Patent Publication Number: US-8111454-B2

Title: Optical communication using shared optical pumps

Description:
BACKGROUND 
     Fiber-optic communication networks serve a key demand of the information age by providing high-speed data between network nodes. Fiber optic communication networks include an aggregation of interconnected fiber-optic links. Simply stated, a fiber-optic link involves an optical signal source that emits information in the form of light into an optical fiber. Due to principles of internal reflection, the optical signal propagates through the optical fiber until it is eventually received into an optical signal receiver. If the fiber-optic link is bi-directional, information may be optically communicated in reverse typically using a separate optical fiber. 
     Fiber-optic links are used in a wide variety of applications, each requiring different lengths of fiber-optic links. For instance, relatively short fiber-optic links may be used to communicate information between a computer and its proximate peripherals, or between local video source (such as a DVD or DVR) and a television. On the opposite extreme, however, fiber-optic links may extend hundreds or even thousands of kilometers when the information is to be communicated between two network nodes. 
     Long-haul and ultra-long-haul optics refers to the transmission of light signals over long fiber-optic links on the order of hundreds or thousands of kilometers. Transmission of optic signals over such long distances presents enormous technical challenges. Significant time and resources may be required for any improvement in the art of long-haul and ultra-long-haul optical communication. Each improvement can represent a significant advance since such improvements often lead to the more widespread availability of communication throughout the globe. Thus, such advances may potentially accelerate humankind&#39;s ability to collaborate, learn, do business, and the like, regardless of where an individual resides on the globe. 
     One of the many challenges that developers of long-haul optic links face involves reliability. The long reach and large carrying capacity of long-haul fiber optic links makes such links heavily relied upon as a functioning component of the Internet, or as a vehicle for communicating voice information. A competing challenge is electrical power consumption. In long-haul optic links, power may not be necessarily available at points in the link (such as repeaters) that might require electrical power. Accordingly, power is often delivered to such points using an electrical conductor that is integrated with, or is associated with the optical cable. Since large distances are involved, much of the electrical power is lost as heat throughout the length of the electrical conductor. 
     BRIEF SUMMARY 
     Embodiments described herein related to the sharing of optical pump units amongst multiple amplifier gain stages even in a single direction of an optical link in an optical communications system. For example, an optical pump unit may output optical pump power that is shared amongst a discrete optical amplification unit and a distributed optical amplification unit (such as a forward and/or backward Raman amplifier). Such sharing has the potential to increase reliability and/or efficiency of the optical communications system. 
     This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe the manner in which the above-recited and other advantages and features can be obtained, a more particular description of various embodiments will be rendered by reference to the appended drawings. Understanding that these drawings depict only sample embodiments and are not therefore to be considered to be limiting of the scope of the invention, the embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  schematically illustrates an example optical communications network including two optically communicating terminals; 
         FIG. 2  schematically illustrates an optical apparatus such as a repeater or a terminal for use in an optical communications system such as that of  FIG. 1 ; 
         FIG. 3  schematically illustrates an optical apparatus such as that of  FIG. 2 , except with an additional pump unit for use with additional Raman amplification; 
         FIG. 4  schematically illustrates an optical apparatus of  FIG. 3 , except with an additional pump unit that supplements or provide redundancy to the optical pump power provided by the second optical pump unit of  FIG. 3 ; 
         FIG. 5  schematically illustrates an optical apparatus of  FIG. 4 , except with additional optical power distribution components that provide additional shared path from the third optical pump unit of  FIG. 4 ; 
         FIG. 6  schematically illustrates an optical communications system that shows that there are multiple optical gain stages in a particular direction of optical communication; 
         FIG. 7  illustrates an example of positive and negative gain slope with respect to wavelength that may be associated with optical gain stages; 
         FIG. 8  illustrates an optical link that connects two nodes in an optical communication network such as that of  FIG. 1 , and that includes a forward optically pumped amplifier, and a backward optically pumped amplifier for each optical communication direction, and in which the optically pumped amplifiers are cross-coupled; and 
         FIG. 9  illustrates an example optical power profile of an eastern optical signal as it travels eastwardly in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with embodiments described herein, the output pump power from optical pump units is cross-distributed amongst multiple amplifier gain stages even in a single direction of an optical link in an optical communications system. For example, an optical pump unit may output optical pump power that is shared amongst a discrete optical amplification unit and a distributed optical amplification unit (such as a forward and/or backward Raman distributed amplification). Such sharing has the potential to increase reliability and/or efficiency of the optical communications system. 
       FIG. 1  schematically illustrates an example optical communications system  100  in which the principles described herein may be employed. In the optical communications system  100 , information is communicated between terminals  101  and  102  via the use of optical signals. For purposes of convention used within this application, optical signals travelling from the terminal  101  to terminal  102  will be referred to as being “eastern”, whereas optical signals traveling from the terminal  102  to the terminal  101  will be referred to as being “western”. The terms “eastern” and “western” are simply terms of art used to allow for easy distinction between the two optical signals traveling in opposite directions. The use of the terms “eastern” and “western” does not imply any actual geographical relation of components in  FIG. 1 , nor to any actual physical direction of optical signals. For instance, terminal  101  may be geographical located eastward of the terminal  102 , even though the convention used herein has “eastern” optical signals traveling from the terminal  101  to the terminal  102 . 
     In one embodiment, the optical signals are Wavelength Division Multiplexed (WDM) and potentially Dense Wavelength Division Multiplexed (DWDM). In WDM or DWDM, information is communicated over each of multiple distinct optical channels called hereinafter “wavelength division optical channels”. Each wavelength division optical channel is allocated a particular frequency for optical communication. Accordingly, in order to communicate using WDM or DWDM optical signals, the terminal  101  may have “n” optical transmitters  111  (including optical transmitters  111 ( 1 ) through  111 ( n ), where n is a positive integer), each optical transmitter for transmitting over a corresponding eastern wavelength division optical channel. Likewise, the terminal  102  may have “n” optical transmitters  121  including optical transmitters  121 ( 1 ) through  121 ( n ), each also for transmitting over a corresponding western wavelength division optical channel. The principles described herein are not limited, however, to communications in which the number of eastern wavelength division optical channels is the same as the number of western wavelength division optical channels. Furthermore, the principles described herein are not limited to the precise structure of the each of the optical transmitters. However, lasers are an appropriate optical transmitter for transmitting at a particular frequency. That said, the optical transmitters may each even be multiple laser transmitters, and may be tunable within a frequency range. 
     As for the eastern channel for optical transmission in the eastern direction, the terminal  101  multiplexes each of the eastern optical signals from the optical transmitters  111  into a single eastern optical signal using optical multiplexer  112 , which may then be optically amplified by an optional eastern optical amplifier  113  prior to being transmitted onto a first fiber link  114 ( 1 ). 
     There are a total of “m” repeaters  115  and “m+1” optical fiber links  114  between the terminals  101  and  102  in each of the eastern and western channels. However, there is no requirement for the number of repeaters in each of the eastern and western channels to be equal. In an unrepeatered optical communication system, “m” would be zero such that there is but a single fiber link  114 ( 1 ) and no repeaters between the terminals  101  and  102 . In a repeatered optical communication system, “m” would be one or greater. Each of the repeaters, if present, may consume electrical power to thereby amplify the optical signals. 
     The eastern optical signal from the final optical fiber link  114 ( m+ 1) is then optionally amplified at the terminal  102  by the optional optical amplifier  116 . The eastern optical signal is then demultiplexed into the various wavelength division optical channels using optical demultiplexer  117 . The various wavelength division optical channels may then be received and processed by corresponding optical receivers  118  including receivers  118 ( 1 ) through  118 ( n ). 
     As for the western channel for optical transmission in the western direction, the terminal  102  multiplexes each of the western optical signals from the optical transmitters  121  (including optical transmitters  121 ( 1 ) through  121 ( n )) into a single western optical signal using the optical multiplexer  122 . The multiplexed optical signal may then be optically amplified by an optional western optical amplifier  123  prior to being transmitted onto a first fiber link  124 ( m+ 1). If the western optical channel is symmetric with the eastern optical channel, there are once again “m” repeaters  125  (labeled  125 ( 1 ) through  125 ( m )), and “m+1” optical fiber links  124  (labeled  124 ( 1 ) through  124 ( m+ 1)). Recall that in an unrepeatered environment, “m” may be zero such that there is only one optical fiber link  124 ( 1 ) and no repeaters  125  in the western channel. 
     The western optical signal from the final optical fiber link  124 ( 1 ) is then optionally amplified at the terminal  101  by the optional optical amplifier  126 . The western optical signal is then demultiplexed using optical demultiplexer  127 , whereupon the individual wavelength division optical channels are received and processed by the receivers  128  (including receivers  128 ( 1 ) through  128 ( n )). Terminals  101  and/or  102  do not require all the elements shown in optical communication system  100 . For example, optical amplifiers  113 ,  116 ,  123 , and/or  126  might not be used in some configurations. Furthermore, if present, each of the corresponding optical amplifiers  113 ,  116 ,  123  and/or  126  may be a combination of multiple optical amplifiers if desired. 
     Often, the optical path length between repeaters is approximately the same. The distance between repeaters will depend on the total terminal-to-terminal optical path distance, the data rate, the quality of the optical fiber, the loss-characteristics of the fiber, the number of repeaters (if any), the amount of electrical power deliverable to each repeater (if there are repeaters), and so forth. However, a typical optical path length between repeaters (or from terminal to terminal in an unrepeatered system) for high-quality single mode fiber might be about 50 kilometers, and in practice may range from 30 kilometers or less to 90 kilometers or more. That said, the principles described herein are not limited to any particular optical path distances between repeaters, nor are they limited to repeater systems in which the optical path distances are the same from one repeatered segment to the next. 
     The optical communications system  100  is represented in simplified form for purpose of illustration and example only. The principles described herein may extend to much more complex optical communications systems. The principles described herein may apply to optical communications in which there are multiple fiber pairs, each for communicating multiplexed WDM optical signals. Furthermore, the principles described herein also apply to optical communications in which there are one or more branching nodes that split one or more fiber pairs and/or wavelength division optical channels in one direction, and one or more fiber pairs and/or wavelength division optical channels in another direction. 
       FIG. 2  illustrates an optical apparatus  200  that may be used in an optical communications system such as the optical communications system  100  of  FIG. 1 .  FIGS. 3 through 5  illustrated optical apparatuses  300 ,  400  and  500 , which in that order added successive components.  FIGS. 2 through 5  will now be described in that order. 
     If used in the optical communications system  100  of  FIG. 1 , the optical apparatus  200  (or any of apparatuses  300 ,  400  or  500 ) may be integrated with one of the eastern repeaters  115  or one of the western repeaters  125 . The optical apparatus  200  (or any of the apparatuses  300 ,  400  or  500 ) may also be integrated with one of the terminals  101  and  102 , regardless of whether the optical communications system  100  is a repeatered optical system or an unrepeatered optical system. The optical signal may be in an eastern optical signal, or a western optical signal, and may be traveling from left to right or from right to left in  FIG. 2 .  FIGS. 2 ,  3 ,  4 , and  5  may include additional components not shown in the figures that may, for example, provide optical isolation or, for example, provide optical depolarization. 
     For example, if the optical apparatus  200  is incorporated into terminal  101  of  FIG. 1 , the discrete amplifier  231  might be the discrete amplifier  113  of  FIG. 1  and the optical fiber span  241  may be the optical fiber span  114 ( 1 ) of  FIG. 1 . In that case, the optical signal would be an eastern optical signal traveling from left to right in  FIG. 2 , and the portion k 1  of the optical pump power would be used to perform forward Raman amplification of the eastern optical signal. Alternatively, the discrete amplifier  231  might be the discrete amplifier  126  of  FIG. 1  and the optical fiber span  241  may be the optical fiber span  124 ( 1 ) of  FIG. 1 . In that case, the optical signal would be a western optical signal traveling from right to left in  FIG. 2 , and the portion k 1  of the optical pump power would be used to perform backward Raman amplification of the western optical signal. 
     On the other hand, if the optical apparatus  200  is incorporated into terminal  102  of  FIG. 1 , the discrete amplifier  231  might be the discrete amplifier  116  of  FIG. 1  and the optical fiber span  241  may be the optical fiber span  114 ( m+ 1) of  FIG. 1 . In that case, the optical signal would be an eastern optical signal traveling from right to left in  FIG. 2  (which means that the optical apparatus  200  would be horizontally flipped in order to fit into the terminal  102  as shown in  FIG. 1 ). In addition, the portion k 1  of the optical pump power would be used to perform backward Raman amplification of the eastern optical signal. Alternatively, the discrete amplifier  231  might be the discrete amplifier  123  of  FIG. 1  and the optical fiber span  241  may be the optical fiber span  124 ( m+ 1) of  FIG. 1 . In that case, the optical signal would be a western optical signal traveling from left to right in  FIG. 2  (which once again means that the optical apparatus  200  would be horizontally flipped in order to fit into the terminal  102  as shown in  FIG. 1 ). In addition, the portion k 1  of the optical pump power would be used to perform forward Raman amplification of the western optical signal. 
     In addition, the optical apparatus  200  may be incorporated into any one of the eastern repeaters  115 ( k ), where k is equal to any integer from 1 to m. In that case, the eastern optical signal could be traveling from left to right in  FIG. 2 , in which case optical fiber span  241  would be optical fiber span  114 ( k+ 1) of  FIG. 1 , and the portion k 1  of the optical pump power would be used to perform forward Raman amplification of the eastern optical signal. Alternatively, if the optical signal is an eastern optical signal traveling from right to left in  FIG. 2  (which means that the optical apparatus  200  would be horizontally flipped to fit into  FIG. 1 ), if the optical apparatus was incorporated into eastern repeater  115 ( k ), the optical fiber span  241  would be the optical fiber span  114 ( k ) of  FIG. 1 , and the portion k 1  of the optical pump power would be used to perform backward Raman amplification of the eastern optical signal. 
     Finally, the optical apparatus  200  may be incorporated into any one of the western repeaters  125 ( k ), where k is equal to any integer from 1 to m. In that case, the western optical signal could be traveling from left to right in  FIG. 2  (which means that the optical apparatus  200  would be horizontally flipped to fit into  FIG. 1 ), in which case optical fiber span  241  would be optical fiber span  124 ( k ) of  FIG. 1 . In that case, the portion k 1  of the optical pump power would be used to perform forward Raman amplification of the western optical signal. Alternatively, if the optical signal is an western optical signal traveling from right to left in  FIG. 2 , if the optical apparatus was incorporated into western repeater  125 ( k ), the optical fiber span  241  would be the optical fiber span  124 ( k+ 1) of  FIG. 1 . In that case, the portion k 1  of the optical pump power would be used to perform backward Raman amplification of the western optical signal. 
     Referring first to  FIG. 2 , the optical apparatus  200  includes an optical pump unit  201  for providing optical pump power that optically powers optical amplifiers. An optical power distribution mechanism  211  (which may be, for example, an optical coupler) distributes a portion (1−k 1 ) (where k 1  is a fraction between 0 and 1, non-inclusive of 0 and 1) of the optical pump power to the discrete optical amplifier  231  (using, for example optical multiplexer  222 ), and a portion (k 1 ) of the optical pump power is injected into optical fiber span  241  (using, for example, optical multiplexer  221 ) for use in performing distributed optical amplification. Instead of, or in addition to, introducing optical pump power to the discrete optical amplifier  231  from the left through the optical multiplexer  222 , the optical pump power may be introduced to the discrete optical amplifier  231  from the right through the optical multiplexer  223 . For simplicity however, the optical multiplexer  223  will not be shown in subsequent drawings that build upon  FIG. 2 , although the optical multiplexer  223  may be used in addition to, or instead of, the optical multiplexer  222  in  FIGS. 3 through 5 . 
     If the optical power distribution mechanism  211  is an optical coupler as is illustrated in  FIGS. 2 through 5 , the optical pump power is distributed in a manner that preserves the frequency characteristics. As applied to the optical coupler  211 , this means that the frequency characteristics of the portion of the optical pump power allocated for the discrete optical amplification unit is approximately the same as the frequency characteristics of the portion of the optical pump power that is for use in the distributed optical amplification. 
     In one embodiment, the discrete optical amplification unit  231  is a rare-earth doped fiber amplifier such as an Erbium-Doped Fiber Amplifier (EDFA), a Semiconductor Optical Amplifier (SOA) or a high-efficiency Raman amplifier. Alternatively, or in addition, the portion k 1  of the optical pump power may be used to perform forward or backward Raman amplification. In that case, since Raman amplification typically takes more optical power than discrete optical amplification, k 1  may be closer to 1 than 0, allowing the majority of the optical power to be used for Raman amplification into the optical fiber span  241 . In one embodiment, in the case of the discrete optical amplifier being an Erbium-Doped Optical Amplifier, k 1  might be, for example, 89%, although the principles described herein are not limited to such a specific embodiment. It may be that if k 1  is higher or lower than the 89%, a more optimum performance might be achieved. If the discrete optical amplifier  231  is an Erbium-Doped Fiber Amplifier, then the optical pump power may be primarily in the range of 1400 nanometers to 1525 nanometers. Furthermore, in the case of Raman amplification being the distributed amplification mechanism, the optical signal should have a wavelength that is longer than at least the majority of the optical pump power. 
       FIG. 3  illustrates an optical apparatus  300  which is similar to the optical apparatus  200  of  FIG. 2 . However, in this case, an additional optical fiber span  341  is illustrated. If the optical signal travels from left to right in  FIG. 2 , the optical signal may travel from the optical fiber span  341 , through an optical mux/demux  322  through the other optical mux/demux  222  where it is combined with a portion of the optical pump power from the optical pump unit  201 . In that case, it is this combination that is then passed into the discrete optical amplifier  231 . The amplified signal is then passed through optical mux demux  221  and onto the optical fiber span  241 . Alternatively, in the case of an optical signal that travels from right to left in  FIG. 2 , the optical signal may travel from the optical fiber span  241 , through the optical mux/demux  221 , through the discrete optical amplifier  231 , through the optical mux demux  222 , through the optical mux/demux  322  and onto the optical fiber span  341 . In any case, the optical signal may include one or more wavelength modulated optical signal channel(s) in the L-band in addition to potentially one or more wavelength modulated optical signal channel(s) in the C-band. Note that the C-band corresponds to optical wavelengths ranging from 1530 nanometers (nm) to 1565 nm, while the L-band corresponds to optical wavelengths ranging from 1565 nm to 1625 nm. In one embodiment, all of the optical wavelengths are greater than 1550 nanometers, with perhaps one or more even exceeding 1567 nanometers. 
     In addition, the optical pump unit  301  provides optical pump power through the optical power distribution mechanism  311 , which may once again be an optical coupler. A portion k 2  (where k 2  is a fraction between, but not including, 0 and 1) of the optical power is provided to the optical mux/demux  322  and thereby propagated to the optical fiber  341  to perform distributed Raman amplification in the optical fiber link  341 . In the case of an optical signal travelling rightward in  FIG. 3 , this distributed Raman amplification would be backward Raman amplification. In the case of an optical signal traveling leftward in  FIG. 3 , this distributed Raman amplification would be forward Raman amplification. Another portion (1−k 2 ) of the optical power is provided to the optical coupler  211 , where it is distributed for use as both distributed Raman amplification pump power in the optical fiber link  241  and discrete optical amplifier pump power  231 . Since distributed Raman amplification typically requires more optical pump power than a discrete optical amplifiers (e.g., an EDFA), k 2  might be closer 1 than 0 (e.g., 95%). 
     In this case, the optical pump power from the optical pump unit  301  is thus used for at least three gain stages, the first Raman amplification gain stage (via the optical coupler  311  and optical multiplexer  322 ), the discrete optical amplification gain stage (via the optical coupler  311 , the optical coupler  211  and the optical multiplexer  222 ), and the second Raman amplification gain stage (via the optical coupler  311 , the optical coupler  211 , and the optical multiplexer  221 ), all in one signal direction. If one considers the possibility of there being an optically pumped amplifier (such as a Remote Optically Pumped Amplifier or “ROPA”) somewhere inline with the optical fiber span  341  or inline with the optical fiber span  241 , there may be four or five optical gain stages all served by optical pump power originating from the optical pump unit  301 . 
     Likewise, the optical pump unit  201  also serves multiple optical gain stages in one signal direction including 1) the discrete optical amplifier  231  (via the optical coupler  211  and the optical multiplexer  222 ), 2) the distributed Raman amplification (via the optical coupler  211  and the optical multiplexer  221 ), and 3) an optional ROPA located in the optical fiber span  241 . 
     The optical sharing of  FIG. 3  provides significant protection against failure. For instance, the optical pump units  201  and  301  may themselves be provided with redundancy. For instance, the optical pump unit  201  might comprises two optical pumps, one perhaps used as a backup, or with one increasing its output should the other optical pump fail. The same is true of the optical pump unit  301 . However, even if all the optical pumps in unit  201  were to fail, the discrete optical amplifier  231  would still be optically powered somewhat by the optical pump unit  301  through the optical couplers  311  and  211 . While the performance of the discrete optical amplifier might degrade in the case of a failure of optical pump  201  or  301 , the contribution of the other optical pump unit  301  or  201  may keep the optical communications system operational. 
       FIG. 4  illustrates an optical apparatus  400  that is similar to the optical apparatus  300  of  FIG. 3 . However, in this case, an additional optical pump unit  401  is provided. A portion k 3  (e.g., 50%) of this optical pump power is joined with the optical pump power from the optical pump unit  301  using the optical mux demux  412 , which may, for example, be a polarization beam combiner, or another optical component to combine two optical inputs into one optical output. Another portion (1−k 3 ) (e.g., 50%) of this optical pump power might be provided to a similarly configured optical pump unit in the opposite direction. This distribution is accomplished via optical coupler  411 . For instance, since the optical pump unit  301  primarily powers the distributed Raman amplification of the eastern optical signal in optical fiber link  341 , the portion (1−k 3 ) (e.g., 50%) of the optical pump power may be used to power the distributed Raman amplification of the western optical signal. In this case, the additional optical pump unit  401  provides additional protection for all optical gain stages in the case of the failure of either or both of optical pump units  201  and  301 . Once again, the optical pump units  201 ,  301  and  401  may provide optical pump power primarily in the same wavelength range (e.g., 1400 to 1525 nanometer, and in one example, 1480 nanometer). In one embodiment of optical apparatus  400 , the portion k 3  of pump unit  401  provides full redundancy for pump unit  301  such that a complete failure of pump unit  301  can be fully compensated for by the portion k 3  of pump unit  401 . In another embodiment of optical apparatus  400 , the portion k 3  of pump unit  401  provides partial redundancy for pump unit  301 . 
     Thus, the same wavelength of optical pump power may be used to power discrete optical amplifiers (such as Erbium-doped optical amplifiers, forward ROPA and/or backward ROPA) and distributed optical amplifiers (such as backwards and/or forwards Raman amplifiers). This commonality in optical pump wavelength permits sharing of optical pump power across multiple gain stages. This also allows for optical power for each gain stage to be obtained from a fewer number of pumps by performing cross-sharing of optical pump power. 
       FIG. 5  illustrates an optical apparatus  500  that is similar to the optical apparatus  400  of  FIG. 4 . However, in this case, two additional optical power distribution components  511  and  512  are provided which may, for example, be optical couplers. The optical coupler  511  draws some (1−k 4 ) of the optical pump power provided from the optical pump unit  401 , and the remainder k 4  is provided to the optical coupler  411 . The portion (1−k 4 ) of the optical pump power from the optical pump unit  401  is then provided to the optical coupler  512  where it may be split a portion k 5  for joining with the optical pump power provided by the optical pump  301  in optical coupler  311 , and a portion (1−k 5 ) for use in perhaps a similar structure in the other direction. In one embodiment, k 4  might be 90% whereas k 5  might be 50%. This has the effect of adding some additional pump power to the discrete optical amplifier  231  which reduces the impact of pump failures. This further revision provides additional protection against failure of either or both of optical pump units  201  and  301 . In the case where optical coupler  411  supplies a portion of the pump power from pump unit  401  to a pump mux/demux in the other signal direction that is similar or symmetric to pump mux demux  412  and where optical coupler  512  supplies a portion of the pump power from pump unit  401  to an optical coupler in the other signal direction that is similar or symmetric to optical coupler  311 , then pump unit  401  provides at least partial redundancy to all other pump units in both directions and therefore at least partially protects all the accessible gain stages in both directions from failures of pumps in other pump units. In one embodiment of optical apparatus  500 , k 1 =89%, k 2 =95%, k 3 =50%, k 4 =90%, and k 5 =50%, which results in an approximate pump sharing as follows (ignoring other transmission factors): 95% [k 2 ] of pump unit  301  is directed to fiber span  341 , 4.5% [(1−k 2 )k 1 ] of pump unit  301  is directed to discrete optical amplifier  231 , 0.6% [(1−k 2 )(1−k 1 )] of pump unit  301  is directed to optical fiber span  241 , 89% [k 1 ] of pump unit  201  is directed to optical fiber span  241 , 11% [(1−k 1 )] of pump unit  201  is directed to discrete optical amplifier  231 , 43% [k 4 k 3 k 2 +(1−k 4 )k 5 (1−k 2 )] of pump unit  401  is directed to optical fiber span  341 , 0.8% [k 4 k 3 (1−k 2 )(1−k 1 )+(1−k 4 )k 5 k 2 (1−k 1 )] of pump unit  401  is directed to optical fiber span  241 , and 6.2% [k 4 k 3 (1−k 2 )k 1 +(1−k 4 )k 5 k 2 k 1 ] of pump unit  401  is directed to discrete optical amplifier  231 . 
     The optical pump units of  FIGS. 2 through 5  may be any optical pump unit, whether presently existing, or whether to be developed in the future. As an example only, the optical pump units may be a pump that uses Fabry-Perot optical cavity with external wavelength-selective gratings. In an alternative embodiment, the optical units of  FIGS. 2 through 5  may be distributed feedback laser which can be more efficient and have lower relative intensity noise than Fabry-Perot based optical pumps. That said, there is no requirement that all of the optical pump units of  FIGS. 2 through 5  be of the same type. 
       FIG. 6  illustrates an optical communications system showing just one direction in an optical communication path  600 . In this case, the particular direction is eastward, but the principles also apply to the western direction. The eastern optical communication path  600  includes potentially up to five or more optically powered gain stages. These are illustrated as including optical amplifiers  601 ,  602 ,  603 ,  604  and  605 . An optical pump network  610  includes one or more pumps. The optical pump network  610  is symbolically illustrated as including two optical pumps  611  and  612 , although the ellipses  613  represents that there may be three or more, or perhaps just one pump unit in the optical pump network  610 . Each pump unit may be configured to optical power two, three, four, five, or even more optical gain stages in a signal optical communication direction. This is represented symbolically by arrows  621  through  625 . 
     In  FIG. 3 , for example, the pump network includes pumps  201  and  301 , where pump unit  201  optically powers up to three optical gain stages including 1) the discrete optical amplifier  231 , 2) the distributed Raman amplifier in fiber span  241 , and 3) the ROPA if present in fiber span  241 . The pump unit  301  in  FIG. 3  optically powers up to five optical gain stages including 1) the first distributed Raman amplifier in fiber span  241 , 2) the first ROPA if present in fiber span  241 , 3) the discrete optical amplifier  231 , 4) the second distributed Raman amplifier in fiber span  341 , and 5) the second ROPA if present in fiber span  341 . In addition, in the cross-coupled configuration that will be described with respect to  FIGS. 8 and 9 , the pump units  201  and  301  may each contribute to optical powering of gain stages in the opposite direction as well. 
     In  FIGS. 4 and 5 , for example, the pump network includes pumps  201 ,  301  and  401 . Pumps  201  and  301  optically pump up to three and five, respectively, optical gain stages as previously mentioned. However, in  FIGS. 4 and 5 , the optical pump  401  provides optical pump power also to all five optically pumped gain stages. 
     One or more of the optical gain stages  601  through  605  might optically amplify different wavelengths of light differently. For example, one or more of the optical gain stages optically amplify longer wavelengths of light with greater gain at least within the wavelength range of the optical signal (hereinafter referred to as “positive gain slope with respect to wavelength” optical gain stages or perhaps simply “positive gain slope” optical gain stages). Another one or more of the optical gain stages optically amplify longer wavelengths of light with lesser gain at least within the wavelength range of the optical signal (hereinafter referred to as “negative gain slope with respect to wavelength” optical gain stages or perhaps simply “negative gain slope” optical gain stages). 
     The pump network  610  may include a tilt control mechanism  615 , which measures whether the overall system has positive or negative wavelength dependency in the overall gain. In the case of there being an overall positive gain slope with respect to wavelength, the tilt control mechanism  615  may respond by increasing the optical power supplied to the negative gain slope optical gain stage(s) and/or decreasing the optical power supplied to the positive gain slope optical gain stage(s). In the case of there being an overall negative wavelength dependency in the gain, the tilt control mechanism  615  may respond by decreasing the optical power supplied to the negative gain slope optical gain stage(s) and/or increasing the optical power supplied to the positive gain slope optical gain stage(s). 
       FIG. 7  shows an example  700  of positive and negative wavelength dependency of optical gain stages when a single optical pump wavelength of 1480 nanometers is used to power the optical gain stages. Curve  701  shows a typical erbium gain profile when a 1480 nm optical pump is applied. Curve  702  shows a Raman gain profile when a 1480 nm optical pump is applied. In  FIG. 3 , for example, the discrete optical amplifier  231  may be erbium gain stage, and a Raman gain stage can be distributed Raman amplification in optical fiber  341 , or  241 . Even though the pump wavelength  301  and  201  are approximately same, wavelength dependency of the pump  301  and  201  can be controlled by adjusting the coupling ratio k 1  and k 2  and/or selecting fiber types of the span  341 ,  241 . For example, high coupling ratio k 2 &gt;0.9 and small core area fiber with higher Raman gain in optical fiber  341  create a positive wavelength dependency on pump  301 ; whereas large core area fiber with lower Raman gain in optical fiber  241  combined with gain from discrete optical amplifier  231  create a negative wavelength dependency on pump  201 . For example, suppose k 1  is selected to be 89%, k 2  is selected to be 95%, the signal direction is from left to right, the fiber span  341  is 40 km of IDF fiber (manufactured by OFS with 30 μm 2  effective area), the fiber span  241  is 40 km of SLA fiber (manufactured by OFS with 106 μm 2  effective area), the ROPA on fiber span  241  is 8 m of Erbium-doped fiber (R37014 manufactured by OFS) located at 40 km from pump mux  221 , the discrete optical amplifier  231  is Erbium-doped fiber (15 m R37003X or similar manufactured by OFS), the pump unit  301  is 0.25 W at 1480 nm, pump unit  201  is 0.25 W at 1480 nm. In that case, the optical gain of pump unit  301  will have a positive wavelength dependency when the pump power is increased and pump unit  201  will have a negative wavelength dependency when the pump power is increased (or the opposite wavelength dependencies on pump power decrease). In this example, the Raman gain is high on fiber span  341  due to the small effective area fiber and the Raman gain is low on fiber span  241  due to the large effective area fiber, whereas the Erbium-doped gain is high and primarily powered by pump unit  201 . 
     Accordingly, the principles described herein provide an effective mechanism for potentially improving the reliability and performance of an optical communications system.  FIGS. 8 and 9  are provided to illustrate a particular environment in which the principles described herein may operate, although the general principles described herein are not limited to that environment. The environment of  FIGS. 8 and 9  provides both forward and backward ROPAs, the forward ROPA for using residual forward Raman optical pump power, and the backward ROPA for using residual backward Raman optical pump power. 
       FIG. 8  illustrates an optical link  800  that connects two nodes  801  and  802  in an optical communication network. For instance, if the optical link  800  is used in the optical communications system  100  of  FIG. 1 , and the optical communications system  100  is an unrepeatered system, the node  801  may be the terminal  101  and the node  802  may be the terminal  102 . However, in a repeatered environment, nodes  801  and  802  may be a terminal on one end and a repeater set on another. For instance, node  801  might be terminal  101  of  FIG. 1 , whereas node  802  might be the repeater set  115 ( 1 ) and  125 ( 1 ) of  FIG. 1 . On the other hand, node  801  might be the repeater set  115 ( m ) and  125 ( m ) of  FIG. 1 , whereas node  802  might be the terminal  102  of  FIG. 1 . If there is more than one repeater (i.e., m&gt;1) in the repeatered environment, it is possible that both nodes  801  and  802  may both be repeater sets. Generally stated, if nodes  801  and  802  are both repeater sets, node  801  may be repeater set  115 ( k ) and  125 ( k ) of  FIG. 1 , whereas node  802  may be repeater set  115 ( k+ 1) and  125 ( k+ 1) of  FIG. 1 , where k is any positive integers from 1 to a maximum of m−1. 
     For instance, nodes  801  and  802  may comprise the optical apparatus  200 ,  300 ,  400 , or  500  of  FIG. 2 ,  3 ,  4 , or  5 , respectively. For example, pump unit  811 A, pump mux  815 A, and fiber span  812 A of  FIG. 8  may correspond to pump unit  201 , pump mux  221 , and fiber span  241  of  FIG. 5 , respectively; pump unit  811 B, pump mux  815 B, discrete amplifier  816 , and fiber span  812 C of  FIG. 8  may correspond to pump unit  301 , pump mux  322 , discrete amplifier  231 , and fiber span  341  of  FIG. 5 , respectively; forward OPA  813 A of  FIG. 8  may correspond to a ROPA in fiber span  241  of  FIG. 5 ; backward OPA  813 B of  FIG. 8  may correspond to a ROPA in fiber span  341  of  FIG. 5 . Nodes  801  and  802  may comprise the pump distribution components  211 ,  311 ,  411 ,  412 ,  511 , and/or  512  of  FIG. 2 ,  3 ,  4 , or  5  although this is not explicitly drawn in  FIG. 8 . Similar correspondence may apply to nodes  801  and  802  for the components in the path of the western optical signal of link  800 . It is understood that the principles described in  FIGS. 2 ,  3 ,  4 , and  5  may apply to optical link  800 . It would be apparent to one of ordinary skill in the art after having read this description that the principles described in  FIGS. 2 ,  3 ,  4 , and  5  may apply to optical link  800 . 
     The optical link  800  is bidirectional and includes an eastern fiber link and a western fiber link. The eastern fiber link propagates the eastern optical signal from the node  801  to the node  802 . The western fiber link propagates the western optical signal from the node  802  to the node  801 . Recall, however, that the terms “eastern” and “western” are used herein merely to distinguish one signal from another and not to represent any sort of actual geographical relation or direction. Components or gain stages within the eastern fiber link will also be sometimes modified herein by the term “eastern”, and components or gain stages within the western fiber link will also be sometimes modified herein by the term “western”. 
     The eastern fiber link transmits the eastern optical signal through the initial eastern optical fiber span  812 A, through the eastern forward Optically Pumped Amplifier (OPA)  813 A, through a first eastern optical multiplexer/demultiplexer (hereinafter, “mux/demux”)  814 A, through the eastern intermediate optical fiber span  812 B, through a second eastern optical mux demux  814 B, through the backward OPA  813 B, and through the final eastern optical fiber span  812 C to the node  802 . In so doing, the optical signal may go through a number of gain stages for each direction. For example, the eastern optical signal may potentially pass through forward Raman amplification gain stage  812 A, forward OPA  813 A, backward OPA  813 B, backward Raman amplification gain stage  812 C, and discrete gain stage  816  in node  802 . 
     Note that the term “forward” and “backward” OPA refers to the direction of the optical pump relative to the signal direction, whereby the optical pump of the “forward” OPA is in the same direction as the signal and the optical pump of the “backward” OPA is in the opposite direction as the signal. 
     As a potential first gain stage for the eastern optical link, the optical fiber span  812 A may serve as a distributed forward Raman amplifier, being powered by the optical pump unit  811 A. The eastern optical signal transmitted from node  801  to node  802  represents the actual information communicated eastward. The pump unit  811 A, on the other hand, transmits optical pump power that has a higher frequency (shorter wavelength) that is outside of the optical signal band. That energy is converted to the signal wavelength(s) to optically amplify the optical signal. The pump unit  811 A provides forward Raman pump power into the optical fiber span  812 A using optical mux demux  815 A to thereby co-propagate with and amplify the optical signal in a distributed manner along the optical fiber span  812 A. 
       FIG. 9  illustrates an example power-distance optical profile diagram  900  showing examples of optical signal power as the optical signal travels through the eastern fiber link of  FIG. 8  in the case where all illustrated gain stages are present. An example power-distance is not shown for the western optical link, although the power-distance profile may be similar, but reversed. That said, there is no requirement for symmetry in optical power profiles in the eastern and western optical fiber links. In  FIG. 9 , a maximum power for the optical fiber link is illustrated as P H , whereas the minimum is illustrated as P L . Positions D 0  and D 3  represent the positions of the node  801  and the node  802 , respectively. Positions D 1  and D 2  represent the positions of the forward OPA  813 A and backward OPA  813 B, respectively. 
     In the first gain stage that occurs between distance D 0  and D 1  in the optical fiber span  812 A, the forward Raman amplification initially slows the attenuation of the optical signal, but as the forward Raman amplification diminishes further from distance D 0 , the approximate logarithmically linear attenuation of the optical fiber begins to dominate. That said, however, even when the optical fiber attenuation dominates, the forward Raman amplification is still sufficient to mitigate the optical fiber attenuation as compared to the attenuation that would occur without forward Raman amplification. In one embodiment, the forward Raman amplification has an on/off gain of at least 1 dB over the distance from D 0  to D 1 , but could be much higher. In this description and in the claims, the “on/off” gain of Raman amplification over a distance refers to the increase in optical signal power of at least one of the one or more signal wavelengths caused by the Raman amplification over that distance as compared to the signal power that would occur without Raman amplification over that distance.  FIG. 9  is not necessarily drawn to scale, and is not necessarily intended to convey an actual optical power-distance profile, but is merely used to describe the power profile from a general perspective. 
     Returning to  FIG. 8 , as a second gain stage, the residual forward Raman optical pump power is then used to power the forward OPA  813 A, which then amplifies the eastern optical signal. Although the forward OPA  813 A is shown as a discrete amplifier, it may be distributed over all or part of fiber span  812 A. The OPAs  813 A,  813 B,  823 A and  823 B illustrated in  FIG. 8  may be what is more commonly referred to as “Remote Optically Pumped Amplifiers” or (ROPAs). However, the term “remote” is not desired for this patent application since the term is relative. In one embodiment, however, the OPAs are at least 30 kilometers in optical path distance from the nearest repeater or terminal, and the optical path distance between nodes  801  and  802  is at least 100 kilometers, but may even be greater than 300 kilometers, perhaps even surpassing 500 kilometers. Referring to  FIG. 9 , the discrete amplification at distance D 1  is a result of the forward OPA  813 A. 
     The OPAs  813 A,  813 B,  823 A and  823 B may each be any optically pumped amplifier. Examples include rare-earth doped fiber amplifiers (such as Erbium-doped fiber amplifiers), optically-pumped semiconductor amplifiers, or perhaps highly efficient Raman amplifiers. 
     Note that in the optical link  800 , there is a forward OPA as well as a backward OPA in each direction. For instance, for the eastern channel, the forward OPA  813 A is more proximate the node  801 , and the backward OPA  813 B is more proximate the node  802 . This allows for more efficient use of the residual forward and backward Raman optical pump power to power the OPAs, and itself represents a significant advancement in the art permitting the distance between nodes  801  and  802  to be extended, all other things being equal. The western channel also has a forward OPA  823 B that is more proximate the node  802  and the backward OPA  823 A that is more proximate the node  801 , resulting in potential efficiency improvement for the western optical channel as well. 
     Returning to the eastern optical fiber link, there is still some residual forward optical pump power remaining even after the forward Raman amplification that occurred in the optical fiber span  812 A, and even after the amplification by the forward OPA  813 A. At least some, and potentially all, of that residual forward optical pump power is diverted to the opposite optical fiber link for use in the backward OPA  823 A. This general diversion of this forward Raman optical pump power is represented generally by the arrow  817 A. The resulting amplification in the backward OPA  823 A may be significantly more than the forward Raman amplification that may have occurred in the eastern intermediate optical fiber span  812 B had the residual forward pump optical power been allowed to continue further in the eastern optical fiber link into the intermediate optical fiber  812 B. 
     To facilitate this diversion, an optical mux/demux  814 A is placed east of the forward OPA  813 A. This optical mux/demux  814 A permits the eastern optical signal (or at least a majority of that signal) to pass through into the intermediate optical fiber span  812 B, but diverts optical pump power towards another optical mux/demux  824 A in the western optical fiber link. The optical mux demux  824 A then injects this residual optical pump power into the backward OPA  823 A for help in powering the backward OPA  823 A. On the other hand, amplification of the forward OPA  813 A may also be assisted by the diversion of residual backward Raman pump optical power from the western optical fiber link. This is represented generally by the arrow  827 B. However, more regarding this diversion will be described further below. 
     Returning to the eastern channel, the eastern optical signal passes into the intermediate optical fiber span  812 B, where it does not experience much, if any, amplification at all. Instead, referring to  FIG. 9 , the optical power attenuates approximately logarithmically linearly in the distanced between D 1  and D 2 , which corresponds to the length and attenuation of the optical fiber span  812 B. 
     As a third optical gain stage, the optical signal passes through the second eastern mux demux  814 B and then is amplified by the backward OPA  813 B. Although the backward OPA  813 B is shown as a discrete amplifier, it may be distributed over all or part of fiber span  812 C. Part of the optical pump power used to supply the backward OPA  813 B is due to a residual amount of backward Raman pump optical power from the pump unit  811 B. A remaining amount is due to diversion of forward Raman pump optical power from the opposite optical fiber link as represented by the arrow  827 A. If the forward Raman pumping of the western optical link is not efficient, then there might be a significant amount of forward optical pump power remaining to be diverted into the eastern optical link. 
     In one embodiment, the backward Raman amplification performed in the optical fiber span  812 C for the eastern signal (and in optical fiber span  822 A for the western signal) is quite efficient allowing strong distributed gain in the optical fiber span  812 C compared to forward Raman amplification of eastern signal in optical fiber span  812 A (and western signal in optical fiber span  822 C). This high gain means, however, that there is relatively little residual optical pump power remaining to power the backward OPA  813 B. Accordingly, the diverted forward Raman pump optical power  827 A from the western optical link (and  817 A from the eastern optical link) helps a great deal when used to optically power the backward OPA  813 B of the eastern optical fiber link (and backward OPA  822 A of the western optical fiber link). In one embodiment, the optical fiber spans  812 C and  822 A are primarily negative chromatic dispersion (D−) fiber, or at least have a relatively smaller effective cross-sectional area for propagation of light. In this description and in the claims, a “DS fiber” is defined as a fiber that has an effective cross-sectional area of less than 65 μm^2. Thus, the optical fiber spans  812 C and  822 A may be comprised of DS fiber. The optical fiber spans  812 A and  822 C, on the other hand, may be positive chromatic dispersion (D+) fiber, or at least have a relatively larger effective cross-sectional area as compared to the optical fiber spans  812 C and  822 A. In this description and in the claims, a “DL fiber” is defined as a fiber that has an effective cross-sectional area of greater than 65 μm^2. Thus, the optical fiber spans  812 A and  822 C may be comprised of DL fiber. In this case, the backward OPA  813 B is helped greatly by the diverted optical pump power from the opposite optical link represented by arrow  827 A. Generally, signal power at the backward OPA  813 B is less than at the forward OPA  813 A due to uncompensated fiber attenuation in span  812 B. Therefore, more amplification can typically be achieved in the backward OPA  813 B compared to the forward OPA  813 A given the same OPA and same amount of pump power. In other words, higher pump power is typically required in forward OPA  813 A to achieve similar gain compared to backward OPA  813 B. 
     As the fourth optical gain stage, and as alluded to already, the pump unit  811 B provides backward Raman pump optical power to thereby perform backward Raman amplification in the optical fiber  812 C. Referring to  FIG. 9 , this results in distributed backward Raman amplification occurring between distances D 2  and D 3 .  FIG. 9  demonstrates one embodiment of a power-distance profile  900  in which the distributed gain between distances D 2  and D 3  is much larger than the distributed gain between D 0  and D 1  due to the use of D− and D+ fiber as described above. In one embodiment, the backward Raman amplification has an on/off gain of at least 5 dB over the distance from D 2  to D 3 , but could be much higher. The backward Raman pump power of pump unit  811 B is injected into the optical fiber span  812 C using the optical mux demux unit  815 B. Following along arrow  817 B, the backward Raman pump optical power is degraded, however, upon performing backward Raman amplification in the optical fiber span  812 C. As previously mentioned, the residual backwards Raman pump optical power is then used to power the backward OPA  813 B. A residual amount remaining after the backward OPA  813 B is then diverted using optical mux/demux  814 B into the western optical fiber link using optical mux/demux  824 B for use in optically powering the forward OPA  823 B in the western optical fiber link. 
     In node  802 , discrete amplifier  816  provides the fifth optical gain stage. For example, discrete amplifier  816  may amplify the optical signal to the next transmission optical fiber (if it is used in a repeater) or to the receiver (if it is located in a terminal). Referring to  FIG. 9 , this discrete amplification may occur at distance D 3 , corresponding to node  802 . If the node  802  is a terminal, the eastern optical signal may then be directed to the terminal receivers such as, for example, receivers  118  of  FIG. 1 . If the node  802  is a repeater, the eastern optical signal may then be transmitted (perhaps after other processing such as, for example, chromatic dispersion compensation, and gain-flattening filtering) to yet other nodes in the optical communication system. Although not shown, there may be optical isolators keeping west bound optical signals from entering or exiting the eastern optical fiber link. 
     As for the western optical link, there may once again be five gain stages. The first potential gain stage is the optical fiber span  822 C which serves as a distributed forward Raman amplifier, being powered by the optical pump unit  821 B. The western optical signal transmitted from node  802  to node  801  represents the actual information communicated westward. The pump unit  821 B, on the other hand, transmits optical pump power that has a higher frequency (shorter wavelength) that is outside of the optical signal band. That energy is converted to the signal wavelength(s) to optically amplify the optical signal. The pump unit  821 B provides that forward Raman pump power into the optical fiber span  822 C using the optical mux/demux  825 B to thereby co-propagate with and amplify the optical signal in a distributed manner along the optical fiber span  822 C. 
     As a second gain stage, the residual forward Raman optical pump power is then used to power the forward OPA  823 B, which then discretely amplifies the western optical signal. 
     In the western optical fiber link, there is still some residual forward optical pump power remaining even after the forward Raman amplification that occurred in the optical fiber span  822 C, and even after the amplification by the forward OPA  823 B. At least some, and potentially all, of that residual forward optical pump power is diverted to the opposite optical fiber link for use in the backward OPA  813 B, as previously mentioned. This general diversion of this forward Raman optical pump power is represented generally by the arrow  827 A. The resulting amplification in the backward OPA  813 B may be significantly more than the forward Raman amplification that may have occurred in the western intermediate optical fiber span  822 B had the residual forward pump optical power been allowed to continue further in the western optical fiber link into the intermediate optical fiber  822 B. 
     To facilitate this diversion, an optical mux demux  824 B is placed west of the forward OPA  823 B. This optical mux demux  824 B permits the western optical signal (or at least a majority of that signal) to pass through into the intermediate optical fiber span  822 B, but diverts optical pump power towards another optical mux/demux  814 B in the eastern optical fiber link. The optical mux demux  814 B then injects this residual optical pump power into the backward OPA  813 B for help in powering the backward OPA  813 B. On the other hand, amplification of the forward OPA  823 B may also be assisted by the diversion of residual backward Raman pump optical power from the eastern optical fiber link, as previously described, and as represented by the arrow  817 B. 
     The western optical signal passes into the intermediate optical fiber span  822 B, where it does not experience much amplification at all. Instead, optical power attenuates approximately logarithmically linearly as optical signals are known to do as they pass through optical fiber without amplification. 
     As a third optical gain stage, the western optical signal passes through the western mux demux  824 A and then is discretely amplified by the backward OPA  823 A. Part of the optical pump power used to supply the backward OPA  823 A is due to a residual amount of backward Raman pump optical power from the pump unit  821 A. A remaining amount is due to diversion of forward Raman pump optical power from the eastern optical fiber link as represented by the arrow  817 A. 
     As the fourth optical gain stage, and as alluded to already, the pump unit  821 A provides backward Raman pump optical power to thereby perform backward Raman amplification in the optical fiber  822 A. The backward Raman pump optical power is injected into the optical fiber span  822 A using the optical mux/demux unit  825 A. Following along arrow  827 B, the backward Raman pump optical power is degraded, however, upon performing backward Raman amplification in the optical fiber span  822 A. As previously mentioned, the residual backwards Raman pump optical power is then used to power the backward OPA  823 A. A residual amount remaining after the backward OPA  823 A is then diverted using optical mux demux  824 A into the eastern optical fiber link using optical mux/demux  814 A for use in optically powering the forward OPA  813 A in the eastern optical fiber link. 
     In node  801 , the fifth gain stage may be the discrete amplifier  826 , which amplifies the optical signal to the next transmission optical fiber or to the receivers if the node  801  is located in terminal. If the node  801  is a terminal, the western optical signal may then be directed to the terminal receivers such as, for example, receivers  128  of  FIG. 1 . The discrete amplifiers  816  and  826  may be any amplifier that is capable of amplifying light, whether powered by electricity or optical power. Examples include rare-earth doped fiber amplifiers (such as Erbium-doped fiber amplifiers), high efficiency Raman amplifiers, and/or a Semiconductor Optical Amplifier (SOA). 
     If the node  801  is a repeater, the western optical signal may then be transmitted (perhaps after other processing such as, for example, chromatic dispersion compensation, and gain-flatten filtering) to yet other nodes in the optical communication system. Although not shown, there may be optical isolators keeping east bound optical signals from entering or exiting the western optical fiber link. 
     Accordingly, in  FIG. 8 , there are four examples of cross fiber optical power diversion as follows:
         A) diversion of forward Raman pump power from the eastern optical fiber link to supplement the optical powering of the backward OPA in the western optical fiber link (hereinafter referred to as “diversion type A”) which is represented in  FIG. 8  by arrow  817 A;   B) diversion of forward Raman pump power from the western optical fiber link to supplement the optical powering of the backward OPA in the eastern optical fiber link (hereinafter referred to as “diversion type B”) which is represented in  FIG. 8  by arrow  827 A;   C) diversion of backward Raman pump power from the eastern optical fiber link to supplement the optical powering of the forward OPA in the western optical fiber link (hereinafter referred to as “diversion type C”) which is represented in  FIG. 8  by arrow  817 B; and   D) diversion of backward Raman pump power from the western optical fiber link to supplement the optical powering of the forward OPA in the eastern optical fiber link (hereinafter referred to as “diversion type D”) which is represented in  FIG. 8  by arrow  827 B.       

     One embodiment of diversion type A, as depicted in  FIG. 8 , comprises both a forward OPA  813 A and a backward OPA  823 A. In another embodiment of diversion type A, only one OPA (either  813 A or  823 A) is employed. One embodiment of diversion type B, as depicted in  FIG. 8 , comprises both a forward OPA  823 B and a backward OPA  813 B. In another embodiment of diversion type B, only one OPA (either  823 B or  813 B) is employed. One embodiment of diversion type C, as depicted in  FIG. 8 , comprises both a forward OPA  823 B and a backward OPA  813 B. In another embodiment of diversion type C, only one OPA (either  823 B or  813 B) is employed. One embodiment of diversion type D, as depicted in  FIG. 8 , comprises both a forward OPA  813 A and a backward OPA  823 A. In another embodiment of diversion type D, only one OPA (either  813 A or  823 A) is employed. 
     In  FIG. 8 , all of the diversion types A, B, C and D are shown. However, the principles described herein may apply if there are fewer than all of these diversion types present as well. For instance, the principles described herein may provide benefits even if just one, two or three of the diversion types A, B, C and D are provided. 
     Referring to  FIG. 8 , the OPAs  813 A and  823 A, and the optical mux/dumuxes  814 A and  814 B may be encompassed within a single assembly  818 A. In that case, the assembly  818 A might be pre-manufactured and may be, for example, a splice box. The box would have at least four ports for each fiber pair; namely, an eastern fiber input terminal (e.g., proximate the forward OPA  813 A), an eastern fiber output terminal (e.g., proximate the optical mux demux  814 A), a western fiber input terminal (e.g., proximate the optical mux/demux  824 A), and a western fiber output terminal (e.g., proximate the backward OPA  823 A). The assembly  818 A has an eastern optical channel and a western optical channel. The eastern optical channel is between the eastern input and output terminals that includes the forward OPA  813 A and the optical mux/demux  814 A. The western optical channel is between the western input and output terminals that includes the optical mux demux  824 A and the backward OPA  823 A. 
     The assembly  818 B also includes a forward OPA  823 B, a backward OPA  813 B, and two optical mux/demuxes  824 B and  814 B, and may be similarly configured as described for the assembly  818 A. However, the assembly  818 A may be simplified in the case where not all of the diversion types A and D are employed. For example, if only diversion type A is employed represented by arrow  817 A, the backward OPA  823 A may be placed to the east of or to the west of the optical multiplexer  824 A. Furthermore, forward OPA  813 A might not be present all. If only diversion type D is employed represented by arrow  827 B, the forward OPA  813 A may be placed to the east of or to the west of the optical multiplexer  814 A. Furthermore, backward OPA  823 A might not be present all. Assembly  818 B may have similar simplifications in the case of there only being one or diversion types B and C. 
     Accordingly, through the use of a dedicated forward and backward OPA for each eastern and western optical fiber link, and through pump optical power coupling between eastern and western optical fiber links, optical power is more efficiently used to perform amplification. Furthermore, pump sharing allows a single pump unit to not only power multiple optical gain stages in one signal direction, but also one or more optical gain stages in the opposite signal direction as well. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.