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 re- configurable add-drop functionality. Photonic, also called op ^¬ tical, 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 transmit ^¬ ted WDM signals. Most of the cross-connects avoid wavelength conversion, because it is cost effective at the expense of re ^¬ duced 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 func ^¬ tion to direct the super-channels to the desired traffic di ^¬ rections via a direction switching unit. PRIOR ART

A photonic cross-connect is disclosed in the patent applica ^¬ tion US 2006/0098981 Al . Each through or express channel has to pass through a WSS (wavelength selective switch) and a mul ^¬ tiplexer. WS-switches are used for realizing cross-connect and add-drop functions. WSS functionality can be realized by dif ^¬ ferent technologies. By means of these technologies, e.g. mi ^¬ cro-electro-mechanical-systems (MEMS) , liquid crystal (LC) or liquid crystal on silicon (LCoS) , an optical WDM signal re ^¬ ceived 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 in ^¬ puts and outputs is possible by applying these elements.

The function of a cyclic filter is explained in "N x N Cyclic- Frequency Router With Improved Performance Based on Arrayed- Waveguide Gratings", Journal of Lightwave Technology, Vol. 27, No. 18, September 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 conten- tionless 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 di ^¬ rect a channel to any traffic direction of the cross-connect and is implemented by connecting an add-drop structure to eve ^¬ ry 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 trans ^¬ mitter can be assigned to any wavelength as long as the number of channels with the same wavelength is not more than the num ^¬ ber of traffic directions in the node. This architecture al ^¬ lows that only one add-drop structure is needed in a node. An M x N WSS is the perfect fit for this architecture, since reusing a wavelength on a fiber is not possible. Today, M ^χ 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 fil ^¬ ters. 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, direc ^¬ tionless, 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 cy ^¬ clic 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 ac ^¬ cording 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 in ^¬ vention are shown. E.g. optical amplifiers, attenuation elements, additional switches or additional filters may be in ^¬ serted in the signal (channel) paths.

A cross-connector comprises a cross-connect section 1 for receiving, cross-connecting and emitting channels. The expres- sion "channel" is used here meaning a signal which is trans ^¬ mitted 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 in ^¬ puts 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 let ^¬ ters indicating the transmitting direction) .

FIG 1 shows for reasons of easier comprehension only representative super-channels denoted wl-4, nl-4, el-4, sl-4 of WDM signals received from the different directions w, n, e, s. Each of the shown super-channels has the same wavelength spec ^¬ trum and carries four "ultra dense" sub-channels. Each receiv ^¬ ing 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 com ^¬ ponents 9 - 12 (WSS or combiner) . According to the received super-channels, only corresponding super-channels Wl-4, Nl-4, El-4, Sl-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 ar ^¬ rangement, it is assumed that only the receiving WSSs 5 and 7 are foreseen by cabling and configuration for dropping the super-channel wl-4 at drop output 5d and/or the super-channel el-4 at drop output 7d 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 5d, 7d is connected to an input 2e and 2w re ^¬ spectively of a M x N = 2 x 4 cyclic filter 21 (M - active) inputs, N - outputs corresponding to the number of sub ^¬ channels/super-channel) arranged in an ivision-multiplex section' 2. In this embodiment, one super-channel may be dropped or both super-channels wl-4 and el-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 el, e2, e3, e4 at a separate output: el at 2a, e2 at 2b, e3 at 2c, and e4 at 2d. Further each of the sub-channels wl, w2, w3, w4 is emitted cyclically shifted at a separate output: wl at 2c, w2 at 2d, w3 at 2a, and w4 at 2b. Hence, always two sub-channels having different wavelength of two super-channels having the same frequency band are dropped at one filter output, el and w3 are emitted at 2a; e2 and w4 are emitted at 2b; e3 and wl at 2c; e4 and w2 at 2d.

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 pe ^¬ riodic' super-channel with a higher or lower frequency spec ^¬ trum 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 2a - 2d because the cyclic filter is periodic in a huge frequency range. The sub-channels emitted at the same filter outputs are sepa ^¬ rated by coherent, preferable tunable receivers RX of a tran ^¬ sponder arrangement 3. But in this embodiment only one super- channel, e.g. el - e4, can be output by the receivers RX be ^¬ cause of the restricted number of four transponders

3a - 3d.

The transmitting part of the cross-connect section 1 is ar ^¬ ranged to add one of two different super-channels El-4 or Wl-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 El-4 or Wl-4. If super-channel El-4 which is inserted via an add input 11a of the WSS 11 on the transmitting side, the add input 9a of WSS 9 is closed within the frequency band of the super-channel or El-4. But it is al ^¬ so possible to transmit the add super-channel in both direc ^¬ tions E and W simultaneously.

If the other super-channel wl-4 is dropped instead of the su ^¬ per-channel el-4, then an add super-channel Wl-4 is added via the transmitting WSS 9.

According to an additional dropped ^x periodic' super-channel an additional ^x periodic' add super-channel may be added and emit ^¬ ted 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 re ^¬ ceivers 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 possi ^¬ ble 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 com ^¬ biner 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 2a - 2d, first outputs of the four splitters are connected to the receiver RX inputs the transponders 3a - 3d, and second outputs of the splitters are connected to receiver inputs of transponders 30a, .... of the expanded transponder arrangement 30.

In the add branch, first and second inputs of four 2 : 1 combin ^¬ ers 55 - 58 are connected to transmitter TX outputs of the transponder arrangements 30, and each cyclic filter 25 output 2W, 2E is connected to an add-input 9a and 11a respectively; shown are only these two connections for reasons of easier un ^¬ derstanding . It is now possible to drop and add two super-channels el-4 and wl-4 simultaneously. The sub-channels el-e4 and wl-w4 are now output at the same cyclic filter 21 outputs 2a - 2d as speci ^¬ fied in the drawing FIG 2. According to their different wave ^¬ lengths the sub-channels are separated by the transpon-ders 3a - 3d and 30a, ..., 30d (30d is not shown in the drawing) . The doubled quantity of tuneable receivers RX (transponders) al ^¬ lows 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 El-4 and Wl-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: El and W3; E2 and W4 ; E3 and Wl; E4 and W2 are combined by the combiners 55 - 58 and fed to the (now) ^x input' ports 2A - 2D of the sec ^¬ ond cyclic filter 25. This filter is "inverse operated": The sub-channels are combined forming two add super-channels El-4, Wl-4, which are emitted at the (now) ^x output' ports 2E and 2W and fed to the add inputs 11a, 9a of the WSSs 11 and 9 re ^¬ spectively.

The shown add-drop-section may be extended to drop and add any two super-channels from and to all four directions simultane ^¬ ously. The drop outputs 6d of WSS 6 and 8d of WSS 8 are then connected to further filter inputs 2s and 2n respectively as indicated by dashed lines; and the filter 25 output ports 2S and 2N are connected to add inputs 12a and 10a respectively. In the add path, according to the allocation of sub-channels and filter 25 ^x input' ports 2A - 2D each generated super- channel can be emitted at any filter ^x output' port 2S, 2E, 2N and 2W and fed to each add input 9a - 12a according to the wavelength of the sub-channels. The dashed lines show the ex- tension 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 di ^¬ rections. But according to the splitters, combiners, and num ^¬ ber 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 ^x periodic' super-channels are processed as explained re ^¬ garding 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 simultane ^¬ ously four super-channels having the same frequency band or different frequency bands. These four super-channels are re ^¬ ceived from one to four directions. Again, for reasons of eas ^¬ ier understanding, super-channels with the same frequency band received from and transmitted in the four directions are re ^¬ garded .

The division-multiplex section 20 comprises again the cyclic filters 21 and 25. The drop outputs 5d, 6d, 7d, 8d of all re ^¬ ceiving WSSs 5, 6, 7, 8 are connected to the inputs 2s, 2e, 2n, 2w of the cyclic filter 21 for dropping super-channels wl- 4, nl-4, el-4, sl-4 of all directions. The filter 21 outputs 2e - 2d 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 drop- ably super-channels) which outputs are connected to inputs of the tunable receivers RX of an enlarged transponder arrange ^¬ ment 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 el, si, wl, nl have the lowest frequency band within the frequency band of the super-channels, the sub-channels e2, s2, w2, n2 and e3, s3, w3, n3 have higher frequency bands, and the sub-channels e4, s4, w4, n4 have the highest frequency bands. Again, be ^¬ cause the drop super-channels are fed to different filter in ^¬ puts 2s, 2e, 2n, 2w, each filter 21 output 2a - 2d 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 el, e2, e3, e4 of the super-channel el-4 are sep ^¬ arated and converted by four receivers, each connected to one output of the four splitters 61 - 64 and tuned to the frequen ^¬ cy bands of the sub-channels el, e2, e3, e4.

Because any tuneable receiver can output any of four received sub-channels (e.g. el, si, wl, n4) the combination of split ^¬ ters and tuneable receivers implies direction switching abil ^¬ ity. In the add path, the 'input' ports 2A - 2D of the inverse op ^¬ erated filter 25 are now connected to outputs of N = 4 combin ^¬ ers 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 2W, 2E, 2N, 2S of the fur ^¬ ther cyclic filter 25 are connected to add inputs 9a, 10a, 11a, 12a of the transmitting WSSs 9 - 12 to add up to four su ^¬ per-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 Sl-4, El-4, Nl-4, and Wl-4.

The allocation of sub-channels forming a super-channel and filter 25 'input' ports 2A - 2D determines the filter "output" port of the combined super-channel. In FIG 3 combining sub ^¬ channels El - E4 to form the add super-channel El-4 is indi ^¬ cated by dashed lines. If the sub-channels are cyclically shifted the add super-channel El-4 is output at a correspond ^¬ ing 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 switch ^¬ ing 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 5d - 8d 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 el-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 in ^¬ put of four combiners 25 - 28. And each output of the four combiners is connected to an add input 9a - 12a of the WSSs 9 - 12 (FIG 2) .

Each cyclic filter 25 - 28 receives the sub-channels of a su ^¬ per-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 2A - 2D, which is determined by the transponders 31 - 46, a super-channel is output at a cer- tain output of the four outputs of a cyclic filter 25 - 28. Hence, this super-channel, e.g. El-4, is fed to only one of the four combiners 55 - 58. According to the sub-channel ar ^¬ rangement 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 switch ^¬ ing 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 addi ^¬ tional 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 num ^¬ ber 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 there ^¬ fore 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

5d - 8d drop outputs

13 cyclic filter

3a-3d, 30a transponder

31 - 46 transponder

9a - 12a 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 el-4 super-channel from direction east el, e2, e3, e4 sub-channels

nl-4 super-channel from direction east sl-4 super-channel from direction south wl-4 super-channel from direction west

2e , 2w (used) cyclic filter inputs

2a, 2b, 2c, 2d cyclic filter outputs

El-4 super-channel emitted in direction E

(east) add sub-channels of El-4

super-channel in direction north super-channel in direction south super-channel in direction west further cyclic filter input ports further cyclic filter output ports