Abstract:
A cross connector for the transparent switching of optical signals is disclosed, comprising a first switching module with one or several switching modules of varying granularity, such as patch panel, wavelength groups or/and wavelengths. A second switching device is connected in series with the first switching device and comprises a polarisation multiplex switching granularity.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/DE03/00795, filed Mar. 12, 2003 and claims the benefit thereof. The International Application claims the benefits of German application No. 10212649.6 filed Mar. 21, 2002, both of the applications are incorporated by reference herein in their entirety. 
     
     FIELD OF INVENTION 
       [0002]    The invention relates to a cross-connector for optical signals. 
       BACKGROUND OF INVENTION 
       [0003]    With regard to WDM networks, in future there will be fully optical (also referred to as photonic), transparent domains. In order to increase the transmission capacity of the individual channels of a WDM or DWDM signal (dense wavelength division multiplexing) it is possible to employ the polarization multiplex technique in such a network. Two differently polarized optical signals whose directions of polarization are normally orthogonal can be transmitted on one wavelength of a channel. Add/drop modules or optical cross-connectors are currently used for the through-connection, addition and/or feeding out of channels in a network element connected to the optical network. These known network elements are simply capable of selectively switching different wavelengths as their smallest granularity. Thus, when using these network elements in the case of optical networks with polarization multiplex, two signals which are situated on one wavelength are always switched. 
         [0004]    In addition to wavelength granularity, current add/drop modules can also switch wavelength group granularity. This means that for example a plurality of channels from the optical network with separate wavelengths are coupled instead of a single channel. The wavelength-selective switching takes place in an additional switching device which is connected to that used for wavelength group switching. 
         [0005]    The normal cross-connectors have the same wavelength granularity and wavelength group granularity as the add/drop modules and also the patch panel granularity which is based on a switching of all or a plurality of channels or of one channel of a WDM signal from one fiber of the optical network to another fiber. 
         [0006]    A cross-connector having a first switching device for patch panel switching (fiber routing switch FR) of signals is described in EP 1 162 855 A1, whose inputs and outputs are linked to outputs and inputs respectively of a second switching device for wavelength switching (wavelength routing switch WR) of channels of a WDM signal. This cross-connector is not however suitable for switching polarized signal components of a polarization multiplexed signal. 
         [0007]    A polarization multiplex switch is known from EP 1 137 308 A2, to whose input one channel with two orthogonal polarization states is fed. A regulated polarization controller regulates the two polarization states of the channel in such a way that two signals having orthogonal directions of polarization are separated by a polarization beam splitter. The regulation of the polarization controller is used in order to optimize the mapping of the two directions of polarization of the outgoing signal from the polarization controller on the separating characteristics of the polarization beam splitter. This polarization demultiplexer forms a drop switching of one of the two orthogonal linearly polarized signals. In addition, the remaining second orthogonal linearly polarized signal passes through a polarization beam coupler tuned to its direction of polarization where it is combined with a new external signal. The new external signal exhibits a direction of polarization which is orthogonal to the direction of polarization of the signal from the polarization beam splitter. 
         [0008]    A switch is also described here in which a WDM signal is fed into a wavelength demultiplexer in order to separate its channels. Some channels are fed into a plurality of add/drop modules and other channels are fed into polarization switches. This switch is therefore not optimally suited in the case of a dynamic optical network having a variable number of channels and/or having variable transmission properties for the channels such as polarization and/or wavelength multiplex because a further item of information from each incoming new channel would be required in order to perform appropriate switching. 
       SUMMARY OF INVENTION 
       [0009]    The object of the invention is thus to specify a cross-connector which allows channels of a WDM signal transmitted as desired with or without polarization multiplex to be switched. In addition, the cross-connector should be fully optically and transparently suitable for an optical dynamic network, whereby a switching should be possible in accordance with one or some to all granularities: patch panel, wavelength group, wavelength and polarization. 
         [0010]    According to the invention this object is achieved by the claims. 
         [0011]    Advantageous developments of the invention are described in the dependent claims. 
         [0012]    On the basis of an optical cross-connector having a first switching device for the through-connection, branching and/or feeding in of optical signals in an optical network, according to the invention a second switching device for polarization multiplex signals is assigned to the first switching device, the inputs and outputs of the second switching device being connected to the outputs and inputs respectively of the first switching device, and the second switching device has at least one polarization demultiplexer and at least one polarization multiplexer with external connection points. 
         [0013]    The granularity of the first switching device is based on patch panel or/and wavelength group or/and wavelength through cascaded connection of switching modules which have at least one of these granularities. 
         [0014]    It is particularly advantageous that an optical signal such as a WDM signal for example is routed transparently from one switching module to another switching module and can be switched in accordance with any desired granularity. After passing through one or several or all switching modules, a channel is fed into the second switching device or so-called polarization multiplex switch and, when polarization multiplex is in use, is split up into two orthogonally polarized signals which are output to an external terminal, for example. 
         [0015]    If the number of channels or the properties of the channels such as wavelength and/or polarization multiplex change as in the case of dynamic networks, the cross-connector according to the invention exhibits a high level of flexibility as a result of its universal switching capabilities and also a low network management resource requirement for the channel switching operations. 
         [0016]    In addition, the cross-connector exhibits bidirectional and transparent transmission or switching as a result of its structural symmetry. The switching arrangement can also be used in respect of signals other than polarization multiplex signals. 
         [0017]    Embodiments of the invention will be described with reference to the drawings. In the drawings: 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  shows a cross-connector according to the invention having at least two granularities, 
           [0019]      FIG. 2  shows a cross-connector according to the invention having two granularities, 
           [0020]      FIG. 3  shows a cross-connector according to the invention having three granularities, 
           [0021]      FIG. 4  shows a cross-connector according to the invention having four granularities, 
           [0022]      FIG. 5  shows a table giving the groupings of the granularities according to the number of switching modules used, and 
           [0023]      FIG. 6  shows a polarization multiplex switch according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0024]      FIG. 1  shows a schematic diagram of a cross-connector according to the invention having at least two granularities which are defined by means of a first switching device SV and by means of a polarization multiplex switch as a second switching device PS. One or more optical signals S are transmitted unidirectionally or bidirectionally by way of an optical network ON whose optical fibers are connected to the first switching device SV. One or more outputs from the first switching device SV are connected to corresponding inputs of the second switching device PS. In the case of a polarization multiplex signal at the input to the second switching device PS, it is possible to couple out both signals of the polarization multiplex signal or to couple out only one signal from the polarization multiplex signal and to through-connect the other signal back to the first switching device SV or to through-connect both signals of the polarization multiplex signal back to the first switching device SV. In the case of signals at the output of the first switching device which have been branched off from the optical network ON and are not polarization multiplex signals, the first switching device SV enables a drop switching to the outside in accordance with its granularity. The granularity provided for the first switching device SV is the wavelength, a group of wavelengths or a patch panel between the different fibers of the optical network ON. The first switching device SV can have one or more switching modules arranged in cascaded fashion with different granularities for the through-connection, branching or/and feeding in of channel signals from the optical signals S. The sequence of the granularities is chosen such that the transmission or the switching of the optical signals S into the cascaded switching modules is performed transparently and with a switching hierarchy of patch panel—wavelength group—individual wavelength up to the second switching device PS. 
         [0025]    Further Add switching facilities are also provided on both switching devices SV, PS, which can feed channel signals for example from an external terminal into the optical network ON for their transmission. 
         [0026]      FIG. 2  shows a cross-connector according to the invention having two granularities according to  FIG. 1  which are defined by a first switching device SV with a first switching module SV 1  and by a polarization switch as a second switching device PS. The switching module SV 1  has a granularity which generates a patch panel switching, a wavelength group switching or a wavelength switching. In order to separate or combine channel signals of the optical signals S, demultiplexers or multiplexers are located in one of the two or between the two switching elements SV 1 , PS if required for example for WDM signals. 
         [0027]      FIG. 3  shows a cross-connector according to the invention having three granularities according to  FIG. 1  or  2 , whereby a second switching module SV 2  having a granularity different from the granularity of the switching module SV 1  is inserted between the switching module SV 1  and the optical network ON. Three different variants of granularity combinations are possible for the cascaded switching modules SV 1 , SV 2 : 
         [0028]    The first switching module SV 1  has a wavelength group granularity and the second switching module SV 2  has a patch panel granularity. 
         [0029]    The first switching module SV 1  has a wavelength granularity and the second switching module SV 2  has a patch panel granularity. 
         [0030]    The first switching module SV 1  has a wavelength granularity and the second switching module SV 2  has a wavelength group granularity. 
         [0031]    The last variant for cascading the switching modules SV 1 , SV 2  is known on the basis of the prior art as an optical add/drop module “OADM” with regard to WDM transmission systems. As a result of the inventive connection with the second switching device PS as a polarization multiplex switch, the polarization granularity is also present. 
         [0032]    In this case also, demultiplexers and multiplexers are located between or in the switching elements SV 1 , SV 2 , PS if required for the separation of the optical signals S into channel signals or for the combination of the channel signals into one or more optical signals S. 
         [0033]      FIG. 4  shows a cross-connector according to the invention having four granularities according to  FIGS. 1 to 3 , whereby the first switching device SV has three cascaded switching modules SV 1 , SV 2 , SV 3  with the respective granularities: patch panel, wavelength group and wavelength. On the basis of the prior art, the first switching device SV is known as an optical cross-connector “OXC”. As a result of the inventive connection with the second switching device PS as a polarization multiplex switch, the polarization granularity is also present. 
         [0034]      FIG. 5  shows a table giving the groupings of the granularities according to the number of switching modules SV 1 , SV 2 , SV 3  used in the first switching device SV according to  FIGS. 1 to 4 . The different switching options are identified as switching W of individual channel signals or as switching GW of groups of channel signals or as switching F of the optical signals S between different fibers of the optical network ON. It is assumed here that the second switching device PS is connected downstream of the first switching module SV 1  as a polarization multiplex switch. The identifiers SV 1 , SV 1 +SV 2  and SV 1 +SV 2 +SV 3  correspond to the different cascaded configurations of the switching modules SV 1 , SV 2 , SV 3  according to  FIGS. 2 to 4 . 
         [0035]      FIG. 6  shows a polarization multiplex switch according to the invention which is used as a second switching device according to  FIGS. 1 to 4 . 
         [0036]    A polarization multiplex signal having two data channels from the first switching device SV or from a demultiplexer is fed by way of an input PSE 1  into a polarization demultiplexer DS with one polarization controller POLCON 1 . The polarization controller POLCON 1  aligns the inbound polarization multiplex signal in such a way that the directions of polarization of both data channels are adjusted to the separating characteristics of a downstream polarization beam splitter PBS 1  in order to effect their separation. The outbound signals from the polarization beam splitter PBS 1  form the Drop signals at the output DROP of the polarization multiplex switch PS. Under certain circumstances, for example if the transmission path suffers from PDL (polarization dependent loss), two polarization controllers and two polarization beam splitters or filters are required in order to allow separation of the two polarization signals. 
         [0037]    Two signal channels for example from an external terminal are fed into a polarization multiplexer AS at further inputs ADD on the polarization multiplex switch PS. The polarization multiplexer AS has two polarization controllers POLCON 2 , POLCON 3  into each of which is fed one of the two channel signals, which is output with orthogonal polarization settings. The polarization settings are defined such that the two channel signals are combined in a downstream polarization coupler PBS 2  to produce a polarization multiplex signal which is routed to an output PSA 1  of the polarization multiplexer AS or of the polarization multiplex switch PS. The polarization multiplex signal generated can then be delivered to the first switching device SV and transmitted onward by the switching modules SV 1 , SV 2 , SV 3 . 
         [0038]    As a general principle, all other polarization multiplex switches operating on the basis of the prior art can also be used as a second switching device PS. 
         [0039]    For bidirectional polarization switching of a channel signal, according to  FIG. 6  two polarization multiplex switches are located in the second switching device PS which in each case are provided for unidirectional operation. 
         [0040]    If the transmission path suffers from PDL (polarization dependent loss) or other characteristics are present which can cancel the orthogonality of the two polarization multiplex signals, the polarization demultiplexer DS has two polarization controllers and two polarization beam splitters, only one output from which is used in each case. One alternative is also to use two polarization filters instead of the polarization beam splitters. A 3 dB coupler connected upstream or another suitable optical component for signal division splits the polarization multiplex signal to the two polarization controllers. 
         [0041]    Signals having any granularities can in particular be switched as optical signals S using the cross-connector according to  FIG. 4 . Signals or channel signals which are not available as polarization multiplex signals or are unpolarized can also be switched transparently by the first switching device SV. This is because of the cascading of the switching elements PS, SV 1 , SV 2 , SV 3  which results in a high level of switching universality for the cross-connector according to the invention. 
         [0042]    A network management facility controls the appropriate switching of the cross-connector according to the invention depending on the required and possible granularity for each optical signal.