Abstract:
Optical bypass node upgrade configurations are disclosed: (1) a configuration where optical taps are pre-positioned in wavelength division multiplex (WDM) line systems terminating at optical-electrical-optical (OEO) core switching nodes to allow for future upgrade of the nodes to degree-two or higher optical bypass; (2) a configuration where the taps are pre-positioned in a degree-two optical bypass node to allow for future upgrade to a degree-N optical bypass node; and (3) a configuration and procedure for upgrading OEO core switching nodes to optical bypass when the taps have not been pre-positioned in the WDM line systems. These configurations do not introduce bit errors for non-upgraded optical paths.

Description:
PROVISIONAL APPLICATION 
     The present application claims priority under 35 U.S.C. § 120 of a provisional application 60/479,181 filed on Jun. 18, 2003, the entirety of which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The field of the invention generally relates to optical networks. More particularly, the field of the invention is directed to a optical configuration that enables optical bypasses to occur without disrupting flow of information. 
     BACKGROUND OF THE INVENTION 
     Optical networks provide a tremendous capacity advantage. Entities wishing to take advantage of the advantages that optical networks offer, must usually make a decision based on their current needs (which may be modest and predictable) and their future needs (which are typically unpredictable). An entity may decide to acquire a network to meet its short-term needs because of the entity&#39;s present financial constraints. 
     However, this approach carries a risk that the network will be insufficient and may cost more in the long run because the entire network has to be replaced due to inadequacies of the network. Also, any upgrades may require the network to be shut down prior to the upgrade. Such a shut down is costly since no service can be provided, which in turn shuts down a revenue stream. In an industry like telecommunications, a shut down can be extremely costly. 
     Another approach is to project a long-term need and acquire a network with capabilities to meet the long-term need. This approach also carries inherent risks as well. In the short run, the investment in the network will be wasted to the extent that there will be excess capacity. In the long run, the needs of the entity may change in a different direction and the acquired network will not be able to handle the changed needs efficiently. 
     Current optical networks typically consist of a collection of point-to-point wavelength division multiplex (WDM) links with optical-electrical-optical (OEO) switches or regeneration sites providing interconnections between links.  FIG. 1  is an example of a conventional optical network  100 . The optical network  100  includes a collection of OEO nodes (for example, nodes  102 ,  104 , and  106 ) and regeneration sites (for example,  108 ). 
     Not all of the optical traffic flowing into an OEO node is destined for that node. For example, some of the optical traffic flowing from the OEO node  102  into the OEO node  104  may actually be destined for the OEO node  106 . However, due to the configuration of the conventional optical network  100 , and in particular due to the configuration of the conventional OEO node, all optical signals flowing into the OEO node undergo optical to electrical conversion and all signals flowing out of the OEO node undergo electrical to optical conversion. Thus, the optical signal traffic from the node  102  to the node  106  via the node  104  undergoes optical-electrical-optical conversion at the OEO node  104 . 
     As the amount of express traffic (traffic that is not terminated or regenerated at a node) increases, it becomes cost effective to keep these channels in the optical domain and bypass the OEO switches. This functionality is termed optical bypass. 
     It is anticipated that the movement to networks with optical bypass will take place gradually by enhancement of existing networks, rather than through builds of complete all-optical networks. In particular, it is expected that bypass capability will only be added to a particular node when the express traffic through that node reaches a capacity level where bypass implementation is cost effective. 
     It will be very important to carriers to not take down existing traffic terminating at the OEO nodes as the upgrade to optical bypass takes place. Thus, it is very desirable to have a modular upgrade path so that networks OEO nodes and regeneration elements can be upgraded in a hitless fashion in the future. 
     An approach is desired where the system deployed is extremely flexible so that future demands on the optical networks, not yet foreseen, may be handled with ease. As the capacity demand grows and changes, it is desirable to provide a flexible system that can meet the increased demand and the type of demand changes. It is also desirable to have the capability to recover previously inaccessible capacity and without service disruptions. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Features of the present invention will become more fully understood to those skilled in the art from the detailed description given herein below with reference to the drawings, which are given by way of illustrations only and thus are not limitative of the invention, wherein: 
         FIG. 1  illustrates a conventional optical network with conventional optical nodes and regeneration sites; 
         FIG. 2  illustrates a first stage bypass-enabled network; 
         FIG. 3  illustrates a second stage bypass-enabled network; 
         FIG. 4  illustrates an optical-electrical-optical (OEO) node with a bypass configuration according to an embodiment of the invention; 
         FIGS. 5A and 5B  illustrate drop taps according to embodiments of the present invention; 
         FIGS. 6A and 6B  illustrate add taps according to embodiments of the present invention; 
         FIGS. 7 and 7A  illustrate an upgrade of an OEO node to a higher degree OEO node according to an embodiment of the present invention; 
         FIG. 8  illustrates a bypass device according to an embodiment of the present invention; 
         FIG. 9  illustrates blocking filters according to an embodiment of the present invention; 
         FIG. 10  illustrates a variant of an upgrade of an OEO node to a higher degree OEO node an embodiment of the present invention; 
         FIGS. 11 and 11A  illustrate an upgrade of a preexisting OEO node to a node that will enable future upgrades in a non-disruptive manner according to an embodiment of the present invention; 
         FIG. 12  illustrates a bypass insertion apparatus of an embodiment of the present invention; and 
         FIG. 13  illustrates a more detailed implementation of the bypass insertion apparatus of an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     For simplicity and illustrative purposes, the principles of the present invention are described by referring mainly to exemplary embodiments thereof. The same reference numbers and symbols in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. The scope of the invention is defined by the claims and equivalents thereof. 
     The expression “optically connects” or “optically communicates” as used herein refers to any connection, coupling, link or the like by which optical signals carried by one optical element are imparted to the “connecting element.” Such “optically communicating” devices are not necessarily directly connected to one another and may be separated by intermediate optical components and/or devices. Likewise, the expressions “connection”, “operative connection”, and “optically placed” as used herein are relative terms and do not necessarily require a direct physical connection. 
     An example of how optical bypass might be implemented into an existing network is detailed utilizing  FIGS. 2 and 3 .  FIG. 2  illustrates an example of a first stage bypass and  FIG. 3  illustrates an example of a second stage bypass. In the first stage of an optical bypass implementation, bypasses are added at certain high capacity junction points so small star networks with all-optical connectivity (no regenerations) are enabled (segments  1 - 4  and  5 - 6  in  FIG. 2 ). 
     In the second phase of bypass upgrade as shown in  FIG. 3 , bypasses are added to nodes where it did not exist (e.g., the addition of segment  9  to segments  1 - 4 ), and nodes with partial bypass are upgraded (e.g., the addition of segments  7  and  8  to connect the two previously enabled transparent domains). 
       FIG. 4  illustrates an optical-electrical-optical node  400  according to an embodiment of the present invention. The configuration of the OEO node  400  is such that add and drop taps are provisioned to enable bypass upgrades in the future without the necessity of disrupting the traffic flow. In other words, with configuration of the OEO node  400 , the network need not be brought down when the future upgrade takes place. 
       FIG. 4  illustrates a degree-2 OEO node, e.g. an optical node with two inputs. However, the scope of the invention is not limited to degree-2 nodes only. 
     The OEO node  400  includes an OEO switch  402 . The example OEO switch  402  includes a plurality of inputs (for example, first and second inputs) that optically communicate with a corresponding plurality of drop taps (for example, first and second drop taps  404 ,  406 ) through a plurality of two-stage demux/mux scheme. For example, the optical signals from the West In source are provided to the first input of the OEO switch  402  via the first drop tap  404 . 
     Each of the first and second drop taps  404  and  406  may be drop taps  500 ,  510  of the type as illustrated in  FIGS. 5A and 5B . A drop tap may include an input and a plurality of outputs. For example, the drop tap  500  as shown in  FIG. 5A  includes an input and first and second outputs. The drop tap  510  as shown in  FIG. 5B  also includes and input and a plurality of outputs (including first and second outputs). Each drop tap  500 ,  510  may be configured to direct optical signals present at its input to each of the outputs including the first and second outputs. 
     The drop tap  500 ,  510  may include an optical splitter  502 ,  512  (or simply “splitter”) configured to direct optical signals present at its input to its plurality of outputs. For example, the splitter  502  of the drop tap  500  (see  FIG. 5A ) directs optical signals present at its input to its first and second outputs. The splitter  512  (see  FIG. 5B ) also directs optical signals from its input to its plurality of outputs, including the first and second outputs. 
     The input of the splitter  502 ,  512  may optically communicate with the input of the drop tap  500 ,  510 . Indeed, the input of the of the splitter  502 ,  512  may serve as the input of the drop tap  500 ,  510 . Likewise, the plurality of outputs of the drop tap  502 ,  512  including the first and second outputs, may optically communicate with the corresponding outputs of the drop tap  500 ,  510 , and may indeed serve as the corresponding outputs of the drop tap  500 ,  510 . 
     The splitter  502 ,  512  may be configured to deliver differing amounts of power to the outputs. For example, the splitter  502  of the drop tap  500  may direct a majority of output power to the first output of the splitter  502 , and consequently to the first output of the drop tap  500 . Generally, low insertion loss on one path is desirable to minimize the number of amplifiers used for the bypass configuration. Indeed, the splitter  502 ,  512  may be such that the amount of power directed to each of the outputs is dynamically tunable. 
     The drop tap may also include a line amplifier to amplify optical signals from its input to its output. Examples of line amplifiers include EDFA (erbium doped fiber amplifier), SoA (semiconductor optical amplifier), and other gain media and optically active materials. 
     For example, the drop tap  500  may include a line amplifier  504  with its input optically communicating with the second output of the splitter  502  and its output optically communicating with the second output of the drop tap  500 . This may be useful in circumstances wherein the amount of optical power of the source is low. For example, this may occur if the splitter  502  is configured to direct a majority of output power to the first output, and consequently delivers only a small amount of power to its second output. 
     While not specifically shown, it should be noted that any of the inputs and outputs of the splitter  502  and/or  512  may be optically connected with a line amplifier as desired. 
     Referring back to  FIG. 4 , the input of the first and second drop taps  404 ,  406  may optically communicate external optical signal sources (West In and East In, respectively) and the first outputs of the of the first and second drop taps  404 ,  406  may optically communicate with the first and second inputs of the OEO switch  402 , respectively. While the drop taps  404 ,  406  are illustrated as being of the type shown in  FIG. 5A , this is not strictly necessary. One or both of the drop taps  404 ,  406  may also be of the type as shown in  FIG. 5B . 
     Note that the labels “West In, East In, West Out, and East Out” are for the convenience of labeling the different sources and destinations of optical signals. The labels do not necessarily indicate the actual direction of flow of the optical signals. 
     Also, while the multiplexing and demultiplexing scheme of the optical signals flowing into and out of the OEO switch  402  illustrated is a two-stage design, the actual scheme of multiplexing and demultiplexing utilized is not limited to this. Any generic multiplexing and demultiplexing scheme surrounding the OEO switch  402  may be used without departing from the scope of the invention. 
     The example OEO switch  402  may also include a plurality of outputs (for example, first and second outputs) that optically communicate with a corresponding plurality of add taps (for example, first and second add taps  408 ,  410 ) through a plurality of two-stage demux/mux scheme. For example, the East Out destination is provided with optical signals first output of the OEO switch  402  via the first add tap  408 . 
     Each of the first and second add taps  408  and  410  may be add taps  600 ,  610  of the type as illustrated in  FIGS. 6A and 6B . An add tap may include a plurality of inputs and an output. For example, the add tap  600  as shown in  FIG. 6A  includes a first and second inputs and an output. The add tap  610  as shown in  FIG. 6B  also includes a plurality of inputs (including first and second inputs) and an output. Each add tap  600 ,  610  may be configured to direct optical signals present at each of its plurality of inputs to its output. 
     The add tap  600 ,  610  may include an optical combiner  602 ,  612  (or simply “combiner”) configured to direct optical signals present at its plurality of inputs to its output. For example, the combiner  602  of the add tap  600  (see  FIG. 6A ) directs optical signals present at its first and second inputs to its output. The combiner  612  (see  FIG. 5B ) also directs optical signals from its plurality of inputs to its output, including from the first and second inputs. 
     The output of the combiner  602 ,  612  may optically communicate with the output of the add tap  600 ,  610 . Indeed, the output of the of the combiner  602 ,  612  may serve as the output of the add tap  600 ,  610 . Likewise, the plurality of inputs of the add tap  602 ,  612  including the first and second inputs, may optically communicate with the corresponding inputs of the add tap  600 ,  610 , and may indeed serve as the corresponding inputs of the add tap  600 ,  610 . 
     The combiner  602 ,  612  may be configured to deliver differing amounts of power from each of the inputs to the output. Indeed, the combiner  602 ,  612  may be such that the amount of power directed to from each of the inputs to the output is dynamically tunable. 
     The add tap may also include a line amplifier to amplify optical signals from its input to its output. For example, the add tap  600  may include a line amplifier  604  with its input optically communicating with the second input of the add tap  600  its output optically communicating with the second input of the combiner  602 . This may be useful in circumstances wherein the amount of optical power delivered to the second input of the add tap  600  is low. 
     The line amplifier may also be utilized to amplify the output signal from the combiner. For example, the add tap  610  may include a line amplifier  614  with its input optically communicating with the output of the combiner  612  and its output optically communicating with the output of the add tap  610 . 
     While not specifically shown, it should be noted that any of the inputs and outputs of the combiner  602  and/or  612  may be optically connected with a line amplifier as desired. 
     Referring back to  FIG. 4 , the output of the first and second add taps  408 ,  410  may optically communicate external optical signal destinations (East Out and West Out, respectively) and the first inputs of the of the first and second add taps  408 ,  410  may optically communicate with the first and second outputs of the OEO switch  402 , respectively. While the add taps  408 ,  410  are illustrated as being of the type shown in  FIG. 6A , this is not strictly necessary. One or both of the add taps  408 ,  410  may also be of the type as shown in  FIG. 6B . 
     The OEO node  400  may also include in-line amplifiers optically communicating with the optical drop taps. The in-line amplifier may configured to amplify optical signals present at its input and direct the amplified signals to its output. 
     In this instance, as illustrated in  FIG. 4 , the outputs of the first and second in-line amplifiers  412 ,  414  may optically communicate with the inputs of the first and second drop taps  404 ,  406 , respectively. Also, the inputs of the first and second in-line amplifiers  412 ,  414  optically communicate with the external optical signal sources West In and East In, respectively. 
     It should be noted that the in-line amplifiers  412 ,  414  are optional. In other words, the in-line amplifiers  412 ,  414  are not necessary for the OEO node  400  to operate. 
     The OEO node  400  may further include boost amplifiers optically communicating with the optical add taps. The boost amplifier may configured to amplify optical signals present at its input and direct the amplified signals to its output. 
     In this instance, as shown in  FIG. 4 , the inputs of the first and second boost amplifiers  416 ,  418  may optically communicate with the outputs of the first and second add taps  408 ,  410 , respectively. Also, the outputs of the first and second boost amplifiers  416 ,  418  optically communicate with the external optical signal destinations East Out and West Out, respectively. 
     It should be noted that the boost amplifiers  416 ,  418  are optional. In other words, the boost amplifiers  416 ,  418  are not necessary for the OEO node  400  to operate. 
     As mentioned previously, the configuration of the OEO node in  FIG. 4  allows for bypass upgrades to take place without the need to bring down the network or other wise disrupt the network.  FIG. 7  illustrates an embodiment of the present invention that includes additional elements that enables an upgrade to take place. In this example, either one or both of first and second bypass devices  702 ,  704  may be inserted. 
       FIG. 7A  illustrates an example of a completed upgrade from a degree-2 node to a degree-3 node. While  FIGS. 7 and 7A  illustrate an upgrade from a degree-2 node, a node of any degree may be upgraded to a node of higher degree by the proper insertion and connection of the bypass devices. 
     One or both of the first and second bypass devices  702 ,  704  may of the type of a bypass device  800  illustrated in  FIG. 8 . The bypass device  800  may include a bypass input, a bypass output, a local output (or a “future-use-bypass output”), and a local input (or a “future-use-bypass input”). The bypass device  800  may be configured to direct zero more optical channels present at its bypass input to its bypass output, direct all optical channels present at its bypass input to its local output, and direct all optical channels present at its local input to its bypass output. 
     Referring back to  FIG. 7 , the bypass input of the first bypass device  702  may optically communicate with the second output of the first drop tap  404  and the bypass output of the first bypass device  702  may optically communicate with the second input of the first add tap  408 . 
     Similarly, the bypass input of the second bypass device  704  may optically communicate with the second output of the second drop tap  406  and the bypass output of the second bypass device  706  may optically communicate with the second input of the second add tap  410 . 
     It can be easily seen that the configuration of the OEO node  400  of  FIG. 4  allows an upgrade to the node  700  of  FIG. 7  to take place without having the network traffic being disrupted. As shown, the optical signal traffic from sources West In and East In to the OEO switch  402  and the optical signal traffic from the OEO switch  402  to the East Out and West Out destinations need not in disrupted while the upgrade takes place. This is due to the provisioning of extra drop outputs of the first and second drop taps  404 ,  406  and the extra inputs of the first and second add taps  408 ,  410 . 
     Note that after the upgrade with the bypass devices  702 ,  704  is completed, further upgrades are possible without disruption in the flow of information due to the provisioning of the future-use-bypass inputs and outputs. 
     Either or both of the bypass devices  702 ,  704  may be of the type of a bypass device  800  as illustrated in  FIG. 8 . In  FIG. 8 , the bypass device  800  may include a drop tap  802  or an add tap  804  or both. In this instance, the drop tap  802  is presented to be of the type of drop tap  500  as illustrated in  FIG. 5A . However, while not shown, the drop tap  802  may also be of type of drop tap  510  as illustrated in  FIG. 5B . Likewise, while the add tap  804  is presented as being of the type of add tap  600  as illustrated in  FIG. 6A , the drop tap  610  as illustrated in  FIG. 6B  may also be utilized. 
     As illustrated in  FIG. 8 , if the drop tap  802  may be present in one or both of the first and second bypass devices  702 ,  704 , the bypass device  800  may be configured such that the input of the drop tap  802  optically communicates with the bypass input of the bypass device  800  and the second output of the drop tap  802  optically communicates with the local output of the bypass device  800 . The first output of the drop tap  802  may optically communicate with the output of the bypass device  800  via a block filter  806  and/or the add tap  804 . 
     Also if the add tap  804  is present in either or both of the first and second bypass devices  702 ,  704 , the bypass device  800  may be configured such that the output of the add tap  804  optically communicates with the output of the bypass device  800  and the second input of the add tap  804  optically communicates with the local input of the bypass device  800 . The first input of the add tap  804  may optically communicate with the input of the bypass device  800  via a block filter  806  and/or the drop tap  802 . 
     Optionally, the bypass device  800  may include a blocking filter  806  configured to direct a subset of optical channels (or wavelengths) presented at its input to its output and block the remaining channels from being transferred. 
       FIG. 9  illustrates examples of blocking filters according to an embodiment of the present invention. As noted, each blocking filter  900 ,  902  may receive optical channels λ 1  . . . λ n  at its input. However, the blocking filter  900 ,  902  severely attenuates (or blocks) optical signals of certain channels while allowing other subset of optical channels to pass through without much attenuation. Blocking filter  902  is a reconfigurable blocking filter and includes a control input. Through the use of the control input, the channels of optical signals selected for blocking may be dynamically tunable. Examples of optical channel blocking technology may be found in U.S. Pat. Nos. 6,141,361; 5,974,207; 6,625,346; 6,687,431 or the like. 
     Referring back to  FIG. 8 , the input of the blocking filter  806  may optically communicate with the bypass input of the bypass device  800 . The optical communication may take place via the first output of the drop tap  802  if the drop tap  802  is present. Similarly, the output of the blocking filter  806  may optically communicate with the bypass output of the bypass device  800 . The optical communication may take place via the first input of the add tap  804  if the add tap  804  is present. 
     As noted above, the drop tap  802  may be of the type of drop tap  500 ,  510  and the add tap  804  may be of the type of add tap  600 ,  610 . That is, the drop tap  802  may include an optical splitter and may also include line amplifier(s). Likewise, the add tap  804  may include a combiner and line amplifier(s). The details of the drop and add taps have already been described with references to  FIGS. 5A ,  5 B,  6 A, and  6 B and thus need not be repeated here. An example of an OEO node  1000  with bypass devices  1002 ,  1004  that includes drop taps  510  and add taps  610  is illustrated in  FIG. 10 . 
     Thus, the method of upgrading an OEO node to a higher degree OEO node where the original OEO node has been pre-enabled for bypass upgrade (for example from OEO node  400  of  FIG. 4  to the OEO  700  node of  FIG. 7  or to OEO node  1000  of  FIG. 10 ) simply requires inserting the appropriate bypass device and making proper optical connections. In this embodiment, the bypass device  702 ,  704  may be of the type of bypass device  800  as shown in  FIG. 8 . 
     Referring back to  FIG. 7 , the bypass device  702 ,  704  may be inserted such that the bypass input of the bypass device  702 ,  704  optically communicates with the bypass drop output (the second output of the optical drop tap  404 ,  406 ) of the OEO node  400 . Also, the bypass output of the bypass device  702 ,  704  may optically communicate with the bypass add input (the second input of the optical add taps  408 ,  410 ) of the OEO node  400 . The method may include adjusting the selection of optical channels to be directed from the bypass input to the bypass output of the bypass device  702 ,  704 , for example, through configuring the blocking filter  806 . 
     It may not always the case where an OEO node has been provisioned for non-disruptive bypass upgrades. When such non-provisioned system undergoes an upgrade, a bypass insertion apparatus may be utilized so that any future bypass upgrades may be performed in a non-disruptive manner. 
       FIG. 11  illustrates an example of an existing OEO node  1100  with no provisions for non-disruptive upgrades. The OEO node  1100  may be upgraded with a bypass insertion device  1200  so that future upgrades to the node  1100  may take place in a non-disruptive manner (see  FIG. 11A ). 
     The upgrade from  FIG. 11  to  FIG. 11A  make take place by inserting the bypass insertion apparatus  1200  as shown in  FIG. 12 . The bypass insertion apparatus  1200  may include a bypass drop device  1202 , a bypass add device  1204 , and optionally a blocking filter  1206 . 
     The bypass drop device  1202  may include a bypass input, a to-OEO-switch output, future-use-bypass output(s), and a to-bypass-add output. The bypass drop device  1202  may be configured to direct optical signals present at its bypass input to each of the to-OEO-switch output, the future-use-bypass output(s), and the to-bypass-add output. The bypass input of the bypass drop device  1202  may optically communicate with an external optical signal source. 
     The bypass add device  1204  may include a bypass output, a from-OEO-switch input, future-use-bypass input(s), and a from-bypass-drop input. The bypass add device  1204  may be configured to direct optical signals present at each of its from-OEO-switch input, the future-use-bypass input(s), and the from-bypass-drop input to its bypass output. The bypass output of the bypass add device  1204  may optically communicate with an external optical signal destination. 
     The to-bypass-add output of the bypass drop device  1202  may optically communicate with the from-bypass-drop input of the bypass add device  1204 . The optical communication between may take place via the blocking filter  1206 . If present, the input of the blocking filter  1206  may optically communicate with the to-bypass-add output of the bypass drop device  1202  and the output of the blocking filter  1206  may optically communicate with the from-bypass-drop input of the bypass add device  1204 . The blocking filter  1206  may be configured to direct a subset of optical channels present at is input to its output (see BF  900  of  FIG. 9 ) and the blocking filter  1206  may be reconfigurable (see RBF  902  of  FIG. 9 ). 
     Further, the bypass insertion apparatus  1200  may optionally include a bypass in-line amplifier  1208  or a bypass boost amplifier  1210  or both. The bypass in-line and boost amplifiers  1208 ,  1210  may be configured to amplify optical signals present at the respective inputs and direct the amplified optical signals to the respective outputs. When present, the input and the output of the bypass in-line amplifier  1208  may optically communicate with the external optical signal source and the bypass input of the bypass drop device  1202 , respectively. Likewise when present, the input and the output of the bypass boost amplifier  1210  may optically communicate with the output of the bypass add device  1204  and the external optical signal destination, respectively. 
       FIG. 13  illustrates an embodiment of a particular implementation  1300  of the bypass insertion apparatus  1200 . In this embodiment, the bypass drop device  1202  (of  FIG. 12 ) may include first and second drop taps  1302 ,  1304 . Each of the first and second drop taps  1302 ,  1304  of the bypass drop device  1202  may be of the type of drop tap  500  or  510  (see  FIG. 5A  or  5 B). In this particular embodiment, both the first and second drop taps  1302 ,  1304  are shown as being of the type of drop tap  500  of  FIG. 5A . 
     Each of the drop taps  1302 ,  1304  may be configured to direct optical signals at the respective inputs to the respective first and second outputs. In this particular embodiment, the input of the first drop tap  1302  may optically communicate with the bypass input of the bypass drop device  1202 , the first output of the first drop tap  1302  may optically communicates with the to-OEO-switch output of the bypass drop device  1202 , the second output of the first drop tap  1302  may optically communicate with the input of the second drop tap  1304 , the first output of the second drop tap  1304  may optically communicate with the to-bypass-add output of the bypass drop device  1202 , and the second output of the second drop tap  1304  may optically communicate with the future-use-bypass output of the bypass drop device. 
     Also in this embodiment, the bypass add device  1204  (of  FIG. 12 ) may include first and second add taps  1306 ,  1308 . Each of the first and second add taps  1306 ,  1308  of the bypass add device  1204  may be of the type of add tap  600  or  610  (see  FIG. 6A  or  6 B). In this particular embodiment, both the first and second add taps  1306 ,  1308  are shown as being of the type of add tap  600  of  FIG. 6A . 
     Each of the first and second add taps  1306 ,  1308  of the bypass add device  1204  may be configured to direct optical signals present at both its first and second inputs to its output. In this particular embodiment, the first input of the first add tap  1306  may optically communicate with the from-OEO-switch input of the bypass add device  1204 , the second input of the first add tap  1306  may optically communicate with the output of the second add tap  1308 , the first input of the second add tap  1308  may optically communicate with the from-bypass-drop input of the bypass add device  1204 , and the second input of the second add tap  1306  may optically communicate with the future-use-bypass input of the bypass add device  1204 . 
     As noted above, each of the drop taps  1302 ,  1304  may be of the type of drop tap  500  or  510  and each of the add taps  1306 ,  1308  may be of the type of add tap  600 ,  610 . That is, the drop taps  1302  and/or  1304  may include an optical splitter and may also include line amplifier(s). Likewise, the add taps  1306  and/or  1308  may include a combiner and line amplifier(s). The details of the drop and add taps have already been described with references to  FIGS. 5A ,  5 B,  6 A, and  6 B and thus need not be repeated here. 
     Thus, the method of upgrading an OEO node to a higher degree OEO node where the original OEO node has not been pre-enabled for bypass upgrade, for example from OEO node of  FIG. 11  to the OEO node of  FIG. 11A  simply requires inserting the appropriate bypass insertion apparatus and making proper optical connections. 
     Referring back to  FIG. 11A , the upgrade method may be accomplished by enabling optical communication between the bypass input of the bypass drop device  1202  with the external optical source; enabling optical communication between the to-OEO-switch output of the bypass drop device  1202  with the input of the OEO node; enabling optical communication between the bypass output of the bypass add device  1204  with the external optical destination; enabling optical communication between the from-OEO-switch output of the bypass add device  1204  with the output of the OEO node; enabling optical communication between the to-bypass-add output of the bypass drop device  1202  to a from-bypass-drop input of the bypass add device  1204 . 
     Optionally, the method may further include inserting a blocking filter  1206  and configuring the blocking filter  1206  if it is a reconfigurable blocking filter. 
     While the invention has been described with reference to the exemplary embodiments thereof, it is to be understood that various modifications may be made to the described embodiments without departing from the spirit and scope of the invention thereof. The terms as descriptions used herein are set forth by way of illustration only and are not intended as limitations.