Abstract:
An integrated reconfigurable planar lightwave add-drop (RPLAD) multiplexer for use in a WDM optical communication system is arranged such that each drop port can receive any wavelength channel and each add port can transmit on any wavelength channel. Drop port reconfigurability is achieved by integrating a cross-connect functionality into the RPLAD, illustratively using optical 1×2 switches to perform “space” switching. The switches are controlled from a remotely located node controller. Add port reconfigurability is achieved by having tunable lasers and a wavelength independent optical power combiner, which may be a star coupler that is integrated with the other above-mentioned elements. The RPLAD has a modular architecture, so that when RPLAD&#39;s are connected by dual unidirectional transmission rings for the purpose of redundancy and failure protection, an RPLAD that fails can be removed and the remaining module can be reconfigured so as to re-route incoming traffic backwards around the still operating optical communication ring.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to optical communications, and more particularly to an arrangement and architecture for a reconfigurable, integrated optical add/drop multiplexer suitable for use in a wavelength division multiplexed (WDM) optical communication system.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical networks traditionally consist of a collection of static, inter-nodal communication links located on a ring or mesh. In such a network, network nodes include (a) transmitters having multiplexers (MUX&#39;s) that combine multiple channels with different wavelengths into a combined WDM optical signal that is transmitted to remote nodes on the network, as well as (b) receivers having demultiplexers (DMUX&#39;s) that recover individual wavelength channels from previously multiplexed WDM optical signals received from remote nodes on the network. The transmitters and receivers typically operate at fixed central frequencies, and the MUX&#39;s and DMUX&#39;s traditionally have fiber port connections that require pre-assigned wavelengths that cannot be varied. Reconfiguring such networks in order to create (or tear down) connections between nodes typically requires human intervention. For example, a technician may need to add or remove equipment (e.g. transmitters) and rearrange fiber connections to various optical components (e.g. MUX&#39;s or DMUX&#39;s) at one or more nodes.  
           [0003]    One approach to achieve reconfigurability involves the use of optical add/drop multiplexers (OADM&#39;s) that can be remotely controlled in order to add or drop an optical signal to a client, and selectively pass an optical signal (of a specific optical wavelength) through the node without being affected. The various major components in such OADM&#39;s currently can be obtained in various discrete technologies. For example, 2×2 switches as well as optical cross-connects (OXC&#39;s) can be made of thermo-optic, electro-optic, or micro-electro-mechanical systems (MEMS). The MUX&#39;s/DMUX&#39;s can be made of multi-layer dielectric filters, arrayed waveguide gratings, or bulk-optic diffraction gratings. Unfortunately, building OADMs out of discrete components is very expensive, and many of these discrete technologies are not well suited for integrated fabrication.  
           [0004]    A recent improvement in the OXC component of an OADM is described in patent application entitled “Integrated Wavelength Router”, Ser. No. 10/035,628 filed Nov. 1, 2001, on behalf of Christopher R. Doerr, which application is assigned to the same assignee as the present application, and which is hereby incorporated herein by reference. In the aforementioned application, a 1×K wavelength-selective cross connect (WSC) comprises a demultiplexer arranged to receive an input WDM signal containing multiple wavelengths, and apply its output, namely, the separated the wavelengths, to a binary tree, i.e., at least two stages, of interconnected 1×2 switches. The switches are integrated, and have their outputs crossing each other at each stage. The outputs of the switches in the final stage are applied to, and combined in, K multiplexers, which provide the outputs of the router. If desired, a set of shutters can be interposed in the waveguides leading to the multiplexer inputs, thereby providing additional isolation. The Doerr arrangement advantageously can be fabricated in a small area and therefore implemented in an integrated fashion.  
           [0005]    Notwithstanding the foregoing, other elements of the OADM have not, to date, been arranged in an integrated architecture, so that the remote reconfiguration functionality that is desired has not been achieved.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the present invention, a reconfigurable planar lightwave add-drop (RPLAD) multiplexer is arranged so that it can be fabricated in an integrated manner. The add-drop multiplexer design is novel in that it exhibits true reconfigurability for both the dropped and added channels. By “true” reconfigurability, we mean that each drop port can receive any wavelength channel and each add port can transmit on any wavelength channel. The drop port reconfigurability is achieved by integrating a cross-connect functionality into the RPLAD, in order to achieve an active spatial routing capability. The cross connect is operated under the control of a remotely located node controller. The add port reconfigurability is achieved using passive power collection, such as by having tunable lasers and a wavelength independent optical power combiner or multiplexer.  
           [0007]    All of this is enabled by using different techniques to perform the add and drop functions of the multiplexer. With respect to drop channels, optical 1×2 switches, interposed in each wavelength channel, control which channels are “through” channels and which channels are “drop” channels. In order to route the wavelength on each channel to a particular desired receiver, an optical cross connect (OXC), which operates as a “space” switch, is used. By way of contrast, with respect to the add channels, tunable lasers are used in the transmitters to generate optical signals at desired specific wavelengths. The individual channels are then combined in a multiplexer, which may be a star coupler that is also integrated with the other above-mentioned elements.  
           [0008]    In accordance with another aspect of the present invention, the RPLAD arrangement of the present invention is also novel in that it is advantageously designed in a modular architecture, which is advantageous when the RPLAD&#39;s are connected by dual transmission rings that operate in both East-to-West (CCW) and West-to-East (CW) directions, for the purpose of redundancy and failure protection. Specifically, the RPLAD includes identical CW and CCW modules, each of which contain a channel-dropping cross connect switch for one direction plus an add power combiner for the opposite direction. Each of the RPLAD&#39;s is arranged to add channels to one of the transmission rings and to drop channels from the other one of the rings. Thus, if one of the modules in an RPLAD unit fails, it can be removed and the remaining module can be reconfigured so as to re-route incoming traffic on one of the optical communication rings backwards around the other optical communication ring. This combination of the channel dropping capability for one direction of traffic and the channel adding capability for the other direction of traffic in the same RPLAD unit makes the RPLAD unit behavior, in the event of a failure, the same as a fiber cut occurring adjacent to the node, which behavior the failure protection arrangement is well-designed to handle.  
           [0009]    In accordance with yet another aspect of the present invention, level balancing, which corrects for imbalances in the power levels present in different WDM channels, brought about, for example, due to unwanted differences in gain or loss in individual channels, is achieved by appropriately controlling variable optical attenuators (VOA&#39;s) that are advantageously integrated into the RPLAD units.  
           [0010]    In accordance with yet another aspect of the present invention, interleavers or multi-dielectric bandsplitters, are provided at each network node, in order to divide a large number of wavelength channels into smaller, more manageable groups. More specifically, the overall optical spectrum is divided into smaller bandwidth segments, each segment accommodating a group of WDM channels and being handled by an individual RPLAD. Advantageously, the interleavers can also be integrated in the same optical device that contains the other components previously described.  
           [0011]    By virtue of the present invention, the manually intensive reconfiguration process normally associated with an add/drop multiplexer is avoided in a remotely reconfigurable optical node that can be fabricated in integrated form. The node includes transmitters with user-selectable optical center frequencies as well as optical cross-connects that can be configured to connect specific user-selectable channels having different optical wavelengths, to (or from) the line system to (or from) the correct port on the MUX (or DMUX). This wavelength or channel reconfigurability enabled by the present invention is especially crucial in the planning and maintenance of networks that have a high degree of churn in network connectivity amongst clients who may have signals of different bit rate and/or protocol. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    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:  
         [0013]    [0013]FIG. 1 is a block diagram of the arrangement of a reconfigurable planar add-drop (RPLAD) multiplexer arranged in accordance with the present invention;  
         [0014]    [0014]FIG. 2 is a block diagram illustrating further details of an embodiment of the invention, arranged using the architecture shown in FIG. 1;  
         [0015]    [0015]FIG. 3 is a block diagram illustrating in yet more detail a possible embodiment of certain elements of FIGS. 1 and 2, for example, demultiplexer  124 , 1×2 switches  126 - 1  through  126 - 8 , optical cross connect  128 , VOA&#39;s  132 - 1  through  132 - 8 , and multiplexer  134 , which may be integrated on a single optical substrate;  
         [0016]    [0016]FIG. 4 is a diagram illustrating an alternate arrangement for the cross-connect portion of the RPLAD of FIGS. 1 and 2;  
         [0017]    [0017]FIG. 5 is a diagram of portions of the add-drop multiplexer of FIGS. 1 and 2, showing schematically the arrangement of elements as they would be fabricated in an integrated RPLAD device;  
         [0018]    [0018]FIG. 6 is a diagram illustrating the manner in which the present invention provides modularity that is useful in reacting to faults that may occur in the optical transmission links connecting WDM add-drop nodes;  
         [0019]    [0019]FIG. 7 is a diagram illustrating the manner in which the modularity and integratability of the present invention can be extended to include the entrance interleavers  110  and  111  of FIG. 2; and  
         [0020]    [0020]FIG. 8 is a diagram illustrating another aspect in which the present invention enables modular operation of a WDM communication system. 
     
    
     DETAILED DESCRIPTION  
       [0021]    Referring first to FIG. 1, there is shown a block diagram of a reconfigurable planar add-drop (RPLAD) multiplexer arranged in accordance with the present invention. Two identical RPLADs  10  and  20  are shown, the former being designated the West RPLAD and the latter the East RPLAD, for reference purposes. Each RPLAD  10 ,  20  includes a drop portion  12 ,  22  and an add portion  14 ,  24  that advantageously, in accordance with the present invention, are integrated on a single optical substrate, respectively. RPLADs  10  and  20  are connected to each other and are also each connected to two WDM optical transmission links, e.g. a west to east link that enters drop portion  12  in RPLAD  10  via fiber  101  carrying a plurality of WDM channels. Drop portion  12  is arranged to selectively output (“drop”) desired channels to particular receivers  16 - 1  to  16 - n , under the control of a remotely located node controller  170 . Drop portion  12  is so designed such that any channel on  101  can be dropped to any receiver chosen by the node controller. Also, the dropped channels are not necessarily terminated immediately at a receiver and may instead enter another optical network. The remaining WDM channels are coupled to add portion  24  in RPLAD  20 , which is arranged to input or “add” desired channels applied via transmitters  28 - 1  to  28 - n , to the WDM signal, which is then output in the east direction via fiber  102 . A series of variable optical attenuators (VOA&#39;s)  29 - 1  to  29 - n , may be included in RPLAD  20  to adjust the levels of the signals applied by transmitters  28 - 1  to  28 - n , respectively. VOA&#39;s  29 - 1  to  29 - n  may also be operated under the control of node controller  170 .  
         [0022]    In the opposite (east to west) direction, operation is similar to that just described. Multiple WDM channels enter RPLAD  20  via fiber  103 . Dropped channels are applied to receivers  26 - 1  to  26 - n  via drop portion  22 , controlled also by node controller  170 . Added channels input from transmitters  18 - 1  to  18 - n  are applied, optionally via VOA&#39;s  19 - 1  to  19 - n , to the WDM signal via add portion  14 , and are output in the west direction via fiber  104 .  
         [0023]    Referring now to FIG. 2, further details of the arrangement of a node in a WDM optical communication system that includes a reconfigurable planar add-drop multiplexer (RPLAD) arranged in accordance with the present invention, are shown. It is again assumed that the node of FIG. 2 is interconnected with other similar nodes in the same bi-directional optical communication system as shown in FIG. 1, such that the node is part of a first west-to-east optical transmission system that enters the node via fiber  101  and exits via fiber  102 , and a second east-to-west optical transmission system that enters the node via fiber  103  and exits via fiber  104 . Again, each of the fibers is arranged to carry multiple WDM wavelength channels, illustratively 32 channels.  
         [0024]    The eastbound channels enter the node via a west entrance interleaver  110 , which, divides the channels into multiple groups each containing multiple WDM channels, and applies each channel group to a respective west RPLAD  120 , only one of which is shown in FIG. 2. (In an illustrative example, the incoming WDM signal on fiber  101  contains 32 wavelength channels; this signal is divided into four groups of eight wavelength channels, one group of eight channels is applied to RPLAD  120  and the three groups of channels are applied to three other west RPLAD&#39;s not shown in FIG. 2.) Likewise, the westbound channels enter the node via an east entrance interleaver  111 , which similarly divides the channels into multiple groups, illustratively four groups of eight channels, and applies each channel group to a respective east RPLAD  140 , only one of which is shown in FIG. 2. (In the illustrative example of four groups of eight wavelength channels, three other east RPLAD&#39;s would be included.) Note that the wavelength channels in each group are preferably not adjacent in terms of wavelength, but rather are dispersed among the several groups, thereby providing better channel separation and the desired interleaving function.  
         [0025]    The eastbound multiplexed optical signal may, if desired, be amplified in an optional amplifier  122  before being applied to a demultiplexer  124 . As will be described more fully hereinafter, demultiplexer  124 , together with many of the other components in FIG. 2, are integrated on a single optical substrate, and are part of the arrangement described in the above-mentioned co-pending application filed on behalf of Doerr.  
         [0026]    Demultiplexer  124  separates the individual wavelength channels in the incoming signal, and in the illustrative case of eight wavelength channels, applies each channel to a respective 1×2 optical switch  126 - 1  through  126 - 8 , which operates under the control of a node controller  170 . If a particular wavelength channel is to be dropped, the appropriate switch  126 - 1  through  126 - 8  is controlled to route that channel to an input of optical cross connect  128 , which is illustratively configured in an 8×8 arrangement to handle up to eight wavelength channels. On the other hand, if a particular wavelength channel is not to be dropped, it is then considered to be a “through” channel, which is coupled to a multiplexer  134  via a corresponding variable optical attenuator  132 - 1  through  132 - 8 . The purpose of these attenuators is to provide suitable level balancing, in order to correct for imbalances in the power levels present in different WDM channels. These imbalances may be brought about, for example, by unwanted differences in gain or loss in individual channels that occur in various portions of the optical transmission system.  
         [0027]    The dropped channels are processed in cross-connect  128  in a manner such that any particular input wavelength channel (output from any of switches  126 - 1  through  126 - 8 ) may be routed to any desired output line  129 - 1  through  129 - 8  of cross-connect  128 , under the control of control signals generated in node controller  170 . This programmability is necessary because the outputs on lines  129 - 1  through  129 - 8  are coupled to individual receivers  167 - 1  through  167 - 8  in drop optical translator units (OTU&#39; s)  160 , and these receivers each are designed to operate at a different, predetermined frequency. Thus, due to the programmability of cross-connect  128 , the appropriate wavelength channel can be applied to each of the receivers, and it is not necessary for a technician to manually reconfigure the receivers. Additional VOA&#39;s  130 - 1  to  130 - 8  are provided in the path between optical cross connect  128  and receivers  167 - 1  to  167 - 8 , again to provide a level balancing capability.  
         [0028]    In addition to drop OTU&#39;s  160 , the arrangement of FIG. 2 includes a second set of drop OTU&#39;s  168 , which perform the same functions as OTU&#39;s  160 , but with respect to wavelengths proceeding westbound on optical transmission medium  103  rather than eastbound on transmission medium  101 . Furthermore, the arrangement of FIG. 2 also includes two add OTU sets  162  and  164 , which are arranged to add or insert wavelength channels onto the optical transmission mediums  104  and  102  proceeding westbound and eastbound, respectively.  
         [0029]    The through channels, as stated previously, are level adjusted in VOA&#39;s  132 - 1  through  132 - 8  and combined in multiplexer  134 , before being applied to one input of a coupler  159 . The second input to the coupler represents the add channels that originate in ones of the transmitters  165 - 1  through  165 - 8  in add OTU&#39;s  164 . These wavelength channels are level adjusted in respective VOA&#39;s  151 - 1  through  151 - 8  before being combined in a multiplexer  153 , which may advantageously be a star coupler. The output of multiplexer  153 , which is a WDM signal, may be level adjusted by optional amplifier  155  and optional VOA  157 , before being applied to the second input to coupler  159 . The output of that coupler represents both the through and add channels, and is applied to the eastbound optical transmission medium  102  via east egress interleaver  113 , which combines the group of channels processed in the RPLAD of FIG. 2 with other channel groups processed in other RPLADS, not shown.  
         [0030]    The arrangement just described is replicated in the node of FIG. 2 such that similar elements to those just described operate in the east to west direction. Specifically, east RPLAD  140  includes an optional amplifier  142  similar to amplifier  122 , a demultiplexer  144  similar to demultiplexer  124 , 1×2 switches  146 - 1  through  146 - 8  similar to switches  126 - 1  through  126 - 8 , a cross connect  148  similar to cross connect  128 , VOA&#39;s  152 - 1  through  152 - 8  similar to VOA&#39;s  132 - 1  through  132 - 8 , VOA&#39;s  150 - 1  through  150 - 8  similar to VOA&#39;s  130 - 1  through  130 - 8 , and multiplexer  154  similar to multiplexer  134 . The drop channels are applied to appropriate receivers  169 - 1  through  169 - 8  on drop OTU&#39;s  168 , while the through channels are applied to a first input to coupler  139 .  
         [0031]    The add channels for the eastbound direction are also arranged in a similar manner to that already described with respect to the westbound direction. Specifically, transmitters  163 - 1  through  163 - 8  in add OTU  162  are coupled to VOA&#39;s  131 - 1  through  131 - 8 , which are similar to VOA&#39;s  151 - 1  through  151 - 8 , already described. The outputs of VOA&#39;s  131 - 1  through  131 - 8  are combined in multiplexer  133 , level adjusted in optional amplifier  135  and VOA  137 , and applied to the second input of coupler  139 . The output of coupler  139  is applied to the westbound optical transmission channel  104  via west egress interleaver  112 .  
         [0032]    Referring now to FIG. 3, there is shown a block diagram illustrating in still more detail a possible embodiment of the arrangement of certain elements of FIGS. 1 and 2, for example, demultiplexer  124 , 1×2 switches  126 - 1  through  126 - 8 , optical cross connect  128 , VOA&#39;s  132 - 1  through  132 - 8 , and multiplexer  134 , which may be integrated on a single optical substrate. This arrangement is called a “wavelength-selective crossconnect” and is contemplated and explained more fully in the above-mentioned co-pending application filed on behalf of Doerr.  
         [0033]    Specifically, in FIG. 3, an integratable lx9 wavelength-selective cross-connect includes a demultiplexer  224  (which corresponds to demultiplexer  124  of FIG. 1), which receives a composite input WDM signal and separates the individual wavelength channels, which are output to 1×2 switches  226 - 1  through  226 - 8  (which correspond to switches  126 - 1  through  126 - 8  of FIG. 2). If any of these switches are in the “down” position, the wavelength channels are designated as through channels, and the switch outputs are combined in multiplexer  234  (which corresponds to multiplexer  134  of FIG. 2). Variable optical attenuators (VOA&#39;s)  232 - 1  through  232 - 8 , which correspond to VOA&#39;s  132 - 1  through  132 - 8  of FIG. 2, are interposed in each optical path, for the purpose of providing desired level balancing.  
         [0034]    If any of the switches  126 - 1  through  126 - 8  are in the “up” position, the wavelength channels are designated as drop channels, and the switch outputs are applied to a three level tree of binary (1×2) switches. The first level consists of one group of eight switches  240 - 1  through  240 - 8 , the second level consists of two groups each of eight switches, switches  241 - 1  through  241 - 16 , and the third level consists of four groups each of eight switches, switches  242 - 1  through  242 - 32 . Collectively, the three switch groups  240 ,  241  and  242 , together with multiplexers  260 - 1  through  2608 , perform the functions of OXC  128  of FIG. 2, in that any desired wavelength can, depending upon the position of the switches in the three groups, be routed to any of the multiplexers, and be available at any of desired receiver  167 - 1  through  167 - 8  in OTU  166 . The positions of the switches are controlled by node controller  170 , possibly indirectly by a secondary controller inside the RPLAD circuit pack. VOA&#39;s  230 - 1  through  230 - 8  correspond to VOA&#39;s  130 - 1  through  130 - 8  of FIG. 2, and provide level balancing in the signals provided to the receivers.  
         [0035]    As explained in the co-pending Doerr application cited above, in order to reduce optical crosstalk encountered in the binary tree that is primarily due to unwanted power transfer in waveguide crossover junctions, optical shutters  270 - 1  through  27032  can be inserted in each optical path. Node controller  170  is then arranged to close the shutter in any path that is not carrying an active wavelength channel.  
         [0036]    The three-level tree of binary (1×2) switches  240 ,  241  and  242  shown in FIG. 3 may be replaced, if desired, by a different arrangement of 1×2 switches as shown in FIG. 4. Here, each wavelength channel, illustratively having eight wavelengths λ 1  to λ 8 , is applied to a separate series connected string of seven 1×2 switches, such as switches  302 - 1 ,  302 - 2  and  302 - 3  in a first string, and switches  304 - 1 ,  304 - 2  and  304 - 3  in a string string, each of which switches has one output connected to the next switch and one output connected to a different one of the multiplexers  260 - 1  through  260 - 7 . One of the outputs of the last switch in the string is also connected to multiplexer  2608 . Depending upon the position of the switches  302 , the optical signal at wavelength λ 1  can thus be directed to a desired one of the multiplexers. In a similar manner, additional strings of switches (such as switches  304 ) are arranged to connect the remaining wavelengths λ 2  to λ 8 , to the desired multiplexers  260 - 1  through  260 - 7 .  
         [0037]    Referring now to FIG. 5, there is shown a diagram of portions of the add-drop multiplexer of FIGS. 1 and 2, showing schematically the arrangement of elements as they would be fabricated in an integrated RPLAD device in accordance with the possible embodiment of FIG. 3. Advantageously, in FIG. 5, the WDM input to the RPLAD is received on line  301 , and the through, drop and add outputs  303 ,  305 - 1  to  305 - n  and  307 , respectively, are all arranged to be physically located on the same (e.g., right) side of the device. (Note that n is an arbitrary integer, representing the total number of possible dropped channels. For the purposes of illustration, it is again assumed that n=8.) The input WDM signal is applied to a demultiplexer  310  consisting of star couplers  311  and  313  interconnected by waveguides in accordance with the arrangement described in the above-referenced application to Doerr. The individual wavelength channels output from demultiplexer  310  are each then applied to a first group of 1×2 switches  320 - 1  to  320 - n , which routes the through channels through shutters  322 - 1  to  322 - n  to a multiplexer  330  consisting of star couplers  331  and  333  interconnected by waveguides. The outputs of switches  320 - 1  to  320 - n  can also be directed to switch groups  340 - 1  to  340 - n , and thence to a desired one of the multiplexers  350 - 1  to  350 - n , which again each consist of star couplers  351  and  353  interconnected by waveguides. Shutters  360  can be interposed in the input to each of the multiplexers, in order to reduce unwanted cross-talk.  
         [0038]    In the add channel, the arrangement is simpler. The individual wavelength channels to be added are applied to respective VOA&#39;s  370 - 1  to  370 - k , and then combined in a single star coupler  372  that operates as a wavelength independent optical power combiner. (Note that k is an arbitrary integer, representing the total number of possible added channels.)  
         [0039]    Several key points are to be noted in connection with the arrangement of FIG. 5. First, it will be appreciated that the majority of the elements in FIG. 5 correspond to the elements of FIG. 2, and thus provide the desired functions via an integrated, easily fabricated optical device. Specifically, using west RPLAD  120  as an example, the input optical signal on line  301  corresponds to the signal applied to demultiplexer  124 ; the output drop signals on outputs  305 - 1  to  305 - n  correspond to the outputs on lines  129 - 1  to  129 - 8  in FIG. 2; the through output  303  corresponds to the output of multiplexer  134  of FIG. 2; and the add output  307  corresponds to the output of multiplexer  133 . Several of the remaining elements shown in FIG. 2 would be considered “off-chip”, and include, for example, RPLAD  120 , amplifiers  122  and  135 . Second, it is to be noted that different arrangements are used with respect to the add and drop channels. The optical cross connect, which is implemented by appropriately controlling switches  340 , operates as a “space” switch. Accordingly, the drop function is performed by active spatial routing. On the other hand, with respect to the add channels, tunable lasers are used in the transmitters to generate optical signals at desired specific wavelengths, and the individual channels are then combined in star coupler  372 . This aspect of the RPLAD therefore operates in the “frequency” (wavelength) domain. It can therefore also be said that the add function is performed by passive power collection.  
         [0040]    Referring now to FIG. 6, there is shown a diagram illustrating the manner in which the present invention provides modularity that is useful in reacting to faults that may occur in the optical transmission links connecting WDM add-drop nodes. West RPLAD  120  and east RPLAD  140 , shown in FIG. 6, are internally the same as shown in FIG. 2. Each of these RPLAD&#39;s is identical to the other, and normally functions in a similar manner when both the east-to-west and west-to-east optical paths are operating properly. In FIG. 6, OTU module  160  of FIG. 2 also includes a pair of 2×2 optical switches,  510  and  520 , that are not shown in FIG. 2. These switches are interconnected with drop OTU &#39;s  166 ,  168  and ADD OTU &#39;s  162 ,  164 , respectively, in such a manner that (a) the signals normally applied from cross connect  105  in west RPLAD  120  to drop OTU  166  can be re-routed to drop OTU  168  by the switching action of switch  510 , (b) the signals normally applied from cross connect  148  in east RPLAD  140  to drop OTU  168  can be re-routed to drop OTU  166  by the switching action of switch  510 , (c) the signals normally applied from add OTU  162  to VOA&#39;s  131  and multiplexer  133  in west RPLAD  120  can be re-routed to VOA&#39;s  151  and multiplexer  153  in east RPLAD  140  by the switching action of switch  520 , and (d) the signals S normally applied from add OTU  164  to VOA&#39;s  151  and multiplexer  153  in east RPLAD  140  can be re-routed to VOA&#39;s  131  and multiplexer  133  in west RPLAD  120 . In this manner, if one of the optical transmission links is out of service, communications using the remaining optical transmission link can nevertheless continue.  
         [0041]    Referring now to FIG. 7, there is shown a diagram illustrating the manner in which the modularity and integrability of the present invention can be extended to include the entrance interleavers  110  and  111  of FIG. 2. In FIG. 7, a module  601  built on a single substrate includes both an RPLAD  603  and an interleaver  602 . A similar arrangement of RPLAD  613  and an interleaver  612  is shown in a second, identical module  611 . On each module, a WDM signal input to the interleaver is separated into multiple individual wavelength channels, and multiple outputs are available from the module, each containing one wavelength channel. In addition, in each module, the input and both through and drop outputs of the RPLAD are available. With this arrangement, each of the modules can advantageously serve different functions. For example, module  601  provides both interleaving and add/drop functionality, by routing some of the wavelength channels to RPLAD  603  and other of the wavelength channels to RPLAD  613  on module  611 . By way of comparison, interleaver  612  on module  611  is not used, and that module provides only add/drop functionality. Although not shown in FIG. 7, it is to be noted that a similar approach can also be used with respect to egress interleavers  112  and  113  of FIG. 2.  
         [0042]    Referring now to FIG. 8, there is shown a diagram illustrating in more detail another aspect in which the present invention enables modular operation of a WDM communication system. In that figure, four nodes  701 - 704  are interconnected via two optical communications rings  711  and  712 , the former operating in a CW direction and the latter operating in a CCW direction. Each of the nodes  701 - 704  is basically similar to the arrangement shown in FIGS. 1 and 2, except that each node is internally arranged to accommodate a different number of add/drop channels. More specifically, nodes  701  and  704  are illustratively each 32×8 nodes, meaning that (a) they are arranged to receive a  32  channel WDM signal, and (b) add/drop  8  WDM channels. To achieve this functionality, cross connects  105  and  148  would of course be 8×8 switches, eight VOA&#39;s  131 ,  151  would be provided, etc. By way of comparison, node  702  is illustratively a 32×16 node, meaning that (a) it is also arranged to receive a 32 channel WDM signal, but it is arranged to (b) add/drop 16 WDM channels. To achieve this functionality, cross connects  105  and  148  would of course be 16×16 switches, sixteen VOA&#39;s  131 ,  151  would be provided, etc. Node  703  is illustratively a 32×4 node, and is therefore arranged to handle  4  add/drop channels.  
         [0043]    By virtue of the arrangement like that shown in FIG. 8, a network designer can easily select a node that has the appropriate capability for the site at which the node is to be used. For high traffic sites, larger nodes are employed, while for sites at which fewer channels are anticipated smaller nodes are sufficient.  
         [0044]    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, additional elements can be integrated on the same substrate as those shown in the figures, and these elements can provide either additional functionality or additional capacity.