Abstract:
A communication system includes a primary module with: first trunk and branch input and output ports, each coupled to first in/out couplers, the first in/out couplers coupled to first circulators, and the circulators in turn coupled to first central couplers; second trunk branch input and output ports, each coupled to second in/out couplers, each of second in/out couplers coupled to second circulators; and first and second bidirectional wavelength-selective switches (WSSes) coupled to the first central couplers. The system also includes a secondary module with first trunk and branch input and output ports, each coupled to first in/out couplers, the first in/out couplers coupled to third circulators, and the circulators in turn coupled to second central couplers; second trunk branch input and output ports, each coupled to second in/out couplers, each of second in/out couplers coupled to fourth circulators, and third and fourth bidirectional wavelength-selective switches (WSSes) coupled to the second central couplers. The system further includes one or more 2×2 switches coupling the primary module WSS outputs with the secondary module WSS outputs.

Description:
[0001]    This application claims priority to Provisional Application 62/144,575 filed Apr. 8, 2015, the content of which is incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    This application is related to a ROADM system. 
         [0003]    Submarine optical networks form the backbone of global communication networks. Submarine optical networks usually consist of main trunks that connect the trunk terminals, which are point-to-point cable links with in-line optical amplifiers to boost the signal power, and branch path that connect to other branch terminals. A branching unit (BU) is a network element that splits the signal between the main trunk and the branch path and vice versa. This allows the signals from different paths to share the same fiber, instead of installing dedicate fiber pairs for each link. The signal splitting and combining function of the BU is usually performed optically, therefore the BU has the similar function as the optical add/drop multiplexer (OADM) in the terrestrial WDM networks.  FIG. 1  shows the schematic of existing submarine communication system that consists of two trunk terminals, one branch terminal and one BU that links the main trunk with the branch path. Bidirectional fiber transmission is illustrated. Note that there are there are two pairs of fiber between the BU and the branch terminal. One pair is used to connect Trunk Terminal 1 to the branching terminal, while the second pair is used between Trunk Terminal 2 and the branch terminal. This allows the reuse of the same wavelengths in the TT1-BT path and the TT2-BT path. 
         [0004]    Typically, a BU consists of two subcomponents, one is called power switched branching unit (PSBU), and the other is an OADM unit. It&#39;s illustrated in  FIG. 2 . The PSBU is inserted in the main trunk, and has several 1×2 switches. It can decide whether there will be add/drop to a branch terminal from this point. If there is no branching path connected, or if add/drop to the branch terminal is not required, the switches will send the signals directly along the trunk path ( FIG. 2( a ) ). This is useful to pre-set a branching point for future use. 
         [0005]    If add/drop is required, and OADM unit is connected to the PSBU, and the 1×2 switches in the PSBU are switched to send the signal to/from the OADM unit, as shown in  FIG. 2( b ) . Inside the OADM unit, a wavelength add/drop subsystem connects the 4 fiber pairs (or 3 pairs, if there&#39;s only one branch fiber pair), and performs wavelength add/drop function according to the network requirement. These 4 fiber pairs and the respective ports are named A, B, C, and D in this document, where ports A and B are connected to the main trunk through the PSBU, and ports C and D are connected to the branch terminal. There are optical signal monitors at each input port, and optical amplifiers at the input and output of each port. The 4-input-4-output OADM unit can be redrawn as  FIG. 3 . 
         [0006]    The key task of designing a BU is to design the optical add/drop subsystem. Therefore in the remaining sections of this document, we ignore the components that are not related to switching or reconfiguration (such as amplifiers, power monitors, and power supplies), and focus on the optical add/drop subsystem (OADM subsystem) design. 
         [0007]    Conventional BU and the submarine network have fixed, pre-determined wavelength arrangement, therefore no reconfiguration is required. However, the traffic in the global communication network is becoming more dynamic as Internet-based traffic becomes more dominating. Therefore the wavelength reconfigurability is required for the next generation submarine network, with reconfigurable BU as the key enabling element. 
         [0008]    Various submarine network BU architectures have been proposed to add reconfigurability and to achieve reconfigurable optical add/drop multiplexing (ROADM) function between the main trunk and branch path in submarine network. The OADM unit becomes a ROADM unit, and the OADM subsystem becomes the ROADM subsystem. These architectures and techniques include using tunable filters, filter array with switch, wavelength-selective switches (WSS), 2×2 switches in bidirectional demultiplexer-switch-multiplexer (DSM) system, 1×2 switches in split-and-select DSM system, 1:2 interleaver with source tuning, 2:2 interleaver with source tuning, etc. Ref [1] describes these architectures, and compares their various features, including reconfigurability, number of branch fiber pairs, latching capability, and broadcasting feature, etc. It also contains more detail reference for each architecture. 
         [0009]    Among these architectures, WSS-based architecture provides the highest level of reconfigurability (2 K  configurations can be achieved, where K is the number of WDM channels at the narrowest channel spacing acceptable in the system). (In comparison, the tunable filter-based architectures can deliver K configurations; the 2×2 switch-based architecture can deliver 2 B  configurations, where B is the number of wavebands, which is a few times smaller than K; and the interleaver-based architecture can deliver 2 K/2  configurations if there&#39;s only one branching split, and fewer configurations at higher split numbers.) 
         [0010]    WSS is also the key optical component for wavelength switching in the terrestrial ROADM nodes, it has reached technology maturity in the past decade and is widely available commercially by multiple key optical component vendors, therefore it is most likely that WSS-based architecture will also be the main solution for submarine reconfigurable branching unit design. This invention is targeting the WSS-based ROADM branching unit. There are 3 key requirements for submarine branching unit design: 
         [0011]    1. Reliable: Due to the physical location and environment (deep sea, ocean bottom), the required time and effort to identify fault and to repair damages in submarine networks is much greater than in the terrestrial network. Therefore the reliability of the branching unit equipment is very important. Several measures are taken to address the reliability issue. For example, submarine-grade devices are used, these devices have low Failure in Time (FIT) rate (which is the number of failures that can be expected in one billion device-hours of operation). Also, it&#39;s desirable to use only passive device or device with latching feature (which means that the switches will maintain their switching setting even after the power is turned off or cut). Since most of the reconfigurable BU require active switches to provide reconfigurability (except very few examples such as in [2] where the reconfigurability is controlled at the terminal node), switches with latching feature is highly desirable. However, despite technology proposals such as [3], latching WSS is still not commercially available yet. Therefore another step to increase the BU equipment&#39;s reliability is to add redundancy, so that when part of the hardware fail, the BU can maintain all or partial functions. 
         [0012]    2. Security: Secure data delivery is another important issue in submarine optical network design. Since submarine networks usually connect multiple countries, the possibility of a terminal (country) receiving non-designated data between other terminals (countries) is a serious security risk and should be prevented. This is even more critical since most submarine network is owned by a consortium of multiple companies and organizations, with ownership distributed among different countries. It is unlikely for a single country or a single company to manage the entire network and control the security setting, especially since the hardware are located at each participating country. Secure data delivery means that only the intended channels will reach each destination terminal (no matter it is a branch terminal or trunk terminal), and thus the data and information carried in each WDM channel cannot be received at unintended terminal. 
         [0013]    3. Low cost: Due to the fact that high reliability submarine-grade devices are much more costly than regular device, and due to other submarine network-specific restrictions and requirements (such as long transmission distance, limited electrical power supply and limited space), the submarine branching unit hardware is more expensive than terrestrial ones. Therefore a design requirement is to keep the cost down as much as possible while meeting the other requirements. In reconfigurable branching unit, the most costly device is the WSS. Therefore keeping the number of WSS to the minimum is one of the design goals. For example, in the bidirectional, 2-branching-pair OADM unit shown in  FIG. 3 , a target is that the total number of WSS does not exceed 4 (one per path). This will also reduce the electrical power requirement and hardware footprint, which are both scarce in branching unit, it will also reduce control complexity, which leads to better reliability. These are all beneficial to branching unit equipment. 
         [0014]    Since all these 3 requirements are important, they need to be met concurrently. However, it is challenging to satisfy all these requirements at the same time. For example, providing redundancy requires additional backup hardware, this is in contradiction to the requirement to keep the hardware cost low. In another example, having secure data delivery means that wavelength-selective device (such as WSS) should be used instead of passive splitting (broadcasting) device (such as optical splitter), but the cost of WSS is more costly then splitter. This invention is aimed at addressing this challenge, namely to meet all these requirements concurrently. 
         [0015]    Since the ROADM subsystems in most submarine branching units are 4-input-4-output ROADM (such as in  FIG. 3 , with full reconfigurability), the target of the ROADM subsystem design can be described more specifically as: Besides meeting the fundamental requirement of providing full reconfigurability through using WSS, the 4-input-4-output ROADM BU should satisfy the following 3 requirements: (A) There are only up to 4 WSS devices in the ROADM unit; (B) When up to 2 WSS fail, the ROADM unit can maintain all the switching (add/drop/through) functions (i.e. with redundancy); (C) Secure data delivery can be maintained at all time, even during device failures. In this invention, this configuration and target is addressed, however the principle can be applied to other configurations (such as 3-input-3-output ROADM). 
         [0016]      FIG. 4  shows the most common configuration of 4-input-4-output WSS-based ROADM subsystem. At each input, an optical splitter is used to split the signal to two paths, one for the opposite trunk terminal, the other for the branch terminal. At each output, a WSS is used to select only the appropriate signal for the destination terminal, therefore it achieves both full reconfigurability and secure data delivery functions. The 4-WSS limit is also satisfied. However, if any single WSS fails, the ROADM function cannot be fully maintained. 
         [0017]    A similar configuration is called switch-coupler (SC) configuration, as shown on  FIG. 5 , where the WSS&#39;s are placed at the input end (1×2 WSS&#39;s, instead of 2×1 WSS&#39;s), and the optical couplers (same device as optical splitters) are placed at the output end. This is the same configuration as in  FIG. 8  of Ref [1]. This configuration has the same feature as the one on  FIG. 4 : full reconfigurability, secure data delivery, no more than 4 WSS, but cannot maintain ROADM function when any WSS fails. 
         [0018]    Therefore it can be noticed that the main limitation of the current ROADM design is that it cannot maintain ROADM function during WSS failure. In other words, providing redundancy protection is a key feature to be added. And it has to be done without requiring additional WSS hardware and without sacrificing secure data delivery feature. 
         [0019]    One conventional design adds redundancy is to use 2×1 switches and additional fiber connections, as shown on  FIG. 6 . Firstly, there is an additional splitter at the input from Trunk Terminal A. The input signal is split into two paths, one is the original path similar to  FIG. 4 , the other is sent to Switch  2  for redundancy purpose. When WSS  1  fails, Switch  1  is set so that the B output takes signal from WSS  2  (which is reconfigured accordingly to provide traffic to Trunk Terminal B), and Switch  2  is also set to take the input A signal from the redundancy splitter for output to Branch Terminal D. If WSS  2  fails, WSS  1  and Switch  1  remain unchanged to provide traffic for Trunk Terminal B, but Switch  2  is switched to take the input A signal from the redundancy splitter for output to Branch Terminal D. Similar protection is used for the section in red for WSS  3  and WSS  4 . 
         [0020]    While being able to provide redundancy during single WSS failures or some cases of simultaneous two WSS failure (one from WSS 1  and WSS 2 , and one from WSS 3  and WSS 4 ), this solution cannot provide traffic security to branch terminal during WSS failure, since the traffic to the relevant branch terminal comes from the input splitter from the trunk terminal and does not go through the selection/filtering process of WSS. Also it cannot handle the case when both WSS 1  and WSS 2  fail at the same time, or when both WSS 3  and WSS 4  fail at the same time. 
         [0021]    Another conventional design uses additional 4×1 WSS as well as four optical splitters, two 2×2 switches, two 2×1 switches, and one 1×2 switches, as shown on  FIG. 7 . All input signals are split into two paths, one normal working path and one for backup. The 4×1 WSS works with other optical switches to provide WSS function in case one of the four original WSS fails. The advantage of this design is that it can maintain traffic security during single WSS failure. However, it requires additional WSS (and 4×1 WSS is usually more costly than 2×1 WSS), and it cannot handle more than one WSS failure at a time. 
       SUMMARY 
       [0022]    A communication system includes a primary module with: first trunk and branch input and output ports, each coupled to first in/out couplers, the first in/out couplers coupled to first circulators, and the circulators in turn coupled to first central couplers; second trunk branch input and output ports, each coupled to second in/out couplers, each of second in/out couplers coupled to second circulators; and first and second bidirectional wavelength-selective switches (WSSes) coupled to the first central couplers. The system also includes a secondary module with first trunk and branch input and output ports, each coupled to first in/out couplers, the first in/out couplers coupled to third circulators, and the circulators in turn coupled to second central couplers; second trunk branch input and output ports, each coupled to second in/out couplers, each of second in/out couplers coupled to fourth circulators, and third and fourth bidirectional wavelength-selective switches (WSSes) coupled to the second central couplers. The system further includes one or more 2×2 switches coupling the primary module WSS outputs with the secondary module WSS outputs. 
         [0023]    Implementations of the system can use optical circulators to perform bidirectional operation to save WSS requirement, so that the spared WSS can provide redundancy protection. Also, it uses small scale optical switches to allow the sharing of protection hardware among different paths, therefore any 2 simultaneous failures can be handled. 
         [0024]    Advantages of the system may include one or more of the following. The ROADM subsystem design enables low cost ROADM BU, while delivering full reconfigurability, redundancy for protection, and secure data delivery. Therefore it is a low cost, fully functional, and high reliability solution. Some features of the exemplary design include:
       Low cost (only require four 1×2 WSS&#39;s)   Allows full reconfigurability   Secure data delivery at all paths   Reuse of dummy light (not requiring light source in the BU to generate dummy light for the outgoing links)   Redundancy protection (for up to any 2 simultaneous WSS failures, all ROADM functions and secure data delivery feature are maintained)       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  shows an exemplary schematic of a submarine communication system with a BU. 
           [0031]      FIG. 2A  shows a structure of a Branching Unit without add/drop while  FIG. 2B  with add/drop to branch terminal. 
           [0032]      FIG. 3  shows an exemplary OADM Unit Configuration. 
           [0033]      FIG. 4  shows an exemplary splitter-switch WSS-based ROADM subsystem. 
           [0034]      FIG. 5  shows an exemplary switch-coupler WSS-based ROADM subsystem. 
           [0035]      FIG. 6  shows an exemplary system adding redundancy protection by splitters and 2×1 switches. 
           [0036]      FIG. 7  shows an exemplary adding redundancy protection by 4×1 WSS 
           [0037]      FIG. 8  shows an exemplary bidirectional WSS-based ROADM subsystem. 
           [0038]      FIG. 9  shows two exemplary types of 2×2 optical switches. 
           [0039]      FIG. 10  shows an exemplary low cost secure ROADM subsystem with redundancy protection using 2×2 bypass switches. 
           [0040]      FIG. 11A-11D  show exemplary working paths when one working WSS and one backup WSS fail simultaneously for ROADM subsystem with 2×2 bypass switches. 
           [0041]      FIG. 12  shows an exemplary low cost secure ROADM subsystem with redundancy protection using 2×2 cross switch. 
           [0042]      FIG. 13  shows exemplary normal working paths for ROADM subsystem with 2×2 cross switch. 
           [0043]      FIG. 14  shows exemplary new working paths when both working path WSS&#39;s fail simultaneously for ROADM subsystem with 2×2 cross switch. 
           [0044]      FIG. 15A-15D  shows exemplary new working paths when one working WSS and one backup WSS fail simultaneously for ROADM subsystem with 2×2 cross switch. 
       
    
    
     DESCRIPTION 
       [0045]    (1) Design 
         [0046]    The exemplary ROADM subsystem architecture is shown on  FIG. 8 . It uses the bidirectional operation feature of WSS, and the unidirectional transmission feature of 3-port optical circulator. The detailed operation principle and features are described in [4]. By using such configuration, two WSS&#39;s are sufficient for the 4-input-4-output ROADM subsystem. It provides full reconfigurability for all paths, and offer secure data delivery feature. However, it does not provide any redundancy protection feature, and since each WSS is used for two paths simultaneously (through bidirectional operation), the failure of each WSS will lead to the failure of two paths, which is half of the BU connections. This is a significant issue for submarine communication, since it will affect large amount of inter-continental traffic and is difficult to repair. In this invention, the redundancy feature is added while maintaining the benefits of this architecture. 
         [0047]    (2) Components 
         [0048]    Besides the components used in the previous architecture shown on  FIG. 8  (i.e. 1×2 bidirectional WSS, 1:2 optical coupler/splitter, and  3 -port optical circulator), the exemplary architecture uses 2×2 optical switches. There are at least two types of 2×2 optical switches available in terms of the switching functions. Each switch has two switching states.  FIG. 9  explains the difference and the symbol used in this document. 
         [0049]    The first type is called “2×2 bypass switch”. In the first state (called the “Bar state”), Input  1  is connected to Output  1 , and Input  2  is connected to Output  2 . In the second state (called the “Bypass state”), Input  2  is connected to Output  1 , while Input  1  and Output  2  are not connected together. In other words, the Input  1  signal is terminated, and Output  2  receives no signal. 
         [0050]    The second type is called “2×2 cross switch”. The first state is the same “Bat state” where Input  1  is connected to Output  2 , and Input  2  is connected to Output  2 . The second state is called “Cross state”, where Input  1  is connected to Output  2 , and Input  2  is connected to Output  1 . 
         [0051]    Both types of switches allow bidirectional operation, which means that the output ports can be used as input ports, and the input ports can be used as output ports. The availability of these two types of optical switches varies, therefore our invention consider each type of switch separately. 
         [0052]    (3) Design 1: using 2×2 Bypass Switches
       (3.1) ROADM Subsystem Design       
 
         [0054]      FIG. 10  is the schematic of the exemplary solution with 2×2 bypass switch ( 101 ). It serves 2 trunk terminals (A ( 102 ) and B ( 103 )), and 2 branch terminals (C ( 104 ) and D ( 105 )). It consists of eight 3-port optical circulators ( 106 - 113 ), four 2×1 WSS&#39;s ( 114  and  117 , also operate as 1×2 WSS&#39;s), two 2×2 bypass switches ( 119  and  119 ), and twelve optical splitters/couplers ( 120  to  131 ). Since optical splitters and couplers are essentially the same device, we will refer to them as “optical couplers” or simply “couplers” in the remaining document). Each 3-port circulator passes signals from Port  1  to Port  2 , and Port  2  to Port  3 , while isolates/blocks signals in the reverse direction (i.e. Port  2  to Port  1 , and Port  3  to Port  2 ). These components are interconnected through optical fiber within the ROADM subsystem. The signals for each source-destination pair are illustrated as colored and shaded bars next to the input and output fibers. 
         [0055]    In this ROADM subsystem, there are 4 circulator-coupler-WSS-switch-circulator paths, such as circulator ( 106 ) to coupler ( 124 ) to WSS ( 114 ) to switch ( 119 ) to circulator ( 110 ). These 4 paths are in parallel, with some cross-connection in between. At any time, two of them work as the working paths, and the other two are not used (either serve as backup paths, or are failed paths). Since these 4 paths are symmetrical in terms of the function, here we assign the upper 2 paths in  FIG. 10  as the working paths, and the lower 2 paths as backup paths. In reality any 2 paths can be used as working paths. 
         [0056]    (3.2) Normal Operation 
         [0057]    The input signal from Trunk Terminal A ( 102 ) is used as the example here. The signal ( 132 ) consists of solid filled red and blue bars, symbolizing different optical channels for different destinations. The signals going to the same destination (such as the solid red bars that are signals from Trunk Terminal A to Trunk Terminal B) do not need to be contiguous in the wavelength/frequency domain, as illustrated. When the signal ( 132 ) enters the ROADM subsystem, it is split by a 1:2 coupler ( 120 ) to two paths. One of them is for the working path, which enters the 3-port optical circulator  106 , and the other is for the backup path, which enters another 3-port optical circulator  108 . For the working path, the signal goes through Port  1  of the 3-port optical circulator ( 106 ), and exits Port  2  to enter the 1:2 coupler ( 124 ). The signal is again split to two paths. The first path goes to Input  1  of a 2×1 WSS ( 114 ). This WSS is configured by the controller of the ROADM subsystem according to the network&#39;s wavelength plan. It picks up only the channels intended for Trunk Terminal B (which are the solid red bars), and discards the remaining channels (the solid blue bars). Since WSS can select each individual channel to either Input  1  or Input  2  independently, all the 2 K  configurations (where K is the number of WDM channels) can be achieved, delivering full reconfigurability. 
         [0058]    The second path (i.e. the second output of coupler  124 ) is sent to Input  1  of another 2×1 WSS ( 115 ), which picks up the solid blue channels as they are intended for Branch Terminal D, and discards the solid red channels that are intended for Terminal B. 
         [0059]    Another terminal that contains signal for Trunk Terminal B is the Branch Terminal C ( 104 ). The input signal ( 133 ) from this terminal consists of the signals for Terminal B (represented by the shaded green bars), as well as some dummy light (represented by the solid black bars), since there is no need to send signal to other terminals (i.e. A and D). This signal goes through the similar device: firstly split by 1:2 coupler  122 , the first path (working path) is then enters Port  1  of circulator  107 , exists Port  2 , then is split by another 1:2 coupler  125  into 2 paths, one of them enters input  2  of WSS  114 . As controlled by the BU controller, the WSS selects the intended channels (shaded green channels), and discards the dummy signals. At the output (Common port) of the WSS, the signal is the combination of the A=&gt;B signal (solid red channels) and C=&gt;B signal (shaded green channels). This combined signal goes through a 2×2 bypass switch ( 119 ), which is at the bar state. The signal enters Input  1  of the switch and exits Output  1 . It then enters Port  2  of the Terminal B circulator ( 110 ) and exits Port  3 , and is combined with signal from backup path (which is sent from circulator  112 ) through the 1:2 coupler  128 . Since the backup path is not used, there is no signal arriving from circulator  112  at this moment. The combined signal ( 134 ) is sent to the output port for Trunk Terminal B ( 103 ). It contains only the intended channels, ensuring secure data delivery. Due to intelligent network planning and wavelength assignment, there are no wavelength contention between the A=&gt;B channels and C=&gt;B channels. 
         [0060]    The second output of the coupler  125  enters Input  2  of 2×1 WSS  115 , which filters out the shaded green channels, and keeps the dummy light. The output (Common port) of the WSS consists of the combination of the A=&gt;D signals (solid blue channels) and dummy light from C. This signal passes through 2×2 bypass switch ( 118 ), which is at the bar state. The signal enters Input  1  of the switch and exits Output  1 . It then enters Port  2  of the circulator  111  and exits Port  3 , and is then combined with backup path signal from circulator  113  (which does not contain any signal during normal operation) through 1:2 coupler  130 . The combined signal  135  is sent to Branch Terminal D ( 105 ). No unintended signal goes to Terminal D, and the dummy light from Terminal C is reused to maintain power balance in the submarine link between the branching unit and Terminal D. 
         [0061]    Similar operation is performed for the signals from B and from D, but in the opposite way (red arrows). The signal from B ( 136 ) is split into working and backup paths by 1:2 coupler  129 . The working path is then sent through circulator  110  (Port  1  to Port  2 ) and the bar-state 2×2 bypass switch  119  to WSS  114 , which now behaves as a 1×2 WSS. The signal intended for Trunk Terminal A (represented by the checked red bars) and the signals intended for Branch Terminal C (represented by the solid green bars) are separated by the WSS at 2 different outputs. 
         [0062]    At the same time, the signal  137  from Terminal D ( 105 ) is split into working and backup paths by 1:2 coupler  131 . The working path is then sent through circulator  111  (Port  1  to Port  2 ) and the bar-state 2×2 bypass switch  118  to WSS  115 , which separated it into two parts, one consists of signal intended for Terminal A (represented by the crossed blue bars), and the other consists of dummy light (represented by the shaded black bars). Through the fiber cross-connection, the signals from these two WSS&#39;s intended for Trunk Terminal A (i.e. the checked red channels, and crossed blue channels) are combined by coupler  124 , which now behaves as a 2:1 coupler, and sent through circulator  106  (Port  2  to Port  3 ) to the output 1:2 coupler  121 . The coupler  121  combines this working path signal with the signal from backup path (which is from circulator  108  and contains no signal during normal operation), and send the combined signal ( 138 ) to the intended destination Terminal A ( 102 ). 
         [0063]    Simultaneously, the solid green signals from Output  2  of WSS  114  intended for Branch Terminal C is combined with the reused dummy light from Terminal D (Output  2  of WSS  115 ) by 1:2 coupler  125 , and sent to through circulator  107  (Port  2  to Port  3 ). This combined working path signal is further combined with the backup path signal from circulator  109  by 1:2 coupler  123 . The combined signal  139  is sent to the intended destination Terminal C ( 104 ). 
         [0064]    Due to the symmetric wavelength assignments, both these two WSS&#39;s have the same wavelength configuration, and the two opposite direction operations of each WSS also have the same wavelength configuration. As represented in the figure, A=&gt;B (solid red) and B=&gt;A (checked red) paths have the same wavelength channels (let&#39;s call it Channel Group I), and A=&gt;D (solid blue), D=&gt;A (checked blue), B=&gt;C (solid green), and C=&gt;B (shaded green) paths have the same wavelength channels (let&#39;s call it Channel Group II). Channel I and Channel II are complementary in the overall channel plan. Due to this feature, these WSS&#39;s can be used simultaneously in both directions. 
         [0065]    The dummy lights (filled-in channels) are used to fill in for the unused channels to maintain the set power level in the transmission link. Therefore the key requirement is the total power level, and not the number of channels or the exact wavelength/frequency. And therefore fewer channels with higher power can be used to substitute more channels with lower power, as illustrated in  FIG. 7 . Within the signal  115  from Branch Terminal C to the branching unit, only the shaded green channels carry useful information. For the remaining parts of the spectrum, dummy lights are used to fill in. It is not necessary to have the dummy light at each empty channel, instead fewer dummy lights (represented by the solid black bars) are used to balance the total power. These dummy lights are reused at the output links from the BU (such as  117 , which reuses the dummy light from  115 ). 
         [0066]    (3.3) Single WSS Failures 
         [0067]    In case that any of the backup path WSS ( 116  and  117  in this example) fails, since they are not used in actual operation, the ROADM subsystem operates normally. The transmission and switching function of the BU is not affected. 
         [0068]    If one of the working path WSS fails (say,  114 ), the backup paths are used. The other working WSS ( 115  in this example) is also switched off, while both backup WSS&#39;s ( 116  and  117 ) are turned on. WSS  116  is set to the same configuration as WSS  114 , and WSS  117  is set to the same configuration as WSS  115 . The respective passive components also change their roles. Here circulator  108  replaces circulator  106 &#39;s function, coupler  126  replaces coupler  124 &#39;s function, circulator  112  replaces circulator  110 &#39;s function, circulator  109  replaced circulator  107 &#39;s function, coupler  127  replaces coupler  125 &#39;s function, and circulator  113  replaces circulator  111 &#39;s function. The 2×2 bypass switches  118  and  119  remain at the bar state. Due to the signal splitting function by splitters  120 ,  122 ,  129  and  131 , as well as signal combing function by couplers  121 ,  123 ,  128 , and  130 , the backup paths have the same signal as the original working path. Therefore the ROADM system operates as usual, i.e. all the ROADM functions are kept, secure data delivery is maintained for all connections, and fully reconfigurability is available. Therefore redundancy protection is achieved. 
         [0069]    The same protection step is taken if the other working path WSS ( 115 ) fails, all the ROADM system&#39;s functions are maintained and redundancy protection is achieved. 
         [0070]    (3.4) Simultaneous Two WSS Failures 
         [0071]    In case that both backup path WSS ( 116  and  117 ) fail at the same time, the ROADM subsystem&#39;s function and operation is not affected. 
         [0072]    In case that both working path WSS ( 114  and  115 ) fail at the same time, the protection step for single WSS failure described above (section 3.3) can be applied, and all ROADM functions with secure data delivery are maintained. 
         [0073]    There are 4 combinations when one of the working path WSS and one of the backup path WSS fail simultaneously: 
         [0074]    (3.4.a) 
         [0075]    If working path WSS  114  and backup path WSS  117  fail at the same time, backup path WSS  116  is turned on and configured with the original configuration of WSS  114 . The 2×2 bypass switch  118  remains at the bar state, and the other 2×2 bypass switch  119  is not used (so can be at any state). The signals from Terminal A to Terminal B will go through splitter  120  and take the backup path. They pass through circulator  108  and splitter  126 , and reach Input  1  of WSS  116 . At the same time, the signals from Terminal C to Terminal B also go through the splitter  122  and take the backup path. They pass through circulator  109  and splitter  127 , and reach Input  2  of WSS  116 . The WSS  116  combines them appropriately according to network channel assignment to form the output signal for Terminal B. This combined signal passes through 2×2 switch  118  (Input  2  to Output  2 ), and reaches Terminal B through circulator  112  and coupler  128 . 
         [0076]    The signals from Terminal B to Terminal A and the signals from Terminal B to Terminal C travel in the opposite direction. 
         [0077]    There is no change for the other signals (from Terminal A to Terminal D, from Terminal C to Terminal D, from Terminal D to Terminal A, and from Terminal D to Terminal C), the same light paths and operations as the normal condition (section 3.2 above) are used. 
         [0078]    (3.4.b) 
         [0079]    If working path WSS  115  and backup path WSS  116  fail at the same time, similar operation as in (3.4.a) is performed to achieve protection. As illustrated in  FIG. 11( b ) , backup WSS  117  is turned on and configured with the original configuration of WSS  115 . 2×2 bypass switch  119  is kept at the bar state. The other 2×2 bypass switch ( 118 ), circulators  111 , and circulator  112  are not used. All ROADM functions with secure data delivery are maintained. 
         [0080]    (3.4.c) 
         [0081]    If working path WSS  114  and backup path WSS  116  fail at the same time, path WSS  117  is turned on and configured with the original configuration of WSS  114 . The signals from Terminal A to Terminal B will go through splitter  120  and take the backup path. They pass through circulator  108  and splitter  126 , and reach Input  1  of WSS  117 . At the same time, the signals from Terminal C to Terminal B also go through the splitter  122  and take the backup path. They pass through circulator  109  and splitter  127 , and reach Input  2  of WSS  117 . The WSS  117  combines them appropriately according to network channel assignment to form the output signal for Terminal B. This combined signal passes through 2×2 switch  119 , which is at the bypass state (Input  2  to Output  1 ), and reaches Terminal B through circulator  110  and coupler  128 . 
         [0082]    The signals from Terminal B to Terminal A and the signals from Terminal B to Terminal C travel in the opposite direction. 
         [0083]    There is no change for the other signals (from Terminal A to Terminal D, from Terminal C to Terminal D, from Terminal D to Terminal A, and from Terminal D to Terminal C), the same light paths and operations as the normal condition (section 3.2 above) are used. The 2×2 bypass switch  118  remains at the bar state. 
         [0084]      FIG. 11( c )  illustrates the new working paths (the unused paths are shown in gray dashes). All ROADM functions with secure data delivery are maintained. 
         [0085]    (3.4.d) 
         [0086]    If working path WSS  115  and backup path WSS  117  fail at the same time, similar operation as in (3.4.c) is performed to achieve protection. As illustrated in  FIG. 11( d ) , backup WSS  116  is turned on and configured with the original configuration of WSS  115 . 2×2 bypass switch  118  is switched to the bypass state, and the other 2×2 bypass switch ( 119 ) remains at the bar state. All ROADM functions with secure data delivery are maintained. 
         [0087]    (4) Design 2: Using 2×2 Cross Switches 
         [0088]    If 2×2 cross switch is available, the number of 2×2 switches can be reduced from 2 to 1,as illustrated in  FIG. 12 . The 2×2 cross switch ( 201 ) is inserted between the “WSS  115  to circulator  111 ” link and the “WSS  116  to circulator  112 ” link (following the notation on  FIG. 10 , same for below). 
         [0089]    We still assume WSS  114  and WSS  115  are for working paths, therefore WSS  116  and WSS  117  serve as backup and not used in the normal operation. During the normal operation, the 2×2 cross switch  201  is switched at the bar state. The operation and light paths of the ROADM subsystem are essentially the same as the ROADM subsystem with 2×2 bypass switches described in (3.2). The working paths are illustrated in  FIG. 13 , where the unused devices and paths are shown in gray dashes. The same setting also covers the situation where either one or both backup WSS&#39;s ( 116  and  117 ) fail. This ROADM subsystem provides full ROADM functions with secure data delivery in all paths. 
         [0090]    If either one or both working path WSS&#39;s ( 114  and  115 ) fail, the backup WSS&#39;s ( 116  and  117 ) are turned on. The 2×2 cross switch remains at the bar state. The new working paths are illustrated in  FIG. 14 . 
         [0091]    If one of the original working path WSS and one of the backup path WSS fail at the same time, there are 4 combinations (4 cases). An appropriate backup WSS is turned on to provide the protection. The new working paths for each case are shown in  FIG. 15( a )  to (d) respectively. 
         [0092]    It can be observed that the 2×2 cross switch ( 201 ) remains at the bar state when both WSS  114  and WSS  117  fail. If both WSS  115  and WSS  116  fail, no light passes through the 2×2 cross switch, therefore its state is irrelevant. For the other 2 cases (both WSS  114  and WSS  116  fail, and both WSS  115  and WSS  117  fail), the 2×2 cross switch is switched to the cross state. 
         [0093]    It can also be noticed that in the case when both WSS  115  and WSS  117  fail ( FIG. 15( c ) ), the WSS  115  is configured to combine traffics for Terminal B, while in all other cases where WSS  115  is operational (including the normal working case), it is configured to combine traffics for Terminal D. 
         [0094]    (5) Comparison and Summary 
         [0095]    Comparing both ROADM subsystem designs, the one using 2×2 cross switch has several advantages: firstly, it reduces the number of 2×2 switches to one; furthermore, there is no insertion loss between WSS  114  and circulator  110 , and there is no insertion loss between WSS  117  and circulator  113 , because there is no optical switch in these two optical links. However since there are still insertion loss between WSS&#39;s  115 / 116  and circulators  111 / 112  due to the presence of the 2×2 switch. Therefore the optical power is not balanced among all paths. This can be easily solved by setting appropriate attenuation levels in WSS  114  and WSS  117 . 
         [0096]    For the ROADM subsystem using 2×2 bypass switch, there is no insertion loss imbalance issue. However it requires two separate 2×2 optical switches. 
         [0097]    In summary, both exemplary ROADM subsystems use only four 1×2 WSS&#39;s, can achieve full 2 K  reconfigurability, and provide secure data delivery in all paths. They offer redundancy protection for up to 2 simultaneous WSS failure. When any two WSS fail, all the ROADM functions and secure data delivery feature are maintained. Therefore it satisfies all the requirements described above. 
         [0098]    Even though the addition of optical circulators will lead to higher insertion loss compared to the 3 prior arts described above, the increase of insertion loss is typically quite low (typically less than 2 dB total for each link), therefore it can be easily compensated by the optical amplifiers at the output of the BU. Another potential issue is the crosstalk from the reverse directional signal, this issue can be mitigated by using high quality optical components, such as optical circulators with high isolation figure, and WSS&#39;s with low back-reflection components. Using direct splicing or angled connectors (instead of flat end connectors) can also mitigate the back-reflection issue. 
         [0099]    These low cost secure ROADM subsystems with redundancy protection can be applied in submarine branching unit (as shown in  FIG. 2( b ) ) to construct low cost secure ROADM branching unit with redundancy protection. 
         [0100]    As described above in detail, the present system increases the reliability of the optical submarine branching unit having the optical path switching function and improves its transmission loss. The communication/transmission can be implemented with a simple structure. 
         [0101]    It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed process and product without departing from the scope or spirit of the invention. For example, alternate relay placement may yield similar logical results. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the invention being indicated by the following claims.