Abstract:
A communication system may include a first control circuit to detect a break on a first optical link; a first optical source to supply a first optical signal on a second optical link in response to detecting the break on the first optical link, the first optical signal propagating in a first direction on the second optical link; a second control circuit to detect a presence of the first optical signal on the second optical link, and output a control signal in response to detecting the presence of the first optical signal; and a second optical source to supply a second optical signal on the second optical link, the second optical signal propagating in a second direction opposite the first direction, where the Raman pump is disabled in response to the control signal.

Description:
BACKGROUND 
       [0001]    Wavelength-division multiplexing (WDM) is a technique where light signals of different wavelengths are combined and simultaneously transmitted over a single optical fiber. Long distance WDM systems often use amplification to amplify the light signals. 
         [0002]    One form of amplification is called Raman amplification. With Raman amplification, the amplification effect is achieved by an interaction between the signal and a pump laser within an optical fiber. One type of Raman amplification is commonly referred to as distributed Raman amplification. Distributed Raman amplification uses the transmission fiber as a gain medium by multiplexing a pump light wavelength with a signal wavelength. The pump light may be coupled into the transmission fiber in the same direction as the signal (co-propagating), in the opposite direction (counter-propagating) as the signal, or both. 
       SUMMARY 
       [0003]    According to one implementation, a communication system may include a first optical source to supply a first optical signal to a first optical link, the first optical signal propagating in a first direction along the first optical link; a second optical source to supply a second optical signal to a second optical link, the second optical signal propagating in the first direction along the second optical link; a first control circuit to detect an absence of the first optical signal on the first optical link, and output a first signal in response to detecting the absence of the first optical signal; a third optical source to supply a third optical signal to the second optical link in response to the first signal, the third optical signal propagating in a second direction different than the first direction; a second control circuit to detect a presence of the third optical signal on the second optical link, and output a second signal in response to detecting the presence of the third optical signal; and a fourth optical source supplying a plurality of fourth optical signals to the second optical link, the plurality of fourth optical signals propagating along the second optical link in the second direction, where the second optical source is disabled in response to the second signal, and the second optical signal provides Raman gain to the fourth optical signal. 
         [0004]    According to another implementation, a communication system may include a first control circuit to detect a break on a first optical link; a pilot laser to supply a first optical signal on a second optical link in response to detecting the break on the first optical link, the first optical signal propagating in a first direction on the second optical link; a second control circuit to detect a presence of the first optical signal on the second optical link, and output a control signal in response to detecting the presence of the first optical signal; and a Raman pump to supply a second optical signal on the second optical link, the second optical signal propagating in a second direction opposite the first direction, where the Raman pump is disabled in response to the control signal. 
         [0005]    According to yet another implementation, a method may include supplying a first optical signal on a first optical link, the first optical signal propagating in a first direction on the first optical link; supplying a second optical signal on the first optical link, the second optical signal propagating in a second direction, that is opposite the first direction, on the first optical link; detecting an absence of the first optical signal on the first optical link; disabling the supplying of the second optical signal on the first optical link in response to detecting the absence of the first optical signal; supplying a third optical signal on a second optical link in response to detecting the absence of the first optical signal, the third optical signal propagating in the second direction on the second optical link; supplying a fourth optical signal on the second optical link, the fourth optical signal propagating in the first direction on the second optical link; detecting a presence of the third optical signal on the second optical link; and disabling the supplying of the fourth optical signal on the second optical link in response to detecting the presence of the third optical signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more implementations described herein and, together with the description, explain these implementations. In the drawings: 
           [0007]      FIG. 1  is a diagram of an exemplary system in which systems and/or methods described herein may be implemented; 
           [0008]      FIG. 2  is a diagram of exemplary components of a Raman amplifier module (RAM) shown in  FIG. 1 ; 
           [0009]      FIG. 3  is another diagram of exemplary components of a RAM shown in  FIG. 1 ; 
           [0010]      FIG. 4  is a diagram of the exemplary system of  FIG. 1  in a situation where a link break occurs on a west-to-east link; 
           [0011]      FIG. 5  is a flowchart of an exemplary process for shutting down components in response to the link break of  FIG. 4 ; 
           [0012]      FIG. 6  is a flowchart of an exemplary process for restoring components in response to repair of the link break of  FIG. 4 ; 
           [0013]      FIG. 7  is a diagram of the exemplary system of  FIG. 1  in a situation where link breaks occur on both a west-to-east link and an east-to-west link; 
           [0014]      FIG. 8  is a flowchart of an exemplary process for shutting down components in response to the link breaks of  FIG. 7 ; 
           [0015]      FIG. 9  is a flowchart of an exemplary process for restoring components in response to repair of the link breaks of  FIG. 7 ; 
           [0016]      FIG. 10  is a diagram of the exemplary system of  FIG. 1  in a situation where a link break occurs on a local transmit link; 
           [0017]      FIG. 11  is a flowchart of an exemplary process for shutting down components in response to the link break of  FIG. 10 ; 
           [0018]      FIG. 12  is a flowchart of an exemplary process for restoring components in response to repair of the link break of  FIG. 10 ; 
           [0019]      FIG. 13  is a diagram of the exemplary system of  FIG. 1  in a situation where a link break occurs on a local receive link; 
           [0020]      FIG. 14  is a flowchart of an exemplary process for shutting down components in response to the link break of  FIG. 13 ; 
           [0021]      FIG. 15  is a flowchart of an exemplary process for restoring components in response to repair of the link break of  FIG. 13 ; 
           [0022]      FIG. 16  is a diagram of another exemplary system in which systems and/or methods described herein may be implemented; 
           [0023]      FIG. 17  is a diagram of the exemplary system of  FIG. 16  in a situation where a link break occurs on a west-to-east link; 
           [0024]      FIG. 18  is a flowchart of an exemplary process for shutting down components in response to the link break of  FIG. 17 ; 
           [0025]      FIG. 19  is a flowchart of an exemplary process for restoring components in response to repair of the link break of  FIG. 17 ; 
           [0026]      FIG. 20  is a diagram of the exemplary system of  FIG. 16  in a situation where a link break occurs on a local link; 
           [0027]      FIG. 21  is a flowchart of an exemplary process for shutting down components in response to the link break of  FIG. 20 ; 
           [0028]      FIG. 22  is a flowchart of an exemplary process for restoring components in response to repair of the link break of  FIG. 20 ; 
           [0029]      FIG. 23  is a diagram of the exemplary system of  FIG. 16  in a situation where link breaks occur on both a west-to-east link and an east-to-west link; 
           [0030]      FIG. 24  is a flowchart of an exemplary process for shutting down components in response to the link breaks of  FIG. 23 ; 
           [0031]      FIG. 25  is a flowchart of an exemplary process for restoring components in response to repair of the link breaks of  FIG. 23 ; 
           [0032]      FIG. 26  is a diagram of the exemplary system of  FIG. 16  in a situation where a link break occurs on a local transmit link; 
           [0033]      FIG. 27  is a flowchart of an exemplary process for shutting down components in response to the link break of  FIG. 26 ; and 
           [0034]      FIG. 28  is a flowchart of an exemplary process for restoring components in response to repair of the link break of  FIG. 26 . 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    The following detailed description refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. Also, the following detailed description does not limit the invention. 
       Overview 
       [0036]    Implementations, described herein, may use counter-propagating and/or co-propagating Raman pumps to perform distributed Raman amplification. Raman pumps may output relatively high power levels (e.g., optical signals with wavelengths in the range of approximately 1420-1460 nanometers (nm)) compared to the power levels of other power-generating components. Due to the relatively high power levels of the Raman pumps, noise associated with the Raman pumps may make it difficult to detect link breaks using a simple power detection of the payload (that may include a number of multiplexed payload channels) and optical supervisory channel (OSC) signal powers. In the description to follow, the payload will be referred to as a “C-band payload” (e.g., optical signals with wavelengths in the range of approximately 1530-1565 nm), though the description is not limited to payload in the C-band. Also, some implementations may not use an OSC signal. 
         [0037]    Accordingly, implementations, described herein, may perform fiber break detection using a dedicated pilot laser that transmits a pilot signal (e.g., an optical signal with a wavelength of approximately 1610 nm) that co-propagates with the payload and the OSC signal. The pilot laser may be modulated to produce one of two tone signals that facilitate in the link shutdown and recovery processes:
       Remote Receive Fault (RRF) signal: may be used to notify the far end of the link about a fiber break in the opposite fiber span and to indicate to the far end should turn off (e.g., disable) its pumps.   Normal (NRM) signal: may be used to notify the far end of the link that no fiber break exists in the opposite fiber span and to indicate that the far end should turn on (e.g., enable) its pumps.
 
These signals may be designed for ease of detection in high loss links. These signals may be generated at the near end of the link and may be detected and used at the far end of the link.
       
 
       Exemplary System 
       [0040]      FIG. 1  is a diagram of an exemplary system  100  in which systems and/or methods described herein may be implemented. While  FIG. 1  shows a particular number and arrangement of components, system  100  may include additional, fewer, different, or differently arranged components than those illustrated in  FIG. 1 . As shown in  FIG. 1 , system  100  may include a west group of components (hereinafter referred to as “group  110 ”) and an east group of components (hereinafter referred to as “group  150 ”) connected by a west-to-east link and an east-to-west link. The west-to-east link and the east-to-west link may include optical fibers. The term “group,” as used herein, may refer to a collection of components that may be located at a same end of a fiber. 
         [0041]    West group  110  may include a band multiplexer module (BMM)/optical amplifier module (OAM) (hereinafter referred to as BMM/OAM)  112  and Raman amplifier module (RAM)  118 . BMM/OAM  112  may include a transmitter (TX) module  114  and a receiver (RX) module  116 . Transmitter module  114  may generally be referred to as an optical source. In one implementation, transmitter module  114  may include an amplifier (e.g., an Erbium-doped fiber amplifier (EDFA)) that may output a C-band payload (e.g., optical signals with wavelengths in the range of approximately 1530-1565 nm), and an OSC source that may output an OSC signal (e.g., optical signal with wavelengths in the range of approximately 1505-1520 nm). Transmitter module  114  may also include a wavelength-division multiplexer (WDM) that may optically combine the C-band payload and the OSC signal for output from BMM/OAM  112 . Receiver module  116  may generally be referred to as an optical signal detector. In one implementation, receiver module  116  may include an amplifier (e.g., an EDFA) that may receive a C-band payload, and an OSC detector that may receive an OSC signal. Receiver module  116  may also include a wavelength-division demultiplexer (also referred to herein as a WDM) that may receive a combined C-band payload and OSC signal at an input of BMM/OAM  112  and optically separate the C-band payload and the OSC signal for presentation to the amplifier and OSC detector, respectively. 
         [0042]    RAM  118  may include components for performing Raman amplification.  FIG. 2  is a diagram of exemplary components of RAM  118  according to one implementation. While  FIG. 2  shows a particular number and arrangement of components, RAM  118  may include additional, fewer, different, or differently arranged components than are illustrated in  FIG. 2 . 
         [0043]    As shown in  FIG. 2 , RAM  118  may include a pilot laser  210 , a WDM  220 , Raman pumps  230 , another WDM  240 , and a controller  250 . Pilot laser  210  may generally be referred to as an optical source. In one implementation, pilot laser  2120  may include a laser that generates, for example, a 1610 nm pilot signal that may be transmitted to co-propagate with the combined C-band payload and OSC signal. In another implementation, pilot laser  210  may generate a light signal of another wavelength that may be selected so as not to interfere with the C-band payload or OSC signal and that may be detected on the far end of the link in the presence of noise from the Raman pumps (on the far end of the link). Pilot laser  210  may modulate the pilot signal to produce one of two tone signals that facilitate in link shutdown and recovery processes. In one implementation, pilot laser  210  may modulate the pilot signal to produce a remove receive fault (RRF) signal or a normal (NRM) signal. The RRF signal may be used to notify the RAM at the far end of the link (e.g., west-to-east link) that a fiber break has been detected in the opposite link (i.e., east-to-west link). The NRM signal may be used to notify the RAM at the far end of the link that no fiber break has been detected in the opposite link. WDM  220  may include an optical multiplexer that may receive the pilot signal from pilot laser  210  and the combined C-band payload and OSC signal from transmitter module  114  ( FIG. 1 ) and optically combine them for output on the west-to-east link. 
         [0044]    Raman pumps  230  may generally be referred to as an optical source that provides distributed Raman amplification. In one implementation, Raman pumps  230  may include one or more pump lasers that produce pump power (also called pump light) (e.g., optical signals with in the range of approximately 1420-1460 nm) that may be transmitted so as to counter-propagate with the C-band payload, OSC signal, and pilot signal. WDM  240  may include an optical demultiplexer that may receive the combined C-band payload, OSC signal, and pilot signal from the east-to-west link and optically separate the C-band payload and OSC signal for output to receiver module  116  ( FIG. 1 ). WDM  240  may also receive the pump power from Raman pumps  230  and output the pump power on the east-to-west link. 
         [0045]    Controller  250  may include a control circuit, such as a field programmable gate array (FPGA), or another type of hardware, that may detect the pilot signals transmitted on west-to-east link and/or east-to-west link. Controller  250  may control the operation of pilot laser  210  to turn on or off and/or to produce the RRF signal or the NRM signal. Controller  250  may also control the operation of Raman pumps  230  to turn on or off. 
         [0046]      FIG. 3  is another diagram of exemplary components of RAM  118  according to another implementation. While  FIG. 3  shows a particular number and arrangement of components, RAM  118  may include additional, fewer, different, or differently arranged components than are illustrated in  FIG. 3 . 
         [0047]    As shown in  FIG. 3 , RAM  118  may include a WDM  310 , an OSC laser  320 , a WDM  330 , a pilot laser  340 , a WDM  350 , Raman pumps  360 , a WDM  370 , and a controller  380 . WDM  310  may include an optical demultiplexer that may receive the combined C-band payload and OSC signal from transmitter module  114  ( FIG. 1 ) and optically separate the C-band payload from the OSC signal. WDM  310  may provide the C-band payload to WDM  330  and the OSC signal to OSC laser  320 . OSC laser  320  may generally be referred to as an optical source. In one implementation, OSC laser  320  may include a regeneration laser that may receive the OSC signal and regenerate the OSC signal for output to WDM  330 . WDM  330  may include an optical multiplexer that may receive the C-band payload and the regenerated OSC signal, optically combine them, and output a combined C-band payload and OSC signal to WDM  350 . 
         [0048]    Pilot laser  340  may generally be referred to as an optical source. In one implementation, pilot laser  340  may include a laser that generates, for example, a 1610 nm pilot signal that may be transmitted to co-propagate with the combined C-band payload and OSC signal. In another implementation, pilot laser  210  may generate a light signal of another wavelength that may be selected so as not to interfere with the C-band payload or OSC signal and that may be detected on the far end of the link in the presence of noise from the Raman pumps (on the far end of the link). Pilot laser  340  may modulate the pilot signal to produce one of two tone signals that facilitate in link shutdown and restoration processes. In one implementation, pilot laser  340  may modulate the pilot signal to produce a RRF signal or a NRM signal. As explained above, the RRF signal may be used to notify the RAM at the far end of the link (e.g., west-to-east link) that a fiber break has been detected in the opposite link (i.e., east-to-west link). As also explained above, the NRM signal may be used to notify the RAM at the far end of the link that no fiber break has been detected in the opposite link. WDM  350  may include an optical multiplexer that may receive the pilot signal from pilot laser  340  and the combined C-band payload and OSC signal from WDM  330  and optically combine them for output on the west-to-east link. 
         [0049]    Raman pumps  360  may generally be referred to as an optical source that provides distributed Raman amplification. In one implementation, Raman pumps  360  may include one or more pump lasers that produce pump power (also called pump light) (e.g., optical signals with in the range of approximately 1420-1460 nm) that may be transmitted so as to counter-propagate with the C-band payload, OSC signal, and pilot signal. WDM  370  may include an optical demultiplexer that may receive the combined C-band payload, OSC signal, and pilot signal from the east-to-west link and optically separate the C-band payload and OSC signal for output to receiver module  116  ( FIG. 1 ). WDM  370  may also receive the pump power from Raman pumps  360  and output the pump power on the east-to-west link. 
         [0050]    Controller  380  may include a control circuit, such as a FPGA, or another type of hardware, that may detect the pilot signals transmitted on west-to-east link and/or east-to-west link. Controller  380  may control the operation of pilot laser  340  to turn on or off and/or to produce the RRF signal or the NRM signal. Controller  380  may also control the operation of Raman pumps  360  to turn on or off. Controller  380  may further control the operation of OSC laser  320  to turn on or off (not shown in  FIG. 3 ). 
         [0051]    Returning to  FIG. 1 , east group  150  may include a BMM/OAM  152  and Raman amplifier module (RAM)  158 . BMM/OAM  152  may include a transmitter (TX) module  154  and a receiver (RX) module  156 . Transmitter module  154  and receiver module  156  may be similar in construction and operation to transmitter module  114  and receiver module  116  described with regard to west group  110 . RAM  158  may be similar in construction and operation to RAM  118  described with regard to west group  110 . 
       Link Break on West-to-East Link 
       [0052]    During normal operation (prior to the link break) (e.g.,  FIG. 1 ), west group  110  and east group  150  may exchange signals on the west-to-east link and the east-to-west link. For example, west group  110  may transmit signals to east group  150  on the west-to-east link, and east group  150  may transmit signals to west group  110  on the east-to-west link. During this time, pilot laser  210 / 340  may be turned on and may output a pilot NRM signal, and Raman pumps  230 / 360  may be turned on and may output pump power. 
         [0053]    Now assume that a link break occurs on the west-to-east link.  FIG. 4  is a diagram of exemplary system  100  in the situation where a link break occurs on the west-to-east link.  FIG. 5  is a flowchart of an exemplary process for shutting down components in response to the link break. In the description to follow, reference will be made to certain components of west group  110  and east group  150 . These components will be referred to as west and east components, respectively. For example, the RAM in west group  110  will be referred to as the west RAM, and the RAM in east group  150  will be referred to as the east RAM. 
         [0054]    The loss of the pilot signal may be detected at the east RAM (block  510 ). For example, due to the link break, the pilot signal outputted by the west RAM cannot reach the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal. Because the pilot signal is generated at 1610 nm, the east controller can differentiate the presence or absence of the pilot signal even in the presence of noise generated by the east Raman pumps, which may be in the range of approximately 1420-1460 nm. 
         [0055]    The east Raman pumps may be turned off (block  520 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal for the east Raman pumps to turn off. The east Raman pumps may turn off in response to the signal from the east controller, as shown in  FIG. 4 . 
         [0056]    The east pilot laser may be modulated to transmit the RRF signal (block  530 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east pilot laser to modulate with the RRF signal. The east pilot laser may modulate with the RRF signal in response to the signal from the east controller, as shown in  FIG. 4 . 
         [0057]    The amplifiers of the east receiver and transmitter modules may be turned off (block  540 ). The east receiver module may detect the loss of the C-band payload and/or OSC signal, as a result of the link break, and may turn off its amplifier. When the east receiver module turns off its amplifier, it may trigger the east transmitter module to turn off its amplifier. The east transmitter module may then turn off its amplifier. 
         [0058]    A backward defect indication (BDI) signal may be transmitted on the east-to-west link (block  550 ). For example, when the east transmitter module turns off its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to generate the BDI signal and output the BDI signal to the east WDM. The east WDM may optically combine the BDI signal with the pilot RRF signal and output the combined BDI signal and RRF signal on the east-to-west link. 
         [0059]    The pilot RRF signal may be detected at the west RAM (block  560 ). For example, the west controller (within the west RAM) may monitor the signals transmitted on the east-to-west link to detect the presence of the pilot signal and to determine whether the pilot signal includes the RRF signal or the NRM signal. In this case, the west controller may detect the presence of the RRF signal. 
         [0060]    The west Raman pumps may be turned off (block  570 ). For example, when the west controller detects the pilot RRF signal from the east RAM, the west controller may send a signal for the west Raman pumps to turn off. The west Raman pumps may turn off in response to the signal from the west controller, as shown in  FIG. 4 . 
         [0061]    The west pilot laser may be modulated to transmit the NRM signal (block  580 ). For example, when the west controller detects the pilot RRF signal from the east RAM, the west controller may send a signal to the west pilot laser to modulate with the NRM signal. The west pilot laser may modulate with the NRM signal in response to the signal from the west controller, as shown in  FIG. 4 . The NRM signal may indicate to the east RAM that the east-to-west link is operable. 
         [0062]    The BDI signal may be detected and the amplifiers of the west receiver and transmitter modules may be turned off (block  590 ). For example, the west receiver module may detect the BDI signal from the east transmitter module, and turn off its amplifier. When the west receiver module turns off its amplifier, it triggers the west transmitter module to turn off its amplifier. The west transmitter module may then turn off its amplifier. Both the west-to-east and the east-to-west links are now completely safe. In other words, a technician may safely repair the link break. 
         [0063]      FIG. 6  is a flowchart of an exemplary process for restoring components in response to repair of the link break. The pilot NRM signal may be detected at the east RAM (block  610 ). For example, now that the link break has been repaired, the pilot NRM signal outputted by the west RAM can reach the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal and detect the NRM signal. 
         [0064]    The east Raman pumps may be turned on (block  620 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal for the east Raman pumps to turn on. The east Raman pumps may turn on in response to the signal from the east controller. 
         [0065]    The east pilot laser may be modulated to transmit the NRM signal (block  630 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east pilot laser to modulate with the NRM signal. The east pilot laser may modulate with the NRM signal in response to the signal from the east controller. 
         [0066]    The amplifiers of the east receiver and transmitter modules may be turned on (block  640 ). The east receiver module may detect the C-band payload and/or OSC signal, as a result of the link break being repaired, detect no BDI signal, and turn on its amplifier. When the east receiver module turns on its amplifier, it triggers the east transmitter module to turn on its amplifier. The east transmitter module may then turn on its amplifier. 
         [0067]    The BDI may no longer be transmitted on the east-to-west link (block  650 ). For example, when the east transmitter module turns on its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to stop generating the BDI signal. 
         [0068]    The pilot NRM signal may be detected at the west RAM (block  660 ). For example, the west controller (within the west RAM) may monitor the signals transmitted on the east-to-west link to detect the presence of the pilot signal and to determine whether the pilot signal includes the RRF signal or the NRM signal. In this case, the west controller detects the presence of the NRM signal. 
         [0069]    The west Raman pumps may be turned on (block  670 ). For example, when the west controller detects the pilot NRM signal from the east RAM, the west controller may send a signal to the west Raman pumps to turn on. The west Raman pumps may turn on in response to the signal from the west controller. 
         [0070]    No BDI signal may be detected on the east-to-west link, and the amplifiers of the west receiver and transmitter modules may be turned on (block  590 ). For example, the west receiver module may detect the removal of the BDI signal from the east transmitter module, and turn on its amplifier. When the west receiver module turns on its amplifier, it triggers the west transmitter module to turn on its amplifier. The west transmitter module may then turn on its amplifier. As a result, system  100  may return to normal operation. 
       Link Breaks on Both West-to-East Link and East-to-West Link 
       [0071]    During normal operation (prior to the link break) (e.g.,  FIG. 1 ), west group  110  and east group  150  may exchange signals on the west-to-east link and the east-to-west link. For example, west group  110  may transmit signals to east group  150  on the west-to-east link, and east group  150  may transmit signals to west group  110  on the east-to-west link. During this time, pilot laser  210 / 340  may be turned on and may output a pilot NRM signal, and Raman pumps  230 / 360  may be turned on and may output pump power. 
         [0072]    Now assume that link breaks occur on both the west-to-east link and the east-to-west link.  FIG. 7  is a diagram of exemplary system  100  in the situation where link breaks occur on both the west-to-east link and the east-to-west link.  FIG. 8  is a flowchart of an exemplary process for shutting down components in response to the link breaks. In the description to follow, reference will be made to certain components of west group  110  and east group  150 . These components will be referred to as west and east components, respectively. For example, the RAM in west group  110  will be referred to as the west RAM, and the RAM in east group  150  will be referred to as the east RAM. 
         [0073]    The loss of the pilot signal may be detected at the east RAM (block  810 ). For example, due to the link break, the pilot signal outputted by the west RAM cannot reach the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal. Because the pilot signal is generated at 1610 nm, the east controller can differentiate the presence or absence of the pilot signal even in the presence of noise generated by the east Raman pumps, which may be in the range of approximately 1420-1460 nm. 
         [0074]    The east Raman pumps may be turned off (block  820 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east Raman pumps to turn off. The east Raman pumps may turn off in response to the signal from the east controller, as shown in  FIG. 7 . 
         [0075]    The east pilot laser may be modulated to transmit the RRF signal (block  830 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east pilot laser to modulate with the RRF signal. The east pilot laser may modulate with the RRF signal in response to the signal from the east controller, as shown in  FIG. 7 . 
         [0076]    The amplifiers of the east receiver and transmitter modules may be turned off (block  840 ). The east receiver module may detect the loss of the C-band payload and/or OSC signal, as a result of the link break, and turn off its amplifier. When the east receiver module turns off its amplifier, it triggers the east transmitter module to turn off its amplifier. The east transmitter module may then turn off its amplifier. 
         [0077]    A BDI signal may be transmitted on the east-to-west link (block  850 ). For example, the east OSC source, associated with the east transmitter module, may generate the BDI signal and output the BDI signal to the east WDM. The east WDM may optically combine the BDI signal with the pilot RRF signal and output the combined BDI signal and RRF signal on the east-to-west link. The combined BDI signal and RRF signal will not arrive at west group  110  due to the link break on the east-to-west link. 
         [0078]    Simultaneously, or substantially simultaneously, blocks  810 - 850  may be performed by west group  110 . As a result, both the west-to-east and the east-to-west links are now completely safe. In other words, a technician may safely repair the link breaks. 
         [0079]      FIG. 9  is a flowchart of an exemplary process for restoring components in response to repair of the link breaks. Assume, for example, that the east-to-west link is repaired first. The pilot RRF signal may be detected at the west RAM (block  905 ). For example, now that the link break on the east-to-west link has been repaired, the pilot RRF signal outputted by the east RAM can reach the west RAM. The west controller (within the west RAM) may monitor the presence of the pilot signal and detect the RRF signal. In response to the pilot RRF signal, the west controller may keep the west Raman pumps turned off. 
         [0080]    The west pilot laser may be modulated to transmit the NRM signal (block  910 ). For example, the west controller may send a signal to the west pilot laser to modulate with the NRM signal. The west pilot laser may modulate with the NRM signal in response to the signal from the west controller. 
         [0081]    The BDI signal may be detected on the east-to-west link and the transmitting of the BDI signal on the west-to-east link may be stopped (block  915 ). For example, the west receiver module may detect the restoration of the OSC power and detect the BDI signal sent from east group  150 . The west receiver module may keep its amplifier turned off and signal the west transmitter module to stop transmitting the BDI signal. 
         [0082]    When the west-to-east link is repaired, the pilot NRM signal may be detected at the east RAM (block  920 ). For example, now that the link break on the west-to-east link has been repaired, the pilot NRM signal outputted by the west RAM can reach the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal and detect the NRM signal. 
         [0083]    The east Raman pumps may be turned on (block  925 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east Raman pumps to turn on. The east Raman pumps may turn on in response to the signal from the east controller. 
         [0084]    The east pilot laser may be modulated to transmit the NRM signal (block  930 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east pilot laser to modulate with the NRM signal. The east pilot laser may modulate with the NRM signal in response to the signal from the east controller. 
         [0085]    The amplifiers of the east receiver and transmitter modules may be turned on (block  935 ). The east receiver module may detect that the OSC power is restored on the west-to-east link, as a result of the link break being repaired, detect no BDI signal, and turn on its amplifier. When the east receiver module turns on its amplifier, it triggers the east transmitter module to turn on its amplifier. The east transmitter module may then turn on its amplifier. 
         [0086]    The BDI may no longer be transmitted on the east-to-west link (block  940 ). For example, when the east transmitter module turns on its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to stop generating the BDI signal. 
         [0087]    The pilot NRM signal may be detected at the west RAM (block  945 ). For example, the west controller (within the west RAM) may monitor the signals transmitted on the east-to-west link to detect the presence of the pilot signal and to determine whether the pilot signal includes the RRF signal or the NRM signal. In this case, the west controller detects the presence of the NRM signal. 
         [0088]    The west Raman pumps may be turned on (block  950 ). For example, when the west controller detects the pilot NRM signal from the east RAM, the west controller may send a signal to the west Raman pumps to turn on. The west Raman pumps may turn on in response to the signal from the west controller. 
         [0089]    No BDI signal may be detected on the east-to-west link, and the amplifiers of the west receiver and transmitter modules may be turned on (block  955 ). For example, the west receiver module may detect the removal of the BDI signal from the east transmitter module, and turn on its amplifier. When the west receiver module turns on its amplifier, it triggers the west transmitter module to turn on its amplifier. The west transmitter module may then turn on its amplifier. As a result, system  100  may return to normal operation. 
       Link Break on Local Transmit Link 
       [0090]    During normal operation (prior to the link break) (e.g.,  FIG. 1 ), west group  110  and east group  150  may exchange signals on the west-to-east link and the east-to-west link. For example, west group  110  may transmit signals to east group  150  on the west-to-east link, and east group  150  may transmit signals to west group  110  on the east-to-west link. During this time, pilot laser  210 / 340  may be turned on and may output a pilot NRM signal, and Raman pumps  230 / 360  may be turned on and may output pump power. 
         [0091]    Now assume that a link break occurs on the local transmit link located between the west BMM/OAM and the west RAM.  FIG. 10  is a diagram of exemplary system  100  in the situation where a link break occurs on the local transmit link.  FIG. 11  is a flowchart of an exemplary process for shutting down components in response to the link break. In the description to follow, reference will be made to certain components of west group  110  and east group  150 . These components will be referred to as west and east components, respectively. For example, the RAM in west group  110  will be referred to as the west RAM, and the RAM in east group  150  will be referred to as the east RAM. 
         [0092]    A loss of C-band payload and/or OSC signal may be detected at the west RAM (block  1110 ). For example, due to the link break, the C-band payload and OSC signal outputted by the west BMM/OAM cannot reach the west RAM. The west controller (within the west RAM) may detect the absence of the C-band payload and/or OSC signal. 
         [0093]    The west Raman pumps and the west pilot laser may be turned off (block  1120 ). For example, when the west controller detects the loss of the C-band payload and/or OSC signal, the west controller may send signals to the west Raman pumps and the west pilot laser to turn off. The west Raman pumps and the west pilot laser may turn off in response to the signals from the west controller, as shown in  FIG. 10 . In the situation where the west RAM includes an OSC laser (e.g., OSC laser  320  in  FIG. 3 ), the west controller may also turn off the west OSC laser at this time. 
         [0094]    The loss of the pilot signal may be detected at the east RAM (block  1130 ). For example, since the west pilot laser has been turned off, there is no pilot signal to reach the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal. Because the pilot signal is generated at 1610 nm, the east controller can differentiate the presence or absence of the pilot signal even in the presence of noise generated by the east Raman pumps, which may be in the range of approximately 1420-1460 nm. 
         [0095]    The east Raman pumps may be turned off (block  1140 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east Raman pumps to turn off. The east Raman pumps may turn off in response to the signal from the east controller, as shown in  FIG. 10 . 
         [0096]    The east pilot laser may be modulated to transmit the RRF signal (block  1150 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east pilot laser to modulate with the RRF signal. The east pilot laser may modulate with the RRF signal in response to the signal from the east controller, as shown in  FIG. 10 . 
         [0097]    The amplifiers of the east receiver and transmitter modules may be turned off (block  1160 ). The east receiver module may detect the loss of the C-band payload and/or OSC signal and turn off its amplifier. When the east receiver module turns off its amplifier, it triggers the east transmitter module to turn off its amplifier. The east transmitter module may then turn off its amplifier. 
         [0098]    A BDI signal may be transmitted on the east-to-west link (block  1170 ). For example, when the east transmitter module turns off its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to generate the BDI signal and output the BDI signal to the east WDM. The east WDM may optically combine the BDI signal with the pilot RRF signal and output the combined BDI signal and RRF signal on the east-to-west link, as shown in  FIG. 10 . 
         [0099]    The BDI signal may be detected and the amplifiers of the west receiver and transmitter modules may be turned off (block  1180 ). For example, the west receiver module may detect the BDI signal from the east transmitter module, and turn off its amplifier. When the west receiver module turns off its amplifier, it triggers the west transmitter module to turn off its amplifier. The west transmitter module may then turn off its amplifier. Both the west-to-east and the east-to-west links are now completely safe. In other words, a technician may safely repair the link break. 
         [0100]      FIG. 12  is a flowchart of an exemplary process for restoring components in response to repair of the link break. A C-band payload and/or OSC signal may be detected at the west RAM (block  1205 ). For example, due to repair of the link break, the C-band payload and OSC signal outputted by the west BMM/OAM can now reach the west RAM. The west controller (within the west RAM) may detect the restoration of the C-band payload and/or OSC signal. In the situation where the west RAM includes an OSC laser (e.g., OSC laser  320  in  FIG. 3 ), the west controller may also turn on the west OSC laser at this time. 
         [0101]    The west pilot laser may be turned on and modulated to transmit the NRM signal (block  1210 ). For example, the west controller may detect the pilot RRF signal from the east RAM. In response, the west controller may send a signal to the west pilot laser to turn on and modulate with the NRM signal. The west Raman pumps may remain turned off. 
         [0102]    The pilot NRM signal may be detected at the east RAM (block  1215 ). For example, now that the west pilot laser has been turned on, the pilot NRM signal outputted by the west RAM can be received by the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal and detect the NRM signal. 
         [0103]    The east Raman pumps may be turned on (block  1220 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east Raman pumps to turn on. The east Raman pumps may turn on in response to the signal from the east controller. 
         [0104]    The east pilot laser may be modulated to transmit the NRM signal (block  1225 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east pilot laser to modulate with the NRM signal. The east pilot laser may modulate with the NRM signal in response to the signal from the east controller. 
         [0105]    The amplifiers of the east receiver and transmitter modules may be turned on (block  1230 ). The east receiver module may detect the C-band payload and/or OSC signal, as a result of the link break being repaired, detect no BDI signal, and turn on its amplifier. When the east receiver module turns on its amplifier, it triggers the east transmitter module to turn on its amplifier. The east transmitter module may then turn on its amplifier. 
         [0106]    The BDI may no longer be transmitted on the east-to-west link (block  1235 ). For example, when the east transmitter module turns on its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to stop generating the BDI signal. 
         [0107]    The pilot NRM signal may be detected at the west RAM (block  1240 ). For example, the west controller (within the west RAM) may monitor the signals transmitted on the east-to-west link to detect the presence of the pilot signal and to determine whether the pilot signal includes the RRF signal or the NRM signal. In this case, the west controller detects the presence of the NRM signal. 
         [0108]    The west Raman pumps may be turned on (block  1245 ). For example, when the west controller detects the pilot NRM signal from the east RAM, the west controller may send a signal to the west Raman pumps to turn on. The west Raman pumps may turn on in response to the signal from the west controller. 
         [0109]    No BDI signal may be detected on the east-to-west link, and the amplifiers of the west receiver and transmitter modules may be turned on (block  1250 ). For example, the west receiver module may detect the removal of the BDI signal from the east transmitter module, and turn on its amplifier. When the west receiver module turns on its amplifier, it triggers the west transmitter module to turn on its amplifier. The west transmitter module may then turn on its amplifier. As a result, system  100  may return to normal operation. 
       Link Break on Local Receive Link 
       [0110]    During normal operation (prior to the link break) (e.g.,  FIG. 1 ), west group  110  and east group  150  may exchange signals on the west-to-east link and the east-to-west link. For example, west group  110  may transmit signals to east group  150  on the west-to-east link, and east group  150  may transmit signals to west group  110  on the east-to-west link. During this time, pilot laser  210 / 340  may be turned on and may output a pilot NRM signal, and Raman pumps  230 / 360  may be turned on and may output pump power. 
         [0111]    Now assume that a link break occurs on the local receive link located between the west BMM/OAM and the west RAM.  FIG. 13  is a diagram of exemplary system  100  in the situation where a link break occurs on the local receive link.  FIG. 14  is a flowchart of an exemplary process for shutting down components in response to the link break. In the description to follow, reference will be made to certain components of west group  110  and east group  150 . These components will be referred to as west and east components, respectively. For example, the RAM in west group  110  will be referred to as the west RAM, and the RAM in east group  150  will be referred to as the east RAM. 
         [0112]    A loss of C-band payload and/or OSC signal may be detected at the west BMM/OAM (block  1410 ). For example, due to the link break, the C-band payload and OSC signal outputted by the east BMM/OAM cannot reach the west BMM/OAM. The west receiver module may detect this loss of the C-band payload and the OSC signal. 
         [0113]    The amplifiers of the west receiver and transmitter modules may be turned off (block  1420 ). For example, the west receiver module may detect the loss of the C-band payload and/or OSC signal and turn off its amplifier. When the west receiver module turns off its amplifier, it triggers the west transmitter module to turn off its amplifier. The west transmitter module may then turn off its amplifier. 
         [0114]    A BDI signal may be transmitted on the west-to-east link (block  1430 ). For example, when the west transmitter module turns off its amplifier, the west transmitter module may trigger the west OSC source, associated with the west transmitter module, to generate the BDI signal and output the BDI signal for transmission on the west-to-east link. 
         [0115]    The BDI signal may be detected and the amplifiers of the east receiver and transmitter modules may be turned off (block  1440 ). For example, the east receiver module may detect the BDI signal from the west transmitter module, and turn off its amplifier. When the east receiver module turns off its amplifier, it triggers the east transmitter module to turn off its amplifier. The east transmitter module may then turn off its amplifier. Both the west-to-east and the east-to-west links are now completely safe. In other words, a technician may safely repair the link break. 
         [0116]      FIG. 15  is a flowchart of an exemplary process for restoring components in response to repair of the link break. A C-band payload and/or OSC signal, and no BDI signal, may be detected at the west BMM/OAM (block  1510 ). For example, due to repair of the link break, the C-band payload and OSC signal outputted by the east BMM/OAM can now reach the west BMM/OAM. The west receiver module may detect this restoration of the C-band payload and the OSC signal. 
         [0117]    The amplifiers of the west receiver and transmitter modules may be turned on (block  1520 ). For example, the west receiver module may detect the restoration of the C-band payload and/or OSC signal and turn on its amplifier. When the west receiver module turns on its amplifier, it triggers the west transmitter module to turn on its amplifier. The west transmitter module may then turn on its amplifier. 
         [0118]    Transmission of the BDI signal may be stopped on the west-to-east link (block  1530 ). For example, when the west transmitter module turns on its amplifier, the west transmitter module may trigger the west OSC source, associated with the west transmitter module, to stop generating the BDI signal. 
         [0119]    No BDI signal may be detected at the east BMM/OAM, and the amplifiers of the east receiver and transmitter modules may be turned on (block  1540 ). For example, the east receiver module may detect that the BDI signal from the west transmitter module has been terminated, and turn on its amplifier. When the east receiver module turns on its amplifier, it triggers the east transmitter module to turn on its amplifier. The east transmitter module may then turn on its amplifier. As a result, system  100  may return to normal operation. 
       Alternative Exemplary System 
       [0120]      FIG. 16  is a diagram of an exemplary system  1600  in which systems and/or methods described herein may be implemented. While  FIG. 16  shows a particular number and arrangement of components, system  1600  may include additional, fewer, different, or differently arranged components than those illustrated in  FIG. 16 . As shown in  FIG. 16 , system  1600  may include a west group of components (hereinafter referred to as “west group  1610 ”) and an east group of components (hereinafter referred to as “east group  1650 ”) connected by a west-to-east link and an east-to-west link. The west-to-east link and the east-to-west link may include optical fibers. 
         [0121]    West group  1610  may include a BMM/OAM  1612 , a RAM  1620 , and a Raman extender module (REM)  1630 . BMM/OAM  1612  may include a transmitter (TX) module  1614  and a receiver (RX) module  1616 . Transmitter module  1614  may generally be referred to as an optical source. In one implementation, transmitter module  1614  may include an amplifier (e.g., an EDFA) that may output a C-band payload (e.g., optical signals with in the range of approximately 1530-1565 nm), and an OSC source that may output an OSC signal (e.g., optical signal with in the range of approximately 1505-1520 nm). Transmitter module  1614  may also include a WDM that may optically combine the C-band payload and the OSC signal for output from BMM/OAM  1612 . Receiver module  1616  may generally be referred to as an optical signal detector. In one implementation, receiver module  1616  may include an amplifier (e.g., an EDFA) that may receive a C-band payload, and an OSC detector that may receive an OSC signal. Receiver module  1616  may also include a WDM that may receive a combined C-band payload and OSC signal at an input of BMM/OAM  1612  and optically separate the C-band payload and the OSC signal for presentation to the amplifier and OSC detector, respectively. 
         [0122]    RAM  1620  may include components for performing Raman amplification. RAM  1620  may include components similar to the components of RAM  118  shown in  FIG. 2  or  FIG. 3 . RAM  1620  differs from RAM  118 , however, in that the controller of RAM  1620  generates an additional signal that was not generated by the controller of RAM  118 . The controller of RAM  1620  generates a shutdown (SD) signal that the controller may output to REM  1630 , as described below. In one implementation, the SD signal may include a signal line that can be asserted or de-asserted to cause certain operations to be performed by REM  1630 . In this case, generating the SD signal may refer to asserting (or de-asserting) the signal line. 
         [0123]    REM  1630  may perform Raman amplification by launching pump power in the same direction as the C-band payload. As shown in  FIG. 16 , REM  1630  may include Raman pumps  1632  and WDM  1634 . Raman pumps  1632  may generally be referred to as an optical source that provides distributed amplification. In one implementation, Raman pumps  1632  may include one or more pump lasers that produce pump power (also called pump light) (e.g., optical signals with in the range of approximately 1420-1460 nm) that may be transmitted so as to co-propagate with the C-band payload, OSC signal, and pilot signal. WDM  1634  may include an optical multiplexer that may receive the combined C-band payload, OSC signal, and pilot signal from RAM  1620  and optically combine the combined C-band payload, OSC signal, and pilot signal with the pump power from Raman pumps  1632  for output on the west-to-east link. 
         [0124]    East group  1650  may include a BMM/OAM  1652 , a RAM  1660 , and a REM  1670 . BMM/OAM  1652  may include a transmitter (TX) module  1654  and a receiver (RX) module  1656 . Transmitter module  1654  and receiver module  1656  may be similar in construction and operation to transmitter module  1614  and receiver module  1616 , respectively, described with regard to west group  1610 . RAM  1660  may be similar in construction and operation to RAM  1620  described with regard to west group  1610 . REM  1670  may include Raman pumps  1672  and WDM  1674 . Raman pumps  1672  and WDM  1674  may be similar in construction and operation to Raman pumps  1632  and WDM  1634  described with regard to west group  1610 . 
       Link Break on West-to-East Link 
       [0125]    During normal operation (prior to the link break) (e.g.,  FIG. 16 ), west group  1610  and east group  1650  may exchange signals on the west-to-east link and the east-to-west link. For example, west group  1610  may transmit signals to east group  1650  on the west-to-east link, and east group  1650  may transmit signals to west group  1610  on the east-to-west link. During this time, pilot laser  210 / 340  may be turned on and may output a pilot NRM signal, and Raman pumps  230 / 360 / 1632 / 1670  may be turned on and may output pump power. 
         [0126]    Now assume that a link break occurs on the west-to-east link.  FIG. 17  is a diagram of exemplary system  1600  in the situation where a link break occurs on the west-to-east link.  FIG. 18  is a flowchart of an exemplary process for shutting down components in response to the link break. In the description to follow, reference will be made to certain components of west group  1610  and east group  1650 . These components will be referred to as west and east components, respectively. For example, the RAM in west group  1610  will be referred to as the west RAM, and the RAM in east group  1650  will be referred to as the east RAM. 
         [0127]    The loss of the pilot signal may be detected at the east RAM (block  1805 ). For example, due to the link break, the pilot signal outputted by the west RAM cannot reach the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal. Because the pilot signal is generated at 1610 nm, the east controller can differentiate the presence or absence of the pilot signal even in the presence of noise generated by the east Raman pumps, which may be in the range of approximately 1420-1460 nm. 
         [0128]    The east RAM Raman pumps may be turned off (block  1810 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east RAM Raman pumps to turn off. The east RAM Raman pumps may turn off in response to the signal from the east controller, as shown in  FIG. 17 . 
         [0129]    The east REM Raman pumps may be turned off (block  1815 ). For example, when the east controller detects the loss of the pilot signal, the east controller may generate the SD signal and send the SD signal to the east REM Raman pumps. The east REM Raman pumps may turn off in response to the SD signal from the east controller, as shown in  FIG. 17 . 
         [0130]    The east pilot laser may be modulated to transmit the RRF signal (block  1820 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east pilot laser to modulate with the RRF signal. The east pilot laser may modulate with the RRF signal in response to the signal from the east controller, as shown in  FIG. 17 . 
         [0131]    The amplifiers of the east receiver and transmitter modules may be turned off (block  1825 ). The east receiver module may detect the loss of the C-band payload and/or OSC signal, as a result of the link break, and turn off its amplifier. When the east receiver module turns off its amplifier, it triggers the east transmitter module to turn off its amplifier. The east transmitter module may then turn off its amplifier. 
         [0132]    A BDI signal may be transmitted on the east-to-west link (block  1830 ). For example, when the east transmitter module turns off its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to generate the BDI signal and output the BDI signal to be optically combined with the pilot RRF signal and output on the east-to-west link. 
         [0133]    The pilot RRF signal may be detected at the west RAM (block  1835 ). For example, the west controller (within the west RAM) may monitor the signals transmitted on the east-to-west link to detect the presence of the pilot signal and to determine whether the pilot signal includes the RRF signal or the NRM signal. In this case, the west controller detects the presence of the RRF signal. 
         [0134]    The west RAM Raman pumps may be turned off (block  1840 ). For example, when the west controller detects the pilot RRF signal from the east RAM, the west controller may send a signal to the west RAM Raman pumps to turn off. The west RAM Raman pumps may turn off in response to the signal from the west controller, as shown in  FIG. 17 . 
         [0135]    The west REM Raman pumps may be turned off (block  1845 ). For example, when the west controller detects the pilot RRF signal from the east RAM, the west controller may generate the SD signal and send the SD signal to the west REM Raman pumps. The west REM Raman pumps may turn off in response to the SD signal from the west controller, as shown in  FIG. 17 . 
         [0136]    The west pilot laser may be modulated to transmit the NRM signal (block  1850 ). For example, when the west controller detects the pilot RRF signal from the east RAM, the west controller may send a signal to the west pilot laser to modulate with the NRM signal. The west pilot laser may modulate with the NRM signal in response to the signal from the west controller, as shown in  FIG. 17 . 
         [0137]    The BDI signal may be detected and the amplifiers of the west receiver and transmitter modules may be turned off (block  1855 ). For example, the west receiver module may detect the BDI signal from the east transmitter module, and turn off its amplifier. When the west receiver module turns off its amplifier, it triggers the west transmitter module to turn off its amplifier. The west transmitter module may then turn off its amplifier. Both the west-to-east and the east-to-west links are now completely safe. In other words, a technician may safely repair the link break. 
         [0138]      FIG. 19  is a flowchart of an exemplary process for restoring components in response to repair of the link break. The pilot NRM signal may be detected at the east RAM (block  1905 ). For example, now that the link break has been repaired, the pilot NRM signal outputted by the west RAM can reach the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal and detect the NRM signal. 
         [0139]    The east RAM Raman pumps may be turned on (block  1910 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east RAM Raman pumps to turn on. The east RAM Raman pumps may turn on in response to the signal from the east controller. 
         [0140]    The east REM Raman pumps may be turned on (block  1915 ). For example, when the east controller detects the pilot NRM signal, the east controller may discontinue the SD signal. The east REM Raman pumps may turn on in response to the discontinuing of the SD signal from the east controller. 
         [0141]    The east pilot laser may be modulated to transmit the NRM signal (block  1920 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east pilot laser to modulate with the NRM signal. The east pilot laser may modulate with the NRM signal in response to the signal from the east controller. 
         [0142]    The amplifiers of the east receiver and transmitter modules may be turned on (block  1925 ). The east receiver module may detect the C-band payload and/or OSC signal, as a result of the link break being repaired, detect no BDI signal, and turn on its amplifier. When the east receiver module turns on its amplifier, it triggers the east transmitter module to turn on its amplifier. The east transmitter module may then turn on its amplifier. 
         [0143]    The BDI may no longer be transmitted on the east-to-west link (block  1930 ). For example, when the east transmitter module turns on its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to stop generating the BDI signal. 
         [0144]    The pilot NRM signal may be detected at the west RAM (block  1935 ). For example, the west controller (within the west RAM) may monitor the signals transmitted on the east-to-west link to detect the presence of the pilot signal and to determine whether the pilot signal includes the RRF signal or the NRM signal. In this case, the west controller detects the presence of the NRM signal. 
         [0145]    The west RAM Raman pumps may be turned on (block  1940 ). For example, when the west controller detects the pilot NRM signal from the east RAM, the west controller may send a signal to the west RAM Raman pumps to turn on. The west RAM Raman pumps may turn on in response to the signal from the west controller. 
         [0146]    The west REM Raman pumps may be turned on (block  1945 ). For example, when the west controller detects the pilot NRM signal, the west controller may discontinue the SD signal. The west REM Raman pumps may turn on in response to the discontinuing of the SD signal from the west controller. 
         [0147]    No BDI signal may be detected on the east-to-west link, and the amplifiers of the west receiver and transmitter modules may be turned on (block  1950 ). For example, the west receiver module may detect the removal of the BDI signal from the east transmitter module, and turn on its amplifier. When the west receiver module turns on its amplifier, it triggers the west transmitter module to turn on its amplifier. The west transmitter module may then turn on its amplifier. As a result, system  1600  may return to normal operation. 
       Link Break on Local Link 
       [0148]    During normal operation (prior to the link break) (e.g.,  FIG. 16 ), west group  1610  and east group  1650  may exchange signals on the west-to-east link and the east-to-west link. For example, west group  1610  may transmit signals to east group  1650  on the west-to-east link, and east group  1650  may transmit signals to west group  1610  on the east-to-west link. During this time, pilot laser  210 / 340  may be turned on and may output a pilot NRM signal, and Raman pumps  230 / 360 / 1632 / 1670  may be turned on and may output pump power. 
         [0149]    Now assume that a link break occurs on the local link connecting the west RAM to the west REM.  FIG. 20  is a diagram of exemplary system  1600  in the situation where a link break occurs on the local link.  FIG. 21  is a flowchart of an exemplary process for shutting down components in response to the link break. In the description to follow, reference will be made to certain components of west group  1610  and east group  1650 . These components will be referred to as west and east components, respectively. For example, the RAM in west group  1610  will be referred to as the west RAM, and the RAM in east group  1650  will be referred to as the east RAM. 
         [0150]    A loss of signals may be detected at west REM (block  2105 ). For example, the west REM may detect the loss of the C-band payload, the OSC signal, and the pilot signal. 
         [0151]    The west REM Raman pumps may be turned off (block  2110 ). For example, in response to the loss of the signals, the west REM may turn off the west REM Raman pumps (regardless of the state of the SD signal), as shown in  FIG. 20 . 
         [0152]    The loss of the pilot signal may be detected at the east RAM (block  2115 ). For example, due to the link break, the pilot signal outputted by the west RAM cannot reach the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal. Because the pilot signal is generated at 1610 nm, the east controller can differentiate the presence or absence of the pilot signal even in the presence of noise generated by the east Raman pumps, which may be in the range of approximately 1420-1460 nm. 
         [0153]    The east RAM Raman pumps may be turned off (block  2120 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east RAM Raman pumps to turn off. The east RAM Raman pumps may turn off in response to the signal from the east controller, as shown in  FIG. 20 . 
         [0154]    The east REM Raman pumps may be turned off (block  2125 ). For example, when the east controller detects the loss of the pilot signal, the east controller may generate the SD signal and send the SD signal to the east REM Raman pumps. The east REM Raman pumps may turn off in response to the SD signal from the east controller, as shown in  FIG. 20 . 
         [0155]    The east pilot laser may be modulated to transmit the RRF signal (block  2130 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east pilot laser to modulate with the RRF signal. The east pilot laser may modulate with the RRF signal in response to the signal from the east controller, as shown in  FIG. 20 . 
         [0156]    The amplifiers of the east receiver and transmitter modules may be turned off (block  2135 ). The east receiver module may detect the loss of the C-band payload and/or OSC signal and turn off its amplifier. When the east receiver module turns off its amplifier, it triggers the east transmitter module to turn off its amplifier. The east transmitter module may then turn off its amplifier. 
         [0157]    A BDI signal may be transmitted on the east-to-west link (block  2140 ). For example, when the east transmitter module turns off its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to generate the BDI signal and output the BDI signal to be optically combined with the pilot RRF signal and output on the east-to-west link. 
         [0158]    The pilot RRF signal may be detected at the west RAM (block  2145 ). For example, the west controller (within the west RAM) may monitor the signals transmitted on the east-to-west link to detect the presence of the pilot signal and to determine whether the pilot signal includes the RRF signal or the NRM signal. In this case, the west controller detects the presence of the RRF signal. 
         [0159]    The west RAM Raman pumps may be turned off (block  2150 ). For example, when the west controller detects the pilot RRF signal from the east RAM, the west controller may send a signal to the west RAM Raman pumps to turn off. The west RAM Raman pumps may turn off in response to the signal from the west controller, as shown in  FIG. 20 . 
         [0160]    The SD signal may be generated (block  2155 ). For example, when the west controller detects the pilot RRF signal from the east RAM, the west controller may generate the SD signal and send the SD signal to the west REM Raman pumps. The west REM Raman pumps are already turned off, and remain turned off, as shown in  FIG. 20 . 
         [0161]    The west pilot laser may be modulated to transmit the NRM signal (block  2160 ). For example, when the west controller detects the pilot RRF signal from the east RAM, the west controller may send a signal to the west pilot laser to modulate with the NRM signal. The west pilot laser may modulate with the NRM signal in response to the signal from the west controller, as shown in  FIG. 20 . 
         [0162]    The BDI signal may be detected and the amplifiers of the west receiver and transmitter modules may be turned off (block  2165 ). For example, the west receiver module may detect the BDI signal from the east transmitter module, and turn off its amplifier. When the west receiver module turns off its amplifier, it triggers the west transmitter module to turn off its amplifier. The west transmitter module may then turn off its amplifier. Both the west-to-east and the east-to-west links are now completely safe. In other words, a technician may safely repair the link break. 
         [0163]      FIG. 22  is a flowchart of an exemplary process for restoring components in response to repair of the link break. Once the local link break is repaired, the west REM detects the C-band payload, OSC signal, and pilot signal. The west REM may keep the west REM Raman pumps shut down because the SD signal is asserted, but may permit the C-band payload, OSC signal, and pilot signal to be outputted on the west-to-east link. 
         [0164]    The pilot NRM signal may be detected at the east RAM (block  2205 ). For example, now that the link break has been repaired, the pilot NRM signal outputted by the west RAM can reach the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal and detect the NRM signal. 
         [0165]    The east RAM Raman pumps may be turned on (block  2210 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east RAM Raman pumps to turn on. The east RAM Raman pumps may turn on in response to the signal from the east controller. 
         [0166]    The east REM Raman pumps may be turned on (block  2215 ). For example, when the east controller detects the pilot NRM signal, the east controller may discontinue the SD signal. The east REM Raman pumps may turn on in response to the discontinuing of the SD signal from the east controller. 
         [0167]    The east pilot laser may be modulated to transmit the NRM signal (block  2220 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east pilot laser to modulate with the NRM signal. The east pilot laser may modulate with the NRM signal in response to the signal from the east controller. 
         [0168]    The amplifiers of the east receiver and transmitter modules may be turned on (block  2225 ). The east receiver module may detect the C-band payload and/or OSC signal, as a result of the link break being repaired, detect no BDI signal, and turn on its amplifier. When the east receiver module turns on its amplifier, it triggers the east transmitter module to turn on its amplifier. The east transmitter module may then turn on its amplifier. 
         [0169]    The BDI may no longer be transmitted on the east-to-west link (block  2230 ). For example, when the east transmitter module turns on its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to stop generating the BDI signal. 
         [0170]    The pilot NRM signal may be detected at the west RAM (block  2235 ). For example, the west controller (within the west RAM) may monitor the signals transmitted on the east-to-west link to detect the presence of the pilot signal and to determine whether the pilot signal includes the RRF signal or the NRM signal. In this case, the west controller detects the presence of the NRM signal. 
         [0171]    The west RAM Raman pumps may be turned on (block  2240 ). For example, when the west controller detects the pilot NRM signal from the east RAM, the west controller may send a signal to the west RAM Raman pumps to turn on. The west RAM Raman pumps may turn on in response to the signal from the west controller. 
         [0172]    The west REM Raman pumps may be turned on (block  2245 ). For example, when the west controller detects the pilot NRM signal, the west controller may discontinue the SD signal. The west REM Raman pumps may turn on in response to the discontinuing of the SD signal from the west controller. 
         [0173]    No BDI signal may be detected on the east-to-west link, and the amplifiers of the west receiver and transmitter modules may be turned on (block  2250 ). For example, the west receiver module may detect the removal of the BDI signal from the east transmitter module, and turn on its amplifier. When the west receiver module turns on its amplifier, it triggers the west transmitter module to turn on its amplifier. The west transmitter module may then turn on its amplifier. As a result, system  1600  may return to normal operation. 
       Link Breaks on Both West-to-East Link and East-to-West Link 
       [0174]    During normal operation (prior to the link break) (e.g.,  FIG. 16 ), west group  1610  and east group  1650  may exchange signals on the west-to-east link and the east-to-west link. For example, west group  1610  may transmit signals to east group  1650  on the west-to-east link, and east group  1650  may transmit signals to west group  1610  on the east-to-west link. During this time, pilot laser  210 / 340  may be turned on and may output a pilot NRM signal, and Raman pumps  230 / 360 / 1632 / 1672  may be turned on and may output pump power. 
         [0175]    Now assume that link breaks occur on both the west-to-east link and the east-to-west link.  FIG. 23  is a diagram of exemplary system  1600  in the situation where link breaks occur on both the west-to-east link and the east-to-west link.  FIG. 24  is a flowchart of an exemplary process for shutting down components in response to the link breaks. In the description to follow, reference will be made to certain components of west group  1610  and east group  1650 . These components will be referred to as west and east components, respectively. For example, the RAM in west group  1610  will be referred to as the west RAM, and the RAM in east group  1650  will be referred to as the east RAM. 
         [0176]    The loss of the pilot signal may be detected at the east RAM (block  2410 ). For example, due to the link break, the pilot signal outputted by the west RAM cannot reach the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal. Because the pilot signal is generated at 1610 nm, the east controller can differentiate the presence or absence of the pilot signal even in the presence of noise generated by the east Raman pumps, which may be in the range of approximately 1420-1460 nm. 
         [0177]    The east RAM Raman pumps may be turned off (block  2420 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east Raman pumps to turn off. The east Raman pumps may turn off in response to the signal from the east controller, as shown in  FIG. 23 . 
         [0178]    The east REM Raman pumps may be turned off (block  2430 ). For example, when the east controller detects the loss of the pilot signal, the east controller may generate the SD signal and send the SD signal to the east REM Raman pumps. The east REM Raman pumps may turn off in response to the SD signal from the east controller, as shown in  FIG. 23 . 
         [0179]    The east pilot laser may be modulated to transmit the RRF signal (block  2440 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east pilot laser to modulate with the RRF signal. The east pilot laser may modulate with the RRF signal in response to the signal from the east controller, as shown in  FIG. 23 . 
         [0180]    The amplifiers of the east receiver and transmitter modules may be turned off (block  2450 ). The east receiver module may detect the loss of the C-band payload and/or OSC signal, as a result of the link break, and turn off its amplifier. When the east receiver module turns off its amplifier, it triggers the east transmitter module to turn off its amplifier. The east transmitter module may then turn off its amplifier. 
         [0181]    A BDI signal may be transmitted on the east-to-west link (block  2460 ). For example, when the east transmitter module turns off its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to generate the BDI signal and output the BDI signal to the east WDM. The east WDM may optically combine the BDI signal with the pilot RRF signal and output the combined BDI signal and RRF signal on the east-to-west link. The combined BDI signal and RRF signal will not arrive at west group  1610  due to the link break on the east-to-west link. 
         [0182]    Simultaneously, or substantially simultaneously, blocks  2410 - 2460  may be performed by west group  1610 . As a result, both the west-to-east and the east-to-west links are now completely safe. In other words, a technician may safely repair the link breaks. 
         [0183]      FIG. 25  is a flowchart of an exemplary process for restoring components in response to repair of the link breaks. Assume, for example, that the east-to-west link is repaired first. The pilot RRF signal may be detected at the west RAM (block  2505 ). For example, now that the link break on the east-to-west link has been repaired, the pilot RRF signal outputted by the east RAM can reach the west RAM. The west controller (within the west RAM) may monitor the presence of the pilot signal and detect the RRF signal. In response to the pilot RRF signal, the west controller may keep the west RAM and REM Raman pumps turned off. 
         [0184]    The west pilot laser may be modulated to transmit the NRM signal (block  2510 ). For example, the west controller may send a signal to the west pilot laser to modulate with the NRM signal. The west pilot laser may modulate with the NRM signal in response to the signal from the west controller. 
         [0185]    The BDI signal may be detected on the east-to-west link and the transmitting of the BDI signal on the west-to-east link may be stopped (block  2515 ). For example, the west receiver module may detect the restoration of the C-band payload and/or the OSC signal, and detect the BDI signal sent from east group  1650 . The west receiver module may keep its amplifier turned off and signal the west transmitter module to stop transmitting the BDI signal. 
         [0186]    When the west-to-east link is repaired, the pilot NRM signal may be detected at the east RAM (block  2520 ). For example, now that the link break on the west-to-east link has been repaired, the pilot NRM signal outputted by the west RAM can reach the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal and detect the NRM signal. 
         [0187]    The east RAM Raman pumps may be turned on (block  2525 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east RAM Raman pumps to turn on. The east RAM Raman pumps may turn on in response to the signal from the east controller. 
         [0188]    The east REM Raman pumps may be turned on (block  2530 ). For example, when the east controller detects the pilot NRM signal, the east controller may discontinue the SD signal. The east REM Raman pumps may turn on in response to the discontinuing of the SD signal from the east controller. 
         [0189]    The east pilot laser may be modulated to transmit the NRM signal (block  2535 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east pilot laser to modulate with the NRM signal. The east pilot laser may modulate with the NRM signal in response to the signal from the east controller. 
         [0190]    No BDI signal may be detected, and the amplifiers of the east receiver and transmitter modules may be turned on (block  2540 ). The east receiver module may detect restoration of the C-band payload and/or the OSC signal on the west-to-east link, as a result of the link break being repaired, detect no BDI signal, and turn on its amplifier. When the east receiver module turns on its amplifier, it triggers the east transmitter module to turn on its amplifier. The east transmitter module may then turn on its amplifier. 
         [0191]    The BDI may no longer be transmitted on the east-to-west link (block  2545 ). For example, when the east transmitter module turns on its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to stop generating the BDI signal. 
         [0192]    The pilot NRM signal may be detected at the west RAM (block  2550 ). For example, the west controller (within the west RAM) may monitor the signals transmitted on the east-to-west link to detect the presence of the pilot signal and to determine whether the pilot signal includes the RRF signal or the NRM signal. In this case, the west controller detects the presence of the NRM signal. 
         [0193]    The west RAM Raman pumps may be turned on (block  2555 ). For example, when the west controller detects the pilot NRM signal from the east RAM, the west controller may send a signal to the west RAM Raman pumps to turn on. The west RAM Raman pumps may turn on in response to the signal from the west controller. 
         [0194]    The west REM Raman pumps may be turned on (block  2560 ). For example, when the west controller detects the pilot NRM signal, the west controller may discontinue the SD signal. The west REM Raman pumps may turn on in response to the discontinuing of the SD signal from the west controller. 
         [0195]    No BDI signal may be detected on the east-to-west link, and the amplifiers of the west receiver and transmitter modules may be turned on (block  2565 ). For example, the west receiver module may detect the removal of the BDI signal from the east transmitter module, and turn on its amplifier. When the west receiver module turns on its amplifier, it triggers the west transmitter module to turn on its amplifier. The west transmitter module may then turn on its amplifier. As a result, system  1600  may return to normal operation. 
       Link Break on Local Transmit Link 
       [0196]    During normal operation (prior to the link break) (e.g.,  FIG. 16 ), west group  1610  and east group  1650  may exchange signals on the west-to-east link and the east-to-west link. For example, west group  1610  may transmit signals to east group  1650  on the west-to-east link, and east group  1650  may transmit signals to west group  1610  on the east-to-west link. During this time, pilot laser  210 / 340  may be turned on and may output a pilot NRM signal, and Raman pumps  230 / 360 / 1632 / 1672  may be turned on and may output pump power. 
         [0197]    Now assume that a link break occurs on the local transmit link located between the west BMM/OAM and the west RAM.  FIG. 26  is a diagram of exemplary system  1600  in the situation where link breaks occur on the local transmit link.  FIG. 27  is a flowchart of an exemplary process for shutting down components in response to the link break. In the description to follow, reference will be made to certain components of west group  1610  and east group  1650 . These components will be referred to as west and east components, respectively. For example, the RAM in west group  1610  will be referred to as the west RAM, and the RAM in east group  1650  will be referred to as the east RAM. 
         [0198]    A loss of signals (e.g., C-band payload and/or OSC signal) may be detected at the west RAM (block  2705 ). For example, due to the link break, the C-band payload and OSC signal outputted by the west BMM/OAM cannot reach the west RAM. The west controller (within the west RAM) may detect the absence of the C-band payload and/or OSC signal. 
         [0199]    The west RAM Raman pumps, the west OSC laser, and the west pilot laser may be turned off (block  2710 ). For example, when the west controller detects the loss of the C-band payload and/or OSC signal, the west controller may send signals to the west RAM Raman pumps, the west OSC laser, and the west pilot laser to turn off. The west RAM Raman pumps, the west OSC laser, and the west pilot laser may turn off in response to the signals from the west controller, as shown in  FIG. 26 . 
         [0200]    The west REM Raman pumps may be turned off (block  2715 ). For example, when the west controller detects the loss of the C-band payload and/or OSC signal, the west controller may generate the SD signal and send the SD signal to the west REM Raman pumps. The west REM Raman pumps may turn off in response to the SD signal from the west controller, as shown in  FIG. 26 . 
         [0201]    The loss of the pilot signal may be detected at the east RAM (block  2720 ). For example, since the west pilot laser has been turned off, there is no pilot signal to reach the east RAM. The east controller (within the east RAM) may monitor the presence or absence of the pilot signal. In this case, the east controller may detect the absence of the pilot signal. 
         [0202]    The east RAM Raman pumps may be turned off (block  2725 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east RAM Raman pumps to turn off. The east RAM Raman pumps may turn off in response to the signal from the east controller, as shown in  FIG. 26 . 
         [0203]    The east REM Raman pumps may be turned off (block  2730 ). For example, when the east controller detects the loss of the pilot signal, the east controller may generate the SD signal and send the SD signal to the east REM Raman pumps. The east REM Raman pumps may turn off in response to the SD signal from the east controller, as shown in  FIG. 26 . 
         [0204]    The east pilot laser may be modulated to transmit the RRF signal (block  2735 ). For example, when the east controller detects the loss of the pilot signal, the east controller may send a signal to the east pilot laser to modulate with the RRF signal. The east pilot laser may modulate with the RRF signal in response to the signal from the east controller, as shown in  FIG. 26 . 
         [0205]    The amplifiers of the east receiver and transmitter modules may be turned off (block  2740 ). The east receiver module may detect the loss of the C-band payload and/or OSC signal and turn off its amplifier. When the east receiver module turns off its amplifier, it triggers the east transmitter module to turn off its amplifier. The east transmitter module may then turn off its amplifier. 
         [0206]    A BDI signal may be transmitted on the east-to-west link (block  2745 ). For example, when the east transmitter module turns off its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to generate the BDI signal and output the BDI signal to the east WDM. The east WDM may optically combine the BDI signal with the pilot RRF signal and output the combined BDI signal and RRF signal on the east-to-west link. 
         [0207]    The BDI signal may be detected and the amplifiers of the west receiver and transmitter modules may be turned off (block  2750 ). For example, the west receiver module may detect the BDI signal from the east transmitter module, and turn off its amplifier. When the west receiver module turns off its amplifier, it triggers the west transmitter module to turn off its amplifier. The west transmitter module may then turn off its amplifier. Both the west-to-east and the east-to-west links are now completely safe. In other words, a technician may safely repair the link break. 
         [0208]      FIG. 28  is a flowchart of an exemplary process for restoring components in response to repair of the link break. Signals (e.g., C-band payload and/or OSC signal) may be detected at the west RAM and west OSC laser may be turned on (block  2805 ). For example, due to repair of the link break, the C-band payload and OSC signal outputted by the west BMM/OAM can now reach the west RAM. The west controller (within the west RAM) may detect the restoration of the C-band payload and/or OSC signal. 
         [0209]    The west pilot laser may be turned on and modulated to transmit the NRM signal (block  2810 ). For example, the west controller may detect the pilot RRF signal from the east RAM. In response, the west controller may send a signal to the west pilot laser to turn on and modulate with the NRM signal. The west RAM and REM Raman pumps may remain turned off. 
         [0210]    The pilot NRM signal may be detected at the east RAM (block  2815 ). For example, now that the link break has been repaired, the pilot NRM signal outputted by the west RAM can reach the east RAM. The east controller (within the east RAM) may monitor the presence of the pilot signal and detect the NRM signal. 
         [0211]    The east RAM Raman pumps may be turned on (block  2820 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east RAM Raman pumps to turn on. The east RAM Raman pumps may turn on in response to the signal from the east controller. 
         [0212]    The east REM Raman pumps may be turned on (block  2825 ). For example, when the east controller detects the pilot NRM signal, the east controller may discontinue the SD signal. The east REM Raman pumps may turn on in response to the discontinuing of the SD signal from the east controller. 
         [0213]    The east pilot laser may be modulated to transmit the NRM signal (block  2830 ). For example, when the east controller detects the pilot NRM signal, the east controller may send a signal to the east pilot laser to modulate with the NRM signal. The east pilot laser may modulate with the NRM signal in response to the signal from the east controller. 
         [0214]    No BDI signal may be detected, and the amplifiers of the east receiver and transmitter modules may be turned on (block  2835 ). The east receiver module may detect the C-band payload and/or OSC signal, as a result of the link break being repaired, detect no BDI signal, and turn on its amplifier. When the east receiver module turns on its amplifier, it triggers the east transmitter module to turn on its amplifier. The east transmitter module may then turn on its amplifier. 
         [0215]    The BDI signal may no longer be transmitted on the east-to-west link (block  2840 ). For example, when the east transmitter module turns on its amplifier, the east transmitter module may trigger the east OSC source, associated with the east transmitter module, to stop generating the BDI signal. 
         [0216]    The pilot NRM signal may be detected at the west RAM (block  2845 ). For example, the west controller (within the west RAM) may monitor the signals transmitted on the east-to-west link to detect the presence of the pilot signal and to determine whether the pilot signal includes the RRF signal or the NRM signal. In this case, the west controller detects the presence of the NRM signal. 
         [0217]    The west RAM Raman pumps may be turned on (block  2850 ). For example, when the west controller detects the pilot NRM signal from the east RAM, the west controller may send a signal to the west RAM Raman pumps to turn on. The west RAM Raman pumps may turn on in response to the signal from the west controller. 
         [0218]    The west REM Raman pumps may be turned on (block  2855 ). For example, when the west controller detects the pilot NRM signal, the west controller may discontinue the SD signal. The west REM Raman pumps may turn on in response to the discontinuing of the SD signal from the west controller. 
         [0219]    No BDI signal may be detected on the east-to-west link, and the amplifiers of the west receiver and transmitter modules may be turned on (block  2860 ). For example, the west receiver module may detect the removal of the BDI signal from the east transmitter module, and turn on its amplifier. When the west receiver module turns on its amplifier, it triggers the west transmitter module to turn on its amplifier. The west transmitter module may then turn on its amplifier. As a result, system  1600  may return to normal operation. 
       Conclusion 
       [0220]    Implementations described herein may facilitate shutdown and recovery in response to link breaks. Table 1 summarizes the operation described above and the effects on the RAM and REM Raman pumps, the OSC laser, and the pilot laser. The OSC laser column is applicable for RAMs that include OSC lasers. The REM pumps column is applicable for configurations that incorporate a REM. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Shutdown 
                 RAM 
                 REM 
                 OSC 
                 Pilot 
               
               
                 State Description 
                 Event 
                 Pumps 
                 Pumps 
                 Laser 
                 Laser 
               
               
                   
               
             
             
               
                 No pilot signal (RRF or 
                 YES 
                 OFF 
                 OFF 
                 ON 
                 RRF 
               
               
                 NRM) detected and C- 
               
               
                 band or OSC signal 
               
               
                 detected at 
               
               
                 BMM/OAM. This 
               
               
                 indicates a fiber break 
               
               
                 in the main fiber span. 
               
               
                 Neither C-band nor 
                 YES 
                 OFF 
                 OFF 
                 OFF 
                 OFF 
               
               
                 OSC detected at the 
               
               
                 BMM/OAM port. This 
               
               
                 indicates a fiber break 
               
               
                 between the local 
               
               
                 BMM/OAM and RAM. 
               
               
                 RRF signal detected 
                 YES 
                 OFF 
                 OFF 
                 ON 
                 NRM 
               
               
                 and C-band or OSC 
               
               
                 signal detected at 
               
               
                 BMM/OAM. This 
               
               
                 indicates a fiber break 
               
               
                 has been detected by 
               
               
                 the RX side of the 
               
               
                 upstream amplifier. 
               
               
                 NRM signal detected 
                 NO 
                 ON 
                 ON 
                 ON 
                 NRM 
               
               
                 and C-band or OSC 
               
               
                 signal detected at 
               
               
                 BMM/OAM. No fiber 
               
               
                 break. 
               
               
                   
               
             
          
         
       
     
         [0221]    When a link break is detected due to the loss of the pilot signal (RRF and NRM), and there is no link break on the local link between the BMM/OAM and the RAM, the RAM may shut down its pumps and modulate its pilot laser to output the RRF signal to the RAM downstream of the break. If applicable, the RAM may also shut down the REM pumps by asserting the SD signal. The pumps are turned off to provide a safe condition as well as to remove any noise from the BMM/OAM receiver. 
         [0222]    When a link break is detected at the BMM/OAM due to a loss of the C-band payload and/or the OSC signal, the RAM may shut down its pumps, OSC laser, and pilot laser. If applicable, the RAM may also shut down the REM pumps by asserting the SD signal. The OSC laser may be shut down because without a valid OSC input signal, the OSC laser may generate noise on the OSC channel. This noise may be detected by the far end BMM/OAM receiver as optical energy that the BMM/OAM may confuse as the OSC signal, causing the BMM/OAM not to output a BDI signal. Without this BDI signal, the amplifier associated with the local transmitter module may not turn off, resulting in an unsafe condition. Accordingly, the pilot laser may be turned off to simulate a link break on the main fiber span for the remote RAM. 
         [0223]    When the RRF pilot signal is detected and there is no link break on local link between the BMM/OAM and the RAM, the RAM may shut down its pumps and modulate its pilot laser to output a NRM signal to the far end RAM. If applicable, the RAM may also shut down the REM pumps by asserting the SD signal. 
         [0224]    When the NRM pilot signal is detected and there is no fiber break on the local link between the BMM/OAM and the RAM, the RAM may turn on its pumps and modulate its pilot laser to output a NRM signal to the far end RAM. If applicable, the RAM may also enable the REM pumps by de-asserting the SD signal. When the RAM de-asserts the SD signal, the REM pumps may be turned on only if there is no link break in the short fiber span between the RAM and the REM. In other words, if the REM detects either the pilot signal, the C-band payload, or the OSC signal from the RAM and the SD signal is de-asserted, then the REM may turn on its pumps. 
         [0225]    The foregoing description provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. 
         [0226]    For example, while series of blocks have been described with regard to  FIGS. 5 ,  6 ,  8 ,  9 ,  11 ,  12 ,  14 ,  15 ,  18 ,  19 ,  21 ,  22 ,  24 ,  25 ,  27 , and  28 , the order of the blocks may be modified in other implementations. Further, non-dependent blocks may be performed in parallel. 
         [0227]    Further, the term “approximately” has been used throughout the description and is intended to mean the exact number or range that follows the term or a number within a particular threshold of the number or range that follows the term. 
         [0228]    Also, while signals of particular exemplary wavelengths have been described, one or more of these signals may have a different wavelength, or operate within a different range of wavelengths, in other implementations. 
         [0229]    Even though particular combinations of features are recited in the claims and/or disclosed in the specification, these combinations are not intended to limit the disclosure of the invention. In fact, many of these features may be combined in ways not specifically recited in the claims and/or disclosed in the specification. Although each dependent claim listed below may directly depend on only one other claim, the disclosure of the invention includes each dependent claim in combination with every other claim in the claim set. 
         [0230]    No element, act, or instruction used in the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Where only one item is intended, the term “one” or similar language is used. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise.