Abstract:
A parallel monitored and controlled optical PMD compensator comprises a branch optical signal split from an optical signal path. A polarization controller (PC) and differential group delay are disposed in each of the paths. A controller adjusts polarization compensation of the PCs in response to PMD dispersion of the branch optical signal. A PMD monitor is preferably disposed in the branch path providing a monitor signal to the controller for use in adjusting the PCs. A polarization rotator may inject a reference signal into the paths with the PC disposed in the branch path acting as a polarization scrambler. A state of polarization (SOP) of the reference signal may be monitored by polarimeters disposed in both paths and the SOP of the reference signal in the branch path may be provided to the controller for adjusting polarization compensation of the inline PC.

Description:
RELATED APPLICATION  
       [0001]    The present invention is related to copending, commonly assigned U.S. patent application Ser. No. 09/940,183, entitled Low Cost Wave Plate Emulator for Polarization control in a Fiber Optic System, filed on Aug. 27, 2001, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention generally relates to fiber optical communications technologies and more specifically to polarization mode dispersion compensator parallel monitoring and control architectures.  
         BACKGROUND OF THE INVENTION  
         [0003]    Optical transmissions inside a single mode fiber are subject to at least two types of fundamental limitations, power loss and dispersion. Such limitations are often represented as a penalty to the distance an optical signal can be transmitted subject to a tolerable signal to noise ratio. Advances in optical amplification technologies using erbium doped fiber amplifiers (EDFA) have provided, to a large extent, effective solutions to overcome loss limited transmission distance problems. On the other hand, solutions to dispersion problems, which are highly bit rate dependent, have not yet been effectively forthcoming.  
           [0004]    A particular dispersion mode problem in fiber optic telecommunications is polarization mode dispersion (PMD) where optical pulses spread out in time. Pulse widths get longer and limit the data rate that can be transmitted. Due to physical birefringence in the fiber which is amplified over multi-kilometer distances, the front part of a pulse and the back part of a pulse will start to separate and develop different polarizations. Birefringence causes one polarization mode to travel slower than the other.  
           [0005]    Prior art polarization mode dispersion compensators (PMDC) are plagued by high order effects such as local sub-optima which can severely degrade the performance of a compensator. Most architectures involve feedback controlled inline compensators, which attempt to track an optimum polarization state as it drifts. Because the line data runs through the compensator, such a PMDC will cause transmission errors if the PMDC deviates from the optimum polarization state. Therefore, prior art PMDCs are not given the freedom to look elsewhere in the optical parameter space for an optimum polarization state, and thus the PMDC may be operating at a local rather than a global optimum polarization state.  
           [0006]    At a receiver, a PMDC may be installed to cancel out the effect of the distortions that occur along the optical transmission fiber line. Many different techniques for compensating polarization mode dispersion (PMD) in fiber optic systems have been proposed. The most commonly reported type is a feedback-based inline optical compensator. With attention directed to FIG. 1, conventional prior art PMDC  100  is shown for reference. Incoming light signal  101  is processed by endless polarization controller (PC)  102  and PMD in the signal is compensated by inline differential group delay (DGD)  103 . The control parameters on endless PC  102  are dithered by controller  104  so that a monitor signal at Monitor (Mon)  105  is always optimized. In such a prior art compensator  100 , consisting of endless polarization controller (PC)  102  in sequence with a differential group delay (DGD)  103 , DGD  103  generally mirrors the PMD of the incoming signal. DGD  103  can be adjusted by manipulating a magnitude of the birefringence of DGD  103 , or more conventionally, manipulating settings for endless PC  102 . The feedback loop of PMDC  100  generally optimizes a monitor signal at MON  105 . Controller  104  searches for the correct parameters for endless PC  102 . By extension, the value of the DGD may be optimized as well. Problematically, architecture  100  is sensitive to parameter space distortions from the presence of high order PMD.  
           [0007]    Polarization controllers may involve one of several prior art technologies, such as lithium niobate based PCs. Problematically, an additional prior art constraint is that the PC employed in prior art PMDCs must be endless, meaning that the PC can transform polarization states which are varying without the need to reset the PC or its control voltages. Minimally, the PC must at least be able to be reset without disrupting the optical signal in order to provide interruption free signal output.  
           [0008]    The DGDs of a prior art PMDC employ the first order of PMD, which results in a differential group velocity delay between two orthogonal states of polarization. A DGD may be comprised of a piece of birefringent polarization maintaining fiber or a birefringent crystal, such as calcite, where an X axis polarization has a larger index of refraction than a Y axis polarization, with the signal propagating along the Z axis. The monitor makes a measure of output signal quality, such as a degree of polarization or state of polarization (SOP). The control is a processor based device which optimizes the monitor signal by dithering the PC controls, such as control voltages.  
           [0009]    Conventional inline compensator  100  tracks a minimum signal distortion state using a feedback based dithering scheme and does not have the freedom to explore other portions of the optical parameter space. This can be problematic if the minimum distortion state turns into a local sub-minimum, or disappears as the optical parameter space evolves with temperature fluctuations and vibrations in the transmission fiber.  
           [0010]    A problem arises in certain cases because the endless PC generally has two or three degrees of freedom. To optimize the degrees of freedom, two or three voltages, or other parameters controlling the degrees of freedom are adjusted until the best monitor signal is obtained. Problematically, more than one setting on the PC may give a good monitor signal. Generally, optimum acceptable peaks for the monitored output signal in the optical parameter space are sought. These optimum signal quality peaks are time dependent based on thermal and acoustic fluctuations of the fiber. There are multiple peaks, some may be higher than others, and essentially prior art feedback schemes attempt to track the peaks as they move around in local parameter space. The optimal control voltages on the PC are maintained to provide the best compensation. However, prior art feedback systems do not ensure that the highest global peak is being employed. The prior art systems only provide local peaks which over time transform. Thus, local peaks may not be the global peak. The prior art has failed to resolve this issue. Prior art systems employ the aforementioned feedback loop assuming that a global peak results, which may or may not be the case. Problematically, a prior art feedback loop does not look to the entire parameter space.  
           [0011]    In prior art FIG. 2, the afore-described feedback control concept is applied to two-section PMDC  200  to control PMD of input optical signal  201 . The prior art illustrated in FIG. 2 provides more degrees of freedom then the structure depicted in FIG. 1. Two compensating DGDs  203  and  207  are employed each having its own polarization controller  202  and  206 , respectively. The structure for monitoring to provide control is similar to FIG. 1. In the two section PMDC  200  of FIG. 2, monitor  205  feeds back an output signal to controller  204 . Signal  201  is optimized by controlling the control voltages on both PCs  202  and  206 . The difference is that more parameters, or degrees of freedom, are provided. However, the aforementioned problem with the local and global optimum peaks is still present.  
           [0012]    A scheme which provides offline analysis of PMD is described in “ Real - Time Principal State Characterization for Use in PMD Compensators, ” by Chou, Fini, and Haus, IEEE PTL vol. 13, no. 6, June 2001, which is incorporated by reference herein in its entirety and which is co-authored by a present inventor. In that work, an optical branch characterizes first order PMD and feeds forward the information to a dither-free polarization controller and compensator. However, the scheme disclosed therein employs a polarization scrambled at the transmitter to provide multiple measurable polarizations. Therefore alteration of the optical signal transmitter is required for such a PMD monitoring system. Furthermore, this scheme employs estimations of PMD derived from measurements of the SOP and degree of polarization (DOP) for an optical signal rather than direct measurements of the signal&#39;s PMD.  
         BRIEF SUMMARY OF THE INVENTION  
         [0013]    The present invention is directed to systems and methods for PMDC parallel architectures used to scan the entire optical parameter space offline to monitor PMD and control PMDCs. The purpose of the architectures described herein is to provide the control processor of a dynamic PMD compensator with full characterization of a compensator&#39;s parameter space. Such a parallel architecture preferably contains a power splitter which allows the PMDC parameter space to be analyzed in a branch path without disturbing the data flowing through the inline path. The branch path contains compensator components, namely at least a PC and DGD. In one embodiment, these components are a reproduction of the inline PC and DGD. The present parallel architecture also includes a signal quality monitor, and may contain other polarization sensing devices for analysis purposes. The present technique can be expanded to include multi-section PMDCs. The present systems and methods do not require altering the optional signal transmitter. Additionally, the methods and systems described herein operate independently of the type of monitoring used. Therefore, the present methods and systems may employ measurements systems that more closely correlated to signal impairment than DOP, thereby improving overall performance and reliability.  
           [0014]    The purpose of this PMDC architecture is to operate the compensator with knowledge of the full parameter space. A parallel architecture is beneficial because analysis of a split off signal branch, a parallel branch, allows application of control parameters, such as control voltages, to a polarization controller which may distort the signal further or in such ways that may not necessarily optimize the signal, but which will facilitate analysis of the entire parameter space. This analysis cannot be carried out with a prior art inline compensator because the signal is passing through the prior art compensator. So if the voltages are varied in a prior art inline compensator to determine global or local peaks, the signal itself is distorted. Preferably, a PC adjusting the actual signal is optimizing the signal at all times. In the present parallel architecture the signal being analyzed is not inline; the signal being analyzed is not actually being received. Therefore, it may be distorted for the purpose of analysis, because the analysis signal is off-line. The present invention allows parameter space to be fully swept, facilitating measurement of the monitored analysis signal at different combinations of control voltages on the PC. This facilitates determination of global maximum and local peak PMD compensation parameters. This information is very useful and is advantageously generated off-line.  
           [0015]    The present invention sweeps out the full optical parameter space. Control electronics looks through the entire parameter space and finds the best PMD compensation value. To sweep out the parameter space, the control voltages of the offline PC are preferably ramped to generate every combination of control voltages. This provides a monitor signal associated with each combination, essentially defining the parameter space. Advantageously, the parallel architecture provides the ability to scan the entire optical parameter space so that problems associated with local sub-optima can be eliminated. For example, scanning the entire space can ensure that the global optimum is selected, not a local one. Additionally, recovery from an outage can be faster if an inline PC does not need to search the entire optical parameter space. As a further advantage, the offline PC in selected embodiments of the present invention need not be endless.  
           [0016]    Another advantage to knowing the entire parameter space is that evolution of the parameter space can be tabulated allowing better decisions to be made by control circuitry about how to vary control, such as control voltages, on a PC over time. For example, it may be desirable to be able to choose an optimal path along the parameter space to avoid the generation of outages. In the prior art, an inline PC is entirely dependent on feedback control. An algorithm to control prior art PCs has a path which is a function of the algorithm itself and how the parameter space is perceived as varying. This leads to resets of non-endless PCs, or a need for complicated endless algorithms applied to avoid resets. This can also lead to non-optimal paths in the parameter space which result in momentary signal outages. Full knowledge of the parameter space allows off-line optimization in real-time without a need for resets or complicated algorithms to avoid resets.  
           [0017]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0018]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:  
         [0019]    [0019]FIG. 1 is a diagramatic representation of a prior art PMD compensator with an integrated inline monitoring system;  
         [0020]    [0020]FIG. 2 is a diagramatic representation of a prior art two step compensator with an integrated inline monitoring system;  
         [0021]    [0021]FIG. 3 is a diagramatic representation of an embodiment of the present PMDC parallel architecture;  
         [0022]    [0022]FIG. 4 is a diagramatic representation of an embodiment of the present PMDC parallel architecture employing a switched reference signal;  
         [0023]    [0023]FIG. 5 is a diagramatic representation of a dither-free embodiment of the present PMDC parallel architecture employing a switched reference signal;  
         [0024]    [0024]FIG. 6 is a diagramatic representation of an embodiment of the present PMDC parallel architecture employing a multi-wavelength reference signal;  
         [0025]    [0025]FIG. 7 is a diagramatic representation of a dither-free embodiment of the present PMDC parallel architecture employing a multi-wavelength fixed reference signal; and  
         [0026]    [0026]FIG. 8 is a diagramatic representation of a multisection dither-free embodiment of the present PMDC parallel architecture employing a multi-wavelength fixed reference signal. 
     
    
     DETAILED DESCRIPTION  
       [0027]    Turning to FIG. 3, there is shown system  300  implementing an embodiment of the present PMDC parallel monitoring and control architecture. Optical signal  301  is split between inline path  308  and branch path  309 , but the strength of signal  301  is not necessarily split between the paths. Optical fiber  310  is used as an optical transmission medium in portions of the system and paths requiring well-controlled polarization transformation may employ free space optical beams  311 , planer optical waveguides or the like. Herein the phrase “free space path” or the like is intended to denote optical paths which preferably have no polarization transformation or at least in which polarizations transformations are well characterized and known. Such “free space paths” may in fact be free space optical beams or they may take the form of planer optical waveguides or the like. Control parameters for inline path  308  may include control voltages applied to PC  302  and, if desired, a value adjustment of DGD  303 . Preferably, within box  300   a,  branch  309  and inline path  308  have matching polarization transformations. Additionally, PCs  302  and  306  preferably match such that matching control voltages result in matching input SOPs at DGDs  303  and  307 , respectively, for any incoming SOP. This configuration allows controller  304  to determine the best inline PC voltages based on measurements in branch  309 . Random problems with local minimal can then be handled in a deliberate manner, rather than relying on statistics of overall PMD over a period of time. In operation, the control parameters are fully swept in branch  309  for branch PC  306 . When the optimum parameters which provide the best PC output for PMD compensator by DGD  307  at MON  305  are found, the parameters of inline PC  302  are adjusted to match optimum parameters found in branch path  309 .  
         [0028]    Alternatively, a branch path may not have a polarization transformation matching that of the inline path, as illustrated in FIGS. 4 through 8. Preferably, in these embodiments, the control parameters are fully swept in the branch, and when the optimum parameters are found, the parameters of the inline compensator are adjusted to emulate the settings in the branch PC(s) by means of sensors in both the inline and branch paths. This sensing can be done in a number of ways, some of which are illustrated in FIGS. 4 through 8.  
         [0029]    [0029]FIG. 4 shows embodiment  400  for a single section parallel architecture PMDC using sensors. A polarization transformer (P-Rot)  412  modulates continuous wave (CW) reference signal  413  at wavelength λ ref  to provide two different input SOPs. Reference signal  413  is injected into optical fiber  410  carrying optical signal  401  at wavelength λ sig . Optical signal  401  and reference signal  413  are tapped to form branch  409  parallel to inline path  408 . Optical signal  401  and reference signal  413  are scrambled by a polarization controller acting as a polarization scrambler (PolScr)  406 . The polarization controller making up polarization scrambler  406  need not necessarily be endless. Control processor  404  may be used to control polarization scrambler  406 . However, a separate control processor may be used to cycle through control voltages for polarization scrambler  406  as there is no need for synchronization with inline PC  402 . The reference signal is monitored by polarimeter SOP2 ref    414  via a filtering splitter Filter2  415 . The polarization transformation in branch  409  can be uniquely identified by two SOPs measured at SOP2ref  414 , corresponding to the two different input SOPs generated by P-Rot  412 . The filtered scrambled signal is sent through compensator  407  in the form of a DGD and the signal distortion level is measured by monitor (Mon)  405 . Control processor  404  records the monitor signal and associated SOPs from SOP2 ref    414  for the full range of polarization transformations induced by PScr  406 , and determines which transformation yields the least PMD after DGD  407 . Control processor  404 , reads SOP1 ref    416 , and dithers inline endless PC  402  so that the inline transformation matches the optimum found in branch  409 . To dither control parameters of endless PC  402 , each control parameter is varied to determine whether a new parameter results in approaching or diverging from the SOP target. Each control parameter is evaluated and the SOP is optimized. For example, with three control voltages, the first one is optimized, then the second one is optimized, then the third, and then the first is optimized again, etc., constantly. Thus, a minimum separation from the target SOP is maintained. PMD at the output should be at the optimal level as long as inline and branch DGDs  403  and  407  are approximately equal.  
         [0030]    In embodiment  400  of FIG. 4, the two branches do not need to match. The SOPs which are tapped off with Filter2  415  and Filter1  416 , can be used to ensure that the optimum polarization transformation found in branch  409  can be applied to endless PC  402 . While sweeping out parameter space with polarization scrambler  406 , SOP2 ref    414  is preferably monitored twice for each combination of control voltages, to take two measurements in order to evaluate the polarization transformation. Generally, a single SOP measurement will not fully characterize a polarization transformation. So reference signal input has polarization rotator  412  which preferably generates two different SOPs. Preferably, the two SOPs do not match and are not orthogonal to each other. By measuring the SOP in branch  409  at SOP2 ref    414  for the two reference signal polarization states, the polarization transformation for branch  409 , up to DGD  407  can be fully characterized. Given the optimum polarization transformation in branch  409 , characterized by measurement at SOP2 ref    414 , control processor  404  only has to compare measurement at SOP2 ref    414  to measurements at SOP1 ref    416  in inline path  408  to provide optimal PMD compensation control settings for endless PC  402 . The correct control parameters on inline endless PC  402  may not be the same as the optimal control parameters found for polarization scrambler  406 . To provide optimal control parameters to inline PC  402 , the two SOPs corresponding to the two different reference polarizations can be matched, thereby matching the polarization transformations in inline path  408  and branch path  409 . Thusly, embodiment  400  avoids the use of matching PCs and many of the free space paths of embodiment  300  of FIG. 3. Free space paths  411  are required at Filter2 ( 415 ) and Filter1 ( 417 ) to insure accurate measurement of SOP2 ref    414  and SOP1 ref    416 , respectively. Dashed outlines  400   a  and  400   b  encompass free space optical beam paths or other well defined optical path in which the polarization transformation between SOP ref  measurements and the inputs of their respective DGDs are known. Preferably, no polarization transformation takes place within each of boxes  400   a  and  400   b.    
         [0031]    [0031]FIG. 5 shows single section PMDC embodiment  500  which does not require dithering of inline endless PC  502 . Inline endless PC  502  should be well characterized and controlled for embodiment  500 , meaning an accurate mapping of control voltages to polarization transformations is required and stored in the memory of control processor  504 . A polarization transformer (P-Rot)  512  modulates continuous wave (CW) reference signal  513  at wavelength λ ref  to provide two different input SOPs. Reference signal  513  is injected into optical fiber  510  carrying optical signal  501  at wavelength λ sig . Optical signal  501  and reference signal  513  are tapped to form branch  509  and scrambled by a polarization controller acting as a polarization scrambler (PolScr)  506 . The polarization controller making up polarization scrambler  506  need not necessarily be endless. Control processor  504  may be used to control polarization scrambler  506 . However, a separate control processor may be used to cycle through control voltages for polarization scrambler  506  as there is no need for synchronization with inline PC  501 . The reference signal is monitored by polarimeter SOP2 ref    514  via filtering splitter, Filter2  515 . The polarization transformation in branch  509  can be uniquely identified by two SOPs measured at SOP2 ref    514 , corresponding to the two different input SOPs generated by P-Rot  512 . The filtered scrambled signal is sent through a compensator in the form of DGD  507  and the signal distortion level is measured by monitor (Mon)  505 . SOP1 ref    516  is measured at the input of inline endless PC  502 , and the control processor  504  calculates the correct setting so that the output of endless PC  502  matches the optimal value determined from measurements in branch  509 . After control processor  504  has found the optimum pair of SOPs to occur at inline DGD  503 , the SOP1 ref    516  is measured in inline path  508 , via filter,  517  before endless PC  502 . Processor  504  calculates what voltages or other control parameters need to be applied to endless PC  502  in order to obtain the desired SOPs after inline endless PC  502 . No dithering is required by embodiment  500  because endless PC  502  is well characterized. Present embodiment  500  employs only a measurement, calculation and application of tabulated voltages found in memory of controller  504 . The preferred optimal reference SOP, determined from measurements at SOP2 ref    514 , is generated by endless PC  502  at the input of the DGD  503 . Free space paths  511   a  and  511   b,  outlined by boxes  500   a  and  500   b,  respectively, are preferably regions in which there is no transformation on the polarization transformation is known. A measurement at SOP1 ref , for example, uniquely identifies the reference SOP entering endless PC  502 .  
         [0032]    [0032]FIGS. 6 and 7 show variations of the configuration in FIG. 3. Instead of modulating the polarization of a single reference signal as embodiments  400  and  500  of FIGS. 4 and 5, a second reference signal is added at a different wavelength. Two filters and SOP monitors are used both inline and in the branch.  
         [0033]    [0033]FIG. 6 shows embodiment  600  of the present PMDC parallel monitoring architecture. Rather than modulating polarization of a single reference signal, a second reference signal is added at a second wavelength (λ ref ) to have two separate known states of polarization in order to uniquely identify the polarization transformation between tap  622  and the reference measurement at MON  605 . Reference signal  613  having wavelengths λ ref1  and λ ref2  is injected into optical fiber  610  carrying optical signal  601  at wavelength λ sig . Optical signal  601  and reference signal  613  are tapped into branch  609  and scrambled by a polarization controller acting as a polarization scrambler (PolScr)  606 . The polarization controller making up polarization scrambler  606  need not necessarily be endless. Control processor  604  may be used to control polarization scrambler  606 . However, a separate control processor may be used to cycle through control voltages for polarization scrambler  606  as there is no need for synchronization with inline endless PC  602 . The two wavelength of reference signal  613  are monitored by polarimeter SOP2 ref1    614  via filtering splitter Filter2 λ ref1    615  and polarimeter SOP2 ref2  via filtering splitter Filter2 λ ref2    619 . The polarization transformation in the branch can be uniquely identified by two SOPs measured at SOP2 ref1  and SOP2 ref2 , corresponding to the two different input reference wavelengths. The filtered scrambled signal is sent through a compensator in the form of a DGD  607  and the signal distortion level is measured by monitor (Mon)  605 .  
         [0034]    The present embodiment has two independent measurements of two different wavelengths, instead of employing a time dependent multiplexing scheme as shown in FIGS. 4 and 5 employing a polarization rotator  412  or  512 . Branch path  609  has polarization scrambler  606  and inline path  608  has endless PC  602 . In each path, before the DGD, two simultaneous SOP measurements are taken. So in branch  609 , Filter2 λref1    615  splits off λ ref1  for measurement by SOP2 ref1    614  and Filter2 λref2    619  splits off λ ref2  for measurements by SOP2 ref2    618 . Similarly, for inline path  608 , two SOP measurements are taken. At inline Filter1 λref1    617 , there is a measurement, SOP1 ref1    616  which is λ 1 , and at Filter1 ref2    621 , SOP1 ref2    620 , whether measurement is at λ 2 . The polarization transformation is matched by the monitoring system all the way from tap  622  to the filters and from the filters to DGDs  607  and  603 . Preferably free space path  611  is employed from the filters to the DGDs as polarization transformations can not be controlled or monitored beyond the filters. The parameter space is swept out in the branch to find the optimum SOP, corresponding SOP measurements are matched in inline path  608  to ensure the correct optimum in the inline path. Dashed regions  600   a  and  600   b  denote free space optical beam paths  611  in which the polarization transformation between SOP ref  measurements and the inputs of their respective DGDs ( 607  and  603 ) are known and preferably static with no polarization transformation.  
         [0035]    [0035]FIG. 7 shows single section PMDC  700  which does not require dithering of endless PC  702 . Endless PC  702  should be well characterized and controlled for embodiment  700 . The SOP, of two reference wavelength, SOP1 ref1  and SOP ref2  are measured at the input of inline Endless PC  702 , and control processor  704  calculates the correct setting so that the output of Endless PC  702  matches the optimal value determined from measurements in branch  709 .  
         [0036]    Reference signal  713  having wavelength λ ref1  and λ ref2  is injected into optical fiber  710  carrying optical signal  701  at wavelength λ sig . Optical signal  701  and reference signal  713  are tapped to provide branch  709  and the signals are scrambled by a polarization controller acting as a polarization scrambler (PolScr)  706 . The polarization controller making up polarization scrambler  706  need not be an endless PC. Control processor  704  may be used to control polarization scrambler  706 . However, a separate control processor may be used to cycle through control voltages for polarization scrambler  706  as there is no need for synchronization with inline PC  702 . Reference signal  713  is monitored by polarimeter SOP2 ref1    714  and SOP2 ref2    718  via filtering splitters Filter2 λref1    715  and Filter2 λref2    719 . The polarization transformation in the branch can be uniquely identified by two SOPs measured at SOP2 ref  and SOP2 ref2 , corresponding to the two different reference wavelengths. The filtered scrambled signal is sent through DGD  707  acting as a compensator and the signal distortion level is subsequently measured by monitor (Mon)  705 .  
         [0037]    For inline path  708 , two SOP measurements are also taken. At inline Filter1 λref1    717 , there is a measurement of SOP1 ref1    716  which is at λ 1 , and at Filter1 ref2    721 , SOP1 ref2    720  is measured at λ 2 , both at the input of inline endless PC  702 . Processor  704  calculates what voltages or other control parameters need to be applied to endless PC  702  in order to obtain the desired SOPs after inline endless PC  702  as determined in branch  709 . Therefore, dithering is not necessary; present embodiment  700  employs only a measurement, calculation and application of tabulated voltages found in memory of controller  704 . Preferably, in order for branch SOP ref  measurements to match the inline states generated by endless PC  702 , there should be no polarization transformations within dashed line boxes  700   a  and  700   b  or any such transformations within boxes  700   a  or  700   b  are known. This may be facilitated by employing free space optical paths  711   a  and  711   b,  or the like  
         [0038]    Alternatively, a multi-sequential section embodiment of the above disclosed parallel architecture embodiments may be employed to monitor and control PMD. By way of example, FIG. 8 illustrates a two section embodiment  800  of PMDC parallel monitoring architecture embodiment  700  of FIG. 7. PMDC  800  does not require dithering of endless PCs  802  and  802   a.  Endless PCs  802  and  802   a  should be well characterized and controlled for this embodiment. The SOP, of two reference wavelength, SOP1 ref1  and SOP ref2  are measured at the input of each inline Endless PC  802  or  802   a,  and-control processor  804  calculates the correct setting so that the output of Endless PCs  802  and  802   a  match the optimal values determined from measurements in branch  809  for each of the respective endless PCs  802  and  802   a.    
         [0039]    Reference signal  813  having wavelength λ ref1  and λ ref2  is injected into optical fiber  810  carrying optical signal  801  at wavelength λ sig . Optical signal  801  and reference signal  813  are tapped to provide branch  809  and the signals are scrambled first by polarization scrambler (PolScr)  806 , preferably comprised of a polarization controller. The polarization controller used as polarization scrambler  806  need not be an endless PC. Control processor  804  may be used to control polarization scrambler  806 . However, a separate control processor may be used to cycle through control voltages for polarization scrambler  806  as there is no need for synchronization with inline PCs  802  or  802   a.  Reference signal  813  is first monitored by polarimeter SOP2 ref1    814  and SOP2 ref2    818  via filtering splitters Filter2 λref1    815  and Filter2 ref2    819 . The polarization transformation in branch  809  can be uniquely identified by two SOPs measured at SOP2 ref  and SOP2 ref2 , corresponding to the two different reference wavelengths. The filtered scrambled signal is sent through a first compensator  807  in the form of a DGD.  
         [0040]    In inline path  808 , two SOP measurements are taken at the input of inline endless PC  802 . At inline Filter1 λref1    817 , there is a measurement of SOP1 ref1    816  which is at λ 1 , and at Filter1 ref2    821 , SOP ref2    820  is measured at λ 2 ,. Control processor  804  calculates what voltages or other control parameters need to be applied to endless PC  802  in order to obtain the desired SOPs after inline endless PC  802  as determined in first section  822  of branch  809 .  
         [0041]    Exiting first section  822  and entering second section  823  optical signal  801  and reference signal  813  in branch  809  are again scrambled, by a second polarization controller acting as polarization scrambler (PolScr)  806   a.  The polarization controller making up polarization scrambler  806   a  also need not be an endless PC. Control processor  804  may also be used to control polarization scrambler  806   a.  However, a separate control processor may be used to cycle through control voltages for polarization scrambler  806   a  as there is no need for synchronization with inline PC,  802  or  802   a.  Reference signal  813  is monitored by polarimeter SOP2 ref1a    814   a  and SOP2 ref2a    818   a  via filtering splitters Filter2 λref1    815   a  and Filter2 ref2    819   a,  respectively. The polarization transformation in branch  809  can again be uniquely identified by two SOPs measured at SOP2 ref  and SOP2 ref2 , corresponding to the two different reference wavelengths. The filtered scrambled signal is sent through compensator  807   a  in the form of a DGD and the signal distortion level is measured by monitor (Mon)  805 .  
         [0042]    Again in inline path  808 , two SOP measurements are taken at the input of inline endless PC  802   a.  At inline Filter1a λref ,  817   a,  there is a measurement of SOP1 ref1a    816   a  which is at λ 1 , and at Filter1a ref2    821   a,  SOP1 ref2a    820   a  is measured at λ 2 ,. Processor  804  also calculates what voltages or other control parameters need to be applied to endless PC  802   a  in order to obtain the desired SOPs after inline endless PC  802 a as determined in second section  823  of branch  809 .  
         [0043]    Dithering is not necessary for embodiment  800 . Measurements, calculations and application of tabulated voltages found in memory of controller  804  are employed to control endless PCs  802  and  802   a.  Preferably free space paths  811   a  and  811   b  are employed from the filters to the DGDs in each section as polarization transformations can not be controlled or monitored beyond the filters. Dashed outlines  800   a,    800   b,    800   c  and  400   d  encompass free space optical beam paths  811   a  and  811   b,  or other well defined optical path, in which the polarization transformation between SOP ref  measurements and the inputs of their respective DGDs are known. Preferably, no polarization transformation takes place within boxes  800   a,    800   b,    800   c  or  800   d.    
         [0044]    By requiring two input reference SOPs or wavelengths. The embodiments of FIGS. 4 through 8 accomplish matching of polarization transformations between inline paths and branches by sensing output polarization states for two distinct input polarizations. Herein, distinct means that the two SOPs are not only different, but are not orthogonal states either this distinction is due to the ambiguity of SOP measurements; an SOP inherently contains two degrees of freedom. Therefore, a single SOP measurement cannot fully describe a polarization transformation. A second input SOP will provide missing information, as long as the second SOP is not orthogonal to the first SOP. When plotted on a Poincaré sphere, the second SOP will ideally occupy a position 90 degrees relative to the first SOP. “Orthogonal” corresponds to a 180 degree relative position on the Poincare sphere.  
         [0045]    The dashed line boxes  400   a,    400   b,    500   a,    500   b,    600   a,    600   b,    700   a,    700   b,    800   a,    800   b,    800   c  and  800   d  are regions in which polarization transformations are preferably known and are preferably static. Within these boxes measured reference SOPs are intended to correlate to signal polarization orientation with respect to the subsequent DGD principal states. The polarization transformations within all fiber connections out side dashed boxes  400   a,    400   b,    500   a,    500   b,    600   a,    600   b,    700   a,    700   b,    800   a,    800   b,    800   c  and  800   d  are preferably free to drift. The present systems and methods are intended to adjust for such variations.  
         [0046]    In present parallel architecture embodiments  300 ,  400 ,  500 ,  600 ,  700  and  800  the branch and inline DGDs do not need to match in terms of optical birefringence. This precludes the DGDs from susceptibility to unequal thermal drifting, making design of a parallel monitoring architecture practical. Additionally, as mentioned above, branch polarization scramblers need not be endless PCs in these embodiments. Instead, the polarization scramblers can be components of lower cost. If desired, a third degree of freedom, tunability of DGDs, can be added to these embodiments. This tunability is denoted by dashed control lines from controls to the DGDs of each embodiment.  
         [0047]    Preferably, PMD monitors (MONs  305 ,  405 ,  505 ,  605 ,  705  and  805 ) are distortion level monitors. For example, a degree of polarization (DOP) measurement system can be employed as a monitor, such a DOP measurement system may be a polarimeter which measures Stokes parameters, from which a DOP can be extracted. Also, a polarization scrambler may be used in conjunction with a polarizer as a monitor. By finding the ratio of the minimum to maximum transmitted power using a scrambler and polarizer the DOP may be extracted. RF measurements may be employed as a means of monitoring PMD. By filtering certain frequencies from a photo-detector which receives the branch or inline optical signal branch or inline distortion levels due to PMD can be extracted from the electrical signal.  
         [0048]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.