Abstract:
A bridge and roll method for doing an in-service switch (less than 50 ms) from one set of inputs and outputs on a primary photonic switch to another set of inputs and outputs on the same or a secondary photonic switch. The bridge and roll method facilitates maintenance, repairs and upgrades on the primary photonic switch. The bridge and roll method may be reversed in order to restore the original inputs and outputs on the primary photonic switch after maintenance, repairs or upgrades have been performed. The invention also provides a method for consolidating multiple wavelengths or multiple bands into single fibers.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to photonic switch systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    The demand for high-speed communication networks has increased dramatically over the last few years. In many situations, communication networks are implemented with electrical interconnections. As desired levels of bandwidth and transmission speed for communication networks increase, it will become more difficult for electrical interconnections to satisfy these levels.  
           [0003]    Optical fiber offers a solution to the difficulties affecting conventional electrical interconnections. For example, optical fiber is less susceptible to inductive or capacitive coupling effects than are electrical interconnections. Optical fiber also offers increased bandwidth and substantial avoidance of electromagnetic interference. The advantages of optical fiber become more important as the transmission rates increase.  
           [0004]    Many communications networks feature hybrid, optical-electrical semiconductor circuits that employ photodetectors, electrical switches, optical modulators and/or lasers. To handle greater data traffic, an alternative approach uses a photonic switch system, which performs switching operations of light pulses or photons (referred to generally as “light signals”) without the need for converting and re-converting signals between the optical domain to the electrical domain.  
           [0005]    However, conventional photonic switches are still subject to a wide variety of disadvantages. Traditional photonic switch systems may not offer any protective features against equipment failures, specifically failures within the photonic switch itself. Moreover, traditional photonic switches do not provide in service, near hitless, bridge and roll capabilities. Bridge and roll capabilities would be useful for switching from a primary photonic switch to a secondary photonic switch to facilitate maintenance, repair and upgrades on the primary photonic switch.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the present invention there is provided a photonic switch system comprising a first photonic switch having a plurality of inputs and outputs, a second photonic switch having a plurality of inputs and outputs, a plurality of head-end modules each having an input port for receiving incoming light signals, a first output port and a second output port, the first output port being connectable to a respective input of the first photonic switch and the second output port being connectable to a respective input of the second photonic switch, a plurality of tail-end modules each having an output port for transmitting outgoing light signals, a first input port and a second input port, the first input port being connectable to a respective output of the first photonic switch and the second output port being connectable to a respective output of the second photonic switch.  
           [0007]    There is further provided a method of performing a bridge and roll operation on incoming optical signals in a photonic switch system having a first photonic switch connected such that an optical path is established only though the first photonic switch, and having a plurality of head-end and tail-end modules and a second photonic switch the method comprising the steps of: connecting a second output port of each module of the plurality of head-end modules to a respective input of the second photonic switch and connecting a second input port of each of the plurality of the tail-end modules to a respective output of the second photonic switch; re-configuring the head-end modules and the tail-end modules so that the incoming light signals that are received on the input ports of the head-end modules are re-routed through the second photonic switch to the output ports of the tail-end modules; and disconnecting a first output port of each head-end module from the first photonic switch and disconnecting a first input port of each tail-end module from the first photonic switch.  
           [0008]    The bridge and roll method facilitates protection, maintenance, repair and upgrade of the primary photonic switch.  
           [0009]    Advantageously, the bridge and roll method may be reversed in order to restore the original inputs and outputs on the primary photonic switch after protection, maintenance, repair or upgrade has been performed.  
           [0010]    The present invention also provides a method for consolidating multiple wavelengths or multiple bands of wavelengths into single fibers.  
           [0011]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of the specific embodiments of the invention in conjunction with the accompanying figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a block diagram of a photonic switch system in accordance with a first embodiment of the present invention;  
         [0013]    [0013]FIGS. 2A, 2B,  2 C,  2 D and  2 E are block diagrams of alternative embodiments of the head-end modules of FIG. 1;  
         [0014]    [0014]FIGS. 3A, 3B,  3 C,  3 D,  3 E and  3 F are block diagrams of alternative embodiments of the tail-end modules of FIG. 1;  
         [0015]    [0015]FIGS. 4, 5, and  6  are block diagrams illustrating a bridge and roll operation of the photonic switch system of FIG. 1;  
         [0016]    [0016]FIG. 7A is a flowchart of the method of performing the bridge and roll operation illustrated in FIGS. 4,5 and  6 ;  
         [0017]    [0017]FIG. 7B is a flowchart of the reverse execution of the method of FIG. 7A;  
         [0018]    [0018]FIGS. 8, 9,  10  and  11  are block diagrams of an alternative embodiment of the present invention;  
         [0019]    [0019]FIG. 12 is a flowchart of the method of performing the bridge and roll operation illustrated in FIGS.  8 , 9 , 10  and  11 ; and  
         [0020]    [0020]FIGS. 13 and 14 are block diagrams of a further embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    Referring to FIG. 1, there is shown a photonic switch system  100  comprising a plurality of photonic switches shown for convenience as a primary photonic switch  102  having a plurality of inputs  102 A, 102 B and a plurality of outputs  102 C, 102 D, and a secondary photonic switch  104  having a plurality of inputs  104 A, 104 B and a plurality of outputs  104 C, 104 D. It should be appreciated that the primary photonic switch  102  is configured to selectively connect any of its inputs to any of its outputs in known fashion and the secondary photonic switch  104  is similarly configured to connect corresponding inputs to corresponding outputs. The photonic switch system  100  also comprises a plurality of head-end modules  106  each having an input port  106 A, a first output port  106 B and a second output port  106 C, and a plurality of tail-end modules  108  each having an output port  108 A, a first input port  108 B and a second input port  108 C.  
         [0022]    The first output port  106 B of each head-end module  106  is connected  114  to a respective input  102 A, 102 B of the primary photonic switch  102  and the first input port  108 B of each tail-end module  108  is connected  116  to a respective output  102 C, 102 D of the primary photonic switch  102 . The number of inputs on the primary photonic switch  102  and the secondary photonic switch  104  may be greater than the number of head-end modules  106 . The number of outputs on the primary photonic switch  102  and the secondary photonic switch  104  may be greater than the number of tail-end modules  108 . The input port  106 A of each head-end module  106  is connectable to a respective input optical fiber  110 . The output port  108 A of each tail-end module  108  is connectable to a respective output optical fiber  112 .  
         [0023]    It should be noted that even though the photonic switch system  100  of FIG. 1 is shown configured with a primary photonic switch  102  and a secondary photonic switch  104 , it should be understood that the photonic switch system  100  may be configured having one or more primary photonic switches and having one or more secondary photonic switches (that is, an M:N system with M primary photonic switches and N secondary photonic switches where M and N are whole numbers) and still be within the scope of this invention. Furthermore, the photonic switch system  100  may be configured wherein the primary photonic switch  102  and secondary photonic switch  104  are integrated into a single unit. That is, a subset of a single photonic switch (not shown) functions as the primary photonic switch  102  and another subset of the single photonic switch functions as the secondary photonic switch  104 .  
         [0024]    Although FIG. 1 and other drawings show light signals flowing in only one direction (left to right) for clarity, it should be appreciated that normally an equivalent photonic switch system (not shown) is provided to carry lights signals flowing in the reverse direction (right to left).  
         [0025]    Referring to FIG. 2A, any one of the head-end modules  106  may comprise, for example, a 1:2 splitter  202  having an input  202 A, a first output  202 B and a second output  202 C. The input  202 A of the 1:2 splitter  202  is connected to the input port  106 A of the head-end module  106 . The first output  202 B of the 1:2 splitter  202  is connected to the first output port  106 B of the head-end module  106 . The second output  202 C of the 1:2 splitter  202  is connected to the second output port  106 C of the head-end module  106 .  
         [0026]    Alternatively, referring to FIG. 2B, any one of the head-end modules  106  may comprise a 1×2 switch  204  having and an input  204 A, a first output  204 B and a second output  204 C. The input  204 A of the 1×2 switch  204  is connected to the input port  106 A of the head-end module  106 . The first output  204 B of the 1×2 switch  204  is connected to the first output port  106 B of the head-end module  106 . The second output  204 C of the 1×2 switch  204  is connected to the second output port  106 C of the head-end module  106 .  
         [0027]    Alternatively, referring to FIG. 2C, any one of the head-end modules  106  may comprise a 1:2 splitter  206  having an input  206 A, a first output  206 B and a second output  206 C, a first variable optical attenuator (VOA)  208  and a second VOA  210 —each VOA  208 , 210  having an input  208 A, 210 A and an output  208 B, 210 B. The input  206 A of the 1:2 splitter  206  is connected to the input port  106 A of the head-end module  106 . The first output  206 B of the 1:2 splitter  206  is connected the input  208 A of the first VOA  208 . The output  208 B of the first VOA  208  is connected to the first output port  106 B of the head-end module  106 . The second output  206 C of the 1:2 splitter  206  is connected to the input  210 A of the second VOA  210 . The output  210 B of the second VOA  210  is connected to the second output port  106 C of the head-end module  106 .  
         [0028]    Alternatively, referring to FIG. 2D, any one of the head-end modules  106  may comprise a 1:2 splitter  212  having an input  212 A, a first output  212 B and a second output  212 C, a first shutter  214  and a second shutter  216 —each shutter  214 , 216  having an input  214 A, 216 A and an output  214 B, 216 B. The topology of this alternative embodiment is the same as the embodiment of FIG. 2C except that each VOA  208 , 210  is replaced with a shutter  214 , 216 .  
         [0029]    Alternatively, referring to FIG. 2E, any one of the head-end modules  106  may comprise a 1×2 switch  218  having an input  218 A, a first output  218 B and a second output  218 C, a first VOA  220  and a second VOA  222 —each VOA  220 , 222  having an input  220 A, 222 A and an output  220 B, 222 B. The topology of this alternative embodiment is the same as the embodiment of FIG. 2C except that the splitter  206  is replaced with the 1×2 switch  218 .  
         [0030]    Referring to FIG. 3A, any one of the tail-end modules  108  may comprise a 2:1 combiner  302  having an output  302 A, a first input  302 B and a second input  302 C. The output of the 2:1 combiner  302  is connected to the output port  108 A of the tail-end module  108 . The first input  302 B of the 2:1 combiner  302  is connected to the first input port  108 B of the tail-end module  108 . The second output  302 C of the 2:1 combiner  302  is connected to the second input port  108 C of the tail-end module  108 .  
         [0031]    Alternatively, referring to FIG. 3B, any one of the tail-end modules  108  may comprise a 2×1 switch  304  having an output  304 A, a first input  304 B and a second input  304 C. The topology of this alternative embodiment is the same as the embodiment of FIG. 3A except that the combiner  302  is replaced with the 2×1 switch  304 .  
         [0032]    Alternatively, referring to FIG. 3C, any one of the tail-end modules  108  may comprise, for example, a 2:1 combiner  306  having an output  306 A, a first input  306 B and a second input  306 C, a first VOA  308  and a second VOA  310 —each VOA  308 , 310  having an input  308 A, 310 A and an output  308 B, 310 B. The output  306 A of the 2:1 combiner  306  is connected to the output port  108 A of the tail-end module  108 . The first input  306 B of the 2:1 combiner  306  is connected to the output  308 B of the first VOA  308 . The input  308 A of the first VOA  308  is connected to the first input port  108 B of the tail-end module  108 . The second input  306 C of the 2:1 combiner  306  is connected to the output  310 B of the second VOA  310 . The input  310 A of the second VOA  310  is connected to the second input port  108 C of the tail-end module  108 .  
         [0033]    Alternatively, referring to FIG. 3D, any one of the tail-end modules  108  may comprise a 2:1 combiner  312  having an output  312 A, a first input  312 B and a second input  312 C, a first shutter  314  and a second shutter  316 —each shutter  314 , 316  having an input  314 A, 316 A and an output  314 B, 316 B. The topology of this alternative embodiment is the same as the embodiment of FIG. 3C except that each VOA  308 , 310  is replaced with a shutter  314 , 316 .  
         [0034]    Alternatively, referring to FIG. 3E, any one of the tail-end modules  108  may comprise a 2×1 switch  318  having an output  318 A, a first input  318 B and a second input  318 C, a first VOA  320  and a second VOA  322 —each VOA  320 , 322  having an input  320 A, 322 A and an output  320 B, 322 B. The topology of this alternative embodiment is the same as the embodiment of FIG. 3C except that the combiner  306  is replaced with the 2×1 switch  318 .  
         [0035]    Alternatively, referring to FIG. 3F, any one of the tail-end modules  108  may comprise, for example, a 2×2 switch  324  having a first input  324 A, a second input  324 B, a first output  324 C and a second output  324 D, and a performance monitor  326  having an input  326 A. The first input  324 A of the 2×2 switch  324  is connected to the first input port  108 B of the tail-end module  108 . The second input  324 B of the 2×2 switch  324  is connected to the second input port  108 C of the tail-end module  108 . The first output  324 C of the 2×2 switch  324  is connected to the input  326 A of the performance monitor  326 . The second output  324 D of the 2×2 switch  324  is connected to the output port  108 A of the tail-end module  108 .  
         [0036]    The preferred combinations of head-end modules  106  and tail-end modules  108  are: the head-end module of FIG. 2A and the tail-end module of FIG. 3B; the head-end module of FIG. 2A and the tail-end module of FIG. 3C; the head-end module of FIG. 2A and the tail-end module of FIG. 3D; the head-end module of FIG. 2B and the tail-end module of FIG. 3A; the head-end module of FIG. 2B and the tail-end module of FIG. 3; the head-end module of FIG. 2C and the tail-end module of FIG. 3A; the head-end module of FIG. 2D and the tail-end module of FIG. 3A; the head-end module of FIG. 2C and the tail-end module of FIG. 3B; the head-end module of FIG. 2D and the tail-end module of FIG. 3B; the head-end module of FIG. 2B and the tail-end module of FIG. 3C; the head-end module of FIG. 2E and the tail-end module of FIG. 3A; the head-end module of FIG. 2A and the tail-end module of FIG. 3F; the head-end module of FIG. 2B and the tail-end module of FIG. 3D; and the head-end module of FIG. 2C and the tail-end module of FIG. 3C. It should be understood that other combinations of head-end modules  106  and tail-end modules  108  may be used and still fall within the scope of the invention.  
         [0037]    Referring to FIG. 1, in operation, the input optical fibers  110  carry light signals that may be, for example, single wavelengths, a band of different wavelengths or a plurality of bands of different wavelengths or any combination thereof. The light signals on the input optical fibers  110  are routed to the output optical fibers  112  via a primary optical path  122 . The primary optical path  122  is from the input ports  106 A of the head-end modules  106 , through the first output ports  106 B of the head-end modules  106 , through the connections  114 , through the primary photonic switch  102 , through the connections  116 , through the first input ports  108 B of the tail-end modules  108  to the output ports  108 A of the tail-end modules  108 . The paths of the light signals through the head-end modules  106  and the tail-end modules  108  are indicated by dotted lines on each module.  
         [0038]    A bridge and roll technique for substituting the second photonic switch  104  for the first photonic switch  102  will now be described with reference to FIGS. 1, 4,  5 ,  6  and the flowchart of FIG. 7A.  
         [0039]    Referring first to FIG. 7A in combination with FIGS. 1 and 4, the second output ports  106 C of the head-end modules  106  are connected  118  to respective inputs  104 A, 104 B of the secondary photonic switch  104 . The second input ports  108 C of the tail-end modules  108  are connected  120  to respective outputs  104 C, 104 D of the secondary photonic switch  104  (Step  702 ). This step  702  is known as bridging.  
         [0040]    Referring next to FIG. 7A and FIG. 5, the head-end modules  106  are re-configured so that the light signals on the input optical fibers  110  are re-routed from the primary optical path  122  to a secondary optical path  124  (Step  704 ). This step  704  is known as rolling. It is preferably executed in a time frame that causes only a small (less than 50 ms) interruption to the light signals. The secondary optical path  124  is from the input ports  106 A of the head-end modules  106 , through the second output ports  106 C of the head-end modules  106 , through the connections  118 , through the secondary photonic switch  104 , through the connections  120 , through the second input ports  108 C of the tail-end modules  108  to the output ports  108 A of the tail-end modules  108 .  
         [0041]    It should be appreciated that the connections  118 , 120  made in the bridging step  702  may be made at any time before the subsequent rolling step  704  such as during the manufacture of the photonic switch system  100  or after installation of the photonic switch system  100  on a customer&#39;s premises.  
         [0042]    In an embodiment where the combination of the head-end module of FIG. 2A and the tail-end module of FIG. 3B is used, the step  704  of rolling is accomplished by re-configuring the 2×1 switch  304  so that the light signals at the output  304 A of the 2×1 switch  304  are derived from the light signals at the second input  304 C instead of the first input  304 B. In this embodiment both photonic switches  102 , 104  receive incoming signals which is useful for monitoring performance of the photonic switches  102 , 104 .  
         [0043]    In an embodiment where the combination of the head-end module of FIG. 2A and the tail-end module of FIG. 3C is used, the step  704  of rolling is accomplished by re-configuring the first VOA  308  and the second VOA  310  so that the first VOA  308  has substantially higher attenuation than the second VOA  310  instead of the second VOA  310  having substantially higher attenuation than the first VOA  308 . In this embodiment the VOAs  308 , 310  also enable nodal output level compensation.  
         [0044]    In an embodiment where the combination of the head-end module of FIG. 2A and the tail-end module of FIG. 3D is used, the step  704  of rolling is accomplished by re-configuring the first shutter  314  and the second shutter  316  so that the first shutter  314  transmits substantially no light signals from the input  314 A of the first shutter  314  to the output  314 B of the first shutter  314  and the second shutter  316  transmits substantially all of the light signals from the input  316 A of the second shutter  316  to the output  316 B of the second shutter instead of the first shutter  314  transmitting substantially all of the light signals from the input  314 A of the first shutter  314  to the output  314 B of the first shutter  314  and the second shutter  316  transmitting substantially none of the light signals from the input  316 A of the second shutter  316  to the output  316 B of the second shutter. The shutters  314 , 316  are less expensive than the VOAs  308 , 310  of the previous embodiment and have lower loss and higher reliability.  
         [0045]    In an embodiment where the combination of the head-end module of FIG. 2B and the tail-end module of FIG. 3A is used, the step  704  of rolling is accomplished by re-configuring the 1×2 switch  204  so that the light signals at the input  204 A of the 1×2 switch  204  are routed to the second output  204 C instead of the first input  204 B. This embodiment has the advantage over the previous three embodiments in that it may reduce back-reflections from a fixed termination.  
         [0046]    In an embodiment where the combination of the head-end module of FIG. 2B and the tail-end module of FIG. 3B is used, the step  704  of rolling is accomplished by re-configuring the 1×2 switch  204  so that the light signals at the input  204 A of the 1×2 switch  204  are routed to the second output  204 C instead of the first input  204 B and re-configuring the 2×1 switch  304  so that the light signals at the output  304 A of the 2×1 switch  304  are derived from the light signals at the second input  304 C instead of the first input  304 B. This embodiment has the lowest transmission loss since switches can have lower loss than splitters and combiners. However this combination may be less reliable, and it requires head-end and tail-end switch coordination.  
         [0047]    In an embodiment where the combination of the head-end module of FIG. 2C and the tail-end module of FIG. 3A is used, the step  704  of rolling is accomplished by re-configuring the first VOA  208  and the second VOA  210  so that the first VOA  208  has substantially higher attenuation than the second VOA  210  instead of the second VOA  210  having substantially higher attenuation than the first VOA  208 . This embodiment is opposite to the embodiment where the combination of the head-end module of FIG. 2A and the tail-end module of FIG. 3C is used. However this embodiment reduces back-reflections from a fixed termination, it also enables nodal input level compensation.  
         [0048]    In an embodiment where the combination of the head-end module of FIG. 2D and the tail-end module of FIG. 3A is used, the step  704  of rolling is accomplished by re-configuring the first shutter  214  and the second shutter  216  so that the first shutter  214  transmits substantially no light signals from the input  214 A of the first shutter  214  to the output  214 B of the first shutter  214  and the second shutter  216  transmits substantially all of the light signals from the input  216 A of the second shutter  216  to the output  216 B of the second shutter instead of the first shutter  214  transmitting substantially all of the light signals from the input  214 A of the first shutter  214  to the output  214 B of the first shutter  214  and the second shutter  216  transmitting substantially none of the light signals from the input  216 A of the second shutter  216  to the output  216 B of the second shutter. This embodiment is opposite to the embodiment where the combination of the head-end module of FIG. 2A and the tail-end module of FIG. 3D is used, however this embodiment reduces back-reflections from a fixed termination.  
         [0049]    In an embodiment where the combination of the head-end module of FIG. 2C and the tail-end module of FIG. 3B is used, the step  704  of rolling is accomplished by re-configuring the first VOA  208  and the second VOA  210  so that the first VOA  208  has substantially higher attenuation than the second VOA  210  instead of the second VOA  210  having substantially higher attenuation than the first VOA  208  and re-configuring the 2×1 switch  304  so that the light signals at the output  304 A of the 2×1 switch  304  are derived from the light signals at the second input  304 C instead of the first input  304 B. This embodiment is opposite to the embodiment where the combination of the head-end module of FIG. 2B and the tail-end module of FIG. 3C is used, however it enables nodal input rather than nodal output level compensation. In general, combinations with active head-end and tail-end modules may have less loss but be more unreliable and require coordination.  
         [0050]    In an embodiment where the combination of the head-end module of FIG. 2D and the tail-end module of FIG. 3B is used, the step  704  of rolling is accomplished by re-configuring the first shutter  214  and the second shutter  216  so that the first shutter  214  transmits substantially no light signals from the input  214 A of the first shutter  214  to the output  214 B of the first shutter  214  and the second shutter  216  transmits substantially all of the light signals from the input  216 A of the second shutter  216  to the output  216 B of the second shutter instead of the first shutter  214  transmitting substantially all of the light signals from the input  214 A of the first shutter  214  to the output  214 B of the first shutter  214  and the second shutter  216  transmitting substantially none of the light signals from the input  216 A of the second shutter  216  to the output  216 B of the second shutter; and re-configuring the 2×1 switch  304  so that the light signals at the output  304 A of the 2×1 switch  304  are derived from the light signals at the second input  304 C instead of the first input  304 B. As above, this embodiment with an active head-end and tail-end has lower transmission loss but also lower reliability, and requires coordination.  
         [0051]    In an embodiment where the combination of the head-end module of FIG. 2B and the tail-end module of FIG. 3C is used, the step  704  of rolling is accomplished by re-configuring the 1×2 switch  204  so that the light signals at the input  204 A of the 1×2 switch  204  are routed to the second output  204 C instead of the first input  204 B and re-configuring the first VOA  308  and the second VOA  310  so that the first VOA  308  has substantially higher attenuation than the second VOA  310  instead of the second VOA  310  having substantially higher attenuation than the first VOA  308 . As above, this embodiment with an active head-end and tail-end has lower transmission loss but also lower reliability, and requires coordination.  
         [0052]    In an embodiment where the combination of the head-end module of FIG. 2E and the tail-end module of FIG. 3A is used, the step  704  of rolling is accomplished by re-configuring the 1×2 switch  204  so that the light signals at the input  204 A of the 1×2 switch  204  are routed to the second output  204 C instead of the first input  204 B and re-cofiguring the first VOA  220  and the second VOA  222  so that the first VOA  220  has substantially higher attenuation than the second VOA  222  instead of the second VOA  222  having substantially higher attenuation than the first VOA  220 . This embodiment is similar to the embodiment where the combination of the head-end module of FIG. 2C and the tail-end module of FIG. 3A is used, however this embodiment has lower transmission loss but lower reliability.  
         [0053]    In an embodiment where the combination of the head-end module of FIG. 2A and the tail-end module of FIG. 3F is used, the step  704  of rolling is accomplished by re-configuring the 2×2 switch  324  so that the light signals at the first input  324 A are transmitted to the second output  324 D and the light signals at the second input  324 B are transmitted to the first output  324 C instead of the light signals at the first input  324 A being transmitted to the first output  324 C and the light signals at the second input  324 B being transmitted to the second output  324 D. This embodiment enables the monitoring of one of the photonic switches  102 , 104 .  
         [0054]    In an embodiment where the combination of the head-end module of FIG. 2B and the tail-end module of FIG. 3D is used, the step  704  of rolling is accomplished by re-configuring the 1×2 switch  204  so that the light signals at the input  204 A of the 1×2 switch  204  are routed to the second output  204 C instead of the first input  204 B and re-cofiguring the first shutter  314  and the second shutter  316  so that the first shutter  314  transmits substantially no light signals from the input  314 A of the first shutter  314  to the output  314 B of the first shutter  314  and the second shutter  316  transmits substantially all of the light signals from the input  316 A of the second shutter  316  to the output  316 B of the second shutter instead of the first shutter  314  transmitting substantially all of the light signals from the input  314 A of the first shutter  314  to the output  314 B of the first shutter  314  and the second shutter  316  transmitting substantially none of the light signals from the input  316 A of the second shutter  316  to the output  316 B of the second shutter. This embodiment is similar to the embodiment where the combination of the head-end module of FIG. 2A and the tail-end module of FIG. 3D is used, however this embodiment has lower transmission loss but lower reliability.  
         [0055]    In an embodiment where the combination of the head-end module of FIG. 2C and the tail-end module of FIG. 3C is used, the step  704  of rolling is accomplished by re-configuring the first VOA  208  and the second VOA  210  so that the first VOA  208  has substantially higher attenuation than the second VOA  210  instead of the second VOA  210  having substantially higher attenuation than the first VOA  208  and re-configuring the first VOA  308  and the second VOA  310  so that the first VOA  308  has substantially higher attenuation than the second VOA  310  instead of the second VOA  310  having substantially higher attenuation than the first VOA  308 . This combination has the disadvantage of lower reliability, duplicated level control and required coordination.  
         [0056]    Referring next to FIG. 7A and FIG. 6, the first output ports  106 B of the head-end modules  106  are left in place or disconnected from the inputs  102 A, 102 B of the primary photonic switch  102 . As well, the first input ports  108 B of the tail-end modules  108  are left in place or disconnected from the outputs  102 C, 102 D of the primary photonic switch  102  (Step  706 ).  
         [0057]    Advantageously, the bridge and roll technique described above can be executed in reverse as described with reference to FIGS. 1, 4,  5 ,  6  and the flowchart of FIG. 7B.  
         [0058]    Referring first to FIG. 7B in combination with FIGS. 5 and 6, the first output ports  106 B of the head-end modules  106  are already in place or connected  114  to respective inputs  102 A, 102 B of the primary photonic switch  102 . The first input ports  108 B of the tail-end modules  108  are already in place or connected to  116  respective outputs  102 C, 102 D of the primary photonic switch  102  (Step  708 ).  
         [0059]    Referring next to FIG. 4, the head-end modules  106  are re-configured so that the light signals on the input optical fibers  110  are re-routed from the secondary optical path  124  to the primary optical path  122 . (Step  710 ).  
         [0060]    Referring next to FIG. 1, the second output ports  106 C of the head-end modules  106  are left in place or disconnected from inputs  104 A, 104 B of the secondary photonic switch  104  and the second input ports  108 C of the tail-end modules  108  are left in place or disconnected from the outputs  104 C, 104 D of the secondary photonic switch  104  (Step  712 ).  
         [0061]    An alternative embodiment of the present invention that is especially useful for consolidating multiple light signals onto a single fiber is described with reference to FIGS. 8, 9,  10 ,  11  and the flowchart of FIG. 12.  
         [0062]    Referring first to FIG. 8, the photonic switch system  800  of FIG. 8 is identical to the photonic switch system  100  of FIG. 1 except that a multiplexor  802  has been added. The multiplexor  802  has an output  802 A and a plurality of inputs  802 B, 802 C where the output  802 A is connected  804  to an input  104 A of the secondary photonic switch  104 .  
         [0063]    A bridge and roll technique for substituting the second photonic switch  102  will now be described with reference to FIG. 9 in combination with FIG. 12. Firstly, the second output ports  106 C of the head-end modules  106  are connected  906  to respective inputs  802 B, 802 C of the multiplexor  802 . All of the second output ports  106 C are shown connected to inputs  802 B, 802 C of the multiplexor  802  for convenience, however only a subset of the second output ports  106 C may be connected to inputs  802 B, 802 C of the multiplexor  802  and still fall within the scope of the invention. The second input ports  108 C of the tail-end modules  108  are connected  120  to respective outputs  104 C, 104 D of the secondary photonic switch  104  (Step  1202 ).  
         [0064]    Following this bridging and referring next to FIG. 10 and FIG. 12, the head-end modules  106  are re-configured so that the light signals on the input optical fibers  110  are re-routed from the primary optical path  122  to the secondary optical path  124  through to photonic switch  104  (Step  1204 ).  
         [0065]    It should be appreciated that the connections  906 , 120  made in the bridging step  1202  may be made at any time before the subsequent rolling step  1204  such as during the manufacture of the photonic switch system  800  or after installation of the photonic switch system  800  on a customer&#39;s premises. It should also be appreciated the connections  906 , 120  may be made manually or automatically under remote control.  
         [0066]    It should be noted that photonic switch  104  is configured such that the input  104 A to which the output  802 A of the multiplexer is connected may be selectively connected to any of the outputs  104 C,  104 D of photonic switch  104 .  
         [0067]    Referring next to FIG. 11 and FIG. 12, the first output ports  106 B of the head-end modules  106  are left in place or disconnected from the inputs  102 A, 102 B of the primary photonic switch  102 . The first input ports  108 B of the tail-end modules  108  are left in place or disconnected from the outputs  102 C, 102 D of the primary photonic switch  102  (Step  1206 ).  
         [0068]    Another embodiment of the present invention is described with reference to FIGS. 13 and 14. Referring first to FIG. 13, a photonic switch system  1300  comprising a plurality of photonic switches shown for convenience as a primary photonic switch  102  having a plurality of inputs  102 A, 102 B and a plurality of outputs  102 C, 102 D, and a secondary photonic switch  104  having a plurality of inputs  104 A, 104 B and a plurality of outputs  104 C, 104 D. The photonic switch system  1300  also comprises: a plurality of head-end 2×2 switches  1306  each having a first input  1306 A, a second input  1306 B, a first output  1306 C and a second output  1306 D; and a plurality of tail-end 2×2 switches  1308  each having a first input  1308 A, a second input  1308 B, a first output  1308 C and a second output  1308 D. The first outputs  1306 A of the head-end 2×2 switches  1306  are connected  1314  to respective inputs  102 A, 102 B of the primary photonic switch  102 . The second outputs  1306 B of the head-end 2×2 switches  1306  are connected  1318  to respective inputs  104 A, 104 B of the secondary photonic switch  104 . The first inputs  1308 A of the tail-end 2×2 switches  1308  are connected  1316  to respective outputs  102 C, 102 D of the primary photonic switch  102 . The second inputs  1308 B of the tail-end 2×2 switches  1308  are connected  1320  to respective outputs  104 C, 104 D of the secondary photonic switch  104 . The first inputs  1306 A of the head-end 2×2 switches  1306  are optically connectable to input optical fibers  1310 A, 1310 C that are carrying high-priority traffic. The second inputs  1306 B of the head-end 2×2 switches  1306  are optically connectable to input optical fibers  1310 B, 1310 D that are carrying low-priority traffic. The first outputs  1308 C of the tail-end 2×2 switches  1308  are optically connectable to output optical fibers  1312 A, 1312 C that are carrying high-priority traffic. The second outputs  1308 D of the head-end 2×2 switches  1308  are optically connectable to output optical fibers  1312 B, 1312 D that are carrying low-priority traffic. The high-priority traffic on the input optical fibers  1310 A, 1310 C is routed from the first inputs  1306 A of the head-end 2×2 switches  1306 , through the first outputs  1306 C of the head-end 2×2 switches  1306 , through the connections  1314 , through the primary photonic switch  102 , through the connections  1316 , through the first inputs  1308 A of the tail-end 2×2 switches  1308  to the first outputs  1308 C of the tail-end switches  1308 . The low-priority traffic on the input optical fibers  1310 B, 1310 D is routed from the second inputs  1306 B of the head-end 2×2 switches  1306 , through the second outputs  1306 D of the head-end 2×2 switches  1306 , through the connections  1318 , through the secondary photonic switch  104 , through the connections  1320 , through the second inputs  1308 B of the tail-end 2×2 switches  1312  to the second outputs  1308 D of the tail-end switches  1312 .  
         [0069]    In the arrangement shown in FIG. 13, the primary photonic switch  102  serves to switch all high priority traffic and the secondary photonic switch  104  serves to switch all low priority traffic. When the photonic switch  102  fails or the reliability is unacceptable it is desirable to change the routing so that all high priority traffic is passed through the secondary photonic switch  104  and all low priority traffic is passed through the primary photonic switch  102 , the head-end 2×2 switches  1306  and tail-end 2×2 switches  1308  are switched over to the positions shown in FIG. 14. As can be seen, the high-priority traffic is now re-routed from the first inputs  1306 A of the head-end 2×2 switches  1306 , through the second outputs  1306 D of the head-end 2×2 switches  1306 , through the connections  1318 , through the secondary photonic switch  104 , through the connections  1320 , through the second inputs  1308 B of the tail-end 2×2 switches  1308  to the first outputs  1308 C of the tail-end switches  1308 ; and the low-priority traffic on the input optical fibers  1310 B, 1310 D is re-routed from the second inputs  1306 B of the head-end 2×2 switches  1306 , through the first outputs  1306 C of the head-end 2×2 switches  1306 , through the connections  1314 , through the primary photonic switch  102 , through the connections  1316 , through the first inputs  1308 A of the tail-end 2×2 switches  1308  to the second outputs  1308 D of the tail-end switches  1308 .  
         [0070]    While the preferred embodiment of the present invention has been described and illustrated, it will be apparent to persons skilled in the art that numerous modifications and variations are possible. The scope of the invention, therefore, is only to be limited by the claims appended hereto.