Abstract:
A bidirectional optical communications system that is operable to dynamically allocate wavelengths for transmission in either direction in an optical fiber. The dynamic allocation is controlled by programmable optical devices. The programmable optical devices may be well known programmable devices such as wavelength selective switches and wavelength blockers or any other programmable optical device capable of dynamically allocating wavelengths between the two directions in the optical fiber. In addition, the programmable optical devices may be any combination of such wavelength selective switches, wavelength blockers or other programmable optical devices with other optical devices such as optical circulators, gain blocks, add/drop multiplexers, or fixed optical filters. Such a bidirectional optical communications system enables the dynamic allocation of bandwidth in an optical fiber without the need to replace components, such as fixed optical filters, and without disturbing communications on all the wavelengths transmitted in the optical fiber.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention is directed to an optical transmission system. More specifically, the present invention is directed to a bidirectional optical transmission system in which bandwidth can be dynamically allocated to transmit information in either direction in an optical fiber.  
         [0002]     It is known how to provide bidirectional optical communications in a single optical fiber. Today, single-fiber bidirectional optical transmission systems generally offer information transfer capacity wherein the wavelengths used to carry information in a given direction through the fiber are fixed and symmetric. That is, half the wavelength channels are permanently assigned, or fixed, to carry information in one direction through the fiber, and half the wavelength channels are permanently assigned to carry information in the other direction through the fiber.  
         [0003]     An example of one such known configuration is illustrated in  FIG. 1 . As shown, a bidirectional optical fiber  11  is coupled to a bidirectional optical fiber  12  through a bidirectional optical amplifier  13 . Bidirectional optical fibers  11  and  12  have properties such that they can transmit, or carry, wavelengths λ 1 . . . n  in both the East and West directions. Bidirectional optical amplifier  13  contains optical circulators  14  and  18 , a 2×1 fixed optical filter  15 , a 1×2 fixed optical filter  17 , and a gain block  16 , all of which are well known in the art. Optical circulators  14  and  18  operate such that light input at port  1  is output at port  2 , and light input at port  2  is output at port  3 . 2×1 Fixed optical filter  15  has the property that only light at wavelengths λ 1 . . . n/2  input at port  1  will be output at port  3 , and only light at wavelengths λ (n+1)/2 . . . n  input at port  2  will be output at port  3 . 1×2 Fixed optical filter  17  has the property that light at wavelengths λ 1 . . . n/2  input at port  1  will be output at port  2 , and light at wavelengths λ (n+1)/2 . . . n  input at port  1  will be out put at port  3 . Through such a configuration of fixed optical filters  15  and  17 , one-half of the wavelengths, λ 1 . . . n/2, are fixed to transmit information in an East direction through bidirectional optical fibers  11  and  12 , and the other half of the wavelengths, λ (n+1)/2 . . . n,  are fixed to transmit information in a West direction through bidirectional optical fibers  11  and  12 .  
         [0004]     The bidirectional optical system of  FIG. 1  is very efficient when the traffic demands on bidirectional optical fibers  11  and  12  are symmetric between the East and West directions. However, in systems in which the traffic demands are asymmetric, the configuration in  FIG. 1  is not efficient. Asymmetric traffic flow could cause the system of  FIG. 1  to not have enough wavelengths to carry information in the busy direction while, at the same time, have unused wavelengths allocated to the other direction. In addition, the configuration of  FIG. 1  cannot adapt to changing traffic demands. That is, since it is fixed, it can not re-allocate the available wavelengths λ 1 . . . n  such that unused wavelengths allocated to one direction are reallocated to carry information in the other direction. Essentially, the only way to achieve a reallocation in the configuration of  FIG. 1  would be to change the configuration altogether. That is, fixed optical filters  15  and  17  would have to be replaced with a new set of fixed optical filters each time a reallocation is desired. Replacing fixed optical filters  15  and  17  each time a reallocation is needed would not only be costly but would disturb or shut down the entire system (i.e. communications on every wavelength λ 1-n ) every time the filters were replaced.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The present invention provides a bidirectional optical communications system that enables the allocation and reallocation of available wavelengths between the East and West directions in an optical fiber without the need for replacing optical components, such as fixed optical filters, and without the need to disturb or shut down communications on the fiber. This is accomplished by using programmable optical devices that are operable to dynamically allocate wavelengths between the East and West directions in the bidirectional optical fiber, as needed. As used herein, the term dynamic allocation refers to the ability to allocate and reallocate wavelengths between the East and West directions in a bidirectional optical fiber without the need to replace optical components, such as fixed optical filters, and without the need to disrupt communications on the wavelengths that are not being reallocated. As used herein, the term programmable optical component refers to subsystems which can be controlled from a signal sent from outside the optical component. For example, the signal can be an electrical digital signal generated from a computer.  
         [0006]     In accordance with the invention, the programmable optical devices may be well known programmable devices such wavelength selective switches and wavelength blockers or any other programmable optical device capable of dynamically allocating wavelengths between the two directions in a bidirectional optical fiber. In addition, the programmable optical devices in accordance with the invention may be any combination of such wavelength selective switches, wavelength blockers, or other programmable optical devices with any other optical devices such as broadband three-port routing elements (including but not limited to optical circulators), gain blocks, add/drop multiplexers, or fixed optical filters.  
         [0007]     In accordance with an embodiment of the invention, two programmable optical devices, a 1×2 wavelength selective switch and a 2×1 wavelength selective switch, are used in combination with two optical circulators and a gain block to provide the dynamic allocation of wavelengths between the East and West directions in a bidirectional optical fiber.  
         [0008]     In another embodiment of the invention, two programmable wavelength blockers are combined with two optical circulators and two gain blocks to form a programmable optical component that is operable to provide dynamic allocation of wavelengths between the East and West directions in a bidirectional optical fiber.  
         [0009]     In yet another embodiment of the invention, a programmable optical component composed of two 2×2 wavelength selective switches, two optical circulators, and one gain block, is combined with a mux/demux structure and a transmitter and receiver bank. The mux/demux structure and the transmitter and receiver bank operate as an add/drop multiplexer that enables wavelengths to be dynamically added to and removed from the bidirectional optical fiber as traffic needs change. Thus, such an embodiment not only provides the ability to dynamically allocate wavelengths between East and West in a bidirectional optical fiber, but it also provides the ability to dynamically add and remove wavelengths as needed.  
         [0010]     These and other advantages of the invention will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows a prior art configuration of an optical communications system including a bidirectional optical amplifier providing a fixed, symmetric allocation of bandwidth over a bidirectional optical fiber.  
         [0012]      FIG. 2  illustrates an embodiment of the present invention.  
         [0013]      FIG. 3  illustrates another embodiment of the present invention.  
         [0014]      FIG. 4  illustrates yet another embodiment of the present invention.  
         [0015]      FIG. 5  illustrates yet another embodiment of the present invention.  
         [0016]      FIG. 6  illustrates yet another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]      FIG. 2  shows an embodiment of a bidirectional communications system  20  in accordance with the present invention. As shown, a programmable optical component  23  is coupled to a bidirectional optical fiber  21  and a bidirectional optical fiber  22 . Bidirectional optical fiber  21  is operable to transmit a set of available wavelengths λ 1 . . . n  to and from a West direction, and bidirectional optical fiber  22  is operable to transmit a set of available wavelengths λ 1 . . . n  to and from an East direction. Programmable optical component  23  has an optical circulator  24 , a 2×1 wavelength selective switch  25 , a gain block  26 , a 1×2 wavelength selective switch  27 , and an optical circulator  28 , all known devices in the art. Gain block  26  may be an erbium doped fiber amplifier or any other amplifier known in the art.  
         [0018]     Wavelengths traveling from West to East, λ East,  on bidirectional optical fiber  21  will enter programmable optical component  23  and be input to port  2  of optical circulator  24 . Optical circulator  24  will output the wavelengths λ East  at port  3  for input to port  1  of 2×1 wavelength selective switch  25 . 2×1 wavelength selective switch  25  is programmed to initially forward wavelengths λ East  input at port  1  to its output port  3  and block any other wavelengths. However, as will be discussed below, 2×1 wavelength selective switch  25  is programmed to dynamically change the set of wavelengths that will be forwarded from its input port  1  to its output port  3 . Thus, initially, 2×1 wavelength selective switch  25  will output wavelengths λ East  through port  3  to gain block  26 . Gain block  26  will amplify wavelengths λ East  and input them to port  1  of 1×2 wavelength selective switch  27 . 1×2 wavelength selective switch  27  is programmed to initially forward wavelengths λ East  (and only wavelengths λ East ) to output port  2 . However, as will be discussed below, 1×2 wavelength selective switch  27  is programmed to dynamically change the set of wavelengths that will be allowed to pass from port  1  to port  2 . Thus, initially, 1×2 wavelength selective switch  27  will forward wavelengths λ East  to circulator  28  which, in turn, will forward wavelengths λ East  through to bidirectional optical fiber  22  for transmission in the East direction.  
         [0019]     Wavelengths traveling from East to West, λ west,  on bidirectional optical fiber  22  will enter programmable optical component  23  and be input to port  2  of optical circulator  28 . Optical circulator  28  will forward wavelengths λ west  through its port  3  to input port  2  of 2×1 wavelength selective switch  25 . 2×1 wavelength selective switch  25  is programmed to initially forward wavelengths λ west  from input port  2  to output port  3  and block any other wavelengths. However, as will be discussed below, 2×1 wavelength selective switch  25  is programmed to dynamically change the set of wavelengths that will be allowed to pass from port  2  to port  3 . Thus, initially, wavelengths λ west  will pass through gain block  26  to input port  1  of 1×2 wavelength selective switch  27 . 1×2 wavelength selective switch  27  is programmed to initially pass wavelengths λ west  input at port  1  to output port  3  (Note: In the preferred embodiment, only wavelengths λ west  are output to port  3 , and wavelengths which are not included in either λ East  or λ west  are blocked). However, as will be discussed below, 1×2 wavelength selective switch  27  is programmed to dynamically change the set of wavelengths that will be allowed to pass from port  1  to port  3 . Thus, initially, 1×2 wavelength selective switch  27  will pass wavelengths λ west  to port  1  of optical circulator  24  which, in turn, will pass wavelengths λ west  to bidirectional optical fiber  21  for transmission in the West direction.  
         [0020]     As mentioned above, 1×2 wavelength selective switch  25  and 2×1 wavelength selective switch  27  are programmed to change the set of wavelengths that are allowed to pass from their input port(s) to their output port(s). It is well known in the art how to program 2×1 wavelength selective switches and 1×2 wavelength selective switch to change the set of wavelengths that are allowed to pass. For example, assuming that initially λ East  is composed of a first set of wavelengths λ 1 . . . n/2  and wavelengths λ west  is composed of a second set of wavelengths λ (n+1)/2 . . . n,  it is well known in the art how to program 2×1 wavelength selective switch  25  and 1×2 wavelength selective switch  27  to dynamically change the wavelengths in the first and second set. That is, wavelength selective switches  25  and  27  can be programmed to dynamically reallocate wavelengths from the first set of wavelengths to the second set of wavelengths, and vice versa. In this way, optical communications system  20  may dynamically change the set of wavelengths allocated for transmission in the East and West directions of bidirectional optical fibers  21  and  22  without affecting communications on wavelengths that are not being reallocated and without the need to replace fixed optical filters.  
         [0021]     Wavelength selective switches are preferably designed to be fully flexible, i.e. any wavelength can be routed from any input port to any output port, regardless of the routing of other wavelengths. However, less flexible devices may be used. Such less flexible wavelength selective switches exist which can dynamically change how wavelengths are routed, subject to constraints—e.g. all wavelengths that are output from the switch must be adjacent to one another (from a continuous band within the optical spectrum). Such wavelength selective switches may be less expensive than a fully flexible wavelength selective switch but still provide the ability to dynamically reallocate wavelengths between the two directions in an optical fiber.  
         [0022]      FIG. 3  shows another embodiment of an optical communications system in accordance with the present invention. As shown, optical communications system  30  has a programmable optical component  33  coupled to a bidirectional optical fiber  31  and a bidirectional optical fiber  32 . Bidirectional optical fiber  31  is operable to transmit a set of available wavelengths λ 1 . . . n  to and from a West direction, and bidirectional optical fiber  32  is operable to transmit a set of available wavelengths λ 1 . . . n  to and from an East direction. Programmable optical component  33  has optical circulators  34  and  37 , gain blocks  35  and  38 , and wavelength blockers  36  and  39 . Optical circulators  34  and  37 , and wavelength blockers  36  and  39  are well known devices in the art. Gain blocks  35  and  38  may be erbium doped fiber amplifiers or any other amplifiers known in the art.  
         [0023]     Wavelengths traveling from West to East, λ East,  on bidirectional optical fiber  31  will enter programmable optical component  33  and be input to port  2  of optical circulator  34 . Optical circulator  34  will pass wavelengths λ East  through port  3  to gain block  35 . Gain block  35  will amplify wavelengths λ East  and input them to wavelength blocker  36 . Initially, wavelength blocker  36  is programmed to pass wavelengths λ East  to optical circulator  37  and block any other wavelengths. However, as will be discussed below, wavelength blocker  36  is programmed to dynamically change the set of wavelengths that will be allowed to pass to optical circulator  37 . Thus, initially, wavelength blocker  36  will output wavelengths λ East  to optical circulator  37  which, in turn, will forward wavelengths λ East  to bidirectional optical fiber  32  for transmission in the East direction.  
         [0024]     Wavelengths traveling from East to West, λ west,  on bidirectional optical fiber  32  will enter programmable optical component  33  and be input to port  2  of optical circulator  37 . Optical circulator  37  will forward wavelengths λ west  through its port  3  to gain block  38 . Gain block  38  will amplify wavelengths λ East  and input them to wavelength blocker  39 . Initially, wavelength blocker  39  is programmed to forward wavelengths λ west  to optical circulator  34  and block any other wavelengths. However, as will be discussed below, wavelength blocker  39  is programmed to dynamically change the set of wavelengths that will be allowed to pass to optical circulator  34 . Thus, initially, wavelengths λ west  will pass through wavelength blocker  39  to optical circulator  34  which, in turn, will pass wavelengths λ west  to bidirectional optical fiber  31  for transmission in the West direction.  
         [0025]     As mentioned above, wavelength blockers  36  and  39  are programmed to change the set of wavelengths that are allowed to pass from their input ports to their output ports. It is well known in the art how to program wavelength blockers to change the set of wavelengths that are allowed to pass. For example, assuming that initially λ East  is composed of a first set of wavelengths λ 1 . . . n/2  and wavelengths λ west  is composed of a second set of wavelengths λ (n+1)/2 . . . n,  it is well known in the art how to program wavelength blockers  36  and  39  to change the wavelengths in the first and second set. Thus, through programming, wavelength blockers  36  and  39  are operable to dynamically reallocate wavelengths from the first set of wavelengths to the second set of wavelengths, and vice versa. In this way, optical communications system  30  may dynamically change the set of wavelengths allocated for transmission in the East and West direction of bidirectional optical fibers  31  and  32  without affecting communications on wavelengths that are not being reallocated and without the need to replace fixed optical filters.  
         [0026]     Another embodiment of the present invention is shown in  FIG. 4 . As shown, optical communications system  40  has a programmable optical component  43  coupled to a bidirectional optical fiber  41 , a bidirectional optical fiber  42 , and an add/drop assembly  65 . Bidirectional optical fiber  41  is operable to transmit a set of available wavelengths λ 1 . . . n  to and from a West direction, and bidirectional optical fiber  42  is operable to transmit a set of available wavelengths λ 1 . . . n  to and from an East direction. Add/drop assembly  65  is composed of a mux/demux structure  49  connected to a transmitter and receiver bank  50 , both of which are known in the art. Programmable optical component  43  has two optical circulators  44  and  48  and two 2×2 wavelength selective switches  45  and  47 , all of which are known in the art, and a gain block  46 . Gain block  46  may be an erbium doped fiber amplifier or any other amplifier known in the art.  
         [0027]     Wavelengths traveling from West to East on optical fiber  41  λ East-in,  in bidirectional optical fiber  41  will enter programmable optical component  43  and be input to port  2  of optical circulator  44 . Optical circulator  44  will output the wavelengths λ East-in  at port  3  for input to port  1  of 2×2 wavelength selective switch  45 . λ East-in  consists of λ East-drop  and λ East-express,  where λ East-drop  includes all wavelengths in λ East-in  carrying signals destined for transmitter and receiver bank  50 , and λ East-express  are the wavelengths in λ East-in  that should be forwarded to optical fiber  42  for transmission further along optical transmission system  40 . Initially, 2×2 wavelength selective switch  45  is programmed to forward wavelengths λ East-express  from its input port  1  to its output port  3 , and to forward wavelengths λ East-drop  to output port  4 , so that these wavelengths can be processed by the add/drop assembly  65 . However, as will be discussed below, 2×2 wavelength selective switch  45  is programmed to dynamically change the set of wavelengths that will be forwarded from its inputs port  1  to its output ports  3  and  4 . Thus, initially, 2×2 wavelength selective switch  45  will output wavelengths λ East-express  through port  3  to gain block  46 , and output wavelengths λ East-drop  through its output port  4  to add/drop assembly  65 .  
         [0028]     Wavelengths traveling from East to West, λ west-in,  on bidirectional optical fiber  42  will be input to programmable optical device  43  and passed through optical circulator  48  to input port  2  of 2×2 wavelength selective switch  45 . λ West-in  consists of λ West-drop  and λ West-express,  where λ west-drop  includes all wavelengths in λ West-in  carrying signals destined for add/drop assembly  65 , and λ West-express  are the wavelengths in λ West-in  that should be forwarded to optical fiber  41  for transmission further along optical transmission system  40  Initially, 2×2 wavelength selective switch  45  is programmed to forward wavelengths λ west-express  from input port  2  to output port  3  and to forward wavelengths λ west-drop  to output port  4 , so that these wavelengths can be processed by the add/drop assembly  65 . However, as will be discussed below, 2×2 wavelength selective switch  45  is programmed to dynamically change the set of wavelengths that will be forwarded from its input port  2  to its output ports  3  and  4 .  
         [0029]     Thus, initially, 2×2 wavelength selective switch  45  will forward wavelengths λ East-express  input at port  1  and wavelengths λ west-express  input at port  2  to output port  3 . Wavelengths λ East-express  and λ west-express  will then be amplified in gain block  46  and passed to input port  1  of 2×2 wavelength selective switch  47 . Signals to be added to optical transmission system  40  will be generated within transmitter and receiver bank  50  at wavelengths λ East-add  and λ west-add.  These wavelengths are passed by mux/demux structure  49  to port  2  of wavelength selective switch  47 . Initially, 2×2 wavelength selective switch  47  is programmed to forward wavelengths λ East-express  input at its input port  1  and λ East-add  input at its input port  2  to its output port  3 , and forward wavelengths λ West-express  input at its input port  1  and λ West-add  input at its input port  2  to its output port  4 . However, as will be discussed below, 2×2 wavelength selective switch  47  is programmed to dynamically change the set of wavelengths that will be allowed to pass from its input port  1  to it output ports  3  and  4 . Thus, initially, 2×2 wavelength selective switch  47  will forward wavelengths λ East-express  input at port  1  and λ East-add  input at its input port  2  through output port  3  to optical circulator  48  which will forward wavelengths λ East-express  and λ East-add  through its port  2  to bidirectional optical fiber  42  for transmission in the East direction. And, it will forward wavelengths λ west-express  and λ west-add  through output port  4  to optical circulator  44  which will pass wavelengths λ west-express  and λ west-add  to bidirectional optical fiber  41  for transmission in the West direction.  
         [0030]     Wavelengths that are part of λ East-drop  can also be assigned to λ East-add.  However they cannot be also be assigned to λ West-add.  In general, wavelength sets λ East-in,  λ West-express  and λ West-add  must all be disjoint, as must, λ West-in,  λ East-express  and λ East-add  In addition, for the embodiment shown in  FIG. 4 , wavelength set λ East-drop  must be disjoint from λ West-drop  and wavelength set λ East-add  must be disjoint from λ West-add.    
         [0031]     As mentioned above, 2×2 wavelength selective switches  45  and  47  are programmed to dynamically change the set of wavelength that are allowed to pass from their input ports to their output ports. It is well known in the art how to program 2×2 wavelength selective switches to change the set of wavelengths that are allowed to pass. For example, assuming that initially λ East-express  is composed of a first set of wavelengths λ 1 . . . n/2  and wavelengths λ west-express  is composed of a second set of wavelengths λ (n+1)/2 . . . n,  it is well known in the art how to program 2×2 wavelength selective switches  45  and  47  to dynamically change the wavelengths in the first and second set. That is, the wavelength selective switches can be programmed to dynamically reallocate wavelengths from the first set of wavelengths to the second set of wavelengths, and vice versa. In this way, optical communications system  40  may dynamically change the set of wavelengths allocated for transmission in the East and West directions of bidirectional optical fibers  41  and  42  without affecting communications on wavelengths that are not being reallocated and without the need to replace fixed optical filters. Similarly, wavelength selective switches  45  and  47  are programmed to dynamically add wavelengths to and remove wavelengths from the sets λ East-drop  λ East-add.  λ West-drop  and λ West-add.    
         [0032]      FIG. 5  shows another embodiment of a bidirectional communications system in accordance with the present invention. As shown, bidirectional optical communications system  51  has a programmable optical component  54  coupled to a bidirectional optical fiber  52  and a bidirectional optical fiber  53 . Bidirectional optical fiber  52  is operable to transmit a set of available wavelengths λ 1 . . . n  to and from a West direction, and bidirectional optical fiber  53  is operable to transmit a set of available wavelengths λ 1 . . . n  to and from an East direction. Programmable optical component  54  has optical circulators  55  and  59 , 1×2 wavelength selective switches  57  and  61 , and optical couplers  58  and  62 , and gain blocks  56  and  60 , all of which are known in the art. Gain blocks  56  and  60  may be an erbium doped fiber amplifier or any other amplifier known in the art.  
         [0033]     Wavelengths traveling from West to East, λ East-in,  on bidirectional optical fiber  52  will enter programmable optical component  54  and be input to port  2  of optical circulator  55 . Optical circulator  55  will output the wavelengths λ East-in  at port  3  for input to gain block  56 . Gain block  56  will amplify wavelengths λ East-in  and pass them to 1×2 wavelength selective switch  57 . Initially, 1×2 wavelength selective switch  57  is programmed to forward wavelengths λ East-express.  input at port  1  to its output port  3  and send wavelengths λ East-drop  through output port  3 . However, as will be discussed below, 1×2 wavelength selective switch  57  is programmed to dynamically change the set of wavelengths that will be forwarded from its input port  1  to its output port  2 , and the set of wavelengths that will be dropped through port  3 . Thus, initially, 1×2 wavelength selective switch  57  will output wavelengths λ East-express  to optical circulator  59  which, in turn, will forward wavelengths λ East-express  to bidirectional optical fiber  53  for transmission in the East direction. Optical coupler  58  may add wavelengths λ East-add  to wavelengths λ East-express,  as needed.  
         [0034]     Wavelengths traveling from East to West, λ West-in,  on bidirectional optical fiber  53  will enter programmable optical component  54  and be input to port  2  of optical circulator  59 . Optical circulator  59  will forward wavelengths λ West-in  through its port  3  to gain block  60 . Gain block  60  will amplify wavelengths λ west  and pass them to 1×2 wavelengths selective switch  61 . Initially, 1×2 wavelength selective switch  61  is programmed to forward wavelengths λ west-express  to output port  2  and drop selected wavelengths λ west-drop  though port  3 . However, as will be discussed below, 1×2 wavelength selective switch  61  is programmed to dynamically change the set of wavelengths that will be allowed to pass from port  1  to port  2 , and the set of wavelengths dropped through port  3 . Thus, initially, wavelengths λ west-express  will pass through 1×2 wavelength selective switch  61  to optical circulator  55 . Optical circulator  55  will pass wavelengths λ west-express  to bidirectional optical fiber  52  for transmission in the West direction. Wavelengths λ west-add  may be added to wavelengths λ west-express  through optical coupler  62 , as needed.  
         [0035]     As mentioned above, 1×2 wavelength selective switches  57  and  61  are programmed to change the set of wavelengths that are allowed to pass from their input ports to their output ports. It is well known in the art how to program 1×2 wavelength selective switches to change the set of wavelengths that are allowed to pass. For example, assuming that initially λ East-in  is composed of a first set of wavelengths λ 1 . . . n/2  and wavelengths the union of λ west-express  and λ west-add  is composed of a second set of wavelengths λ (n+1)/2 . . . n,  it is well known in the art how to program a 1×2 wavelength selective switches  57  and  61  to dynamically change the wavelengths in the first and second set. That is, the wavelength selective switches can be programmed to dynamically reallocate wavelengths from the first set of wavelengths to the second set of wavelengths, and vice-versa. Wavelengths can also be reassigned from λ East-expess  to λ East-add  and/or λ East-drop  and vice-versa. In this way, optical communications system  51  may dynamically change the set of wavelengths allocated for transmission in the East and West directions of bidirectional optical fibers  52  without affecting communications on wavelengths that are not being reallocated and without the need to replace fixed optical filters. Similarly, the wavelengths in sets λ West-in,  λ East-express  and λ East-add  can be reassigned.  
         [0036]      FIG. 6  shows another embodiment of an optical communications system in accordance with the present invention. As shown, optical communications system  70  has a programmable optical component  73  coupled to a bidirectional optical fiber  71  and a bidirectional optical fiber  72 . Bidirectional optical fiber  71  is operable to transmit a set of available wavelengths λ 1 . . . n  to and from a West direction, and bidirectional optical fiber  32  is operable to transmit a set of available wavelengths λ 1 . . . n  to and from an East direction. Programmable optical component  33  has optical circulators  74  and  79 , gain blocks  75  and  80 , wavelength blockers  77  and  87 , optical couplers  76 ,  78 ,  81 , and  83 ., Optical circulators  74  and  79 , wavelength blockers  77  and  87 , optical couplers  76 ,  78 ,  81 , and  83  are known devices in the art. Gain blocks  75  and  80  may be erbium doped fiber amplifiers or any other amplifiers known in the art.  
         [0037]     Wavelengths traveling from West to East, λ East-in,  on bidirectional optical fiber  71  will enter programmable optical component  73  and be input to port  2  of optical circulator  74 . Optical circulator  74  will output the wavelengths λ East-in  at port  3  for input to gain block  75 . Gain block  75  will amplify wavelengths λ East-in  and input them through coupler  76 , to wavelength blocker  77 . Wavelength blocker  77  is programmed to initially pass wavelengths λ East-express  through coupler  78  to optical circulator  79  and block any other wavelengths. However, as will be discussed below, wavelength blocker  77  is programmed to dynamically change the set of wavelengths that will be allowed to pass to optical circulator  79 . Thus, initially, wavelength blocker  77  will output wavelengths λ East-express  through coupler  78  to optical circulator  79  which, in turn, will forward wavelengths λ East-express  to bidirectional optical fiber  72  for transmission in the East direction. Selected wavelengths may be added to wavelengths λ East-express  through optical coupler  78 . Optical coupler  76  diverts some power from every wavelength in λ East-in.  This allows the wavelengths intended for drop at this location to be selected and received  
         [0038]     Wavelengths traveling from East to West, λ West-in,  on bidirectional optical fiber  72  will enter programmable optical component  73  and be input to port  2  of optical circulator  79 . Optical circulator  79  will forward wavelengths λ West-in  through its port  3  to gain block  80 . Gain block  80  will amplify wavelengths λ West-in  and input them through optical coupler  81  to wavelength blocker  87 . Wavelength blocker  87  is programmed to initially forward wavelengths λ west-express  through optical coupler  83  to optical circulator  74  and block all other wavelengths. However, as will be discussed below, wavelength blocker  87  is programmed to dynamically change the set of wavelengths that will be allowed to pass through coupler  83  to optical circulator  74 . Thus, initially, wavelengths λ West-express  will pass through wavelength blocker  87 , optical coupler  83  to optical circulator  74  which, in turn, will pass wavelengths λ west-express  to bidirectional optical fiber  71  for transmission in the West direction. Wavelengths λ west-add,  which must be a disjoint set of wavelengths from λ west-express,  and λ East-in,  can be added to optical transmission system  70  through the add port of optical coupler  83 .  
         [0039]     As mentioned above, wavelength blockers  77  and  87  are programmed to change the set of wavelengths that are allowed to pass from their input ports to their output ports. It is well known in the art how to program wavelength blockers to change the set of wavelengths that are allowed to pass. For example, assuming that initially λ East-in  is composed of a first set of wavelengths λ 1 . . . n/2  and the union of wavelengths λ west-add,  and λ west-express  is composed of a second set of wavelengths λ (n+1)/2 . . . n,  it is well known in the art how to program wavelength blockers  77  and  87  to change the wavelengths in the first and second set. Thus, through programming, wavelength blockers  77  and  87  are operable to dynamically reallocate wavelengths from the first set of wavelengths to the second set of wavelengths, and vice verse. In this way, optical communications system  70  may dynamically change the set of wavelengths allocated for transmission in the East and West direction of bidirectional optical fibers  71  and  72  without affecting communications on wavelengths that are not being reallocated and without the need to replace fixed optical filters.  
         [0040]     The embodiments described in  FIG. 5  and  FIG. 6  have the additional advantage over the embodiment of  FIG. 4  in that they are not subject to the limitation that wavelength set λ East-drop  must be disjoint from λ West-drop  and wavelength set λ East-add  must be disjoint from λ West-add.  The embodiment described in  FIG. 4  has the additional advantage over the embodiments of  FIG. 5  and  FIG. 6  that it allows a single transmitter and receiver bank to serve wavelengths coming from/going to both the East-facing and the West-facing fibers.  
         [0041]     To one skilled in the art, it will be clear that some or all of the gain blocks shown in  FIGS. 1-6  could be replaced by other types of unidirectional, multiwavelength optical signal processing elements known in the art. In particular, gain blocks might be replaced by, or might include, elements for chromatic dispersion compensation, gain equalization, wavelength conversion, or all-optical regeneration, or other optical signal processing functions. Similarly, the optical circulators shown in  FIGS. 1-6  could be replaced by optical couplers or other broadband, three-port routing elements known in the art.  
         [0042]     The foregoing Detailed Description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Detailed Description, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention. Those skilled in the art could implement various other feature combinations without departing from the scope and spirit of the invention.