Abstract:
An optical fiber network comprises a plurality of communications nodes. Each node is able to communicate utilizing multiplexed optical signals comprising a plurality of optical wavelength components. A plurality of optical fiber links interconnects the nodes. A dynamic balancer is inserted into each corresponding optical fiber links. Each dynamic balancer adjusts the intensities of a plurality of optical wavelength components in multiplexed optical signals transmitted over the optical fiber link.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention pertains to communications systems utilizing optical fiber, in general, and to apparatus and methods for the dynamic balancing of multiple optical wavelengths in such systems, in particular.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical fiber communications networks to date have been configured as point-to-point networks with single point-to-point routing between geographic locations. As the optical fiber network evolves, it is highly likely that point-to-point networks will become more of a mesh type configuration with multiple paths possible between end points. In such mesh networks, multiple optical communications paths interconnect each network node. In a wavelength-multiplexed infrastructure, that utilizes a mesh type network, the paths that the different optical wavelengths traverse may vary by significant amounts. The length of a path has an effect on the power level or intensity of optical signals. The effect of such different path lengths is a lack of uniformity in the power or intensity of multiplexed wavelengths at various nodes in the network.  
           [0003]    It is desirable that the multiplexed wavelengths at each node in the network be at the same level or intensity. The present invention provides a method and arrangement for the dynamic balancing or equalization of power for each optical wavelength in a wavelength multiplexed system.  
         SUMMARY OF THE INVENTION  
         [0004]    In accordance with the invention, an optical fiber network comprises a plurality of communications nodes. Each node is able to communicate utilizing multiplexed optical signals comprising a plurality of optical wavelength components. A plurality of optical fiber links interconnects the nodes. A dynamic balancer is inserted into each corresponding optical fiber links. Each dynamic balancer adjusts the intensities of a plurality of optical wavelength components in multiplexed optical signals transmitted over the optical fiber link.  
           [0005]    In accordance with one aspect of the invention each dynamic balancer adjusts the intensities to be substantially the same level. Each dynamic balancer comprises a control loop for each optical wavelength component. The control loop is used to adjust the intensities.  
           [0006]    In accordance with another aspect of the invention, each dynamic balancer comprises a single micro controller utilized for all control loops.  
           [0007]    In accordance with another aspect of the invention, each dynamic balancer comprises a plurality of variable optical attenuators, each of which is operable to adjust the intensity of one corresponding optical wavelength component. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]    The invention will be better understood from a reading of the following detailed description in conjunction with the drawing figures in which like reference indicators are used to identify like elements, and in which:  
         [0009]    [0009]FIG. 1 is a network diagram of a mesh network to which the invention is advantageously applied:  
         [0010]    [0010]FIG. 2 is a wavelength intensity diagram;  
         [0011]    [0011]FIG. 3 is a second wavelength intensity diagram;  
         [0012]    [0012]FIG. 4 is a wavelength intensity diagram illustrating the intensity of wavelengths at a network node in accordance with the principles of the invention:  
         [0013]    [0013]FIG. 5 is dynamic equalizer in accordance with the principles of the invention;  
         [0014]    [0014]FIG. 6 is a diagram of a variable optical reflector/attenuator in accordance with the principles of the invention;  
         [0015]    [0015]FIG. 7 is a diagram of a second variable optical reflector isolator in accordance with the principles of the invention;  
         [0016]    [0016]FIG. 8 is a cross-section of a non-reciprocal phase shifter in accordance with the invention; and  
         [0017]    [0017]FIG. 9 is a cross-section of a second non-reciprocal phase shifter in accordance with the principles of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]    [0018]FIG. 1 illustrates a mesh network  100  illustrative of the present invention. Mesh network  100  includes a plurality of communications nodes  101 , 102  . . .  109 . Each node  101 , 102  . . .  109  is typically at a different geographic location, however, as those skilled in the art will appreciate, nodes may be collocated in the same geographic location and at the same or different physical locations. However, in the illustrative embodiment of the invention, mesh network  100  represents a portion of a national network for the United States. Considering a typical situation, each node might represent a different switching or routing office or a different city. Node  101  may, for example, be located in San Francisco, Calif.; node  102  may be located in Phoenix, Ariz.; node  108  may be located in New York, N.Y.; and node  109  may be located in Atlanta, Ga. In the optical communications system utilizing the mesh network  100 , communications occurs over multiple optical wavelengths that are multiplexed. Fiber optic links  121 ,  122 ,  123 , . . . ,  135  link the various nodes  101 , 102  . . .  109  to form mesh network  100 . Each link may include multiple optical fibers.  
         [0019]    Multiplexing of optical wavelengths is known and various arrangements are available in the prior art to provide multiplexed optical wavelength communications. In the example shown in FIG. 1, only four multiplexed wavelengths λ 1 , λ 2 , λ 3 , λ 4  are shown for purposes of clarity. Each wavelength is represented by arrows to indicate the routing of that wavelength component in mesh network  100 . However, the invention is applicable to network arrangements in which any numbers of optical wavelengths are multiplexed together. The particular problem to which the present invention provides a solution is illustrated in FIG. 1. Two wavelength components λ 1 , λ 2  carry information from node  101 . Wavelength λ 1  carries information for node  108 , and wavelength λ 2  carries information for node  109 . Similarly, two wavelengths λ 3 , λ 4  carry information from node  102 . Wavelength λ 3  carries information for node  108 , and wavelength λ 4  carries information for node  109 . Thus wavelengths λ 1 , λ 3  carry information for node  108  and wavelengths λ 2 , λ 4  carry information for node  109 . Wavelength λ 1  travels a network route from node  101  to node  104  via link  121 , from node  104  to node  105  via link  130 , from node  105  to node  106  via link  131 , and from node  106  to node  108  via node  123 . Wavelength λ 3  travels a network route from node  102  to node  103  via link  118 , from node  103  to node  105  via link  129 , from node  105  to node  106  via link  131 , from link  106  to node  108  via link  123 . Wavelength λ 2  travels a network route from node  101  to node  104  via link  121 , from node  104  to node  106  via link  106 , from node  106  to node  107  via link  133 , and from node  107  to node  109  via link  135 . Wavelength λ 4  travels a network route from node  102  to node  103  via link  118 , from node  103  to node  107  via link  132 , and from node  107  to node  109  via link  135 . Because the network path lengths over the links from node  101  to node  108  and over the links from node  102  to node  108  are different, the power levels or intensities of wavelength component λ 1  and wavelength component λ 3  at node  108  will be different. This is shown in graphical form in FIG. 2, which illustrates the intensity of wavelengths received at node  108 . Similarly, the power or intensity levels between wavelength λ 2  and wavelength λ 4  received at node  109  are different because of the differences in path lengths for the different wavelength components as illustrated in FIG. 3. It is undesirable to have wavelength components of different intensities. In accordance with the principles of the invention, the intensity of all wavelength components received at a node are adjusted to be at the same intensity level as illustrated in FIG. 4.  
         [0020]    Turning back to FIG. 1, in accordance with one aspect of the present invention, an optical communications network that utilizes multiplexing of wavelength component signals and provides a plurality of paths interconnecting a plurality of nodes includes dynamic equalizers disposed in the different network path segments to provide for dynamic balancing of wavelength components. In FIG. 1 four dynamic equalizers  400  are shown. It will be understood by those skilled in the art that every network path segment or link may include one or more dynamic equalizers  400  to provide for appropriate balancing of wavelength component intensity. For clarity, dynamic equalizers  400  are also identified as dynamic equalizers E 1 , E 2 , E 3 , E 4 . Dynamic equalizers  400  each provide an adjustment to the wavelength component signals to equalize the path lengths in the network for all communications paths. Thus, equalizer E 1  operates on wavelength λ 3  from nodes  102 ,  103 . Equalizer E 2  operates on wavelengths λ 1 , λ 3 . Equalizer E 3  operates on wavelengths λ 2 , λ 4 . Equalizer E 4  operates on wavelengths λ 4  from nodes  102 ,  103 . Each dynamic equalizer E 1 , E 2 , E 3 , E 4  is identical and includes two ports  401 ,  403  that are coupled into the network links or paths. The wavelength intensity plot shown in FIG. 2  represents the input to equalizer E 2 . The wavelength intensity plot shown in FIG. 3 represents the input to equalizer E 3 . The output wavelength intensity plot for both equalizers E 2 , E 3  is shown in FIG. 4. As can be seen from FIG. 4, equalizers E 2 , E 3  operate on the input wavelength signal components to balance or equalize all the wavelength components to the same level of intensity.  
         [0021]    Turning now to FIG. 5, a dynamic balancing equalizer  400  is shown. Dynamic balancing equalizer  400  has ports  401 ,  403  and includes a circulator  410 , a multiplexer/de-multiplexer  420 , a plurality of variable optical reflector/attenuators  430 , a plurality of detectors  440  and a micro-controller  450 . Circulator  410  has ports  411 ,  413 ,  415 . Arrow  417  indicates the circulation direction. Port  401  receives unbalanced input wavelength component signals from the mesh network. Port  413  is coupled to multiplexer/de-multiplexer  420  and port  403  is coupled into mesh network  100  and provides balanced wavelength signal components back into mesh network  100 . Multiplexer/de-multiplexer  420  are a bi-directional unit that can couple or de-multiplex each one of n wavelength components to a corresponding one of a plurality of variable optical reflector/ attenuator units  430 . In addition, reflected signals from each of the corresponding variable optical reflector/attenuator units  430  are coupled or multiplexed by multiplexer/de-multiplexer  420  back to port  413  of circulator  410 . Circulator  410  couples the wavelength component signals from multiplexer/de-multiplexer  420  to port  403 .  
         [0022]    Each variable optical reflector/attenuator units  430  is coupled to a corresponding one detector  440 . Each detector  440  is utilized to provide a signal representative of the intensity of the particular wavelength component that its corresponding variable optical reflector/attenuator  430  receives. Each detector  440  couples signals representative of the detected wavelength intensity to a micro-controller  450 . Micro-controller  450  utilizes the signals to determine the amount of adjustment to each variable optical reflector/attenuator  430  necessary to cause all wavelength component outputs at output port  415  to be the same. Detectors  440  are part of an intensity control loop that includes micro-controller  450  to determine the intensities of output wavelength components. The structure shown and described in FIG. 5 is unidirectional in that wavelength components flow only in a direction of from port  401  to port  403 .  
         [0023]    In another embodiment of the invention, circulator  410  is replaced by the equivalent of a bidirectional circulator that allows wavelength component signals at port  401  to circulate to port  413  and reflected signals to circulate from port  413  to port  403  and further allows wavelength signal components receive at port  403  to circulate to port  413 , and reflected signals to circulate form port  413  to port  401 . As will be appreciated by those skilled in the art, such a bi-directional circulator may be implemented easily with a pair of conventional unidirectional circulators and isolators. Such a device is referred to herein as a bi-directional circulator.  
         [0024]    Detectors  440  may be detectors of a type known in the art. Micro-controller  450  may be a micro-controller of a type known in the art. Micro-controller  450  utilizes signals from each detector  440  to generate control signals at output  451  to control the intensity of each corresponding wavelength component to a desired predetermined level. Micro-controller  450  may utilize any of the known methods of correlating input signals to adjustment signals for each wavelength. The known methods include, but are not limited to table look up methods and algorithm based methods. In any event, micro-controller  450  is part of n control loops for each of the corresponding n wavelengths to adjust the intensity of each on a dynamic basis to produce the desired predetermined output level.  
         [0025]    Variable optical attenuator/reflector  430  of the invention is configured similarly to a Sagnac Interferometer. As shown in FIG. 6, variable optical attenuator/reflector  430  includes a non-reciprocal phase shifter  511  disposed in an optical fiber loop  501 . A coupler  503  having ports  502 ,  504 ,  506 ,  508  is utilized. Coupler  503  is a 50/50 coupler. Input wavelength component signals received at port  502  are attenuated by a desired amount and reflected back out to port  502 . Portions of the wavelength components are outputted at port  504 . When utilized as a variable attenuator/reflector, port  504  may not be used. Non-reciprocal phase shifter  511  creates a +Φ phase shift for light propagating in a clockwise direction in loop  501  and a −Φ phase shift for counter-clockwise propagating light. The reflection rate depends on the power ratio between Ithru and Iin, which in turn depends on the Φ phase shift produced by NRPS  51 . For a phase shift of Φ=0°, Ithru=0% and Iref=100%. For a phase shift of Φ=45°, Ithru =50% and Iref=50%. For a phase shift of Φ=90°, Ithru=100% and Iref=0%. Varying the control signal at input  613  varies the phase shift angle Φ and accordingly varies the amount of light reflected back to the input port. Non-reciprocal phase shifter  511  thus controls the intensity of the output at through port  502 .  
         [0026]    [0026]FIG. 7 illustrates a modification to the arrangement of FIG. 6 to provide monitoring capability for the coupling of optical signals to detectors. In the arrangement of FIG. 7, a second coupler  811  is utilized to provide a tap for monitor signals. Coupler  811 , extracts a small amount of light, typically 1% to 5%. The extracted light is coupled to a corresponding detector  440 . In all other respects, operation of the arrangement of FIG. 7 is like that of the arrangement of FIG. 6.  
         [0027]    [0027]FIG. 8 illustrates a non-reciprocal phase shifter (NRPS)  511  in accordance with the invention. NRPS  511  is a hermetically sealed unit that includes tubular aluminum housing  901  that has a plurality of heat radiating fins  903  disposed on its outer surface. An inner support sleeve or tube  905  is positioned concentric with housing  901 . Tube  905  is also of aluminum in the illustrative embodiment. Support washers  907 ,  909 ,  911 , support tube  905  within housing  901 . Disposed within tube  905  are two magneto-optic Faraday rotation devices that are thin film Bismuth Iron Garnet (BIG) crystals  913 ,  915  Optical signals are coupled to and from the non-reciprocal phase shifter  511  via optical waveguides  921 ,  923 , which in the particular embodiment shown are optical fiber. However, in other embodiments, one or both of the waveguides  921 ,  923  may be waveguides formed on a substrate and the non-reciprocal phase shifter may be formed on the substrate also as an integrated optic device. Optical fiber  921  extends through a housing cap washer  925  to couple to collimator  929 . Epoxy  931  is used to bond fiber  921  in place. Similarly, optical fiber  923  extends through housing cap washer  927  to couple to collimator  933 . Epoxy  935  is used to bond fiber  923  in place. Boots  937 ,  939  are positioned on each housing cap washer  925 ,  927 , respectively to support fibers  921 ,  923 .  
         [0028]    A ring shaped permanent magnet  941  is positioned concentric with crystal  913 . An electromagnet  943  is disposed proximate crystal  915 . A wire coil forms electromagnet  943 .  
         [0029]    In operation, crystal  915  is fixed at a predetermined rotation angle and crystal  913  is switched from a second predetermined rotation angle to a third predetermined rotation angle to provide for switching of NRPS  511 . In the illustrative embodiment of the invention, permanent magnet  941  biases crystal  915  to either +45 degrees or −45 degrees of rotation. The current supplied to electromagnet  943  varies so as to vary the magnetic flux produced and to change its magnetic polarity to vary the Faraday rotation in crystal  113  between +45 degrees and −45 degrees. The combined result is that varying current to electromagnet  943  produces a 0 to 90 degree phase shift by NRPS  511 .  
         [0030]    The non-reciprocal phase shifter  511  of FIG. 8 is simply assembled, with construction similar to that of optical isolators. Advantageously, non-reciprocal phase shifter  511  provides low insertion loss of 1 dB or less, low cost and small size. More specifically the device of FIG. 1 is 48 mm in length and has an outside diameter of 10 mm without fins  903 . With elliptical fins  903 , the outside diameter is 28 mm×16 mm.  
         [0031]    [0031]FIG. 9 illustrates a second non-reciprocal phase shifter  511   a  in accordance with the principles of the invention. Non-reciprocal phase shifter  511   a  differs in operation from non-reciprocal phase shifter  511  in that it utilizes a pair of permanent magnets in place of the electromagnet of the structure of FIG. 8.  
         [0032]    NRPS  511   a  is a hermetically sealed unit that includes tubular aluminum housing  201 . Because no heat generating components are included in NRPS  511   a , heat-dissipating fins are not needed. An inner support sleeve or tube  205  is positioned concentric with housing  201 . Tube  205  is also of aluminum in the illustrative embodiment. Support washers  107 ,  109  support tube  105  within housing  101 . Disposed within tube  105  are two magneto-optic Faraday rotation devices, i.e., thin film BIG crystals  213 ,  215 . Crystal  215  is supported at one end of tube  205 , and crystal  213  is disposed within tube  205 . Optical signals are coupled to and from the non-reciprocal phase shifter  200  via optical waveguides  221 ,  223 , which, in the particular embodiment shown, are optical fiber. In other embodiments, one or both of the waveguides  221 ,  223  may be waveguides formed on a substrate and the non-reciprocal phase shifter may be formed on the substrate also as an integrated optic device. Optical fiber  221  extends through a housing cap washer  225  to couple to collimator  229 . Epoxy  231  is used to bond fiber  221  in place. Similarly, optical fiber  223  extends through housing cap washer  227  to couple to collimator  233 . Epoxy  235  is used to bond fiber  223  in place. Boots  237 ,  239  are positioned on each housing cap washer  225 ,  227 , respectively to support fibers  221 ,  223 .  
         [0033]    A ring shaped permanent magnet  241  is positioned concentric with crystal  215 . A pair of ring shaped magnets  255 ,  257  is positioned on and longitudinally movable on tube  205 . Magnets  255 ,  257  produce the same magnetic flux density, but are aligned to be of opposite magnetic polarity. Magnets  255 , 257  are movable from the position shown in FIG. 2 where magnet  255  is concentric with crystal  255  to a second position where Magnet  257  is concentric with crystal  213 , and back to the first position. In the first position, magnet  255  causes crystal  213  to produce a first predetermined Faraday rotation in one direction. In the second position, magnet  257  causes crystal  213  to produce a second predetermined Faraday rotation in the opposite direction Movement of the pair of magnets  255 ,  257  to positions intermediate the first and second positions produces Faraday rotations in between the first and second predetermined Faraday rotations. The advantage to this arrangement is that magnets  255 ,  257  may be moved by mechanical means such as pressurized air or vacuum in ports  261 ,  263  that are provided in housing  201 . The magnet positions are stable in all positions and accordingly, the magnets will; latch in any of the positions intermediate the first and second positions. Advantageously, no continuous energy must be expended to maintain the magnets  255 , 257  in any position.  
         [0034]    In operation, crystal  215  is fixed at a predetermined Faraday rotation angle. Crystal  213  is varied from a second predetermined Faraday rotation angle to a third predetermined Faraday rotation angle to provide for varying the non-reciprocal phase shift of NRPS  511   a . In the illustrative embodiment of the invention, permanent magnet  241  biases crystal  915  to either +45 degrees or −45 degrees of rotation Magnets  955 ,  257  are movable to vary the magnetic field at crystal  913  between two predetermined rotation angles of +45 degrees and 45 degrees. The combined result is that movement of magnets  255 ,  257  produces a cumulative phase shift in non-reciprocal phase shifter  511   a  that may be varied from 0 degrees to 90 degrees. Non-reciprocal phase shifter  511   a  is latchable.  
         [0035]    Non-reciprocal phase shifter  511   a  of FIG. 9 is also simply assembled, with construction similar to that of optical isolators. Advantageously, non-reciprocal phase shifter  200  provides low insertion loss of 1 dB or less, low cost and small size.  
         [0036]    As will be appreciated by those skilled in the art, various modifications can be made to the embodiments shown in the various drawing figures and described above without departing from the spirit or scope of the invention In addition, reference is made to various directions in the above description. It will be understood that the directional orientations are with reference to the particular drawing layout and are not intended to be limiting or restrictive. It is not intended that the invention be limited to the illustrative embodiments shown and described. It is intended that the invention be limited in scope only by the claims appended hereto.