Network Node for a Coherent Optical WDM Transmission Network

Abstract

Provided is a network node for a coherent optical wavelength division multiplex (WDM) transmission network including at least one remote port which is adapted to receive from and/or output to neighboring network nodes an optical WDM signal including one or more optical channel signals each lying within an optical channel of an optical WDM transmission band and a predetermined number N of local ports, each local port being adapted to receive from a dedicated coherent optical transmitter an optical add channel signal which is to be integrated in an optical WDM signal that is output at a remote port and/or each local port being adapted to output to a respective dedicated optical receiver an optical drop channel signal. The network node includes an optical router device that defines the at least one remote port and further defines an internal remote port, the optical router device being configured to route one or more selected or all optical channel signals included in the optical WDM signal received at a selected remote port as optical drop channel signals to the internal remote port and/or to route one or more optical add channel signals received at the internal remote port to one or more selected remote ports or to all remote ports. The network node further includes a passive optical filter device which is connected, at an internal remote port of the passive optical filter device, to the internal remote port of the optical router device, and which further defines the predetermined number N of local ports. The passive optical filter device is configured to define N optical bandpass filter functions each of which describes a bandpass filter characteristic between the internal remote port of the passive optical filter device and a selected, respectively differing local port. The passband of each optical bandpass filter function covers two or more neighboring optical channels. The passbands of all optical bandpass filter functions differ from each other with respect to the optical channels covered.

Claims

1. A network node for a coherent optical wavelength division multiplex (WDM) transmission network comprising (a) one remote port which is adapted to receive from and/or output to neighboring network nodes an optical WDM signal including one or more optical channel signals each lying within an optical channel of an optical WDM transmission band and (b) a predetermined number N of local ports, each local port being adapted to receive from a dedicated coherent optical transmitter an optical add channel signal which is to be integrated in an optical WDM signal that is output at a remote port and/or each local port being adapted to output to a respective dedicated optical receiver an optical drop channel signal, (c) wherein that the network node comprises an optical router device that defines the at least one remote port and further defines an internal remote port, the optical router device being configured (i) to route one or more selected or all optical channel signals included in the optical WDM signal received at a selected remote port as optical drop channel signals to the internal remote port and/or (ii) to route one or more optical add channel signals received at the internal remote port to one or more selected remote ports or to all remote ports, and (d) wherein the network node further comprises a passive optical filter device which is connected, at an internal remote port of the passive optical filter device, to the internal remote port of the optical router device, and which further defines the predetermined number N of local ports, (e) wherein the passive optical filter device is configured to define N optical bandpass filter functions each of which describing a bandpass filter characteristic between the internal remote port of the passive optical filter device and a selected, respectively differing local port, and (f) wherein the passband of each optical bandpass filter function covers two or more neighboring optical channels and wherein the passbands of all optical bandpass filter functions differ from each other with respect to the optical channels covered.

2. The network node according to claim 1, wherein the passbands of the optical bandpass filter functions cover the total transmission band of the WDM transmission network and, preferably, reveal essentially the same width.

3. The network node according to claim 1, wherein neighboring optical bandpass filter functions are chosen in such a way that the passbands of respective two neighboring optical bandpass filter functions are separated by a guard band, in which the filter attenuation of the two neighboring optical bandpass filter functions is higher than a predetermined threshold value, so that power portions of any optical signal, including noise, that lies within the guard band are suppressed.

4. The network node according to claim 3, wherein neighboring optical bandpass filter functions are chosen in such a way that no optical channel lies within a guard band.

5. The network node according to claim 1, wherein the passband of each of the optical bandpass filter functions covers a predetermined number of optical channels.

6. The network node according to claim 5, wherein the predetermined number of optical channels is in the region of four to twelve, preferably six to ten.

7. The network node according to claim 1, wherein the passbands of neighboring optical bandpass filter functions overlap and that at least one optical channel lies in a respective overlap region.

8. The network node according to claim 1, wherein the passive optical filter device realizes a cyclic filter characteristic, wherein the passbands an essentially constant frequency spacing and essentially the same width.

9. The network node according to claim 1, wherein the passive optical filter device comprises a first optical filter defining a first number of local ports and a common port, wherein first optical bandpass filter functions describe the transmission characteristics of the first optical filter between its common port and the respective local ports, and a second optical filter defining a second number of local ports and a common port, wherein the second optical bandpass filter functions describe the transmission characteristics of the second optical filter between its common port and the respective local ports, and that the passive optical filter device further comprises a 1?2 coupler defining a common port and two coupling ports, wherein the common port realizes the internal remote port of the passive optical filter device, and wherein one of the coupling ports is connected to the common port of first optical filter and wherein the respective other of the coupling ports is connected to the common port of second optical filter.

10. The network node according to claim 9, wherein the passbands of the first optical bandpass filter functions and the passbands of the second optical bandpass filter functions realize a fully interleaved structure, wherein the passbands of the interleaved first and second optical bandpass filter functions do not overlap.

11. The network node according to claim 9, wherein the passbands of the first optical bandpass filter functions and the passbands of the second optical bandpass filter functions realize an overlapping interleaved structure, wherein the passbands of interleaved first and second optical bandpass filter functions are shifted versus each other in such a way that the passbands (PB-i) of the second optical bandpass filter functions fully cover guard bands between the passbands of the first optical bandpass filter functions and vice versa.

12. The network node according to claim 11, wherein the passbands of the first and second optical bandpass filter functions have an essentially identical frequency spacing.

13. The network node according to claim 12, wherein the passbands of the first and second optical bandpass filter functions are shifted by half of the frequency spacing.

14. The network node according to claim 1, wherein the network node comprises at least one coherent optical receiver which is optically connected to a local port, wherein the optical receiver is configured to select a predetermined optical channel signal from two or more optical channel signals that are output at the local port for reception by adjusting a local oscillator frequency of an optical local oscillator comprised by the coherent optical receiver to an optical carrier frequency of the optical channel signal selected.

15. A coherent optical wavelength division multiplex transmission network comprising two or more network nodes according to claim 1, wherein all passive optical filter devices comprised by the network nodes have an identical routing property, and wherein the passive optical filter devices are especially identical.

Description

[0045] In the following, the invention will be described in detail with reference to the drawings. In the drawings,

[0046] FIG. 1 shows a schematic block diagram of a section of a coherent optical WDM transmission network comprising three network nodes according to the invention having an add/drop capability;

[0047] FIG. 2 shows a diagram illustrating the frequency characteristics of the passive optical filter devices comprised by each of the nodes in FIG. 1, which define the local (add/drop) ports;

[0048] FIG. 3 shows a block diagram of a passive optical filter device comprising two cyclic optical filters the frequency characteristics of which define an interleaved passband structure; and

[0049] FIG. 4 shows a diagram illustrating the frequency characteristics of the cyclic optical filters of the passive optical filter device in FIG. 3.

[0050] The schematic block diagram shown in FIG. 1 illustrates a portion of a coherent optical WDM transmission network 100 comprising three neighboring network nodes 102 each having a western remote port 102a, an eastern remote port 102b and a number of N local ports 102c-1 to 102-N. The directly neighboring network nodes are connected by an optical path 104, e.g. an optical single-mode fiber, wherein the optical path connects the respective Eastern and Western ports 102a and 102b.

[0051] The remote ports 102a, 102b of each network node are defined by an optical router device 106 comprised by the optical network node 102. Each router device 106 is configured to receive, at its Western remote port, an eastbound optical WDM signal S.sub.WDM,E and, at its eastern port, a westbound optical WDM signal S.sub.WDM,W.

[0052] The optical WDM signals S.sub.WDM,E, S.sub.WDM,W may especially be optical dense wavelength division multiplex (DWDM) signals, that comprise, at the maximum, all M optical channel signals S.sub.ch-i (wherein the index i is an integer number greater than or equal to land lower than or equal to M) of M optical channels CH.sub.i that are defined for the coherent optical WDM transmission network 100. For example, in case the coherent optical WDM transmission network shall make use of the whole extended C-band, 96 optical channels having a channel spacing of 100 GHz may be used.

[0053] Each optical router device 106 is further configured to feed selected optical channel signals S.sub.ch-i of each of the optical WDM signals S.sub.WDM,W, S.sub.WDM,E and to output the selected channel signals at an internal remote port 106a. In the simplest case, the optical router device 106 may be realized by an optical 1?2 coupler which taps off a predetermined power portion of the respective optical WDM signal S.sub.WDM,W, S.sub.WDM,E, i.e. all optical channel signals S.sub.ch-i are (statically) selected and fed to the internal remote port 106a. However, usually, the optical device will be realized as a means that is configured to statically select or select on-demand predetermined optical channel signals S.sub.ch-i, i.e. the optical router device 106 is realized as an optical add/drop multiplexer (OADM) or as a reconfigurable optical add/drop multiplexer (ROADM). For this purpose, one or more wavelength selective switches (WSS) may be used within the optical router device

[0054] The router device 106 may be configured to output optical channel signals S.sub.ch-i that have been selected from the western and eastern optical WDM signals S.sub.WDM,W, S.sub.WDM,E at a single physical internal remote port 106a. However, in this case it is not allowed to select the optical channel signals S.sub.ch-i that are transmitted within the same optical channel CH.sub.i. Therefore, the optical router device 106 usually realizes the internal remote port 106a as two or more physical ports, wherein each of the physical ports is used to output the optical channel signals S.sub.ch-i that are selected from one of the remote ports 102a or 102b, respectively, that is assigned to a selected physical port. Of course, in case of a network node having three or more remote ports, the internal remote port may be realized by a corresponding number of physical ports.

[0055] The internal remote port 106a of the optical router device 106 of each network node 102 is connected to an internal remote port 108a of a passive optical filter device 108 comprised by each network node 102. Each optical filter device 108 defines the N local ports 102c-1 to 102c-N. Of course, if, as described above, the optical router device 106 defines two or more physical internal remote ports that realize the internal remote port 106a, a corresponding number of two or more optical filter devices 108 will be required and comprised by the network node 102, wherein each of the optical filter devices 108 is used to process the optical channel signals S.sub.ch-i that have been selected from the optical WDM signal S.sub.WDM,W, S.sub.WDM,E received at a dedicated remote port 102a, 102b. This property of the network nodes 102 is indicated in FIG. 1 by the representation of the optical filter devices 108 through two trapezoids that are slightly shifted versus each other.

[0056] The optical filter devices are configured to route the optical channel signals S.sub.ch-i to the local ports as will be described below so that each of the optical channel signals S.sub.ch-i is output at at least one dedicated local port.

[0057] Each local port 102c-1 to 102c-N defined by each optical filter device 108 may be connected to a coherent optical transceiver 110 which is configured to receive the optical channel signals S.sub.ch-i output at the respective local port 102c-1 to 102c-N. Likewise, the coherent optical transceivers 110 may also be configured to transmit a corresponding optical channel signal S.sub.ch_I which is received at the respective local port 102c-1 to 102c-N of the optical filter device 108 and output at the respective internal remote port of the optical filter device 108.

[0058] Of course, the network nodes 102 may in general also be configured to exclusively drop optical channel signals S.sub.ch-i, i.e. output, at the local ports 102c-1 to 102-N, optical channel signals S.sub.ch-i selected from an optical WDM signal received at a remote port, or to exclusively add optical channel signals S.sub.ch-I, i.e. receive, at the local ports 102c-1 to 102-N, optical channel signals S.sub.ch-i and integrate same into an optical WDM signal output at a remote port. In such a case, instead of coherent optical transceivers, coherent optical receivers or coherent optical transmitters may be provided.

[0059] According to a further embodiment (not shown in the drawings), separate optical filter devices 108 may be provided for optical channel signals to be dropped and the optical channel signals to be added. It goes without saying that also in this case separate coherent optical receivers and transmitters may be used or coherent optical transceivers that provide separate physical (optical) receive and transmit ports.

[0060] In the following, the optical characteristics of the passive optical filter device(s) 108 will be described.

[0061] In general, the invention aims at a segmentation of the whole optical band that is used by the coherent optical WDM transmission network 100 in order to define the local ports 102c-1 to 102-cN of the network nodes. The band segments, each of which is assigned to a selected different local port, are chosen in such a way that, on the one hand, a desired number of local ports is obtained and, on the other hand, sufficient noise filtering is achieved. Further, two or more optical channels CH.sub.i would be available at each local port. Selecting a desired optical channel signal S.sub.ch,i (even if two or more optical channel signals are output at the respective local port 102c-1 to 102c-N) may be done by the coherent receiver (simply by adjusting the local oscillator frequency to the center frequency of the desired optical channel signal S.sub.ch,i).

[0062] As already mentioned, preferably a segmentation of the whole transmission band that is used by the coherent optical WDM transmission network is desirable. In other words, the whole transmission band used is segmented in such a way that the segments (i.e. the passbands of all filter functions FF-1 to FF-N) fully cover the transmission band.

[0063] In order to achieve this segmentation, CWDM (multiplexer/demultiplexer) may be used. However, as apparent from FIG. 2, such optical filter devices reveal a cyclic filter characteristic, wherein the transmissivity (i.e. the magnitude of the filter function) is low in rather broad bandwidth regions between the passbands PB-1 to PB-N. In FIG. 2, these bandwidth regions (which are designated as guard bands in this description) are illustrated in grey. In a CWD transmission network, these guard bands are necessary in order to avoid crosstalk between neighboring channels. Further, especially such low-cost CWDM filter devices have rather flat filter slopes so that the guard bands become large. This is, in general, no severe problem for CWDM transmission networks, either, as the frequency spacing of the CWDM channels is high and the bandwidth of the channel signals is decisively lower than this frequency spacing.

[0064] FIG. 2 shows a diagram illustrating, as an example, the magnitude of the N optical filter (transfer) functions FF-1 to FF-N between the internal remote port and each of the N local ports of the optical filter device 108 of each network node 102, wherein the optical filter device 108 is realized by a CWDM filter device. As apparent from this Figure, the optical filter device 108 shows the behavior of a cyclic optical bandpass filter (for the superimposed filter functions) having an essentially constant frequency spacing ?fs, wherein each passband PB-1 to PB-N is of essentially identical width. The passbands are defined by a threshold value, which must be exceeded by the magnitude of the respective optical filter function FF-1 to FF-N. In FIG. 2, this threshold value is chosen rather high as in modern coherent optical transmission systems acceptable insertion losses are low due to low launch power and low receive sensitivity of the coherent optical transceivers. As a consequence, the guard bands become broad.

[0065] If such an optical filter device 108 is used in order to define the local ports 102c-1 to 102c-N, only optical channel signals lying within the passbands PB-1 to PB-N can be correctly received at the corresponding local ports 102c-1 to 102c-N. Also transmitting optical channel signals S.sub.ch,i lying within guard bands is to be avoided as such channel signals would encounter high attenuation so that they might not be correctly received at the end of the respective transmission link in another network node.

[0066] As an example, FIG. 2 shows four optical channels CH.sub.i or channel signals S.sub.ch,1 to S.sub.ch,4 that lie within the passband of the optical filter function FF-1, which describes the transmission characteristic between local port 102c-1 and the internal remote port 108a of the passive optical filter device 108. Due to the broad guard bands, a rather high number of optical channels could not be used with an optical filter device having the transmission characteristic according to FIG. 2. Thus, the coherent optical WDM transmission network would be restricted to the number of optical channels CH.sub.i which are covered by the passbands PB-1 to PB-N of the optical filter functions FF-1 to FF-N in case all optical filter devices 108 of the network nodes 102 revealed such identical transmission characteristics.

[0067] A solution to this problem would be to design a passive optical filter device 108 that has very narrow guard bands, which would require extremely steep filter flanks outside the passbands. In general, it would be possible to manufacture such passive optical filter devices. However, the manufacturing costs would be decisively higher as compared to corresponding low-cost optical filter devices having broad guard bands.

[0068] Another solution will now be described with reference to FIGS. 3 and 4. FIG. 3 shows an embodiment of a passive optical filter device 108 that comprises two cyclic passive optical filter devices 108-I, 108-II, which may be realized as CWDM multiplexer/demultiplexer devices or in general as low-cost broadband multiplexer/demultiplexer devices. The optical filter device 108-I realizes optical filter functions according to the upper part of the diagram shown in FIG. 4, wherein the X-axes indicate the optical frequency and the Y-axes indicate the magnitude of the optical transfer function (i.e. the transmissivity) of the respective optical filter device. Likewise, the optical filter device 108-II realizes the optical filter functions according to the lower part of the diagram shown in FIG. 4. As indicated in FIG. 3, the optical filter device 108-I provides odd local ports 102c-1, 102c-3, . . . , 102c-11 and the optical filter device 108-II provides even local ports 102c-2, 102c-4, . . . , 102c-12, wherein, as an example, this embodiment realizes twelve local ports in total. However, of course, any arbitrary number of local ports may be realized in an analog manner.

[0069] Each of two multiplex/demultiplex ports 108-Ia, 108-IIa of the optical filter devices 108-I, 108-II is connected to a coupling port 112a, 112b of an optical 1?2 coupler 112. Each of the optical filter devices 108-I, 108-II is configured to receive, at each of its (odd or even) local ports 102c-1 to 102c-N, an optical channel signal S.sub.CH,i lying within a selected one of two or more optical channels CH.sub.i, which are assigned to the respective local port. All optical channel signals S.sub.CH,i received are multiplexed into a partial optical WDM signal S.sub.WDM,I, S.sub.WDM,I that is output at the respective multiplex/demultiplex port 108-Ia, 108-IIa. The optical coupler 112 combines the partial optical WDM signals S.sub.WDM,I, S.sub.WDM,I into a combined partial optical WDM signal that is output at the internal remote port 108a that is defined by the optical coupler 112. Likewise, the optical coupler 112 according to FIG. 3 is capable of receiving any optical channel signal S.sub.ch,i, which may be integrated in a partial optical WDM signal, at the internal remote port 108a and of demultiplexing this partial optical WDM signal by feeding same to each of the multiplex/demultiplex ports 108-Ia, 108-IIa of the optical filter devices 108-I, 108-II, wherein the respective optical filter device 108-I, 108-II demultiplexes the partial optical WDM signal received according to its filter characteristic. As a result, each of the optical channel signals CH.sub.i is output at the or any local port 102c-1 to 102c-12 that covers the respective optical channel Ch.sub.i.

[0070] Of course, as explained above, the passive optical filter device 108 may be used for both transmission directions, i.e. for receiving, multiplexing and outputting optical channel signals received at the local ports 102c-1 to 102c-12 and for receiving a partial optical WDM signal that includes optical channel signals S.sub.ch,i to be dropped and outputting the demultiplexed optical channel signals S.sub.ch,i at the dedicated local ports 102c-1 to 102c-12.

[0071] As apparent from FIG. 4, the optical filter functions FF-i that realize the multiplex/demultiplex functionality of the passive optical filter devices 108-I and 108-II may reveal rather broad guard bands and rather flat filter edges. The optical filter functions FF-i (i=1, 3, . . . , 11) defining the odd ports and the optical filter functions FF-I (i=2, 4, . . . , 12) defining the even ports reveal the same shape, i.e. define passbands of equal width, and have a respective identical frequency spacing ?fs. However, these odd and even optical filter functions are shifted versus each other by half of the (constant) frequency spacing ?fs. In this way, it is achieved that the passbands of the odd optical filter functions realized by the passive optical filter device 108-I fully cover the guard bands between the even optical filter functions realized by the passive optical filter device 108-II and vice versa. All passbands PB-I superimposed cover the whole optical band that is used by the respective coherent optical WDM transmission network. For example, as mentioned as an example above, the whole optical band may be the extended C-band and the number N of optical channels CH.sub.i may be 96. That is, in the example according to FIGS. 3 and 4, in which twelve local ports are defined by the passive optical filter device 108, a group of (at least) eight neighboring different channels must be assigned to each local port 102c-1 to 102c-12.

[0072] It is, however, possible to assign more than eight neighboring channels to each local port due to an overlap of neighboring filter functions FF-I, as illustrated in FIG. 4. According to FIG. 4, each passband PB-i covers not only the respective eight neighboring different channels Ch.sub.i but, due to a rather large overlap, ten neighboring optical channels CH.sub.i. Thus, the last optical channel covered by a given passband PB-i, i.e. the optical channel having the highest center or carrier frequency, is also covered by the immediately neighboring passband PB-(i+1) and the first optical channel covered by a given passband PB-i, i.e. the optical channel having the lowest center or carrier frequency, is also covered by the immediately neighboring passband PB-(i?1). It would, of course, also be possible to even increase the overlap so that the overlap covers not only one, but two or more optical channels (at each side of a passband, except the lower side of the first passband and the higher side of the last passband, which have no neighboring passbands).

[0073] In this way, it is possible to use low-cost filter devices having flat filter edges as passive optical filter devices 108-I and 108-II, respectively. Even such flat filter edges are sufficient to provide sufficient noise filtering in the transmit direction. As explained above, selecting a predetermined optical channel, from the group of optical channels covered by the passband of a given optical filter function, as reception channel, i.e. receiving the optical channel signal included within a predetermined optical channel may be effected by the coherent optical receiver 110 (as explained above, by adjusting the local oscillator frequency accordingly).

[0074] As illustrated in FIG. 4, it might even be possible to receive an optical channel signal S.sub.CH,i of an optical channel that lies outside the passband of a given filter function FF-i if the filter edge of the respective optical filter function is so flat that the respective optical channel signal is output with a sufficiently high optical power (see the channel signal illustrated by a dashed line in filter functions FF-1 and FF-2 in FIG. 4). Of course, as the case may be, such optical channel signals might even be received at both neighboring ports.

[0075] It shall be noted that the embodiment referred to in FIGS. 3 and 4 shall merely serve as an example and that the invention may also be realized using optical filter devices 108-I and 108-II having filter functions of passbands having different widths. In this case, the number of channels per passband that realize the odd and even ports may be different.

[0076] As explained above, the invention provides a simplified structure of a network node for a coherent WDM transmission network by segmenting the, preferably full, optical band that is used by the WDM transmission network into rather broad passbands assigned to the local ports. In this way, groups of two or more neighboring optical channels are assigned to each local port. The further selection of a specific channel as a receive channel is effected by the coherent optical receiver. The width of the passbands is chosen in such a way that a sufficient noise filtering is achieved in the transmit direction. The number of segments or passbands, i.e. the number of ports, is chosen in such a way that a passive optical filter device that is configured to carry out the desired multiplex/demultiplex function reveals a sufficiently low insertion loss (between the common or remote port and each of the local ports).

LIST OF REFERENCE SIGNS

[0077] 100 coherent optical WDM transmission network [0078] 102 network node [0079] 102a western remote port [0080] 102b eastern remote port [0081] 102c-1 to 102c-N local port [0082] 104 optical path (optical fiber) [0083] 106 router device [0084] 106a internal remote port (of router device 106) [0085] 108 passive optical filter device [0086] 108a internal remote port (of passive optical filter device 108) [0087] 108-I passive optical filter device [0088] 108-Ia multiplex/demultiplex port [0089] 108-II passive optical filter device [0090] 108-IIa multiplex/demultiplex port [0091] 110 coherent optical transceiver [0092] 112 optical 1?2 coupler [0093] 112a coupling port [0094] 112b coupling port [0095] M number of optical channels [0096] N number of local ports [0097] CH.sub.i optical channel (1?i?M) [0098] PB-1 to PB-N passband [0099] ?fs frequency spacing [0100] S.sub.ch,i optical channel signal (1?i?M) [0101] S.sub.WDM,W western optical WDM signal [0102] S.sub.WDM,E eastern optical WDM signal