Abstract:
The alteration of the bandwidth of an optical amplifier. Before alteration, optical signals having a first set of wavelengths are provided through a gain medium of the optical amplifier. In addition, a first pump having a set of pump wavelengths is propagated through the gain medium to thereby amplify the optical signals. After alteration, optical signals having at least a partially different set of wavelengths are able to be optically amplified by coupling a second pump into the optical medium. The second pump is at least partially distinct from the first pump in that the second pump includes at least one pump wavelength that was not included in the first pump.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 61/019,577, entitled “SYSTEM AND METHOD FOR EXPANDING THE BANDWIDTH OF AN OPTICAL AMPLIFIER”, filed Jan. 7, 2008, by DO-IL Chang et al. This application also claims benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 61/019,574, entitled “EFFICIENT DISCRETE AMPLIFICATION”, filed Jan. 7, 2008, by DO-IL Chang et al. This application claims benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 61/019,467, entitled “OPTICAL AMPLIFIER CAPABLE OF AMPLIFYING OPTICAL SIGNALS THAT TRAVERSE SEPARATE TRANSMISSION FIBERS”, filed Jan. 7, 2008, by Wayne S. Pelouch et al. 
     
    
     BACKGROUND 
       [0002]    Conventional optical communication systems typically implement one or more discrete in-line amplifiers to amplify an optical communication signal as it traverses a communication span or transmission fiber. Due to the relatively high cost of an optical amplifier, the expense of optical communication systems can be a barrier to entry into the communications market. In some cases, a company may desire installation of a system having only a relatively limited bandwidth that is sufficient to satisfy the present needs of a company at lower cost. The downside to this approach is that, in conventional systems, when the company desires to expand its bandwidth to handle additional traffic, the expansion can be expensive. This expense may arise because bandwidth expansion typically requires replacement of the optical amplifiers or additional optical amplifiers to support the increased bandwidth. 
       BRIEF SUMMARY 
       [0003]    Embodiments described herein relate to the alteration of the bandwidth of an optical amplifier. Before alteration, optical signals having a first set of wavelengths are provided through a gain medium of the optical amplifier. In addition, a first pump having a set of pump wavelengths is propagated through the gain medium to thereby amplify the optical signals. After alteration, optical signals having at least a partially different set of wavelengths are able to be optically amplified by coupling a second pump into the optical medium. The second pump is at least partially distinct from the first pump in that the second pump includes at least one pump wavelength that was not included in the first pump. In one embodiment, the alteration of bandwidth is an expansion of bandwidth, and the first and second pumps are used after expansion to amplify additional wavelength channels of the optical signal. Although not required, the optical amplification may be Raman amplification. 
         [0004]    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 
         [0005]    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: 
           [0006]      FIG. 1  illustrates a block diagram showing at least a portion of an optical communication system operable to facilitate communication of one or more multiple wavelength signals; 
           [0007]      FIG. 2  is an optical pump and signal wavelength schema for an optical amplifier whose amplification bandwidth has been expanded; 
           [0008]      FIG. 3  is an optical pump and signal wavelength schema for an optical amplifier whose amplification bandwidth has been expanded; 
           [0009]      FIG. 4  illustrates a block diagram showing at least a portion of an optical amplifier capable of having its bandwidth expanded; 
           [0010]      FIGS. 5-7  illustrate embodiments of components and/or functions depicted in  FIG. 4 ; 
           [0011]      FIG. 8   a  illustrates a block diagram of one example of a modular optical amplifier capable of having its bandwidth expanded; 
           [0012]      FIG. 8   b  shows example gain and noise figure characteristics of bandwidth expansion of the example amplifier of  FIG. 8   a  including a transmission fiber; 
           [0013]      FIG. 9  is a block diagram of one example of a modular optical amplifier capable of having its bandwidth expanded; 
           [0014]      FIG. 10  is a block diagram of one example of a modular optical amplifier capable of having its bandwidth expanded; 
           [0015]      FIG. 11   a  is a block diagram of one example of a discrete modular optical amplifier capable of having its bandwidth expanded; and 
           [0016]      FIG. 11   b  shows example gain and noise figure characteristics of bandwidth expansion of the example amplifier of  FIG. 11   a  including a transmission fiber. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Particular examples and values (such as dimensions and wavelengths) specified throughout this document are for illustrative purposes only, and are not intended to limit the scope of the present disclosure. In particular, this disclosure is not limited to any particular type of optical communication system. The teachings of the present disclosure may be used in any optical communication system where it is desired to expand the bandwidth of an existing amplification scheme. Moreover, the illustrations in  FIGS. 1 through 11   b  are not intended to be to scale. 
         [0018]      FIG. 1  is a block diagram showing at least a portion of an example optical communication system  10  operable to facilitate communication of one or more multiple wavelength signals  16 . In some embodiments, system  10  may comprise the entire optical communication system. In other embodiments, system  10  may comprise a portion of a larger optical communication system. 
         [0019]    In this example, system  10  includes a plurality of transmitters  12   a - 12   n  operable to generate a plurality of optical signals (or channels)  15   a - 15   n , each comprising a center wavelength of light. In some embodiments, each optical channel  15  comprises a center wavelength that is substantially different from the center wavelengths of other channels  15 . As used throughout this document, the term “center wavelength” refers to a time-averaged mean of the spectral distribution of an optical signal. The spectrum surrounding the center wavelength need not be symmetric about the center wavelength. Moreover, there is no requirement that the center wavelength represent a carrier wavelength. Transmitters  12  can comprise any device capable of generating one or more optical channels. Transmitters  12  can comprise externally modulated light sources, or can comprise directly modulated light sources. 
         [0020]    In one embodiment, transmitters  12  comprise one or a plurality of independent light sources each having an associated modulator, with each source being operable to generate one or more optical channels  15 . Alternatively, transmitters  12  could comprise one or more light sources shared by a plurality of modulators. For example, transmitters  12  could comprise a continuum source transmitter operable to generate a multitude of optical signals. In that embodiment, a signal splitter receives the continuum and separates the continuum into individual channels each having a center wavelength. In some embodiments, transmitters  12  can also include a pulse rate multiplexer, such as a time division multiplexer, operable to multiplex pulses received from a mode locked source or a modulator to increase the bit rate of the system. 
         [0021]    Transmitters  12 , in some cases, may comprise a portion of an optical regenerator. That is, transmitters  12  may generate optical channels  15  based on electrical representations of electrical or optical channels received from other optical communication links. In other cases, transmitters  12  may generate optical channels  15  based on information received from sources residing locally to transmitters  12 . Transmitters  12  could also comprise a portion of a transponder assembly (not explicitly shown), containing a plurality of transmitters and a plurality of receivers. 
         [0022]    In various embodiments, transmitters  12  may include a forward error correction (FEC) encoder/decoder module capable improving the Q-factor of channels  15  and the bit-error rate of system  10 . For example, the FEC module may encode an FEC sequence, such as, Reed Solomon coding, Turbo Product Codes coding, Concatenated Reed-Solomon coding, or other algorithms capable of improving the Q-factor of channels  15  and the bit error rate of system  10 . As used throughout this document, the term “Q-factor” refers to a metric for determining the quality of the signal communicated from a transmitter. The “Q-factor” associated with optical channels  15  communicated from transmitters  12  refers to the difference of the mean value of the high signal values (M H ) and the mean value of the low signal values (M L ) associated with an optical signal over the sum of the standard deviation of the multiple highs (Δ H ) and the multiple lows Δ L ). The value of the Q-factor can be expressed in dB 20 . In equation form, this relationship is expressed as: 
         [0000]        Q=[M   H   −M   L ]÷[Δ H +Δ L ] 
         [0023]    In some cases, multiple wavelength signals  16  can carry wavelength signals  15   a - 15   n  ranging across a relatively wide bandwidth. In some implementations, wavelength signals  15   a - 15   n  may even range across different communications bands (e.g., the short band (S-band), the conventional band (C-band), and/or the long band (L-band)). 
         [0024]    In the illustrated embodiment, system  10  also includes combiners  14  operable to receive optical channels  15   a - 15   n , and to combine those signals into multiple wavelength channels  16 . As one particular example, combiners  14  could comprise a wavelength division multiplexer (WDM). The terms wavelength division multiplexer and wavelength division demultiplexer as used herein may include equipment operable to process wavelength division multiplexed signals and/or equipment operable to process dense wavelength division multiplexed signals. 
         [0025]    System  10  communicates multiple wavelength signal  16  over optical communication spans  20   a - 20   n . Communication span  20  can comprise, for example, standard single mode fiber (SMF), dispersion shifted fiber (DSF), non-zero dispersion shifted fiber (NZDSF), dispersion compensating fiber (DCF), pure-silica core fiber (PSCF), or another fiber type or combination of fiber types. In various embodiments, span  20   a - 20   n  can comprise any span length. In some embodiments, communication span  20  could comprise, for example, a unidirectional span or a bi-directional span. Span  20  could comprise a point-to-point communication link, or could comprise a portion of a larger communication network, such as a ring network, a mesh network, a star network, or any other network configuration. For example, communication span  20  could comprise one span or link of a multiple link system, where each link couples to other links through, for example, optical regenerators or wavelength selective switches. A link refers to a group of one or more spans with optical communication between two points through the spans. 
         [0026]    One or more spans of communication medium  20  can collectively form an optical link. In the illustrated example, communication media  20  includes a single optical link  25 , respectively, comprising numerous spans  20   a - 20   n . System  10  could include any number of additional links coupled to links  25 . For example, optical link  25  could comprise one optical link of a multiple link system, where each link is coupled to other links through, for example, optical regenerators or wavelength selective switches. 
         [0027]    Optical link  25  could comprise point-to-point communication links, or could comprise a portion of a larger communication network, such as a ring network, a mesh network, a star network, or any other network configuration. 
         [0028]    System  10  may further include one or more access elements  27 . For example, access elements  27  could comprise an add/drop multiplexer, a cross connect, or another device operable to terminate, cross connect, switch, route, process, and/or provide access to and from optical link  25  and another optical link or communication device. System  10  may also include one or more lossy elements (not explicitly shown) and/or gain elements capable of at least partially compensating for the lossy element coupled between spans  20  of link  25 . For example, the lossy element could comprise a signal separator, a signal combiner, an isolator, a dispersion compensating element, a circulator, or a gain equalizer. 
         [0029]    In this embodiment, separators  26  separates individual optical signals  15   a - 15   n  from multiple wavelength signals  16  received at the end of link  25 . Separator  26  may comprise, for example, a wavelength division demultiplexer (WDM). Separator  26  communicates individual signal wavelengths or ranges of wavelengths to a bank of receivers  28  and/or other optical communication paths. One or more of receivers  28  may comprise a portion of an optical transceiver operable to receive and convert signals between optical and electrical formats. 
         [0030]    In the illustrated embodiment, transmitters  12  and receivers  28  reside within terminals  11  and  13 , respectively. Terminals  11  and  13  can include both transmitters and receivers without departing from the scope of the present disclosure. Additionally, terminals  11  and  13  may include any other optical component, such as, combiner  14 , booster amplifier  18 , pre-amplifier  24 , and/or separator  26  without departing from the scope of the present disclosure. In some cases, terminals  11  and  13  can be referred to as end terminals. The phrase “end terminal” refers to devices operable to perform optical-to-electrical and/or electrical-to-optical signal conversion and/or generation. 
         [0031]    System  10  includes a plurality of optical amplifiers coupled to communication span  20 . In this example, system  10  includes booster amplifier  18  operable to receive and amplify wavelengths of signals  16  in preparation for transmission over communication span  20 . Where communication system  10  includes a plurality of fiber spans  20   a - 20   n , system  10  can also include one or more in line amplifiers  22   a - 22   m  with or without co-propagating and/or counter-propagating (relative to the signal direction) distributed Raman amplification. In line amplifiers  22  couple to one or more spans  20   a - 20   n  and operate to amplify signals  16  as they traverse communication span  20 . The illustrated example also implements a preamplifier  24  operable to amplify signals  16   b  received from final fiber span  20   n  prior to communicating signals  16  to separator  26 . Although optical link  25  is shown to include one or more booster amplifiers  18  and preamplifiers  24 , one or more of the amplifier types could be eliminated in other embodiments. 
         [0032]    Amplifiers  18 ,  22 , and  24  could each comprise, for example, one or more stages of discrete Raman amplification stages, distributed Raman amplification stages, rare-earth-doped amplification stages, such as erbium-doped or thulium-doped stages, semiconductor amplification stages or a combination of these or other amplification stage types. Throughout this document, the term “amplifier” denotes a device or combination of devices operable to at least partially compensate for at least some of the losses incurred by signals while traversing all or a portion of optical link  25 . Likewise, the terms “amplify” and “amplification” refers to offsetting at least a portion of losses that would otherwise be incurred. 
         [0033]    An amplifier may, or may not impart a net gain to a signal being amplified. Moreover, the terms “gain” and “amplify” as used throughout this document do not (unless explicitly specified) require a net gain. In other words, it is not necessary that a signal experiencing “gain” or “amplification” in an amplifier stage experience enough gain to overcome all losses in the amplifier stage or in the fiber connected to the amplifier stage. As a specific example, distributed Raman amplifier stages often do not experience enough gain to offset all of the losses in the transmission fiber that serves as a gain medium. Nevertheless, these devices are considered “amplifiers” because they offset at least a portion of the losses experienced in a transmission fiber. 
         [0034]    Depending on the amplifier types chosen, one or more of amplifiers  18 ,  22 , and/or  24  could comprise a wide band amplifier operable to amplify all signal wavelengths  15   a - 15   n  received. Alternatively, one or more of those amplifiers could comprise a parallel combination of narrower band amplifier assemblies, wherein each amplifier in the parallel combination is operable to amplify a portion of the wavelengths of multiple wavelength signals  16 . In that case, system  10  could incorporate signal separators and/or signal combiners surrounding the parallel combinations of amplifier assemblies to facilitate amplification of a plurality of groups of wavelengths for separating and/or combining or recombining the wavelengths for communication through system  10 . 
         [0035]    In this or other embodiments, system  10  may implement one or more dispersion management techniques to compensate for dispersion of signals  16 . For example, system  10  can implement a pre-compensation, in-line compensation, and/or a post-compensation technique. These dispersion compensation techniques can include, for example, electronic dispersion compensation techniques, optical dispersion compensation techniques, or any other appropriate dispersion compensation technique. In various embodiments, terminals  11  and  13  can include one or more dispersion compensating elements capable of at least partially compensating for chromatic dispersion associated with signals  16 . In some embodiments, the dispersion compensating element can comprise a dispersion length product that approximately compensates for the dispersion accumulated by optical signals  16  while traversing span  20  of system  10 . In other embodiments, at least a portion of a gain medium of amplifier  24  may comprise a dispersion compensating fiber that is capable of at least partially compensating for chromatic dispersion associated with signals  16 . In those embodiments, the dispersion compensating fiber can comprise a slope of dispersion that is equal to and opposite from the slope of chromatic dispersion associated with multiple wavelength signals  16  in spans  20 . 
         [0036]    In certain embodiments, the bandwidth of system  10  can be expanded by adding one or more pump sources to amplifiers  18 ,  22 , and/or  24 . The pump source can comprise any device or combination of devices capable of generating one or more pump wavelengths at desired power levels and wavelengths. For example, the pump source can comprise a solid state laser, such a Nd:YAG or Nd:YLF laser, a semiconductor laser, a laser diode, a cladding-pumped fiber laser, or any combination of these or other light sources. 
         [0037]    In those embodiments, each of the pump sources may be capable of generating one or more pump wavelengths. The pumps can each comprise one or more pump wavelengths, each of the one or more pump wavelengths comprising a center wavelength of light. In some embodiments, each of the one or more pump wavelengths within a particular pump can comprise a center wavelength that is substantially different from the center wavelengths of the other pump wavelengths within the particular pump. The new pump wavelengths may be shorter, longer, or interspersed with the original (or “core”) pump wavelengths. 
         [0038]    In some embodiments, the new pumps can co-propagate through span  20  in relation to signal  16 . In other embodiments, the new pumps can counter-propagate through span  20  in relation to optical signal  16 . In yet other embodiments, some of the new pumps can co-propagate through span  20  in relation to signal  16 , while other new pumps can counter-propagate through span  20 . As used throughout this document, the term “co-propagates” or “co-propagating” refers to a condition where, for at least some time at least a portion of the pump propagates through the gain medium in the same direction as at least one wavelength of the optical signal being amplified. In addition, the term “counter-propagates” or “counter-propagating” refers to a condition where at least a portion of a pump propagates through a gain medium of an optical device in a direction counter to the direction of the optical signal being amplified. 
         [0039]    One aspect of this disclosure recognizes that the bandwidth of one or more of amplifier  18 ,  22 , and  24  can be advantageously expanded by adding at least one new pump wavelength that is shorter than the longest core pump wavelength. Another aspect of this disclosure presents an algorithm for bandwidth expansion in which the signal power ripple and noise figure is minimally degraded as the bandwidth of the amplifier is increased from the minimum to the maximum value. A further advantage of this disclosure is that the pump power is minimized for the minimum-bandwidth configuration, thus lowering initial installed cost. Another aspect of this disclosure recognizes that adding the new pump wavelengths on the opposite side of the gain spool as the core pumps may allow more flexibility in wavelength selection since this creates two pump multiplexer sections, which are not dependent on each other. This pump expansion can be accomplished with a number of different pump configurations, as described in further detail below. 
         [0040]      FIG. 2  is an optical pump and signal wavelength schema for an optical amplifier  200  whose amplification bandwidth has been expanded. The optical amplifier  200  can be substantially similar in structure and function to amplifiers  18 ,  22 , and/or  24  of  FIG. 1 . The particular wavelengths and/or combinations of wavelengths illustrated in  FIG. 2  is intended for illustrative purposes only and is not intended to limit the scope of the present disclosure. It should be appreciated that other embodiments or combinations of wavelengths may be used without departing from the scope of the present disclosure. 
         [0041]    In this example, amplifier  200  comprises a discrete or distributed Raman amplifier capable of amplifying one or more optical signals in Bandwidth B 1 . Although amplifier  200  in this example includes a Raman amplifier, any other optical amplifier can be used without departing from the scope of the present disclosure. For example, amplifier  200  could comprise a multi-stage discrete amplifier having one or more rare-earth-doped amplification stage and one or more Raman amplification stages. System  200  also includes a first pump source that generates one or more pumps P 1  for introduction to the Raman gain fiber of amplifier  200 . Although  FIG. 2  illustrates two pumps P 1 , one or any number of pumps could be used without departing from the scope of the present disclosure. 
         [0042]    In this example, it is desired to expand the amplification bandwidth of amplifier  200  to include new amplification bandwidth B 2 . Thus, amplifier assembly  200  also includes a new second pump source that generates one or more pumps P 2  for introduction to the Raman gain fiber of amplifier  200 . Although  FIG. 2  illustrates two pumps P 2 , one or any number of pumps could be used without departing from the scope of the present disclosure. In this particular embodiment, new pump wavelengths P 2  are introduced to the Raman gain medium of amplifier  200  by introducing at least one pump wavelength P 2   a  that is shorter than the shortest wavelength of P 1  (indicated by P 1   a ). Optionally, a pump wavelength P 2   b  that is longer than the longest wavelength of P 1  (indicated by P 1   b ) can be added. 
         [0043]    In this particular embodiment, the additional amplification bandwidth B 2  comprises a plurality of wavelengths longer than the wavelengths in bandwidth B 1 . In other embodiments, the additional amplification bandwidth could comprise a plurality of wavelengths shorter than the wavelengths in bandwidth B 1 . In some embodiments, the additional amplification bandwidth B 2  could comprise a plurality of wavelengths longer than the wavelengths in bandwidth B 1  and a plurality of wavelengths shorter than wavelengths in bandwidth B 1 . 
         [0044]    In various embodiments, the exact configuration of pumps could depend on an optimization algorithm, involving factors such as achieving low noise figures, minimizing channel power ripple, and minimizing pump power. Further, it may be necessary in certain embodiments to switch off some pump wavelengths during bandwidth expansion in order to satisfy one of the above optimization factors or any other optimization factor. 
         [0045]      FIG. 3  is an optical pump and signal wavelength schema for an optical amplifier  300  whose amplification bandwidth has been expanded. The optical amplifier  300  can be substantially similar in structure and function to amplifiers  18 ,  22 , and/or  24  of  FIG. 1 . The particular wavelengths and/or combinations of wavelengths illustrated in  FIG. 3  is intended for illustrative purposes only and is not intended to limit the scope of the present disclosure. It should be appreciated that other embodiments or combinations of wavelengths may be used without departing from the scope of the present disclosure. 
         [0046]    In this example, amplifier  300  comprises a discrete or distributed Raman amplifier capable of amplifying one or more optical signals in Bandwidth B 1 . Although amplifier  300  in this example includes a Raman amplifier, any other optical amplifier can be used without departing from the scope of the present disclosure. For example, amplifier  300  could comprise a multi-stage discrete amplifier having one or more rare-earth-doped amplification stage and one or more Raman amplification stages. System  300  also includes a first pump source that generates one or more pumps P 1  for introduction to the Raman gain fiber of amplifier  300 . Although  FIG. 3  illustrates two pumps P 1 , any number of pumps could be used without departing from the scope of the present disclosure. 
         [0047]    In this example, it is desired to expand the amplification bandwidth of amplifier  300  to include new amplification bandwidth B 2 . Thus, amplifier assembly  300  also includes a new second pump source that generates one or more pumps P 2  for introduction to the Raman gain fiber of amplifier  300 . Although  FIG. 300  illustrates two pumps P 2 , one or any number of pumps could be used without departing from the scope of the present disclosure. In this particular embodiment, new pump wavelength P 2  are introduced to the Raman gain medium of amplifier  300  by introducing at least one pump wavelength P 2   a  that is in between the shortest pump wavelength of P 1  (indicated by P 1   a ) and the longest pump wavelength of P 1  (indicated by P 1   b ). Optionally, a pump wavelength P 2   b  that is longer than the longest pump wavelength P 1   b  can be added. 
         [0048]    In this particular embodiment, the additional amplification bandwidth B 2  comprises a plurality of wavelengths longer than the wavelengths in bandwidth B 1 . In other embodiments, the additional amplification bandwidth could comprise a plurality of wavelengths shorter than the wavelengths in bandwidth B 1 . In some embodiments, the additional amplification bandwidth B 2  could comprise a plurality of wavelengths longer than the wavelengths in bandwidth B 1  and a plurality of wavelengths shorter than wavelengths in bandwidth B 1 . 
         [0049]    In various embodiments, the exact configuration of pumps could depend on an optimization algorithm, involving factors such as achieving low noise figures, minimizing channel power ripple, and minimizing pump power. Further, it may be necessary in certain embodiments to switch off some pump wavelengths during bandwidth expansion in order to satisfy one of the above optimization factors or any other optimization factor. 
         [0050]      FIG. 4  is a block diagram showing at least a portion of an optical amplifier  2000  capable of having its bandwidth expanded. The optical amplifier  2000  can be substantially similar in structure and function to amplifiers  18 ,  22 , and/or  24  of  FIG. 1 .  FIG. 4  shows one example of the means by which expansion pumps  2091  may be added to the optical amplifier  2000  and shows one example of how the expansion pumps  2091  may be used to expand the bandwidth of the optical amplifier  2000 . 
         [0051]    Amplifier  2000  comprises at least one fiber unit  2010   a . Optionally, fiber units  2010   b  and/or  2010   c  may interact with expansion pumps  2091  or in some embodiments may be removed. Fiber units  2010  are further depicted in  FIG. 7 . Amplifier  2000  comprises at least one WDM unit in either the backward direction (WDM-b)  2020  which is positioned after fiber unit  2010   a  and/or in the forward direction (WDM-f)  2025  which is positioned before fiber unit  2010   a . The term forward and backward are relative to the signal direction  2001 . Each WDM unit  2020  and/or  2025  further comprises at least one WDM function  2021  and/or  2026 , respectively. WDM functions are further depicted in  FIG. 6 . Each WDM function  2021  and/or  2026  receives the output of pump multiplexer (mux) function  2050 , which is further depicted in  FIG. 5 . Optical amplifier  2000  has at least one pump mux function  2050  within the original pump mux section  2090  and optionally may add at least one pump mux function  2050  from the expansion pump section  2091 . The expansion pump section  2091  depicts pumps that may be added to optical amplifier  2000  in order to expand the bandwidth of optical amplifier  2000 . Pump mux function  2050  is further depicted in  FIG. 5 . The other depicted components and functions are optional in certain embodiments, including pump reflector  2030 , pump demultiplexer (dmux)  2027 , pump dmux  2022 , pump terminator  2040 , and expansion pump connector  2060 . The position of WDM-b functions  2021  and optional pump reflector  2030  within WDM-b unit  2020  can be in any order with respect to the signal path  2001  along with their dependent pump mux  2050  inputs. The position of WDM-f functions  2026  and optional pump reflector  2030  within WDM-f unit  2025  can be in any order with respect to the signal path  2001  along with their dependent pump mux  2050  inputs. Pump mux  2050  in the expansion pump section  2091  may connect directly to WDM functions  2021  and/or  2026  or may connect to other pump mux function  2050  within the original pump section  2090 . Optional pump dmux  2027  is located before WDM-f unit  2025  and may be located before fiber unit  2010   c  (which may allow one or more backward pumps to travel through fiber unit  2010   c ) or after fiber unit  2010   c . Optional pump dmux  2022  is located after WDM-b unit  2020  and may be located before fiber unit  2010   b  or after fiber unit  2010   b  (which may allow one or more forward pumps to travel through fiber unit  2010   b ). Expansion pump mux  2050  in section  2091  (expansion pumps) may be attached to optical amplifier  2000  through optional optical connectors  2060  or by other means. 
         [0052]    It should be noted in the following example description of expansion pump paths that WDM-f is optional if WDM-b exists and that WDM-b is optional if WDM-f exists, or both WDM-b and WDM-f may be used. In the following example, the one or more pump mux  2050  in original pump mux section  2090  may attach to either WDM-b and/or WDM-F, independent of the location of pump mux  2050  in expansion pump section  2091 . 
         [0053]    Expansion pumps that eventually connect to WDM-b unit may travel backward with respect to signal direction  2001  through fiber unit  2010   a , and potentially: (1) pass through optional WDM-f unit, or (2) reflect off of optional pump reflector  2030  within WDM-f or couple through any optional WDM-f function  2026  (towards dependent pump mux  2050 ), whose path may contain a pump reflector  2030  capable of reflecting one or more expansion pump wavelengths. The reflected expansion pumps then, if applicable, could be directed forward into fiber unit  2010   a . If route (1) was taken above, then the expansion pumps (3) may further be directed by optional pump dmux  2027  into optional pump reflector  2030  capable of reflecting one or more expansion pump wavelengths. The reflected expansion pumps then, if applicable, would be directed forward into fiber unit  2010   a , or (4) may travel through optional fiber unit  2010   c , or (5) may travel through optional fiber unit  2010   c  and then may be directed by optional pump dmux  2027  into optional pump reflector  2030  capable of reflecting one or more expansion pump wavelengths. The reflected expansion pumps then, if applicable, would be directed back through fiber unit  2010   c  in the forward direction, through optional WDM-F, and be directed forward into fiber unit  2010   a . Thus, expansion pumps from WDM-b may travel backward through fiber unit  2010   a , optionally travel backward through fiber unit  2010   c , and/or optionally be reflected in the forward direction to pass through either fiber unit  2010   a  or both fiber units  2010   c  and  2010   a . Expansion pumps from WDM-b that were reflected into the forward direction and pass through fiber unit  2010   a  in the forward direction may either (1) retrace their path back into pump mux  2050  (potentially terminating at an isolator), or (2) if the WDM function  2021  that expansion pumps are coupled to is a circulator  2221   b  of  FIG. 6  and is the last WDM-b function  2021   n  with respect to signal direction  2001 , then expansion pumps will continue to travel in the forward direction and either (2a) be directed from pump dmux  2022  into pump terminator  2040 , or (2b) travel forward through optional fiber unit  2010   b  in which case one or more expansion pump wavelengths may further reflect off of optional pump dmux  2022  and pump reflector  2030  after fiber unit  2010   b  into the backward direction into fiber unit  2010   b  and eventually terminate in said circulator. Thus, expansion pumps from WDM-b that were reflected into the forward direction and passed through fiber unit  2010   a  in the forward direction may pass through optional fiber unit  2010   b , and optionally be reflected in the backward direction through fiber unit  2010   b.    
         [0054]    Expansion pumps that eventually connect to WDM-f unit may travel forward with respect to signal direction  2001  through fiber unit  2010   a . The expansion pumps may take the symmetric paths discussed in the preceding paragraph, noting that the functions and components of optical amplifier  2000  are symmetric from fiber unit  2010   a  with respect to signal direction  2001  except for the circulator. Thus, expansion pumps from WDM-f will travel forward through fiber unit  2010   a , optionally travel forward through fiber unit  2010   b , and/or optionally be reflected in the backward direction to pass through either fiber unit  2010   a  or both fiber units  2010   b  and  2010   a.    
         [0055]    It is understood that any combination of alternative components that perform substantially similar functions as those depicted in  FIG. 4  may be substituted with those of  FIG. 4  without departing from the scope of the present disclosure. As one example, pump dmux  2022  and pump reflector  2030  below pump dmux  2022  may be substituted with a pump FBG reflector in the signal path. 
         [0056]      FIG. 5  depicts some example embodiments of pump mux function  2050  of  FIG. 4 . Pump mux function  2050  has an input of one or more pump wavelengths and/or one or more groups of pump wavelengths, potentially from other pump mux functions  2050 . Pump mux function  2050  may comprise a wavelength division multiplexer (WDM)  2150   a  which combines pump 1  and pump 2  into one output, where pump 1  and pump 2  are one or more pump wavelengths; a polarization division multiplexer (PDM)  2150   b  which combines pump 1  of polarization  1  and pump 2  of polarization  2  into one output, where pump 1  and pump 2  are one or more pump wavelengths; a time division multiplexer (TDM)  2150   d  which combines pump 1  of one pulsed format and pump 2  of another pulsed format into one output, where pump 1  and pump 2  are one or more pump wavelengths; a piece of optical fiber  2150   c  that transmits pump; an optical isolator  2150   e  that protects pump transmitter from back-reflections; and/or a depolarizer  2150   f  that reduces the degree of polarization of the pump. It is understood that any combination of the components  2150  and similar components known to those skilled in the art may be combined to perform pump mux function  2050 , an example of which is pump mux combination  2151  which has five pump inputs of one or more pump wavelengths and one pump output that multiplexes the inputs. It is understood that in certain embodiments other mux components may have more than two inputs. Pump wavelengths are added at the input of the pump mux  2050 . 
         [0057]      FIG. 6  depicts some example embodiments of WDM-b function  2021 , WDM-f function  2026 , and pump reflector function  2030  of  FIG. 4 . WDM-b function  2021  combines and/or separates pump 1 , of one or more pump wavelengths, with/from signal, of one or more signal wavelengths, and with/from pump 2 , of zero or more pump wavelengths. WDM-b function  2021  may comprise WDM  2221   a  and/or circulator  2221   b . If circulator  2221   b  is used as WDM-b function  2021 , then in some embodiments it must be the last WDM function in WDM-b with respect to the signal direction  2001  of  FIG. 4  since it does not allow any pumps to be transmitted in the backward direction (i.e., it will terminate any pumps traveling backward into the circulator). WDM-f function  2026  combines and/or separates pump 1 , of one or more pump wavelengths, with/from signal, of one or more signal wavelengths, and with/from pump 2 , of zero or more pump wavelengths. WDM-f function  2026  may comprise WDM  2226 . In certain embodiments, pump reflect function  2030  transmits pump 1 , of zero or more pump wavelengths, in a first direction; reflects pump 2 , of one or more pump wavelengths, traveling from a second direction back into the first direction; and transmits signal, of zero or more signal wavelengths, traveling in either direction. Pump reflect function  2030  may comprise a fiber Bragg grating  2030   a  reflective at pump 2  wavelength(s), a WDM similar to  2221   a , and a broadband reflector  2230   b . It is understood that any combination of components known to those skilled in the art may be combined to perform pump reflect function  2030 . 
         [0058]      FIG. 7  depicts some example embodiments of fiber unit  2010  of  FIG. 4 . Fiber unit  2010  comprises one or more fiber sections capable of providing gain when one or more pump wavelengths travel in any direction at least part of the way through the fiber section; each section may comprise one or more fiber types and/or lengths. Fiber unit  2010  may comprise, for example, Raman gain fiber, dispersion-compensation fiber, rare-earth-doped fiber (such as erbium-doped or thulium-doped fiber), and/or transmission (or “line”) fiber (examples of which are standard single mode fiber (SMF), dispersion shifted fiber (DSF), non-zero dispersion shifted fiber (NZDSF), dispersion compensating fiber (DCF), and pure-silica core fiber (PSCF)). Fiber sections of fiber unit  2010  may comprise discrete fiber spools, hybrid fiber spools, and/or cabled transmission fiber used for optical communication. Examples of fiber unit  2010  are shown in  FIG. 7 : a single section of fiber  2310 ; multiple sections of possibly dissimilar fiber  2311 ; and/or combinations of fiber sections and connectorized fiber spools  2312  which may be added ( 2312   b ) or removed ( 2312   a ). In addition, fiber unit  2010  may comprise other optical components such as isolators and/or WDMs. It may be beneficial, in certain embodiments, to place such optical components between two or more sections of fiber. An example  2313  of this is to place an isolator between spools with a WDM on each side of the isolator to allow pump, of one or more wavelengths, to bypass the isolator and travel in either direction, but allow signal, of one or more wavelengths, to travel in only one direction. One advantage of  2313  is that pump may travel from right to left through both fiber sections while signal may only travel from left to right. One advantage of this may be lower multiple-path interference in the signal wavelength range and another advantage may be that amplified spontaneous emission from the right fiber section does not travel into the left fiber section. 
         [0059]      FIG. 8   a  illustrates a block diagram of one example of a modular optical amplifier  422  capable of having its bandwidth expanded. In various embodiments, the structure and function of optical amplifier  422  can be substantially similar to the structure and function of amplifiers  18 ,  22 , and/or  24  of  FIG. 1 . In this example, initial bandwidth (B 1  in  FIGS. 2 and 3 ) is approximately 1543.33 to 1567.13 nm and expanded bandwidth (B 2  in  FIGS. 2 and 3 ) is approximately 1567.54 to 1592.10 nm. Amplifier  422  includes original pump sources  150 ,  152 ,  154 , and  156  capable of generating pump wavelengths at 1441 nm, 1460, nm, 1434 nm, and 1468 nm, respectively. Although each of pump sources  150 ,  152 ,  154 , and  156  generate particular pump wavelengths in this example, other pump wavelengths can be used without departing from the scope of the present disclosure. In addition, although each of pump sources  150 ,  152 ,  154 , and  156  generate one pump wavelength in this example, pump sources  150 ,  152 ,  154 , and  156  can generate one or more pump wavelengths without departing from the scope of the present disclosure. Pump sources  150 ,  152 ,  154 , and  156  can comprise any device or combination of devices capable of generating one or more pump wavelengths at desired power levels and wavelengths. For example, pump sources  150 ,  152 ,  154 , and  156  may comprise a depolarizer, a polarization division multiplexer (PDM) with two orthogonally polarized pumps, a solid state laser, such a Nd:YAG or Nd:YLF laser, a semiconductor laser, a laser diode, a cladding pump fiber laser, or any combination of these or other light sources. In this particular embodiment, pump source  150 ,  152 ,  154 , and  156  comprise laser diodes with PDM or depolarizer. 
         [0060]    Amplifier  422  also includes combiners  104   a - 104   n  operable to receive the pumps generated by pump sources  150 ,  152 ,  154 , and  156 , and to combine those pumps into multiple wavelength pump  170 . As one particular example, combiners  104  could comprise a wavelength division multiplexer (WDM). In this example, amplifier  422  also includes couplers  106   a  and  106   b  operable to couple pumps  170   a  and  170   b , respectively, to a gain medium  172 . In this example, pumps  170  counter-propagate through gain media  172  with respect to optical signal direction  116 . Although pumps  170  counter-propagate through gain media  172  in this example, one or more of the pump wavelengths of pumps  170  could co-propagate through gain media  172  without departing from the scope of the present disclosure. 
         [0061]    To expand the bandwidth of amplifier  422 , new pump sources  160  and  162  are added to amplifier  422 . In this example, new pump sources  160   a  introduce a pump wavelength at 1425 nm, pump sources  160   b  introduce a pump wavelength at 1480 nm, pump sources  162   a  introduce a pump wavelength at 1454 nm, and pump sources  162   b  introduce a pump wavelength at 1494 nm. Although each of pump sources  160  and  162  generate particular pump wavelengths in this example, other pump wavelengths can be used without departing from the scope of the present disclosure. In addition, although each of pump sources  160  and  162  generate one pump wavelength in this example, pump sources  160  and  162  can generate one or more pump wavelengths without departing from the scope of the present disclosure. The structure and function of pump sources  160  and  162  can be substantially similar to the structure and function of pump sources  150 - 156 . 
         [0062]    In this particular embodiment, new pump wavelengths from pump sources  160  are combined with the pump wavelengths of pump sources  150  and  152  to broaden the distributed Raman amplification bandwidth in transmission line  172   b  of amplifier  422 . In addition, new pump wavelengths from pump sources  162  are combined with the pump wavelengths of pump sources  154  and  156  to broaden discrete Raman amplification bandwidth in discrete fiber  172   a  of amplifier  422 . In addition, pumps may be passed through pump isolators  108   a - 108   n . Pump isolators may be used to prevent pump cross-talk or instabilities through feedback. 
         [0063]      FIG. 8   b  shows example gain and noise figure characteristics of bandwidth expansion of the example amplifier of  FIG. 8   a  including an 80-km transmission fiber. The original (before expansion) gain and noise figure are shown for amplification of 60 channels. The gain and noise figure are also shown for the expanded 120-channel amplification which has similar gain, gain ripple, and noise figure as the 60-chan case. 
         [0064]      FIG. 9  is a block diagram of one example of a modular optical amplifier  522  capable of having its bandwidth expanded. Amplifier  522  is similar in function to amplifier  422  of  FIG. 8   a . However, the pump wavelengths used in this and in other figures in this disclosure are examples only; other pump wavelength values may be used in any of the various embodiments. The design in  FIG. 9  uses additional multiplexers  106  in the implementation of the design, but uses a fewer number of pump connectors  102  and pump isolators  108 . Initial and expansion pump wavelength signals pass through one or more ports  102  and/or one or more pump multiplexers (mux)  104 . As with other designs, expansion pump wavelengths can be placed above, below, or in between the initial pump wavelengths. 
         [0065]      FIG. 10  is a block diagram of one example of a modular optical amplifier  622  capable of having its bandwidth expanded. Amplifier  622  is similar in function to amplifier  422  of  FIG. 8   a . Initial and/or expansion pumps may pass through pump muxes  104 , isolators  108 , ports  102 , and/or multiplexers  106 . In addition, in certain embodiments a pump mux like mux  106   a  may be used to combine two or more pumps, which provides less component loss for the signal path than multiplexer  106   a  of  FIG. 9 . 
         [0066]      FIG. 11   a  is a block diagram of one example of a discrete modular optical amplifier  1022  capable of having its bandwidth expanded by adding the new pump wavelengths on the opposite side of the gain spool relative to the original pump. One benefit of this example is two pump multiplexer sections, which are not dependent on each other, allowing more flexibility in wavelength selection and reducing the number of couplers  104  in the core amplifier. Amplifier  1022  is similar in function to amplifier  422  of  FIG. 8   a . In this example, the initial bandwidth is approximately 1543.33 to 1567.13 nm and the expanded bandwidth is approximately 1531.51 to 1542.94 nm and 1567.54 to 1592.10 nm. Amplifier  1022  includes original pump sources  150  and  152  capable of generating pump wavelengths at 1440 nm and 1471 nm, respectively. Although each of the pump sources  150  and  152  generate particular pump wavelengths in this example, other pump wavelengths can be used without departing from the scope of the present disclosure. 
         [0067]    When bandwidth expansion is desired, pump wavelengths  160 ,  162 ,  164  and  166  can be added. In this example  160 ,  162 ,  164  and  166  introduce pump wavelengths at 1432 nm, 1450 nm, 1481 nm and 1498 nm, respectively. Original and/or new pump wavelengths may pass through one or more couplers  104 , isolators  116 , and multiplexers  106 . In this and other embodiments, one or more pump reflectors  118  may be used to reflect one or more pump wavelengths of the pump in one direction and direct it to the opposite direction. Optical circulators  120  are non-reciprocal devices that redirect light from port to port sequentially in only one direction. In particular, pump from port  1  is directed to port  2  in the backward direction in this example, pump and/or signal entering port  2  transmit to port  3  in the forward direction in this example, but ports  2  to  1  and  3  to  1  are isolated. Optical circulators are 3-port coupling devices that are made to be polarization independent and have low insertion loss. 
         [0068]      FIG. 11   b  shows example gain and noise figure characteristics of bandwidth expansion of the example amplifier of  FIG. 11   a  including an 80-km transmission fiber. The original (before expansion) gain and noise figure are shown for amplification of 60 channels. The gain and noise figure are also shown for the expanded 150-channel amplification which has similar gain, gain ripple, and noise figure as the 60-chan case. 
         [0069]    Although the present disclosure has been described with several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present disclosure encompass such changes, variations, alterations, transformations, and modifications as fall within the scope of the appended claims.