fiber link, and a programmable multiplexer having an output port to which wavelength λ is destined. 
     If each of the input ports  410 ,  420  and  430  and each of the output ports  454 ,  464 , and  474  can carry N optical channels, then the total switching capacity of the cross connect switch is (K+1)*N optical channels. 
     An exemplary path for an optical channel λ 5  originating at input port  410  and destined for output port  464  is through the programmable demultiplexer  412 , which routes λ 5  to the port that is connected to the desired line system. In our example, the optical channel would emerge on demultiplexer port  412 - 2 , which routes  5  to multiplexer port  460 - 1  of programmable multiplexer  462  via a fiber link. The programmable multiplexer reconfigures to interconnect multiplexer port  460 - 1  to output port  464  for λ 5 , completing the signal path. Since the programmable multiplexer/demultiplexer devices can carry multiple optical channels on every demultiplexed port, the fact that λ 5  is traversing through the fiber that interconnects programmable demultiplexer  412  to programmable multiplexer  462  does not prevent other optical channels from sharing the same pathway via the interconnecting fiber link, thus concurrently supporting all the optical channels that are interchanged between the same input and output ports at the cross connect. 
     The switch architecture of FIG. 4 has several desirable properties. Typically when an optical cross connect node is first put into service, it interconnects relatively few optical line system, each carrying relatively few optical channels. When the network operator decides to add more optical channels to the system, a field technician is not required to modify the cross connect, since all the optical channels can advantageously be handled internally by the programmable multiplexers and demultiplexers. This is not the case in the prior art illustrated in FIGS. 1 and 2. 
     With the arrangement of the present invention, when a new line system is installed and attaches to the node, the only modification that is required is to attach the new system&#39;s demultiplexer and multiplexer to the existing multiplexers and demultiplexers, respectively, via fiber links, and to connect control inputs to these elements from controller  400 . This is typically very simple, as the number of line systems connected to a node is typically a small number. When the cross connect is fully populated by line systems (K+1 systems), the number of fibers that interconnect the input and output systems is only K(K+1). This compares very favorably with the solutions of FIGS. 1 and 2, which require 2*(K+1)*N fibers (as K&lt;&lt;N, typically). The cost of the cross connect also grows proportionally to the number of input/output line systems, such that the initial cost is low, when few ports are populated. This allows for a low start-up cost and future revenues as the system expands (a pay as you grow architecture). 
     FIG. 5 illustrates a cross-connect switch with reconfigurable optical channel add and drop capability. In this embodiment, the input and output ports  410 ,  420  and  430 , and  454 ,  464  and  474 , as well as the programmable demultiplexers  412 ,  422  and  432 , programmable multiplexers  452 ,  462  and  472 , and the interconnecting linking fibers, are largely the same as in FIG. 4, and the same numbering is retained. However, in this embodiment, at least one output port of each programmable demultiplexer  412 ,  422  and  432  is devoted for the drop channels, and at least one input port of each programmable multiplexer  452 ,  462  and  472  is devoted to the add channels. The dropped optical channels that are routed to the programmable demultiplexer output port designated for the drop channels are demultiplexed by a second demultiplexer that can be programmable or conventional, while the added optical channels that are routed to the programmable multiplexer input port designated for the add channels are first multiplexed by a second multiplexer that can also be programmable or conventional. Thus, a large number of added channels can be inserted at the input ports of the second programmable multiplexers, and at the output ports of the second programmable demultiplexer, a large number of dropped channels can be directly detected. 
     The optical channels to be dropped arrive at one of the input ports  410 ,  420  or  430 , and are applied by the corresponding one of the programmable multiplexers  412 ,  422  or  432  to the programmable multiplexer output port  414 -K,  424 -K or  434 -K that is assigned to dropped channels. These programmable multiplexer output ports are connected, via linking fibers, to respective second programmable demultiplexer  514 ,  524  or  534 , connected the dropped channel to an available output port  518 - 1  through  518 -K′,  528 - 1  through  528 -K′ or  538 - 1  through  538 -K′. An example path of a dropped channel entering the cross connect of FIG. 5 from input port  420  is through programmable demultiplexer  422  to programmable demultiplexer output port  424 -K, which is connected via a linking fiber to the programmable demultiplexer input port  520  of programmable demultiplexer  524  that assigns the dropped channel to an available one of the programmable demultiplexer output ports  528 - 1  through  528 -K′. Note that the programmable demultiplexer output port count of the second programmable demultiplexer  524  does not have to be identical to that of the first programmable demultiplexer  422 , and, as indicated above, a conventional demultiplexer can also be used. 
     The added channels undergo a similar process to that described for the drop channels, but use two-stage multiplexing rather than two-stage demultiplexing. The added optical channels are inserted at one of the programmable multiplexer input ports  540 - 1  to  540 -K′,  550 - 1  to  550 -K′, or  560 - 1  to  560 -K′ of a first programmable multiplexer  544 ,  554  or  564 , which is connected to an input port  470 -K,  460 -K or  450 -K of a corresponding second programmable multiplexer  472 ,  462  or  452 , that is designated to receive added channels. An example path of an added channel that is destined for output port  474  is to originate at one of the programmable multiplexer input ports  540 - 1  to  540 -K′ of programmable multiplexer  544 , whose programmable multiplexer output port  548  is connected to the input port  470 -K of programmable multiplexer  472  that multiplexes all the channels to the desired output port  474 . The added channels can utilize tunable wavelength laser sources at programmable multiplexer input ports  540 - 1  to  540 -K′, allowing the cross-connect with add/drop capability to fit well in a networking environment where optical channels may change often. Note, as indicated above, that conventional multiplexers can also be used with fixed wavelength lasers. Note that some or all of the programmable multiplexer input ports  540 - 1  to  540 -K′,  550 - 1  to  550 -K′ and  560 - 1  to  560 -K′ can simultaneously receive more than one wavelength or channel, i.e., the ADD channels input to one or more programmable multiplexer input ports of one or more programmable multiplexers  544 ,  554  and  564  can itself be a multiplexed optical signal. Likewise, some or all of the programmable demultiplexer output ports  414 - 1  to  414 -K,  424 - 1  to  424 -K and  434 - 1  to  434 -K can simultaneously receive more than one wavelength or channel, i.e., the DROP channels input to one or more demultiplexers  510 ,  520  and  530  can itself be a multiplexed optical signal. Also note that optical detection may, instead of being performed directly at the drop port, be performed at a remote location, such as at a customer&#39;s premises. In that case, several dropped channels can be assigned to the drop port that leads to the customer for demultiplexing and detection of the multiple optical channels. 
     The add-drop arrangement of FIG. 5 (as well as the arrangement of FIG. 6, described below) can be easily modified to perform wavelength conversion or regeneration, by (a) first dropping one or more wavelength channels, (b) converting the existing wavelengths to different wavelengths, or regenerating the modulated signals on the same wavelength carriers, in separate converters or regenerators, not shown, which operate either optically or through an optical/electrical/optical conversion, and (c) then adding the new wavelength channels or the regenerated signals so that they are available at a cross connect output port. 
     FIG. 6 illustrates a variation of the previously described cross-connect with add/drop capability shown in FIG. 5, and retains the same designations for like elements. The arrangement of FIG. 6 may be preferable to better utilize installed transmitter and receiver equipment associated with the add/drop channels. Specifically, in the arrangement of FIG. 5, each of the programmable demultiplexer output ports  518 - 1  through  518 -K′,  528 - 1  through  528 -K′ and  538 - 1  through  538 -K′ normally has an associated drop channel receiver, and each of the programmable multiplexer input ports  540 - 1  through  540 -K′,  550 - 1  through  550 -K′ and  560 - 1  through  560 -K′ normally has an associated add channel transmitter. These drop channel receivers and add channel transmitters are, in the implementation of FIG. 5, “dedicated”, in the sense that each input port  410 ,  420  or  430 , which is typically connected to a different input line system, can only route drop channels to certain receivers, and each output port  454 ,  464  and  474 , which is typically connected to a different output line system, can only receive add channels from certain transmitters. However, it may be wiser to share the receivers and transmitters across all input/output ports, as add or drop channel demand may shift, over time, from one input or output line system to another line system. 
     Still referring to FIG. 6, additional cross-connect switches  610  and  640  that respectively function as a concentrator for the drop channels (many input ports, fewer output ports) and as a distributor for the add channels (fewer input ports, many output ports) may be included. With respect to the drop channels, the programmable demultiplexer output ports  518 - 1  through  518 -K′,  528 - 1  through  528 -K′ and  538 - 1  through  538 -K′ of the programmable demultiplexers  514 ,  524  and  534  enter concentrator cross connect switch  610 , which has a number of input ports equal to the cumulative number output ports of all of the programmable demultiplexers  514 ,  524  and  534 . The number of output ports P of concentrator cross connect  610  is determined by the number of receivers available at the node, which is a system design consideration, and will in any event permit the receivers to be shared. Thus, any dropped channel from any line system can appear at any one of the output drop ports  620 - 1  through  620 -P. 
     With respect to the add channels, the programmable multiplexer input ports  5401  through  540 -K′,  550 - 1  through  550 -K′ and  560 - 1  through  560 -K′ of the programmable multiplexers  544 ,  554  and  564  exit concentrator cross connect switch  640 , which has a number of output ports equal to the cumulative number input ports of all of the programmable multiplexers  544 ,  554  and  564 . The number of input ports Q of concentrator cross connect  640  is determined by the number of transmitters available at the node, which is also a system design consideration, and will in any event permit the transmitters to be shared. Thus, any added channel from any line system can originate at any one of the input add ports  630 - 1  through  630 -Q. Note that P and Q are integers that can be equal or unequal. 
     Note that it is possible to carry out the implementation of FIG. 6 with conventional (i.e., static) demultiplexers and multiplexers connected to the concentrator and distributor cross connects, instead of programmable multiplexers and programmable demultiplexers. However this solution would require a large port count concentrator and distributor cross connect. 
     Referring now to FIG. 7, there is shown an illustration of an embodiment of an optical cross connect switch arranged in accordance with the present invention to use programmable demultiplexers and passive combiners. This type of arrangement (and that described below in connection with FIG. 8) can be a replacement for the arrangement of FIG. 4 when it is desired to reduce cost by substituting passive devices in place of the programmable multiplexers (or in the case of FIG. 8, in place of the programmable demultiplexers) However, it is to be noted that in this arrangement, optical losses can be greater, due to the inherent lossy nature of passive splitters and couplers. 
     In FIG. 7, each of the programmable multiplexers  452 ,  462  and  472  of FIG. 4 is replaced by a respective passive combiner  752 ,  762  and  772 . The passive combiners, which may be multi-port couplers or other similar passive devices, simply merge all of the wavelengths received at their respective input ports onto a single output port  754 ,  764  and  774 , respectively, but have a higher optical loss than the programmable multiplexers of FIG.  4 . Control signals  451 ,  461  and  471  of FIG. 4 are not required in this embodiment. The control signals  411 ,  421  and  431  controlling programmable demultiplexers  412 ,  422  and  432  are the same as in FIG.  4 . 
     In FIG. 8, each of the programmable demultiplexers  412 ,  422  and  432  of FIG. 4 is replaced by a respective passive splitter  712 ,  722  and  732 . The passive splitters, which may be multi-port couplers or other similar passive devices, simply replicate all of the wavelengths received at their respective input ports  710 ,  720  and  730  onto all of their output ports, but have a higher optical loss than the programmable demultiplexers of FIG.  4 . Control signals  411 ,  421  and  431  of FIG. 4 are not required in this embodiment. The control signals  451 ,  461  and  471  controlling programmable multiplexers  452 ,  462  and  472  are the same as in FIG.  4 . In this embodiment, programmable multiplexers  452 ,  462  and  472  serve to select the desired wavelength to be coupled from its programmable multiplexer input ports  450 - 1  through  450 -K,  460 - 1  through  460 -K, and  470 - 1  through  470 -K to each one of its programmable multiplexer output ports  454 ,  464  and  474 . 
     It should be further understood that the arrangements shown in FIGS. 5 and 6 can each also be modified so that either the programmable multiplexers or the programmable demultiplexers are replaced by passive optical combiners or optical splitters, respectively, in a manner analogous to that just described. Furthermore, the add side channels can be multiplexed by a passive optical combiner, such as a multiport coupler or a conventional multiplexer. Moreover, the drop side channels can be separated by a conventional demultiplexer. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims. For example, it is possible to cascade the programmable multiplexers and demultiplexers in the various cross connect embodiments described above, in order to increase the output port count of the devices from K to K 2  when two devices are cascaded.