Patent Publication Number: US-6907199-B2

Title: Method for polarization mode dispersion compensation

Description:
RELATED APPLICATIONS 
     The present application Ser. No. 10/036,987 is related to the co-pending, commonly assigned application entitled, “System for All Order Dispersion Compensation”, Applicants&#39; filed co-currently with the present application, which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and system for polarization mode dispersion compensation of optical signals. In particular, the present invention relates to a method for polarization mode dispersion compensation using at least two linearly chirped Bragg gratings to selectively tune the reflection points of two polarization resolved signals, creating a variable polarization dependent delay. 
     Present day telecommunication systems require that optical signals be conveyed over very long distances. In an optical communications signal, data are sent in a series of optical pulses. Real signal pulses are composed of a distribution of wavelengths and polarizations, each of which travels at its own characteristic velocity. This variation in velocity leads to pulse spreading and thus signal degradation. Degradation due to the wavelength dependence of the velocity is known as chromatic dispersion, while degradation due to the polarization dependence is known as polarization mode dispersion. 
     Mathematically, the speed of light v in a waveguide is given by 
             v   =     c   n             (   1   )             
 
where c is the velocity of light in free space and n is the effective index of refraction in the waveguide. Normally, the effective index, n, of the optical mode is dependent upon the wavelength. Thus components of light having different wavelengths will travel at different speeds. In addition to being dependent upon wavelength, the effective index in a waveguide may also be dependent upon the polarization of the optical signal. Even in “single-mode” fiber, two orthogonal polarizations are supported, and, in the presence of birefringence, the polarizations travel at different speeds. Birefringence in the fiber may arise from a variety of sources including both manufacturing variations and time-dependent environmental factors. The speed difference results in a polarization-dependent travel time or “differential group delay” (DGD) between the 2 different polarization modes within the birefringent fiber. In real systems, the degree of birefringence, and the orientation of the birefringent axes, varies from place to place along the fiber. This results in a more complex effect on the optical signal, which is characterized by the concept of “principal states of polarization” or PSPs. PSPs are defined as the two polarization states that experience the maximum relative DGD, and they uniquely characterize the instantaneous state of the system.
 
     Polarization mode dispersion (PMD) is the distortion arising from the statistical sum of the different group velocities of the two components of polarization as the signal propagates through the different sections of the optical communications system. PMD includes first order PMD and higher order PMD and is non-deterministic. First order PMD is the differential polarization group delay at a given wavelength. The instantaneous value for a long fiber can vary over both long time intervals (due to slow variations, such as temperature drift) and short time intervals (due to fast variations, such as mechanical vibration induced polarization fluctuations). The coefficient describing the mean value of first order PMD can vary from &gt;2 ps/km 1/2  for relatively poor PMD performance fiber to &lt;0.1 ps/km 1/2  for relatively good PMD performance fiber. 
     Second order PMD arises from two sources: i.) a first order PMD that varies with wavelength; ii.) a change of the system PSP (principal state of polarization) orientation with wavelength, which results in a variation of PMD with wavelength. Second order PMD results in a wavelength dependent group delay, which is equivalent in effect to variable chromatic dispersion, and, can have either a negative or positive sign. The speed of fluctuation is similar to that of first order PMD. 
     Dispersion imposes serious limitations on transmission bandwidth, especially across long distances, such as in transoceanic routes. Dispersion issues become much more important at higher bit rates, where the separation between the optical pulses is less and where shorter pulses result in a wider signal spectral bandwidth, exacerbating chromatic and second order PMD effects. At bit rates greater than or equal to 40 Gb/s, even for good fiber (&lt;0.1 ps/km 1/2  PMD) long length links are deemed to require PMD compensation. PMD can become an inhibiting factor either limiting overall system length or increasing system costs due to the need for additional optical-to-electrical-to-optical signal conversion sites to permit electrical signal regeneration. 
     One approach to compensation for first order PMD is to introduce a DGD of equal magnitude and opposite sign to the first order PMD in the system. In general, time delays in an optical system can be described in terms of optical path length (OPL) defined by
 
Δ=nL   (2)
 
where Δ is the OPL, L is the physical length of the medium, and n is the index of refraction of the material.
 
     As may be appreciated from equation (2) above, the OPL of an optical waveguide may be lengthened by increasing the index of refraction of the medium or by increasing the physical length of the waveguide. Similarly, the OPL of an optical waveguide may be shortened by decreasing the index of refraction or by decreasing the physical length of the waveguide. Thus, to generate a DGD for PMD compensation, the two orthogonal PSPs of the signal can be sent down two separate paths with different OPLs. If the delayed polarization of the signal is sent down a path with a shorter path length than the leading polarization of the signal, the amount of differential group delay between the two polarizations will be reduced. 
     A variety of alternatives have been presented to attempt to compensate for first order PMD effects. One proposed system includes a polarization controller and a length of high birefringence polarization maintaining (PM) fiber. A photodetector samples the output signal and attempts to drive the controller using control loop techniques. A long coil of PM fiber (e.g., 50 meters) is necessary to achieve adequately large DGD for dispersion compensation. More importantly, the amount of PMD correction is fixed because of the fixed DGD of the PM fiber, limiting the adaptability and applicability of the system. 
     Another proposed system attempts to address the problem of adaptability by employing a movable prism element, which generates a variable DGD by varying the distance traveled by one polarization. There are a number of disadvantages to this scheme. For example, optical path losses must be very closely balanced to prevent polarization-dependent loss (PDL). In addition, the overall speed of the variable delay element will be slow due to the mechanical movement of the optics. Furthermore, since the variable delay is created outside of fiber there may be issues of cost and stability due to the complexity associated with the required active alignment of the optical beam. 
     Another proposed approach to a variable DGD element consists of a single non-linearly chirped grating in a PM fiber. Chirped gratings are gratings in which the spacing of the grating elements varies with position along the grating, so that the effective position at which a signal is reflected depends on its wavelength. In this case, the application of axial strain to the fiber changes the reflection location of each polarization at a different rate, thus changing the delay between them. However, such a design can only achieve a limited range of delays because the differential delay is proportional to the small birefringence of the fiber, and is limited by the small range of strain that the fiber can withstand before breaking. Additionally, this approach induces a varying chromatic dispersion that must be separately compensated. 
     Yet another proposed approach to generating a differential group delay (DGD) consists of a polarization beam splitter coupled to a pair of single-mode (SM) optical fibers each having a linearly chirped Bragg reflection grating and a controllable extension means for differentially axially straining the fibers. This dual grating approach has the advantage that the two polarization components experience the same chirp when reflected, thereby experiencing matched chromatic dispersion so that polarization-dependent chromatic dispersion is not introduced. However, such a system does not account for polarization fading effects which will require dynamic polarization control in each of the SM fiber grating paths to assure that all light returns properly through the polarization splitter; this will substantially add to the complexity and cost of the system. Furthermore, such a system does not address the difficulty of balancing the two legs during manufacturing, or of properly biasing the system when the compensator is first turned on. 
     The need remains for a reliable, wide-dynamic range, dynamically tunable PMD system. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a dynamically tunable polarization dispersion compensator including a polarization controller and a variable differential polarization delay unit. The polarization controller converts incoming light of an arbitrary polarization to a controlled output signal having a desired state of polarization. The differential polarization delay unit is optically coupled to receive the controller output signal. The differential polarization delay unit includes a polarization beam splitter element, a differential delay element, and a polarization combiner element. The polarization beam splitter has a first input port coupled to receive the controller output signal, a split point, and a first and a second output port, where at the split point the controller output signal is split into a first and a second orthogonal polarization signals. The first and the second polarization signals are directed to the first and second output ports of the splitter respectively. The differential delay element includes a first waveguide and a second waveguide. The waveguides are birefringent, thereby suppressing coupling between the two polarization modes in each. The first waveguide and the first output port of the splitter are optically coupled and aligned by matching their cores and polarization axes. The first waveguide has a first chirped grating tuned to reflect the first polarization signal at a first reference reflection point. The second waveguide is optically coupled and aligned to the second output port of the splitter. The second waveguide has a second chirped grating tuned to reflect the second polarization signal and has a second reference reflection point. The chirp of the gratings may be linear, or may have a more complex spatial dependence. 
     At least one tuning mechanism is coupled to at least one of the gratings. The tuning mechanism is capable of variably adjusting the optical path length of one or both of the reference points, with respect to the split point. The tuning mechanism may include: applying axial mechanical stress to stretch the gratings, applying electric fields to electro-optically control the grating index, applying heat to thermo-optically control the grating index, or using other tuning mechanisms known in the art. In one embodiment, the tuning mechanism includes a first tuning device and a second tuning device. The first tuning device is coupled to both the first and second gratings and tunes both gratings generally simultaneously and in equal amounts. The second tuning device independently tunes only one of the gratings. 
     The initial position of the first and second reference reflection points with respect to the split point (i.e., the optical path length of the segment) may be tailored to the particular application. In applications where the expected DGD does not exceed the range of the tuning mechanism, the first and second reference reflection points may be at substantially the same optical path length with respect to the split point. Alternatively, one or the other reference reflection points may be biased, that is, have a different optical path length, to compensate for all or part of the PMD. A combiner element recombines the two reflected orthogonal polarization signals into a delay line output. In a preferred embodiment, a splitter/combiner performs the functions of both the splitter and the combiner elements. 
     A circulator may be used to route the input and output signals. The circulator has an input port optically coupled to receive the polarization controller output signal, a recirculation port optically coupled to transmit the polarization controller output signal to the differential polarization delay unit and to receive the delay unit output, and an output port optically coupled to transmit the delay unit output. 
     An optical tap coupler may be coupled to the output port of the circulator to provide a signal analyzer with a sample of the output signal. The analyzer evaluates the quality of the delay line output signal and provides control signals to the polarization controller and the differential polarization delay unit. 
     Different components of the present invention may be integrated into an integrated optical device, such as a LiNbO 3  chip, which contains birefringent waveguides. In one embodiment, the polarization controller and the differential polarization delay unit are integrated onto a single LiNbO 3  chip. In another embodiment, the polarization dispersion compensator components from neighboring channels in a WDM system may be integrated onto a single LiNbO 3  chip. Obviously, integrated optical devices based on other materials systems could also be used. 
     The present invention further relates to a method for compensating for polarization mode dispersion of an incoming optical communications signal. The incoming optical communications signal is first passed through a polarization controller, which aligns the states of the signal polarization to the optical axes of a differential polarization delay unit. In the delay unit, the communications signal is split into a first and second orthogonal principal state of polarization at a split point. The first polarization state is directed to a first waveguide having a first chirped grating having a first reference reflection point The second polarization state is directed to a second waveguide having a second chirped grating having a second reference reflection point. The first and second reflections of the optical communications signal are recombined at the polarization combiner. The output signal may be sampled using a signal analyzer to determine its quality. The state of polarization of the incoming signal and/or the optical path length location of the reflection points may then be variably adjusted to optimize the quality of the output signal. 
     In exemplary embodiments, the waveguides of the differential polarization delay unit are birefringent single-mode optical fiber, or birefringent waveguides in an integrated optical device. Birefringent single mode fiber includes polarization maintaining fiber, polarizing fiber, shaped birefringent fiber, and photonic band gap optical fiber. 
     In a particular embodiment, the fibers are polarization-maintaining (PM) or polarizing (PZ) single mode silica-based optical fibers and the gratings are linearly chirped and have substantially similar length and chirp patterns. In a specific embodiment for compensation of PMD in the range of 100 ps (for up to one bit period at 10 Gb/sec data rate) in a single wavelength channel of a dense wavelength division multiplexed (DWDM) telecommunications system, the first and second gratings measure at least five (5) cm long, with an optical chirp rate that may be set to accommodate the level of chromatic dispersion of the incoming signal. A specific exemplary embodiment for polarization delay of 100 ps includes first and second gratings having a length of 5 cm and a chirp rate of 680 ps/nm for a signal of wavelength 1550 nm. 
     The optical path length location of one or both of the reference reflection points is adjustably varied to compensate for polarization dispersion between the first and second orthogonal states of polarization. In an alternative embodiment, prior to the step of adjustably varying the optical path length from the second reflection point, the optical path length of at least one of the gratings may be pre-tuned such that one of the reflection points is either at substantially the same optical path length as the other reflection point or slightly ahead or behind (shorter or longer optical path length). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic component flow diagram of the polarization mode dispersion compensation method of the present invention. 
         FIG. 2  is a schematic diagram of a dynamically tuned polarization mode dispersion compensator in accordance with the present invention. 
         FIG. 3  is a schematic diagram of a dynamically tuned polarization mode dispersion compensator showing three types of tuning in accordance with the present invention. 
         FIG. 4  is a cut-away drawing of a delay line waveguide made with shaped PM fiber in accordance with the present invention. 
         FIG. 5  is a schematic diagram of a dynamically tuned polarization mode dispersion compensator where the system circulator is placed before the system polarization controller. 
         FIG. 6  is a schematic diagram of an adaptive polarization compensator partly integrated onto a LiNbO 3  chip in accordance with the present invention. 
         FIG. 7  is a schematic diagram of an integrated multi-channel PMD compensator for a WDM system. 
         FIG. 8  is a schematic diagram showing the case where a tap coupler is integrated onto the LiNbO 3  chip for each channel in a WDM system. 
         FIG. 9  is a schematic diagram for a higher order PMD compensation and variable chromatic dispersion compensation system where the dual grating 1 st  order PMD compensator is followed by a tuned non-linearly chirped grating. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to a method for compensation and a system for compensation for polarization mode dispersion (PMD).  FIG. 1  illustrates a component flow diagram of a method for compensating for polarization mode dispersion of an incoming optical communications signal  12 . A polarization controller  10 , that adjusts the state of polarization to minimize the distortion of the signal, receives the incoming signal  12 . The output  14  of the polarization controller  10  is optically coupled and aligned to a differential polarization delay unit  30 , which includes a polarization splitter  20 , a differential delay line  90  and a polarization combiner  40 . The polarization splitter  20  receives the output  14  and splits the communication signal into a first and a second orthogonal state of polarization  22 . The polarization states  22  are directed into the differential delay line  90  that compensates for PMD by delaying the “fast” component of polarization with respect to the “slow” component of polarization. The delayed signals  32  are coupled to the polarization combiner  40 , which recombines the signals into an output signal  42 . 
     The output signal  42  is sampled through a tap coupler  50 . The sample signal  54  is coupled to a signal analyzer device  60 , while the PMD compensated signal  52  exits the system. The signal analyzer device  60  assesses the quality of the output signal and issues control loop commands  62  to the polarization controller  10  and the differential delay line  90 . This affords first order compensation when the incoming signal polarization is properly aligned to the polarization dependent delay device. 
     Alternately, a second tap coupler (not illustrated) could be placed before the polarization controller  10 , to sample incoming signal  12 , and via a signal analyzer  60 , provide control loop commands  62  to the polarization controller  10  and differential delay line  90 . 
     A PMD compensator of this embodiment may be applied to a WDM system, where a demultiplexer is used to separate the individual channels of the communications system, a separate PMD compensator of the present method is used to compensate for delay in each channel (or group of channels), and a multiplexer is used to recombine the signal. 
       FIG. 2  is a schematic diagram of a polarization mode dispersion compensation system  102  in accordance with the present invention. The system  102  includes a polarization controller  110 , a circulator  170 , a differential polarization delay unit  130 , a tap coupler  150  and a signal analyzer  160 . 
     The polarization controller  110  includes an input port  111  that receives the incoming communication signal  112  and output port  113  that outputs an output signal with its states of polarization properly oriented. 
     The circulator  170  includes an input port  172  that is coupled to the output  114  of the polarization controller, a circulator port  174  that is optically coupled to the differential polarization delay unit  130 , and an output port  176  that is coupled to an input port  152  of the tap coupler  150 . The circulator  170  receives the signal  114  and couples it as signal  116  to the differential polarization delay unit  130 . 
     The differential polarization delay unit  130  includes a port  123 , a polarization splitter/combiner  125 , and a differential delay line  190 . The splitter/combiner  125  performs the functions of the polarization splitter  20  and the polarization combiner  40  illustrated in FIG.  1 . The function of the polarization beam splitter/combiner  125  may be performed by a joint splitter/combiner as shown, or alternatively by a separate splitter and a combiner using circulators or Faraday rotators with a 2×2 polarization splitter arrangement. The polarization beam splitter/combiner  125  has an optical polarization split point  124 , and a first and a second port,  126  and  128 , respectively. The signal  116  enters through the port  123 . At the split point  124 , the input signal  116  is split into two orthogonal polarization signals  118  and  119  each polarization signal being directed to one of the ports  126  and  128 . 
     Differential delay line  190  includes a first optical waveguide  132  having a first chirped Bragg grating  134  and a second optical waveguide  136  including a second chirped Bragg grating  138 . Both the first and the second waveguides  132  and  136  are birefringent to prevent coupling between the polarization modes. The birefringent waveguides may be birefringent fibers or birefringent integrated optical waveguides. Exemplary birefringent fiber includes polarization maintaining fiber, polarizing fiber, and shaped birefringent fiber. The first and second waveguides  132  and  136  are connected to the first and second ports of the polarization beam splitter/combiner  126  and  128 , respectively. In one particular embodiment gratings  134  and  138  are long-length gratings having a length of 15 cm and a chirp rate of 1360 ps/nm. 
     Both the first grating  134  and the second grating  138  include a reflection reference point, schematically illustrated as  135  and  139 , respectively, which represents the effective distance into the gratings  134  and  138  which the optical signals travel before being reflected. In reality, the signal is reflected over some finite length of each grating, but will experience a delay related to the reflection reference points. This reference point is determined during fabrication of the gratings  134  and  138 . The optical path length in each leg, and the corresponding time delay, is determined by the reflection reference points  135  and  139 , as well as the length of the waveguides  132  and  136  leading to the gratings  134  and  138  from the split point  124 . 
     When the differential polarization delay line  190  is manufactured, the relative delay between the reflections from the two gratings may be adjusted to zero, or biased to a non-zero value. A non-zero bias may be introduced either by fabricating non-identical gratings that have different reflection reference points, or by adjusting the length of the waveguides  132  and  136  between the split point  124  and the gratings  134  and  138 . This bias may be used to compensate for the average differential group delay (DGD) expected in the deployed link. 
     In the present embodiment, the gratings  134  and  138  are substantially identical in reflectivity and chirp rate, to avoid polarization dependent losses and residual chromatic dispersion. The reflection points  135  and  139  of the first and second gratings are configured so as to have a predetermined optical path length difference. The chirp in the grating allows the reflection point of the signal within the grating to be changed when the grating is tuned. The optical position of the reflection reference point of at least one of the gratings may be tuned using a tuning mechanism  180 . Embodiments of the tuning mechanism  180  include mechanical (e.g. via piezoelectric or solenoid actuators) thermal, electro-optic, acousto-optic, magnetostrictive, controlled bending of the grating(s) (i.e. gluing to a plate which is bent) and other suitable embodiments known in the art. 
     A further advantage of the present invention is afforded by considering the case where the differential delay line gratings  134  and  138  and polarization beam splitter/combiner  125  are fabricated in the same birefringent waveguides. Specifically, for a differential polarization delay unit fabricated in birefringent fiber, the polarization splitter  125  could be fabricated by side fusion of the two pieces of PM fiber that make up the waveguides  132  and  136 . Gratings  134  and  138  could then be simultaneously written into the two waveguides  132  and  136 . Such a process would ensure that the gratings  134  and  138  had similar chirp rates and similar path length with respect to the split point  124 . It may be possible with such an arrangement to completely avoid pre-tuning the paths lengths, thereby greatly reducing the cost and complexity of connecting the polarization beam splitter/combiner output ports  126  and  128  to the differential delay line first and second optical waveguides  132  and  136 , respectively. Furthermore, such an arrangement may allow shorter gratings (less than 4-5 cm) to be used by eliminating the need to bias the differential delay line  190  initially by tuning one or both of the gratings  134  and  138 . Alternatively, fabrication of the PM fiber-based polarization beam splitter/combiner could be performed by the same fusion method after the gratings  134  and  138  have been written in waveguides  132  and  136 . This would be facilitated by using shaped PM fiber, such as V-shaped PM fiber, to allow passive alignment of the PM fibers while maintaining the proper orientation of their birefringent axes. 
     The polarization signals  118  and  119  enter the differential delay line  90  and are each reflected by gratings  134  and  138 , respectively. The differential delay line  190  delays the two signals with respect to the other as desired. The reflected signals are recombined at the splitter/combiner  125  into an output signal  120 . 
     The circulator port receives the recombined signal  120 , which is directed out of the circulator  170 , through the output port  176 . The input port  152  of the tap coupler  150  is connected to the output port  176  of the circulator  170 . The tap coupler  150  redirects a sample of the optical signal  120  from the tap output port  154  to the input port  162  of the signal analyzer  160 . The signal  120  passes through the tap coupler  150  and out the system  102  through the tap coupler output port  156 . 
     The signal analyzer  160  takes the output of the tap coupler  150 , through input port  162  and uses it to determine the quality of the communications signal. Using this information, the signal analyzer  160  provides control signals  164  and  166 , which are used to adjust the polarization controller  110  and the differential polarization delay unit  130  to minimize output signal distortion. Since the amount of PMD in the system may change continuously, the settings of the polarization controller  110  and the differential polarization delay unit  130  may be continuously changed using the control signals  164  and  166  from the signal analyzer. The signals to the polarization controller  110  and the differential polarization delay unit  130  are changed either by dithering or directionally changing the signals at a speed that is significantly faster than the required compensation speed. As these quantities are changed, the signal analyzer  160  monitors the signal from the tap coupler  150  to indicate the amount of residual PMD in the signal. This quantity may be related to the degree of polarization of the signal or to various frequency components of the detected signal. If the control signals are dithered, the signal analyzer forces their levels to a point where the dithering produces no or minimal change in the monitored signal, typically through integration, indicating a minimization of the DGD in the system. If the control signals are changed directionally, software decides in which direction the control signal levels are to change until the signal distortion due to PMD is minimized. Generally, this method is less preferred since such a scheme requires a computer or microprocessor. 
     In contrast with previous PMD compensation devices using traditional gratings, embodiments of the present invention may alleviate the difficulty of device biasing by using long (e.g., greater than 4-5 cm) and continuous gratings  134  and  138 . Using long gratings allows the communication signal to experience reflection far from the ends of the gratings, reducing distortion upon reflection. Additionally, long continuous gratings avoid stitching errors that may result from a non-continuously written longer grating. In an exemplary embodiment, the reference reflection point of the signal would fall generally along the middle portion of the grating initially. At least 1 cm of grating on each side of this center point would be desirable for the signal to move under operation for a PMD compensation range of 100 ps. In addition, at least another 1 cm of grating on each side of the center point is desirable to account for differences in the delay line optical path lengths, defined as the waveguide length from the split point  124  to the reference reflection points  135  and  139 , caused by fusion splice position errors or from differing initial lengths of fiber waveguides  132  and  136 . Thus, in an embodiment of the present invention, an exemplary grating for a 1550 nm wavelength signal having a PMD compensation range of 100 ps measures 5 cm. 
     Another advantage of using long gratings is that almost any desired chirp pattern may be applied to the gratings. Using a linearly chirped Bragg grating results in a fixed amount of chromatic dispersion, which is set by the amount of chirp within the grating. Therefore, the present invention could be used simultaneously as both a fixed chromatic and a variable polarization mode dispersion compensation device. For example, the chirp of the gratings  134  and  138  may be chosen such that the gratings compensate for any chromatic dispersion that has accumulated on the incoming communications signal  112 . Such large chirp gratings (700-1400 ps/nm or approx. 7-14 pm/mm) will have a very wide signal reflection band. That is, for a 1360 ps/nm grating, an incoming 10 Gb/s signal will effectively be reflected over approximately 1.2 cm of grating length. This distance increases to 4.8 cm at 40 Gb/s. In the exemplary embodiment of a 680 ps/nm grating and 10 Gb/s operation, for ease of manufacturing purposes, the gratings may be written 50 cm in length such that one part may be applied to eight neighboring wavelength channels in a 100 GHz WDM system and there will be adequate distance to account for pre-tuning and operation at either 10 or 40 Gb/s. 
     Another advantage of the present invention is obtained through the use of two linearly chirped gratings. For a differential delay line made using a single non-linearly chirped grating, a much higher strain is required to tune the grating compared to the present approach. In the non-linearly chirped grating, the delay generation comes from the birefringence in the fiber creating reflection point differences of the two polarizations at different locations in the fiber. However, because the birefringence of polarization maintaining fiber is very low (˜1 e-3), the separation between the reflection points may be small. Additionally, the change in separation of the reference reflection points with applied tuning is small. For comparison, using mechanical tuning, the amount of stretch required for the dual grating approach of the current invention would depend on the chirp rate of the grating, but in all cases would be small. In an exemplary embodiment, with a chirp rate of 1000 ps/nm, a wavelength of 1550 nm, and a required delay generation of 100 ps, the amount of stretch can be calculated using the following equation:
 
Δ P/P (%)=Δλ/λ*100   (3)
 
where ΔP/P is the required strain, Δλ is the required change in wavelength from the original reference reflection point to the desired reference reflection point, and λ is the free-space wavelength. Using this equation, a strain on the fiber of only 0.0064% would be required. Under these same conditions, a non-linear chirped grating in a PM fiber, with a chirp rate that varied from 500 ps/nm to 1500 ps/nm, would produce a delay of only 0.1 ps, instead of the 100 ps obtained with the dual grating approach of the current invention, due to the low birefringence difference. Therefore, the amount of strain required to compensate for PMD using a non-linearly chirped grating in PM fiber may be prohibitive.
 
       FIG. 3  is a schematic of a second embodiment of a dynamically tuned PMD compensator  202 . The compensator  202  includes a polarization controller  210 , a recirculator  270 , a differential delay unit  230 , a tap coupler  250  and a signal analyzer  260 . The differential delay unit  230  includes a polarization splitter combiner  225  and a differential delay line  290 . The differential delay line  290  includes a first birefringent waveguide  232  having a first reflection grating  234  having a first reflection reference point  235 . The first grating  234  is mounted on a tunable mechanism  280   a . The differential delay line also includes a second birefringent waveguide  236  having a second reflection grating  238  having a second reflection reference point  239 . The second grating  238  is mounted on a tunable mechanism  280   b , which can tune the optical position of the second reflection reference point with respect to split point  224 . In turn, both gratings  234  and  238  may be simultaneously tuned by a third tuning mechanism  280   c . Both the first grating  234  and the second grating  238  are long-length, chirped gratings (&gt;4-5 cm.). 
     As shown in  FIG. 3 , in the exemplary long-length fiber grating embodiment of the present invention, an initial optical path length difference in the two waveguides  232  and  236  may be compensated for by initially biasing the first grating  234 , such as by stretching the grating. This biasing does not need to be dynamic. If grating  234  is designed such that the channel wavelength is reflected at the center, under stretch the reflection point will initially change. Thus, the grating needs to be long enough to account for this biasing An additional 1 cm of grating on each side of center reference reflection point  235  may be incorporated to account for biasing, thus resulting in a grating at least 5 cm in length. Alternately, both of the gratings  234  and  238  may be tuned dynamically. For example, the first grating  234  could be statically tuned with tuning mechanism  280   a  to account for the initial balancing of the OPL of the two waveguides  232  and  236 , as described earlier. The second grating  238  may be dynamically tuned with tuning mechanism  280   b  to compensate for the differential group delay, and both gratings  234  and  238  could be tuned simultaneously by a common tuning mechanism  280   c , such as temperature tuning. This could alleviate any temperature effects and assure that both gratings  234  and  238  are operating well within their operating range. To avoid polarization fading effects, the gratings  234  and  238 , shown in  FIG. 3 , are both written in a polarization preserving or polarizing fiber. This is an advantage over the previous art because it eliminates the need for two additional polarization controllers that would be required in a differential polarization delay unit to correct for mode coupling in the single-mode fiber gratings. The issue of mode coupling becomes more apparent when using long gratings (&gt;4-5 cm), where stresses that may induce mode coupling need to be avoided. Such long length gratings benefit from a method to assure that the polarization is indeed preserved throughout. 
     EXEMPLARY EMBODIMENTS 
     Exemplary chirp rates may be either 680 ps/nm or 1360 ps/nm as these correspond to 40 km or 80 km DCM chirp rates (for standard SMF-28 fiber).
         1. Exemplary minimum length grating (single channel PMD compensator) at 10 Gb/s with 680 ps/nm chirp. 
               Length   =       ⁢       2   *     (       l   ⁢   ength     ⁢           ⁢     f   ⁢   or     ⁢           ⁢   100   ⁢           ⁢   ps   ⁢           ⁢   delay     )       +     2   *         ⁢                           ⁢       (     length   ⁢           ⁢     fo   ⁢   r     ⁢           ⁢     pre-tuning       )     +     2   *     (     length  for  signal  reflection     ⁢                                 ⁢     assuming  signal bandwidth  is  0.1  nm)                 Length   =       ⁢       2   *     (     1   ⁢           ⁢   cm     )       +     2   *     (     1   ⁢           ⁢   cm     )       +     2   *     (     0.54   ⁢           ⁢   cm     )                     Length   =       ⁢       5.08   ⁢           ⁢   cm     =     &gt;     5.0   ⁢           ⁢   cm                   
       

     Factors of 2 are used to signify a ‘+ or −’ situation.
         2. Exemplary long-length grating (eight channel compensator) at either 10 or 40 Gb/s with 680 ps/nm chirp. 
             Length   =       ⁢       7   *     (     channel   ⁢           ⁢   bandwidth   ⁢           ⁢   converted   ⁢           ⁢   to   ⁢           ⁢   length   ⁢           ⁢   via   ⁢           ⁢   chirp     )       +                     ⁢       2   *     (     length   ⁢           ⁢   for   ⁢           ⁢   pre   ⁢     -     ⁢   tuning     )       +     2   *     (     length   ⁢           ⁢   for   ⁢           ⁢   100   ⁢           ⁢   ps     ⁢                                   ⁢   delay   )     +           ⁢     2   *     (     length   ⁢           ⁢   for   ⁢           ⁢   signal   ⁢           ⁢   reflection   ⁢           ⁢   for   ⁢           ⁢   40   ⁢           ⁢   Gb   ⁢     /     ⁢   s     )                   Length   =       ⁢       7   *     (     5.7   ⁢           ⁢   nm   *   680   ⁢           ⁢   ps   ⁢     /     ⁢   nm   *   1   ⁢           ⁢   cm   ⁢     /     ⁢   100   ⁢           ⁢   ps     )       +                     ⁢       2   *     (     1   ⁢           ⁢   cm     )       +     2   *     (     1   ⁢           ⁢   cm     )       +     2   *     (   2.176   )                     Length   =       ⁢       47.11   ⁢           ⁢   cm     =     &gt;     50   ⁢           ⁢   cm                   
   3. Exemplary calculation for higher-order case (dual grating section+single nonlinear chirped FBG for variable chromatic and 2nd order PMD compensation)
           Assume + or −500 ps/nm tunability on the variable chromatic dispersion section.   Assume 80 km of fiber (1360 ps/nm of fixed chromatic dispersion for which to compensate).   Know that for the nonlinear chirped FBG all dispersion values must be positive (cannot have a zero crossing for it to operate correctly).   Pick a chirp range from 200 ps/nm up to 1200 ps/nm (this will give the + or −500 ps/nm about an average value of 700 ps/nm).   Fixed PMD compensator must compensate for remaining chromatic dispersion which is 1360 ps/nm−700 ps/nm=660 ps/nm.   
               

     In one particular embodiment, shown in  FIG. 4 , a V-shaped polarization maintaining fiber waveguide  332  with a long grating  334  is wrapped around a grooved piezoelectric mandrel  345 . Epoxy may be used to secure the entire mechanism. For this embodiment, the stress  382  applied during tuning is always in the same direction and along one of the major axes of the polarization-preserving fiber, such that minimal mode coupling occurs. Such a tuning technique will also alleviate the problems of stretching a long grating in a limited amount of space equally along its distance. 
     An alternative embodiment of the present invention is shown in FIG.  5 . Similar elements to those in  FIG. 3  share the same last two reference numerals. In this case the circulator  470  is placed before the polarization controller  410 . The circulator input port  472  would receive the incoming communications signal. The circulator output port  474  is coupled to the polarization controller input port  411 . The polarization controller output port  413  is then coupled to the input port  423  of the differential polarization delay unit  430 . 
       FIGS. 6-8  illustrate alternative embodiments of the present invention where the compensator of the present invention, or portions thereof, is integrated onto a lithium niobate, or other suitable birefringent electro-optical material, chip. Again, similar elements to those in  FIG. 3  share the same last two reference numerals. 
     To allow for polarization splitting, and to achieve a high electro-optic effect for the tuning of waveguide gratings, the orientation of the lithium niobate crystal is x-cut, y-propagating. Polarization controllers may be fabricated in this crystal orientation as well. Preliminary calculations estimate that only ˜50 V may be required for the tuning of gratings, depending on the design. Since the fastest time changes for this device are expected to be on the order of 100&#39;s of microseconds, significantly higher voltages may be acceptable, allowing for further design flexibility. There are a number of possibilities for making the gratings, such as etching, ion implantation, diffusion doping, or using a high-refractivity index layer patterned on the wafer surface. 
     In one particular embodiment of the present invention, shown in  FIG. 6 , a polarization controller  510  and a differential polarization delay unit  530  are integrated into a LiNbO 3 , or other suitable birefringent electro-optical material, chip  595 . Previous methods for polarization mode dispersion compensation utilizing Lithium Niobate used ‘single-path’ techniques, where both polarizations of the signal travel the same path and the differential delay comes from the birefringence of the crystal. However, only ˜25 psec maximum differential delay may be possible in a single pass of a 4″ wafer. Additionally, large minimum bend radii (˜3 cm) make multiple pass configurations difficult. The design of the present invention greatly enhances the possible amount of differential delay from an integrated Lithium Niobate waveguide. For example, to create a differential delay of 300 psec (using a push-pull configuration where both gratings are tuned in the opposite direction) a linearly chirped grating length on the order of only 3 cm would be required. The current embodiment has a further advantage. Because the polarization splitter  525  and the differential delay line waveguides  532  and  536  are fabricated on the lithium niobate together, no waveguide OPL biasing is required for the purpose of accommodating splicing tolerances. However, bias may be purposefully introduced to build in a fixed DGD offset value. 
     Furthermore, a PMD compensator of this embodiment can be applied to a WDM system, where a demultiplexer is used to separate the individual channels of the system, a separate compensation system is used for each channel, and a multiplexer is used to recombine the signal. 
       FIG. 7  shows an alternate embodiment to further reduce the number of components in a WDM system utilizing PMD compensation for each wavelength channel. In this embodiment, PMD compensators  604 - 606  from neighboring WDM channels are integrated onto one Lithium Niobate chip  695 . Each individual compensator is based on the embodiment shown in FIG.  6 . The signals from the compensators  604 - 606  are multiplexed (demultiplexer) by a WDM MUX/DEMUX  690 . A plurality of signal analyzers  660  coupled to a WDM demultiplexer  692  monitor the output and help control each compensator  604 - 605 . Such a scheme allows the use of only one circulator  670  and one tap coupler  625 . Furthermore, only one optical packaging step may be required for all the channels integrated onto the chip, reducing the overall cost of packaging. 
       FIG. 8  shows an alternate embodiment of the multi-channel WDM design shown in FIG.  7 . In this embodiment, tap couplers  725  for each wavelength channel are integrated along with the PMD compensator components  704 ,  705 ,  706  (illustrated detail in  FIG. 6 ) onto the Lithium Niobate (LiNbO 3 ) chip  795 . The tap coupler may be integrated in the reflected path, either before or after the polarization controller, such that the optical packaging may be performed in one step. Doing this would further integrate the functions of the PMD compensator and greatly reduce overall cost. 
       FIG. 9  shows an advanced embodiment for compensating first and higher orders of dispersion, both chromatic and polarization mode. Similar elements to those in  FIG. 2  share the same last two reference numerals. The second order PMD term is quantitatively equivalent to the term for varying chromatic dispersion. This technique is therefore attractive because many of the new 40 Gb/s systems to be deployed will require tunable chromatic dispersion compensation as well as PMD compensation due to reduced dispersion tolerance, and due to the presence of add/drop and switching nodes which will cause link lengths to vary considerably as a function of time. In  FIG. 9 , the communications signal  812  is first sent through a polarization controller  810 , a circulator  870 , and a differential polarization delay unit  830 , which also acts as a fixed chromatic dispersion compensator when the appropriate chirp in the gratings is chosen. Then, before sampling the output to determine the signal quality, the signal is sent through a variable chromatic/2 nd  order PMD compensator  803 , which includes a standard non-linearly chirped grating  837  that can be tuned to adjust the level of chirp. The output signal from the variable chromatic/2 nd  order PMD compensator  803  is then sampled by tap coupler  850  and sent back out into the system. Signal analyzer  860  is used to analyze the quality of the sampled output signal. In an exemplary embodiment, with a standard link length chromatic dispersion component (1360 ps/nm) and with +/−500 ps/nm of tunability, a dual gratings differential delay line  830  with a chirp rate of 660 ps/nm and a non-linearly chirped grating in the variable chromatic/2 nd  order PMD compensator  803  with a chirp rate that varies from 200 ps/nm to 1200 ps/nm would compensated for higher-order dispersion. 
     Referring to  FIGS. 1 and 2 , a method for compensating for polarization mode dispersion of an incoming optical communications signal according to the present invention includes the step of adjusting the state of polarization of the incoming optical communications signal  12  to a state that minimizes the distortion of the signal after it has passed through the differential polarization delay unit. The adjusted communications signal  14  is split into a first and second orthogonal principal states of polarization  22  at the split point  124 . For some splitter devices, the split point may be an optical abstraction that indicates the optical path position where the polarization mode split occurs, rather than necessarily a physical split point. The first of the polarization states is directed to the first birefringent waveguide  132  having the first chirped grating  134  having the first reference reflection point  135 . The second of the polarization states is directed to the second birefringent waveguide  136  having the second chirped grating  138  having a chirp pattern substantially similar to that of the first chirped grating and having the second reference reflection point  139 . 
     The optical path length from the second reflection point  139  to the split point  124  may be variably adjusted to compensate for polarization dispersion between the first and second orthogonal states of polarization. The polarization modes are reflected by the Bragg gratings  134  and  138  back towards the split point  124 . The polarizing beam splitter/combiner  125  recombines the first and second polarization states into output signal  42 . 
     Those skilled in the art will appreciate that the present invention may be used in a variety of applications where correction of polarization mode dispersion is present. Alternatively, it may even be used to introduce polarization mode dispersion in a controlled manner. While the present invention has been described with a reference to exemplary preferred embodiments, the invention may be embodied in other specific forms without departing from the spirit of the invention. Accordingly, it should be understood that the embodiments described and illustrated herein are only exemplary and should not be considered as limiting the scope of the present invention. Other variations and modifications may be made in accordance with the spirit and scope of the present invention.