Abstract:
An optical amplification mechanism that introduces optical pump(s) into one port of an optical circulator. The optical circulator directs the optical pumps from that port into another port that is coupled to the output of a gain stage. The optical pump(s) then pass from the output to the input of the gain stage while amplifying an optical signal passing from the input to the output of the gain stage. A residual amount of optical pump(s) that exits the input of the gain stage is reflected back into the input of the gain stage. The reflected optical pump(s) then further assists in the amplification of the optical signal. Other embodiments are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application 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 also 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. 61/019,467, entitled “OPTICAL AMPLIFIER CAPABLE OF AMPLIFYING OPTICAL SIGNALS THAT TRAVERSE SEPARATE TRANSMISSION FIBERS”, filed Jan. 7, 2008, by Wayne S. Pelouch et al. 
    
    
     BACKGROUND 
     In-line or discrete optical amplifiers used in optical communication networks are capable of amplifying wavelength channels as those channels traverse the communication network. Conventional discrete optical amplifiers have typically been limited in how efficiently they use the pump power provided to the gain medium because of their inherent performance characteristics. In other words, pump sources typically provided a relatively high power pump signal (“pump”) to the gain medium and the limitations associated with the characteristics of the optical amplifier prevented it from using all or a substantial portion of the pump power. Consequently, conventional discrete optical amplifiers dump a significant amount of the residual power from the pump. 
     BRIEF SUMMARY 
     Embodiments described herein relate to optical amplification using a gain stage. 
     In one embodiment, an optical pump source introduces optical pump(s) into one port of an optical circulator. The optical circulator directs the optical pumps from that port into another port that is coupled to the output of a gain stage. The optical pump(s) then pass from the output to the input of the gain stage while amplifying an optical signal passing from the input to the output of the gain stage. A residual amount of optical pump(s) that exits the input of the gain stage is reflected back into the input of the gain stage. The reflected optical pump(s) then further assists in the amplification of the optical signal as the optical pump(s) travels from the input to the output of the gain stage. 
     In another embodiment, optical pump(s) are emitted into the output of a first gain stage so that the optical pump(s) propagates from the output to the input of the first gain stage while amplifying an optical signal travelling from the input to the output of the first gain stage. An optical element (such as a pump bypass filter) allows the optical signal to transmit from the input port of the optical element to the output port of the optical element and into the input of the first gain stage. The optical element also isolates the input port of the optical element from optical signals present at the output port of the optical element. However, the optical element allows the optical pump(s) to be communicated from the output port to the input port and into the output of a second gain stage. While the optical pump(s) pass from the output of the second gain stage to the input of the second gain stage, the optical pump(s) assist in the amplification of optical signals propagating from the input to the output of the second gain stage. The optical pumps exiting the input of the second gain stage are then reflected back into the second gain stage to further assist in amplification of the optical signal. 
     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  is a block diagram showing at least a portion of an optical communication system operable to facilitate communication of one or more multiple wavelength signals; 
         FIGS. 2A-2B  are block diagrams of example embodiments of an efficient discrete optical amplifier; 
         FIGS. 3A-3F  are block diagrams of example embodiments of efficient discrete optical amplifiers; 
         FIGS. 4A-4B  are block diagrams of example embodiments of efficient discrete optical amplifiers illustrates another embodiment of a discrete optical amplifier comprising a circulator, a reflector, and a dispersion compensation unit; 
         FIG. 5  is a block diagram of a conventional discrete optical amplifier; and 
         FIG. 6  contains a table of example efficiency and optical performance metrics that demonstrate some of the advantages of the current disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     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 efficiently amplify optical signals traversing an optical fiber. Moreover, the illustrations in  FIGS. 1 through 6  are not intended to be to scale. 
       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. 
     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. 
     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. 
     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. 
     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:
 
 Q=[M   H   −M   L ]÷[Δ H +Δ L ]
 
     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)). 
     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. 
     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 bidirectional 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. 
     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. 
     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. 
     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. 
     In this embodiment, separator  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. 
     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. 
     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. 
     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. 
     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 the transmission fiber. 
     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 . 
     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 . 
     One or more of amplifiers  18 ,  22 , or  24  can comprise an efficient discrete optical amplifier. An efficient discrete optical amplifier may, in some circumstances, use a substantial portion of available pump power to amplify one or more communication signals. Efficient discrete optical amplifiers may use a much greater percentage of available power than some existing designs, thus wasting less power. 
     In some embodiments, an efficient discrete optical amplifier may comprise a highly efficient gain fiber and a composite Raman gain medium. A highly efficient gain fiber is defined as having a small-signal gain of greater than 5 dB per 100 mW (0.05 dB/mW) of depolarized pump power which is more than twice the efficiency of Raman amplification in typical dispersion compensation fiber and typical line fiber. The gain fiber may comprise, for example, a rare-earth-doped amplifier. For example, erbium doped fiber typically has an efficiency of greater than 0.5 dB/mW. In some embodiments, the length of rare-earth-doped fiber may be short enough to provide net gain with a relatively low amount of pump power. In some embodiments, Raman amplification can then be used to further amplify the communication signal. One or more Raman gain media may be used to amplify one or more communication signals, with designs that use a substantial portion of the available pump power. Certain embodiments may also lower a noise figure and/or reduce multi-path interference. 
     Described below are a number of configurations that may be used to create an efficient discrete optical amplifier. In some embodiments, a circulator may be placed between two Raman gain media, which can reduce multi-path interference and increase a lasing threshold. The circulator can replace one or more components of an amplifier, providing not only more efficient use of available pump power but also cost savings. In certain other embodiments, the insertion loss of a dispersion compensation unit, capable of providing Raman amplification, may be lowered by a more efficient use of pump power. In other embodiments, reflectors may be used at either end of a gain medium to increase the efficiency of the amplifier. 
       FIG. 2A  is a block diagram of one example embodiment of an efficient discrete optical amplifier assembly  122  capable of amplifying optical signals that traverse optical fibers or communication spans. Amplifier assembly  122  may, for example, be useful in system  10  of  FIG. 1  as one of amplifiers  18 ,  22 , and/or  24 . In this example, amplifier assembly  122  includes a pump source  144  that generates a pump  141  having one or more pump wavelengths. Pump assembly  144  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 assembly  144  can comprise 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. 
     Pump  141  can 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 pump  141  can comprise a center wavelength that is substantially different from the center wavelengths of any other pump wavelengths within particular pump  141 . Although one pump source  144  and pump  141  are illustrated in this example, any other number of pump sources and/or pumps can be used without departing from the scope of the present disclosure. 
     In some embodiments, pump  141  can co-propagate through amplifier assembly  122  in relation to one or more optical signals, such as signal  16  described in  FIG. 1 . In other embodiments, pump  141  can counter-propagate through amplifier assembly  122  in relation to one or more optical signals. In yet other embodiments, at least a portion of pump  141  can co-propagate through amplifier assembly  122  in relation to one or more optical signals, while another portion of pump  141  can counter-propagate through amplifier assembly  122 . 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. 
     Amplifier assembly  122  includes one or more couplers  142  capable of coupling and/or decoupling pump  141  to/from optical signal path  116 . Couplers  142  can comprise any optical coupler capable of adding or removing pump  141  from optical signal path  116 . For example, couplers  142  can comprise a wavelength division multiplexer (WDM) or an optical add/drop multiplexer (OADM). Throughout this disclosure the terms “add/drop,” “adding/dropping,” and “added/dropped” refer to either the operation of adding one or more wavelength signals, dropping one or more wavelength signals, or adding wavelength signals and dropping others. Those terms are not intended to require both add and drop operation, but are also not intended to exclude add and drop operations. The terms are merely used as a convenient way to refer to either adding or dropping or both adding and dropping operations. Pump couplers  142  can be used to combine one or more pumps and/or signals in amplifier assembly  122 . 
     In this example, amplifier assembly  122  further includes a reflector  140  capable of substantially reflecting pump  141 . Reflector  140  may comprise, for example, an optical filter, a mirror, a Bragg grating, or any other optical device capable of substantially reflecting one or more pump wavelengths. In some cases, reflector  140  may allow additional pumps to be coupled into one of the one or more Raman gain mediums  150  and  152 . Reflector  140  can be located on either side of Raman gain medium  150  or  152 ; in certain embodiments, more than one reflector  140  may be used. 
     In this particular embodiment, amplifier assembly  122  also includes a relatively short length of a rare-earth-doped fiber  146 . In some embodiments, the length of rare-earth-doped fiber may be short enough to provide net gain with a relatively low amount of pump power. Rare-earth-doped fiber  146  can comprise, for example, an erbium-doped or thulium-doped amplifier. In this embodiment, fiber  146  may comprise 2.1 meters of erbium-doped fiber (OFS, R37105XL). Although fiber  146  comprises a length of 2.1 meters in this example, any other desired length of fiber can be used without departing from the scope of the present disclosure. In another embodiment, doped fiber  146  may comprise 3.7 meters erbium-doped fiber (OFS, R37003X). Rare-earth-doped fiber  146  can be located on either side of Raman gain medium  150  or  152 ; in certain embodiments, more than one rare-earth-doped fiber  146  may be used. 
     Amplifier assembly  122  can also includes a first Raman gain medium  150  and a second Raman gain medium  152 . Although this example includes two Raman gain media  150  and  152 , any number of gain media can be used without departing from the scope of the present disclosure. In various embodiments, Raman gain medium  150  could comprise 3.8 km OFS low-loss-micro-DK (LLMicroDK) fiber. In another embodiment, Raman gain medium  152  could comprise 4.0 km of LLMicroDK fiber. Raman gain media  150  and  152  may provide Raman amplification to an optical signal traversing a communication fiber. 
     Amplifier assembly  122  further includes optical isolators  154  that prevent the transmission of optical signals and/or pumps in specific directions. In some cases, isolators  154  may be used to prevent optical signal cross-talk or instabilities through feedback. Optical isolators  154  can comprise any device capable of passing an optical signal in one direction and capable of substantially blocking an optical signal from passing in another direction. Amplifier assembly  122  may comprise additional couplers  142   c  and  142   d  to allow pump wavelengths, but not signal wavelengths, to bypass isolator  154   c  through path  160  and travel in either direction. Component combination  148  is referred to as a “pump bypass filter”. 
     One aspect of this disclosure recognizes that placing optical isolator  154   c  and couplers  142   c ,  142   d  between Raman gain medium  150  and  152  advantageously prevents lasing within the Raman amplification region of amplifier assembly  122  while allowing pump  141  to be directed to second gain fiber  150 . 
     In this particular embodiment, pump  141  comprises a pump power of approximately 167 mw at a wavelength of 1442 nm and a pump power of 353 mW at a wavelength of 1468 nm to support 80 km (loss of 21 dB) standard single mode transmission fiber (SSMF). Pumps  141  first traverses Raman gain medium  152  in a counter-propagating direction and provides Raman gain to optical signal  116 . Pumps then traverse path  160 , bypassing isolator  154   c , and then operate to amplify optical signal  116  in Raman gain medium  150 . In this example, pump  141  still has sufficient power after traversing Raman gain medium  152  to amplify optical signal  116  in gain medium  150 . 
     After traversing gain medium  150 , pump  141  may contain approximately 15% of its original power. One aspect of this disclosure recognizes that the efficiency of amplifier assembly  122  can be improved by implementing rare-earth doped fiber  146  to amplify optical signal  116  using any residual power remaining in pump  141  after traversing Raman gain media  150  and  152 . 
     In this example, fiber  146  can provide a fixed gain over a particular set of wavelengths. Raman gain media  150  and/or  152  may then provide Raman amplification when pumped with a pump from pump unit  144 , which in certain embodiments can be a much larger amplification than provided by rare-earth-doped fiber  146 . In certain other embodiments, rare-earth-doped fiber  146  can be connected to pump bypass filter  148 , or located on either side of Raman gain medium  152 . In certain other embodiments, pump bypass filter  148  may be positioned between fibers  146  and  150 . Amplifier assembly  122  further comprises optical isolators  154 A and  154 B. Pump isolator  154 A may be used to prevent pump cross-talk or instabilities through feedback. Signal isolator  154 B may be used, in certain embodiments, to isolate the signal in a communication fiber. Amplifier assembly  122  further comprises reflector  140  capable of substantially reflecting pump  141  to gain fiber  146 ,  150  and  152  to achieve additional gain. Reflector  140  may comprise, for example, an optical filter, a mirror, a fiber Bragg grating (FBG), or any other optical device capable of substantially reflecting one or more pump wavelengths. Coupler  142   a  and reflector  140  may be replaced by an FBG in the signal path  116  before fiber  146 . In some embodiments, reflector  140  may allow additional pumps to be coupled into one or more gain fibers  146 ,  150 , and/or  152 . 
       FIG. 2B  is another embodiment of amplifier  122 . In  FIGS. 2A and 2B , like numerals denote like components. In  FIG. 2B , pump  144  is coupled into signal path  116  in the co-propagating direction. 
       FIG. 3A  is a block diagram of one example embodiment of an efficient discrete optical amplifier assembly  322  that implements an optical circulator  160 . In this example, amplifier assembly  322  includes a pump source  144  that generates a pump  141  having one or more pump wavelengths. Pump assembly  144  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 assembly  144  can comprise 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. 
     Pump  141  can 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 pump  141  can comprise a center wavelength that is substantially different from the center wavelengths of any other pump wavelengths within particular pump  141 . Although one pump source  144  and pump  141  are illustrated in this example, any other number of pump sources and/or pumps can be used without departing from the scope of the present disclosure. 
     In some embodiments, pump  141  can co-propagate through amplifier assembly  322  in relation to one or more optical signals, such as signal  16  described in  FIG. 1 . In other embodiments, pump  141  can counter-propagate through amplifier assembly  322  in relation to one or more optical signals. In yet other embodiments, at least a portion of pump  141  can co-propagate through amplifier assembly  322  in relation to one or more optical signals, while another portion of pump  141  can counter-propagate through amplifier assembly  322 . 
     Amplifier assembly  322  includes a coupler  142  capable of coupling or decoupling a pump to/from optical signal path  116 . In various embodiments, the structure and function of coupler  142  can be substantially similar to the structure and function of coupler  142  in  FIGS. 2A and 2B . 
     Amplifier assembly  322  further includes a reflector  140  capable of substantially reflecting one or more pump wavelengths of pump  141 . Reflector  140  may comprise, for example, an optical filter, a mirror, a Bragg grating, or any other optical device capable of substantially reflecting one or more pump wavelengths. In some cases, reflector  140  may allow additional pumps to be coupled into one of the one or more Raman gain mediums  150  and  152  or fiber  146 . 
     In this particular embodiment, amplifier assembly  322  also includes a relatively short length of a rare-earth-doped fiber  146 . In some cases, rare-earth-doped fiber can advantageously provide a fixed initial gain over a particular set of wavelengths. Rare-earth-doped fiber  146  can comprise, for example, an erbium-doped or thulium-doped amplifier. In this embodiment, fiber  146  may comprise a 2.0 meters of erbium-doped fiber (OFS, R37105XL). Although fiber  146  comprises a length of 2.0 meters in this example, any other desired length of fiber can be used without departing from the scope of the present disclosure. In another embodiment, doped fiber  146  may comprise 3.7 meters of erbium-doped fiber (OFS, R37003X). 
     Amplifier assembly  322  can also includes a first Raman gain medium  150  and a second Raman gain medium  152 . Although this example includes two Raman gain media  150  and  152 , any number of gain media can be used without departing from the scope of the present disclosure. In various embodiments, Raman gain medium  150  could comprise 3.8 km of OFS LLMicroDK fiber. In another embodiment, Raman gain medium  152  could comprise 4.0 km of OFS LLMicroDK fiber. Raman gain media  150  and  152  may provide Raman amplification to an optical signal traversing a communication fiber. 
     In this example, amplifier assembly  322  also includes an optical circulator  160  that is capable of introducing pump  141  to Raman gain medium  150  in a counter-propagating direction and capable of transmitting reflected pump  141  to Raman gain medium  152  in a co-propagating direction. Optical circulators are typically non-reciprocal devices that redirect light from port to port sequentially in only one direction. In particular, input from port  1  is redirected to port  2 . However, a reverse signal entering port  2  totally transmits to port  3  as a usable signal. Ports  1  and  3  are completely isolated. Optical circulators are 3-port coupling devices that are made to be polarization independent and with low insertion loss. Use of circulator  160  can provide better gain control than certain other embodiments. With circulator  160 , a higher Raman gain fiber in Raman gain medium  150  can be more efficiently used. Circulator  160  can also create a better noise figure due to gain being higher in Raman gain medium  150  than in  FIGS. 2A and 2B . In this example, circulator  160  operates to isolate Raman gain medium  150  from Raman gain medium  152 . By isolating the Raman gain media, circulator effectively splits the Raman gain media into two parts and operates to prevent lasing from within the Raman gain media. 
     The one or more pumps then may travel through multiplexer  142  and to reflector  140 , where the one or more pumps may be reflected and sent back through multiplexer  142  and doped fiber  146 . The one or more pumps may then travel through Raman gain medium  150 , circulator  160 , and Raman gain medium  152 . The one or more pumps may be used to amplify one or more communication signals traveling through a transmission fiber. Circulator  160  can also provide cost savings by replacing one or more other components in amplifier assembly  322 . 
     In this particular embodiment, pump  141  comprises a pump power of approximately 215 mw at a wavelength of 1442 nm and 365 mW at a wavelength of 1468 nm to support 80 km (loss of 21 dB) standard single mode transmission fiber (SSMF). In operation, pump  141  is received by circulator  160  and is communicated to Raman gain medium  150  in a counter-propagating direction. Pump  141  interacts with optical signal  116  and provides Raman gain to optical signal  116 . After traversing gain medium  150 , pump contains approximately 33% of its original power. 
     Pump  141  then traverses rare-earth doped fiber  146 , resulting in the amplification of optical signal  116 . Pump  141  is then reflected back to signal path  116  by reflector  140  and traverses fiber  146  and Raman gain medium  150  in a co-propagating direction. Circulator  160  receives pump  141  and introduces any residual power from pump  141  into Raman gain medium  152 . 
     In this example, fiber  146  can provide a fixed gain over a particular set of wavelengths due to saturation of the pump absorption. Raman gain media  150  and/or  152  may then provide Raman amplification when pumped with a pump from pump unit  144 , which in certain embodiments can be a much larger amplification than provided by rare-earth-doped fiber  146 . In certain other embodiments, rare-earth-doped fiber  146  can be connected to circulator  160 , or located on either side of Raman gain medium  150  or  152 . 
     One aspect of this disclosure recognizes that implementing circulator  160  in amplifier assembly  322  advantageously reduces the number of optical components required. Compared with the example of  FIGS. 2A and 2B , circulator  160  performs the functions of isolator  154   a , coupler  142   b , and pump bypass filter  148 . 
     This configuration also advantageously allows the pump power of pump  141  to be efficiently used in amplifying optical signal  116 . 
     In another embodiment of  FIG. 3A , fiber  146  comprises one of more sections of highly efficient gain fiber such as high-gain Raman fiber capable of providing higher gain per pump power than either Raman gain media  150  or  152 . One example of a high-gain Raman fiber is a Raman fiber with a smaller core diameter and/or higher Raman gain coefficient than either Raman gain media  150  or  152 . Fiber  146  may comprise a combination of one or more sections of high-gain Raman fiber and one of more rare earth-doped fibers. In another embodiment of  FIG. 3A , fiber  146  may comprise a fiber-coupled semiconductor amplifier. In another embodiment of  FIG. 3A , fiber  146  may comprise an optical parametric fiber amplifier. 
       FIGS. 3B to 3F  are block diagrams of example embodiments of efficient discrete optical amplifier assemblies that implement optical circulators  160 . Amplifier assemblies  422 ,  522 ,  622 ,  722  and  822  are similar in function to amplifier  322  of  FIG. 3A . In  FIGS. 3A to 3F , like numerals denote like components. 
       FIG. 3B  is a block diagram of an example embodiment of efficient discrete optical amplifier assembly  422  that implements an optical circulator  160 . Amplifier assembly  422  comprises a second reflector, reflector  140 B, and a second multiplexer  142 B. When one or more pumps have traveled through Raman gain medium  152  in the forward direction with respect to signal  116 , the one or more pumps may then travel through multiplexer  142 B and reflector  140 B and be reflected back towards Raman gain medium  152 . In another embodiment, circulator  160  is a 4-port optical circulator used to dump the residual pump power to a power dump  145  traveling in the backward direction with respect to signal  116  into port  3  of circulator  160 . 
       FIG. 3C  is a block diagram of an example embodiment of efficient discrete optical amplifier assembly  522  that implement an optical circulator  160 . In  FIG. 3C , a grating, such as fiber Bragg grating  158 A, is used as a pump reflector instead of reflector  140 , as in  FIG. 3A , for example. Grating  158 A can be used to reflect one or more pump wavelengths in amplifier assembly  522 . Grating  158 A can comprise, as one example, a grating such as TERAXION® C52465-0001, which reflects a wavelength band with a center wavelength of approximately 1471 nm. Amplifier assembly  522  in  FIG. 3C  may also comprise, in some embodiments, a grating  158 B for use as a pump reflector. Grating  158 B can operate to reflect one or more pump wavelengths back towards Raman gain medium  152 . 
       FIG. 3D  is a block diagram of an example embodiment of efficient discrete optical amplifier assembly  622  that implement an optical circulator  160 . In this embodiment, doped fiber  146  is connected to Raman gain medium  152 , instead of Raman gain medium  150 , as in some other described embodiments. In  FIG. 3D , one or more pumps from pump unit  144  travel a path similar to that described with respect to  FIG. 3A . The pumps are reflected off of reflector  140 , and then travel through Raman gain medium  150 , circulator  160 , and Raman gain medium  152 . One or more pumps may then travel through doped fiber  146 . 
       FIG. 3E  is a block diagram of an example embodiment of efficient discrete optical amplifier assembly  722  that implement an optical circulator  160 . This embodiment is similar in operation to  FIG. 3D , but highly efficient gain fiber  146  is connected between circulator  160  and Raman gain medium  152 . In other embodiments, one or more Raman gain mediums may reside on either or both sides of highly efficient gain fiber  146 . In certain other embodiments, highly efficient gain fiber  146  may be positioned between fiber  150  and circulator  160 . 
       FIG. 3F  is a block diagram of an example embodiment of efficient discrete optical amplifier assembly  822  that implement an optical circulator  160 . This embodiment lacks a doped fiber  146 . One or more pumps travel a similar path as described above with respect to  FIG. 3A . One or more pumps from pump unit  144  travel through circulator  160 , Raman gain medium  150 , and multiplexer  142  and are reflected off of reflector  140 . Pumps can then travel back through multiplexer  142 , Raman gain medium  150 , circulator  160 , and Raman gain medium  152 . 
       FIG. 4A  is a block diagram of an example embodiment of an efficient discrete optical amplifier assembly  922  that implements an optical circulator  160  and a dispersion compensation unit  162 . Amplifier assembly  922  is similar in function to amplifier  322  of  FIG. 3A . In  FIGS. 3A ,  4 A and  4 B, like numerals denote like components. In  FIG. 4A , one or more pumps travel a similar path as described above with respect to  FIG. 3A . One or more pumps from pump unit  144  travel through circulator  160 , Raman gain medium  150 , doped fiber  146 , and multiplexer  142  and are reflected back by reflector  140 . Pumps can then travel back through multiplexer  142 , doped fiber  146 , Raman gain medium  150 , and circulator  160 . At this point, residual pump power can be sent to dispersion compensation unit (“DCU”)  162 . The residual pump power can be used to pump the DCU, lowering the insertion loss by means of Raman amplification. The residual pump power in the DCU may also act to amplify one or more signals traversing the communication fiber. 
     In various embodiments, the DCU may at least partially compensate for chromatic dispersion associated with one or more signals. In some embodiments, the dispersion compensating element can comprise a dispersion length product that approximately compensates for the dispersion accumulated by one or more optical signals while traversing a communication fiber. In some embodiments, a dispersion compensating fiber can comprise a slope of dispersion that is equal to and opposite from the slope of chromatic dispersion associated with the one or more signals traversing a communication fiber. In this particular embodiment, pump  141  comprises a pump power of approximately 230 mw at a wavelength of 1442 nm and 373 mW at a wavelength of 1468 nm to support 80 km (loss of 21 dB) standard single mode transmission fiber (SSMF). 
     In some embodiments, assembly  922  may comprise 3.7 meter rare-earth-doped fiber (OFS, R37003X)  146 , 3.9 km LLMicroDK in first Raman fiber  150 , 3.9 km LLMicroDK in the DCU  162 , a pump of 514 mW at 1440 nm and 210 mW at 1471 nm to support 80 km, 21 dB SSMF span. 
       FIG. 4B  is a block diagram of an example embodiment of efficient discrete optical amplifier assembly  1022  that implement an optical circulator  160  and a dispersion compensation unit  162 .  FIG. 4B  operates similarly to  FIG. 4A .  FIG. 4B  comprises a fiber Bragg grating  158 , used as a pump reflector instead of reflector  140 , as in  FIG. 4A , for example. Grating  158  can be used to reflect one or more pump wavelengths in amplifier assembly  1022 . Also, in certain embodiments, amplifier assembly  1022  may comprise one or more doped fibers  146 , such as doped fibers  146 A and  146 B. In certain embodiments, doped fiber  146 A may comprise 3.5 m erbium-doped fiber R37003X, and doped fiber  146 B may comprise 0.7 m of erbium-doped fiber R37105XL. 
       FIG. 5  is a block diagram of one example embodiment of a conventional Raman discrete amplifier. In certain embodiments, more than one segment of Raman gain fiber is used to reduce multi-path interference. Gain fiber  150  is pumped by pump source  148  and gain fiber  152  is pumped by pump source  144 . Residual pump power is dumped into terminators  170   a  and/or  170   b.    
       FIG. 6  shows a table of efficiency and performance metrics of the example efficient amplifiers of  FIGS. 2A ,  3 A,  4 A and the conventional amplifier of  FIG. 5  under similar operating conditions (80 km SSMF transmission fiber of 21 dB loss with optical signal of 60 channels). It should be noted that (1) the total pump power required for the efficient amplifiers of  FIGS. 2A ,  3 A, and  4 A is significantly lower than that of the conventional amplifier of  FIG. 5 , (2) the residual pump power that is unused (terminated) is less in the efficient amplifiers compared to the conventional amplifiers, (3) the pump usage efficiency is higher in the efficient amplifiers compared to the conventional amplifiers, and (4) the noise figure (NF) is lower in the efficient amplifiers compared to the conventional amplifiers. 
     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.