Abstract:
A method for regenerating and amplifying optical signals includes determining a source optical signal, adding a first pump optical signal and a second pump optical signal to the source optical signal to yield an intermediate optical signal, duplicating the intermediate optical signal to yield a first duplicate signal and a second duplicate signal, phase-shifting the first duplicate signal, passing the phase-shifted first duplicate signal and the second duplicate signal bi-directionally through a nonlinear optical element, and performing degenerate phase-sensitive amplification on the phase-shifted first duplicate signal and the second duplicate signal.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to optical communication networks and, more particularly, to optical quadrature phase-shift-keying signal regeneration and amplification. 
     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 are 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 in one circle 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. 
     SUMMARY 
     In one embodiment, a method for regenerating and amplifying optical signals includes determining a source optical signal, adding a first pump optical signal and a second pump optical signal to the source optical signal to yield an intermediate optical signal, duplicating the intermediate optical signal to yield a first duplicate signal and a second duplicate signal, phase-shifting the first duplicate signal, passing the phase-shifted first duplicate signal and the second duplicate signal bi-directionally through a nonlinear optical element, and performing degenerate phase-sensitive amplification on the phase-shifted first duplicate signal and the second duplicate signal. 
     In another embodiment, a system for regenerating optical signals includes an input configured to accept a source optical signal, a dual-pump source configured to generate a first pump optical signal and a second pump optical signal, a coupler communicatively coupled to the input and the dual-pump source configured to add the first pump optical signal and the second pump optical signal to the source optical signal to yield an intermediate optical signal, a wavelength selective processor configured to accept the intermediate optical signal, and a nonlinear optical element communicatively coupled at two ends to the wavelength selective processor. The first wavelength selective processor is configured to duplicate the intermediate optical signal to yield a first duplicate signal and a second duplicate signal, phase-shift the first duplicate signal, and send the phase-shifted first duplicate signal and the second duplicate signal through the nonlinear optical element in opposite directions. The nonlinear optical element is configured to perform degenerate phase-sensitive amplification on the phase-shifted first duplicate signal and the second duplicate signal. The wavelength selective processor includes a wavelength selective switch. 
    
    
     
       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 for optical QPSK signal regeneration and amplification; 
         FIG. 2  is an illustration of an example embodiment of an optical amplifier for use with a QPSK input signal; 
         FIG. 3  is an illustration of an example embodiment of an optical amplifier for use with a DP-QPSK signal; 
         FIG. 4  is an illustration of another example embodiment of an optical amplifier for use with a DP-QPSK signal; and 
         FIG. 5  is an example embodiment of a method for optical signal regeneration and amplification of signals modulated with QPSK or DP-QPSK modulation formats. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example embodiment of a system  100  configured for optical QPSK signal regeneration and amplification. In one embodiment, system  100  may include components with a wavelength-selective processor to conduct optical QPSK signal regeneration and amplification. In a further embodiment, such wavelength selective processors may be reconfigurable. In another embodiment, system  100  may include a bi-directional phase-sensitive amplifier to conduct optical QPSK signal regeneration and amplification. In a further embodiment, such phase-sensitive amplification may be degenerate. The optical signal regeneration and amplification of optical QPSK modulation formats may be conducted by one or more optical amplifiers, such as optical amplifier  102 . 
     Optical amplifier  102  may be configured to regenerate and amplify optical signals in system  100 . System  100  may include an input signal  110  to be regenerated and 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 regeneration and amplification as described herein. Example implementations of all or part of optical amplifier  102  may include amplifiers  200 ,  300 , and  400  as shown in  FIGS. 2 ,  3 , and  4 , respectively. Optical amplifier  102  may include a processor  104  coupled to a memory  106 . In one embodiment, to perform optical signal regeneration and 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 signals to adjust signal information such as phase, power and chromatic dispersion. In another embodiment, to perform optical signal regeneration and amplification, optical amplifier  102  may include components for performing dual-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 nonlinear optical element. In yet another further embodiment, passing such signals bi-directionally may include separately and simultaneously processing the input signal&#39;s imaginary and real components in each such direction by passing the signal in a given direction through the nonlinear optical element. 
     Specifically, optical amplifier  102  may generate pump laser signals, which may be used to create idler signals which are then added to the input signal. The resulting input signal and idler signals may become degenerate after wave mixing. Optical amplifier  102  may be configured to conduct four-wave mixing (“FWM”) which amplifies input signal  110  and accepts the symmetric idler signals. The wavelengths of the pump and idler signals may be equidistant (or nearly equidistant) from the wavelength of input signal  110 . Optical amplifier  110  may be configured to apply FWM to input signal  110  and to the pump signals and thus reduce the phase noise on the symbols of input signal  110 . The equidistant or nearly equidistant wavelengths may include wavelengths that are, for example, perfectly equidistant or approximately equidistant such that overall performance is not impacted significantly. Such approximately equidistant wavelengths may include wavelength differences between the idler signals and input signal  110  that are approximately equal, or wavelength differences between the pump signals and input signal that are approximately equal. In one embodiment, approximately equal wavelength differences may include wavelength differences that vary less than ten percent of in terms of wavelength in relation to the wavelength of input signal  110 . 
     Input signal  110  may include an optical signal modulated through any suitable QPSK or dual-polarization (“DP”) QPSK. 
     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. 
     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 regenerate and amplify input signal  110  and output the result as output signal  114 . 
     Some methods of regenerating a signal to overcome phase noise may include optical-electrical-optical (“OEO”) regeneration methods. Such methods may include, for example, converting optical signals into electronic signals. Such conversion may occur after demultiplexing. The electronic signals may be switched and then converted back into optical signals, which may then be multiplexed onto optical networks. In one embodiment, optical amplifier  102  may not use OEO regeneration methods. In another embodiment, optical amplifier  102  may regenerate a received signal using optical mechanisms and without converting the information in the optical mechanisms to electronic format. 
       FIG. 2  is an illustration of an example embodiment of an optical amplifier  200  for use with a QPSK input signal. 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 QPSK signal  202 . The input signal may be communicatively coupled to dual-pump source  208  through coupler  210 . Coupler  210  may be configured to couple input QPSK signal  202  and dual-pump source  208  and provide the output to optical circulator  214 , which may be communicatively coupled on a first input/output line to a wavelength selective processor  216  and on a second input/output line to a bandpass filter  238 . Wavelength selective processor  216  may be communicatively coupled through two outputs to either end of an optical nonlinear element  218 . The output of bandpass filter  238  may be communicatively coupled to output QPSK signal  244 , which may implement fully or in part output signal  114  of  FIG. 1 . 
     Input QPSK signal  202  and output QPSK signal  244  may include, for example, optical signals modulated using QPSK or DP-QPSK. 
     Optical circulator  214  may include any suitable mechanism for selective routing of inputs and outputs according to the present disclosure. For example, optical circulator  214  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 may operate in clockwise fashion such that the input from coupler  210  is output to wavelength selective processor  216 , and input from wavelength selective processor  216  is output to bandpass filter  238 . 
     Dual-pump source  208  may include any suitable mechanism for outputting two optical pump signals of given wavelengths and frequencies. Dual-pump source  208  may be implemented with configurable laser sources. The configuration of dual-pump source  208  may be set by a processor of amplifier  200 , such as that represented as processor  104  in  FIG. 1 . In one embodiment, the configuration of dual-pump source  208  may be set by feedback and control unit  242 . Dual-pump source  208  may be configured to respond to the specific kind of input QPSK signal  202  received by amplifier  200 . 
     In one embodiment, dual-pump source  208  may be configured to produce two pump signals that are symmetrically located on each side of a signal to be amplified, such as input QPSK signal  202 . Each pump signal may be equidistant, or approximately equidistant, from the source signal in terms of wavelength. In another embodiment, each of the pump signals may have exactly the same phase. In a further embodiment, each of the pump signals may have a phase of zero. An example output of coupling the dual-pump signals and the original source signal may be illustrated in diagram  206 . This output may be routed to wavelength selective processor  216  by optical circulator  214 . In one example, each pump of dual-pump source  208  may have a 200-mW power and 350-GHz spacing. 
     Wavelength selective processor  216  may include one or more wavelength selective switches configured to perform optical switching. Such wavelength selective switches may be implemented by any suitable mechanism, including optical components, for conducting such optical switching. Furthermore, wavelength selective processor  216  may include modules, circuitry, or software configured to adjust phase and power levels of components of signals. In addition, wavelength selective processor  216  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  216  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  216  may be configured to divide its input into two signals such that imaginary and real portions of input QPSK signal  202  may be divided and four-wave-mixing performed on each simultaneously. To perform such four-wave mixing, wavelength selective processor  216  may be configured to divide its input into two signals wherein one such signal may include input QPSK signal  202  phase-shifted by 90°. For example, wavelength selective processor  216  may produce the output shown in diagram  220  onto one branch of input to optical nonlinear element  218 —including input QPSK signal  202  phase-shifted by 90° and the pump signals—and produce the output shown in diagram  222  onto the other branch of input to optical nonlinear element  218 —including input QPSK signal  202  and the pump signals. 
     Wavelength selective processor  216  may be configurable to handle a variety of types of high-level modulation formats, such as QPSK or DP-QPSK. To conduct such handling, wavelength selective processor  216  may be configured to determine the type of signal to be handled through, for example, analyzing input signals, referencing user settings, referencing system settings, or receiving input from feedback and control unit  242 . Furthermore, wavelength selective processor  216  may be configured to filter unused idler signals, based on the type of signal in use. 
     Optical nonlinear element  218  may be configured to bi-directionally amplify and regenerate signals passing through either end of optical nonlinear element  218  using four-wave mixing. Such bi-directional amplification may be performed on signals passing simultaneously through optical nonlinear element  218  in each direction. In one embodiment, any nonlinear element that can support bi-directional propagation and nonlinear processing may be used to implement optical nonlinear element  218 . For example, optical nonlinear element  218  may include an optical, highly nonlinear fiber (“HNLF”) of length of two hundred meters, nonlinear 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, optical nonlinear element  218  may include waveguides configured to produce the desired output. In yet other examples, optical nonlinear element  218  may include a silicon waveguide, III-V waveguide, or periodically poled Lithium Niobate (“PPLN”). 
     The input into optical nonlinear element  218  from wavelength selective processor may be illustrate by diagrams  220 ,  222  as discussed above as well as diagrams  224 ,  228 . Diagrams  224 ,  228  illustrate the imaginary and real components of input QPSK signal  202  that have been included by wavelength selective processor  216 . The output of optical nonlinear element  218  may be illustrated in diagrams  226 ,  230 . Diagram  224  may illustrate the real and imaginary components of diagram  204  as they have been phase-shifted. Diagram  228  may illustrate the real and imaginary components of diagram  204  has it was originally provided to wavelength selective processor  216 . In diagram  226 , the imaginary component of input QPSK signal  202  (phase-shifted to the real axis of the input signal illustrated in diagram  202 ) has been regenerated and amplified as a result of its input—the combination of input QPSK signal  202  phase-shifted by 90° and the pump signals—being passed through optical nonlinear element  218 . In diagram  230 , the real component of input QPSK signal  202  has been regenerated and amplified as a result of its input—the combination of input QPSK signal  202  and the pump signals—being passed through optical nonlinear element  218 . 
     The combination of wavelength selective processor  216  and optical nonlinear element  218  may be bi-directional in that signals pass from wavelength selective processor  216  to optical nonlinear element  218  and back to wavelength selective processor  216  in both directions (clockwise and counter-clockwise). By performing separate processing of imaginary and real components of input QPSK signal  202 , 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 nonlinear elements. 
     Wavelength selective processor  216  may be configured to couple the outputs of optical nonlinear element  218  and selectively block or filter the pump signals. Wavelength selective processor  216  may be configured to send the resultant signal to bandpass filter  238  by way of optical circulator  214 . 
     Bandpass filter  238  may be configured to remove the idler signals from the result of FWM by only allowing signals with the wavelength of the original input QPSK signal  202  to pass. Bandpass filter  238  may be implemented in any suitable manner, such as with digital or analog circuitry. Bandpass filter  238  may be configured to generate output QPSK signal  244 . The result of optical amplification and regeneration in output QPSK signal  244  may be illustrated in diagram  232 , wherein the noise present in input QPSK signal  202  as illustrated in diagram  204  has been reduced. 
     Feedback and control unit  242  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 QPSK signal  202 , signals from dual-pump source  208 , or output QPSK signal  244 . Such information may include, for example, wavelength, power, residual chromatic dispersion, and optical signal-to-noise ratio. Based on such information, feedback and control unit  242  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, dual-pump source  208 , wavelength selective processor  216 , and input QPSK signal  202 . Feedback and control unit  242  may be configured to adjust the phase, power, and chromatic dispersion of signals in such portions before passing signals through optical nonlinear element  218 . Control of dual-pump source  208  may be made to match pump signals to be nearly equidistant in wavelength distance from input QPSK signal  202 . Wavelength-selective processor  216  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 optical nonlinear element  218 . Further, wavelength-selective processor  216  may be configured to adjust the phase level of its input signals in order to increase or optimize amplification and regeneration. 
     In operation, input QPSK signal  202  may be received by optical amplifier  200  and communicatively coupled to the output of dual-pump source  208 . Dual-pump source  208  may output two pump signals with wavelengths equidistant from the wavelength of input QPSK signal  202 . The wavelength, power, residual chromatic dispersion, and optical signal-to-noise ratio of input QPSK signal  202  and the pump signals may be monitored by feedback and control unit  242 . Feedback and control unit  242  may adjust the phase, power, and chromatic dispersion of input QPSK signal  202  and the pump signals to maximize or optimize amplification and regeneration. 
     The resultant combination of input QPSK signal  202  and the pump signals may be routed by optical circulator  214  to wavelength selective processor  216 . Wavelength selective processor  216  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 optical nonlinear element  218 , or adjust its input signals&#39; phase levels in order to maximize or optimize amplification and regeneration. 
     Wavelength selective processor  216  may duplicate its input signals and, for one such duplicate, selectively shift the phase of the portion corresponding to input QPSK signal  202  by 90°. Wavelength selective processor  216  may output one such signal to a first terminal of optical nonlinear element  218  and the other such signal to a second terminal of optical nonlinear element  218 . 
     Optical nonlinear element  218  may conduct amplification and regeneration on each received signal. The amplification may include phase-sensitive amplification. The phase-sensitive amplification may be degenerate. In addition, optical nonlinear element  218  may conduct four-wave mixing on each received signal. Such four-wave mixing may cause the regeneration and amplification of the signal. For the received signal including input QPSK signal  202  phase-shifted by 90°, optical nonlinear element  218  may amplify and regenerate the real components of the received signal. For the received signal including input QPSK signal  202  without the phase shifting, optical nonlinear element  218  may amplify and regenerate the imaginary components of the received signal. Optical nonlinear element  218  may send the resultant signals back to wavelength selective processor  216 . 
     Wavelength selective processor  216  may recombine the results received from optical nonlinear element  218  and send the resultant signals to bandpass filter  238  through optical circulator  214 . Bandpass filter  238  may remove the pump signals from the received signal and send the result as output QPSK signal  244 . 
       FIG. 3  is an illustration of an example embodiment of an optical amplifier  300  for use with a DP-QPSK signal. Optical amplifier  300  may implement fully or in part optical amplifier  102  of  FIG. 1 . Optical amplifier  300  may be configured to regenerate and amplify QPSK modulated signals that have been further modulated with dual polarizations. Further, optical amplifier  300  may be configured to split an input signal such as DP-QPSK input signal  302  into x-polarization and y-polarization components and process each such polarization using bi-directional phase sensitive amplification. The implementation and operation of optical amplifier  300  may be otherwise similar to optical amplifier  200  of  FIG. 2 . Coupler  310 , dual-pump source  308 , optical circulators  312 ,  314 , wavelength selective processors  316 ,  320 , optical nonlinear elements  318 ,  322 , bandpass filters  324 ,  328 , and feedback and control unit  342  may be implemented wholly or in part by or in similar fashion to coupler  210 , dual-pump source  208 , optical circulator  214 , wavelength selective processor  216 , optical nonlinear element  218 , bandpass filter  238 , and feedback and control unit  242 , respectively, of  FIG. 2 . 
     Optical amplifier  300  may include a mechanism for accepting an input signal such as input DP-QPSK signal  302 . The input signal may be communicatively coupled to dual-pump source  308  through coupler  310 . Coupler  310  may be configured to couple input DP-QPSK signal  302  and dual-pump source  308  and provide the output to a polarization beam splitter  306 . 
     Polarization beam splitter  306  may be configured to split its input signal according to x-polarizations and y-polarizations. For example, DP-QPSK input signal  302  may include an x-polarization and a y-polarization component as illustrated in diagram  304 . Thus, polarization beam splitter  306  may be configured to output the x-polarization of the combination of DP-QPSK input signal  302 &#39;s x-polarization and the pump signals and to output the polarization of the combination of DP-QPSK input signal  302 &#39;s y-polarization and the pump signals. Polarization beam splitter  306  may be configured to output each polarization to a different amplification and regeneration stage. Each such amplification and regeneration stage may be implemented in similar fashion to the combination of optical circulator  214 , wavelength selective processor  216 , optical nonlinear element  218 , and bandpass filter  238  as illustrated in  FIG. 2 . Polarization beam splitter  306  may be implemented in any suitable manner for splitting its input signals into x-polarization and y-polarization components. 
     In one embodiment, polarization beam splitter  306  may be configured to pass the resultant x-polarization signal to optical circulator  314  and the resultant y-polarization signal to optical circulator  312 . In another embodiment, polarization beam splitter  306  may be configured to pass the resultant y-polarization signal to optical circulator  314  and the resultant x-polarization signal to optical circulator  312 . 
     The configuration, operation, and implementation of optical circulator  314 , wavelength selective processor  316 , optical nonlinear element  318 , and bandpass filter  324  may be performed in similar fashion to the configuration, operation, and implementation of optical circulator  214 , wavelength selective processor  216 , optical nonlinear element  218 , and bandpass filter  238  of  FIG. 2 . Thus, optical circulator  314 , wavelength selective processor  316 , optical nonlinear element  318 , and bandpass filter  324  may be configured to amplify and regenerate, through bi-directional phase-sensitive amplification, an x-polarization or y-polarization of input DP-QPSK signal  302 . Optical circulator  314 , wavelength selective processor  316 , optical nonlinear element  318 , and bandpass filter  324  may output an x-polarization or y-polarization of a regenerated and amplified signal. Optical amplifier  300  may include a delay  326  to correct for resulting behavior from polar-specific operations such as drift. 
     The configuration, operation, and implementation of optical circulator  312 , wavelength selective processor  320 , optical nonlinear element  322 , and bandpass filter  328  may be performed in similar fashion to the configuration, operation, and implementation of optical circulator  214 , wavelength selective processor  216 , optical nonlinear element  218 , and bandpass filter  238  of  FIG. 2 . Thus, optical circulator  312 , wavelength selective processor  320 , optical nonlinear element  322 , and bandpass filter  328  may be configured to amplify and regenerate, through bi-directional phase-sensitive amplification, an x-polarization or y-polarization of input DP-QPSK signal  302 . Optical circulator  312 , wavelength selective processor  320 , optical nonlinear element  322 , and bandpass filter  328  may output an x-polarization or y-polarization of a regenerated and amplified signal. Accordingly, optical amplifier  300  may include a delay  326  to correct for resulting behavior from polar-specific operations such as drift. 
     Polarization beam combiner  330  may be configured to receive the output from bandpass filter  328  and bandpass filter  324  and recombine the x-polarization and y-polarization signals that have been amplified and regenerated. Further, polarization beam combiner  330  may be configured to output DP-QPSK output signal  332 . The result of bi-directional amplification and regeneration of DP-QPSK input signal  302  may be illustrated in diagram  334 , wherein noise in DP-QPSK input signal  302  as shown in diagram  304  has been reduced. 
     Feedback and control unit  342  may be configured to monitor performance of optical amplifier  300  and its signals. Such monitoring may be conducted in real-time and may include, for example, information regarding input DP-QPSK signal  302 , signals from dual-pump source  308 , output of polarization beam splitter  306 , output of bandpass filter  328 , or output of bandpass filter  324 . Such information may include, for example, wavelength, power, residual chromatic dispersion, and optical signal-to-noise ratio. Based on such information, feedback and control unit  342  may be configured to adjust or control the operation of various portions of optical amplifier  300  to enhance or optimize performance of optical amplifier  300 . Such portions may include, for example, dual-pump source  308 , wavelength selective processors  316 ,  320 , input DP-QPSK signal  302 , polarization beam splitter  306 , and polarization beam combiner  330 . Feedback and control unit  342  may be configured to adjust the phase, power, and chromatic dispersion of signals in such portions before passing signals through optical nonlinear elements  318 , 322 . Control of dual-pump source  308  may be made to match pump signals to be nearly equidistant in wavelength distance from input DP-QPSK signal  302 . Wavelength-selective processors  316 ,  320  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 optical nonlinear elements  318 ,  322 . Further, wavelength-selective processors  316 ,  320  may be configured to adjust the phase level of its input signals in order to increase or optimize amplification and regeneration. 
     In operation, DP-QPSK input signal  302  may be received by optical amplifier  300  and communicatively coupled to dual-pump source  308 . Dual-pump source  308  may output two pump signals with wavelengths equidistant from the wavelength of input DP-QPSK signal  302 . The wavelength, power, residual chromatic dispersion, and optical signal-to-noise ratio of input QPSK signal  302  and the pump signals may be monitored by feedback and control unit  342 . Feedback and control unit  342  may adjust the phase, power, and chromatic dispersion of input DP-QPSK signal  302  and the pump signals to maximize or optimize amplification and regeneration. 
     The resultant signal may be split into x-polarization and y-polarization signals by polarization beam splitter  306 . In one embodiment, the x-polarization of the resultant signal may be routed by optical circulator  314  to wavelength selective processor  316  and the y-polarization of the resultant signal may be routed by optical circulator  312  to wavelength selective processor  320 . In another embodiment, the y-polarization of the resultant signal may be routed by optical circulator  314  to wavelength selective processor  316  and the x-polarization of the resultant signal may be routed by optical circulator  312  to wavelength selective processor  320 . 
     Wavelength selective processors  316 ,  320  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 optical nonlinear elements  318 ,  322 , or adjust their input signals&#39; phase levels in order to maximize or optimize amplification and regeneration. 
     Wavelength selective processors  316 ,  320  may duplicate their input signals and, for one such duplicate, selectively shift the phase of the portion corresponding to input DP-QPSK signal  302  by 90°. Wavelength selective processors  316 ,  320  may each output one such signal to a first terminal of optical nonlinear elements  318 ,  322 , respectively, and the other such signal to a second terminal of optical nonlinear elements  318 ,  322 , respectively. 
     Optical nonlinear elements  318 ,  322  may each conduct amplification and regeneration on each received signal. The amplification may include phase-sensitive amplification. The phase-sensitive amplification may be degenerate. In addition, optical nonlinear elements  318 ,  322  may conduct four-wave mixing on each received signal. Such four-wave mixing may cause the regeneration and amplification of the signal. For the received signals including input DP-QPSK signal  302  phase-shifted by 90°, optical nonlinear elements  318 ,  322  may amplify and regenerate the real components of the received signals. For the received signals including input DP-QPSK signal  302  without the phase shifting, optical nonlinear elements  318 ,  322  may amplify and regenerate the imaginary components of the received signals. Optical nonlinear elements  318 ,  322  may send the resultant signals back to wavelength selective processors  316 ,  320 , respectively. 
     Wavelength selective processors  316 ,  320  may recombine the results received from optical nonlinear elements  318 ,  322 , respectively, and send the resultant signals to bandpass filters  328 ,  324  through optical circulators  314 ,  312 , respectively. Bandpass filters  328  may remove the pump signals from the received signals and send the results to polarization beam combiner  330 . Bandpass filter  324  may remove the pump signals from the received signals and send the results to delay  326 . Delay  326  may compensate for conditions such as drift introduced between the x-polarization and y-polarization signals. Delay  326  may send the resulting signal to polarization beam combiner  330 . Polarization beam combiner  330  may reassemble the amplified and regenerated x-polarization and y-polarization components into DP-QPSK output signal  332 . 
       FIG. 4  is an illustration of another example embodiment of an optical amplifier  400  for use with a DP-QPSK signal. Optical amplifier  400  may implement fully or in part optical amplifier  102  of  FIG. 1 . Optical amplifier  400  may be configured to produce the same or similar results as optical amplifier  300  of  FIG. 3  but may vary from optical amplifier  300  in terms of its components. Optical amplifier  400  may be configured to regenerate and amplify QPSK modulated signals that have been further modulated with dual polarizations. Further, optical amplifier  400  may be configured to split an input signal such as DP-QPSK input signal  402  into x-polarization and y-polarization components and process each such polarization using bi-directional phase sensitive amplification. 
     DP-QPSK input signal  402 , coupler  410 , dual-pump source  408 , optical circulator  414 , wavelength selective processor  418 , polarization beam splitters  424 ,  426 , optical nonlinear elements  420 ,  422 , bandpass filter  430 , feedback and control unit  442 , and DP-QPSK output signal  432  may be implemented wholly or in part by or in similar fashion to DP-QPSK input signal  302 , coupler  310 , dual-pump source  308 , optical circulators  314 ,  312 , wavelength selective processors  316 ,  320 , polarization beam splitter  306 , optical nonlinear elements  318 ,  320 , bandpass filters  328 ,  324 , feedback and control unit  342 , and DP-QPSK output signal  332  respectively, of  FIG. 3 . 
     Optical amplifier  400  may be configured to accept DP-QPSK input signal  402 , which may include an x-polarization and a y-polarization component as illustrated in diagram  404 . The input signal may be communicatively coupled to dual-pump source  408  through coupler  410 . Coupler  410  may be configured to couple input DP-QPSK signal  402  and dual-pump source  408  and provide the output to a polarization controller  416  optical via optical circulator  414 . 
     Polarization controller  416  may be configured to adjust the x-polarization and y-polarization components of its input signals to maximize or increase the effects of amplification and regeneration to be performed by, for example, optical nonlinear elements  420 ,  422 . Such adjustments may include a polarization-shifting of either the x-polarization and y-polarization components. Furthermore, polarization controller  416  may be configured to adjust such components after they have been amplified and regenerated. Polarization controller  416  may be implemented in any suitable manner to perform such adjustments. 
     Polarization controller  416  may be configured to send its resulting signals to wavelength selective processor  418 . Wavelength selective processor  418  may be configured to duplicate its input signals, phase-shift a portion corresponding to DP-QPSK input signal  402  of one such resulting signal, and send one resulting signal to polarization beam splitter  424  and the other such resulting signal to polarization beam splitter  426 . The phase-shifted portion corresponding to DP-QPSK input signal  402  may be phase-shifted by 90°. In one embodiment, wavelength selective process  418  sends the phase-shifted signal to polarization beam splitter  424  and the non-phase-shifted signal to polarization beam splitter  426 . In another embodiment, wavelength selective processor  418  sends the phase-shifted signal to polarization beam splitter  426  and the non-phase-shifted signal to polarization beam splitter  424 . 
     Polarization beam splitters  426 ,  424  may each be configured to split its received signal into x-polarization and y-polarization components. In one embodiment, polarization beam splitters  426 ,  424  may be configured to route the resulting x-polarization components to optical nonlinear element  420  and the resulting y-polarization components to optical nonlinear element  422 . In another embodiment, polarization beam splitters  426 ,  424  may be configured to route the resulting y-polarization components to optical nonlinear element  420  and the resulting x-polarization components to optical nonlinear element  422 . 
     The configuration, operation, and implementation of wavelength selective processor  418  and optical nonlinear elements  420 ,  422  may otherwise be performed in similar fashion to the configuration, operation, and implementation of, for example, wavelength selective processor  316  and optical nonlinear element  318  or of wavelength selective processor  320  and optical nonlinear element  322  of  FIG. 3 . Thus, wavelength selective processor  418  and optical nonlinear elements  420 ,  422  may be configured to amplify and regenerate, through bi-directional phase-sensitive amplification, an x-polarization and y-polarization of input DP-QPSK signal  402 . Optical amplifier  400  may include a delay  426  configured to correct for resulting behavior from polar-specific operations such as drift. 
     Wavelength selective processor  418  may be configured to receive and combine the resulting amplified and regenerated signals from an individual branch of execution corresponding to optical nonlinear element  420  or optical nonlinear element  422 . Such a set of amplified and regenerated signals may correspond to an individual set of one of the x-polarization or y-polarization components. Wavelength selective processor  418  or polarization controller  416  may be configured to recombine the resulting amplified and regenerated x-polarization and y-polarization components together. Polarization controller  416  may be configured to adjust the x-polarization or y-polarization components as necessary to account for effects such as drift or signal manipulation. Polarization controller  416  may be configured to send the resultant signals to bandpass filter  430  through optical circulator  414 . 
     Bandpass filter  430  may be configured to selectively remove pump or other signals from its received signals and to output DP-QPSK output signal  432 . The result of bi-directional amplification and regeneration of DP-QPSK input signal  402  may be illustrated in diagram  434 , wherein noise in DP-QPSK input signal  402  as shown in diagram  404  has been reduced. 
     Feedback and control unit  442  may be configured to monitor performance of optical amplifier  400  and its signals. Such monitoring may be conducted in real-time and may include, for example, information regarding input DP-QPSK signal  402 , signals from dual-pump source  408 , output of polarization beam splitters  424 ,  426 , or output of bandpass filter  430 . Such information may include, for example, wavelength, power, residual chromatic dispersion, and optical signal-to-noise ratio. Based on such information, feedback and control unit  442  may be configured to adjust or control the operation of various portions of optical amplifier  400  to enhance or optimize performance of optical amplifier  400 . Such portions may include, for example, dual-pump source  408 , wavelength selective processor  418 , polarization beam splitters  424 ,  426 , polarization controller  416 , delay  428 , and bandpass filter  430 . Feedback and control unit  442  may be configured to adjust the phase, power, and chromatic dispersion of signals in such portions before passing signals through optical nonlinear elements  420 ,  422 . Control of dual-pump source  408  may be made to match pump signals to be nearly equidistant in wavelength distance from input DP-QPSK signal  402 . Wavelength-selective processor  418  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 optical nonlinear elements  420 ,  422 . Further, wavelength-selective processor  418  may be configured to adjust the phase level of its input signals in order to increase or optimize amplification and regeneration. Polarization controller  418  and polarization beam splitters  424 ,  426  may be adjusted to maximize or increase signal regeneration and amplification by subsequently adjusting x-polarization or y-polarization components of signals. Bandpass filter  430  may be adjusted to selectively filter the output of dual-pup source  408 . 
     In operation, DP-QPSK input signal  402  may be received by optical amplifier  400  and communicatively coupled to dual-pump source  408 . Dual-pump source  408  may output two pump signals with wavelengths equidistant from the wavelength of input DP-QPSK signal  402 . The wavelength, power, residual chromatic dispersion, and optical signal-to-noise ratio of input QPSK signal  402  and the pump signals may be monitored by feedback and control unit  442 . Feedback and control unit  442  may adjust the phase, power, and chromatic dispersion of input DP-QPSK signal  402  and the pump signals to maximize or optimize amplification and regeneration. 
     The resultant signal may be routed to polarization controller  416  through optical circulator  414 . The x-polarization and y-polarization components of the signal may be adjusted by polarization controller  416 , which may receive input for such adjustments from feedback and control unit  442 . Polarization controller  416  may send the resulting signals to wavelength selective processor  418 . 
     Wavelength selective processor  418  may pre-compensate its 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 optical nonlinear elements  420 ,  422 , or adjust their input signals&#39; phase levels in order to maximize or optimize amplification and regeneration. 
     Wavelength selective processor  418  may duplicate its input signals and, for one such duplicate, selectively shift the phase of the portion corresponding to input DP-QPSK signal  402  by 90°. Wavelength selective processor  418  may output one such signal to a first polarization beam splitter  424  to then be output to a first terminal of optical nonlinear elements  420 ,  422 , respectively, and the other such signal to a second polarization beam splitter  426  to then be output to a second terminal of optical nonlinear elements  420 ,  424 , respectively. 
     Polarization beam splitters  424 ,  426  may split their received signals into x-polarization and y-polarization components. In one embodiment, polarization beam splitter  424  may route the x-polarization component of its received signal to a first terminal of optical nonlinear element  420  and the y-polarization component of its received signal to a first terminal optical nonlinear element  422 , and polarization beam splitter  426  may route the x-polarization component of its received signal to a second terminal of optical nonlinear element  420  and the y-polarization component of its received signal to a second terminal optical nonlinear element  422 . In another embodiment, polarization beam splitter  424  may route the y-polarization component of its received signal to a first terminal of optical nonlinear element  420  and the x-polarization component of its received signal to a first terminal optical nonlinear element  422 , and polarization beam splitter  426  may route the y-polarization component of its received signal to a second terminal of optical nonlinear element  420  and the x-polarization component of its received signal to a second terminal optical nonlinear element  422 . 
     Optical nonlinear elements  420 ,  422  may each conduct amplification and regeneration on each received signal. The amplification may include phase-sensitive amplification. The phase-sensitive amplification may be degenerate. In addition, optical nonlinear elements  420 ,  422  may conduct four-wave mixing on each received signal. Such four-wave mixing may cause the regeneration and amplification of the signal. For the received signals including input DP-QPSK signal  402  phase-shifted by 90°, optical nonlinear elements  420 ,  422  may amplify and regenerate the real components of the received signals. For the received signals including input DP-QPSK signal  402  without the phase shifting, optical nonlinear elements  420 ,  422  may amplify and regenerate the imaginary components of the received signals. Optical nonlinear elements  420 ,  422  may send the resultant signals back to wavelength selective processor  418  respectively. Delay  428  may be used to delay the x-polarization or the y-polarization components in order to compensate for any length mismatch between the two paths to nonlinear elements  420 ,  422 . 
     Wavelength selective processor  418  may recombine the results received from optical nonlinear elements  420 ,  422  resultant signals to polarization controller  416 , which may send the result to bandpass filter  430  through optical circulator  414  Bandpass filter  430  may remove the pump signals from the received signals. Bandpass filter  430  may produce DP-QPSK output signal  432 . 
       FIG. 5  is an example embodiment of a method  500  for optical signal regeneration and amplification of signals modulated with QPSK or DP-QPSK modulation formats. Method  500  may include using wavelength selective processors and phase-sensitive amplification. 
     In step  505 , a signal to be regenerated and amplified may be determined. The signal may be modulated using, for example, QPSK or DP-QPSK modulation techniques. Compensation for noise and errors in input signal and in the equipment used to regenerate and amplify the input signal may be determined and applied. Determinations of equipment of operation of equipment necessary to regenerate and amplify the signal may be made. Based on the determination of the signal to be regenerated and amplified through, for example, analysis of the signal, software settings, or user settings, the physical embodiment of the system or optical amplifier may be programmed to handle the type of signal. For example, the pump signals generated and selected in step  510  or whether to engage beam splitters and combiners for a dual-polarized QPSK signal may be based upon such programming. 
     In step  510 , a dual-pump signal may be generated and communicatively coupled to the input signal. The pump signals may have a wavelength near the source signal. The difference in wavelength between the each of the pump signals and the source signal may be the same or nearly the same. In one embodiment, the first pump signal may have a shorter wavelength than the input signal while the second pump signal may have a longer wavelength than the input signal. The pump signals may lie symmetrically around the input signal in terms of wavelength. The input signal may be added to the first pump signal. In one embodiment, the pump signal may have no phase elements. 
     In step  515 , if the input signal is a DP-QPSK signal, the input signal and pump signals may be divided into x-polarization and y-polarization components. Each such component may be handled, for example, by parallel performance of steps  520 - 540 . In one embodiment, step  515  may be performed before operation of step  520  to perform duplication of input signals and phase-shifting of one such copy. In another embodiment, step  515  may be performed after operation of step  520 . Processing of x-polarization and y-polarization components may be performed in parallel using, for example, the configurations shown in  FIG. 3  or  4 . Delay may be applied to the x-polarization or y-polarization components to compensate for effects such as drift or path mismatch. 
     In step  520 , the input signal and pump signals may be duplicated. In step  525 , one such signal may be phase-shifted by 90°. In step  530 , one such resulting copy may be may be sent to a first terminal of an optical nonlinear element and the other copy may be sent to a second terminal of the optical nonlinear element. 
     In step  535 , the optical nonlinear element, or other suitable mechanism, may perform bi-directional amplification and signal generation on the two signals. The optical nonlinear element may utilize FWM or degenerate phase-sensitive amplification. The resulting signals may include an amplified signal with reduced phase and amplitude noise. The processing of the input signal that has been phase-shifted may be applicable to amplify and regenerate the real components of the input signal. The processing of the input signal that has not been phase-shifted may be applicable to amplify and regenerate the imaginary components of the input signal. 
     In step  540 , the signals may be reassembled. Such reassembly may include reassembling the real and imaginary portions of the amplified and regenerated signal or reassembling the x-polarization and y-polarizations after amplification and regeneration. In step  545 , the pump signals may be filtered out of the resulting signal. In step  550 , the resulting signal may be output. 
     Method  500  may be implemented using the system and optical amplifiers of  FIGS. 1-4 , or any other system or device operable to implement method  500 . As such, the preferred initialization point for method  500  and the order of the steps comprising method  500  may depend on the implementation chosen. In some embodiments, some steps may be optionally omitted, repeated, or combined. In certain embodiments, method  500  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.