Patent Publication Number: US-8542992-B2

Title: System and method for mitigating polarization hole burning

Description:
TECHNICAL FIELD 
     This invention relates generally to the field of optical communication networks and more specifically to mitigating the effects of polarization hole burning. 
     BACKGROUND 
     A communication network includes paths of nodes that route packets through the network. Optical amplifiers perform an important function within these networks by amplifying an optical signal in order to increase the operational length of an optical network. In some configurations, the efficiency of an optical communication network may be compromised by an effect known as polarization hole burning. 
     Optical communication systems designed to operate over long distances may suffer from multiple polarization-dependent effects that reduce the operational efficiency of the system. Polarization hole burning (PHB) is one of these phenomena. PHB may seriously reduce the performance of rare-earth doped fiber optical amplifiers, such as an erbium doped fiber amplifier (EDFA), used to amplify signal strength within the communication system. 
     PHB occurs when a strong, polarized optical signal is launched into an EDFA. This strong signal can cause anisotropic saturation of the amplifier. This saturation effect, which is related to the population inversion dynamics of the EDFA, depresses the gain of the EDFA for light with the same state of polarization (SOP) as the saturating signal. Thus, PHB causes a signal having a SOP orthogonal to the saturating signal to have a gain greater than that of the saturating signal. 
     As a result, amplified spontaneous emission (ASE) noise in the SOP orthogonal to the saturating signal may accumulate faster than in the SOP of the saturating signal. In a communication system utilizing a chain of EDFAs operating at or near saturation, ASE noise may accumulate at each amplifier stage. As the noise builds up over the course of the system, the signal-to-noise ratio (SNR) for a signal with a SOP orthogonal to the saturating signal may rise to unacceptable levels. The SNR in such cases can then cause errors in the received data stream. Accordingly, mitigating the effects of PHB in amplified optical systems is desirable. 
     One of the causes of the undesirable PHB effect is operating an EDFA in a way that leads to gain compression. Gain compression (“Cp”) is a measure of the difference of the amplifier&#39;s non-saturated gain (“Go,” or the gain when operating on a low power signal) and the amplifier&#39;s saturated operating gain (“G”). The operating gain, in decibels, can be measured by taking the difference between the saturated output power (“So”) and the input power of a saturating signal (“Si)”, as follows:
 
 G=So−Si.  
 
     The corresponding gain compression may be calculated as the difference between the non-saturated gain and the saturated operating gain:
 
 Cp=Go−G.  
 
     The gain in the SOP orthogonal to a saturating signal may be measured using a probe signal with an input signal orthogonal to the saturating signal by measuring the input power (“Pi”) and output power (“Po”) of the probe signal:
 
 Po−Pi=G+ΔG.  
 
     The “ΔG” in the above formula represents the amount of PHB in the SOP orthogonal to the saturation signal. This is a result of operating the amplifier with a saturating signal. As gain compression of an amplifier increases, so does the amount and effect of PHB. For instance, a single EDFA operating at a gain compression of about 3 dB may produce a PHB of about 0.08 dB. However, when that EDFA operates in a more saturated condition, with Cp=9-10 dB, the PHB may rise to about 0.2 dB. 
     The degree of PHB may also be affected by other factors, such as the degree of polarization of the saturating signal. If a signal&#39;s SOP varies over time, the effects of PHB may be reduced. 
     While the degree of PHB may be small for a single EDFA, these effects may be seriously compounded in communication systems that chain together a series of EDFAs. A number of arrangements have been proposed for reducing the effects of PHB in optical communication systems. However, such arrangements continue to suffer from drawbacks such as an inability to deal with arbitrary channel loading, expense and difficulty of implementation, and the innate stability characteristics of rare-earth doped fiber amplifiers. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with the present invention, disadvantages and problems associated with previous techniques for mitigating the effects of polarization hole burning in optical amplifiers may be reduced or eliminated. 
     According to one embodiment of the present invention, a system is provided for mitigating the effects of polarization hole burning in an optical communication system. The system includes an optical input signal comprising one or more traffic channels, a measurement module configured to check for the existence of ghost channels around the traffic channels, and a ghost channel generation module configured to generate a ghost channel around the traffic channels from amplified spontaneous emission noise of the optical input signal. 
     Certain embodiments of the invention may provide one or more technical advantages. A technical advantage of one embodiment may be that creating ghost channels around a saturating signal in an optical communication network increases the signal to noise ratios of those signals with adjacent ghost channels. In some embodiments, the creation of one or more ghost channel(s) on one side or both sides of an optical signal may provide a low-cost implementation for polarization hole burning mitigation. 
     Certain embodiments of the invention may include none, some, or all of the above technical advantages. One or more other technical advantages may be readily apparent to one skilled in the art from the figures, descriptions, and claims included herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates an optical communication network system, in accordance with certain embodiments of the present disclosure; 
         FIG. 2  illustrates a node with a plurality of degrees, in accordance with certain embodiments of the present disclosure; 
         FIG. 3  illustrates a graph of a traffic channel surrounded on either side by ghost channels, in accordance with certain embodiments of the present disclosure; 
         FIG. 4  illustrates an optical amplification scheme for generating ghost channels, in accordance with certain embodiments of the present disclosure; 
         FIG. 5  is a flowchart illustrating one embodiment of a method of mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure; 
         FIG. 6  illustrates a table for storing information regarding the validity of a channel incoming to a degree of node, in accordance with certain embodiments of the present disclosure; 
         FIG. 7  illustrates a table for storing information regarding the validity and signal strength of a channel incoming to a particular degree of node, in accordance with certain embodiments of the present disclosure; 
         FIG. 8  illustrates a series of tables representing the consolidated information received from other degrees of node, in accordance with certain embodiments of the present disclosure; 
         FIG. 9  illustrates a table for storing information regarding the current ghost channel load for degrees in node, in accordance with certain embodiments of the present disclosure; 
         FIG. 10  is a flowchart illustrating one embodiment of a method of managing the selection of ghost channels in mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure; 
         FIG. 11  is a flowchart illustrating one embodiment of a method of selecting ghost channels for use in mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure; 
         FIG. 12  is a flowchart illustrating one embodiment of a method of selecting the degree of node from which to select the appropriate ghost channels for mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure; and 
         FIG. 13  is a flowchart illustrating one embodiment of a method of optimizing a ghost channel selection routine in order to mitigate the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention and its advantages are best understood by referring to  FIGS. 1 through 13  of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
       FIG. 1  illustrates an optical communication network system, in accordance with certain embodiments of the present disclosure. Optical network system  10  includes components such as network nodes  22 . In general, a network node  22  may include any suitable arrangement of components operable to perform the operations of the network node. As an example, a network node may include logic, an interface, memory, other component, or any suitable combination of the preceding. “Logic” may refer to hardware, software, other logic, or any suitable combination of the preceding. Certain logic may manage the operation of a device, and may comprise, for example, a processor. “Processor” may refer to any suitable device operable to execute instructions and manipulate data to perform operations. 
     “Interface” may refer to logic of a network node operable to receive input for the network node, send output from the network node, perform suitable processing of the input or output or both, or any combination of the preceding, and may comprise one or more ports, conversion software, or both. 
     “Memory” may refer to logic operable to store and facilitate retrieval of information, and may comprise Random Access Memory (RAM), Read Only Memory (ROM), a magnetic drive, a disk drive, a Compact Disk (CD) drive, a Digital Video Disk (DVD) drive, removable media storage, any other suitable data storage medium, or a combination of any of the preceding. 
     Network system  10  communicates information through signals, such as an optical signal. As an example, an optical signal may have a frequency of approximately 1550 nanometers and a data rate of 10, 20, 40, or over 40 gigabits per second. 
     According to the illustrated embodiment, network system  10  may include one or more networks. A network may include nodes  22  coupled by fibers  26  in a mesh topology as shown in  FIG. 1  or any other suitable topology, such as a liner or ring topology. 
     The components of network system  10 , coupled together by the optical fibers  26 , may include one or more reconfigurable optical add/drop multiplexers (ROADM), one or more amplifiers, and one or more splitters, as described in more detail below with reference to  FIG. 2 . Network system  10  may be used in any optical communication network, or any other suitable network or combination of networks. Optical fibers  26  comprise any suitable type of fiber, such as a Single-Mode Fiber (SMF), Enhanced Large Effective Area Fiber (E-LEAF), or TrueWave® Reduced Slope (TW-RS) fiber. 
     In a given topology, each node  22  will have an associated number of “degrees.” The number of degrees of node  22  may be defined to be the number of links incident to node  22 . In the illustrated embodiment, a portion of a mesh topology consisting of four nodes  22  is shown. Each node  22  has four degrees: three links to the other nodes  22  of network system  10  and a link to the remaining portion of network system  10 . The number of degrees may be any number, depending on the particular topology and implementation chosen. 
     In some embodiments, network system  10  may be designed to assign each incoming signal to a particular “channel,” or carrier wavelength. The number of channels and the wavelengths assigned may vary depending on the chosen implementation. As an illustrative example, network system  10  may carry 88 channels in the 1550 nm wavelength band, with a channel separation of 50 GHz (˜0.4 nm). That is, network system  10  may potentially communicate information on carrier wavelengths between 1528.77 nm (196.1 THz) and 1563.45 nm (191.75 THz). In some embodiments, network system  10  may include some means of dynamically allocating incoming signals to various wavelengths, depending on the design needs, such that none, some, or all channels are in use at one time. 
     The process of communicating information over multiple channels of a single optical path is referred to in optics as wavelength division multiplexing (WDM). Dense wavelength division multiplexing (DWDM) refers to the multiplexing of a larger (denser) number of wavelengths, usually greater than forty, onto a fiber. WDM, DWDM, or other multi-wavelength transmission techniques are employed in optical networks to increase the aggregate bandwidth per optical fiber. Without WDM or DWDM, the bandwidth in networks would be limited to the bit rate of solely one wavelength. With more bandwidth, optical networks are capable of transmitting greater amounts of information. Referring back to  FIG. 1 , network system  10  is operable to transmit disparate channels using WDM, DWDM, or some other suitable multi-channel channel multiplexing technique, and to amplify the multiplexed wavelengths  40 . 
       FIG. 2  illustrates a node  22  with a plurality of degrees  200 , in accordance with certain embodiments of the present disclosure. In the illustration, four degrees  200  are shown, but more or fewer degrees  200  may be present in a given configuration. Each degree  200  of node  22  may include splitter  202 , wavelength selective switch (WSS)  204 , optical channel monitor (OCM)  208 , and one or more amplifier(s)  206 . In operation, degree  200  of node receives the multiplexed wavelengths  40  from another degree  200  of node  22 , another node  22 , or some other portion of network system  10 . The channels comprising the multiplexed wavelengths  40  incoming to degree  200  of node  22  may be added, dropped, and/or amplified before exiting degree  200  of node  22 . 
     Amplifier  206  may be used to amplify the multiplexed wavelengths  40 . Amplifier  206  may be positioned before and/or after certain lengths of fiber  26 . Amplifier  206  may comprise an optical repeater that amplifies the optical signal. This amplification may be performed without opto-electrical or electro-optical conversion. In some embodiments, amplifier  206  may comprise an optical fiber doped with a rare-earth element. When a signal passes through the fiber, external energy is applied to excite the atoms of the doped portion of the optical fiber, which increases the intensity of the optical signal. As an example, amplifier  206  may comprise an erbium-doped fiber amplifier (EDFA). However, any other suitable amplifier  206  may be used. In the illustrated embodiment, each degree  200  of node  22  has a plurality of amplifiers  206  on an amplifier card  212 . Amplifier card  212  may, in some embodiments, also be configured to gather information about multiplexed wavelengths  40 , as described in more detail below with respect to  FIGS. 6-10 . Although the figure shows each degree  200  with its own amplifier card  212 , there may be a single amplifier card  212  for some or all degrees  200  of node  22  or multiple nodes  22 . 
     When the optical strength of the multiplexed wavelengths  40  reaches a certain point, amplifier  206  may reach its maximum linear response. Past this point, amplifier  206  may behave in a non-linear fashion (referred to as being “saturated”). At these levels, the multiplexed wavelengths  40  may be referred to as a “saturating signal.” When amplifier  206  is supplied with a saturating signal, it may experience greater degrees of certain negative effects such as polarization hole burning (PHB). PHB may act to create a difference in the amount of amplified spontaneous emission noise in the same SOP as the saturating signal and the amplified spontaneous emission noise in the SOP orthogonal to the saturating signal. This difference leads to an overall decrease in the signal-to-noise ratio. Additionally, the magnitude of the decrease is dependent on the polarization state of the saturating signal and thus varies over time. This can result in both a reduction of signal quality and a time-varying signal quality at later nodes  22 . The effects of PHB can be mitigated through the use of ghost channels generated from the Amplified Spontaneous Emission (ASE) noise present within network system  10 , as described in more detail below with reference to  FIGS. 3-13 . After amplification, if required, the multiplexed wavelengths  40  may then pass to splitter  202 . 
     Splitter  202  may include any device or component of a device which may be configured to reproduce the multiplexed wavelengths  40  (at greater or lesser magnitudes) before passing on multiple copies of the multiplexed wavelengths  40  to other components of node  22  or to other nodes  22  of network system  10 . One such copy of multiplexed wavelengths  40  may be passed to WSS  204 . WSS  204  may include any device or component of a device which may be configured to receive, combine, add, drop, and/or amplify the component channels of the incoming multiplexed wavelengths  40  transmitted from other degrees  200  of node  22  or other nodes  22  of network system  10 . In some embodiments, WSS  204  may be configured to generate ghost channels and/or amplify or attenuate previously generated ghost channels, as described in more detail below with reference to  FIGS. 3-5 . 
     OCM  208  may include any component or set of components operable to provide information on the optical power of the individual optical channels comprising the multiplexed wavelengths  40 . In some embodiments, OCM  208  may be an integrated part of switch card  214 , such as that found on the Fujitsu Flashwave 7500 ROADM and Flashwave 9500 ROADM. In other embodiments, OCM  208  may be a stand-alone component, or the functions of OCM  208  may be performed by WSS  204  or any other appropriately configured component of node  22 . In operation, OCM  208  may measure at least the optical power of the signal component channels in order to provide this power information to other components of node  22  for the appropriate generation, selection, management, and optimization of ghost channels for mitigating the effects of PHB. Switch card  214 , or a component of switch card  214 , may also be configured to provide the functions of splitter  202 , WSS  204 , OCM  208 , or any other necessary functions, such as the maintenance of information regarding the current ghost channel load of degree  200 , as described in more detail below with reference to  FIGS. 6-10 . 
       FIG. 3  illustrates a graph  300  of a traffic channel  304  surrounded on either side by ghost channels  302 , in accordance with certain embodiments of the present disclosure. In the illustrated embodiment, traffic channel  304  and ghost channels  302  are shown at specific wavelengths and specific amplitudes. These and other specific properties of traffic channel  304  and ghost channels  302  are intended for illustrative purposes only, and are in no way intended to limit the scope of the present disclosure. 
     In some embodiments, node  22  comprises certain modules configured to generate ghost channels  302  surrounding traffic channel  304 . These ghost channels  302  may operate to reduce the deleterious effects of PHB. A ghost channel  302  generally refers to the propagation of optical energy through a communication channel of network system  10  without transmitting any signal information. Ghost channels  302  may be treated like other channels in need of amplification and/or propagation, but do not carry signal information. 
     Ghost channels  302  have been amplified to have an optical strength at or near the optical strength of traffic channel  304  as described in more detail below with reference to  FIG. 4 . Traffic channel  304  is a channel within a multi-channel optical signal that is carrying information along its carrier wavelength at a given moment. Generation, selection, management, and optimization of the ghost channels are discussed in more detail below with reference to  FIGS. 4-13 . 
     In some embodiments, traffic channel  304  may have only a single ghost channel  302  on either side. In other embodiments, there may be two, four, or any number less than the current capacity of network system  10 . In some configurations, the use of higher numbers of ghost channels  302  may result in the possibility of feedback loops within network system  10 . A given implementation may balance the desire for increasing the number of ghost channels  302  (and the corresponding decrease in the effects of PHB) and the desire to avoid feedback. 
     A given channel may have one or more “neighbor” channels. Neighbor channels are defined generally to be those that are within a certain wavelength distance of a given channel. For instance, if network  10  has implemented a channel separation of 50 GHz (˜0.4 nm), then channels at 1544.92 nm and 1545.72 nm may be the neighbors of the channel at 1545.32 nm. In other embodiments, a neighbor channel may be defined to be within an optical bandwidth equivalent to two or more times the channel spacing bandwidth (e.g., 50 GHz in the illustrative example), depending on the system&#39;s design criteria (such as the fear of signal interference and the intrinsic properties of the chosen carrier wavelength). Neighbor channels may also be said to “surround” a signal channel. 
     The effects of PHB may be most severe in situations where a particular traffic channel  304  is isolated from other traffic channels  304 . If network system  10  is operating at full capacity, with all channels carrying information at the same time, then each operating channel is surrounded by other operating channels. In such a case, the surrounding channels function to mitigate the effects of PHB for any given channel. However, rarely does network system  10  operate at such a full capacity. Often only a percentage of the communication channels carry information at any given time. 
     Some prior solutions have randomly rotated the SOP of communication signals so that PHB effects will not be severe in any particular state of polarization. However, such solutions may be cost prohibitive to implement and, depending on the chosen implementation for rotating the SOP of the saturating signal, may not be able to successfully handle dynamic loading of channels across network system  10  (situations where the number and identity of information-carrying channels changes over time). In such situations, PHB effects may be mitigated by surrounding a communication channel with ghost channels. 
     In some embodiments, traffic channel  304  may be surrounded by ghost channels if it does not have another traffic channel  304  sufficiently nearby. This determination may be made in accordance with a set of predetermined rules. For instance, if, for a first optical signal, the next nearest optical signal is more than 20 channels away, then the first optical signal may require accompanying ghost channels in order to mitigate the effects of polarization hole burning. The more isolated a signal, the stronger the need for ghost channels. Below are example situations in which it might be desirable to produce ghost channels  302  for a given traffic channel  304  (denoted by λ i ).
         if . . . λ i ε{ch 1 , . . . , ch 22 } . . . and . . . λ i−1  &amp; λ i+1 ε/{ch 1 , . . . , ch 23 } and N={1, . . . , 42},   λ i−1 , λ i+1  are channels adjacent to λ i  and N is the total number of channels propagating in a span.   if . . . λ i ε{ch 23 , . . . , ch 44 } . . . and . . . λ i−1  &amp; λ i+1 ε/{ch 22 , . . . , ch 44 }, and N={1, . . . , 42}   λ i−1 , λ i+1  are channels adjacent to λ i . and N is the total number of channels propagating in a span.   if . . . λ i , λ i+1 ε{ch 1 , . . . , ch 22 } . . . and . . . λ i−1  &amp; λ i+2 ε/{ch 1 , . . . , ch 23 }, and N={2, . . . , 42}   λ i−1 , λ i+1  are channels adjacent to λ i  and λ i , λ i+2  are channels adjacent to λ i+1 . N is the total number of channels propagating in a span.   if . . . λ i , λ i+1 ε{ch 23 , . . . , ch 44 } . . . and . . . λ i−1  &amp; λ i+2 ε/{ch 22 , . . . , ch 44 }, and N={2, . . . , 42}   λ i−1 , λ i+1  are channels adjacent to λ i  and λ i , λ i+2  are channels adjacent to λ i+1 . N is the total number of channels propagating in a span.       

     In some embodiments, there may also be rules to determine when ghost channels  302  may not be generated. For example, if traffic channel  304  (denoted by λ i ) falls within the rule described below, then it may not be sufficiently isolated to warrant the generation of ghost channels  302 .
         if . . . n( . . . , λ i−1 , λ i , λ i+1 , . . . )≧3 . . . and ( . . . , λ i−1 , λ i , λ i+1 , . . . )ε{ch 1 , . . . , ch 44 }, and N={3 . . . , 44}   . . . , λ i−1 , λ i , λ i+1 , . . . are neighbouring channels, n is the number of neighbouring channels, and N is the total number of channels propagating in a span.       

       FIG. 4  illustrates an optical amplification scheme  400  for generating ghost channels  302 , in accordance with certain embodiments of the present disclosure. In the illustrated embodiment, traffic channel  304  and ghost channels  302  are shown at specific amplitudes and separations. These and other specific properties of traffic channel  304  and ghost channels  302  are intended for illustrative purposes only, and are in no way intended to limit the scope of the present disclosure. When traffic channel  304  is amplified, e.g., by amplifier  206  of node  22 , a certain amount of noise is introduced through a phenomenon known as spontaneous emission. The amplification of traffic channel  304  may also amplify this noise, resulting in amplified spontaneous emission (ASE), an undesirable and problematic noise source, particularly for long-haul systems where traffic channel  304  may be amplified multiple times along its path. ASE is typically absorbed or extracted from network system  10  in order to maintain an acceptable signal to noise ratio. 
     However, controlling the location and magnitude of ASE may provide a source for the generation of ghost channels. Referring back to  FIG. 4 , four nodes  22  of network system  10  are shown, labeled as nodes  22   a ,  22   b ,  22   c , and  22   d . These labels are intended for clarity of discussion and are in no way intended to limit the scope of this disclosure. A traffic channel  304  may be added at node  22   a . At node  22   a , ASE at all incoming channels may be blocked, as shown in loss diagram  402   a . Power spectrum  404   a  demonstrates that, as traffic channel  304  is transmitted from node  22   a  to node  22   b , only traffic channel  304  is transmitted. Loss diagrams  402  and power spectrums  404  are provided for illustrative purposes only. As an example, in some embodiments, there may be multiple traffic channels  304  at varying separations. 
     When traffic channel  304  is amplified at node  22   b , the ASE in the channels neighboring traffic channel  304  are not absorbed or extracted. The ASE in these neighboring channels may be allowed to grow to a certain point in order to provide the appropriate ghost channels  302  surrounding traffic channel  304 . Loss diagram  402   b  shows that the block levels applied to the neighbor channels have been reduced to a substantially lower level. This may allow the ASE in the neighbor channels to grow, generating a ghost channel, as shown in power spectrum diagram  404   b.    
     At this stage, ghost channels  302  may have a low optical strength relative to traffic channel  304 . As traffic channel  304  and ghost channels  302  are passed through a third node  22   c , ghost channels  302  may be amplified to a greater optical strength. Loss diagram  402   c  shows that the block levels applied to the neighbor channels are at the same substantially lower level as those in loss diagram  402   b . This may allow the ASE in the neighbor channels to continue to grow, allowing ghost channels  302  to gain optical power, as shown in power spectrum diagram  404   c.    
     This amplification may continue through a fourth node  22   d  until the optical strength of ghost channels  302  is at or near the optical strength of traffic channel  304 . At this stage ghost channels  302  may be at their most efficient in mitigating the effects of polarization hole burning without overcoming traffic channel  304 . Loss diagram  402   d  shows that the block levels applied to the neighbor channels have been raised relative to loss diagram  402   c , but are still at a substantially lower level than in loss diagram  402   a . This may allow the ASE in the neighbor channels to be capped at a certain optical power level, allowing ghost channels  302  to propagate at an optical power level substantially equal to traffic channel  304 , as shown in power spectrum diagram  404   d.    
     Although this figure depicts this process taking place over the course of four nodes  22   a -D, a particular implementation may take more or fewer nodes to get ghost channels  302  to an appropriate optical strength. 
     In some embodiments, degree  200  of node  22  may amplify ghost channels  302 . As part of the amplification, degree  200  may also determine whether ghost channel  302  needs to be amplified, or if it is already of sufficient magnitude. For instance, in some embodiments, it may be desirable to cap amplification of a ghost channel  302  as described in more detail below with reference to  FIGS. 5-13 . In other embodiments, the amplification and measurement may be done by more or different components of network system  10 . 
       FIG. 5  is a flowchart illustrating one embodiment of a method  500  of mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure. Method  500  includes checking an incoming traffic channel  304  for the existence of neighboring ghost channels  302 , generating ghost channels  302  if necessary, and amplifying ghost channels  302  if necessary. 
     According to one embodiment, method  500  preferably begins at step  502 . Teachings of the present disclosure may be implemented in a variety of configurations of nodes  22  and network system  10 . As such, the preferred initialization point for method  500  and the order of steps  502 - 512  comprising method  500  may depend on the implementation chosen. As described in more detail above with reference to  FIG. 1 , node  22  may be associated with a number of degrees  200 , each receiving multiplexed wavelengths  40  from a different portion of network system  10 . Depending on the implementation chosen, method  500  may be performed on some, all, or none of degrees  200  of node  22 . Additionally, multiplexed wavelengths  40  may be a multi-channel signal which may be demultiplexed into its component channels. Depending on the implementation chosen, method  500  may be performed on some, all, or none of the traffic channels  304  of multiplexed wavelengths  40 . 
     At step  502 , degree  200  of node  22  receives multiplexed wavelengths  40 . After receiving multiplexed wavelengths  40 , method  500  may begin to analyze a first channel constituting multiplexed wavelengths  40 . After analyzing that channel, method  500  may proceed to step  503 , where node  22  may determine whether the channel under consideration is a traffic channel  304 . In some embodiments, determining whether a given channel is a traffic channel  304  may constitute examining the WCS and WCF bits for that channel, as described in more detail below with reference to  FIGS. 6-10 . 
     If the channel is not a traffic channel  304 , then method  500  may proceed to step  512 , where method  500  may proceed to examine the next channel before returning to step  502 . If the channel under consideration is a traffic channel  304 , then method  500  may proceed to step  504 , where node  22  may determine whether there are existing ghost channels  302  for traffic channel  304 . In some embodiments, step  504  may be performed by software controlling wavelength selection switch  204  of node  22 , or any other appropriately configured measurement module, as described in more detail above with reference to  FIGS. 1-4 . In other embodiments, step  504  may be performed by hardware, firmware, or any other software module, including the operating system controlling node  22  configured to determine the presence of ghost channels  302 . 
     If no ghost channels  302  are currently present, method  500  may proceed to step  506 , where node  22  may generate ghost channels  302 . In some embodiments, step  506  may be performed by wavelength selection switch  204  of node  22 , or any other ghost channel generation module configured to modify the blocking level of ASE in appropriate channels, as described in more detail above with reference to  FIGS. 1-4 . After generating ghost channels  302 , method  500  may proceed to step  508 . If, in step  504 , method  500  determined that there were extant ghost channels  302 , method  500  may proceed directly to step  508 . 
     At step  508 , method  500  may determine the power of ghost channels  302 , as measured by optical channel monitor  208  or any other appropriately configured power monitor, as described in more detail above with reference to  FIGS. 1-4 . Method  500  may then compare the power of ghost channels  302  to the power of the associated traffic channel  304 . In some embodiments, this comparison may be performed by wavelength selection switch  204  of node  22 , or any other appropriately configured comparator, as described in more detail above with reference to FIGS.  1 - 4 . If ghost channels  302  are not of a sufficient magnitude, then method  500  may proceed to step  510 , where ghost channels  302  are amplified, as described in more detail above with reference to  FIGS. 1-4 . After amplification, method  500  may proceed to step  512 . 
     If, in step  508 , method  500  determined that extant ghost channels  302  were already of a sufficient magnitude, then method  500  may proceed directly to step  512 . At step  512 , method  500  may proceed to examine the next channel before returning to step  502 . 
     Although  FIG. 5  discloses a particular number of steps to be taken with respect to method  500 , method  500  may be executed with more or fewer steps than those depicted in  FIG. 5 . In addition, although  FIG. 5  discloses a certain order of steps comprising method  500 , the steps comprising method  500  may be completed in any suitable order. For example, in the embodiment of method  500  shown, node  22  determines whether ghost channels  302  are substantially equal in magnitude to traffic channel  304 . However, in short-distance systems where design considerations may not require concern over potential overgrowth of ghost channels, these steps may not be necessary. Additionally, method  500  may also include additional steps concerning the determination of how close a ghost channel  302  may be in order to decide if additional ghost channels  302  may be necessary. 
     Certain embodiments of the invention may provide one or more technical advantages. A technical advantage of one embodiment may be that using ghost channels  302  to mitigate the effects of polarization hole burning allows for a more robust solution to PHB effects in dynamically loaded optical communication network systems  10 . Another advantage may be that, since the methods and systems disclosed herein may be implemented in pre-existing hardware and/or software, the implementation costs and difficulties may be substantially reduced. 
     Effectively mitigating the effects of polarization hole burning in an optical communication system may require systems and/or methods of effectively managing, selecting, and/or optimizing the generated ghost channels  302 . Referring again to  FIG. 2 , in order to effectively manage ghost channel generation, it may be desirable to have the degrees  200  of node  22  communicate with one another. Intranodal communication may be used for many purposes, including the direction of incoming traffic over different output paths. Intranodal communication may also be used for communicating the current state of particular channels passing through each degree  200  of node  22 . This information can be important in the management of ghost channels  302  used for polarization hole burning mitigation. 
     In some embodiments, a degree  200  may need to add a ghost channel  302 , or pass-through (and potentially amplify) an existing ghost channel  302  from another degree  200  of node  22 . In order to effectively balance the load of the ghost channels across node  22 , it may be important to know the current and potential sources for ghost channels. However, the information required to effectively manage the ghost channel sources may not be readily available at each degree  200  of node  22 . For instance, the splitter input power for a given channel at Degree  1  may not be available to WSS  204  of Degree  4 . Without this information, Degree  4  may be unable to correctly determine whether to source a ghost channel from that given channel. In order to overcome these obstacles, degrees  200  may share information. 
     In some embodiments, degree  200  may collect certain pieces of information regarding the channels incoming to that degree  200 . That information may include a wavelength channel signal bit (“WCS”), a wavelength channel failure indicator (“WCF”), and the splitter input power for each channel. The WCS and WCF bits may be collected by amplifier card  212  of degree  200  and splitter input power may be collected by OCM  208  of degree  200 . However, these functions may be performed by the same component, different components, or any appropriately configured channel information module. 
     The WCS bit may be used to indicate whether a particular wavelength is intended to be present, e.g., whether information is being sent through that channel. In some embodiments, a “1” may indicate that a wavelength is intended to be present and a “0” may indicate that a wavelength is not intended to be present. The WCF bit may be used to indicate whether a wavelength is actually present. Although a wavelength may be intended to be present, some failure may have occurred. In some embodiments, a “1” may indicate that actual light is present, while a “0” may indicate that no light is present. These indicators, or others like them, may work together to indicate that a particular channel is “valid” for the purposes of ghost channel sourcing. In the illustrated configuration, a valid channel is one that is both intended to carry information and actually carrying information. However, other configuration may be based on different design decisions and define a valid channel differently without departing from the scope of this disclosure. 
     In some embodiments, each degree  200  may assemble a table for the desired channel validity information, with an entry for each incoming channel. These tables are discussed below in further detail with reference to  FIGS. 5-9 . Each degree  200  may then transmit these tables to every other degree  200  of node  22  in order to maximize information sharing and subsequent decision making. 
       FIG. 6  illustrates a table  600  for storing information regarding the validity of a channel incoming to a degree  200  of node  22 , in accordance with certain embodiments of the present disclosure. Table  600  may comprise a plurality of entries  602 , with each entry  602  corresponding to an incoming channel. For each entry  602  table  600  may store one or more value(s)  604 . In some embodiments, values  604  are the WCS and WCF values for each incoming channel. WCS and WCF values are discussed in more detail above with reference to  FIG. 3 . As an illustrative example, entry  602  for channel  1  may have a value of “1” for WCS and “0” for WCF. This may indicate that channel  1  is intended to be operational, but that no light is present. Entry  602  for channel  2  may have a “1” for WCS, indicating that it is intended to be operational, and a “1” for WCF, indicating that light is, in fact, present. The channel validity information of table  600  may then be passed on to a portion of degree  200  configured to measure optical strength of incoming channels. In some embodiments, the gathering of data in table  600  is performed by amplifier card  212  within degree  200  of node  22 . Amplifier card  212  may then send table  600  to switch card  214  in order to gather information regarding the optical power of the incoming channels as described in more detail below with reference to  FIGS. 7-9 . However, in other embodiments, table  600  may be generated within switch card  214  of degree  200  or by any channel information module of node  22  configured to gather the appropriate channel validity information. 
       FIG. 7  illustrates a table  700  for storing information regarding the validity and signal strength of a channel incoming to a particular degree  200  of node  22 , in accordance with certain embodiments of the present disclosure. Table  700  may comprise a plurality of entries  702 , with each entry  702  corresponding to an incoming channel. For each entry  702 , table  700  may store one or more value(s)  704 . In some embodiments, values  704  are the WCS, WCF, and splitter input power for each incoming channel. WCS and WCF values are discussed in more detail above with reference to  FIGS. 2 and 6 . OCM  208  of degree  200  may measure the splitter input power for each channel and record the information in table  700 . Table  700  may then be broadcast to every other degree  200  of node  22 . 
     In some embodiments, switch card  214  of degree  200  receives channel validity information from amplifier card  212  (or some other channel information module), as described in more detail above with reference to  FIG. 6  (e.g., in the form of table  600 ) and appends to that information regarding the optical power of the incoming channels from OCM  208  (e.g., in the form of splitter input power in table  700 ). However, in other embodiments, these functions may be performed by the same component. For instance, the channel information module and switch card  214  may be an integral component configured to determine WCS bits, WCF bits, and to measure splitter input power at the same time without the need for separate tables  600  and  700 . 
     Once the information illustrated in table  700  has been broadcast to other degrees  200  of node  22 , it may be necessary or efficient for each degree  200  to consolidate the tables  700  received from each of the other degrees  200 . 
       FIG. 8  illustrates a series of tables  800  representing the consolidated information received from other degrees  200  of node  22 , in accordance with certain embodiments of the present disclosure. In some embodiments, a degree  200  of node  22  may receive tables from other degrees  200  of node  22  containing information regarding the validity and optical power of certain channels, as described in more detail above with reference to  FIGS. 6-7 . Aggregating such data in a way such that the data is available for each channel may be desirable, such as is depicted in table(s)  800 . It may also be desirable to include “freshness” data to indicate how recently the data has been received. 
     Each table  800  may represent the aggregated information for a particular channel. In some embodiments, there is a table  800  corresponding to each incoming channel (as an example only, the illustrated embodiment includes eighty-eight channels, so there are eighty-eight tables  800 ). Each table  800  may include a plurality of entries  802 , with each entry  802  corresponding to a degree  200  in node  22 . Some configurations may also find it more desirable or efficient to combine one or more table(s)  800  into a single table  800 , or to split a single table  800  into smaller tables. 
     For each entry  802  table  800  may store one or more value(s)  804 . In some embodiments, values  804  are the WCS bit, WCF bit, splitter input power, and freshness values for each degree. WCS, WCF, and splitter input power values are described in more detail above with reference to  FIGS. 6-7 . The “Fresh” value of table  800  may be used to indicate whether the remaining values  804  of table  800  have been recently updated (or sufficiently “fresh”). In some embodiments, switch card  214  of degree  200  may determine whether the data stored in table  800  has been refreshed within a predetermined amount of time, e.g., five seconds. If the data has been received within that time period, then the Fresh value  804  may be marked with a “1” to indicate that the data is sufficiently fresh; if is has not, then it may be marked with a “0” to indicate that the data is stale. 
     In addition to information regarding the validity and freshness of the incoming channels at each degree  200  of node  22 , it may also be desirable to know what, if any, ghost channels are already sourced from a degree  200 . 
       FIG. 9  illustrates a table  900  for storing information regarding the current ghost channel load for degrees  200  in node  22 , in accordance with certain embodiments of the present disclosure. Table  900  may include a plurality of entries  902 , with each entry  902  corresponding to a degree  200  in node  22 . For each entry  902  table  900  may store one or more value(s)  904 . In some embodiments, value  904  is the current ghost channel count for each degree  200  in node  22 . The ghost channel count is the number of ghost channels that are currently sourced from a particular degree. The data in current ghost count  904  may be used to efficiently balance the ghost channel load within node  22 . This information may allow the distribution of ghost channels across degrees  200  within node  22 , which may in turn improve operational efficiency of node  22  and network system  10 . Table  900  may, in some embodiments, be stored and managed by the software code that operates WSS  204  of degree  200 . In other embodiments, table,  900  may be managed by an overarching node management system, and implemented in software, hardware, or firmware, or some combination thereof. The information in table  900  regarding the current ghost channel load of degrees  200  may be combined with the channel input information in table  600  to better manage the ghost channels used to mitigate the effects of polarization hole burning. 
       FIG. 10  is a flowchart illustrating one embodiment of a method  1000  of managing the selection of ghost channels in mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure. Method  1000  includes collecting validity and optical power data for optical communication channels, transmitting that data to all degrees  200  within a node  22 , aggregating received data, and collecting data concerning the current ghost channel loading. 
     According to one embodiment, method  1000  preferably begins at step  1002 . Teachings of the present disclosure may be implemented in a variety of configurations of node and network system  10 . As such, the preferred initialization point for method  1000  and the order of steps  1002 - 1010  comprising method  1000  may depend on the implementation chosen. 
     At step  1002 , method  1000  collects validity data for an optical communication channel at a first degree  200  (as an example only, the WCS and WCF bits for the optical communication channel). In some embodiments, this step may be performed by a channel information module as described in more detail above with reference to  FIGS. 6-9 . After collecting this information method  1000  may proceed to step  1004 . 
     At step  1004 , method  1000  collects optical power data for the optical communication channel at each degree  200 . In some embodiments, step  1004  may be performed by OCM  208  as described in more detail above with reference to  FIGS. 6-9 . After collecting this information method  1000  may proceed to step  1006 . 
     At step  1006 , method  1000  may transmit all of the collected validity and optical power data to all other degrees  200  within node  22 . Step  1006  may be performed by switch card  214  as described in more detail above with reference to  FIGS. 6-9 . After transmitting this data, method  1000  may proceed to step  10101008 . 
     At step  1008 , method  1000  may aggregate the channel validity and optical power data received at each degree  200  from all other degrees  200  such that a composite picture of the data for a given channel may be formed. The aggregated validity and power data may be combined with a freshness value, indicating how recently the data had been retrieved. Step  1008  may be performed by switch card  214  as described in more detail above with reference to  FIGS. 6-9 . After aggregating and collecting this data, method  1000  may proceed to step  1010 . 
     At step  1010 , method  1000  may collect data regarding the current ghost channel load within node  22 . This data may comprise information detailing which degrees  200  within node  22  are currently being used as ghost channel sources, as described in more detail above with reference to  FIG. 9 . After collecting this data, method  1000  may return to step  1002  to begin the data collection cycle again. In some embodiments, there may be a time delay, such as five seconds, before the cycle begins again. Such a time delay would be a design determination to suit the particular implementation of network system  10 . 
     Although  FIG. 10  discloses a particular number of steps to be taken with respect to method  1000 , method  1000  may be executed with more or fewer steps than those depicted in  FIG. 10 . In addition, although  FIG. 10  discloses a certain order of steps comprising method  1000 , the steps comprising method  1000  may be completed in any suitable order. For example, in the embodiment of method  1000  shown, degree  200  of node  22  collects channel validity data and optical power data in two separate steps. In some embodiments, it may be desirable to separate these steps to have separate components of degree  200  perform these tasks. However, in other embodiments these steps may be performed simultaneously and/or by the same component of degree  200 . 
     Certain embodiments of the invention may provide one or more technical advantages. A technical advantage of one embodiment may be that effective management of ghost channel selection can allow a network system to effectively balance the load that ghost channels may place on the system. 
     Along with managing the load balance of ghost channels  302  within node  22 , it may also be beneficial to select the appropriate source for ghost channels  302  in order to reduce or eliminate the potential deleterious effects of feedback when mitigating the effects of polarization hole burning. 
       FIG. 11  is a flowchart illustrating one embodiment of a method  1100  of selecting ghost channels for use in mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure. Method  1100  includes checking each channel to see if it is a traffic channel, a blocked channel, a current ghost channel, or if its neighbor channels are add or pass-through channels. In operation, method  1100  determines whether, for a given channel, either neighbor channel will have a signal when it leaves degree  200 . If it does, then the current channel may need to be blocked from use as a ghost channel in order to prevent undesirable feedback within network system  10 . One point at which it may be effective to reduce feedback is at the point where a traffic channel is added. If the traffic channel is being added at node  22 , then ghost channel  302  may not be immediately introduced at node  22 . If a traffic channel is a pass through channel, then ghost channel  302  may be generated with less concern for feedback. 
     According to one embodiment, method  1100  preferably begins at step  1102 . Teachings of the present disclosure may be implemented in a variety of configurations of communication system  10 . As such, the preferred initialization point for method  1100  and the order of steps  1102 - 1116  comprising method  1100  may depend on the implementation chosen. In some embodiments, the steps of method  1100  may be performed by the software that manages WSS  204  of each degree  200 . In other embodiments different steps may be performed by different pieces of software or different software modules within one piece of software, or may be implemented in hardware or firmware or any appropriate combination thereof configured to perform the method within each degree  200 . 
     Beginning with a first channel, at step  1102 , method  1100  determines whether, the first channel is a traffic channel. In some embodiments, determining whether a given channel is a traffic channel  304  may constitute examining the WCS and WCF bits for that channel, as described in more detail above with reference to  FIGS. 6-10 . If the channel is a traffic channel, then method  1100  may proceed to step  1108 , wherein method  1100  advances to the next channel before returning to step  1102 . If the channel is not a traffic channel, then method  1100  may proceed to step  1104 . 
     At step  1104 , method  1100  determines whether either channel neighboring the channel under consideration is an add channel. In some embodiments, an add channel is a channel on which traffic is added at WSS  204  of the degree  200  performing the method. In such a situation, sourcing a ghost channel may lead to undesirable feedback within network system  10 . If either neighbor channel is an add channel, then method  1100  may proceed to step  1106 . If neither neighbor channel is an add channel, then method  1100  may proceed to step  1112 . 
     At step  1106  method  1100  may determine whether the channel under consideration is currently blocked. If it is blocked, then method  1100  may proceed to step  1108 , wherein method  1100  advances to the next channel before returning to step  1102 . If the current channel under consideration is not blocked, then method  1100  may proceed to step  1110 , wherein the channel is blocked, before proceeding to step  1108 . 
     At step  1112 , method  1100  may determine whether either neighbor channel is a pass-through channel. In some embodiments, a pass-through channel is a channel on which traffic is received at the degree  200  and which is passed through WSS  204  of that degree  200 . In the case of pass-through channels, sourcing a ghost channel from the channel under consideration may not run as high a risk of undesirable feedback within network system  10 . If either neighbor channel is a pass-through channel, then method  1100  may proceed to step  1114 . If neither neighbor channel is a pass-through channel, then method  1100  may proceed to step  1106 . 
     At step  1114 , method  1100  may determine whether the current channel is already being used to source a ghost channel. If it is, then method  1100  may proceed to step  1108 , wherein method  1100  advances to the next channel before returning to step  1102 . If it is not, then method  1100  may proceed to step  1116 . At step  1116 , method  1100  may select the current channel to source a ghost channel. The selection method may be simple or complex, depending on the particular implementation. In some embodiments, the ghost source may be selected from a calculated set of degrees with appropriate validity and optical power criteria, as described below in more detail with reference to  FIG. 12 . 
     Although  FIG. 11  discloses a particular number of steps to be taken with respect to method  1100 , method  1100  may be executed with more or fewer steps than those depicted in  FIG. 11 . In addition, although  FIG. 11  discloses a certain order of steps comprising method  1100 , the steps comprising method  1100  may be completed in any suitable order. For example, in the embodiment of method  1100  shown, channels with neighbors that will carry add channels after leaving degree  200  may be blocked. However, in some configurations it may be less important to guard against feedback and these steps may be reduced in scope or eliminated. 
       FIG. 12  is a flowchart illustrating one embodiment of a method  1200  of selecting the degree  200  of node  22  from which to select the appropriate ghost channels for mitigating the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure. Method  1200  includes selecting the source degree  200  for the ghost channel  302  identified above with reference to  FIG. 11  by examining the validity and optical power values of channels within the degrees  200  of node  22 , in order to maintain a desired level of load balancing across network system  10 . 
     In operation, method  1200  may select the degree  200  of node  22  that can source a ghost channel  302  with the highest initial optical power (e.g., the ASE noise at the highest initial level) and/or the degree with the lowest current ghost channel count. 
     According to one embodiment, method  1200  preferably begins at step  1202 . Teachings of the present disclosure may be implemented in a variety of configurations of communication system  10 . As such, the preferred initialization point for method  1200  and the order of steps  1202 - 1220  comprising method  1200  may depend on the implementation chosen. In some embodiments, the steps of method  1200  may be performed by the software that manages WSS  204  of degree  200 . In other embodiments different steps may be performed by different pieces of software or different software modules within one piece of software, or may be implemented in hardware or firmware or any appropriate combination thereof configured to perform the method within degree  200 . 
     At step  1202 , method  1200  may determine whether a given channel is valid, fresh, and has a splitter input power greater than or equal to a predetermined threshold. Channel validity, freshness, and optical power information are discussed in more detail above with reference to  FIGS. 6-10 . The predetermined threshold may be any value determined by the particular implementation of network system  10  to produce a likelihood of finding an appropriate ghost channel while maintaining the balance of ghost channel load across degrees  200  of node  22 . The set of degrees  200  meeting both criteria is depicted in method  1200  by the letter “D.” After examining each degree  200 , the method may proceed to step  1204 . 
     At step  1204 , method  1200  may determine whether there are any degrees  200  within the set that met both criteria in step  1202  (e.g., whether D=0). If there are any sufficiently valid, fresh, and powerful channels in any degree  200  (D does not equal 0), then method  1200  may proceed to step  1206 . If there are not, then method  1200  may proceed to step  1212 . 
     At step  1206 , method  1200  may determine which degree currently has the least number of ghost channels currently sourced. The gathering, collecting, and transmission of current ghost channel load information is described above in more detail with reference to  FIGS. 6-10 . In  FIG. 12 , this degree is denoted by the letter “d.” After determining the appropriate degree, method  1200  may proceed to step  1208 . At step  1208 , the controller may switch in the designated ghost channel from the designated degree d. Method  1200  may then proceed to step  1210 , where the ghost channel count for the designated degree is incremented, at which point method  1200  may proceed to step  1220 , where method  1200  terminates. 
     If there are no sufficiently valid, fresh, powerful channels found at step  1204  (e.g., D=0), then method  1200  may proceed to step  1212 , where method  1200  may limit the determination to the number of sufficiently valid and fresh channels (denoted by the letter “D.”). After determining the number of valid and fresh channels, method  1200  may proceed to step  1214 . At step  1214 , the controller may determine if there were any valid and fresh channels found in step  1212 . If there were no valid and fresh channels (D=0), then method  1200  may proceed to step  1216 , where the channel under examination is blocked as a ghost source. After blocking the channel, method  1200  may proceed to step  1220 , where method  1200  terminates. 
     If, in step  1214 , there were one or more valid and fresh channels, then method  1200  may proceed to step  1218 . At step  1218 , method  1200  may determine which degree has the channel with the highest splitter input power. This degree is denoted by the letter “d” in the drawing. After making this determination, method  1200  may proceed to step  1208 . At step  1208 , method  1200  may switch in the ghost channel from the designated degree. As discussed above, after switching in the ghost channel, method  1200  may then proceed to step  1210 , where the ghost count is incremented for the designated degree, and then to step  1220 , where method  1200  terminates. 
     Although  FIG. 12  discloses a particular number of steps to be taken with respect to method  1200 , method  1200  may be executed with more or fewer steps than those depicted in  FIG. 12 . In addition, although  FIG. 12  discloses a certain order of steps comprising method  1200 , the steps comprising method  1200  may be completed in any suitable order. For example, in the embodiment of method  1200  shown, a choice may be made to switch in a channel with a splitter input of less than the full ghost threshold. However, in some configurations it may be desirable to only source ghost channels from channels with a splitter input greater than or equal to the full ghost threshold. 
     In order to maintain the effectiveness of a load balanced, ghost channel polarization hole burning mitigation scheme, it may be necessary or desirable to optimize that scheme for continued performance. 
       FIG. 13  is a flowchart illustrating one embodiment of a method  1300  of optimizing a ghost channel selection routine in order to mitigate the effects of polarization hole burning, in accordance with certain embodiments of the present disclosure. Method  1300  includes periodically running a ghost channel selection algorithm in order to ensure that the most effective degree  200  of node  22  is the current source for a ghost channel  302 . 
     According to one embodiment, method  1300  preferably begins at step  1302 . Teachings of the present disclosure may be implemented in a variety of configurations of communication system  10 . As such, the preferred initialization point for method  1300  and the order of steps  1302 - 1308  comprising method  1300  may depend on the implementation chosen. In some embodiments, the steps of method  1300  may be performed by the software that manages WSS  204  of degree  200 . In other embodiments different steps may be performed by different pieces of software or different software modules within one piece of software, or may be implemented in hardware or firmware or any appropriate combination thereof configured to perform the method within degree  200 . 
     At step  1302 , method  1300  may run a ghost selection routine for a first channel. The ghost selection routine may be simple or complex, depending on the configuration of network system  10 . In some embodiments, the ghost source may be selected from a calculated set of degrees with appropriate validity and optical power criteria, as described in more detail above with reference to  FIGS. 11-12 . Once the ghost selection routine has run for the first channel, method  1300  may proceed to step  1304 . 
     At step  1304 , method  1300  may determine the current source degree for the ghost channel. Method  1300  may then proceed to step  1305 . At step  1305 , method  1300  may determine whether the ghost channel source degree returned from the selection routine is different from the current ghost channel source degree. If it is not, then method  1300  may proceed to step  1308 , wherein method  1300  may proceed to the next ghost channel and repeat the method by returning to step  1302 . If the ghost channel selection routine returns a different source than what is currently used, then method  1300  may proceed to step  1306 . At step  1306 , method  1300  may switch the source of the current ghost channel from the degree  200  currently being used to the degree  200  returned from the ghost channel selection routine in step  1302 . After switching the ghost channel source, method  1300  may proceed to step  1308 , wherein method  1300  may proceed to the next ghost channel and repeat the method by returning to step  1302 . 
     Although  FIG. 13  discloses a particular number of steps to be taken with respect to method  1300 , method  1300  may be executed with more or fewer steps than those depicted in  FIG. 13 . In addition, although  FIG. 13  discloses a certain order of steps comprising method  1300 , the steps comprising method  1300  may be completed in any suitable order. For example, in the embodiment of method  1300  shown, the ghost channel selection routine is run for every ghost channel. However, in some configurations it may not be desirable or efficient to run the routine for every ghost channel continuously. It may be more efficient to, for instance, only run the routine for every other ghost channel the first time through method  1300 , and run the routine for the other half of ghost channels the second time through method  1300 . 
     While this disclosure has been described in terms of certain embodiments and generally associated methods, alterations and permutations of the embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.