Patent Publication Number: US-7212739-B2

Title: Protection switching arrangement for an optical switching system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. application Ser. No. 09/726,027 filed on Nov. 30, 2000 now U.S. Pat. No. 6,999,677, assigned to the Assignee of the present invention and hereby incorporated by reference herein. The present application is also related in subject matter to the following U.S. applications, which are assigned to the Assignee of the present invention and hereby incorporated by reference herein: U.S. application Ser. No. 09/511,065, entitled “Switch For Optical Signals”, filed on Feb. 23, 2000, now U.S. Pat. No. 6,606,427; U.S. application Ser. No. 09/593,697, entitled “Optical Switching Device”, filed on Jun. 15, 2000, now U.S. Pat. No. 6,366,716; U.S. application Ser. No. 09/703,631, entitled “Optical Switching System for Switching Optical Signals in Wavelength Groups”, filed on Nov. 2, 2000, now U.S. Pat. No. 6,882,800; U.S. application Ser. No. 09/648,767, entitled “Method, System and Signal for Carrying Overhead Information in a Transport Network Employing Optical Switching Nodes” and filed on Aug. 28, 2000; and U.S. application Ser. No. 09/580,495, entitled “Optical Switch with Power Equalization” and filed on May 30, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to systems for switching optical signals and more particularly to protection switching arrangements for such systems. 
     BACKGROUND OF THE INVENTION 
     Dense Wavelength Division Multiplexing (DWDM) of optical signals is a technique used to carry many optical signals on a single optical fiber. In DWDM systems, the transmission spectrum, for example 1520 nm to 1550 nm, is divided into many channel wavelengths with adequate spacing left between each channel wavelength to allow for separation of the DWDM signal into its constituent channel wavelengths, also referred to as lambdas, in a demultiplexer, feeding an array of receivers. Each optical signal in the DWDM signal has a unique wavelength representing a particular frequency, which has been assigned to the carrier signal of that channel, the carrier signal having been modulated at a high bit-rate, for example 10 Gb/s, by data to be transmitted on that channel wavelength. This creates optical sidebands above and below the carrier frequency. These determine the densest practical spacing in a WDM system since they must not overlap. As improvements in DWDM related techniques are made, for example improvements in modulation of carrier signals and demultiplexing of DWDM signals into their constituent optical signals, it is feasible that more optical signals, each of a higher bit-rate, will be carried on a single fiber. For example, systems are currently envisioned that will transmit up to 160 channel wavelengths, each carrying up to 10 Gb/s of data, on a single fiber. As these advances are made, switches for switching the DWDM signals will be required to switch a larger number of optical signals both with a granularity of an individual wavelength and as groups of wavelengths, resulting in larger and more complex switch architectures. 
     With any optical switching system there is a concern of data loss due to a breakdown of the physical path through the optical switching system. The use of DWDM techniques amplifies this concern due to the increased number of channels involved in any physical path breakdown. 
     In view of the above, there is a need for a protection switching arrangement for optical switching system that addresses the protection requirements of optical switching systems using DWDM techniques described above. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved protection switching arrangement for optical switching systems. 
     Accordingly, the protection switching arrangement for the optical switching system provides a way to protect the dense traffic in DWDM and WDM optical signals by providing alternative switching paths. Conveniently optical wavelengths may be referred to as lambdas and groups of optical wavelengths as lambda groups. 
     According to an aspect of the present invention there is provided a protection switching arrangement for optical switching systems comprising: a plurality of optical switching matrices having multiple inputs and multiple outputs and being operable to optical channel signals from any one of a plurality of the inputs to any one of a plurality of the outputs; a plurality of wavelength division demultiplexers coupled at its outputs to the inputs of the plurality of optical switching matrices for dividing a composite optical signal into optical channel signals and providing each optical channel signal to a corresponding optical switching matrix; a spare wavelength division demultiplexer coupled at its outputs to the inputs of the plurality of optical switching matrices for dividing a composite optical signal into optical channel signals; and at least one optical protection switch having a plurality of inputs and a plurality of straight-through outputs and at least one protection output and coupled at each of its straight-through outputs to an input of a respective one of the plurality of wavelength division demultiplexers and coupled at its protection output the inputs of the spare wavelength division demultiplexer. 
     An advantage of embodiments of the invention is the provision of in situ testing during various phases of implementing and recovering from a protection switch condition for the 2×N and 3×N protection switches. 
     In accordance with another aspect of the present invention there is provided a protection switching arrangement comprising: a first logical layer for switching optical channels; a second logical layer for switching a group of optical channels; and a first coupler for grouping together optical channels of the first logical layer and coupling them to the second logical layer; a second coupler for ungrouping grouped optical channels of the second logical layer and coupling them to the first logical layer; a first protection switch providing an alternative switch path for at least one of the grouped optical channels from the first logical layer in the second logical layer. 
     In accordance with another aspect of the present invention there is provided a protection switching arrangement for optical switching systems comprising an optical protection switch including: a first column of deployable mirrors, each mirror operable for deflecting an optical signal from an optical signal input path to a protection path; and a second column of deployable mirrors, each mirror operable for deflecting an optical test signal from an optical test signal input path to an optical switch testing path wherein for each mirror of the first column and corresponding mirror of the second column, the respective optical signal input path and optical switch test path are substantially aligned. 
     In accordance with another aspect of the present invention there is provided protection switching arrangement for optical switching systems comprising an optical protection switch including: a first column of deployable mirrors, each mirror operable for deflecting an optical signal from an optical signal input path to a protection path; a second column of deployable mirrors, each mirror operable for deflecting an optical test signal from an optical test signal input path to an optical switch testing path; and a third column of deployable mirrors, each mirror operable for deflecting an optical signal from an optical signal input path to a protection path; wherein for each mirror of the first and third column and corresponding mirror of the second column, the respective optical signal input path and optical switch test path are substantially aligned. 
     Other aspects of the invention include combinations and sub combinations of the features described above other than the combinations described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be further understood from the following detailed description of embodiments of the invention with reference to the drawings, in which: 
         FIG. 1  illustrates in a functional block diagram a known optical switch protection arrangement; 
         FIG. 2  illustrates in a functional block diagram a lambda-plane switch of a co-pending application of the present applicant; 
         FIG. 3  illustrates in a functional block diagram lambda-plane optical switch of  FIG. 2  with a protection switching arrangement in accordance with an embodiment of the present invention; 
         FIG. 4  illustrates a protection switch in accordance with an embodiment of the present invention; 
         FIG. 5  illustrates a protection switch in accordance with a further embodiment of the present invention; 
         FIG. 6  illustrates a protection switching arrangement using the protection switch of  FIG. 5 ; 
         FIG. 7  illustrates a protection switch in accordance with another embodiment of the present invention; 
         FIG. 8  illustrates further detail of the lambda-plane optical switch with a protection switching arrangement of  FIG. 3 ; 
         FIG. 9  illustrates a multiple-granularity multiple-plane optical switch core of a co-pending application of the present applicant; 
         FIG. 10  illustrates in a functional block diagram a protection arrangement for the multiple granularity multiple plane optical switch core of  FIG. 9  in accordance with an embodiment of the present invention; 
         FIG. 11  illustrates in a functional block diagram a protection arrangement for the switch core and ports of  FIG. 9  in accordance with an embodiment of the present invention; 
         FIG. 12  illustrates in a functional block a protection arrangement for the multiple granularity multiple plane optical switch core of  FIG. 9  with the plane switches implemented as six-port MEMS in accordance with an embodiment of the present invention; 
         FIG. 13  illustrates in a functional block diagram a protection arrangement for the switch core and ports of a combined switch core in accordance with an embodiment of the present invention; and 
         FIG. 14  illustrates the optical switch of  FIG. 13  modified to provide fiber plane switch protection using the same protection switch plane as used for lambda and lambda group protection in  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1  there is illustrated in a block diagram a known optical switch protection arrangement. The known protection arrangement provides a first switch core  1  and a second switch core  2 . A plurality of inputs  3   a – 3   q  are connected to both switch cores via 3 dB splitters  4   a – 4   q . A plurality of outputs  5   a – 5   q  are connected to one of switch cores  1  and  2  via an array of switches  6   a – 6   q . For simplicity, none of the control circuitry required is illustrated with only the optical paths being shown. 
     In operation, one switch core is active, for example switch core  1 , while the other is inactive or on standby, in the present example switch core  2 . Upon detection of a fault in switch core  1 , the array of switches  6   a – 6   q  are activated to switch to the standby core switch  2 . As can be appreciated, there are concerns associated with this arrangement. The arrangement requires duplication of the core switch, thereby doubling the cost of the switch core. The arrangement requires switching every output, which for large cores may involve hundreds of connections, thereby causes transient losses of data for all connects in the switch. The splitters introduce an additional 3 dB loss per line that must be compensated. However, the use of the 3 dB splitter permits traffic to be fed into both switch fabrics SW 1  and SW 2 , thereby facilitating routine testing of the protection fabric, e.g. SW 1 , to insure that it has not failed. However, it does not permit the injection of any optical test signals. 
     Referring to  FIG. 2  there is illustrated in a functional block diagram a WDM optical switching system  10  of a co-pending application of the present applicant. The system  10  switches the individual optical carriers or groups of carriers of throughput Dense WDM (DWDM) optical signals, each signal consisting of M optical signal channels. Each of the M channels carries an optical signal modulated on an optical carrier of a wavelength unique to that channel. Incoming DWDM optical signals are split, or demultiplexer, into their component optical signal channels, which are then, switched by the system  10 , and then combined, or multiplexed, into outgoing DWDM optical signals. The system  10  has N input ports and N output ports to receive and transmit the incoming and outgoing DWDM optical signals, respectively. 
     The system  10  includes a wavelength-plane optical switching sub-system  12 , a plurality N of 1 to M demultiplexers  16 , a plurality N of M to 1 multiplexers  18 , a wavelength converting switch  14  and a controller  20 . After input preamplifier  21 , a plurality N of fibers  22  are coupled to the plurality N of demultiplexers  16  at the ingress of the system  10 , each fiber  22  coupled to a respective demultiplexer  16 . Each of the demultiplexers  16  has one input and M outputs. For the purpose of example M=40 and N=24. A plurality N of array of optical interconnections  24 , each of width M couple the N×M outputs of the demultiplexers  16  to M×N port inputs (Pi) of the optical switching sub-system  12 . Similarly, a plurality N of array of optical interconnections  26 , each of width M, couple N×M port outputs (Po) of the optical switching sub-system  12  to N×M inputs of the multiplexers  18 . Each of the N multiplexers  18  has M inputs and one output. A plurality N of fibers  28  are coupled to the plurality of multiplexers  18  at the egress of the system  10 , each fiber  28  coupled to a respective multiplexer  18  and an output preamplifier  29 . The switching sub-system  12  is a wavelength plane structure in that it includes a distinct switching matrix, or matrices, for switching each one of the M unique wavelengths or wavelength groups. 
     The optical switching sub-system  12  includes a plurality K of matrix output ports (Mo) and a plurality K of matrix input ports (Mi) for coupling optical signal channels to the wavelength converting switch  14 . A plurality M of optical interconnection arrays  30 , each of width K, couple K×M matrix output ports (Mo) to the wavelength converting switch  14  at its ingress. Similarly, the egress of the wavelength converting switch  14  is coupled to K×M matrix input ports (Mi) via a plurality M of optical buses  32 , each of width K. The wavelength converting switch  14  has a plurality R of inputs for adding optical signal channels  34  and a plurality R of outputs for dropping optical signal channels  36 . 
     Referring to  FIG. 3  there is illustrated in a functional block diagram a WDM optical switching system  10  of  FIG. 2  including a protection switching arrangement in accordance with a first embodiment of the present invention. To protect a photonic switch having N inputs and M planes requires the addition of an WDM demux  16   n + 1 , a protection switch  56   n + 1 , an additional switch plane  12   m + 1  together with additional input and output on all switch planes (N×N) such that they become (N+1)×(N+1) switch planes, a WDM mux  18   n + 1 , a protection switch  58   n + 1 , and input and output 1×N protection switches  52  and  54 . A protection switch controller  50  activates the protection switches in response to measurements indicative of fault conditions. 
     In operation, the protection switch controller  50 , responsive to conditions in the switch indicative of a fault condition, actives the appropriate protection switches. For example, a fault in one of the port cards carrying for demux  16   b  and mux  18   b  would cause the protection switch controller  50  to activate the protection switch  52  to switch the entire incoming DWDM signal to the spare demux  16   n + 1 , at the same time the protection switch  54  would switch from the corresponding mux  18   b  to the spare mux  18   n + 1  to complete the protection switch. A fault in the switch core, for example optical switch matrix  12   b  would cause activation of all protection switches  56  and  58  to switch the traffic from  12   b  representing the entire switch traffic at that lambda to the spare switch matrix  12   m + 1 . Further detail of the operation of this protection switching arrangement is described with regard to  FIG. 8 . 
     Referring to  FIG. 4  there is illustrated a protection switch in accordance with an embodiment of the present invention. The protection switch  56  includes a four-port 1×M MEMS optical switch  70  having a linear array of erectable mirrors fabricated by known techniques (see Lih Y. Lin who describes such a device in the form of a Micro-Electro-Mechanical System (MEMs) in an article entitled “Free-Space Micromachined Optical-Switching Technologies and Architectures” in OFC99 Session W14–1 Proceedings published Feb. 24, 1999). Substrates  72  and  74  are provided with V-grooves for aligning input fibers  76   a–m  and output fibers  78   a–m  with rod lenses  80   a–m  and  82   a–m , respectively. A protection output fiber  84  is coupled to the 1×M MEMS optical switch  70 , via a rod lens  86 , a fiber  88  and an optical amplifier  90 . The rod lens  86  and the fiber  88  are aligned on substrate  92 . A test channel is provided via a test fiber  94  and rod lens  96  aligned on substrate  98  such that they are aligned with the rod lens  86  and the fiber  88 . The rod lenses  80   a – 80   m  and the rod lenses  82   a – 82   m  are aligned such that, when a MEMS mirror (e.g. mirror  103 ) is not erect, the light from the respective fiber  76   a – 76   m , having been collimated through rod lenses  80   a – 80   m  and projected across the intervening gap, is refocused by rod lenses  82   a – 82   m  such that it is coupled into fibers  78   a – 78   m . A similar situation exists for the fiber  94 , rod lenses  96  and  86  and the fiber  88 . The MEMS mirrors are positioned to deflect one of the collimated light beams from  80   a – 80   m  into the rod lens  86  so that the light center in the appropriate lens in the set  80   a – 80   m  from its associated fiber is coupled by lens  86  into the fiber  88 , thereby completing a switched optical path. 
     In operation, when no fault is detected, optical signals pass from the input to the output, for example from input  76   a  to output  78   a  as indicated by an arrow  100 . Since the optical path is very short, this path can be designed with have a very low optical loss. Also, a test signal may be input across the 1×M MEMS optical switch  70  via the test fiber  94  and the rod lens  96 , passing over the M inactivated mirrors of the MEMS and reaching the rod lens  86  as shown by an arrow  102 , thereby allowing periodic testing of the protection path. 
     When a fault is detected the appropriate mirror, for example a mirror  104 , is deployed and an optical signal input via the fiber  76   c  is deflected as shown by a broken-line arrow  106  to the protection path. This protection path is longer, hence introduces higher optical loss (typically for a 20:1 switch the loss would be 3 dB compared to 1 dB for the unprotected path). Consequently the optical amplifier  90  (EDWA, Erbium doped waveguide amplifier) is introduced in the protection path to compensate for the loss. A single amplifier can compensate for the loss in protection switches on both sides of the protected entity, since the concatenated extra loss of these switches (typically about 4 dB) is well within the gain of a low performance EDWA. 
     Referring to  FIG. 5  there is illustrated a protection switch in accordance with a further embodiment of the present invention. The protection switch of  FIG. 5  is similar to that of  FIG. 4 , except that the 1×M MEMS  70  is replaced by 2×M MEMS  110 , having a first column of mirrors  111  oriented such that they couple light to/from ports  76   a – 76   m  and a second column of mirrors  112  oriented in a different plane to the mirrors of  111  such that they couple light to/from the second set of ports  78   a–m . The second column of mirrors  112  is aligned with a fiber  114  and a rod lens  116  and a rod lens  118  and a fiber  120  forming a test channel. In the normal unprotected state, the traffic optical paths flow straight across this port as indicated by arrow  100 , as similarly shown in  FIG. 4 . As shown in  FIG. 4 , in a protection switch state when a mirror  104  is deployed in the first column of mirrors  111 , a corresponding mirror  122  in the second column of mirrors  112  may be deployed to allow a test signal, input via the fiber  120  to be injected to the output  78   c  for testing the faulty component, as indicated by the line-broken arrow  124 . Normally, when protection switching is not required, a test signal, as indicated by an arrow  113 , may be passed over the first mirror column  111 , from rod lens  96  and another test signal as indicated by arrow  126  may be passed over the second mirror column  112 , from rod lens  96  for test purposes, in a similar way to the test signal indicated by the arrow  102  was used in  FIG. 4 . Alternatively, by providing hairpin connection between fiber  94  and fiber  114  as indicated by broken line half circle  115 , an optical test signal injected at the fiber  120  can be used both for testing a failed switch plane, in protection mode and for testing the protection switch  110  in normal mode. 
     Referring to  FIG. 6  there is illustrated a protection switching arrangement using the protection switch of  FIG. 5 . In this figure the active mirrors are shown as diagonal lines while the diamonds represent mirrors that are not active. The protection switching arrangement involves tributary cards  128   a  and  128   b , switch cards  12   a  to  12   m , protection switch card  12   m + 1 , a test source  130  and a test receiver  132 . By way of example a switch plane  12   m  has failed in some way. Mirrors  134  and  136  of 2×M MEMS  10   a  and  110   b , respectively, are deployed to establish a protection path  138  through the protection switch card  12   m + 1 . At the same time a corresponding mirrors  140  and  142  of 2×M MEMS  110   a  and  110   b , respectively, are deployed to establish a test path  144  between the test source  130  and the test receiver  132  and through the faulty switch plane for the purpose of exercising the switch plane to determine the nature of the failure. Note, if the protection path is now removed from service and traffic is returned to the normal (unprotected) state, the test generator in automatically connected to the protection card to continuously verify its availability. 
     Referring to  FIG. 7  there is illustrated a protection switch in accordance with another embodiment of the present invention. The protection switch of  FIG. 7  is similar to that of  FIG. 5 , except that the 2×M MEMS is replaced by a 3×M MEMS  146 , having an third column of mirrors  148 , aligned with a fiber  150  and a rod lens  152  and a rod lens  154  and a fiber  156 ; A splitter  158  is connected to both fibers  94  and  150 , an optical amplifier  160  is connected to the fiber  156  and a combiner  162  connects fibers  88  and a fiber  164  from the optical amplifier  160 . 
     In operation,  FIG. 7 , when a mirror  104  fails to deploy in the protection switch condition, a mirror  166  in the third column of mirrors  152  is deployed to allow a optical signal from input fiber  76   c , to be diverted to the protection path via the rod lens  154  and the fiber  156 . The optical amplifier  160  is activated when the primary protection switch mirrors are determined to be inoperative. The secondary protection switch mirrors, third column  148  are placed in front of the primary protection switch mirrors, first column  111 , so that failures can be bypassed by deploying mirrors in the third column of mirrors  148 . Downtime would still be required to replace the failed protection switch, however the auxiliary row of mirrors would allow restoration of service between the time of failure and the repair period. 
     As shown in  FIG. 7 , in a protection switch state when a mirror  166  is deployed in the third column of mirrors  148 , a corresponding mirror  122  in the second column of mirrors  112  may be deployed to allow a test signal, input via the fiber  120  to be injected to the output  78   c  for testing the faulty component, as indicated by the line-broken arrow  124 . 
     Normally, when protection switching is not required, a test signal, as indicated by arrows  113  and  167 , may be passed over the first and third mirror columns  111  and  148 , respectively, for test purposes, in a similar way to the test signal indicated by the arrow  113  was used in  FIG. 5 . Alternatively, by providing hairpin connection between splitter  158  and fiber  114  as indicated by broken line  168 , an optical test signal injected at the fiber  120  can be used both for testing a failed switch plane, in protection mode and for testing the protection switch  146  in normal mode. 
       FIG. 8  illustrates further detail of the lambda-plane optical switch with a protection switching arrangement of  FIG. 3 .  FIG. 8  includes detail on the control circuitry for monitoring of the photonic switch and for activating the protection switches. To protect a photonic switch having N inputs and M planes requires the addition of an WDM demux  16   n + 1 , a protection switch  56   n + 1 , an additional switch plane  12   m + 1  together with additional input and output on all switch planes (N×N) such that they become. (N+1)×(N+1) switch planes, a WDM mux  18   n + 1 , a protection switch  58   n + 1 , and input and output 1×N protection switches  52  and  54 . Alternatively, if full N×N switching were not required [(N−1)+1]×[(N−1)+1] could be provided without increasing the dimensions of the switch matrices. 
     The simplified protection switch controller  50  of  FIG. 3  is replaced by a maintenance and protection processor  170  and a switch verification/equalization block  172  having a connection map check function  174  and a power spectrum equalization function  176 . The processor  170  is connected to each of the protection switches (just as controller  50  was in  FIG. 3 ) and receives input from the connection map check function  174  and the power spectrum equalization function  176 . Both the connection map check function  174  and the power spectrum equalization function  176  are fed from selective front ends  175  and  177  connected to a tap on the input and output amplifiers  23  and  29  of respective input and output fibers connected to the switches. This selective front end cycles round all of the inputs and outputs in any one of various methods (further detail provided in above referenced co-pending application) to allow each wavelength of each fiber in and out to be connected to the analysis block for some or all of the time depending upon the design of the selective front end. The connection map check function  174  and the power spectrum equalization function  176  are the subject matter of related co-pending patent applications, hence details of these functions are not duplicated here. The maintenance and protection processor  170  takes the connection status input and the power spectrum measurement input and determines therefrom when a failure condition exists in the optical switch and where that failure is located. 
     A small percentage of both the input and output signals are tapped (typically 5%) and these is fed into the switch verification/equalization block  172 . Within the connection map function  174 , the connection map actually being implemented by the switch (as opposed to the connection map being sent from the control processor to the switch) is determined by a process of comparing outputs with inputs. At the same time the output signal parameters, especially that of output power is determined by the power spectrum equalization function  176  (and used to flatten the output spectrum by adjusting the gain on the per lambda EDWAs). Both the path check and signal integrity check are implemented on a per lambda basis. The results from switch verification/equalization block  172  functions connection map check function  174  and the power spectrum equalization function  176  are fed to the maintenance and protection processor  170  where they are analyzed. An output signal may be out-of-range in amplitude (i.e. cannot be compensated for by changing the EDWA gain) because of several reasons such as equipment failure within the switch (e.g. high loss in crosspoint) or due to loss-of-input low input. The failures in the switch have to be differentiated from failures of incoming signals and this is done in the maintenance processor  170  by, in this case, examining both the input power to the switch node and the output power to determine whether the switch loss/gain is within specification. Similarly, connection mismatches are analyzed as described in further detail in another above-referenced co-pending application. In the event that the maintenance and protection processor  170  determines that a switch plane has failed, it tests the spare protection path, then, if good, switches the spare protecting plane in place of the switch card that is suspected to have failed. Once this is done, the switch card that is now out-of-service can be tested by using the second row of mirrors to inject test signals to localize the failure. In the event that the maintenance and protection processor  170  determines that it is likely that a port card has failed that can also be removed from service, then tested. Clearly, it is important that the analysis/assessment of the maintenance and protection processor  170  as to which unit has failed is as accurate as possible before it triggers a protection switch, since the protection switch operation causes a service-affecting transient during the switching operation. 
     Referring to  FIG. 9  there is illustrated a multiple-granularity multiple plane optical switch core of a co-pending application of the present applicant. Briefly, the optical switch core  180  includes a fiber plane optical switch  182 , a plurality J of lambda group plane optical switches  184 , each associated with a plurality L of lambda plane optical switches  186 , where J×L=M, resulting in a total of M lambda plane switches. In  FIG. 9 , for example, J=4, L=3, hence M=12. Each plane switch is implemented in 4-port MEMS having input ports I and output ports O and expansion input ports Ei and expansion output ports Eo. The plane switches can also be implemented as six-port MEMS as described in detail in the above-referenced co-pending application. Six-port MEMS will be discussed in further detail with regard to  FIG. 12 . Also included are a plurality N of 1 to J lambda band demultiplexers or de-interleavers  190 , a plurality N of J to 1 lambda band multiplexers or interleavers  192 . J pluralities of N 1 to L per lambda channelized demultiplexers are represented by  194   a  through  194   d  and J pluralities of N L to 1 per lambda channelized multiplexers are represented by  196   a  through  196   d . For simplicity a controller for setting up switched paths through individual optical switches  182 ,  184 , and  186  is not shown in  FIG. 9 . An array of low cost, low gain optical amplifiers  185  between the lambda group switch and the fiber switch and another array of similar optical amplifiers  187  between the lambda switches  16  and lambda group switches  184  are included. The purpose of arrays of amplifiers  185  and  187  is to equalize the loss through the various optical paths so that the output spectrum of the recombined WDM stream has a similar optical power spectrum in each wavelength. This is done to ensure that the emerging WDD streams contain a similar optical power in each spectral line, which will contribute to longer reach for optical transmission. A plurality N of fibers  200  are coupled to the plurality N of input ports (I) of the fiber plane switch  182  at the ingress of the system  180 , each fiber  200  coupled to a respective input port. Output ports (O) of fiber plane optical switch  182  are coupled to a plurality N of fibers, with each fiber  202  coupled to a respective output port. The fiber plane optical switch  182  also has a plurality N of expansion output ports (Eo)  204  coupled to a plurality N of 1:J demultiplexers. Each demultiplexer separates the DWDM signal, having M optical channels therein, into J groups of L optical channels or lambdas where J×L=M. For simplicity,  FIG. 9  shows four (4) lambda groups plane optical switches, that is for illustrative purposes only, J=4. The expansion output ports of the lambda planes  186   a–m  are coupled to a wavelength converting switch  14  that is not part of the photonic switch core of system  180 . 
     Referring to  FIG. 10  there is illustrated in a functional block diagram a protection arrangement for the multiple granularity multiple plane optical switch core of  FIG. 9  in accordance with an embodiment of the present invention. The plane switches are implemented as four-port MEMS. Due to the complexity of the drawing, some of the reference characters common to  FIGS. 9 and 10  have been dropped in  FIG. 10  to make room for reference characters needed for additions to  FIG. 10 .  FIG. 10  adds protection switches  210 ,  212 , and  214  and  216  at the input and outputs of the fiber plane switch  182 , the lambda group plane switches  184  and the lambda plane switches  186 , respectively. The other additions include protection switch planes  182   p ,  184   p  and  186   p  and  186   p   2  for the fiber, lambda group and lambda layers, respectively. The protection switches used may be any of those previously described in connection with  FIGS. 4 ,  5 , or  7 . The arrangement for  FIG. 10  provides 1:1 protection for the fiber layer, J:1 protection for the lambda group layer and (M/2):1 protection for the lambda layer. Hence the multiple granularity switch core has the advantage of allowing the degree of protection provided to increase with increasing density of the optical signal. 
     Operationally, the protection arrangement of  FIG. 10  is similar to that described for the lambda plane switch of  FIGS. 3 and 8 . The tapping of optical signals for the purposes of connectivity checking and power spectrum measurement would also include tapping of the intermediate optical amplifier arrays  185  and  187 , between the lambda group and fiber planes and the lambda and lambda group planes, respectively. Note due to the complexity of  FIGS. 9 and 10 , the input and output optical amplifiers shown in  FIG. 3 and 8  and any switch matrix associated EDWAs as shown in  FIG. 8 , are not shown in  FIGS. 9 and 10 . 
     Specifically, the fiber plane is protected by protection switches  210   a ,  210   b ,  210   c  and  210   d  at the inputs, outputs, expansion outputs and expansion inputs, respectively. For simplicity, each protection switch  210  is drawn as a single block, however there are N fiber inputs. Hence, all of the blocks  210   a ,  210   b ,  210   c , and  210   d  represent N (y×1) optical switches, where y=1, 2, or 3, depending upon the embodiment of protection switch chosen (recall that y=2 or 3 allows the defective plane switch to be tested in situ during the protection switch condition. 
     Similarly, the lambda group planes are protected by protection switches  212   a ,  212   b ,  212   c  and  212   d  at the inputs, outputs, expansion outputs and expansion inputs, respectively. For simplicity, each protection switch  212  is drawn as a single block, however there are N demuxs  190  inputting to each lambda group plane  184   a–d . Hence, all of the blocks  212   a ,  212   b ,  212   c , and  212   d  represent N (y×J) optical switches, where y =1, 2, or 3, depending upon the embodiment of protection switch chosen. 
     Similarly, the lambda planes are protected by two sets of protection switches: protection switches  214   a ,  214   b ,  214   c  and  214   d  at the inputs, outputs, expansion outputs and expansion inputs of the top M/2 lambda planes, respectively and protection switches  216   a ,  216   b ,  216   c  and  216   d  at the inputs, outputs, expansion outputs and expansion inputs of the bottom M/2 lambda planes, respectively. For simplicity, each protection switch  214  and  216  is drawn as a single block, however there are N demuxs  194  inputting to each lambda plane  186   a–m . Hence, all of the blocks  214   a ,  214   b ,  214   c ,  214   d ,  216   a ,  216   b ,  216   c  and  216   d , represent N (y×M/2) optical switches, where y=1, 2, or 3, depending upon the embodiment of protection switch chosen. 
     Referring to  FIG. 11  there is illustrated in a functional block diagram a protection arrangement for the switch core and ports of  FIG. 9  in accordance with an embodiment of the present invention. Due to the complexity of the drawing, some of the reference characters common to  FIGS. 9 and 10  have been dropped in  FIG. 11  to make room for reference characters needed for additions to  FIG. 11 . The port protection arrangement of  FIG. 11  provides 1:N sparing for each port at the lambda group and lambda layer. Specifically,  FIG. 11  adds protection switches  220  and  221  between outputs of the fiber plane switch protection switches  210   c  and the lambda group demuxs  190 , and between the lambda group plane protection switches  212   c  and the lambda demuxs  194 , respectively. The other additions include at the lambda group level: sparing of demuxs  190  and protection switches  212   a–d , and muxs  192 , plus addition of a row and column of mirrors on each of the switch planes  184   a–d  and, a spare switch plane  184   p . Similarly, The other additions include at the lambda level: sparing of demuxs  194   a–d  and protection switches  214   a–d ,  216   a–d  and muxs  196   a–d , plus an addition of a row and column of mirrors on each of the switch planes  186   a–m  and, a spare switch planes  186   p  and  186   p   2 . 
     In order to keep the drawing from becoming too complicated, only the protection switch  220 , demuxs  190 , protection switches  212   a , protection switches  221   a–d , demuxs  194   a–b  and protection switches  214   a  are shown in expanded form. However it is to be understood that similar sparing is required at the remaining inputs and outputs of the lambda group and lambda plane switches as indicated partially by circles on  FIG. 11 , for example protection switches  222   a–d,.    
     The protection switches used may be any of those previously described in connection with  FIGS. 4 ,  5 , or  7 . 
     Operationally, the protection arrangement of  FIG. 11  is similar to that described for the lambda plane switch of  FIGS. 3 and 8 . The tapping of optical signals for the purposes of connectivity checking and power spectrum measurement would also include tapping of the intermediate optical amplifier arrays  185  and  187 , between the lambda group and fiber planes and the lambda and lambda group planes, respectively. Note due to the complexity of  FIG. 11 , the input and output optical amplifiers shown in  FIG. 3 and 8  are not shown in  FIG. 11 . 
     Referring to  FIG. 12  there is illustrated in a functional block diagram a protection arrangement for the multiple granularity multiple plane optical switch core of  FIG. 9  with the plane switches implemented as six-port MEMS in accordance with an embodiment of the present invention. Due to the complexity of the drawing, some of the reference characters common to  FIGS. 9 and 10  have been dropped in  FIG. 12  to make room for reference characters needed for additions to  FIG. 12 . 
     The plane switches are implemented as six-port MEMS. A representation of a six-port MEMS is shown in the lower left hand corner of  FIG. 12 . Unlike the four port MEMS used in  FIGS. 9–11 , where the ports used to return signals to a switch plane are the expansion ports, in which the number of ports on the input ports Ei and output ports Eo are equal to those of the primary inputs I and outputs O and exhibit a fixed mapping between I, Eo and Ei, O. A six-port MEMS provides matrix inputs and outputs Mi and Mo, that have a fewer number of ports, and a suitable mapping between I, Mo and Mi,O, while the mapping between I, Eo and Ei, O remain fixed. The use of 6-port MEMS allows a layered switch core that tapers, in umber of ports from the fiber layer to the lambda group layer to the lambda layer.  FIG. 12  adds protection switches  210 ,  212 , and  214  and  216  at the input and outputs of the fiber plane switch  182 , the lambda group plane switches  184  and the lambda plane switches  186 , respectively. The other additions include protection switch planes  182   p ,  184   p  and  186   p  and  186   p   2  for the fiber, lambda group and lambda layers, respectively. The protection switches used may be any of those previously described in connection with  FIGS. 4 ,  5 , or  7 . The arrangement for  FIG. 12  provides 1:1 protection for the fiber layer, J:1 protection for the lambda group layer and (M/Q):1 protection for the lambda layer, where Q is the number of switches per tributary (e.g. Q=2 for the present example). This is done: to control differential loss that would be incurred in a longer protection switch; to allow a lower protection ratio than M:1; and to allow the switch to be physically partitioned. Hence the multiple granularity switch core has the advantage of allowing the degree of protection provided to increase with increasing density of the optical signal. The core protection is that same as that shown in  FIG. 10  for the four-port MEMS implementation. The difference that becomes apparent is in considering protection at the port level. While the four-port MEMS implementation of  FIG. 11  required additional protection switches  221  and  222 , the six-port MEMS version eliminates these protection switches by providing the additional paths within the switch planes by increasing the number of Mi and Mo ports. Hence the protection arrangement of  FIG. 12 , is the functional equivalent of that provided by  FIG. 11 . 
     Referring to  FIG. 13  there is illustrated in a functional block diagram a protection arrangement for the switch core and ports of a combined switch core in accordance with an embodiment of the present invention. For convenience the same reference characters used in earlier figures have been used in  FIG. 13  for similar elements. In  FIG. 13 , a logically layered switch is embodied, that maps a lambda and a lambda group onto a single physical plane, for example  184   a / 186   a ,  184   b / 186   e ,  184   c / 186 I and  184   d / 186   m . This is accomplished in a tributary cards (TRIB)  201   a–n . The TRIB card  201   a  includes the lambda group demux  190  and lambda demuxs  194   a–d  as shown in previous drawings. However the TRIB card  200   a  also includes 1×1 four port MEMS switches  223   a–d . The placement of these switches allows lambda plane switching (for that group) when not activated and lambda group switching when activated. For example, when 2×2 MEMS switch  223   a  is not activated lambda group 1 passes straight through from the input I to the output Eo to demux  24   a , whose first output in input to Ei for straight through output to output O and on to lambda plane switch  16   a . If lambda group 1 is to be switched, 1×1 four port MEMS switch  223   a  is activated and the lambda group at input I is switched to output O and onto lambda group plane  184   a . However in this configuration lambda plane switch  186   a  and lambda group plane switch  184   a  are the same plane switch. Consequently the TRIB card  201   a  couples either a lambda to each plane in a lambda group or the lambda group to the first lambda plane in the lambda group. Note this is a per tributary card function, hence λ1s or λgroup 1s from other TRIB cards  201   b–n  could be input to the switch plane  184   a / 186   a  and hence co-exist thereon as long as the provisioning of the tributary cards at the individual λ, λ-group level and the permissible cross-connect mapping between tributaries through the affected switch cores is consistent. This is achieved by linking the tributary card mapping status and the cross-connect map in the control processor, so that cross-connects are only permitted between tributary cards that are like-provisioned in respect to λ, λ-groups. Also shown is an array of EDWA (Erbium Doped Waveguide Amplifiers)  224  and  225   a–d  similar to those discussed with regard to  FIG. 8 . The EDWA  224  compensates for the increased loss of the lambda-group switching path relative to the fiber path. The EDWAs  225   a–d  compensate for the losses of the per lambda paths (the loss in  194   a–d  and the inverse function on the other side of the switch (not shown in  FIG. 13  for simplicity) relative to the respective lambda group. Each protection switch  228  includes an EDWA  229  to compensate for losses in the protection path. 
     The port protection arrangement of  FIG. 13  provides 1:N sparing for each port at the fiber layer by providing a spare trib card (not shown) for trib cards  201   a–n  and switch plane protection by providing a spare switch plane  186   s . Specifically,  FIG. 13  adds protection switches  226  and  228  between outputs of the fiber plane switch and the demuxs  190 / 194 , respectively. The spare trib card is not shown due to the complexity of  FIG. 13 , but would be similar in configuration to the trib cards  201   a–n  and the protection channel to the spare TRIB card is indicated by an arrow  230 . 
     Operationally, the protection arrangement of  FIG. 13  is similar to that described for the multiple granularity switch of  FIGS. 10 and 11 . The tapping of optical signals for the purposes of connectivity checking and power spectrum measurement would also include tapping of the intermediate optical amplifier arrays  224  and  225 . Note due to the complexity of  FIG. 12 , the input and output optical amplifiers shown in  FIG. 3 and 8  are not shown. 
     Referring to  FIG. 14  there is illustrated the optical switch of  FIG. 13  modified to provide fiber plane switch protection using the same protection switch plane as used for lambda and lambda group protection in  FIG. 13 . Specifically,  FIG. 14  adds a protection switch  232  at the fiber inputs to the tributary cards, an additional mirror on protection switch  228  and a 2×2 MEMS switch  234  (similar to those on the tributary cards). 
     In operation, the protection switch  232  at the fiber inputs provides N:1 protection port protection for the fiber ports by protection switching the spare tributary card as indicated by an arrow  236 . The fiber plane switch  182  is protected by activation the additional mirror at the top of the protection switch  229  and by activation of the 2×2 MEMS  234 . In the event of a failure in the fiber plane switch  182 , the fiber input is diverted down the protection switch  228  (in the manner illustrated with regard to  FIGS. 4 ,  5  and  7 ), amplified by EDWA  229  and coupled to spare switch plane  186   s , Mo outputs are couple back through the 2×2 MEMS switch  234  to tributary card inputs  224   a–n.    
     Throughout this above description the terms multiplexers and demultiplexers have been used, however one of ordinary skill would recognize that between one layer and another layer, interleavers and de-interleavers could alternatively be used. For simplicity, all drawings show unidirectional paths through the plane switches, however as would be appreciated by one of ordinary skill, the optical plane switches can carry bi-directional traffic with suitable input and output components. 
     Modifications, variations and adaptations to the embodiments of the invention described above are possible within the scope of the invention, which is defined by the claims.