Abstract:
System and methods for upgrading an optical add-drop multiplexer (OADM) to higher degree wavelength selective router (WSR)/wavelength selective switch (WSS) are disclosed. For example, an OADM of degree-2 may be provisioned for upgrades to degree-3 and higher. The existing links power and signal-to-noise (SNR) budgets are not significantly affected when the node is upgraded to a higher degree WSS/WSR. Cascaded power dividers and combiners may be used in conjunction with optical amplifiers and reconfigurable blocking filters to increase the number of paths over which an optical signal can be routed/switched without affecting the paths already utilized. Prior to enabling service on either an input or output fiber, taps and combiners are pre-provisioned so as to ensure that at least one additional transmission path is always available.

Description:
PROVISIONAL APPLICATION 
   The present application claims priority under 35 U.S.C. § 120 of a provisional application 60/479,180 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 method and apparatus for enabling in-service upgrades of optical add-drop multiplexers (OADM) to wavelength selective switches (WSS) or routers (WSR) of higher degrees. 
   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 it&#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 such as telecommunications, 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 include wavelength selective switches (WSS) and routers (WSR). A WSS/WSR of degree N is an apparatus which switches wavelengths from N input fibers to N output fibers. Most practical applications require only switching N inputs to (N−1) outputs. The switching/routing operation occurs entirely within the optical domain, although the control signals for the WSS are electrical. 
     FIG. 1  illustrates a conventional design for a WSS/WSR apparatus  100 . In this instance the WSS/WSR apparatus  100  is a degree 4 WSS/WSR. The WSS/WSR apparatus  100  includes a plurality of optical splitters  102 , a plurality of wavelength filters  104 , and a plurality of optical combiners  106 . 
   Each optical splitter  102  is a 1:4 splitter (one input, four outputs) and each optical combiner is a 4:1 combiner (four inputs, one output). If fully connected, then there may be as many as sixteen wavelength filters  104  (combination of 4 inputs and 4 outputs). However, for simplicity, only the connections to the first optical combiner  106  are illustrated. 
   A WSS/WSR of degree N with capability to drop wavelengths from any of the N inputs and add wavelengths to any of the N outputs is an OADM of degree N. The simplest and most common type of OADM is a degree-2 OADM. 
     FIG. 2  illustrates a conventional OADM  200 . In this instance, the OADM  200  is a degree 2 OADM. The OADM  200  includes first and second optical splitters  202  and  204 , first and second optical combiners  206  and  208 , first and second wavelength filters  210  and  212 , and first and second wavelength demux/mux devices  214  and  216 . 
   As shown, optical signal sources FROM-WEST and FROM-EAST are connected to the input of the first and second optical splitters  202 ,  204 , respectively. Each optical splitter  202 ,  204  splits the optical signals and directs the signals to the respective wavelength filters  210 ,  212  and to the respective wavelength demux/mux devices  214 ,  216 . The first and second optical combiners  206 ,  208  receive optical signals from the respective wavelength filters  210 ,  212  and from the respective wavelength demux/mux devices  214 ,  216  and output the combined optical signals to the TO-EAST and TO-WEST optical signal destinations, respectively. 
   As noted previously, WSS/WSR may be constructed from one or more OADMs. Indeed, an OADM providing connectivity between more than two fibers is considered to be a WSS or WSR. Typically, network connectivity evolves from network elements—such as OADM/WSS/WSR—from a lower degree (degree 2 being the most common) to a higher degree. 
   However, if the conventional OADM  200  as shown in  FIG. 2  is to be upgraded to a higher degree OADM/WSS/WSR, it is clear that disruption of the signals traversing the OADM  200  will occur since any upgrade will require the OADM  200  to be shut down. In other words, an in-service upgrade, where disruption of service does not occur, cannot take place. 
   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 DRAWINGS 
     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 design for a wavelength selective switch apparatus; 
       FIG. 2  illustrates a conventional optical add/drop multiplexer (OADM); 
       FIG. 3  illustrates an OADM according to an embodiment of the present invention; 
       FIGS. 4A and 4B  illustrate drop taps according to embodiments of the present invention; 
       FIGS. 5A and 5B  illustrate blocking filters according to embodiments of the present invention; 
       FIGS. 6A and 6B  illustrate add taps according to embodiments of the present invention; 
       FIG. 7  illustrates an example of an upgrade of an WSS/WSR to a higher degree WSS/WSR according to an embodiment of the present invention; and 
       FIG. 8  illustrates an example of a further upgrade of a WSS/WSR according to 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. 
     FIG. 3  illustrates an OADM  300 , which is a degree 2 OADM. The OADM  300  may be pre-provisioned with add/drop taps to allow a future in-service upgrade to a degree N WSS/WSR, (N arbitrary, &gt;2). The OADM  300  may include first and second input drop taps  302 ,  304  and first and second provisional drop taps  310 ,  312 . 
   Each of the drop taps  302 ,  304 ,  310 ,  312  may be a drop tap  400 ,  410  of the types as illustrated in  FIGS. 4A and 4B . A drop tap may include an input and a plurality of outputs. For example, the drop tap  400  as shown in  FIG. 4A  includes an input and first and second outputs. The drop tap  410  as shown in  FIG. 4B  also includes and input and a plurality of outputs (including first and second outputs). Each drop tap  400 ,  410  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  400 ,  410  may include an optical splitter  402 ,  412  (or simply “splitter”) configured to direct optical signals present at its input to its plurality of outputs. For example, the splitter  402  of the drop tap  400  (see  FIG. 4A ) directs optical signals present at its input to its first and second outputs. The splitter  412  (see  FIG. 4B ) also directs optical signals from its input to its plurality of outputs, including the first and second outputs. 
   The input of the splitter  402 ,  412  may optically communicate with the input of the drop tap  400 ,  410 . Indeed, the input of the of the splitter  402 ,  412  may serve as the input of the drop tap  400 ,  410 . Likewise, the plurality of outputs of the drop tap  402 ,  412  including the first and second outputs, may optically communicate with the corresponding outputs of the drop tap  400 ,  410 , and may indeed serve as the corresponding outputs of the drop tap  400 ,  410 . 
   The splitter  402 ,  412  may be configured to deliver differing amounts of power to the outputs. For example, the splitter  402  of the drop tap  400  may direct a majority of output power to the first output of the splitter  402 , and consequently to the first output of the drop tap  400 . Generally, low insertion loss on one path is desirable to minimize the number of amplifiers used for the bypass configuration. Indeed, the splitter  402 ,  412  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  400  may include a line amplifier  404  with its input optically communicating with the second output of the splitter  402  and its output optically communicating with the second output of the drop tap  400 . 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  402  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  402  and/or  412  may be optically connected with a line amplifier as desired. 
   Referring back to  FIG. 3 , the input of the first and second input drop taps  302 ,  304  may optically communicate external optical signal sources FROM-WEST and FROM-EAST, respectively. 
   It should be noted that the labels FROM-WEST and FROM-EAST (and likewise TO-EAST and TO-WEST) 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. 
   The OADM  300  may also include first and second blocking filters  318  and  320 . The inputs of the first and second blocking filters  318 ,  320  may optically communicate with the first outputs of the first and second input drop taps  302 ,  304 , respectively. 
   Each of the blocking filters  318  and  320  may be a blocking filter  500 ,  510  of the type as illustrated in  FIGS. 5A and 5B . As illustrated, each blocking filter  500 ,  510  may receive optical channels λ 1  . . . λ n  at its input. However, the blocking filter  500 ,  510  severely attenuates (or blocks) optical signals of certain channels while allowing other subset of optical channels to pass through without much attenuation. Generally, the channels are blocked to prevent overlap of optical channels that may be added on the add path. 
   The blocking filter  510  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. 3 , the second outputs of the first and second input drop taps  302 ,  304  may optically communicate with the inputs of the first and second provisional drop taps  310 ,  312 . In this instance, the provisional drop taps  310 ,  312  are shown as being of the type of drop tap  400  as illustrated in  FIG. 4A . However, as noted above, one or both of the provisional drop taps  310 ,  312  may also be of the type of drop tap  410  as illustrated in  FIG. 4B . 
   For each of the first and second provisional drop taps  310 ,  312 , one output may be provisioned for future in-service upgrades. For example, as illustrated in  FIG. 3 , the second outputs of the provisional drop taps  310 ,  312  may be provisioned for such future use (first and second provisional outputs). 
   The first outputs of the first and second provisional drop taps  310 ,  312  may optically communicate with corresponding inputs of the first and second wavelength demux/mux devices  322 ,  324 . The optical signals flowing into the wavelength demux/mux devices  322 ,  324  may be demultiplexed and provided for local destinations. Similarly, optical signals from local sources may be multiplexed and output from the first and second wavelength demux/mux devices  322 ,  324 . 
   The OADM  300  may also include first and second output add taps  306 ,  308  and first and second provisional add taps  314 ,  316 . Each add tap  306 ,  308 ,  314 , and  316  may be an add tap  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 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 combiner  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. 3 , the outputs of the first and second wavelength demux/mux devices  322 ,  324  may optically communicate with the first inputs of the second and first provisional add taps  316 ,  314 , respectively. The outputs of the first and second provisional add taps  314 ,  316  may optically communicate with the second inputs of the first and second output add taps  306 ,  308 , respectively, and the outputs of the first and second blocking filters  318 ,  320  may optically communicate with the first inputs of the first and second output add taps  306 ,  308 , respectively. Further, the outputs of the first and second output drop taps  306 ,  306  may optically communicate external optical signal destinations TO-WEST and TO-EAST, respectively. 
   Similar to the situation described above for the first and second provisional drop taps  310 ,  312 , one output of each of the first and second provisional add taps  314 ,  316  may be provisioned for future in-service upgrades. For example, as illustrated in  FIG. 3 , the second inputs of the provisional add taps  314 ,  316  may be provisioned for such future use (first and second provisional inputs). 
   Signals from one or both of the FROM-WEST and FROM-EAST sources may be amplified. As shown in  FIG. 3 , the OADM  300  may include a first in-line amplifier  326  or a second in-line amplifier  328  or both. Each inline amplifier  326 ,  328  may be configured to amplify optical signals present at its input and direct the amplified optical signals to its output. 
   If present, the input of the first in-line amplifier  326  may optically communicate with the first input of the OADM  300  (FROM-WEST source) and the output of the first in-line amplifier  326  may optically communicates with the input of the first input drop tap  302 . 
   Also if present, the input of the second in-line amplifier  328  may optically communicate with the second input of the OADM  300  (FROM-EAST source) and the output of the second in-line amplifier  328  may optically communicate with the input of the second input drop tap  304 . 
   Also, the signals to one or both of the TO-WEST and TO-EAST destinations may be amplified. As shown in  FIG. 3 , the OADM  300  may include a first boost amplifier  330  or a second boost amplifier  332  or both. Each boost amplifier  330 ,  332  may be configured to amplify optical signals present at its input and direct the amplified optical signals to its output. 
   If present, the output of the first boost amplifier  330  may optically communicate with the first output of the OADM  300  (TO-EAST destination) and the input of the first boost amplifier  330  may optically communicates with the output of the first output add tap  306 . 
   Also if present, the output of the second boost amplifier  332  may optically communicate with the second output of the OADM  300  (TO-WEST destination) and the input of the second boost amplifier  332  may optically communicate with the output of the second output add tap  308 . 
   As noted previously, the OADM  300  allows future in-service upgrades to take place, i.e. without disrupting the traffic of information flow. The upgrades may take place by optically connecting other elements to the provisional input(s) and output(s). Because the provisional input(s) and output(s) are utilized, the OADM  300  itself need not be shut down and thus, traffic flow of information to and from the wavelength demux/mux devices  322 ,  324  are not disrupted. 
   As an aside, the signal output from the first provisional output is the signal from the source FROM-WEST and the signal output from the second provisional output is the signal from the source FROM-EAST. Conversely, any future optical signals provided on the first provisional input will be delivered to the destination TO-EAST and any future optical signals provided on the second provisional input will be delivered to the destination TO-WEST. 
   By analyzing the differences between  FIGS. 2 and 3 , a general method to upgrade from an OADM/WSS/WSR that is not provisioned for future in-service upgrades (e.g. OADM  200 ) to an OADM/WSS/WSR that is provisioned for future in-service upgrades (e.g. OADM  300 ) may be determined. In  FIG. 3 , the first and second provisional inputs and first and second provisional outputs allow the degree of the OADM  300  to be increased. 
   However, strictly speaking, the degree of the WSS/WSR may be increased even if only one pair of provisional input and output exists. For example, it is only necessary that the first provisional input and the first provisional output be provided. 
   From this perspective, a method to upgrade an OADM which has not been provisioned for future in-service upgrades to an OADM/WSS/WSR that allows for future in-service upgrades may be as follows. First, a provisional drop tap (such as the first provisional drop tap  310 ) may be inserted so that the input of provisional drop tap  310  optically communicates with the second output of the first input drop tap  302  and the first output of the provisional drop tap  310  optically communicates with the input of the first wavelength demux/mux device  322 . In this instance, the second output of the provisional drop tap  310  may serve as the provisional output for future in-service upgrades. 
   Second, a provisional add tap (such as the first provisional add tap  314 ) may be inserted so that the output of provisional add tap  314  optically communicates with the second input of the first output add tap  306  and the first input of the provisional add tap  314  optically communicates with the output of the second wavelength demux/mux device  324 . In this instance, the second input of the provisional add tap  314  may serve as the provisional input for future in-service upgrades. 
   It should be noted that the order of insertion need not be performed as stated above. Also, provisional drop tap  310  may be either type of drop taps  400 ,  410  and the provisional add tap  314  may be either type of add taps  600 ,  610 . 
     FIG. 7  illustrates an example upgrade of the OADM  300  of  FIG. 3  to a higher degree WSS/WSR  700 . In  FIG. 7 , the WSS/WSR  700  is a degree 4 WSS/WSR due to the mirror like and pair-wise insertions of expansion drop taps ( 702  and  704 ), expansion add taps ( 706  and  708 ), expansion splitters ( 710  and  712 ), expansion combiners ( 714  and  716 ), and blocking filters ( 718 ,  720 ,  722 , and  724 ). 
   However, as indicated above, the pair-wise insertion is not strictly necessary to increase the degree of the OADM/WSS/WSR. For this reason and also for clarity, the WSS/WSR  700  will be described with references to additional elements of the top half of  FIG. 7 . 
   As shown, the WSS/WSR  700  illustrated in  FIG. 7  may include all elements of the OADM  300  illustrated in  FIG. 3 . In addition, the WSS/WSR  700  may include the expansion drop tap  702 . While shown as being of the type of drop tap  400 , the drop tap  702  may also be of the type of drop tap  410 . The input of the expansion drop tap  702  may optically communicate with the second output of the provisional drop tap  310 . 
   The WSS/WSR  700  may also include the expansion optical splitter  710  configured to direct optical signals present at its input and direct the optical signals to its first and second outputs. The splitter  710  may be of the type of splitter  402  described above. Also, while the splitter  710  is illustrated as delivering equal amounts of output power to the first and second outputs in  FIG. 7 , like the splitter  402 , the amount of output power delivered to each output may be tunable and need not be evenly split. 
   The first and second outputs of the splitter  710 , both of which provide optical signals from the FROM-WEST source, may optically communicate with other destinations. The first output of the expansion drop tap  702  may optically communicate with the input of the splitter  710 . 
   Note that after this part of the upgrade is completed, the new provisional output is the second output of the expansion drop tap  702 . In effect, the expansion drop tap  702  becomes the new provisional drop tap so that further future in-service upgrades may take place. 
   The WSS/WSR  700  may include the expansion add tap  706 . While shown as being of the type of add tap  600 , the add tap  706  may also be of the type of add tap  610 . The output of the expansion add tap  706  may optically communicate with the second input of the provisional add tap  314 . 
   The WSS/WSR  700  may also include an expansion optical combiner  714  configured to direct optical signals present at its input and direct the optical signals to its first and second outputs. The combiner  714  may be of the type of combiner  602  described above. Also, while the combiner  714  is illustrated as delivering equal amounts of output power from the first and second inputs to the output in  FIG. 7 , like the splitter  602 , the amount of output power delivered from each input may be tunable and need not be even. 
   The first and second inputs of the combiner  714 , the optical signals of which will be delivered to the TO-EAST destination, may optically communicate with other optical signal sources. The first input of the expansion add tap  706  may optically communicate with the output of the combiner  714 . 
   Note that after this part of the upgrade is completed, the new provisional input is the second input of the expansion add tap  706 . In effect, the expansion add tap  706  becomes the new provisional add tap so that further future in-service upgrades may take place. 
   The WSS/WSR  700  may further include a first expansion blocking filter  718  whose output may optically communicate with the first input of the expansion combiner  714  and/or a second expansion blocking filter  720  whose output may optically communicate with the second input of the expansion combiner  714 . One or both of the expansion blocking filters  718 ,  720  may be of the type of blocking filter  600  or  610  described above. 
   By analyzing  FIG. 7 , a general method to upgrade from an OADM/WSS/WSR which has been provisioned for in-service upgrades (e.g. OADM  300 ) to an OADM/WSS/WSR of a higher degree (e.g. WSS/WSR  700 ) may be determined. 
   The method may include inserting an expansion drop tap  702  such that the input of the expansion drop tap  702  optically communicates with the second output of the provisional drop tap  310  and inserting an expansion splitter  710  such that the first output of the expansion drop tap  702  optically communicates with the input of the expansion splitter  710 . If desirable or necessary, the amount of power delivered to each of the first and second outputs of the expansion splitter  710  may be tuned. 
   The method may also include inserting an expansion add tap  706  such that output of the expansion add tap  706  optically communicates with the second input of the provisional add tap  314  and inserting an expansion combiner  714  such that the first input of the expansion add tap  706  optically communicates with the output of the expansion combiner  714 . If desirable or necessary, the amount of power delivered from each of the first and second inputs of the expansion combiner  714  may be tuned. 
   The method may further include inserting a first expansion blocking filter  718  or a second expansion blocking filter  720  or both. If inserted, the output of the first expansion blocking filter  718  may optically communicate with the first input of the expansion combiner  714 . If inserted, the output of the second expansion blocking filter  720  may optically communicate with the second input of the expansion combiner  714 . If desired or necessary, the selection of optical channels to be blocked by the expansion filters  718 ,  720  may be controlled individually. 
     FIG. 8  illustrates an example WSS/WSR  800  which has been upgraded from the WSS/WSR  700 . Because the method used to upgrade from the WSS/WSR  700  to WSS/WSR  800  is similar to the method used to upgrade from OADM  300  to WSS/WSR  700 , the detailed description is omitted. 
   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.