Abstract:
A photonic cross-connect includes a plurality of ingress WS-switches with drop outputs outputting WDM drop signals including odd channels and a plurality of egress WS-switches for receiving WDM add signals including add channels. Wavelength selective elements in a drop path and in an add path comply with a double frequency spacing and about a double bandwidth of the ingress WS-switches or egress WS-switches. If both odd and even channels are dropped and added, separate add and drop paths are provided for the odd and for the even channels.

Description:
CLAIM FOR PRIORITY 
       [0001]    This application claims the benefit of priority to European Application No. 07022625, filed Nov. 21, 2007, the contents of which hereby incorporated by reference. 
       TECHNICAL FIELD OF THE INVENTION 
       [0002]    The invention is directed to a photonic cross-connect with add-drop functions. 
       BACKGROUND OF THE INVENTION 
       [0003]    Photonic (also called optical) cross-connects are based on the idea that a channel (more exactly a single signal) 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 and, if optical-electrical conversion and 3R-regeneration is not used, reduces the signal quality. 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. 
         [0004]    To singularize single channels (single signals) of a DWDM signal and to combine a plurality of channels to a DWDM signal, it is necessary to use very narrow filters or similar wavelength selecting elements. This results in a reduction of signal quality. For dropping and adding channels (single signals) from and respectively to a DWDM signal, wavelength selecting elements are also necessary, but neither the quality of the dropped channels nor the quality of the added channels should be reduced. Beside this, a certain switching flexibility is also necessary for outputting drop signals and for inserting add signals. 
         [0005]    Modern photonic cross-connects comprise optical multiplexers, demultiplexers, tuneable filters and wavelength selective switches. 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. This can be accepted in most cases. But each add and each drop signal has to pass through three narrow filter elements, this means that each signal (referred to as channels) has to pass three narrow filter elements in an add path and at least three narrow filter elements in an drop path even in a short point to point connection. The result is a significant reduction of signal quality. 
         [0006]    WS-switches are used for realizing cross-connect and add-drop functions. A WSS functionality can be realized by different technologies. By means of these technologies, e.g. micro-electro-mechanical-systems (MEMS), 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. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention discloses a cross-connect offering high signal quality cross-connections and sufficient add-drop flexibility. 
         [0008]    The improvement in signal quality is obtained by separate processing of odd and even drop channels and odd and even add channel. The drop and the add signals have to pass wide filters in the whole drop path, e.g. a deinterleaver, a drop arrangement and a demultiplexer, respectively in the add path, e.g. a multiplexer and an add arrangement. 
         [0009]    The impairment of signal quality of the through/express signals is reduced by substituting the ingress WS-switches by power splitters. 
         [0010]    Full add and drop capability is achieved if odd and even channels are dropped, where odd and even channels are also processed separately. 
         [0011]    In one aspect, power splitters and deinterleavers are used instead of ingress WS-switches for dropping odd and even channels. 
         [0012]    If only a few add and drop channels are needed, simple but upgradeable add and drop arrangements reduce the costs for a basic network. 
         [0013]    The switching flexibility on the client side is improved if add and drop arrangements with switching capability are used. The drop and the add arrangement can be upgraded to cross-connect-arrangements. 
         [0014]    In another aspect of the invention, the power splitters are substituted by additional WS-switches to improve crosstalk properties. 
         [0015]    In another aspect, cost-effective drop and add arrangements can be even used, if only a few odd and even channels are dropped and inserted. 
         [0016]    I another aspect it is advantageous to use at least one demultiplexer with the standard grid or the offset grid to singularize odd or even drop channels and at least one multiplexer with the standard grid or the offset grid to combine odd channels or even channels. 
         [0017]    In still another aspect of the invention, it is advantageous to use at least two different demultiplexers, whose inputs are connected to outputs of the drop arrangements and at least two different multiplexers, whose outputs are connected to inputs of the add arrangement. A plurality of switching possibilities of the drop and add arrangement and of the demultiplexers and multiplexers enables the dropping and adding of channels (signals) carrying signals of the same frequency. At least two different demultiplexers and multiplexers are always necessary because odd and even channels have to be processed separately. 
         [0018]    In yet another aspect of the invention, which refers to drop and add arrangements for odd and even channels, it is advantageous to use at least two demultiplexers, whose inputs are connected to outputs of the first drop arrangements and at least two further demultiplexers, whose inputs are connected to outputs of the second drop arrangements and at least two multiplexers and two further multiplexers, whose outputs are connected to inputs of the at least two add arrangement. Odd and even channels are processed in different demultiplexers and multiplexers with standard and offset grid separately to avoid an impairment of signal quality. 
         [0019]    The switching capability for client drop signals and client add can be improved by using optical or electrical switch matrices. 
         [0020]    It is another aspect to integrate signal quality monitors to initiate protection switching. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0021]    Examples of the invention of preferred embodiments are described below with reference to accompanying figures, wherein: 
           [0022]      FIG. 1  shows a drawing of a first embodiment of a photonic cross-connect comprising ingress and egress wavelength selective switches. 
           [0023]      FIG. 2  shows a drawing of a second embodiment with ingress power splitters instead of ingress WS-switches. 
           [0024]      FIG. 3  shows a drawing of a third embodiment with improved cross-connect possibilities. 
           [0025]      FIG. 4  shows a drawing of a fourth embodiment with ingress power splitters instead of ingress WS-switches with improved cross-connect possibilities. 
           [0026]      FIG. 5  shows a drawing of a further embodiment of an add or drop arrangement. 
           [0027]      FIG. 6  shows a drawing of a simplified third embodiment with less switching capabilities of the drop and add arrangements. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]      FIG. 1  shows a schematic drawing of a first embodiment of the invention. Elements for amplifying or attenuating signals are not shown for clarity reasons and also not mentioned in the description. The basic cross-connect shown in this figure has a plurality of ingress wavelength selective switches and egress wavelength selective switches, which are further down referred to as WSS or WS-switches. Two ingress WS-switches  1  and  3  and two egress WS-switches  2  and  4  are shown. The first WS-switch  1  receives at an ingress port  11  a first DWDM signal E 1  (dense wavelength division signal; dense is here not restricted to a certain channel spacing) from a first direction, e.g. “east”, and the first egress WSS  2  transmits a second DWDM signal E 2  in this direction at an egress port  23 . Both ports represent a bidirectional port for exchanging DWDM signals in the first direction “east”. The other couples of ingress and egress WS-switches, e.g. ingress WSS  3  and egress WSS  4 , exchange DWDM signals W 3 , W 4  via an ingress port  31  and an egress port  33  in a second direction “west”. 
         [0029]    An ingress WS-switch  1 ,  3  has a plurality of outputs. The through or express channels of the received DWDM signals are output at “through outputs”. Each through output  12  and  32  is connected to a “through input”  2   1  or  4   1  respectively of an egress WSS  2  or  4 , which transmits a DWDM signal W 4 , E 2  in another direction as in the direction from which the DWDM signal is received. A through channel (or more exact through/express signal) received at the ingress WSS  1  from the direction east is e.g. transferred to the egress node  4  and send in the direction west. Cross-connects are usually designed for more than two directions, but only the connections for two directions are shown for clarity reasons. 
         [0030]    Each ingress WSS  1  or WSS 3  has beside the through outputs at least one additional output  1   3 ,  3   3 , which is called a “drop output” used to output a WDM drop signal. According to this embidiment e.g. odd channels (channel  1 ,  3 ,  5 , . . . ) are dropped at a WSS drop output, this is every second channel of the DWDM signal or, in other words: These are the channels of the standard grid for WDM signals with a frequency spacing of 100 GHz or the “offset” channels (channel  2 ,  4 ,  6 , . . . ) shifted by 50 GHz. While this cross-connect is designed for dropping and adding odd channels the next cross-connect in a transmission line can be designed for dropping the even channels (of course, also e.g. every fourth or sixth channel can also be output). 
         [0031]    Drop outputs  1   3 ,  3   3  of the ingress WS-switches  1  and  3  are connected with drop inputs of a drop arrangement  5 , 6 , 7 , 8 . Because only two ingress WS-switches are shown in  FIG. 1  for clarity reasons, only few elements of the drop arrangement are illustrated. A common drop arrangement for a two directions cross-connect comprises two drop power splitters  5  and  6 , each receiving a WDM drop signal D 1  or D 3  respectively, and two drop WS-switches  7  and  8 . Outputs of the power splitters  5  and  6  are connected with an input of each drop WS-switch  7  or  8  respectively. Therefore each drop WSS  7  or drop WSS  8  receives a combination of a first and a second WDM drop signal D 1  and D 3 . The drop arrangement has a possibility to select certain channels and to feed the selected channels, referred to as “WDM client drop signals”, to a certain demultiplexer  13  or  14 , which outputs singularized client drop signals CD 13 , CD 14 . 
         [0032]    If the switching flexibility of this “drop matrix” is not needed, or if a system is installed for only a few clients, all the WDM drop signals of the same WDM grid can be combined and output by one demultiplexer  13 . A basic cross-connect as shown in  FIG. 1  can be upgraded easily if power splitters with a plurality outputs and WS-switches with a plurality of inputs are used. Each drop signal passes once a narrow filter of an ingress WSS. 
         [0033]    A common add arrangement  9 , 10 , 11 , 12  is designed similar to the drop arrangement and comprises WS-switches  9 ,  10  and power splitters  11 ,  12  used as power combiners. Two add WS-switches  9  and  10  receive multiplexed client add signals CA 15 , CA 16  of the same frequency grid, referred to as “WDM client add signals”, from multiplexers  15  and  16  respectively. Outputs of each WSS  9 ,  10  are connected with an input off all power combiners  11 ,  12 , an add output of the combiner  11  is connected to an add input  2   2  of the first egress WSS  3  and an add output of the combiner  12  is connected to an add input  42  of the second egress WSS  4 . 
         [0034]    The received client add signals CA 15 , CA 16  meet also the requirements of the WDM standard grid and therefore correspond to the odd channels. These channels are combined by multiplexers  15  and  16 , pre-selected by the add WS-switches  9 ,  10 —both have the frequency spacing of the standard grid (100 GHz) and the broad pass-band of the standard grid channels, which is about the double bandwidth of the DWDM channels—and fed to the power combiners  11  and  12  to be combined to WDM add signals A 2 , A 4 , which are fed to the add inputs  22  and  42  of the egress WS-switches. 
         [0035]    The add arrangement has also a switching flexibility and can feed the clients signals to a selected egress WSS. The WDM add signals A 2 , A 4  are inserted into the transmitted DWDM signals E 2 , W 4  by the egress WS-switches  2  and  4 . Each client add signal CA 15 , CA 16  passes once a narrow filter of an egress WSS. 
         [0036]    To summarize, all through channels are selected by the ingress WS-switches  1  and  3  and fed to another egress WSS  4  or  2  (“another” egress WSS, e.g.  2 , is defined as an WSS, which transmits a DWDM signal, e.g. W 4 , in another user-defined direction, e.g. west”, and not in the direction, e.g. “east”, from which the selected channel is received by the ingress WSS, e.g. WSS 1 , as part of the DWDM signal E 1 . As mentioned before, all WDM drop signals D 1 , D 3  are also selected by the ingress WS-switches  1  and  3  and fed to the drop arrangement. The received client add channels are combined to WDM add signals in the add arrangement and fed to the egress WS-switches. 
         [0037]    If the cross-connect is designed for more than two directions more WS-switches with additional outputs and inputs are necessary and the drop and add arrangements are also upgraded. If, in addition, the cross-connect is designed for more traffic directions and therefore comprising additional ingress WS-switches and additional egress WS-switches (but still only odd or even) WDM signals, then it is necessary to upgrade the drop and the add arrangement. Additional power splitters with a further outputs and an additional WS-switches with further inputs are inserted in the drop arrangement and additional demultiplexers and power combiners in the add arrangement. For every additional ingress WSS (or for each further direction) an additional power splitter and an additional WSS and also an additional demultiplexer is necessary. The add arrangement is upgraded in an appropriate way. Of course there are also some less complex solutions but less flexibly solutions possible. 
         [0038]    The embodiment of  FIG. 1  shows a drop arrangement comprising a first monitoring device  41  and a second monitoring device  42 . 
         [0039]    The quality of the WDM drop signals is monitored. Protection switching of important channels is initiated if a received DWDM signal, e.g. E 1 , has a very bad quality or if the transmission line of WSS  1  is interrupted. Protection switching is of course also possible in the other embodiments of the invention. 
         [0040]      FIG. 2  shows a second embodiment of the invention. The ingress WS-switches  1 ,  3  are substituted by ingress power splitters  17  and  18 . An interleaver  19 , respectively  20  is inserted downstream of the drop output of each power splitter (the received DWDM signal E 1 , respectively W 2  is output at all through and outputs of the power splitters) to output an odd WDM drop signal D 1   o , D 3   o  comprising odd drop channels (or even drop channels), which are fed to the drop inputs of the drop arrangement  7 , 8 , 5 , 6 . The power splitters  5 ,  6  and the WS-switches  7 ,  8  have interchanged their positions (also the arrangement described before can be used). The drop channels are now elected by the WS-switches  7 ,  8  and the selected drop channels are combined by the power splitters  5 ,  6 , and fed to the demultiplexers  13  and at least 14. 
         [0041]    Also in the add arrangement are the positions of the power splitters  11 ,  12  and the WS-switches  9 ,  10  are interchanged (but the arrangement described before can also be used). The combined client signals are fed to both add WS-switches  9 ,  10 , which output the first WDM add signal A 2  and the second WDM add signal A 4  respectively. These signals are fed to the add inputs  2   2  and  4   2  of the egress WS-switches  2  and  4  respectively. 
         [0042]    The advantage of this arrangement is obvious: Also the through channels have to pass a narrow bandwidth filter element in the egress WSS once, but the switching flexibility of the ingress WS-switches is missing. 
         [0043]      FIG. 3  shows a third embodiment with full drop and full add capability. At least two outputs of each ingress WSS 1  and at least WSS 3  are used as drop outputs, the first “odd drop output”  1   3 ,  3   3  outputs a first “odd WDM drop signal” D 1  representing odd channels according to the 100 GHz ITU standard grid and a second drop output, the “even drop output”  14 ,  34  drops a shifted first “even WDM drop signal” D 2  representing even channels according to a 50 GHz shifted “offset grid”. The ingress WSS 2  outputs a second “odd WDM drop signal” D 3  and a second “even WDM drop signal” D 4 . 
         [0044]    Odd WDM drop signals D 1 , D 3  with odd channels are fed to the first drop arrangement  5 , 6 , 7 , 8 , and shifted even WDM drop signals D 2 , D 4  with even channels are fed to a second drop arrangement  21 , 22 , 23 , 24 . Odd and even channels are separately combined with odd and respectively even channels received from other directions (not shown) as described before. Additional demultiplexers  29  and  30  are used to singularize the even drop channels. 
         [0045]    According to the ingress WS-switches  1 ,  3  egress WS-switches  2 ,  4  with two “add inputs” each are applied, one add input for odd add channels and one add input  2   4 ,  4   4  for even add channels. An appropriate second add arrangement  25 , 26 , 27 , 28  and additional multiplexers  34 ,  35  are used to combine the even client add signals and to select two shifted WDM add signals A 3  and A 5 . These signals are fed to the two second add inputs  2   4 ,  4   4  off the egress WS-switches  2 ,  4  and inserted into the transmitted DWDM signals E 2 , W 4 . 
         [0046]    Because odd and even channels are processed separately in the add and in the drop arrangements, add signals and drop signals have to pass a narrow filter element once in an ingress WSS or in an egress WSS respectively. 
         [0047]    The third embodiment shows in addition optical switch matrices (arrays)  31 ,  32 , which are connected to output terminals of the demultiplexers  13 ,  14 ,  29 ,  30 . An additional M(N×K) switch matrices  33  are inserted downstream the switch matrices  31 ,  32 . In case that only two switch matrices  31 ,  32  and no additional outputs are used, then N=K=2 and M×N is the number of drop channels. In this arrangement each drop channel could be output at each output terminal of the N×N switch matrix. The same switch arrangement with a M(N×N) switch matrix  38  and switching matrices  36 ,  37  can be used at the add site. Each add channel can be inserted in each of the WDM add signals. Of course, at the drop side also switch matrices  33  with more outputs (K&gt;N) could be used and at the add side switch matrices  38  with more inputs could be used. The switching matrices can be substituted by electrical switch matrices and transponders if optical-electrical and electrical-optical conversion is used. 
         [0048]      FIG. 4  shows a fourth embodiment, where, according to  FIG. 2 , the ingress WS-switches  1 ,  3  are replaced by optical splitters  17 ,  18 , but in addition to the embodiment shown in  FIG. 2  both outputs of the deinterleavers  19 ,  20  are used to drop the odd channels outputted at a first interleaver output as well as the even channels outputted at a second interleaver output. The odd WDM drop signals D 1   o  and D 3   o  comprising odd channels are fed to the power splitters  5  and  6  respectively, split and fed to the inputs of the WS-switches  7  and  8 . Even WDM drop signals comprising even channels are fed via the power splitters  21  and  22  to the inputs of the WS-switches  23  and  24 . The selection of odd and even channels is executed in the drop arrangements by the drop WS-switches  7 ,  8  and  23 ,  24 . A drop arrangement according to  FIG. 3  with the same functionality could be also applied in this embodiment. 
         [0049]      FIG. 5  illustrates a further embodiment of a drop arrangement. The power splitters  17  and  18  are replaced by WS-switches  51  and  61 . Since two WS-switches with filter characteristics are connected in series, the influence of neighbour channels is reduced. 
         [0050]      FIG. 6  illustrates an embodiment of the cross-connect as shown in  FIG. 3  with simplified drop and add arrangements. The drop arrangement comprises power splitters  5 ,  6  (or WS-switches instead of these power splitters/combiners, not shown). Odd drop channels are combined by the power combiner  5  and even drop channels are combined the power combiner  6  respectively, which are directly connected to the odd and even drop outputs of the ingress WS-switches  1  and  3 . The ingress WS-switches  1  and at least 3 are responsible for the selection of all drop channels. The selection of the through channels is also carried out by the ingress WS-switches. The outputs of the power splitters  5 ,  6  are connected to e.g. tuneable demultiplexers  13  and  14  respectively. The drop arrangement can be upgraded by applying combiners with more inputs. 
         [0051]    The add arrangement is designed in the same way and comprises at least two power splitters  11 ,  12 . Each power splitter receives odd or even client add signals from the (tuneable) multiplexers  15  and  16  and distributes the odd WDM add signals A 2 , A 4  and the even WDM add signals A 3 , A 5  to the odd or even add inputs off the egress WS-switches, which are responsible for the selection of all add channels. The add arrangement could also be upgraded by applying splitters with more outputs. 
         [0052]    A drop switch matrix  39  connected to the outputs of the demultiplexers  13 ,  14  and an add switch matrix  40  connected upstream the inputs of the multiplexers  15 ,  16  enable a satisfactory switching flexibility for many applications.