Abstract:
A photonic cross-connect arrangement is presented which is able to cope with the transmission of super-channels, wherein complete super-channels are dropped and added to change a direction of transport. At least a cyclic filter is used in a drop-branch of a cross-connect for dividing a super-channel into sub-channels and/or at least a further cyclic filter is used in an add-branch to configure a super-channel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 U.S.C. 371 national stage filing of International Application No. PCT/EP2013/076565, filed on Dec. 13, 2013, which claims priority to European Patent Application No. 13151917.5, filed on Jan. 18, 2013. The contents of the aforementioned applications are hereby incorporated by reference in their entireties. 
     FIELD OF THE INVENTION 
     The invention refers to a PHOTONIC CROSSCONNECT WITH RECONFIGURABLE ADD-DROP-FUNCTIONALITY. 
     BACKGROUND OF THE INVENTION 
     The invention is directed to a photonic cross-connect with reconfigurable add-drop functionality. Photonic, also called optical, cross-connects are based on the idea that a channel from a plurality of received WDM signals (wavelength division multiplex signals) can be redirected into any one of transmitted WDM signals. Most of the cross-connects avoid wavelength conversion, because it is cost effective at the expense of reduced signal quality, if optical-electrical conversion and 3R-regeneration are not used. Another restriction for the signal quality comes from the wavelength selective elements. These problems increase, if DWDM signals (dense wavelength division multiplex signals) are transmitted. 
     In future, high data-rates signals will be transported and routed through a network in frequency slots filled with a set of sub-channels at spectrally disjunctive optical frequencies. Such group of sub-channels is denoted as super-channel in the following. A super-channel is generated by combining e.g. 4 sub-channels by a pluggable module having several optical line side ports, or is generated directly by appropriate modules. 
     The optical cross-connect has to provide the functions to add and drop such sub-channels via tributary ports, and the function to direct the super-channels to the desired traffic directions via a direction switching unit. 
     PRIOR ART 
     A photonic cross-connect is disclosed in the patent application US 2006/0098981 A1. Each through or express channel has to pass through a WSS (wavelength selective switch) and a multiplexer. WS-switches are used for realizing cross-connect and add-drop functions. WSS functionality can be realized by different technologies. By means of these technologies, e.g. micro-electro-mechanical-systems (MEMS), liquid crystal (LC) or liquid crystal on silicon (LCOS), an optical WDM signal received by an input port can be switched frequency selective to a plurality of output ports and vice versa. The realisation of a frequency-selective switching matrix with a plurality of inputs and outputs is possible by applying these elements. 
     The function of a cyclic filter is explained in “N×N Cyclic-Frequency Router With Improved Performance Based on Arrayed-Waveguide Gratings”, Journal of Lightwave Technology, Vol. 27, No. 18, Sep. 15, 2009. 
     An article “Flexible Architectures for Optical Transport Nodes and Networks” Steven Gringeri et al., IEEE Communication Magazine, July 2010 presents architectures and various cross-connect (ROADM reconfigurable optical add-drop multiplexer) implementations including colorless, directionless and contentionless add-drop structures. 
     In a colorless design any wavelength (signal) can be assigned to an add-drop port. 
     A directionless add-drop structure provides the freedom to direct a channel to any traffic direction of the cross-connect and is implemented by connecting an add-drop structure to every direction. This can be realized by e.g. adding another optical coupler to the add structure and another WSS to the drop structure. 
     A contentionless ROADM design removes wavelength restrictions from the add-drop portion of the ROADM node so that a transmitter can be assigned to any wavelength as long as the number of channels with the same wavelength is not more than the number of traffic directions in the node. This architecture allows that only one add-drop structure is needed in a node. An M×N WSS is the perfect fit for this architecture, since reusing a wavelength on a fiber is not possible. Today, M×N WSSs are not yet commercially available, but the function can be built using many smaller switches. 
     An alternative design uses optical splitters, couplers, arrays of photonic switches with small port counts, and tunable filters. In this way the number of add-drop ports is scalable, while the full flexibility of a contentionless function is maintained. It should be noted that with a colorless, directionless, and contentionless ROADM constraints on wavelength assignment are only removed from the add-drop structure. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a cross-connect arrangement to cope with the new transmission of super channels. 
     According to the invention complete super-channels are dropped and added to change a direction of transport. At least a cyclic filter is used in a drop-branch of a cross-connect for dividing a super-channel into sub-channels and/or at least a further cyclic filter is used in an add-branch to configure a super-channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Presently preferred examples of the invention are described below with reference to accompanying drawings, where 
         FIG. 1  shows a basic arrangement of a of a cross-connect according to the invention, 
         FIG. 2  shows a second embodiment of a basic arrangement of the invention dropping/adding 2 super-channels, 
         FIG. 3  shows an advanced add-drop arrangement dropping/adding 4 super-channels simultaneously, and 
         FIG. 4  shows another advanced add-drop arrangement. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a block diagram of a cross-connect. Only the functional elements relating to a basic embodiment of the invention are shown. E.g. optical amplifiers, attenuation elements, additional switches or additional filters may be inserted in the signal (channel) paths. 
     A cross-connector comprises a cross-connect section  1  for receiving, cross-connecting and emitting channels. The expression “channel” is used here meaning a signal which is transmitted with an adequate wavelength in this channel. Usually, a receiving part comprises drop outputs for dropping single channels (signals), and a transmitting part comprises add inputs for adding channels. 
     The depicted cross-connect comprises in the receiving part wavelength selective switches WSSs as distributing components  5 - 8  receiving super-channels from four directions w, n, e, s (west, north, east, south), and comprises in the transmitting part further WSSs as combiner components  9 - 12  emitting super-channels in the four directions W, N, E, S (the capital letters indicating the transmitting direction). 
       FIG. 1  shows for reasons of easier comprehension only representative super-channels denoted w 1 - 4 , n 1 - 4 , e 1 - 4 , s 1 - 4  of WDM signals received from the different directions w, n, e, s. Each of the shown super-channels has the same wavelength spectrum and carries four “ultra dense” sub-channels. Each receiving distributing component  5 - 8  (WSS or splitter) is arranged to connect via its outputs a received super-channel to one of a plurality of inputs of one of the transmitting combiner components  9 - 12  (WSS or combiner). According to the received super-channels, only corresponding super-channels W 1 - 4 , N 1 - 4 , E 1 - 4 , S 1 - 4  are emitted in different directions. 
     The receiving part of the cross-connect has in addition drop outputs for dropping super-channels. In the depicted basic arrangement, it is assumed that only the receiving WSSs  5  and  7  are foreseen by cabling and configuration for dropping the super-channel w 1 - 4  at drop output  5   d  and/or the super-channel e 1 - 4  at drop output  7   d  respectively. The dropped super-channels are referred to as “drop super-channels”. The drop-outputs of the WSSs may perform a pre-selection of the drop super-channels. 
     Each drop output  5   d ,  7   d  is connected to an input  2   e  and  2   w  respectively of a M×N=2×4 cyclic filter  21  (M-active) inputs, N-outputs corresponding to the number of sub-channels/super-channel) arranged in an ‘division-multiplex section’  2 . In this embodiment, one super-channel may be dropped or both super-channels w 1 - 4  and e 1 - 4  may be dropped simultaneously (which can be avoided by the WSSs design or by another appropriate configuration). The cyclic filter separates and emits each of the N=4 sub-channels e 1 , e 2 , e 3 , e 4  at a separate output: e 1  at  2   a , e 2  at  2   b , e 3  at  2   c , and e 4  at  2   d . Further each of the sub-channels w 1 , w 2 , w 3 , w 4  is emitted cyclically shifted at a separate output: w 1  at  2   c , w 2  at  2   d , w 3  at  2   a , and w 4  at  2   b . Hence, always two sub-channels having different wavelength of two super-channels having the same frequency band are dropped at one filter output, e 1  and w 3  are emitted at  2   a ; e 2  and w 4  are emitted at  2   b ; e 3  and w 1  at  2   c ; e 4  and w 2  at  2   d.    
     In case that the two super-channels have only N=2 sub-channels, two super-channels instead of one super-channel could be terminated by such a cyclic filter. Such use creates spectral blocking though: For a given direction such N=2 super-channel cannot access any frequency slot but only those which are accessible for the two cyclic filter drop ports. 
     A received WDM signal comprises usually a plurality of super-channels with different (higher or lower) ‘periodic’ frequency bands named here ‘periodic’ super-channels. An additional ‘periodic’ super-channel with a higher or lower frequency spectrum of the WDM signal incoming from the same direction may be received, selected and dropped by the allocated WSS. Then the second ‘periodic’ sub-channels is additional output cyclical arranged at the same cyclic filter outputs  2   a - 2   d  because the cyclic filter is periodic in a huge frequency range. 
     The sub-channels emitted at the same filter outputs are separated by coherent, preferable tunable receivers RX of a transponder arrangement  3 . But in this embodiment only one super-channel, e.g. e 1 -e 4 , can be output by the receivers RX because of the restricted number of four transponders  3   a - 3   d.    
     The transmitting part of the cross-connect section  1  is arranged to add one of two different super-channels E 1 - 4  or W 1 - 4 , referred to as “add super-channels”. In the add branch a star-coupler  29  is used for combining the sub-channels of a single add super-channel E 1 - 4  or W 1 - 4 . If super-channel E 1 - 4  which is inserted via an add input  11   a  of the WSS  11  on the transmitting side, the add input  9   a  of WSS  9  is closed within the frequency band of the super-channel or E 1 - 4 . But it is also possible to transmit the add super-channel in both directions E and W simultaneously. 
     If the other super-channel w 1 - 4  is dropped instead of the super-channel e 1 - 4 , then an add super-channel W 1 - 4  is added via the transmitting WSS  9 . 
     According to an additional dropped ‘periodic’ super-channel an additional ‘periodic’ add super-channel may be added and emitted in the same direction. 
     The optical combiner  29  may be substituted by a further cyclic filter which is “inverse” operated as will be explained below. 
     In addition, this embodiment may be upgraded to drop and add additional super-channels from additional directions (dashed lines). Also, in case that each of the two super channels have only 2 instead of 4 sub-channels, all 4 sub-channels could be terminated. 
     Usually the dropped sub-channels (signals) emitted by the receivers are converted by a digital signal processer  4  into client signals and output at client ports CPR. The incoming client signals at client ports CPT are converted into add sub-channels/signals. 
       FIG. 2  shows a second embodiment of the invention for dropping and adding two super-channels simultaneously. Only for reasons of clarity the  FIG. 2  also shows only two (of four possible) dropped and two (of four possible) added super-channels possible super. Only for reasons of easier explanation the two regarded super-channels occupy the same frequency band and are attributed to different directions. The cross-connect section  1  remains as depicted in  FIG. 1 . 
     In a second division-multiplex section  20  is the optical combiner  29  is substituted by a second cyclic filter  25 . 
     In addition, a splitter-combiner arrangement  50  is inserted between the cyclic filters  21 ,  25  and an extended transponder arrangement  30 . The inputs of four 1:2 splitters  51 - 54  are connected to filter  21  outputs  2   a - 2   d , first outputs of the four splitters are connected to the receiver RX inputs the transponders  3   a - 3   d , and second outputs of the splitters are connected to receiver inputs of transponders  30   a , . . . of the expanded transponder arrangement  30 . 
     In the add branch, first and second inputs of four 2:1 combiners  55 - 58  are connected to transmitter TX outputs of the transponder arrangements  30 , and each cyclic filter  25  output  2 W,  2 E is connected to an add-input  9   a  and  11   a  respectively; shown are only these two connections for reasons of easier understanding. 
     It is now possible to drop and add two super-channels e 1 - 4  and w 1 - 4  simultaneously. The sub-channels e 1 -e 4  and w 1 -w 4  are now output at the same cyclic filter  21  outputs  2   a - 2   d  as specified in the drawing  FIG. 2 . According to their different wavelengths the sub-channels are separated by the transpon-ders  3   a - 3   d  and  30   a , . . . ,  30   d  ( 30   d  is not shown in the drawing). The doubled quantity of tuneable receivers RX (transponders) allows to drop any two super-channels of four possible super channels at a time and to emit these sub-channels at different receiver RX outputs. Hence, the tuneable receivers imply also direction switching ability. 
     In the add path, the sub-channels of two super-channels E 1 - 4  and W 1 - 4  having the same frequency band are generated by the transmitters TX of the transponder arrangement  30 . Always two of the sub-channels with different frequency bands: E 1  and W 3 ; E 2  and W 4 ; E 3  and W 1 ; E 4  and W 2  are combined by the combiners  55 - 58  and fed to the (now) ‘input’ ports  2 A- 2 D of the second cyclic filter  25 . This filter is “inverse operated”: The sub-channels are combined forming two add super-channels E 1 - 4 , W 1 - 4 , which are emitted at the (now) ‘output’ ports  2 E and  2 W and fed to the add inputs  11   a ,  9   a  of the WSSs  11  and  9  respectively. 
     The shown add-drop-section may be extended to drop and add any two super-channels from and to all four directions simultaneously. The drop outputs  6   d  of WSS  6  and  8   d  of WSS  8  are then connected to further filter inputs  2   s  and  2   n  respectively as indicated by dashed lines; and the filter  25  output ports  2 S and  2 N are connected to add inputs  12   a  and  10   a  respectively. In the add path, according to the allocation of sub-channels and filter  25  ‘input’ ports  2 A- 2 D each generated super-channel can be emitted at any filter ‘output’ port  2 S,  2 E,  2 N and  2 W and fed to each add input  9   a - 12   a  according to the wavelength of the sub-channels. The dashed lines show the extension for dropping and adding super channels from and to all directions. 
     Hence, tunable transmitters TX in combination with the cyclic filter  25  imply also direction switching ability, and the arrangement can drop and add super-channels from and to all directions. But according to the splitters, combiners, and number of transponders this embodiment is restricted to drop and add only two super-channels having the same frequency band simultaneously. 
     Two or more super-channels having different frequency bands (super-channels of a WDM signal) may also be received from the same direction or from different directions. In this case, their sub-channels would also have different frequency bands. These ‘periodic’ super-channels are processed as explained regarding the embodiment of  FIG. 1 . A selection of the dropped and added super-channels may be performed by the WSSs. The number of dropped or added super-channels is again restricted by the number of transponders and by the number of splitter outputs and combiner inputs. 
       FIG. 3  shows an embodiment extended to drop and add simultaneously four super-channels having the same frequency band or different frequency bands. These four super-channels are received from one to four directions. Again, for reasons of easier understanding, super-channels with the same frequency band received from and transmitted in the four directions are regarded. 
     The division-multiplex section  20  comprises again the cyclic filters  21  and  25 . The drop outputs  5   d ,  6   d ,  7   d ,  8   d  of all receiving WSSs  5 ,  6 ,  7 ,  8  are connected to the inputs  2   s ,  2   e ,  2   n ,  2   w  of the cyclic filter  21  for dropping super-channels w 1 - 4 , n 1 - 4 , e 1 - 4 , s 1 - 4  of all directions. The filter  21  outputs  2   e - 2   d  are now connected to inputs of N=4 splitters  61 - 64  (N corresponding to four sub-channels/super-channel) with M=4 outputs (M corresponding to the number simultaneously dropably super-channels) which outputs are connected to inputs of the tunable receivers RX of an enlarged transponder arrangement  300 . 
     Now, the function of cyclic filter  21  is regarded when four super-channels with the same frequency band received from four directions are dropped. E.g. the sub-channels e 1 , s 1 , w 1 , n 1  have the lowest frequency band within the frequency band of the super-channels, the sub-channels e 2 , s 2 , w 2 , n 2  and e 3 , s 3 , w 3 , n 3  have higher frequency bands, and the sub-channels e 4 , s 4 , w 4 , n 4  have the highest frequency bands. Again, because the drop super-channels are fed to different filter inputs  2   s ,  2   e ,  2   n ,  2   w , each filter  21  output  2   a - 2   d  emits four sub-channels having different frequency bands. The output sub-channels are listed in  FIG. 3 . Naturally, also all four outputs of each splitter  61 - 64  carry the same listed sub-channels. 
     The number of transponders has to be also increased according to the number of simultaneously dropped or added sub-channels. And because of the different frequency bands the 16 receivers RX of an enlarged transponder arrangement  300  can separate all 16 sub-channels of the four dropped super-channels. E.g. the sub-channels e 1 , e 2 , e 3 , e 4  of the super-channel e 1 - 4  are separated and converted by four receivers, each connected to one output of the four splitters  61 - 64  and tuned to the frequency bands of the sub-channels e 1 , e 2 , e 3 , e 4 . 
     Because any tuneable receiver can output any of four received sub-channels (e.g. e 1 , s 1 , w 1 , n 4 ) the combination of splitters and tuneable receivers implies direction switching ability. 
     In the add path, the ‘input’ ports  2 A- 2 D of the inverse operated filter  25  are now connected to outputs of N=4 combiners  65 - 68  with M=4 inputs, whereat the combiner inputs are connected to outputs of the transmitters TX of the transponder arrangement  300 . The ‘output’ ports  2 W,  2 E,  2 N,  2 S of the further cyclic filter  25  are connected to add inputs  9   a ,  10   a ,  11   a ,  12   a  of the transmitting WSSs  9 - 12  to add up to four super-channels. 
     The transmitters TX of the enlarged transponder arrangement  300  generate sub-channels (signals) which are combined by the combiners  65 - 68 . The output combinations of the sub-channels are listed in  FIG. 3 . The ‘inverse’ operated cyclic filter  25  receives these combinations and outputs four add super-channels S 1 - 4 , E 1 - 4 , N 1 - 4 , and W 1 - 4 . 
     The allocation of sub-channels forming a super-channel and filter  25  ‘input’ ports  2 A- 2 D determines the filter “output” port of the combined super-channel. In  FIG. 3  combining sub-channels E 1 -E 4  to form the add super-channel E 1 - 4  is indicated by dashed lines. If the sub-channels are cyclically shifted the add super-channel E 1 - 4  is output at a corresponding filter  25  ‘output’ port. 
     Again, because the wavelengths of the sub-channels/signals are generated by the transmitters TX, the combination of tuneable transmitters TX and the cyclic filter  25  implies also switching ability. 
     As aforesaid, also ‘periodic’ super-channels may be dropped and added. And if additional ‘periodic’ super-channels should be dropped and added the number of transponders, the splitters and combiners have to be adapted. The digital signal converter  4  converts optical signals into client signals and vice versa. 
       FIG. 4  shows a variation of an enlarged splitter-combiner arrangement  60  and an advanced division-multiplex section  200  with corresponding flexibility. The drop path is realized with optical 1:4 splitters  61 - 64  and optical cyclic filters  21 - 24  connected downstream in series. Each drop output  5   d - 8   d  of the WSSs  5 - 8  is connected with an input of one optical splitter  61 - 64 . And each output of the optical splitters is connected with an input of one of the cyclic filters  21 - 24 . As already described, because the super-channels are fed to different filter inputs the output sub-channels are cyclically shifted at the filter outputs. Receivers of transponders (RTX)  31 - 46  are connected to all 16 outputs of the cyclic filters  21 - 24 . 
     Hence, regarding the drop branch, each group of four transponders, e.g.  31 - 34 , is able to select each of the four dropped super-channels (having the same frequency bands), e.g. comprising the sub-channels e 1 - 4 . 
     The add branch is designed according to the drop branch with four cyclic filters  25 - 28 , receiving sub-channels from transmitters of the transponders  31 - 46 . Each of the four outputs of each cyclic filter  215 - 218  is connected to an input of four combiners  25 - 28 . And each output of the four combiners is connected to an add input  9   a - 12   a  of the WSSs  9 - 12  ( FIG. 2 ). 
     Each cyclic filter  25 - 28  receives the sub-channels of a super-channel. The ‘inverse’ functions of the cyclic filters  25 - 28  are used, to combine the received sub-channels to super-channels. According to the sequential arrangement of the sub-channels at the filter ports  2 A- 2 D, which is determined by the transponders  31 - 46 , a super-channel is output at a certain output of the four outputs of a cyclic filter  25 - 28 . Hence, this super-channel, e.g. E 1 - 4 , is fed to only one of the four combiners  55 - 58 . According to the sub-channel arrangement the generated super-channel can be output at any of the cyclic filter ports and fed to any of the transmitting WSSs. In other words, the cyclic filters in combination with tunable transmitters/receivers are used as direction switching matrix. 
     The output of each combiner  65 - 68  is connected to one add input of the WSSs  9 - 12 . According to this embodiment each of the four added super-channels can be send in each direction. 
     If less super-channels have to be dropped simultaneously, the number of splitter outputs/combiner inputs and the number of cyclic filters can be reduced. And if only channels from/to certain directions have to be dropped/added the number of splitters and combiners can be also reduced. 
     The present invention is not limited to the details of the above described principles. The drop branch and add branch may be extended by additional or modified cyclic filters and additional splitters and combiners for dropping additional super-channels from and to additional directions. The number of transceivers has to be enlarged to drop and add a greater number of signals. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalents of the scope of the claims are therefore to be embraced by the invention. 
     REFERENCE SIGNS 
     
         
           1  cross-connect section 
           2  division-multiplex section 
           3  transponder arrangement 
           3 ,  31  transponder arrangements 
           4  digital signal converter 
           5 - 8  (distributer components; receiving wavelength selective switches 
           9 - 12  combiner components; further wavelength selective switches 
           5   d - 8   d  drop outputs 
           13  cyclic filter 
           3   a - 3   d ,  30   a  transponder 
           31 - 46  transponder 
           9   a - 12   a  add input 
           20  division-multiplex section 
           200  enlarged division-multiplex section 
           21 - 24  cyclic filter 
           25 - 28  further cyclic filter 
           29  optical combiner 
           30  extended transponder arrangement 
           50  splitter-combiner arrangement 
           60  enlarged splitter-combiner section 
           30  extended 
           300  enlarged transponder arrangement 
           80  advanced splitter-combine arrangement 
         e 1 - 4  super-channel from direction east 
         e 1 , e 2 , e 3 , e 4  sub-channels 
         n 1 - 4  super-channel from direction east 
         s 1 - 4  super-channel from direction south 
         w 1 - 4  super-channel from direction west 
           2   e ,  2   w  (used) cyclic filter inputs 
           2   a ,  2   b ,  2   c ,  2   d  cyclic filter outputs 
         E 1 - 4  super-channel emitted in direction E (east) 
         E 1 , E 2 , E 3 , E 4  add sub-channels of E 1 - 4   
         N 1 - 4  super-channel in direction north 
         S 1 - 4  super-channel in direction south 
         W 1 - 4  super-channel in direction west 
           2 A- 2 D further cyclic filter input ports 
           2 S,  2 E,  2 N,  2 W further cyclic filter output ports