SE524214C2 - Add-drop node with small loss for WDM - Google Patents

Description

25 30 35 524 214 2 add / drop band fi lter, the band of which covers two adjacent channels. In this way, the number of add / drop ilter in the transmission fi bem is not greater than the amount of filter needed in each transmission fl ber in a WDM ring according to the state of the art, as shown in fi g. 2, and can generally be kept to a minimum, ie in fact as small as possible.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described as non-limiting embodiments with reference to the accompanying drawings, in which: - Fig. 1 is a block diagram of a general optical WDM ring network according to the prior art and with protective function, - Fig. 2 is a block diagram of a part of an optical WDM network according to the prior art comprising a pair of optical add / drop nodes arranged for communication in transmission bridges, which pass through the nodes, Fig. 3 is a block diagram similar to that in Figs. 2, the nodes being arranged for bidirectional communication or duplex communication in a single transmission carrier, Figs. Fig. 4a is a view of an add / drop filter according to the prior art, which shows the numbering of ports, - Fig. 4b - 4d is a diagram of the transmission between different ports in a single-channel add / drop filter as a function of the wavelength of light, which the gates, - Figs. Sa, Sb are diagrams of the transmission as a function of the wavelength between two gates, gates nos. "drain ter lter") in the nodes according to fi g. 3, the transmission being shown with a dashed line, - Figs. 6a - 6c are images of three different types of two-channel add / drop filters, - Figs. 6d - 6f are diagrams of the transmission as a function of the wavelength between two ports, ports no. 2 and 3, in two-channel add / drop filters according to fi g . 6a - 6c, - F ig. 7a is an image of a ring-type optical WDM network having a name node or master node and using a single fi ber as the transmission fi ber or main fi path and enabling bidirectional communication between the name node or the main node and other nodes connected in the transmission path, the network having protection function, - F ig. 7b is an image similar to that of fig. Fig. 7a but with two parallel fi paths, - Fig. 8a is an image of a ring-type optical WDM network, which has a trace pattern with connections between all nodes ("meshed traf p c pattem") and uses a transmission fi ber or main fi bearing path, - Fig. 8b is an image of a ring-type WDM optical network, which only travels between adjacent nodes and uses a transmission ber or main path, - Fig. 8c is an image of a ring-type WDM optical network having a tracing pattern with connections between all nodes and using two parallel transmission paths or main paths and enabling bidirectional communication between nodes connected in the transmission path, the network having a protection function, 524 214 3 - Fig. 9a is a view of a part of an add / drop node comprising a Mach-Zehnder filter with Bragg grating, - Fig. 9b is a view of a combined filter comprising a Bragg grating between two optical circulators for use in an add / drop node, - Fig. 10 is a block diagram of a part of an optical WDM network similar to that of Fig. 3, the nodes being arranged for bidirectional communication or duplex communication in a single transmission fi ber in fl your channels, and - Fig. 11 is a block diagram of a part of a WDM optical network similar to that in fi g. Fig. 12 is a block diagram of a part of a WDM optical network similar to that in Figs. 10 including nodes of a further alternative construction.

DETAILED DESCRIPTION The general structure of a typical WDM ring network is shown in the figure in Fig. 1. Two parallel ring paths 1a, 1b of optical fibers transport light signals, which propagate in opposite directions. In each ring road, the light, which includes the signals, is called the path in the road.

In each ring ring, the signals are transmitted in several different wavelength bands or wavelength channels. In the annular paths, fl your nodes 3 are connected, the nodes also being called add / drop nodes. In each such node, signals are taken or drained from the lane in at least one of the annular paths and signals are emitted or added to the lane in at least one of the ring roads. For the case shown, in which two parallel ring roads transport traffic in opposite directions to enable bidirectional communication between the nodes, redundancy or protection function is obtained. Thus, for transmission from a transmitting node to a receiving node, two paths may be used, a first path being part of a first of the fiber rings and a second path being part of the second bearing, so that the second path corresponds to or is parallel to the part of the first ring, which is not used for the first road. Normally only one of the first and second paths is used, the second path being used only for the protection function and thus only being used when the first path is incorrect.

In the schematic picture in fi g. 2 shows communication according to known technology between a node A and a node B in, for example, a ring network of the i fi g. 1 showed the battle. Each of the two beams 1a, 1b used for transport in the network, referred to herein as transmission beams, carries light signals which travel in only one direction, and the directions of the light signals in the two beams are opposite to each other.

Usually the same wavelength channel, as shown wavelength channel No. 1, is used in the two transmission fibers. A total of four add / drop filters 21, 41 connected in the path of the transmission fibers 1a, 1b are needed to achieve the bidirectional communication between the two nodes A, B. The two nodes also comprise optical transmitter 91 and optical receivers 111 for channel no. 1.

However, bidirectional communication between a pair of nodes, such as nodes A and B i fi g. 2, is achieved by means of a single transmission fi ber 1 as shown in the schematic diagram in fi g. 3. The transmission fiber 1 is used to transport light signals in the two opposite directions. In transmission fi bem 1 in node A, a two-channel add / drop filter 51,12, for example an add / drop band fi l- ter, is connected to drain light signals in two, preferably adjacent or adjacent, wavelength channels nos. 1 and 2, which arrives at node A from the east, assuming that node B is connected directly to the right or east side of node A, so that nodes A and B are only connected by an unbroken fi bar. In general, light with wavelengths different from those used for the bidirectional communication between nodes A and B, ie different from the wavelengths used in channels no. 1 and 2 in the example according to the in transmission with small loss. To the branch port of the two-channel add / drop filter 5e1-2, an internal single-channel drop filter 72 for draining channel no. 2 is connected. It has an express port to which signals outside the drained channel, i.e. outside the channel no. 2, pass and to this express port is connected in node A a source of light signals in wavelength channel no. 1, an optical transmitter 91 such as a laser. A light receiver or optical receiver 112 for receiving signals in channel no. 2 is connected to the branch port of the single-channel drop filter 72.

Node B has a structure that is similar but complementary to the structure of node A. Thus, the two-channel add / drop filter Sw 1-2 is arranged to branch off traffic coming from the west, the inner single-channel drop filter 71 picks up signals in wavelength channel no. 1, the light signal source 92 transmits light only in wavelength channel no. 2 and the receiver 111 is arranged receive signals in wavelength channel no. 1.

The light signal source 91 in node A generates signals only in wavelength channel no. 1, which are introduced into the express port of the single-channel drop filter 72 in node A. The signals in channel no. -2 connected in the transmission fi bern 1. Due to the branching direction of the two-channel add / drop filter, the signals in channel no. 3, until they reach the node B. In node B, the signals in channel no. 1 in the two-channel add / drop filter 5w1-2 in node B are drained and connected therefrom to the inner single-channel drop filter 71 in node B. All light in wavelength channel no. 1 is drained. in the inner single channel drop filter and transmitted to the receiver 111 in node B.

In the case of signals in the opposite direction, in node B the light signal source 92 transmits light signals only in wavelength channel no. - / drop filter 5w1.2 and propagated in a westerly direction or in a left-hand direction in the transmission fiber 1 together with another tractor in the same direction. In node A, the signals in the considered channel no. 2 in the two-channel add / drop filter 561.2 are drained, connected to the single-channel drop filter 72 and drained in this filter and connected to the receiver 112 in node A.

If the two-channel add / drop filters 5@1.2 and 5w1-2 are of the type with add-drop band function, the wavelength channels used for bidirectional communication between a node pair, i.e. channels no. 1 and 2 in the example described above, should be as indicated above , preferably be adjacent or adjacent, so that thereby these filters 561-2, 5w1-2 block as few other wavelength channels as possible and preferably none at all.

Add / drop filters for wavelengths different from or outside the add / drop wavelengths of the two-channel add / drop filters 561-2, 5w1-2 in nodes A and B can be connected in transmission fi bem l without limiting the function of the above-described bidirectional communication.

The two-channel add / drop filters The Sen, 5w1-2 and single-channel drop filters 71, 72 all have three carrier ports.

They can, for example, be of the tower interference type from, for example, E-TEK, product type DWFI, or from Coming or Oplink. Function of a single-channel add / drop filter is shown in fi g. 4a - 4d.

The single channel drop filter has three ports numbered 1-3, of which port No. 2 is the normal port or typical input / output port port No. 1 the express port and port No. 3 is the branch port or branch port or add / drop port. From port no. 1 to port no. 2 and in the opposite direction, ie from port no. 2 to port no. 1, signals pass in all wavelength channels, except those in the drained channel, with little loss, see the diagram in fi g. 4b over the transmission as a function of the wavelength. From port no. 2 to port no. 3 and in the opposite direction, ie from port no. 3 to port no. 2, only signals pass in the drained channel with a small loss, see the diagram in fi g. 4c. From gate no. 1 to gate no. 3 and in the opposite direction, ie from gate no. 3 to gate no. 1, the signals pass in all wavelength channels with high loss, see fi g. 4d.

The ports of the fl-channel add / drop filter and add / drop band filter can be numbered in a manner corresponding to that shown in fi g. 4a. An add / drop filter of an an channel generally branches only the wavelengths in at least two channels, while all other channels pass through the filter, between its ports no. 1 and 2, with little loss. Add / dropband filters in the sense used have the same basic function as single-channel drop filters, but have a wider wavelength range, which is drained, so that your channels are drained, ie branched between ports nos. 2 and 3 and blocked between ports nos. 1 and 2.

The wavelength dependence of the four-channel add / drop filters 561-2, Swi-z, when these are add / drop band filters, and of the single-channel drop filters 72, 71 in the nodes A and B, as used in the nodes according to the schematic diagram in fi g. 3, is shown by the diagrams in fi g. In resp. 5b, in which the curves show the transmission between the filters nos. 2 and 3 of the filters.

I fi g. 6a - 6c show examples of different types of two-channel add / drop filters, while the spectral response, ie the transmission between ports no. 2 and 3 as a function of the wavelength, is shown in the diagrams in resp. fi g. 6d - 6f. That i fi g. 6a, the two-channel add / dropband filter is of the thin film interference filter type. The filter adds / drops two adjacent channels, as seen in fi g. 6d.

In it i fi g. 6b shows the two-channel add / drop filter, the central element is an optical circulator 17. Two Bragg grating l9N1, 19N2 are connected to the express port of the filter, each grating reflecting wavelengths in a specific wavelength channel no. N1, N2. I fi g. 6c shows an add / drop filter with a sampled Bragg grating l9S connected to the express port and an optical circulator 17 as in the filter according to fi g. 6b. The grating 19S reflects the wavelengths in every third channel, so that these channels are drained in the filter, as can be seen in the diagram in fi g. 6f.

An example of a WDM ring network built using it in fi g. 3 is shown in the block diagram in fi g. 7a, in which only the two-channel add / drop filters in the nodes are shown.

Only one transmission ring 1 is used. The network has a central node or main node, the name node H, which has a direct optical connection to each of the satellite nodes, which are four in this example and are denoted by the letters A to D. In the left half of the name node H, four two-channel add - / drop filter SW, one for communication with each satellite node A - D and all arranged for communication, for example in the same direction, counterclockwise direction or western direction in the figure, seen from the name node. Eight wavelength channels are required, in the figure channels no. 1 - 8.

Obviously, no ring configuration is required for this communication between the name node and each of the satellite nodes, but the "independent" ring is used to provide redundancy or protection in the network. Thus, for protection purposes, the name node H is shown to have four additional add / dropband filters. See, the regular add / dropband filters are shown as gray squares and those used for protection purposes are shown as black squares. The add / drop filters for the protection function are preferably also all connected for communication in one direction, in the direction opposite to that selected for the regular filters, for the name node H in the eastern direction or clockwise direction seen in the figure. For example, the regular communication from name node H to node B in wavelength channel no. 3, and the response from node B to name node goes in wavelength channel no. 4. This means that the two-channel add / drop filters in both name node H and in node B are arranged to drain channels no. 3 and 4 for this communication, that the inner single-channel drop filter, not shown in fi g. 7a, in the name node H is arranged to drain channel no. 4 and that the inner single-channel drop filter, not shown, in node B is arranged to drain channel no. 3.

In a ring ring network there are two possible paths between the name node H and each satellite node A - D, these two paths using different, complementary segments of the ring, a right segment and a left segment seen in fi g. 7a, which is obtained by dividing the ring at the name node and the satellite node under consideration. In the example according to fi g. 7a runs for normal traffic, when each component in the network works, the traffic from the name node H to the satellite nodes A - D in the counterclockwise direction along the ring fiber and the traffic from the satellite nodes to the name node goes in the opposite direction or clockwise, so that this determines the connection direction of the two channels droplets, which are used for regular communication.

If a fi ber break occurs, for example, between nodes A and B, as these are connected according to fi g. 7a, the path between the name node H and the satellite nodes B, C and D is unaffected, passes in the left segment of the ring, while the path between the name node H and the satellite node A can be redirected to the second, right segment of the ring. This is achieved by using two-channel add / drop filters for protection function, which are used for the protected bidirectional communication between the name node H and the satellite node A. These two-channel add / drop filters for the protection function use the same or corresponding wavelength channels as the regular filters but sends / receives traffic in the opposite direction, as also mentioned above. Alternatively, if no protection function is needed in a ring network as shown in Fig. 7a, the communication paths between the satellite nodes A, B, C and D and the name node H, which run in the right segment of the ring, can be used for others. traction channels, so that the total capacity of the ring is thereby increased.

It is advantageous to transmit signals in the same wavelength channel from a node for both normal traffic and traffic triggered by the protection function. The reason is that some of the light to be completely drained in an add / dropband felt always leaks along the ring fiber and mixes with the signal added to the same node, and this is an undesirable effect. The unwanted residual light propagates together with the desired signal and finally reaches the receiver in the same node from which it was originally emitted. If then the desired wavelength channel and the unwanted channel, which leaks through, have different wavelengths, the unwanted light is filtered once more in the single-channel drop filter and thereby the crosstalk effect is greatly reduced.

If two or more ring transmission beams are used instead of only one ring transmission beam, the number of two-channel add / drop filters connected in each ring fiber can be reduced, see the network shown in Fig. 7b, in which two parallel ring transmission beams are located. This means that the optical losses in each ring can be significantly reduced, and thus the size of the ring can be increased without the use of optical amplifiers, or alternatively the traction channels can be transported in the network without increasing the optical losses. It should also be noted that the wavelengths in the two rings are independent. For example, the channel pairs 1/2 and 3/4 in Fig. 7b may be the same two wavelengths, since they propagate in different fi widths. This can be advantageous for keeping the number of different components small or for making better use of the wavelength range which is more effectively amplified by means of erbium-doped fi amplifiers. In general, the links used for the bidirectional communication between the nodes, ie if the bidirectional communication is transported in the first or second ring fiber, should be chosen so that the highest loss of traction in the two rings is as small as possible, in that the general case of the two or fl your parallel transmission fi rings will be as small as possible. In practice, this usually means that there is an equal number of add / drop filters connected in the two or more fiber rings.

The bidirectional communication described with reference to Fig. 3 can also be used in WDM networks with traffic patterns with connections between all nodes, as shown by the image in fi g. 8a. Here, all traffic in both directions is transported in a single ring transmission fiber and no protection function is used. If only traffic between adjacent nodes is possible in a network, which can be called strictly adjacent traffic, only two channels are required for the bidirectional communication, as shown in the picture in fi g. 8b.

A WDM ring network, which has a traffic pattern with connections between all nodes and uses bidirectional communication, as described with reference to fi g. 3 and also has a protection function, is shown as an example of the principal image in fi g. 8c. Here, two ring transmission transmitters 1a, 1b are used, each of which transports traffic in both directions, in order to obtain lower losses, which are caused by the losses in the two-channel add / drop filters in each direction. The gray squares denote add- / drop fi lter, which are used for normal or regular traffic. The black squares 10 15 20 25 30 35 524 214 8 denote the add / drop belt used for traction triggered by the protection function. It can be observed that the wavelength channel number of the channels used in the inner fi bem la and written in bold in fi g. 8c, and the wavelength channel numbers used in the outer fi bem lb and written in the normal font may denote channels which have the same channel wavelength. In the same way as for the network according to fi g. 7b, the rings used for the bidirectional communication between each of the nodes shall be selected so that the maximum loss of traffic in the two rings is as small as possible.

The two add / drop filters in a node, which are used for normal traction and for traction triggered by the protection function in the networks shown in Figs. can be replaced with a single device.

Thus, in the block diagram Fig. 9a a node is shown comprising instead of two two-channel add / drop filter a 4-port add / drop filter 21 with reflecting Bragg grating 23 in the two parallel paths in a Mach-Zehnder interferometer 25. This kind of add / drop filters are sold by, for example, Arroyo Optics under the product name FiberFilter AD 200.

This type of Mach-Zehnder 21lter 21 with Bragg grid has four ports or tenninals, of which two ports 27, called express ports, are used to connect the combined filter in the main path and two ports 29, called add / drop ports , for connection to the internal drop filters (71, 72 in fi g. 3) in the node. As can be seen in fi g. 9a, the signals added / dropped in the left add / drop port are reflected to / from the part of the transmission fi bem connected to the left side of the node, and the signals added / dropped in it the right add / drop port, is re-rectified to / from the part of the transmission fi bern, which is connected to the right side of the node. All other wavelength channels except the two channels, which are added / drained, pass with little loss through the combined filter from one part of the transmission beam or main path to the other part.

Another simple device, which can replace the two add / drop filters in a node, which are used for normal traction and for traction triggered by the protection function or general traction in opposite directions from the node, is shown by the image in fi g. 9b. In the device 31, a Bragg grating 33 is connected between two optical circulators 35. The device has a function substantially similar to the function of the Mach-Zehnder filter shown in Fig. 9a with Bragg grating.

The one with reference to fi g. 3 can be modified to enable bidirectional communication in your channels, see Fig. 10. Several channels, eg channels no. 1 - 4 in node A and channels no. 5 - 8 in node B, are added to the traffic in trans. - mission children 1 by means of an additional channel add / drop filter, 561-3 or 5w1-8, and your channels, such as channels no. using the same channel add / drop filter. The channels of the anerchannel add / drop filter 531-8 or Swns, which may be an add / dropband filter, include the wavelengths of all the channels used, eg channels no. 1 - 8 in the example shown in the example. The wavelengths of the added channels can be separated from the wavelengths of the tapped channels, by, for example, allowing one group to be long wavelengths and the other to be short wavelengths. The inner single channel drop filter 72 in node A i 10 15 20 25 30 35 524 214 9 fi g. 3 is then generally replaced by a drop channel drop filter, such as a wavelength band splitter or a drop band filter 75-3 with a suitable bandwidth, so that in the example the drained channels include channels no. 5 - 8. Single channel drop filter 71 in node B in fi g. 3 is similarly replaced by a drop channel filter 714 as a drip band terlter with a band comprising the wavelengths of the channels Nos. 1-4.

Alternatively, the wavelengths of the channels added to a node (A or B in the figure) can be separated from the wavelengths of the lost channels by letting one group be channels with odd numbers and the other group be channels with even numbers. , when the numbering of the channels is performed in sequence for, for example, increasing wavelengths. Then use instead of the inner band split filters or drop filters 75-1; and 71.4 optical interleavers, which allow each other channel to pass and branch each other channel, for example, branch or deflect the channels Nos. 2, 4, 6, 8 in node A and branch the channels Nos. .

With the inner channel drop filter, ie the band split filter 75-1; or 714, alternatively the optical interleaver filter, is an optical multiplexer 1314 or 135-1; connected to receive and combine the added channels and an optical demultiplexer 155.1; or 151.4 for filtering out the individual drained channels. The optical multiplexers are thus connected to receive light from light sources such as transmitters 91, 92, for respective channels and to combine the light emitted by them to form a single combined signal. The optical demultiplexers are connected to transmit the demultiplexed light signals to light receivers 111, 112, for the respective channels.

A first advantage of it in fi g. The node structure shown is that fewer add- / dropband filter are needed in transmission fis for nodes with communication in many channels. This means a lower optical loss for the traffic in the transmission beam and thus a greater range for signals, which are propagated in the transmission beam.

A second advantage is that an optical interleaver kanlter can be used instead of a band splitter såsomlter such as intermediate or internal filters in the nodes. This enables greater channel separation in the multiplexers and demultiplexers used and thereby simpler devices at a lower cost.

Another to modify it with reference to fi g. The node structure described in Fig. 3 to enable bidirectional communication in more than one channel is shown by the one in fi g. 11 showed the image. This structure can be seen as an extension or generalization of it in fi g. 3 showed the node structure. In the i fi g. 11, two bidirectional channels are added / drained in the two-channel add / drop filters 5614 and 5w1.4 in the transmission brem 1, these add / drop filters thus branching out all the wavelengths used, as in the other node structures described above. Instead of single-channel drop filters inside the nodes, two-channel filters 71.2 and 73.4 are used, eg drop band filters, to drain two channels such as channels no. 1 and 2 and channels no. 3 and 4 for node A and 3, respectively. node B in the example shown in the figure. The two-channel drip filters are each connected to structures similar to the internal structure of those in fi g. 3, i.e. to structures with a single channel drop filter 7'1, 7 "3 connected to a light signal source 91, 93 and to a light receiver 112, 114 in node A in the example shown in the example .

The construction of the i fi g. 11 nodes can be easily extended to include more than two duplex channels.

The advantage of it in fi g. 11 shown is the node structure, as for those with reference to fi g. Described in the node constructions, that the node for which the communication requires W your WDM channels, introduces minimal loss in the transmission fi bern.

Yet another way to modify it with reference to fi g. The node structure described in Fig. 3 to enable bidirectional communication in more than one channel is shown by the image in fi g. 12. As in the construction according to fi g. Two bi-directional links are added / drained in the four-channel add / drop filters 5614 and 5w1.4 connected in the transmission brakes. The drip ports of the single-channel drip filters are connected to receivers 111, 113 for the drained channel.

The express ports of the drop filters are connected to other single channel drop filters 7 ';> _, 7'4, which have their add / drop ports connected to receivers 112, 114 for the channels which are drained in these filters.

The express ports of the other drop filters are connected to addlter 35 ("additional filter") or combiner, which receives light signals from light transmitters 93, 94 in node A and 91, 92 in node B.

In addition, the design of the fi g. 12, the nodes are easily extended to be used for bidirectional communication in more than two logical channels.

In all the examples described above, all wavelength channels that have not been used for bidirectional connections on a single äg path can be used for other bidirectional connections on a single erv path or for standard one-way WDM connections.

In all the cases described above, a WDM transmitter can be either a WDM transmitter such as a laser integrated with customer equipment or a part of a WDM transmitter end transponder, which receives its input signal optically from a customer equipment.

In all the cases described above, a WDM receiver can be either a receiver integrated with customer equipment or part of a WDM receiver end transponder, which receives an optical signal from the WDM network and retransmits it optically to a customer equipment.

Claims (20)

10 15 20 25 30 35 524 214 ll PATENT REQUIREMENTS 1. l. Optical add / drop node for bidirectional communication in a WDM optical network, characterized by two main ports for connection in a main path and a wavelength-selective add / drop filter connected between the two main ports, whereby add / drop the drip filter branches or deflects light arriving at the node on a first of the two main ports and has wavelengths in first WDM channels, to an inner first drip filter, the inner first drip filter being connected to branch or deflect light to at least one receiver and to receive light from at least one source and pass it to the add / drop filter to pass through it to the first of the two main ports to be emitted from there to the main path, when the node is connected in a WDM optical network. Optical add / drop node according to claim 1, characterized in that the inner first drop filter is arranged by branching or deflecting light with wavelengths in at least one second WDM channel, which is included in the first WDM channels, to said at least one receiver and to receive light with wavelengths in at least one third WDM channel, which is included in the first channels and is not included in the at least one second channel from said at least one source. Optical add / drop node according to one of Claims 1 to 2, characterized in that the add / drop filter comprises an add / drop band filter and / or that the inner first drop filter comprises a drop band filter. Optical add / drop node according to any one of claims 1 to 2, characterized in that the add / drop filter comprises an add / drop band filter with a first band and the inner first drop filter comprises a drop band filter with a second band. , the inner first drop filter deflecting light with wavelengths within the second band to said at least one receiver and from said at least one source receiving light with wavelengths outside the second band and within the first band. Optical add / drop node according to any one of claims 1 to 4, characterized in that said at least one source comprises a plurality of light sources connected to an optical multiplexer and said at least one receiver comprises a plurality of optical receivers connected to an optical demultiplexer. Optical add / drop node according to any one of claims 1 - 4, characterized in that said at least one source and / or said at least one receiver comprises an internal second drop filter connected to at least one receiver and to at least one source. Optical add / drop node according to one of Claims 1 to 6, characterized in that two add / drop band filters with the first band are connected between the two main ports, a first of the add / drop band filters branching or deflecting light which arrives at the node on a first of the two main ports and a second of the add / dropband fi filters branch or deflect light arriving at the node on a second of the two main ports. Optical add / drop node according to claim 7, characterized in that the two add / drop band filters are included in a single interferometric Mach-Zehnder filter comprising Bragg grating. Optical add / drop node according to claim 7, characterized in that the two add / drop band filters are included in a single device comprising a Bragg grating connected between two optical circuits. 10 15 20 25 30 35 524 214 12 A WDM optical network, characterized by a number of nodes, each comprising at least one add / drop node according to claim 1, connected by two or two parallel optical paths, wherein bidirectional communication channels are arranged between pairs of the nodes over links. , which are parts of one of the two or fl optical fi paths, the bidirectional communication channel between the nodes in each of said pairs being transported in two different WDM wavelength channels, a separate wavelength channel for each direction. WDM optical network according to claim 10, characterized in that the wavelengths of the two wavelength channels used for the bidirectional communication channel between the nodes in each of said pairs are added / drained in each node by means of a wavelength selective add - / drop fi lter. WDM optical network according to claim 11, characterized in that the wavelength selective add / drop filter is an add / drop band filter. WDM optical network according to any one of claims 10 to 12, characterized in that the optical fi pathways form closed rings and that the links are selected to be parts of suitable of the fiber paths, so that the highest loss of traction in the two or fl your rings will be as small as possible. WDM optical network according to any one of claims 10 to 13, characterized in that the number of nodes comprises a main node and a number of satellite nodes, wherein bi-directional communication channels are arranged between the main node and each of the satellite nodes in at least one of the two or fl are parallel optical fi pathways, the bidirectional communication channels between the main node and each of the satellite nodes being transmitted in two different WDM wavelength channels, the two different WDM wavelength channels used by a satellite node being separate from the two different WDM wavelength channels used by each other satellite node. WDM optical network according to claim 14, characterized in that the main node is connected to the two ends of each of the two or fl parallel optical fi paths, so that rings are formed, which may be interrupted at the main node, the satellite nodes being arranged for bidirectional communication with the main node in only one of the two or fl parallel optical fi paths and in each of the two complementary segments of said one of the optical fi paths, in which this äg path is divided by the satellite node and the main node. WDM optical network according to claim 15, characterized in that the satellite nodes are arranged to use for the bidirectional communication in a first of the segments the same two different WDM wavelength channels as for the bidirectional communication in a second of the segments. . WDM optical network according to any one of claims 10 to 16, characterized in that the two different WDM wavelength channels used by a first pair of nodes, which have bidirectional communication with each other in one of the two or two parallel optical fiber paths, are different from the two different WDM wavelength channels used by a second pair of nodes, which have bidirectional communication with each other in the same of the two or fl your parallel optical fi paths. WDM optical network according to any one of claims 10 to 17, characterized in that each of the two or fl parallel optical erv pathways is connected to form a closed ring, wherein a pair of nodes, arranged for bidirectional communication with each other in a first of the parallel fi pathways, are arranged for bidirectional communication in each of the two complementary segments of the first optical opt path, in which the path is divided by the node pair. 19. A WDM optical network according to claim 18, characterized in that the nodes in the pair are arranged to use the same two different WDM wavelength channels for the bidirectional communication in the two complementary segments. WDM optical network according to any one of claims 10 to 19, characterized in that the part of one of the parallel optical fi pathways used for the bidirectional communication channel between the nodes in a first pair of nodes has no part in common with the part of the same optical erv path used for the bidirectional communication channel between the nodes in a second node pair, the two different WDM wavelength channels used for the bidirectional communication channel between the nodes in the first node pair also being used for the bidirectional communication channel between the nodes in the other pair of nodes.