Abstract:
A reconfigurable directional optical system comprises N ports that can be configured individually as inputs or outputs. A system of this kind comprises at least one optical device having unidirectional inputs and outputs the total number of which is equal to or less than N, and an N×N optical switch such that each of the ports can be coupled interchangeably either to one of the inputs or to one of the outputs of the optical device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based on French Patent Application No. 01 12 596 filed Oct. 1, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. §119.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to the technology of transmission on optical fibers and more particularly to a reconfigurable directional optical system.  
           [0004]    2. Description of the Prior Art  
           [0005]    Using optical fibers for new communication networks has become the norm. Large quantities of optical fibers have already been installed, often, as shown in FIG. 1A, in the form of cables  110  combining a plurality of fibers  120 . The cables interconnect optical communication equipment units, for example an optical cross-connect (OXC) system  100  for distributing traffic between the fibers of three cables  110 ,  112 ,  114  in the particular example shown in FIG. 1A.  
           [0006]    A communication network based on optical fibers can include other optical devices. FIG. 1B shows a device that is often needed, namely an optical add/drop multiplexer (OADM)  130  for optically adding and dropping local traffic. In this case, the OADM drops the portion of the traffic incoming on one or more fibers  132  and intended for local use and adds traffic  136  generated locally. Thus the traffic outgoing on one or more fibers  138  comprises all the incoming traffic  132  less the traffic  134  dropped locally plus the traffic  136  generated locally.  
           [0007]    Another type of device used in optical networks, shown in FIG. 1C, is a light amplifier  140 , often of the erbium-doped fiber amplifier (EDFA) type, which amplifies the level of the incoming light signals  142 . If the latter must be propagated over great distances, the amplifier produces a level at the output  144  sufficient to process them locally or to relay them to a more distant destination.  
           [0008]    The reader cannot have failed to notice, on examining FIGS. 1A, 1B and  1 C, that the various optical devices shown always use the installed fibers in the same propagation direction. Thus, in FIG. 1A, the four fibers of the cable  114  on the left-hand side of the figure propagate light signals toward the right in the case of two of the fibers and toward the left in the case of the other two fibers. The situation is similar for the other two cables  110 ,  112 . Thus the resources in terms of fibers are allocated in a fixed manner, which has the consequence that the resources in one direction must be allocated for the peak traffic in that direction. Persons skilled in the art of communication networks are well aware that the traffic at a node of a network can be highly asymmetric, one direction having to carry a much greater quantity of information than the other. This is the case with video distribution networks in particular, which are beginning to be installed and in which the quantity of information to be distributed, in the form of pictures, is always infinitely greater than the quantity of information contained in initial requests received from users. Also, the asymmetry of the traffic can change with time, for example over a period of 24 hours. This is the case with international data communications, for example between Europe and North America, because of time differences.  
           [0009]    Thus the fixed allocation of resources in terms of optical fibers of a network presupposes that they be dimensioned for the peak traffic in both directions even though the traffic peaks may never occur simultaneously, with the result that some of the resources are always unused.  
           [0010]    This is why the object of the invention is to provide a directional optical system that is reconfigurable so that network resources in terms of optical fibers can be allocated dynamically.  
         SUMMARY OF THE INVENTION  
         [0011]    The invention therefore provides a reconfigurable directional optical system having N ports divided between a plurality of cables, each port being individually configurable as an input or an output and the system including:  
           [0012]    at least one optical device having unidirectional inputs and outputs whose total number is less than or equal to N, and  
           [0013]    an N×N optical switch such that each of the ports can be coupled interchangeably either to one of the inputs or to one of the outputs of the optical device, the N×N optical switch including first and second connection layers based on smaller n×p optical switches, where n and p are less than N,  
           [0014]    in which system the number of small switches of the first switching layer is equal to the number of cables and the number of small switches of the second switching layer is equal to whichever is the greater of the number of inputs and the number of outputs of the optical device, so that an input and an output of the optical device can be connected to two separate cables from the plurality of cables.  
           [0015]    The system according to the invention can change the propagation direction of light signals in a network of optical fibers to avoid having to dimension the network according to the peak traffic between two nodes of the network in both directions.  
           [0016]    The objects, aims, features and advantages of the invention will emerge more clearly from the detailed description of a preferred embodiment of the invention, illustrated by the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIGS. 1A, 1B and  1 C are examples of the prior art in which optical signals are always propagated in the same direction.  
         [0018]    [0018]FIG. 2 shows a general embodiment of a system according to the invention.  
         [0019]    [0019]FIG. 3 shows a particular embodiment of an optical system according to the invention.  
         [0020]    [0020]FIGS. 4A, 4B and  4 C show other particular embodiments of an optical system according to the invention simplified in various ways as a function of the optical devices used.  
         [0021]    [0021]FIG. 5 shows an embodiment of an optical system according to the invention using optical amplifiers. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    The general principle of the invention is described with reference to FIG. 2, which shows a set of optical fiber cables  200  in which the propagation direction of each individual fiber, for example the fiber  210 , is not allocated in a fixed manner, in contradistinction to the prior art previously discussed with reference to FIGS. 1A to  1 C. The optical fibers must access a device  230  consisting of a plurality of subdevices D 1 , D 2 , D 3  for processing optical signals that they transport. These devices include devices of the type shown in FIGS. 1A to  1 C, i.e. OXC, OADM and optical amplifiers. Of course, the above list is not limiting on the invention and refers only to standard optical devices to which the invention may be relevant. According to the invention, access to these devices is obtained via an N×N optical switch  260  which is capable of optically connecting any of N upper inputs/outputs  270  and any N lower inputs/outputs  210 . In this instance, N corresponds to the total number of inputs and outputs of the optical device  230 , of which there are 16 in this particular example, divided into seven outputs  240  and nine inputs  250  connected to the upper inputs/outputs  270  of the optical switch  260 . Various kinds of optical device  230  are available for processing information circulating on all the cables  200  and thus on at least N fibers, 16 fibers in this example being connected to the lower inputs/outputs  210  of the optical switch. Accordingly, the optical fibers can be distributed individually to make best use of the optical devices as a function of the traffic to be transported at a given time. This implies that the direction of circulation of the light signals  115  in the optical switch  260  is interchangeable between the top inputs/outputs  270  and the bottom inputs/outputs  210 . FIG. 2 is merely one particular example of the use of the optical switch  260 , of course.  
         [0023]    Based on the FIG. 2 general solution, FIG. 3 shows a more specific embodiment of the invention using smaller optical switches which therefore do not individually have the necessary  2 ×N inputs/outputs. The switch  260  in FIG. 2 can therefore be implemented in practice using two groups of smaller switches, for example. A first group comprises four 4×9 switches  224  constituting a first switching layer feeding a second layer comprising 4×2 switches  228  and 4×1 switches  227  (the 4×1 switches can in practice be 4×2 switches of which only one output is used). This provides the same functionality as FIG. 2, but with small switches. It will be noted in particular that the first layer of switches associates a switch with each cable, thus authorizing the switching of any fiber of a cable to any switch of the second layer. The latter associates a switch with each input/output pair of the directional device  230 . If there are more inputs than outputs, as is the case in the FIG. 3 example, in which there are nine inputs and seven outputs, the second layer must further associate a switch  227  with each supplementary input, of which there are two in this example. The converse is equally true. If the directional device  230  has more outputs than inputs, a switch must be associated with each supplementary output. Accordingly, each of these switches of the second layer can connect an input and an output separately to two separate cables via the switches of the first layer.  
         [0024]    The invention can be implemented in various ways without in any way departing from the spirit of the invention. In particular, simplifications can be made as a function of the optical devices that are accessed by the optical fibers. FIG. 4A shows a series of optical add and drop multiplexers (OADM)  300 ,  310  and  320  such as might be found in the successive nodes of a ring network, for example. In this case, as the loop comprises cables with four fibers  330 , one 4×4 optical switch  340  is sufficient, with the four 2×2 switches  350 , to rearrange the circulation direction of the fibers, whereas an 8×8 switch was necessary in the FIG. 2 general case and two 4×4 switches were necessary in FIG. 3. FIG. 4A is merely one example of the arrangements possible with this configuration, in which the optical devices are OADM or similar devices, in which example each 2×2 switch is connected on one side directly to an input and to an output of the optical devices  300 ,  310 ,  320  and on the other side to each of the two cables, directly in the case of one of them and via a 4×4 switching matrix  340  in the case of the other one.  
         [0025]    [0025]FIGS. 4B and 4C show other possible arrangements using the same result in terms of the number of elements necessary. FIG. 4B shows the example of an 8×8 switch  360  placed in front of the eight inputs of an optical device  370  of the FIG. 4A type while two cables each with eight fibers are connected to eight 2×2 optical switches similar to those of FIG. 4A. If this particular embodiment is compared to the general solution described with reference to FIG. 2, it will immediately be noted that a 16×16 switch would normally have had to be used. The embodiment shown in FIG. 4B can also be compared to that described with reference to FIG. 3 and which would have required the use of two 8×8 switches, whereas only one is necessary here. Also, it will be noted that, compared to FIG. 4A, it is merely a question of a different position of the switch  360 . Whereas in FIG. 4A the switch  340  is placed in front of the 2×2 switches  350 , in FIG. 4B it is placed between the 2×2 switches and the optical devices  370 .  
         [0026]    [0026]FIG. 4C shows another position for the 8×8 switch  360  from the previous figure, the switch here being placed at the output of the optical devices  370 .  
         [0027]    [0027]FIG. 5 shows another embodiment of the invention which achieves even greater simplification in the case of optical amplifier type devices  400 . In this case, the ports are divided between two cables and only 2×2 optical switches  410  are necessary to be able to use the fibers and the amplifiers in either direction. Also, each of the 2×2 switches is connected directly to an input and an output of the amplifier.  
         [0028]    [0028]FIG. 3, FIGS. 4A, 4B and  4 C, and FIG. 5 are intended to show that the invention can be implemented in many forms with particular advantages as a function of the optical devices used and the optical fiber cable configurations to be interconnected. The person skilled in the art will understand the advantages of using, in a particular context, one or the other of those forms of the invention.