Abstract:
A device includes a first band coupler, a first reconfigurable optical add-drop multiplexer (ROADM), a second ROADM, and a second band coupler. The first band coupler is configured to decouple a regular band and an extended band. The first ROADM is configured to add or drop one or more frequencies in the decoupled regular band to produce a first output in the regular band. The second ROADM is configured to add or drop one or more frequencies in the decoupled extended band to produce a second output in the extended band. The second band coupler is configured to couple the first output and the second output to produce a third output occupying the regular band and the extended band.

Description:
BACKGROUND INFORMATION 
     With the growing demand for network bandwidths, the amount of network traffic will soon test the limits of existing optical fiber network systems. To further increase the capacity of networks, communication technology companies and research institutions are developing optical fibers that can support greater network bandwidths. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates different approaches for increasing network bandwidths; 
         FIG. 2  illustrates an exemplary optical network in which concepts described herein may be implemented; 
         FIG. 3  is a diagram of an exemplary portion of the optical network of  FIG. 2  according to an implementation based on a single communication band; 
         FIG. 4  is a diagram of the exemplary portion of the optical network of  FIG. 2  according to an implementation based on two or more communication bands; 
         FIG. 5  illustrates a regular band and an extended band for the network of  FIG. 4 ; 
         FIG. 6A  is a diagram of exemplary components of an exemplary reconfigurable optical add-drop multiplexer (ROADM) module of  FIG. 4  according to one implementation; 
         FIG. 6B  is a diagram of exemplary components of the ROADM module of  FIG. 4  according to another implementation; 
         FIG. 7  illustrates an exemplary operation of an exemplary band coupler of  FIG. 6A ; 
         FIG. 8  is a diagram of exemplary components of an amplifier module of  FIG. 4  according to one implementation; 
         FIG. 9  illustrates an exemplary operation of the exemplary pump coupler of  FIG. 8 ; 
         FIG. 10  illustrates exemplary Raman amplification in the amplifier module of  FIG. 8 ; 
         FIG. 11  is a diagram of exemplary components of an amplifier module of  FIG. 4  according to another implementation; and 
         FIG. 12  is a flow diagram of an exemplary process for upgrading an existing ROADM system to enable the system to use an additional communication band. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following detailed description refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. 
     As described below, a multi-band reconfigurable add-drop multiplexer (ROADM) system uses multiple communication bands: a regular band and extended bands. For example, in one embodiment, a two-band ROADM system uses both the C-band (wavelengths between 1530 nanometers (nm)-1560 nm) as well as the L-band (wavelengths between 1565 nm-1625 nm). For a given network, in a two-band ROADM system, two ROADMS are used in place of each ROADM that would be installed in the network to build a single-band ROADM system. More generally, in a multi-band ROADM system, multiple ROADMS are used in place of each ROADM that would be installed in the network to build a single-band ROADM system. 
       FIG. 1  illustrates a number of different technological approaches for increasing network bandwidths. As shown, optical network bandwidths may be increased by increasing modulation levels  102 , the number of cores per optical fiber  104 , the size of core  106 , the number of cables per fiber  108 , the number of modes per optical fiber  110 , and the number of bands per optical fiber  112 . 
     Modulation level  102  refers to the number of bits encoded based on phases states of a given modulation method. Different modulation methods include: Differential Phase Shift Keying (DPSK) (modulation level 1-1 bit encoded by each phase state); Differential Quadrature Phase Shift Keying (DQPSK) (modulation level 2-2 bits encoded by each phase state); Dual Polarization State Quadrature Phase Shift Keying Modulation (modulation level 4-4 bits encoded by each phase state), etc. 
     For a physical optical network, capacity varies linearly with the number of cores per fiber  104 , optical fibers per cable  108  and modes per optical fiber  110 . Therefore, increasing the number of cores per fiber  104 , optical fibers per cable  108 , and modes per optical fiber  110  increases the capacity directly in proportion to the number of cores, optical fibers, and modes. Increasing the size of a core  106  may reduce nonlinear penalty, which, in turn, allows higher levels of modulation and increased optical power. 
     Network capacity may also be affected by the number of bands  112  that are used for communication in a single-core optical fiber. Currently, in many systems, only one communication band is used. If additional bands (herein referred to as “extended bands”) were used in addition to the original band (herein referred to as “regular band”), the network bandwidth would increase by the amount associated with the extended bands. 
     For current 100 gigabits per second (referred to as 100 G) systems,  FIG. 1  shows a map  114  of currently deployed technologies. More specifically, map  114  is a graphical plot of modulation level  102 , cores per fiber  104 , size of core  106 , number of optical fibers per cable  108 , number of modes per optical fiber  110 , and number of bands  112 . Map  114  shows the modulation level  102  to be at 2, the number of cores per optical fiber  104  at 1, the size of core  106  at 130 or 80 microns (μm), the number of optical fibers  108  at 48, the number of modes  110  at 1, and the number of bands  112  at 1. 
     As also indicated in  FIG. 1 , attempts to further increase network capacity by changing the modulation level  102  may soon hit a technological limit. Furthermore, increasing the network capacity by increasing the number of cores per fiber  104 , the size of each core  106 , the number of optical fibers per cable  108 , and the number of modes per optical fiber  110  requires deployment of new optical fibers, which is costly. This leaves increasing the number of bands  112  per optical fiber as an attractive low cost-to-benefit approach for increasing the network capacity/bandwidth. To convert a system that uses a single optical communication band into one that uses two or more extended communication bands, new ROADMS may be installed in the system. In addition, Raman pumps may be installed in the system as necessary to amplify the signals in the extended bands. 
       FIG. 2  shows an exemplary optical network  200  in which the concepts described herein may be implemented. As shown, optical network  200  may include metro/regional networks  202  and  204 , long haul or ultra-long haul optical lines  206 , and edge network  208 . Depending on the implementation, optical network  200  may include additional, fewer, or different optical networks and optical lines than those illustrated in  FIG. 2 . For example, in one implementation, optical network  200  may include additional edge networks and/or metro/regional networks that are interconnected by Synchronous Optical Network (SONET) rings. 
     Metro/regional network  202  may include optical fibers and central office hubs that are interconnected by the optical fibers. The central office hubs, one of which is illustrated as central office hub  210 , may include sites that house telecommunication equipment, including switches, optical line terminals, ROADMS, etc. In addition to being connected to other central offices, central office hub  210  may provide telecommunication services to subscribers, such as telephone service, access to the Internet, cable television programs, etc., via optical line terminals. 
     Metro/regional network  204  may include similar components as metro/regional network  202  and may operate similarly. In  FIG. 2 , metro/regional network  204  is illustrated as including central office hub  212 , which may include similar components as central office hub  210  and may operate similarly. 
     Long haul optical lines  206  may include optical fibers that extend from metro/regional optical network  202  to metro/regional optical network  204 . Edge network  208  may include optical networks that provide user access to metro/regional optical network  204 . As shown in  FIG. 2 , edge network  208  may include access points  214  (e.g., office buildings, residential area, etc.) via which end customers may obtain communication services from central office hub  212 . 
       FIG. 3  is a diagram of an exemplary portion  300  of optical network  200  according to an implementation based on a single communication band. Network portion  300  may be part of metro/regional network  202 / 204  or long haul optical lines  206 . As shown, portion  300  may include ROADMS  302 - 1  through ROADMS  302 - 8  (collectively “ROADMS  302 ” and individually “ROADM  302 ”), optical amplifiers  304 - 1  through  304 - 8  (collectively “optical amplifiers  304 ” and individually “optical amplifier  304 ”), and optical fibers that connect ROADMS  302  and optical amplifiers  304 . 
     ROADM  302  may select and/or inject one or more frequencies of light from/into a beam whose frequencies span a regular band. ROADM  302  may include wavelength selective switches to separate light into different frequencies and to select individual frequencies. Wavelength selective switches may be constructed from liquid crystal (LC), micro mirrors (micro-electromechanical system (MEMS) type), and liquid crystal on silicon (LCOS). 
     In some implementations, ROADM  302  may also include amplifiers and a channel monitor for detecting errors. In addition, in some implementations, ROADM  302  may be colorless (tunable wavelength switching), directionless (add/drop wavelength can be routed to any direction), and contentionless (each add/drop structure in ROADM  302  can have multiple transponders with at the same wavelength). ROADM  302  may typically be installed at certain locations in long haul optical line  206  and in metro/regional network  204 , such as, for example, central office  212  and access points  214 . 
     Optical amplifier  304  may include, for example, a doped-fiber amplifier, semiconductor amplifier, Raman amplifier, etc. Given an input signal, optical amplifier  304  may generate a corresponding output signal with a gain (i.e., the output signal has more power than the input signal). 
     In  FIG. 3 , the number of components and the specific interconnections are exemplary. In a different implementation, network portion  300  may include additional, fewer, or a different arrangement of components than those illustrated in  FIG. 3 . Furthermore, although not illustrated in  FIG. 3  for simplicity, network portion  300  may include components other than those illustrated in  FIG. 3  (e.g., regenerators). 
       FIG. 4  is a diagram of an exemplary portion  400  of optical network of  200  according to an implementation based on two or more communication bands. As shown, network portion  400  may include ROADM modules  402 - 1  through  402 - 8  (collectively “ROADM modules  402 ” and individually “ROADM module  402 ”), amplifier modules  406 - 1  through  406 - 8  (collectively “amplifier modules  406 ” and individually “amplifier module  406 ”), and optical fibers that connect ROADM modules  402  and amplifier modules  406 . The implementation of  FIG. 4  may be obtained by replacing each of ROADMS  302  and optical amplifiers  304  in network portion  300  with a ROADM module  402  and an amplifier module  406 . In other implementations, however, network portion  400  may include additional, fewer, or different components than those illustrated in  FIG. 4 . 
     ROADM module  402  may select and/or inject one or more frequencies of light from/into a beam whose frequencies span two or more communication bands: the regular band and extended bands. As described below, ROADM module  402  may include a ROADM  302 , one or more additional ROADMS, as well as other components. 
     Amplifier module  406  may include optical amplifier  304  for providing gain to the optical signal in the regular band and/or a pump (e.g., Raman pump) for providing the energy for amplification of the optical signals in the extended band(s). 
       FIG. 5  illustrates a regular band  502  and an extended band  504  for network  400 . In one implementation, regular band  502  corresponds to the C-band and extended band  504  corresponds to the L-band. As indicated above, regular band  502  is used in both network portion  300  and network portion  400 . Extended band  504  is used in network portion  400  but not in network portion  300 . The combination of regular band  502  and extended band  504  of network portion  400  accommodates additional communication channels, and accordingly, network portion  400  supports greater network traffic than network portion  300  (e.g., approximately twice the capacity of network portion  300 ). Although not illustrated, in some implementations, network portion  400  may use additional extended bands. 
       FIG. 6A  is a diagram of exemplary components of a ROADM module  402 - 1  according to one implementation. Each of other ROADM modules  402  in  FIG. 4  may include similar components and may be configured similarly as ROADM module  402 - 1 . 
     In  FIG. 6A , ROADM module  402 - 1  is implemented as a dual ROADM module. As shown, ROADM module  402 - 1  may include a ROADM  302 - 1 , a ROADM  404 - 1 , a band coupler  602 - 1 , a band coupler  602 - 2 , and optical path/fiber segments  604 - 1  through  604 - 6  (collectively “optical paths/fiber segments  604 ” and individually “optical path/fiber segment  604 ”). Depending on the implementation, ROADM module  402 - 1  may include additional, fewer, different, or a different arrangement of components than those illustrated in  FIG. 6A . 
     Band coupler  602 - 1  may combine input signals in regular band  502  and in extended band  504  or segregate signals in the two communication bands  502  and  504 , depending on the direction of the input and output signals. Assume that input signals are provided via optical paths  604 - 2  and  604 - 5  and band coupler  602 - 1  sends output signals on optical path  604 - 1 . In this case, band coupler  602 - 1  combines signals in regular band  502 , from optical path  604 - 2 , and signals of extended band  504 , from optical path  604 - 5 , and outputs the aggregated signals of two bands  502  and  504  on optical path  604 - 1 . 
     Conversely, assume that input optical signals of two bands are provided to band coupler  602 - 1  via optical path  604 - 1 . In this case, band coupler  602 - 1  segregates the input signals into signals that belong to regular band  502  and signals that belong to extended band  504 . Band coupler  602 - 1  outputs the signals of each band to optical paths  604 - 2  and  604 - 5 , respectively. 
     Band coupler  602 - 2  may operate similarly as band coupler  602 - 1 , but with respect to optical paths  604 - 3 ,  604 - 4 , and  604 - 6 . That is, band coupler  602 - 2  may combine optical signals from optical paths  604 - 3  and  604 - 5  and output the combined signals on optical path  604 - 4 . Band coupler  602 - 2  may also segregate optical signals from optical path  604 - 4  into signals in regular band  502  and signals in extended band  504 , and output the segregated signals on optical paths  604 - 3  and  604 - 6 , respectively. 
     ROADM  302 - 1  adds/drops optical signals, which belong to regular band  502 , on/from optical paths  604 - 2  and  604 - 3 . Similarly, ROADM  402 - 1  adds/drops optical signals, which belong to extended band  504 , on/from optical paths  604 - 5  and  604 - 6 . 
     Optical paths  604  carry optical signals from one optical component/element to another. 
       FIG. 6B  is a diagram of exemplary components of ROADM module  402 - 1  according to another implementation. In this implementation, network portion  400  uses regular band  502  and N−1 extended bands (e.g., extended band 1, extended band 2, . . . extended band N-1, where N is an integer greater than 2). As shown, ROADM module  402 - 1  includes ROADMS  612 - 1  through  612 -N (collectively “ROADMS  612 ” and individually “ROADM  612 ”), band couplers  614 - 1  through  614 -(N−1) (collectively “band couplers  614 ” and individually “band coupler  614 ”), and band couplers  616 - 1  through  616 -(N−1) (collectively “band couplers  616 ” and individually “band coupler  616 ”). For simplicity, optical paths in  FIG. 6B  are not labeled. The optical paths in  FIG. 6B  carry optical signals from one optical component to another. 
     Band coupler  614 - 1  may combine input signals in regular band  502  and in one of N−1 extended bands or segregate signals in the two communication bands, depending on the direction of the input and output signals. Other band couplers  614  may operate similarly. 
     Each of band couplers  616  may operate similarly as a corresponding band coupler  614 , but with respect to different optical paths. 
     ROADM  612 - 1  adds/drops optical signals, which belong to regular band  502 , on/from optical paths. Similarly, each of other ROADMS  612  adds/drops optical signals, which belong to one of extended bands 1 through N−1, on/from optical paths. 
       FIG. 7  illustrates an exemplary operation of band coupler  602 - 1 . As indicated above, band coupler  602 - 2  (or other band couplers (e.g., band couplers  614 ) in a network of ROADM modules  402 ) may operate similarly band coupler  602 - 1 . 
     Assuming that optical signals are propagating from the right to left, optical path  604 - 2  carries optical signals in regular band  702 , and optical path  604 - 5  carries optical signals in extended band  704 . Band coupler  602 - 1  combines the signals and outputs the combined signals on optical path  604 - 1 , in regular band  706  and extended band  708 . 
     For optical signals travelling from the left to right, optical path  604 - 1  carries optical signals in regular band  706  and extended band  708 . Band coupler  602 - 1  segregates the signals into signals that belong to regular band  702  and signals that belong to extended band  704 . Band coupler  602 - 1  outputs the signals that belong to regular band  702  on optical path  604 - 2  and outputs the signals that belong to extended band  704  on optical path  604 - 5 . 
     Band coupler  602 - 1  has the property that it injects only a minimal insertion loss in signals traveling on optical paths  604 - 1  to  604 - 2  (or vice versa), to reduce the impact on signals in regular band  702  (e.g., insertion loss is less than a particular threshold). Band coupler  602 - 2  (or other band couplers) has a similar property. 
       FIG. 8  is a diagram of exemplary components of amplifier module  406 - 1  according to one implementation. In this implementation, amplifier module  406 - 1  amplifies optical signals that travel from the left to right (from optical path  806 - 1  to optical path  806 - 3 ). As shown, amplifier module  406 - 1  may include amplifier  304 - 1 , pump coupler  802 - 1 , Raman pump  804 , and optical paths/fiber segments  806 - 1  through  806 - 4  (collectively “optical paths/fiber segments  806 ” and individually “optical path/fiber segment  806 ”). 
     Pump coupler  802  receives optical signals via optical path  806 - 1  and pump signals from Raman pump  804  via optical path  806 - 4 , combines the signals, and outputs the combined signals on optical path  806 - 2 . 
     Raman pump  804  generates Raman signals and outputs the Raman signals on optical path  806 - 4 . Amplifier  304 - 1  receives optical signals in the regular band from optical path  806 - 2 , amplifies the signals, and outputs the amplified signals on optical path  806 - 2 . In one implementation, amplifier  304 - 1  does not amplify the signals in the extended band(s), and simply passes such signals from optical path  806 - 2  to  806 - 3 . 
     Optical paths  806  convey optical signals from one optical component/element to another. Optical path  806 - 1  may include a segment of Raman fiber. The segment of Raman fiber may transfer the energy in the Raman signals from Raman pump  804  to the signals in extended band  504 , thus amplifying the signals in extended band  504 . 
     In operation, the power level of Raman pump  804  and the gain of optical amplifier  304 - 1  may be tuned/adjusted such that the signals in regular band  502  and extended bands are proportionately amplified. Because pump couplers  802  and band couplers  602  introduce insertion loss, the signals in regular band  502  may degrade by an extent greater than the amount anticipated based on fiber loss. 
     Although  FIG. 8  shows amplifier module  406 - 1  as including Raman pump  804  and amplifier  304 - 1 , depending on the implementation and system properties, amplifier module  406 - 1  may include additional, fewer, different, or a different arrangement of components than those illustrated in  FIG. 8 . For example, in one implementation, pump coupler  802 - 1  may be installed to the right of amplifier  304 - 1 , rather than to the left of amplifier  304 - 1  as shown in  FIG. 8 . In another example, amplifier module  406  may include only a Raman amplifier (for amplifying signals of a particular extended band) and a corresponding pump coupler, but may not include amplifier  304 - 1 . In yet another example, amplifier module  406  may include multiple Raman amplifiers and corresponding pump couplers. In still yet another example, amplifier module  406  may include only an optical amplifier  304 . 
       FIG. 9  illustrates an exemplary operation of pump coupler  802 - 1 . In  FIG. 9 , optical signals travel from the left to right (from optical path  806 - 1  to optical path  806 - 2 ), and optical path  806 - 4  carries Raman signal  902  from Raman pump  804 . Raman signal  903  injected in optical path  806 - 1  propagates from the right to left, in the direction opposite to that of the optical signals in regular band  904  and extended band  906 . Raman signal  803  couples with the optical signals in extended band  906  via a Raman fiber in optical path  806 - 1 . Consequently, the signal in regular band  904  experiences no gain, but the signal in extended band  906  experiences a gain. This is illustrated in  FIG. 9 . On optical path  806 - 2 , the energy in extended band  906  is illustrated as being greater than that in regular band  904 . 
     Pump coupler  802 - 1  has the property that it only injects a minimal insertion loss in signals traveling on optical paths  806 - 1  to  806 - 2 - 2  (or vice versa), to reduce the impact on signals in regular band  904  (e.g., insertion loss of pump coupler  802 - 1 &lt;a particular threshold). Pump coupler  802 - 2  (or other pump couplers) has a similar property (e.g., insertion loss of pump coupler  602 - 2 &lt;a particular threshold). Each of other amplifier modules  406  may include similar components and may be configured similarly as amplifier module  406 - 1 . 
     Although  FIG. 9  shows the operation of pump coupler  802 - 1  for extended band  906  that is adjacent to regular band  904 , in other implementations, pump coupler  802 - 1  may couple Raman signals that amplify signals in another extended band. 
       FIG. 10  illustrates exemplary Raman amplification in amplifier module  406 - 1 . Optical path  806 - 1  ( FIG. 9 ) may include a Raman fiber. Accordingly, as the optical signals propagate from the left to right and Raman signal  903  travel from the right to left, optical path  906 - 1  may transfer energy from Raman signal  903  ( FIG. 9 ) to extended band  906 . As a consequence of the energy transfer, as shown in  FIG. 10 , extended band  906  becomes extended band  1002 . That is, the optical signals in extended band  906  are amplified. The signals in regular band  904  are not amplified via Raman amplification, but via optical amplifier  304 - 1 . 
       FIG. 11  is a diagram of exemplary components of amplifier module  406 - 1  according to another implementation. In this implementation, amplifier module  406 - 1  amplifies optical signals that travel from the right to left (signals that travel from optical path  806 - 5  to optical path  806 - 1 ) as well as signals that travel from the left to right (signals that travel from optical path  806 - 1  to optical path  806 - 5 ). As shown in  FIG. 11 , in addition to the components shown in  FIG. 8 , amplifier module  406 - 1  includes pump coupler  802 - 2 , Raman pump  1104 , and optical paths  806 - 5  and  806 - 6 . 
     In  FIG. 11 , pump coupler  802 - 1  couples Raman signals from Raman pump  804  to optical signals traveling from optical path  806 - 1  to optical path  806 - 2 . Of the coupled signals, those in extended band  906  are Raman amplified via the Raman fiber in optical path  806 - 1 . Among the signals on optical path  806 - 2 , the signals in regular band  904  are then amplified by optical amplifier  304 - 1 . The Raman amplified signals and the signals amplified by optical amplifier  304 - 1  then pass through pump coupler  802 - 2  (via optical path  806 - 3 ), onto optical path  806 - 5  without further amplification. 
     Pump coupler  802 - 2  couples Raman signals from Raman pump  1104  to optical signals traveling from optical path  806 - 5  to optical path  806 - 3 . Of the coupled signals, those in extended band  906  are Raman amplified via the Raman fiber on optical path  806 - 5 . Among the signals on optical path  806 - 3 , the signals in regular band  904  are then amplified by optical amplifier  304 - 1 . The Raman amplified signals and the signals amplified by optical amplifier  304 - 1  then pass through pump coupler  802 - 1 . The amplified signal passes through pump coupler  802 - 1  (via optical path  806 - 3 ), onto optical path  806 - 1  without further amplification. 
       FIG. 12  is a flow diagram of an exemplary process  1200  for upgrading an existing ROADM system to enable the system to use an additional extended band. Assume that a network engineer or an operator wishes to convert an optical network or a portion of the optical network to so that the optical network uses the extended optical band. As shown, process  1200  may include selecting a route in the network portion (block  1202 ). 
     The network operator/engineer may shift important traffic (e.g., packets with high priority) to alternate routes (block  1204 ). For the un-shifted traffic, the network operator/engineer may notify the users (block  1204 ) of service interruptions. 
     The network operator/engineer may disconnect fiber links to existing ROADMS and optical amplifiers in the selected route (block  1206 ). The network operator/engineer may insert band couplers and pump couplers (block  1208 ) at certain locations in the route. Inserting the band couplers and pump couplers re-links the existing ROADMS and the optical amplifiers to the selected route. 
     The locations for the band couplers may be chosen such that when new ROADMS are installed in combination with the band couplers and existing ROADMS, the band couplers, the new ROADMS, and the already installed ROADMS form ROADM modules  402  in the route. Similarly, the locations for the pump couplers may be chosen such that Raman pumps can be installed at various points in the route to provide Raman amplification to optical signals in the extended band(s). 
     Thereafter, the network operator/engineer may wait for the modified system to recover (block  1210 ). That is, the network operator may wait for the network traffic to reach an operational state and the interconnected components to be functional. After the waiting period, the network operator/engineer may shift/return the traffic back to the route, from the alternate routes (block  1212 ). 
     The network operator/engineer may install new ROADMS (block  1214 ) in combination with the band couplers to build ROADM modules  402 . In addition, the network operator/engineer may also install Raman pumps into the route (block  1214 ). To use the extended band, the network operator/engineer may add new channels in the extended band (block  1216 ). 
     By upgrading an existing ROADM system to a new ROADM system with one or more additional extended bands in accordance with process  1200 , performance impact on the existing, working band can be kept relatively small. Because the extended band is powered by Raman pumps, the upgrade has a minimal impact on the existing band. 
     As described above, a multi-band ROADM) system uses multiple communication bands: a regular band and one or more extended bands. For example, in one embodiment, a two-band ROADM system uses both the C-band as well as the L-band. For a given network, in a two-band ROADM system, two ROADMS are used in place of each ROADM that would be installed in the network to build a single-band ROADM system. More generally, in a multi-band ROADM system, multiple ROADMS are used in place of each ROADM that would be installed in the network to build a single-band ROADM system. 
     In this specification, various preferred embodiments have been described with reference to the accompanying drawings. It will, however, be evident that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the broader scope of the invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative rather than restrictive sense. 
     For example, while a series of blocks have been described with regard to the process illustrated in  FIG. 12 , the order of the blocks may be modified in other implementations. In addition, non-dependent blocks may represent blocks that can be performed in parallel. 
     No element, block, or instruction used in the present application should be construed as critical or essential to the implementations described herein unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise.