Abstract:
A method for amplifying optical signals includes determining a source optical signal, generating a first resultant signal including a pump signal and the source optical signal, sending the first resultant signal through a non-linear element to generate a second resultant signal including the first resultant signal and an idler signal, and sending the second resultant signal through a non-linear element to perform phase-sensitive amplification. The phase-sensitive amplification results in a third resultant signal including an amplified source optical signal, the pump signal, and the idler signal. The method also includes filtering the third resultant signal to remove the pump signal and the idler signal and outputting the amplified source optical signal.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to optical communication networks and, more particularly, to optical phase-sensitive amplifier for dual-polarization modulation formats. 
     BACKGROUND 
     Telecommunications systems, cable television systems and data communication networks may use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information may be conveyed in the form of optical signals through optical fibers. Optical fibers may comprise thin strands of glass capable of communicating the signals over long distances with very low loss. Optical networks often employ modulation schemes to convey information in the optical signals over the optical fibers. Such modulation schemes may include phase-shift keying (“PSK”), frequency-shift keying (“FSK”), amplitude-shift keying (“ASK”), and quadrature amplitude modulation (“QAM”). 
     In PSK, the information carried by the optical signal may be conveyed by modulating the phase of a reference signal, also known as a carrier wave. The information may be conveyed by modulating the phase of the signal itself using differential phase-shift keying (“DPSK”). 
     In QAM, the information carried by the optical signal may be conveyed by modulating both the amplitude and phase of the carrier wave. PSK may be considered a subset of QAM, wherein the amplitude of the carrier waves is maintained as a constant. 
     PSK and QAM signals may be represented using a complex plane with real and imaginary axes on a constellation diagram. The points on the constellation diagram representing symbols carrying information may be positioned with uniform angular spacing around the origin of the diagram. The number of symbols to be modulated using PSK and QAM may be increased and thus increase the information that can be carried. The number of signals may be given in multiples of two. As additional symbols are added, they may be arranged in uniform fashion around the origin. PSK signals may include such an arrangement in a circle on the constellation diagram, meaning that PSK signals have constant power for all symbols. QAM signals may have the same angular arrangement as that of PSK signals, but include different amplitude arrangements. QAM signals may have their symbols arranged around multiple circles, meaning that the QAM signals include different power for different symbols. This arrangement may decrease the risk of noise as the symbols are separated by as much distance as possible. A number of symbols “m” may thus be used and denoted “m-PSK” or “m-QAM.” 
     Examples of PSK and QAM with a different number of symbols can include binary PSK (“BPSK” or “2-PSK”) using two phases at 0° and 180° (or 0 and π) on the constellation diagram; or quadrature PSK (“QPSK”, “4-PSK”, or “4-QAM”) using four phases at 0°, 90°, 180°, and 270° (or 0, π/2, π, and 3π/2). Phases in such signals may be offset. Each of 2-PSK and 4-PSK signals may be arranged on the constellation diagram. 
     M-PSK signals may also be polarized using techniques such as dual-polarization QPSK (“DP-QPSK”), wherein separate m-PSK signals are multiplexed by orthogonally polarizing the signals. M-QAM signals may also be polarized using techniques such as dual-polarization 16-QAM (“DP-16-QAM”), wherein separate m-QAM signals are multiplexed by orthogonally polarizing the signals. 
     SUMMARY 
     In one embodiment, a method for amplifying optical signals includes determining a source optical signal, generating a first resultant signal including a pump signal and the source optical signal, sending the first resultant signal through a non-linear element to generate a second resultant signal including the first resultant signal and an idler signal, and sending the second resultant signal through a non-linear element to perform phase-sensitive amplification. The phase-sensitive amplification results in a third resultant signal including an amplified source optical signal. The method also includes filtering the third resultant signal to remove the pump signal and the idler signal and outputting the amplified source optical signal. 
     In another embodiment, a system for amplifying optical signals includes an input configured to accept a source optical signal, a pump source configured to generate a pump signal, a coupler configured to add the pump signal to the source optical signal to yield a first resultant signal, a first controller, a second controller, and a filter. The first controller is configured to split the first resultant signal into an x-polarization component and a y-polarization component and send the x-polarization component of the first resultant signal and the y-polarization component of the first resultant signal bi-directionally through a first non-linear element to generate an x-polarization component and a y-polarization component of a second resultant signal including the first resultant signal and an idler signal. The second controller is configured to send the x-polarization component of the second resultant signal and the y-polarization component of the second resultant signal bi-directionally through a second non-linear element to perform phase-sensitive amplification. The phase-sensitive amplification results in a third resultant signal including an amplified source optical signal. The filter is configured to filter the third resultant signal and output the amplified source optical signal. 
     In yet another embodiment, a system for amplifying optical signals includes an input configured to accept a source optical signal, a pump source configured to generate a pump signal, a coupler configured to add the pump signal to the source optical signal to yield a first resultant signal, a controller, and a filter. The controller is configured to split the first resultant signal into an x-polarization component and a y-polarization component, and route the x-polarization component of the first resultant signal bi-directionally through a first non-linear element to yield an x-polarization component of a second resultant signal. The first non-linear element is configured to perform phase-sensitive amplification on the x-polarization component of the first resultant signal. The controller is also configured to route the y-polarization component of the first resultant signal bi-directionally through a second non-linear element to yield a y-polarization component of the second resultant signal. The second non-linear element is configured to perform phase-sensitive amplification on the y-polarization component of the first resultant signal. The controller is also configured to combine the x-polarization component of the second resultant signal and the y-polarization component of the second resultant signal to yield the second resultant signal. The filter is configured to filter the second resultant signal and output an amplified source optical signal. 
     In still yet another embodiment, a system for amplifying optical signals includes an input configured to accept a source optical signal, a pump source configured to generate a pump signal, a coupler configured to add the pump signal to the source optical signal to yield a first resultant signal, a polarization-maintaining non-linear element communicatively coupled to the coupler, and a filter and configured to filter an amplified signal and output an amplified source optical signal. The polarization-maintaining non-linear element is configured to add an idler signal to the first resultant signal as the first resultant signal passes through the polarization-maintaining non-linear element in a first direction that yields a second resultant signal, sends the second resultant signal to be rerouted to the polarization-maintaining non-linear element, receives the second resultant signal, and performs phase-sensitive amplification on the second resultant signal as the second resultant signal passes through the polarization-maintaining non-linear element in a second direction, yielding the amplified signal. The first direction and second direction are opposites. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates an example embodiment of a system configured to provide optical phase-sensitive amplification for dual-polarization modulation formats; 
         FIG. 2  is an illustration of an example embodiment of a system with an optical amplifier for conducting optical phase-sensitive amplification; 
         FIGS. 3A and 3B  are an illustration of the operation of an example embodiment of an optical amplifier for conducting optical phase-sensitive amplification on a single channel; 
         FIGS. 4A and 4B  are an illustration of the operation of an example embodiment of an optical amplifier for conducting optical phase-sensitive amplification on wavelength division multiplexing (WDM) channels; 
         FIG. 5  is an illustration of an another example embodiment of system with an optical amplifier for conducting optical phase-sensitive amplification; 
         FIG. 6  is an illustration of the operation of another example embodiment of an optical amplifier for conducting optical phase-sensitive amplification on a single channel; 
         FIG. 7  is an illustration of the operation of another example embodiment of an optical amplifier for conducting optical phase-sensitive amplification on WDM channels; 
         FIG. 8  is an illustration of a yet another example embodiment of an optical amplifier for conducting optical phase-sensitive amplification; 
         FIG. 9  is an illustration of the operation of yet another example embodiment of an optical amplifier for conducting optical phase-sensitive amplification on a single channel; 
         FIG. 10  is an illustration of the operation of yet another example embodiment of an optical amplifier for conducting optical phase-sensitive amplification on WDM channels; and 
         FIG. 11  is an example embodiment of a method for optical phase-sensitive amplification. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example embodiment of a system  100  configured to provide optical phase-sensitive amplification for dual-polarization modulation formats. In one embodiment, system  100  may include components with a wavelength selective processor to conduct optical signal amplification. Such implication may be performed on any suitable signal such as a QPSK signal. In a further embodiment, such wavelength selective processors may be reconfigurable. In another embodiment, system  100  may include bi-directional phase-sensitive amplification to conduct optical phase-sensitive amplification for dual-polarization modulation formats. In a further embodiment, such phase-sensitive amplification may be degenerate. One or more optical amplifiers, such as optical amplifier  102 , may conduct the optical phase-sensitive amplification for dual-polarization modulation formats. 
     Optical amplifier  102  may be configured to amplify optical signals in system  100 . System  100  may include an input signal  110  to be amplified as output signal  114  by optical amplifier  102 . Signals may be transmitted in system  100  over an optical network  108 , which may include one or more optical fibers  112  of any suitable type. System  100  may include optical amplifier  102  in any suitable portion of system  100  or an optical network, such as in a transmission line between two optical components or in a reconfigurable optical add-drop multiplexer (“ROADM”). Furthermore, optical amplifier  102  may be configured to operate as a stand-alone device or as part of another piece of optical transmission equipment. Optical amplifier  102  may be placed a distance d from a subsequent piece of optical equipment. 
     Optical amplifier  102  may include any suitable number and kind of components configured to perform optical signal amplification as described herein. Example implementations of all or part of optical amplifier  102  may include amplifiers  200 ,  500 , and  800  as shown in  FIGS. 2 ,  5 , and  8 , respectively, and as shown in operation in  FIGS. 3-4 ,  6 - 7 , and  9 - 10 , respectively. Optical amplifier  102  may include a processor  104  coupled to a memory  106 . In one embodiment, to perform optical signal amplification, optical amplifier  102  may include components for configuring optical amplifier  102  to monitor, adjust, and pre-compensate input signals and other system characteristics such as pump signal to adjust signal information such as phase, power and chromatic dispersion. In another embodiment, to perform optical signal amplification, optical amplifier  102  may include components for performing one-pump optical four-wave mixing. In a further embodiment, such four-wave mixing may be accomplished by passing the input signal, or filtered portions thereof, bi-directionally through a non-linear optical element. In yet another further embodiment, passing such signals bi-directionally may include separately and simultaneously processing the input signal&#39;s polarization components in each such direction by passing an x-polarization component signal in a given direction through the non-linear optical element and a y-polarization component signal in the opposite direction through the non-linear element. 
     Optical amplifier  102  may be configured to utilize two optical processing stages. In a first stage, optical amplifier  102  may be configured to generate a conjugate signal of input signal  110 . In a second stage, optical amplifier  102  may be configured to conduct phase-sensitive non-degenerate four-wave mixing (“FWM”). Such FWM may transfer the energy from the pump signal to the input signal  110  and to its conjugate idler signals. 
     Specifically, optical amplifier  102  may generate pump laser signals, which may be used to create idler signals that are then added to the input signal. The idler signals may include conjugate signals to input signal  110 . The resulting input signal and idler signals might not become degenerate after wave mixing, and thus the amplification may include non-degenerate phase-sensitive amplification. Optical amplifier  102  may be configured to conduct FWM that amplifies input signal  110  and based on the symmetric idler signals. The wavelengths of input signal  110  and idler signals may be equidistant (or nearly equidistant) from the wavelength of the pump signal. The equidistant or nearly equidistant wavelengths may include wavelengths that are, for example, perfectly equidistant or approximately equidistant such that overall performance may not be impacted significantly. Such approximately equidistant wavelengths may include wavelength differences between the idler signals and pump signal that are approximately equal, or wavelength differences between the pump signal and input signal  110  that are approximately equal. In one embodiment, approximately equal wavelength differences may include wavelength differences that vary less than ten percent between the wavelength differences. Idler signals may include a phase that may be the inverse of the phase of input signal  110 . 
     Input signal  110  may include an optical signal modulated through any suitable method, such as m-QAM or m-PSK. Input signal  110  may include dual-polarization components. Optical amplifier  102  may be configured to accept dual-polarization signals in any suitable manner. Optical amplifier  102  may be configured to split input signal  110  into x-polarization and y-polarization components. Such split components may be processed independently. In one embodiment, a single non-linear element may be used for bi-directional signal conversion of the x-polarization and y-polarization components. In another embodiment, a single non-linear element may be used for bi-directional non-degenerate FWM for phase-sensitive amplification of the x-polarization and y-polarization components. In yet another embodiment, the x-polarization and y-polarization components may share the elements of the first and second stages, wherein crosstalk and path mismatch are avoided between the two polarizations. 
     Optical amplifier  102  may include performance monitoring and wavelength selective processors to dynamically control the operation of optical amplifier  102 . Information regarding input signal  110 , such as wavelength, power, residual chromatic dispersion, and optical signal-to-noise ratio (“OSNR”) may be monitored. Furthermore, information regarding the operation and output of the components of optical amplifier  102  may be monitored. According to monitored information, phase and power levels of various portions of optical amplifier  102  may be dynamically changed, such as the phase and power levels of the input signal, pump signal, and conjugate signals. 
     Optical amplifier  102  may be configured to accept wavelength division multiplexing (“WDM”) signals. The first stage of optical amplifier  102  may be configured to generate idler signals for each WDM component of input signal  110 . Furthermore, the second stage of optical amplifier  102  may be configured to perform FWM for each pair of signals within input signal  110  and its idler signal counterpart generated from the first stage. When WDM signals are used in optical amplifier, each idler signal may be equidistant (or nearly equidistant) from the pump signal as its input signal  110  counterpart. 
     Processor  104  may comprise, for example a microprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), or any other digital or analog circuitry configured to interpret and/or execute program instructions and/or process data. In some embodiments, processor  104  may interpret and/or execute program instructions and/or process data stored in memory  106  to carry out some or all of the operation of optical amplifier  102 . Memory  106  may be configured in part or whole as application memory, system memory, or both. Memory  106  may include any system, device, or apparatus configured to hold and/or house one or more memory modules. Each memory module may include any system, device or apparatus configured to retain program instructions and/or data for a period of time (e.g., computer-readable media). Memory  106  may be non-transitory. One or more portions or functionality of optical amplifier  102  may be implemented by the execution of instructions resident within memory  106  by processor  104 . 
     Optical network  108  may include one or more optical fibers  112  operable to transport one or more optical signals communicated by components of the optical network  108 . Optical network  108  may be, for example, a point-to-point optical network with terminal nodes, a ring optical network, a mesh optical network, or any other suitable optical network or combination of optical networks. Optical network  108  may be used in a short-haul metropolitan network, a long-haul inter-city network, or any other suitable network or combination of networks. The capacity of optical network  108  may include, for example, 100 Gbit/s, 400 Gbit/s, or 1 Tbit/s. Optical fibers  112  may include any suitable type of fiber, such as a Single-Mode Fiber (“SMF”), Enhanced Large Effective Area Fiber (“E-LEAF”), or TrueWave® Reduced Slope (“TW-RS”) fiber. Optical network  108  may include devices, such as optical amplifier  102 , operable to transmit optical signals over optical fibers  112 . Information may be transmitted and received through optical network  108  by modulation of one or more wavelengths of light to encode the information on the wavelength. 
     In operation, optical amplifier  102  may be operating on optical network  108 . Input signal  110  may arrive on optical network  108  through fibers  112 . Optical amplifier may amplify input signal  110  and output the result as output signal  114 . 
       FIG. 2  is an illustration of an example embodiment of an optical amplifier  200  for conducting optical phase-sensitive amplification. In one embodiment, optical amplifier  200  may be configured to support dual-polarization modulation formats. Optical amplifier  200  may implement fully or in part optical amplifier  102  of  FIG. 1 . 
     Optical amplifier  200  may include a mechanism for accepting an input signal such as input signal  202 . Input signal  202  may include a plurality of WDM channels. Each such channel may correspond to a different wavelength. Furthermore, each such channel may correspond to a different modulation format. Such a wavelength may be denoted by α i . For each such channel, input signal  202  may include an x-polarization and a y-polarization component. Input signal  202  may implement input signal  110  of  FIG. 1 . Input signal  202  may be communicatively coupled to wavelength selective switch  206 . Wavelength selective switch  206  may be configured to select what portions of input signal  202  are to be amplified with optical amplifier  200 . Wavelength selective switch  206  may thus be configured to select the desired channels of input signal  202  to be processed on, for example, a per-wavelength basis. Such switching may be performed to select what portions of input signal  202  that are to be amplified. Wavelength selective switch  206  may be implemented in any suitable manner, such as by active or passive configurable filters, array waveguides, electromechanical devices, or crystals. Wavelength selective switch  206  may be communicatively coupled to control module  238 . Control module  238  may be configured to adjust the operation of wavelength selective switch  206  to, for example, select what portion of input signal  202  is to be amplified by optical amplifier  200 . Such adjustments may be based upon, for example, the nature or kind of input signal  202 , detected output of optical amplifier  200 , or detected output of wavelength selective switch  206 . 
     Wavelength selective switch  206  may be communicatively coupled to polarization-mode dispersion compensator  208 . Polarization-mode dispersion compensator  208  may be configured to compensate for residual polarization-mode dispersion in input signal  202 . Polarization-mode dispersion compensator  208  may be implemented in any suitable manner, such as a module, optical device, or electronic device. Polarization-mode dispersion compensator  208  may be communicatively coupled to control module  238 . Control module  238  may be configured to adjust the operation of polarization-mode dispersion compensator. Such adjustments may be based upon, for example, the nature or kind of input signal  202 , detected output of optical amplifier  200 , or detected output of polarization-mode dispersion compensator  208 . 
     Optical amplifier  200  may include a first stage configured to perform signal conversion to generate a conjugate signal of input signal  202 . In addition, optical amplifier  200  may include a second stage configured to conduct phase-sensitive non-degenerate FWM. Such non-degenerate FWM may be performed upon input signal  202  and the conjugate signal. 
     The first stage of optical amplifier  200  configured to generate a conjugate signal of input signal  202  may include a pump  210 , coupler  212 , optical circulator  222 , polarization controller  214 , polarization beam controller  216 , polarization controller  218 , and non-linear element  220 . 
     Pump  210  may be configured to provide a pump signal with a wavelength and strength that may be set in relation to the wavelength of input signal  202  that is to be amplified. The output of polarization-mode dispersion compensator  208  may be communicatively coupled to coupler  212 , along with the output of pump  210 . Pump  210  may be implemented in any suitable manner for providing an appropriate pump signal in optical amplifier  200 , such as by a configurable laser source. Pump  210  may be communicatively coupled to control module  238 . Control module  238  may be configured to adjust the wavelength, power, phase, or other aspects of the operation of pump  210 . Such adjustments may be based upon, for example, the nature or kind of input signal  202 , detected output of pump  210 , or detected output of optical amplifier  200 . 
     Coupler  212  may be configured to couple the output of polarization-mode dispersion compensator  208  (including input signal  202  compensated for polarization-mode dispersion) and the output of pump  210 . Coupler  212  may be communicatively coupled to optical circulator  222  and configured to provide its output thereto. Coupler  212  may be implemented in any suitable manner for coupling the inputs as described. 
     Optical circulator  222  may be communicatively coupled on a first input/output line to polarization controller  214  and on a second input/output line to a second stage of optical amplifier  200 , which may include a wavelength selective processor  224 . Optical circulator  222  may be communicatively coupled on an input line to coupler  212  and configured to receive its output. Optical circulator  222  may include any suitable mechanism for selective routing of inputs and outputs according to the present disclosure. For example, optical circulator  222  may include a plurality of sequentially identified optical input-output ports and may allow light to travel in only one direction. An optical signal entered into a first port will exit the second port, while a signal entering the second port will exit the third port. The sequential identification of the first, second, and third port, and thus the input-output behavior, may be schematically identified with a clockwise or counter-clockwise indicator. In the example of  FIG. 2 , optical circulator  222  may operate in clockwise fashion such that the input from coupler  212  is output to polarization controller  214 , and input from polarization controller  214  may be output to wavelength selective processor  224 . 
     Polarization controllers  214 ,  218  may be configured to adjust the x-polarization and y-polarization components of its input signals with respect to polarization beam controller  216  to maximize or increase the effects of conjugate signal generation as performed by, for example, non-linear element  220 . Such adjustments may include a polarization shifting of the x-polarization and y-polarization components. Furthermore, polarization controllers  214 ,  218  may be configured to adjust such components after a conjugate signal has been generated for the components. Polarization controllers  214 ,  218  may be implemented in any suitable manner to perform such adjustments. Polarization controllers  214 ,  218  may be communicatively coupled to control module  238 . Control module  238  may be configured to adjust the operation of polarization controllers  214 ,  218 . Such adjustments may be based upon, for example, the nature or kind of input signal  202 , detected output of polarization controllers  214 ,  218 , or detected output of optical amplifier  200 . 
     Polarization controller  214  may be configured to receive its input from optical circulator  222 , perform adjustments on the x-polarization and y-polarization components if necessary, and output the results to polarization beam controller  216 . 
     Polarization beam controller  216  may be configured to split an input signal according to x-polarization and y-polarization components, and to combine x-polarization and y-polarization components that were previously split. For example, input signal  202  may include an x-polarization component and a y-polarization component. Thus, polarization beam controller  216  may be configured to output the x-polarization component of the combination of input signal  202  and the pump signal and to output the y-polarization component of the combination input signal  202  and the pump signal. Polarization beam controller  216  may be configured to output each polarization bi-directionally to the same non-linear element  220  for signal conversion. For example, the x-polarization component may be provided to non-linear element  220  in the clockwise circuit loop in  FIG. 2  and the y-polarization component may be provided to non-linear element  220  in the counter-clockwise circuit loop in  FIG. 2 . Polarization beam controller  216  may be implemented in any suitable manner for splitting its input signals into x-polarization and y-polarization components. Polarization beam controller  216  may be communicatively coupled to control module  238 . Control module  238  may be configured to adjust the operation of polarization beam controller  216 . Such adjustments may be based upon, for example, the nature or kind of input signal  202 , detected output of optical amplifier  200 , or detected output of polarization beam controller  216 . 
     Polarization controller  218  may be configured to receive its input from polarization beam controller  214  or non-linear element  220  and, if necessary, adjust the polarization components of its inputs and output the result to non-linear element  220  or polarization beam controller  214 , respectively. Further, polarization beam controller  216  may be configured to combine the x-polarization and y-polarization components as they are received after passing bi-directionally through non-linear element  220 . Polarization beam controller  216  may receive the x-polarization input from polarization controller  218 . Furthermore, polarization beam controller  216  may output the combination of the x-polarization and y-polarization to polarization controller  214  and on to optical circulator  222 . 
     Non-linear element  220  may be configured to bi-directionally provide signal conversion for signals passing through either end of non-linear element  218 . Non-linear element  220  may include an optical non-linear element. Such signal conversion may be performed on signals passing simultaneously through optical non-linear element  220  in each direction, such as x-polarization and y-polarization components from polarization beam controller  216 . In one embodiment, any non-linear element that can support bi-directional propagation and non-linear processing may be used to implement non-linear element  220 . For example, non-linear element  220  may include an optical, highly non-linear fiber (“HNLF”) of length of two hundred meters, non-linear coefficient (γ=9.2 (1/W·km)), dispersion slope (S=0.018 ps/km/nm 2 ), and zero-dispersion wavelength (“ZDW”) at 1550 nm. In another example, non-linear element  220  may include waveguides configured to produce the desired output. In yet other examples, non-linear element  220  may include a silicon waveguide, III-V waveguide, or periodically poled Lithium Niobate (“PPLN”). 
     Non-linear element  220  may be configured to provide signal conversion based upon the nature of its input signals, which may include the combination of input signal  202  and pump signal from pump  210 . In one embodiment, non-linear element  220  may be configured to cause an idler signal to be added to the combination of input signal  202  and the pump signal. In a further embodiment, the idler signal and input signal  202  may be equidistant, or nearly equidistant, from the pump signal. Thus, the idler signal and input signal  202  may be symmetrically, or nearly symmetrically, located on each side of the pump signal. In another embodiment, the generated idler signal may include the inverse phase as input signal  202 . If input signal  202  includes multiple WDM components, non-linear element may generate an idler signal for each such WDM component. Each idler signal and the corresponding WDM component may be equidistant, or nearly equidistant, from the pump signal in terms of wavelength. Thus, the WDM components of input signal  202  and the corresponding idler signals may be symmetric, or nearly symmetric, around the pump signal. 
     By performing separate processing of x-polarization and y-polarization components, optical amplifier  200  may avoid crosstalk or path mismatch between the components. Further, by performing the processing bi-directionally, optical amplifier  200  may achieve hardware efficiency by lessening the need for additional optical non-linear elements. 
     The second stage of optical amplifier  200  configured to perform FWM on the combination of input signal  202 , pump signal, and idler signal may include a wavelength selective processor  224 , optical circulator  226 , polarization controllers  228 ,  232 , polarization beam controller  230 , and non-linear element  234 . 
     Wavelength selective processor  224  may be configured to receive a signal from the first stage of optical amplifier  200 . Such reception may be made from, for example, optical circulator  222 . The received signal may include a combination of input signal  202 , pump signal, and idler signals. Wavelength selective processor  224  may be configured to select which portions of the received signal are to be amplified using FWM. Such selection may be made based on, for example, wavelength. For example, generation of idler signals in conjunction with non-linear element  220  may have caused unnecessary or unused idler signals for the purposes of amplification. Thus, wavelength selective processor  224  may be configured to filter out these unused idler signals. Wavelength selective processor  224  may be implemented in any suitable manner to perform optical switching according to the present disclosure. For example, wavelength selective processor  224  may include one or more wavelength selective switches implemented by any suitable mechanism, including optical components. Furthermore, wavelength selective processor  224  may include modules, circuitry, or software configured to adjust phase and power levels of components of signals. For example, the phases of input signal  202 , the pump signal, and the idler signal may be adjusted to facilitate FWM. In addition, wavelength selective processor  224  may include automation software configured to control the operation of wavelength selective switches. Any suitable automation software may be used. The automation software may include instructions resident upon a computer-readable medium for execution by a processor. Wavelength selective processor  224  may include a microprocessor, microcontroller, DSP, ASIC, or any other digital or analog circuitry for executing the instructions resident upon a computer-readable medium or for otherwise performing control of wavelength selective switches. Wavelength selective processor  224  may be communicatively coupled to control module  238 . Control module  238  may be configured to adjust the operation of wavelength selective processor  224 , such as adjustment of power or phases of signals received by wavelength selective processor  224 , or adjustment of signals that will be filtered by wavelength selective processor  224 . Furthermore, wavelength selective processor  224  may be adjusted to pre-compensate its input signals for the input signals&#39; residual chromatic dispersion or for dispersion slope of HNLF resident within non-linear elements. Such adjustments may be based upon, for example, the nature or kind of input signal  202 , detected output of wavelength selective processor  224 , or detected output of optical amplifier  200 . 
     If input signal  202  includes WDM signals, wavelength selective processor  224  may be configured to select a range including the WDM signals to be amplified, the pump signal, and the range of idler signals corresponding to each of the WDM signals. 
     Wavelength selective processor  224  may be configured to output its results to optical circulator  226 . Optical circulator  226  may be implemented in similar fashion as optical circulator  222 , as discussed above. Optical circulator  226  may be configured to route input from wavelength selective processor  224  to polarization controller  228 . Furthermore, optical circular  226  may be configured to route input polarization controller  228  to filter  236 . 
     Polarization controllers  228 ,  232  may be implemented in similar fashion to polarization controllers  214 ,  218  as described above. Polarization controllers  228 ,  232  may be communicatively coupled to control module  238 . Control module  238  may be configured to adjust the operation of polarization controllers  228 ,  232 . Such adjustments may be based upon, for example, the nature or kind of input signal  202 , detected output of polarization controllers  228 ,  232 , or detected output of optical amplifier  200 . 
     Polarization controller  228  may be communicatively coupled to optical circulator  226  and polarization beam controller  230 . Polarization controller  228  may adjust x-polarization or y-polarization components in signals received from optical circulator  226  and send the result to polarization beam controller  230 . Furthermore, polarization controller  228  may adjust x-polarization or y-polarization components in signals received from polarization beam controller  230  and send the result to optical circulator  226 . 
     Polarization beam controller  230  may be implemented in similar fashion to polarization beam controller  216 , as described above. Polarization beam controller  230  may be communicatively coupled to two ends of non-linear element  234 . Polarization beam controller  230  may be configured to output the x-polarization of the output of wavelength selective processor  224  (which may be processed by polarization controller  228 ) to one end of non-linear element  234  and output the y-polarization of the output of wavelength selective processor  224  to another end of non-linear element  234 . Thus, polarization beam controller  230  may be configured to output each polarization bi-directionally to the same non-linear element  234  for signal amplification. Polarization beam controller  230  may be communicatively coupled to control module  238 . Control module  238  may be configured to adjust the operation of polarization beam controller  230 . Such adjustments may be based upon, for example, the nature or kind of input signal  202 , detected output of optical amplifier  200 , or detected output of polarization beam controller  230 . 
     Polarization controller  232  may be configured to receive its input from polarization beam controller  230  or non-linear element  234  and, if necessary, adjust the polarization components of its inputs and output the result to non-linear element  234  or polarization beam controller  230 , respectively. Further, polarization beam controller  230  may be configured to combine the x-polarization and y-polarization components as they are received after passing bi-directionally through non-linear element  234 . Polarization beam controller  230  may receive the x-polarization input from polarization controller  232 . Furthermore, polarization beam controller  230  may output the combination of the x-polarization and y-polarization to polarization controller  228  and on to optical circulator  232 . 
     Non-linear element  234  may be configured to bi-directionally amplify and amplify signals passing through either end of non-linear element  234  using FWM. Such signals may include both input signal  202  and its idler signals. In one embodiment, non-linear element  234  may perform non-degenerate FWM. Such bi-directional amplification may be performed on signals passing simultaneously through non-linear element  234  in each direction. In one embodiment, any non-linear element that can support bi-directional propagation and non-linear processing may be used to implement non-linear element  234 . For example, non-linear element  234  may include an optical, highly non-linear fiber (“HNLF”) of length of two hundred meters, non-linear coefficient (γ=9.2 (1/W·km)), dispersion slope (S=0.018 ps/km/nm 2 ), and zero-dispersion wavelength (“ZDW”) at 1550 nm. In another example, non-linear element  234  may include waveguides configured to produce the desired output. In yet other examples, non-linear element  234  may include a silicon waveguide, III-V waveguide, or periodically poled Lithium Niobate (“PPLN”). 
     The combination of polarization beam controller  230  and non-linear element  234  may be bi-directional in that signals pass from polarization beam controller  230  to non-linear element  234  and back to polarization beam controller  230  in both directions (clockwise and counter-clockwise). By performing separate processing of x-polarization and y-polarization components, optical amplifier  200  may avoid crosstalk or path mismatch between the components. Further, by performing the processing bi-directionally, optical amplifier  200  may achieve hardware efficiency by lessening the need for additional non-linear elements. 
     The FWM performed by non-linear element  234  may utilize the equidistant, or nearly equidistant, arrangement of input signal  202  and its idler signals around the pump signal. Furthermore, the FWM performed by non-linear element  234  may utilize the performance of the idler signals as conjugate signals to input signal  202 . 
     If input signal  202  includes WDM signals, non-linear element  234  may amplify the range of WDM signals and the range of the idler signals corresponding to the WDM signals. 
     Optical amplifier  200  may include a filter  236  configured to remove the idler signals and pump signal from the result of FWM. Filter  236  may include a bandpass filter. Filter  236  may be communicatively coupled to optical circulator  226 . Furthermore, filter  236  may be configured to only allow signals with the wavelength of the original input signal (input signal  202 ) to pass. Filter  236  may be implemented in any suitable manner, such as with digital or analog circuitry. Filter  236  may be configured to generate output signal  204 . The result of optical amplification in output signal  204  may implement output signal  114  of  FIG. 1 . 
     Filter  236  may be communicatively coupled to control module  238 . Control module  238  may be configured to adjust the operation of filter  236 . Such adjustments may be based upon, for example, the nature or kind of input signal  202 , detected output of optical amplifier  200 , or detected output of filter  236 . 
     Control module  238  may be configured to monitor performance of optical amplifier  200  and its signals. Such monitoring may be conducted in real-time and may include, for example, information regarding input signal  202 , output signal  204 , wavelength selective switch  206 , polarization-mode dispersion compensator  208 , pump  210 , polarization controllers  214 ,  218 ,  228 ,  232 , polarization beam controllers  216 ,  230 , wavelength selective processor  224 , and filter  236 . Such information may include, for example, wavelength, power, residual chromatic dispersion, and optical signal-to-noise ratio. Based on such information, control module  238  may be configured to adjust or control the operation of various portions of optical amplifier  200  to enhance or optimize performance of optical amplifier  200 . Such portions may include, for example, wavelength selective switch  206 , polarization-mode dispersion compensator  208 , pump  210 , polarization controllers  214 ,  218 ,  228 ,  232 , polarization beam controllers  216 ,  230 , wavelength selective processor  224 , and filter  236 . 
     In operation, input signal  202  may be received by optical amplifier  200  and filtered by wavelength selective switch  206 . The signal may be compensated for dispersion by polarization-mode dispersion compensator  208 . The output of polarization-mode dispersion compensator  208  may be communicatively coupled to the output of pump  210  by coupler  212 . 
     In a first stage of optical amplifier  200 , signal conversion may be performed on the combination of the pump signal and input signal  202  to yield an idler signal. The resultant combination of input signal  202  and the signal of pump  210  may be routed by optical circulator  222  to polarization controller  214 . Polarization controller  214  may adjust the polarization components and send the results to polarization beam controller  216 . Polarization beam controller  216  may split its input into x-polarization and y-polarization components. The x-polarization component may be routed clockwise to non-linear element  220  and the y-polarization component may be routed counter-clockwise to non-linear component  220 . Polarization controller  218  may adjust the polarization components and send the results to non-linear element  220 . Non-linear element  220  may simultaneously perform signal conversion on the x-polarization component and the y-polarization component. The signal conversion may yield an idler signal added to the combination of the pump signal and input signal  202 . The idler signal and input signal  202  may be equidistant, or nearly equidistant, from the pump signal in terms of wavelength. The x-polarization component may be routed to polarization controller  218 , which may adjust the polarization component and route the result to polarization beam controller  216 . The y-polarization component may be routed counter-clockwise to polarization beam controller  216 . Polarization beam controller  216  may combine the x-polarization and the y-polarization components and route the result to optical circulator  222 , which may in turn route the result to wavelength selective processor  224 . 
     In a second stage of optical amplifier  200 , non-degenerate phase-sensitive amplification may be performed on the combination of pump signal  210 , input signal  202 , and idler signal generated by the first stage. Wavelength selective processor  224  may pre-compensate its input signals for the signals&#39; residual chromatic dispersion, pre-compensate its input signals for the dispersion slope of any HNLF or other components of non-linear element  234 , or adjust its input signals&#39; phase levels in order to maximize or optimize amplification. Wavelength selective processor  224  may select or filter which portions of the combination of the pump signal, input signal  202 , and idler signal will be amplified. The results may be routed to polarization controller  228 , which may adjust the polarization components and route the result to polarization beam controller  230 . Polarization beam controller  230  may split its input into x-polarization and y-polarization components. Polarization beam controller  230  may route the y-polarization component counter-clockwise through polarization controller  232 , which may adjust the component, to non-linear element  234 . Furthermore, polarization beam controller  230  may route the x-polarization component clockwise to non-linear element  234 . Non-linear element  234  may perform phase-sensitive non-degenerate FWM upon the x-polarization and y-polarization components simultaneously. The result of the FWM may be to amplify input signal  202  and the corresponding idler signal. Non-linear element  234  may send the amplified y-polarization component counter-clockwise to polarization beam controller  230  and send the amplified x-polarization component clockwise to polarization beam controller  230 . The x-polarization component may be adjusted by polarization controller  232 . Polarization beam controller  230  may combine the x-polarization and y-polarization components and send the result to polarization controller  228 . Polarization controller  228  may route the result to filter  236  through optical circulator  226 . Filter  236  may remove all portions of the signal other than those wavelengths originally present in input signal  202 . For example, the pump signal and idler signals may be removed. Filter  236  may output the result as output signal  204 . 
     Control module  238  may continuously monitor performance of optical amplifier  200  and its signals and adjust various components of optical amplifier  200  in real-time. Control module  238  may monitor information regarding input signal  202 , output signal  204 , wavelength selective switch  206 , polarization-mode dispersion compensator  208 , pump  210 , polarization controllers  214 ,  218 ,  228 ,  232 , polarization beam controllers  216 ,  230 , wavelength selective processor  224 , and filter  236 . Control module  238  may adjust or control the operation of various portions of optical amplifier  200  to enhance or optimize performance of optical amplifier  200 , such as wavelength selective switch  206 , polarization-mode dispersion compensator  208 , pump  210 , polarization controllers  214 ,  218 ,  228 ,  232 , polarization beam controllers  216 ,  230 , wavelength selective processor  224 , and filter  236 . Adjustments may be made to match the nature, modulation format, polarization, frequency, or other aspects of input signal  202 , or to optimize or increase amplification. 
       FIGS. 3A and 3B  are an illustration of the operation of optical amplifier  200  for conducting optical phase-sensitive amplification, which may include amplification of dual polarization input signals.  FIGS. 3A and 3B  illustrate the status of signals at various reference points in optical amplifier  200  as illustrated in  FIG. 2 . 
     At (A), a degraded input signal  202  and the output of pump  210  may have been combined. Both polarizations are shown in combination. Input signal  202  may be separated from the pump signal by a wavelength distance α. 
     At (B), the combined input signal  202  and pump signal may have been divided into x-polarization and y-polarization components by polarization beam controller  216 . The signals may be prepared to be sent through non-linear element  220  bi-directionally for signal conversion. 
     At (C), an idler signal has been added to the x-polarization and y-polarization components of the combined signals as a result of bi-directionally passing through non-linear element  220 . The idler signal may be separated from the pump signal by a wavelength distance β. The wavelength distances α and β may be equal, or nearly equal. 
     At (D), the x-polarization and y-polarization components of the combined signals may have been recombined into a single signal set by polarization beam controller  216 . 
     At (E), the x-polarization and y-polarization components of the combined signals may have been separated by polarization beam controller  230 . Furthermore, each of the x-polarization and y-polarization components of the combined signals may have been passed bi-directionally through non-linear element  234  to perform non-degenerate FWM to cause phase-sensitive amplification. As a result, input signal  202  and the idler signal may have been amplified. 
     At (F), the x-polarization and y-polarization components of the combined signals may have been recombined by polarization beam controller  230 . Furthermore, the resultant signal set may have been filtered by filter  236  to remove the idler signal and the pump signal, leaving only the amplified version of input signal  202 , which may be output signal  204 . 
       FIGS. 4A and 4B  are an illustration of the operation of optical amplifier  200  for conducting optical phase-sensitive amplification using WDM, which may include amplification of dual-polarization signals.  FIGS. 4A and 4B  illustrate the status of signals at various reference points in optical amplifier  200  as illustrated in  FIG. 2 . 
     At (A), a degraded set of WDM signals of input signal  202  and the output of pump  210  may have been combined. Both polarizations are shown in combination. Each of the WDM signals of input signal  202  may each be separated from the pump signal by a wavelength distance, such as α 0 , α 1 , and α 2 , respectively. In one embodiment, each wavelength distance may be a multiple of another wavelength distance. 
     At (B), a combined input signal  202  and pump signal may have been divided into x-polarization and y-polarization components by polarization beam controller  216 . The signals may be prepared to be sent through non-linear element  220  bi-directionally for signal conversion. 
     At (C), a set of idler signals has been added to the x-polarization and y-polarization components of the combined signals as a result of bi-directionally passing through non-linear element  220 . Each of the idler signals illustrated may correspond to a WDM component of input signal  202 . Each of the idler signals may be separated from the pump signal by a wavelength distance such as β 0 , β 1 , and β 2 , respectively. The wavelength distances α i  and β i  may be equal, or nearly equal. 
     At (D), the x-polarization and y-polarization components of the combined signals may have been recombined into a single signal set by polarization beam controller  216 . 
     At (E), the x-polarization and y-polarization components of the combined signals may have been separated by polarization beam controller  230 . Furthermore, each of the x-polarization and y-polarization components of the combined signals may have been passed bi-directionally through non-linear element  234  to perform non-degenerate FWM to cause phase-sensitive amplification. As a result, each WDM component of input signal  202  and the corresponding idler signals may have been amplified. 
     At (F), the x-polarization and y-polarization components of the combined signals may have been recombined by polarization beam controller  230 . Furthermore, the resultant signal set may have been filtered by filter  236  to remove the set of idler signals and the pump signal, leaving only the amplified version of the WDM components of input signal  202 , which may be output signal  204 . 
       FIG. 5  is an illustration of an example embodiment of an optical amplifier  500  for conducting optical phase-sensitive amplification. In one embodiment, optical amplifier  500  may be configured to support dual-polarization modulation formats. Optical amplifier  500  may implement fully or in part optical amplifier  102  of  FIG. 1 . 
     Optical amplifier  500  may include a mechanism for accepting an input signal such as input signal  502 . Input signal  502  may include a plurality of WDM channels. Each such channel may correspond to a different wavelength. Such a wavelength may be denoted by α i . For each such channel, input signal  502  may include an x-polarization and a y-polarization component. Input signal  502  may implement input signal  110  of  FIG. 1 . Input signal  502  may be communicatively coupled to wavelength selective switch  506 . Wavelength selective switch  506  may be implemented in a similar manner to wavelength selective switch  206  of  FIG. 2 , and may be configured to select what portions of input signal  502  are to be amplified with optical amplifier  500 , perform wavelength demultiplexing, and switch signals on a per-wavelength basis. Wavelength selective switch  506  may be communicatively coupled to control module  534 . Control module  534  may be configured to adjust the operation of wavelength selective switch  506  to, for example, select what portion of input signal  502  is to be amplified by optical amplifier  500 . Such adjustments may be based upon, for example, the nature or kind of input signal  502 , detected output of optical amplifier  500 , or detected output of wavelength selective switch  506 . 
     Wavelength selective switch  506  may be communicatively coupled to polarization-mode dispersion compensator  508 . Polarization-mode dispersion compensator  508  may be implemented in a similar manner as polarization-mode dispersion compensator  208  of  FIG. 2 , and configured to compensate for residual polarization-mode dispersion in input signal  502 . Polarization-mode dispersion compensator  508  may be communicatively coupled to control module  534 . Control module  534  may be configured to adjust the operation of polarization-mode dispersion compensator. Such adjustments may be based upon, for example, the nature or kind of input signal  502 , detected output of optical amplifier  500 , or detected output of polarization-mode dispersion compensator  508 . 
     Optical amplifier  500  may include a first stage configured to perform signal conversion to generate a conjugate signal of input signal  502 . In addition, optical amplifier  500  may include a second stage configured to conduct non-degenerate FWM. Such non-degenerate FWM may be performed upon input signal  502  and the conjugate signal. 
     The first stage of optical amplifier  500  configured to generate a conjugate signal of input signal  502  may include a pump  510 , coupler  512 , optical circulator  514 , polarization controller  516 , polarization beam controller  518 , and non-linear elements  520 ,  526 . 
     The second stage of optical amplifier  500  configured to perform non-degenerate FWM for phase-sensitive amplification may include reflectors  524 ,  530 , wavelength selective processors  522 ,  528 , non-linear elements  520 ,  526 , and polarization beam controller  518 . Thus, in one embodiment, non-linear elements  520 ,  526  may be used in both stages of optical amplifier  500 . 
     Pump  510  may be configured and implemented in a similar manner as pump  210  of  FIG. 2  by providing a pump signal with a wavelength and strength that is set in relation to the wavelength of input signal  502  that is to be amplified. The output of polarization-mode dispersion compensator  508  may be communicatively coupled to coupler  512 , along with the output of pump  510 . Pump  510  may be communicatively coupled to control module  534 . Control module  534  may be configured to adjust the wavelength, power, phase, or other aspects of the operation of pump  510 . Such adjustments may be based upon, for example, the nature or kind of input signal  502 , detected output of pump  510 , or detected output of optical amplifier  500 . 
     Coupler  512  may be configured to couple the output of polarization-mode dispersion compensator  508  (including input signal  502  compensated for polarization-mode dispersion) and the output of pump  510 . Coupler  512  may be communicatively coupled to optical circulator  514  and configured to provide its output thereto. Coupler  512  may be implemented in any suitable manner for coupling the inputs as described. 
     Optical circulator  514  may be communicatively coupled on a first input/output line to polarization controller  516  and on a second input/output line to filter  532 . Optical circulator  514  may be communicatively coupled on an input line to coupler  512  and configured to receive its output. Optical circulator  514  may be implemented in a similar manner as optical circulator  222  of  FIG. 2 . 
     Polarization controller  516  may be implemented in a similar manner as polarization controller  214  of  FIG. 2 . Polarization controller  516  may be configured to adjust the x-polarization and y-polarization components of its input signals to maximize or increase the effects of conjugate signal generation to be performed by, for example, non-linear elements  520 ,  526 . Such adjustments may include a polarization shifting of the x-polarization and y-polarization components. Polarization controller  516  may be communicatively coupled to control module  534 . Control module  534  may be configured to adjust the operation of polarization controller  516 . Such adjustments may be based upon, for example, the nature or kind of input signal  502 , detected output of polarization controller  516 , or detected output of optical amplifier  500 . Polarization controller  516  may be configured to receive its input from optical circulator  514 , perform adjustments on the x-polarization and y-polarization components if necessary, and output the results to polarization beam controller  518 . 
     Polarization beam controller  518  may be implemented in a similar manner as polarization beam controller  216  of  FIG. 2 . Polarization beam controller  518  may be configured to split an input signal according to x-polarization and y-polarization components and to output each component. Polarization beam controller  518  may be configured to send the x-polarization component to non-linear element  520  and the y-polarization component to non-linear element  526 . Polarization beam controller  518  may be communicatively coupled to control module  534 . Control module  534  may be configured to adjust the operation of polarization beam controller  518 . Such adjustments may be based upon, for example, the nature or kind of input signal  502 , detected output of optical amplifier  500 , or detected output of polarization beam controller  518 . 
     Each of non-linear element  520  and non-linear element  526  may be implemented in a similar manner to non-linear elements  220 ,  234  of  FIG. 2 . Each of non-linear element  520  and non-linear element  526  may be configured to perform signal conversion on a polarization component received from polarization beam controller  518  and to send the result to wavelength selective processors  522 ,  528 , respectively. 
     Each of non-linear element  520  and non-linear element  526  may be configured to provide signal conversion based upon the nature of its respective input signals, which may include the combination of input signal  502  and pump signal from pump  510 . In one embodiment, each of non-linear element  520  and non-linear element  526  may be configured to cause an idler signal to be added to the combination of input signal  502  and the pump signal. In a further embodiment, the idler signal and input signal  502  may be equidistant, or nearly equidistant, from the pump signal. Thus, the idler signal and input signal  502  may be symmetrically, or nearly symmetrically, located on each side of the pump signal. In another embodiment, the generated idler signal may include the inverse phase as input signal  502 . If input signal  502  includes multiple WDM components, non-linear element may generate an idler signal for each such WDM component. Each idler signal and the corresponding WDM component may be equidistant, or nearly equidistant, from the pump signal in terms of wavelength. Thus, the WDM components of input signal  502  and the corresponding idler signals may be symmetric, or nearly symmetric, around the pump signal. 
     By performing separate processing of x-polarization and y-polarization components, optical amplifier  500  may avoid crosstalk or path mismatch between the components. 
     After performing signal conversion, non-linear element  520  may be configured to yield the x-polarization of a combination of input signal  502 , the idler signals generated, and the pump signal. Non-linear element  520  may be configured to send this polarization to wavelength selective processor  522 . Furthermore, non-linear element  526  may be configured to yield the y-polarization of a combination of input signal  502 , the idler signals generated, and the pump signal. Non-linear element  526  may be configured to send this polarization to wavelength selective processor  528 . 
     Each of wavelength selective processors  522 ,  528  may be configured to receive a signal from the first stage of optical amplifier  500 . Such reception may be made from, for example, non-linear elements  520 ,  526 , respectively. The received signal may include a combination of input signal  502 , the pump signal, and the idler signals. Wavelength selective processors  522 ,  528  may be implemented in a similar manner as wavelength selective processor  224  of  FIG. 2 . Wavelength selective processors  522 ,  528  may be configured to select which portions of the received signal are to be amplified using FWM. Such selection may be made based on, for example, wavelength wherein unused idler signals and other signals are filtered out. Wavelength selective processors  522 ,  528  may be communicatively coupled to control module  534 . Control module  534  may be configured to adjust the operation of wavelength selective processors  522 ,  528 , such as adjustment of power or phases of signals received by wavelength selective processors  522 ,  528 , or adjustment of signals that will be filtered by wavelength selective processors  522 ,  528 . Furthermore, wavelength selective processors  522 ,  528  may be adjusted to pre-compensate their input signals for the input signals&#39; residual chromatic dispersion or for dispersion slope of HNLF resident within non-linear elements. Such adjustments may be based upon, for example, the nature or kind of input signal  502 , detected output of wavelength selective processors  522 ,  528 , or detected output of optical amplifier  500 . 
     If input signal  502  includes WDM signals, wavelength selective processors  522 ,  528  may be configured to select a range including the WDM signals to be amplified, the pump signal, and the range of idler signals corresponding to each of the WDM signals. 
     Each of wavelength selective processors  522 ,  528  may be communicatively coupled to a respective reflector  524 ,  530 . Furthermore, each of wavelength selective processors  522 ,  528  may be configured to send its output to the respective reflector  524 ,  530 . 
     Each of reflector  524 ,  530  may be configured to reflect perfectly, or nearly perfectly, each wavelength of a set of received signals back to its source. Reflectors  524 ,  530  may be implemented in any suitable manner for reflecting its input signals back as output. The signals input into reflectors  524 ,  530  may be returned to wavelength selective processors  522 ,  528 . In one embodiment, wavelength selective processors  522 ,  528  may be configured to allow the reflected signals to pass through from reflectors  524 ,  530  to non-linear elements  520 ,  526 . In another embodiment, wavelength selective processors  522 ,  528  may be configured to allow the reflected signals to pass through from non-linear elements  520 ,  526  to reflectors  524 ,  530 , and to perform their designated operations upon the reflected signals as they return from reflectors  524 ,  530  to non-linear elements  520 ,  526 . 
     Upon receipt of the reflected signals containing input signal  502 , the idler signals, and the pump signal, non-linear elements  520 ,  526  may be configured to perform phase-sensitive amplification through non-degenerate FWM. Input signal  502  and idler signals will be amplified. Thus, non-linear elements  520 ,  526  may be configured to bi-directionally provide signal conversion and phase-sensitive amplification for the x-polarization and y-polarization components, respectively. The FWM performed by non-linear elements  520 ,  526  may utilize the equidistant, or nearly equidistant, arrangement of input signal  502  and its idler signals around the pump signal. Furthermore, the FWM performed by non-linear elements  520 ,  526  may utilize the performance of the idler signals as conjugate signals to input signal  502 . If input signal  502  includes WDM signals, non-linear elements  520 ,  526  may amplify the range of WDM signals and the range of the idler signals corresponding to the WDM signals. 
     Non-linear elements  520 ,  526  may be configured to send their respective amplified x-polarization and y-polarization components to polarization beam controller  518 , which may be configured to combine the respective polarization components. Polarization beam controller  518  may be configured to route the result to polarization controller  516 , which may be configured to perform adjustments, such as phase adjustments, to the respective components and route the result to optical circulator  514 . Optical circulator  514  may be configured to route the result to filter  532 . 
     Optical amplifier  500  may include filter  532  configured to remove the idler signals and pump signal from the result of FWM. Filter  532  may be implemented in a similar manner as filter  236  of  FIG. 2 . Filter  532  may be configured to only allow signals with the wavelength of the original input signal (input signal  502 ) to pass. Filter  532  may be configured to generate output signal  504 . The result of optical amplification in output signal  504  may implement output signal  114  of  FIG. 1 . Filter  532  may be communicatively coupled to control module  534 . Control module  534  may be configured to adjust the operation of filter  532 . Such adjustments may be based upon, for example, the nature or kind of input signal  502 , detected output of optical amplifier  500 , or detected output of filter  532 . 
     Control module  534  may be implemented in a similar manner as control module  238  of  FIG. 2 . Control module  534  may be configured to monitor performance of optical amplifier  500  and its signals including, for example, information regarding input signal  502 , output signal  504 , wavelength selective switch  506 , polarization-mode dispersion compensator  508 , pump  510 , polarization controller  516 , polarization beam controller  518 , wavelength selective processors  522 ,  528 , and filter  532 . Such information may include, for example, wavelength, power, residual chromatic dispersion, and optical signal-to-noise ratio. Based on such information, control module  534  may be configured to adjust or control the operation of various portions of optical amplifier  500  to enhance or optimize performance of optical amplifier  500 . Such portions may include, for example, wavelength selective switch  506 , polarization-mode dispersion compensator  508 , pump  510 , polarization controller  516 , polarization beam controller  518 , wavelength selective processors  522 ,  528 , and filter  532 . 
     In operation, input signal  502  may be received by optical amplifier  500  and filtered by wavelength selective switch  506 . The signal may be compensated for dispersion by polarization-mode dispersion compensator  508 . The output of polarization-mode dispersion compensator  508  may be communicatively coupled to the output of pump  510  by coupler  512 . 
     In a first stage of optical amplifier  500 , signal conversion may be performed on the combination of the pump signal and input signal  502  to yield an idler signal. The resultant combination of input signal  502  and the signal of pump  510  may be routed by optical circulator  514  to polarization controller  516 . Polarization controller  516  may adjust the polarization components and send the results to polarization beam controller  518 . Polarization beam controller  518  may split its input into x-polarization and y-polarization components. The x-polarization component may be routed to non-linear element  520  and the y-polarization component may be routed to non-linear component  526 . Non-linear element  520  may perform signal conversion on the x-polarization component. Non-linear element  526  may perform signal conversion on the y-polarization component. The signal conversion may yield an idler signal added to each respective polarization component and be combined with the pump signal and input signal  502 . The idler signal and input signal  502  may be equidistant, or nearly equidistant, from the pump signal in terms of wavelength. The x-polarization component may be routed to wavelength selective processor  522 . The y-polarization component may be routed to wavelength selective processor  528 . 
     In a second stage of optical amplifier  500 , non-degenerate phase-sensitive amplification may be performed on the combination of the pump signal, input signal  502 , and idler signal generated by the first stage. Wavelength selective processors  522 ,  526  may pre-compensate their input signals for the signals&#39; residual chromatic dispersion, pre-compensate their input signals for the dispersion slope of any HNLF or other components of non-linear elements  520 ,  526 , or adjust their input signals&#39; phase levels in order to maximize or optimize amplification. Wavelength selective processors  522 ,  526  may select or filter which portions of the combination of the pump signal, input signal  502 , and idler signal will be amplified. The results may be routed to reflectors  524 ,  530 , which may reflect the signals back to wavelength selective processors  522 ,  526 . Wavelength selective processors  522 ,  526  may pass the reflected signals through to non-linear elements  520 ,  526 . Non-linear elements  520 ,  526  may perform non-degenerate FWM upon the x-polarization and y-polarization components, respectively. The result of the FWM may be to amplify input signal  502  and the corresponding idler signal. Non-linear elements  520 ,  526  may send the amplified x-polarization and y-polarization components to polarization beam controller  518 , which may reassemble the polarization components and route them to polarization controller  516 . The polarization components may be adjusted by polarization controller  516 , and then routed through optical circulator  514  to filter  532 . Filter  532  may remove all portions of the signal other than those wavelengths originally present in input signal  502 . For example, the pump signal and idler signals may be removed. Filter  532  may output the result as output signal  504 . 
     Control module  534  may continuously monitor performance of optical amplifier  500  and its signals and adjust various components of optical amplifier  500  in real-time. Control module  534  may monitor information regarding input signal  502 , output signal  504 , wavelength selective switch  506 , polarization-mode dispersion compensator  508 , pump  510 , polarization controller  516 , polarization beam controller  518 , wavelength selective processors  522 ,  528 , and filter  532 . Control module  534  may adjust or control the operation of various portions of optical amplifier  500  to enhance or optimize performance of optical amplifier  500 , such as wavelength selective switch  506 , polarization-mode dispersion compensator  508 , pump  510 , polarization controller  516 , polarization beam controller  518 , wavelength selective processors  522 ,  528 , and filter  532 . Adjustments may be made to match the nature, modulation format, polarization, frequency, or other aspects of input signal  502 , or to optimize or increase amplification. 
       FIG. 6  is an illustration of the operation of optical amplifier  500  for conducting optical phase-sensitive amplification, which may include amplification of dual-polarization signals.  FIG. 6  illustrates the status of signals at various reference points in optical amplifier  500  as illustrated in  FIG. 5 . 
     At (A), a degraded input signal  502  and the output of pump  510  may have been combined. Both polarizations are shown in combination. Input signal  502  may be separated from the pump signal by a wavelength distance α. 
     At (B), the combined input signal  502  and pump signal may have been divided into an x-polarization component by polarization beam controller  518 . The signals may have been sent through non-linear element  520  for signal conversion, yielding an idler signal. The idler signal may be separated from the pump signal by a wavelength distance β. The wavelength distances α and β may be equal, or nearly equal. The combined signals may be prepared to be passed through wavelength selective processor  522 , reflected by reflector  524 , and reentered into non-linear element  520  for phase-sensitive amplification. 
     At (C), the combined input signal  502  and pump signal may have been divided into a y-polarization component by polarization beam controller  518 . The signals may have been sent through non-linear element  526  for signal conversion, yielding an idler signal. The idler signal may be separated from the pump signal by a wavelength distance β. The wavelength distances α and β may be equal, or nearly equal. The combined signals may be prepared to be passed through wavelength selective processor  528 , reflected by reflector  530 , and reentered into non-linear element  526  for phase-sensitive amplification. 
     At (D), the x-polarization component may have been passed through non-linear element  520  and phase-sensitive amplification may have been performed with FWM. As a result, input signal  502  and the idler signal may have been amplified. 
     At (E), the y-polarization component may have been passed through non-linear element  526  and phase-sensitive amplification may have been performed with FWM. As a result, input signal  502  and the idler signal may have been amplified. 
     At (F), the x-polarization and y-polarization components of the combined signals may have been recombined by polarization beam controller  518 . Furthermore, the resultant signal set may have been filtered by filter  532  to remove the idler signal and the pump signal, leaving only the amplified version of input signal  502 , which may be output signal  504 . 
       FIG. 7  is an illustration of the operation of optical amplifier  500  for conducting optical phase-sensitive amplification using WDM, which may include amplification of dual-polarization signals.  FIG. 7  illustrates the status of signals at various reference points in optical amplifier  500  as illustrated in  FIG. 5 . 
     At (A), a degraded set of WDM signals of input signal  502  and the output of pump  510  may have been combined. Both polarizations are shown in combination. Each of the WDM signals of input signal  502  may each be separated from the pump signal by a wavelength distance, such as α 0 , α 1 , and α 2 , respectively. In one embodiment, each wavelength distance may be a multiple of another wavelength distance. 
     At (B), the combined input signal  502  and pump signal may have been divided into an x-polarization component by polarization beam controller  518 . The signals may have been sent through non-linear element  520  for signal conversion, yielding a set of idler signals. Each of the idler signals illustrated may correspond to a WDM component of input signal  502 . Each of the idler signals may be separated from the pump signal by a wavelength distance such as β 0 , β 1 , and β 2 , respectively. The wavelength distances α i  and β i  may be equal, or nearly equal. The combined signals may be prepared to be passed through wavelength selective processor  522 , reflected by reflector  524 , and reentered into non-linear element  520  for phase-sensitive amplification. 
     At (C), the combined input signal  502  and pump signal may have been divided into a y-polarization component by polarization beam controller  518 . The signals may have been sent through non-linear element  526  for signal conversion, yielding a set of idler signals. Each of the idler signals illustrated may correspond to a WDM component of input signal  502 . Each of the idler signals may be separated from the pump signal by a wavelength distance such as β 0 , β 1 , and β 2 , respectively. The wavelength distances α i  and β i  may be equal, or nearly equal. The combined signals may be prepared to be passed through wavelength selective processor  528 , reflected by reflector  530 , and reentered into non-linear element  526  for phase-sensitive amplification. 
     At (D), the x-polarization component of the combined signals may have been passed through non-linear element  520  and phase-sensitive amplification may have been performed with FWM. As a result, each WDM component of input signal  502  and the corresponding idler signal may have been amplified. 
     At (E), the y-polarization component of the combined signals may have been passed through non-linear element  526  and phase-sensitive amplification may have been performed with FWM. As a result, each WDM component of input signal  502  and the corresponding idler signal may have been amplified. 
     At (F), the x-polarization and y-polarization components of the combined signals may have been recombined by polarization beam controller  518 . Furthermore, the resultant signal set may have been filtered by filter  532  to remove the set of idler signals and the pump signal, leaving only the amplified version of the WDM components of input signal  502 , which may be output signal  504 . 
       FIG. 8  is an illustration of a yet another example embodiment of an optical amplifier  800  for conducting optical phase-sensitive amplification. In one embodiment, optical amplifier  800  may be configured to support dual-polarization modulation formats. Optical amplifier  800  may implement fully or in part optical amplifier  102  of  FIG. 1 . 
     Optical amplifier  800  may include a mechanism for accepting an input signal such as input signal  802 . Input signal  802  may include a plurality of WDM channels. Each such channel may correspond to a different wavelength. Such a wavelength may be denoted by α i . For each such channel, input signal  802  may include an x-polarization and a y-polarization component. Input signal  802  may implement input signal  110  of  FIG. 1 . Input signal  802  may be communicatively coupled to wavelength selective switch  806 . Wavelength selective switch  806  may be implemented in a similar manner to wavelength selective switch  206  of  FIG. 2 , and may be configured to select what portions of input signal  802  are to be amplified with optical amplifier  800 , perform wavelength demultiplexing, and switch signals on a per-wavelength basis. Wavelength selective switch  806  may be communicatively coupled to control module  826 . Control module  826  may be configured to adjust the operation of wavelength selective switch  806  to, for example, select what portion of input signal  802  is to be amplified by optical amplifier  800 . Such adjustments may be based upon, for example, the nature or kind of input signal  802 , detected output of optical amplifier  800 , or detected output of wavelength selective switch  806 . 
     Wavelength selective switch  806  may be communicatively coupled to polarization-mode dispersion compensator  808 . Polarization-mode dispersion compensator  808  may be implemented in a similar manner as polarization-mode dispersion compensator  208  of  FIG. 2 , and configured to compensate for residual polarization-mode dispersion in input signal  802 . Polarization-mode dispersion compensator  808  may be communicatively coupled to control module  826 . Control module  826  may be configured to adjust the operation of polarization-mode dispersion compensator. Such adjustments may be based upon, for example, the nature or kind of input signal  802 , detected output of optical amplifier  800 , or detected output of polarization-mode dispersion compensator  808 . 
     Optical amplifier  800  may include a first stage configured to perform signal conversion to generate a conjugate signal of input signal  802 . In addition, optical amplifier  800  may include a second stage configured to conduct non-degenerate FWM. Such non-degenerate FWM may be performed upon input signal  802  and the conjugate signal. 
     The first stage of optical amplifier  800  configured to generate a conjugate signal of input signal  802  may include a pump  810 , coupler  812 , optical circulator  814 , polarization controller  816 , and non-linear element  818 . 
     The second stage of optical amplifier  800  configured to perform non-degenerate FWM for phase-sensitive amplification may include reflector  822 , wavelength selective processor  820 , non-linear element  818 , and polarization controller  816 . Thus, in one embodiment, non-linear element  818  may be used in both stages of optical amplifier  800 . 
     Pump  810  may be configured and implemented in a similar manner as pump  210  of  FIG. 2  by providing a pump signal with a wavelength and strength that is set in relation to the wavelength of input signal  802  that is to be amplified. The output of polarization-mode dispersion compensator  808  may be communicatively coupled to coupler  812 , along with the output of pump  810 . Pump  810  may be communicatively coupled to control module  826 . Control module  826  may be configured to adjust the wavelength, power, phase, or other aspects of the operation of pump  810 . Such adjustments may be based upon, for example, the nature or kind of input signal  802 , detected output of pump  810 , or detected output of optical amplifier  800 . 
     Coupler  812  may be configured to couple the output of polarization-mode dispersion compensator  808  (including input signal  802  compensated for polarization-mode dispersion) and the output of pump  810 . Coupler  812  may be communicatively coupled to optical circulator  814  and configured to provide its output thereto. Coupler  812  may be implemented in any suitable manner for coupling the inputs as described. 
     Optical circulator  814  may be communicatively coupled on a first input/output line to polarization controller  816  and on a second input/output line to filter  832 . Optical circulator  814  may be communicatively coupled on an input line to coupler  812  and configured to receive its output. Optical circulator  814  may be implemented in a similar manner as optical circulator  222  of  FIG. 2 . 
     Polarization controller  816  may be implemented in a similar manner as polarization controller  214  of  FIG. 2 . Polarization controller  816  may be configured to adjust the x-polarization and y-polarization components of its input signals to maximize or increase the effects of conjugate signal generation to be performed by, for example, non-linear element  818 . Such adjustments may include a polarization shifting of the x-polarization and y-polarization components. Polarization controller  816  may be communicatively coupled to control module  826 . Control module  826  may be configured to adjust the operation of polarization controller  816 . Such adjustments may be based upon, for example, the nature or kind of input signal  802 , detected output of polarization controller  816 , or detected output of optical amplifier  800 . Polarization controller  816  may be configured to receive its input from optical circulator  814 , perform adjustments on the x-polarization and y-polarization components if necessary, and output the results to non-linear element  818 . 
     Non-linear element  818  may be implemented in a similar manner to non-linear elements  220 ,  234  of  FIG. 2 , except that non-linear element  818  may be further configured as a polarization-maintaining non-linear element. For example, if non-linear element  818  is implemented using fiber, such fiber may include polarization-maintaining optical fiber. Non-linear element  818  may maintain the polarization of its inputs as the signals are propagated through non-linear element  818  through maintenance of the orientation of the x-polarization and y-polarization components. Non-linear element  818  may be configured to perform signal conversion on the combined polarization components of its inputs received from polarization controller  816  to send the result to wavelength selective processor  820 . 
     Non-linear element  818  may be configured to provide signal conversion based upon the nature of its input signals, which may include the combination of input signal  802  and pump signal from pump  810 . In one embodiment, non-linear element  818  may be configured to cause an idler signal to be added to the combination of input signal  802  and the pump signal. In a further embodiment, the idler signal and input signal  802  may be equidistant, or nearly equidistant, from the pump signal. Thus, the idler signal and input signal  802  may be symmetrically, or nearly symmetrically, located on each side of the pump signal. In another embodiment, the generated idler signal may include the inverse phase as input signal  802 . If input signal  802  includes multiple WDM components, non-linear element may generate an idler signal for each such WDM component. Each idler signal and the corresponding WDM component may be equidistant, or nearly equidistant, from the pump signal in terms of wavelength. Thus, the WDM components of input signal  802  and the corresponding idler signals may be symmetric, or nearly symmetric, around the pump signal. After performing signal conversion, non-linear element  818  may be configured to route its outputs to wavelength selective processor  820 . 
     Wavelength selective processor  820  may be configured to receive a signal from the first stage of optical amplifier  800 . Such reception may be made from, for example, non-linear element  818 . The received signal may include a combination of input signal  802 , the pump signal, and the idler signals. Wavelength selective processor  820  may be implemented in a similar manner as wavelength selective processor  224  of  FIG. 2 . Wavelength selective processor  820  may be configured to select which portions of the received signal are to be amplified using FWM. Such selection may be made based on, for example, wavelength wherein unused idler signals and other signals are filtered out. Wavelength selective processor  820  may be communicatively coupled to control module  826 . Control module  826  may be configured to adjust the operation of wavelength selective processor  820 , such as adjustment of power or phases of signals received or adjustment of signals that will be filtered. Furthermore, wavelength selective processor  820  may be adjusted to pre-compensate their input signals for the input signals&#39; residual chromatic dispersion or for dispersion slope of HNLF resident within non-linear elements. Such adjustments may be based upon, for example, the nature or kind of input signal  802 , detected output of wavelength selective processor  820 , or detected output of optical amplifier  800 . 
     If input signal  802  includes WDM signals, wavelength selective processor  820  may be configured to select a range including the WDM signals to be amplified, the pump signal, and the range of idler signals corresponding to each of the WDM signals. 
     Wavelength selective processors  820  may be communicatively coupled to a reflector  822 . Furthermore, wavelength selective processor  820  may be configured to send its output to reflector  822 . 
     Reflector  822  may be configured to reflect perfectly, or nearly perfectly, each wavelength of a set of received signals back to its source. Reflector  822  may be implemented in any suitable manner for reflecting its input signals back as output. The signals input into reflector  822  may be returned to wavelength selective processor  820 . In one embodiment, wavelength selective processors  820  may be configured to allow the reflected signals to pass through from reflector  822  to non-linear element  818 . In another embodiment, wavelength selective processor  820  may be configured to allow the reflected signals to pass through from non-linear element  818  to reflector  822 , and to perform their designated operations upon the reflected signals as they return from reflector  822  to non-linear element  818 . 
     Upon receipt of the reflected signals containing input signal  802 , the idler signals, and the pump signal, non-linear element  818  may be configured to perform phase-sensitive amplification through non-degenerate FWM. Input signal  802  and idler signals will be amplified. Thus, non-linear element  818  may be configured to bi-directionally provide signal conversion and phase-sensitive amplification for the x-polarization and y-polarization components. The FWM performed by non-linear element  818  may utilize the equidistant, or nearly equidistant, arrangement of input signal  802  and its idler signals around the pump signal. Furthermore, the FWM performed by non-linear element  818  may utilize the performance of the idler signals as conjugate signals to input signal  802 . If input signal  802  includes WDM signals, non-linear element  818  may amplify the range of WDM signals and the range of the idler signals corresponding to the WDM signals. 
     Non-linear element  818  may be configured to its respective combined x-polarization and y-polarization components to polarization controller  816 , which may be configured to perform adjustments, such as phase adjustments, to the respective components and route the result to optical circulator  814 . Optical circulator  814  may be configured to route the result to filter  824 . 
     Optical amplifier  800  may include filter  824  configured to remove the idler signals and pump signal from the result of FWM. Filter  824  may be implemented in a similar manner as filter  236  of  FIG. 2 . Filter  824  may be configured to only allow signals with the wavelength of the original input signal (input signal  802 ) to pass. Filter  824  may be configured to generate output signal  804 . The result of optical amplification in output signal  804  may implement output signal  114  of  FIG. 1 . Filter  824  may be communicatively coupled to control module  826 . Control module  826  may be configured to adjust the operation of filter  824 . Such adjustments may be based upon, for example, the nature or kind of input signal  802 , detected output of optical amplifier  800 , or detected output of filter  824 . 
     Control module  826  may be implemented in a similar manner as control module  238  of  FIG. 2 . Control module  826  may be configured to monitor performance of optical amplifier  800  and its signals including, for example, information regarding input signal  802 , output signal  804 , wavelength selective switch  806 , polarization-mode dispersion compensator  808 , pump  810 , polarization controller  816 , wavelength selective processor  820 , and filter  824 . Such information may include, for example, wavelength, power, residual chromatic dispersion, and optical signal-to-noise ratio. Based on such information, control module  826  may be configured to adjust or control the operation of various portions of optical amplifier  800  to enhance or optimize performance of optical amplifier  800 . Such portions may include, for example, wavelength selective switch  806 , polarization-mode dispersion compensator  808 , pump  810 , polarization controller  816 , wavelength selective processor  820 , and filter  822 . 
     In operation, input signal  802  may be received by optical amplifier  800  and filtered by wavelength selective switch  806 . The signal may be compensated for dispersion by polarization-mode dispersion compensator  808 . The output of polarization-mode dispersion compensator  808  may be communicatively coupled to the output of pump  810  by coupler  812 . 
     In a first stage of optical amplifier  800 , signal conversion may be performed on the combination of the pump signal and input signal  802  to yield an idler signal. The resultant combination of input signal  802  and the signal of pump  810  may be routed by optical circulator  814  to polarization controller  816 . Polarization controller  816  may adjust the polarization components and send the results to non-linear element  818 . Non-linear element  818  may process received signals while maintaining polarization. Non-linear element  818  may perform signal conversion on the combined x-polarization and y-polarization components. The signal conversion may yield an idler signal added to the combined x-polarization and y-polarization components and be combined with the pump signal and input signal  802 . The idler signal and input signal  802  may be equidistant, or nearly equidistant, from the pump signal in terms of wavelength. The combined x-polarization and y-polarization components may be routed to wavelength selective processor  820 . 
     In a second stage of optical amplifier  800 , non-degenerate phase-sensitive amplification may be performed on the combination of the pump signal, input signal  802 , and idler signal generated by the first stage. Wavelength selective processor  820  may pre-compensate its input signals for the signals&#39; residual chromatic dispersion, pre-compensate its input signals for the dispersion slope of any HNLF or other components of non-linear element  818 , or adjust its input signals&#39; phase levels in order to maximize or optimize amplification. Wavelength selective processor  820  may select or filter which portions of the combination of the pump signal, input signal  802 , and idler signal will be amplified. The results may be routed to reflector  822 , which may reflect the signals back to wavelength selective processor  820 . Wavelength selective processor  820  may pass the reflected signals through to non-linear element  818 . Non-linear elements  818  may perform non-degenerate FWM upon the combined x-polarization and y-polarization components. The result of the FWM may be to amplify input signal  802  and the corresponding idler signal. Non-linear element  818  may send the amplified x-polarization and y-polarization components to polarization controller  816 . The polarization components may be adjusted by polarization controller  816 , and then routed through optical circulator  814  to filter  824 . Filter  824  may remove all portions of the signal other than those wavelengths originally present in input signal  802 . For example, the pump signal and idler signals may be removed. Filter  824  may output the result as output signal  804 . 
     Control module  826  may continuously monitor performance of optical amplifier  800  and its signals and adjust various components of optical amplifier  800  in real-time. Control module  834  may monitor information regarding input signal  802 , output signal  804 , wavelength selective switch  806 , polarization-mode dispersion compensator  808 , pump  810 , polarization controller  816 , wavelength selective processor  820 , and filter  824 . Control module  826  may adjust or control the operation of various portions of optical amplifier  800  to enhance or optimize performance of optical amplifier  800 , such as wavelength selective switch  806 , polarization-mode dispersion compensator  808 , pump  810 , polarization controller  816 , wavelength selective processor  820 , and filter  824 . Adjustments may be made to match the nature, modulation format, polarization, frequency, or other aspects of input signal  802 , or to optimize or increase amplification. 
       FIG. 9  is an illustration of the operation of optical amplifier  800  for conducting optical phase-sensitive amplification, which may include amplification of dual-polarization signals.  FIG. 9  illustrates the status of signals at various reference points in optical amplifier  800  as illustrated in  FIG. 8 . 
     At (A), a degraded input signal  802  and the output of pump  810  may have been combined. Both polarizations are shown in combination. Input signal  802  may be separated from the pump signal by a wavelength distance α. 
     At (B), the combined input signal  502  and pump signal may have been sent through non-linear element  818  for signal conversion, yielding an idler signal. The idler signal may be separated from the pump signal by a wavelength distance β. The wavelength distances α and β may be equal, or nearly equal. The combined signals may be prepared to be passed through wavelength selective processor  820 , reflected by reflector  822 , and reentered into non-linear element  818  for phase-sensitive amplification. 
     At (C), the combined signals may have been passed through non-linear element  818  and phase-sensitive amplification may have been performed with FWM. As a result, input signal  802  and the idler signal may have been amplified. 
     At (D), the combined signals may have been filtered by filter  824  to remove the idler signal and the pump signal, leaving only the amplified version of input signal  802 , which may be output signal  804 . 
       FIG. 10  is an illustration of the operation of optical amplifier  800  conducting optical phase-sensitive amplification using WDM, which may include amplification of dual-polarization signals.  FIG. 10  illustrates the status of signals at various reference points in optical amplifier  800 . 
     At (A), a degraded set of WDM signals of input signal  802  and the output of pump  810  may have been combined. Both polarizations are shown in combination. Input signal  802  may be separated from the pump signal by a wavelength distance, such as α 0 , α 1 , and α 2 , respectively. In one embodiment, each wavelength distance may be a multiple of another wavelength distance. 
     At (B), the combined input signal  802  and pump signal may have been sent through non-linear element  818  for signal conversion, yielding a set of idler signals. Each of the idler signals illustrated may correspond to a WDM component of input signal  802 . Each of the idler signals may be separated from the pump signal by a wavelength distance such as β 0 , β 1 , and β 2 , respectively. The wavelength distances α i  and β i  may be equal, or nearly equal. The combined signals may be prepared to be passed through wavelength selective processor  820 , reflected by reflector  822 , and reentered into non-linear element  818  for phase-sensitive amplification. 
     At (C), the combined signals may have been passed through non-linear element  818  and phase-sensitive amplification may have been performed with FWM. As a result, each WDM component of input signal  802  and the corresponding idler signal may have been amplified. 
     At (D), the combined signals may have been filtered by filter  824  to remove the idler signal and the pump signal, leaving only the amplified version of the WDM components of input signal  802 , which may be output signal  804 . 
       FIG. 11  is an example embodiment of a method  1100  for optical phase-sensitive amplification. In one embodiment, method  1100  may be applied to signals with dual-polarization modulation formats. 
     In  1105 , an input signal to be amplified may be determined. The signal may be modulated using, for example, m-QAM or m-PSK modulation techniques. The signal may include an x-polarization and a y-polarization component. Furthermore, the signal may be multiplexed using WDM. Based on the determination of the signal, parameters for performing other elements of method  1100  may be determined. Portions of the input signal may be filtered or selected such that a desired portion of the input signal will be amplified. Analysis of the signal, software settings, user settings, and the physical embodiment of the system or optical amplifier may be used to determine how to handle the type of signal. For example, the pump signal generated and selected may be based upon such analysis. 
     In  1110 , a pump signal may be generated and added to the input signal. The pump signal may be a given wavelength distance from the input signal. 
     In  1115 , signal conversion may be applied to the resultant signal. Such signal conversion may be performed to generate a conjugate, idler signal. The idler signal and the input signal may be equidistant, or nearly equidistant, from the pump signal in terms of wavelength. Signal conversion may be applied, for example, by passing the resultant signal through an optical non-linear element. In one embodiment, the resultant signal may be divided into an x-polarization component and a y-polarization component, and the polarization components may be passed through the optical non-linear element bi-directionally. In another embodiment, the resultant signal may be divided into an x-polarization component and a y-polarization component, and the polarization components may be passed through two separate optical non-linear elements. In yet another embodiment, the resultant signal may be preserved with both x-polarization component and a y-polarization component and passed through a polarization maintaining non-linear element. If the input signal includes WDM components, idler signals may be generated for each WDM component. Each pair of WDM component and idler signal may be equidistant, or nearly equidistant, from the pump signal in terms of wavelength. In  1120 , the generated idler signal may be added to the resultant signal. In one embodiment,  1115  and  1120  may be performed simultaneously. 
     In  1125 , the resultant signal may be adjusted to optimize phase-sensitive amplification. For example, the phase of each of the pump signal, idler signal, and input signal may be adjusted. Furthermore, phase or dispersion may be compensated and adjusted. Furthermore, specific wavelengths to be amplified may be selected, wherein additional unnecessary idler signals are filtered. 
     In  1130 , phase-sensitive amplification may be performed on the resultant signal. The idler signal and the input signal may be amplified. If the input signal includes WDM components, each WDM component and the corresponding idler signal may be amplified. Phase-sensitive amplification may be performed by conducting non-degenerate FWM by passing the resultant signal through an optical non-linear element. In one embodiment, phase-sensitive amplification may be performed by dividing the resultant signal into an x-polarization component and a y-polarization component, and passing the polarization components through an optical non-linear element bi-directionally. In another embodiment, the resultant signal may be divided into an x-polarization component and a y-polarization component, and the polarization components may be passed through two separate optical non-linear elements. In such an embodiment, a single optical non-linear element may be used to conduct both signal conversion and phase-sensitive amplification for a given polarization by performing the actions bi-directionally. To facilitate such bi-directional actions, the resultant signal may be reflected after exiting the optical non-linear element to then be returned to the same optical non-linear element. In yet another embodiment, the resultant signal may be preserved with both x-polarization component and a y-polarization component and passed through a polarization maintaining non-linear element. 
     In  1135 , the resultant signal may be filtered. The pump signal and idler signals may be removed, leaving an amplified version of the original input signal. In  1140 , the amplified signal may be output. Method  1100  may repeat as necessary. 
     Method  1100  may be implemented using the system, optical amplifiers, and operation of  FIGS. 1-10 , or any other system or device operable to implement method  1100 . As such, the preferred initialization point for method  1100  and the order of the steps comprising method  1100  may depend on the implementation chosen. In some embodiments, some steps may be optionally omitted, repeated, or combined. In certain embodiments, method  1100  may be implemented partially or fully in software embodied in computer-readable media. 
     For the purposes of this disclosure, computer-readable media may include any instrumentality or aggregation of instrumentalities that may retain data and/or instructions for a period of time. Computer-readable media may include, without limitation, storage media such as a direct access storage device (e.g., a hard disk drive or floppy disk), a sequential access storage device (e.g., a tape disk drive), compact disk, CD-ROM, DVD, random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), and/or flash memory; as well as other tangible, non-transitory media; and/or any combination of the foregoing. 
     Although the present invention has been described with several embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.