Abstract:
Current optical networks are engineered to handle amplifier noise and chromatic dispersion. Polarization mode dispersion occurs in optical networks due splitting of the light energy of a pulse propagating in a fiber into two modes. Compensating for polarization mode dispersion is a difficult and expensive task and hence only few commercial systems have been deployed to deal with this issue. A polarization mode dispersion compensation module according to an example embodiment of the present invention compensates for polarization mode dispersion by determining a performance metric related to an error rate of an optical signal in at least one polarization mode in a filtered state. Based on the performance metric, a control vector is determined to control the optical signal in the at least one polarization mode in the filtered state. The control vector is then applied to a polarization effecting device to compensate for polarization mode dispersion.

Description:
BACKGROUND 
       [0001]    Current optical networks are engineered to handle amplifier noise and chromatic dispersion. Polarization Mode Dispersion (PMD) is a phenomenon that occurs due to splitting of energy of an optical pulse propagating in a fiber into two polarization modes. Since these two modes have slightly different refractive indices, the two modes travel at different velocities and thus, PMD results in pulse spreading. PMD compensation (PMDC) is a difficult and expensive task and, hence, fiber links with high PMD coefficients are largely avoided for high speed transmission. However, if there are no other available options for dealing with PMD than to use a high PMD link, the operator has to pay a high price for deploying costly regenerators designed to zero out the dispersion. 
         [0002]    Due to the difficulty and high cost associated with PMDC, only a few commercial systems have been deployed. Ten Gigabit per seconds (10 Gbps) transmission is fairly tolerant to PMD because of its long, 100 picoseconds (ps), symbol period. A general rule of thumb in optical network engineering is that having a PMD level up to a third of the symbol period can be tolerated. A third of the symbol period for a 10 Gbps transmission translates to having the symbol period set at 33 ps. Hence, network operators engineer their networks to keep the PMD below this level. 
         [0003]    Higher rate channels are making their way into networks initially designed for 10 Gbps. These higher rate services of 40 Gbps and someday 100 Gbps have shorter signaling periods and, as such, are much more susceptible to PMD. For this reason and also to solve the problem of lower rates, such as 10 Gbps, on a high PMD fiber, there is a need to compensate for PMD. 
       SUMMARY 
       [0004]    A method or corresponding apparatus in an example embodiment of the present invention compensates for polarization mode dispersion by determining a performance metric related to bit error rate of an optical signal in at least one polarization mode in a filtered state. Based on the performance metric, a control vector is determined to control the optical signal in the at least one polarization mode in the filtered state. The control vector is then applied to a polarization effecting device to compensate for polarization mode dispersion. 
         [0005]    Another example embodiment of the present invention compensates for polarization mode dispersion by determining a performance metric related to a bit error rate of an optical signal. If the performance metric is below a threshold, a control vector is determined to control polarization mode of an optical signal based on optical signal power. Based on the performance metric, the control vector is then applied to a polarization effecting device to compensate for polarization mode dispersion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0007]      FIG. 1  illustrates an example of an optical communication network with a receiver that employs a polarization mode dispersion compensation module according to an example embodiment of the present invention; 
           [0008]      FIG. 2  illustrates an example of an optical communication network employing a polarization mode dispersion compensation module using an example embodiment of the present invention; 
           [0009]      FIG. 3  illustrates an example embodiment of the present invention employing a polarization mode dispersion compensation module with a polarizer; 
           [0010]      FIG. 4  is a plot of receive polarization power as a function of polarization controller degrees of freedom; 
           [0011]      FIG. 5  is a plot of receive polarization performance as a function of polarization controller degrees of freedom; 
           [0012]      FIG. 6  is a flow diagram of an example embodiment of the polarization mode dispersion compensation module; and 
           [0013]      FIG. 7  is a flow diagram of an example embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    A description of example embodiments of the invention follows. 
         [0015]    An example embodiment of the present invention relates to compensating for Polarization Mode Dispersion (PMD). 
         [0016]    PMD arises because an optical signal on a fiber exists in two polarization modes such that the modulated bit stream exists on two separate electromagnetic waves that are orthogonal to each other. The two polarization modes are identical at the beginning of the path. However, after some distance and depending on the symmetry of the fiber, the two modes begin to shift in time, which manifest itself as a pulse broadening effect. The pulse broadening effect of dispersion causes signals in adjacent bit periods to overlap, a phenomenon referred to as inter-symbol interference (ISI). In other words, one mode travels at a slightly faster speed than the other, so the two modes begin to shift in time with respect to each other. 
         [0017]    In the receiver, both modes are mixed together as the optical signal is converted to an electrical signal. The shift between polarization modes creates a smeared electrical signal. In cases where the shift between polarization modes is severe, this shift may create multiple images of the signal. 
         [0018]    The above phenomenon is referred to as first order PMD or Differential Group Delay (DGD). There are other higher orders of PMD that cause further undesirable effects, but receiver impairments are dominated by differential group delay. 
         [0019]    PMD Compensation (PMDC) technology attempts to correct for the time shift between the modes. It has been the subject of much research and some product development, but little of it has made its way to commercial use due to its complexity and expense. 
         [0020]    An example embodiment of the present invention relates to compensating for undesirable effects due to first order PMD or differential group delay. The example embodiment may control the effective differential group delay to less than half of a symbol period in order to compensate for PMD. 
         [0021]      FIG. 1  illustrates an example of an optical communications network  100  with a receiver  140  that employs a polarization mode dispersion compensation module (not shown) according to an example embodiment of the present invention. In this example embodiment, an optical transmitter node  110  transmits a 40 Gbps optical signal  120  through a transmission path, which includes a fiber  150 . The optical signal  120  propagates through such fibers  150  in different modes, with each mode traveling at a slightly different velocity. This difference in the propagation of the optical signal  120  results in pulse spreading  130  due to PMD. 
         [0022]    In addition to the fiber, PMD can be caused by individual components in the optical communications network  100 . Additionally, factors such as mechanical stress due to movement of the fiber may result in PMD. For example, PMD may be caused by daily or seasonal temperature changes that result in cooling or heating of the optical fibers. PMD may be caused by vibrations in the fiber from nearby elements. For instance, vibrations arising from highways, railroad tracks, and fans in a central office, located near optical fibers, may result in PMD in the optical fibers. 
         [0023]    In order to compensate for the PMD in the optical fiber, the optical communications network  100  employs the receiver  140  with the polarization mode dispersion compensation module according to an example embodiment of the invention. 
         [0024]    The PMD compensation module employs measurements of the optical receiver  140  performance to determine a performance metric based on an error rate, such as a bit error rate, of the optical signal  120 . The optical receiver  140  then uses this determined performance metric to compensate for the PMD dispersion. 
         [0025]    In accordance with the foregoing, a method or corresponding apparatus in an example embodiment of the present invention compensates for polarization mode dispersion by determining a performance metric related to error rate, such as bit error rate, of an optical signal in at least one polarization mode in a filtered state. Based on the performance metric, a control vector is determined to control the optical signal in the at least one polarization mode in the filtered state. The control vector is then applied to a polarization effecting device to compensate for polarization mode dispersion. 
         [0026]    Another example embodiment of the present invention compensates for polarization mode dispersion by determining a performance metric related to error rate of an optical signal. If the performance metric is below a threshold, a control vector is determined to control polarization mode of an optical signal based on optical signal power. Based on the performance metric, the control vector is then applied to a polarization effecting device to compensate for polarization mode dispersion. 
         [0027]    Yet another example embodiment of the present invention includes a computer program product including a computer readable medium having computer readable code stored thereon, which, when executed by a processor, causes the processor to determine a performance metric related to an error rate of an optical signal in at least one polarization mode in a filtered state. Based on the performance metric, a control vector is determined to control the optical signal in the at least one polarization mode in the filtered state. The control vector is then applied to a polarization effecting device to compensate for polarization mode dispersion. 
         [0028]    In the view of the foregoing, the following description illustrates example embodiments and features that may be incorporated into a system for compensation of PMD, where the term “system” may be interpreted as a system, subsystem, device, apparatus, method, or any combination thereof. 
         [0029]    The system may determine the control vector based on the performance metric and optical power of the optical signal of the at least one polarization mode in the filtered state. The system may determine the control vector based on a combination of optical powers, including optical power of the optical signal of the at least one polarization mode in the filtered state and of the optical signal of a different polarization mode in an unfiltered state. The system may determine the control vector based on the power of the at least one filtered polarization mode during startup and following startup, determine the control vector based on the performance metric. The system may determine the control vector based on the power of the at least one filtered polarization mode if the performance metric falls below a first predetermined threshold, and following the performance metric rising above a second predetermined threshold, the system may determine the control vector based on the performance metric. The system may determine the control vector as a function of determining the control vector based on two polarization modes. The system may determine the control vector by applying dither control. 
         [0030]    The system may determine the performance metric as a function of bit error rate. Additionally, the system may determine the performance metric as a function of at least one of the followings: eye opening, eye height, eye width, or Q-Factor. The system may determine a performance metric related to bit error rate of the optical signal in the least one filtered polarization mode to control polarization. The system may determine the bit error rate from a forward error correction function. 
         [0031]    The system may compensate for polarization mode dispersion in a single mode fiber. 
         [0032]    The system may control effective differential group delay in the single mode fiber to less than half of a symbol period. 
         [0033]    The system may filter the optical signal to produce the optical signal in a filtered state in the at least one filtered polarization mode. 
         [0034]    The system may perform polarization beam splitting of the optical signal to produce the optical signal in the at least one filtered polarization mode. 
         [0035]    The optical signal to the system may be a received optical signal. The optical signal to the system may be a partially received optical signal. 
         [0036]    The system may compensate for polarization mode dispersion using a stand alone polarization mode dispersion compensator. 
         [0037]      FIG. 2  illustrates an example of an optical communications network  200  employing a node  260  that provides polarization mode dispersion compensation module using an example embodiment of the present invention. In this example embodiment, the optical transmitter node  210  transmits a 40 Gbps optical signal  220  through a first transmission path  250   a , which includes a fiber  250 . The optical signal  220  propagates through the fiber  250  in different modes, each of which travels at a slightly different velocity. This difference in the propagation of the optical signal  220  results in pulse spreading  230  due to PMD. 
         [0038]    In order to compensate for the PMD in the optical fiber, a polarization mode dispersion compensation module  260  according to an example embodiment of the present invention is employed. The PMD compensation module  260  employs a performance metric calculated based on the bit error rate of the optical signal  220  to compensate for PMD. They PMD compensation module may employ at least one amplifier (not shown) to amplify the light signal while performing PMD compensation, thus acting as a repeater in case losses are caused by optical elements (not shown) used to perform the PMD compensation. 
         [0039]    After PMD compensation is complete, the optical signal with compensated PMD  230  is then transmitted to an optical receiver node  240 . 
         [0040]      FIG. 3  illustrates an example embodiment  300  of the present invention employing polarization mode dispersion compensation with a polarization effecting device (PED)  351 . 
         [0041]    For illustration purposes,  FIG. 3  only considers the case in which a single impulse of light, interchangeably referred to herein as optical signal  320 , is transmitted down an optical path  350 , e.g., optical fiber. The optical signal  320  propagates on the optical path  350  (i.e., an optical fiber) in two different polarization modes  301 ,  302  about a time axis  303 , such that the modulated bit stream exists on two separate electro-magnetic waves that are orthogonal to each other. Due to the two different paths of propagation, the optical signal  320  is affected by polarization mode dispersion. At the top of  FIG. 3 , arrows represent these two polarization modes  301 ,  302  about the time axis  303  at various points in the example embodiment. 
         [0042]    Initially, the two modes  301 ,  302  are split along the time axis  303  with the horizontal mode  302  delayed with respect to the vertical mode  301  about the time axis  303 . Additionally, the two modes  301  and  302  always remain orthogonal to each other along the time axis  303 . 
         [0043]    A polarization effecting device  351  can rotate the polarization of the optical signal  320  under control of a polarization mode compensation controller  360  to any particular orientation. The controller  360  rotates the polarization of the optical signal  350  so that one of the polarization modes (e.g., horizontal mode  302 ) is aligned with the vertical polarizer inside of a polarization beam splitter  355  along an optical path  350  and the other (e.g., vertical mode  301 ) is aligned to the horizontal polarizer. In this example embodiment, a polarization beam splitter  355  separates the horizontal mode  302  and sends it to an optical receiver  340 . Thus, the configuration of this example embodiment  300  does not employ a time shifter (not shown) to shift the vertical mode  301 , nor does this example embodiment  300  recombine the horizontal and vertical modes  301 ,  302  to create a corrected signal. It should be understood that the polarization beam splitter  355  may alternatively be a filter or other optical element(s) used to separate the polarization modes as described herein. 
         [0044]    The polarization effecting device  351  in this example embodiment may be used to maximize optical power entering the optical receiver  340 . Unlike traditional polarization compensators, the polarization effecting device  351  of this example embodiment is not controlled based on polarization alignment with the polarization beam splitter  355 , which simplifies control and reduces both costs and computational complexity. 
         [0045]    Further, in this example embodiment, the controller&#39;s  360  controlling the polarization effecting device  351  may not be entirely based on a measurement of the optical receiver  380  performance but also based on measurements of optical power by an optional vertical polarization mode power detector  370  or a horizontal polarization mode power detector  375 . Specifically, the receiver  380  determines a performance metric  385  based on an error rate, such as a bit error rate, of the optical signal  340 . 
         [0046]    The performance metric  385  may be determined as a function of at least one of the followings: eye opening, eye height, eye width, or Q-Factor. Eye opening, height, and width are factors relating to an “eye” diagram, which is a useful tool for analysis of the signal in digital communication. The eye diagram is essentially the oscilloscope display of the received optical signal, sampled repetitively and applied to the vertical input signal, resulting in a pattern resembling a series of “eyes”. The eye diagram may be analyzed to measure the performance of the system. Eye opening, measure of height of an eye from peak to peak, is a parameter used for measuring the amount of additive noise in the signal. Similarly, eye width and eye height may be used as measures of distortion, synchronization, and jitter effects. 
         [0047]    The polarization mode dispersion compensation of this example embodiment  300  then employs the performance metric  385  along with possible optical power metric measurements  372 ,  377 , or possibly at least the horizontal polarization mode power metric  377 , obtained from the input and output of the polarization beam splitter  355  to compensate for the dispersion. 
         [0048]    The receiver  380  of this example embodiment may include modules (not shown), such as an optical front end, photo detectors, clock recovery module, decision circuit, and/or forward error correction module. When forward error correction is employed, the receiver employs arithmetic or algebraic structure of the optical signal to detect and correct possible errors in the signal. The receiver  340  generates the receive bit stream  399  based on the digital representation of the optical signal bit stream. 
         [0049]    In one example embodiment, the control system  360  can maximize a difference between the horizontal power and the vertical power measures  372 ,  377  to ease the procedure of making differential power measurement. 
         [0050]    In this example embodiment, two degrees of freedom are used to control the polarization effecting device  351 , resulting in reduction of cost and simplified control. Although, theoretically, employing two degrees of freedom may not perform as well as configurations with higher degrees of freedom, this configuration  300  is cost effective, computationally feasible, and performs well enough to be used in many network applications. 
         [0051]    The polarization controller  360  may employ available control system methods in the literature, such as a proportional integral derivative (PID) control or any form of digital control. 
         [0052]    An example embodiment of the present invention may determine the control vector by applying dither control. Dither control is typically used in situations where the relationship between the control variables and the plant being controlled are unpredictable. The control variables are changed in small steps in random directions while plant performance is monitored. Future control decisions are biased along the directions that produced performance improvements in the past. Polarization control has a high degree of unpredictability and complexity that make dither control attractive for practical control applications. 
         [0053]    Another example embodiment of the present invention may employ a fiber squeezer (not shown) to change physical dimension(s) of the optical path  350  to control the polarization in the optical path  350 . 
         [0054]    Yet another example embodiment of the present invention may determine the bit error rate from a forward error correction function. 
         [0055]    Another example embodiment of this invention may determine the performance metric as a function of at least one of eye opening, eye height, eye width, or Q-Factor, where each of these performance metrics is a measure of an optical logical one to an optical logical zero, represented by photons in the optical signal  320  as determined by the optical receiver  380 . 
         [0056]    Yet another example embodiment of the present invention may determine a control vector based on the power of at least one of the polarization modes (e.g., horizontal) during startup. Following startup the example embodiment may determine the control vector  362  exclusively based on the performance metric. 
         [0057]    Another example embodiment of the present invention may switch from using power measurements (i.e., power metric  372  or  377 ) to control the polarization effecting device  351  as follows. After a predetermined amount of time has passed (or a threshold has been crossed), the example embodiment may switch the control from power control to some other form of control based on receiver performance, to maximize some measure of receiver performance. 
         [0058]    Yet another example embodiment of the present invention may employ measures of the bit error rate of the receiver using forward error correction and eye opening penalty in order to determine the performance metric. 
         [0059]    Other example embodiments of the present invention may use measures such as signal-to-noise ratio. The measure may be calculated using linear or logarithmic calculations. Similarly, other factors may be calculated using linear or logarithmic calculations to determine the performance metric. 
         [0060]    Another example embodiment of the present invention may employ a stand alone configuration of the polarization mode dispersion compensation. The stand alone polarization mode dispersion compensation configuration operates much like the example embodiment illustrated in  FIG. 3  with the receiver  380  replaced with a partial receiver (not shown). The partial receiver contains circuitry needed to produce the performance measurement for control purposes and not full transceiver circuitry. Using a stand alone configuration may be lower cost when it is needed to zero out the polarization mode control on a dispersive path. Additionally, having a stand alone configuration may be more cost effective than having a full back-to-back transmitter-responder (transponder) used to receive, amplify, and retransmit the optical. 
         [0061]    Another example embodiment of the present invention may be employed for non-return to zero applications. In non-return to zero applications, the pulse portion of the signal for a “1” bit occupies the entire bit interval, and no pulse is used for a “0” bit. The advantage having a non-return to zero application is in that the signal occupies a small bandwidth. An example embodiment of the present invention may be used for 40 Gbps non-return to zero optical transmission systems to improve the tolerance of the system to PMD. 
         [0062]    Yet another example embodiment of the present invention may be employed for differential phase shift keying (DPSK) modulation format. 
         [0063]      FIG. 4  is a plot of receive horizontal polarization power as a function of polarization controller&#39;s two degrees of freedom. This illustrates typical fiber behavior where the polarization power is a smooth function of the polarization effecting device  351  and has a single, global maximum. In this example, the power is maximized when the first degree (d 0 ) is controlled to π and the second degree (d 1 ) is controlled to π/2. 
         [0064]      FIG. 5  is a plot of receive polarization performance as a function of polarization controller degrees of freedom. This plot illustrates an example embodiment, in which eye opening penalty (in decibels, on the vertical axis) is used as the performance metric. This performance plot is more complex than the smooth power plot shown in  FIG. 4 . It illustrates typical fiber behavior where the presence of local maximums make finding the global maximum difficult. As illustrated in this plot, the performance of the system is maximized when d 0  takes values close to π and d 1  takes values close to zero. Hence, the performance of this example embodiment improves as the value of eye opening approaches zero. 
         [0065]      FIGS. 4 and 5  illustrate that performance is maximized at a different control operating point than where the power is maximized. Hence both measures may be used to control the polarization controller. 
         [0066]      FIG. 6  is a flow diagram of an example embodiment  600  of the polarization mode dispersion compensation module. The example embodiment  600  compensates for polarization mode dispersion by determining the performance metric  685  in a module for determining the performance metric  680 . The performance metric  685  is calculated as a function of received bit error rate  610  of an optical signal  620 . 
         [0067]    A module for determining the control vector  660  subsequently determines the control vector  660  as a function of the performance metric  685 . The control vector  662  is then applied to a polarization effecting device to compensate for polarization mode dispersion  640 . 
         [0068]      FIG. 7  is a flow diagram of an example embodiment  700  of the polarization mode dispersion compensation module. The example embodiment  700  compensates for polarization mode dispersion by determining the performance metric  785  in a module for determining the performance metric  780 . The performance metric  785  is calculated as a function of received bit error rate  710  of an optical signal  720 . 
         [0069]    If the performance metric  785  is below a certain threshold, a module for determining the control vector  760  subsequently determines the control vector  760  as a function of the performance metric  785  and the optical signal power  770 . The control vector  762  is then applied to a polarization effecting device to compensate for polarization mode dispersion  740 . 
         [0070]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.