Abstract:
High optical communication rates are making their way into networks initially designed for 10 Gigabits per seconds (Gbps). These higher rates of 40 Gbps and higher have shorter signaling periods and are more susceptible to differential group delay (DGD). A method and corresponding apparatus in an example embodiment of the present invention compensates for polarization state sensitivity of a receiver by determining a performance metric relating to an error rate due to transmission and reception of a modulated optical signal in a medium introducing DGD. Based on the performance metric, a control vector is determined to control a polarization state of the modulated optical signal. The control vector is applied to a polarization effecting device to compensate for the DGD and the polarization state sensitivity of the receiver. Communication rates of 40 Gbps and higher can be used in transmission mediums that introduce DGD through use of embodiments presented.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/052,745, filed on May 13, 2008, the entire teachings of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Optical transmission techniques deployed to date have been mostly based on intensity modulated signals. Amplitude-shift keying (ASK), also known as on-off keying (OOK), is a simple form of modulation that represents the digital data as variations in the amplitude of its carrier wave. In optical transmission, ASK encodes a bit to a logical “one” in the presence of a pulse of light and to a logical “zero” in the absence or near absence of a light pulse. 
         [0003]    Optical dual binary (ODB) is another form of modulation, where the optical signal, at the receiving side of the transmission fiber, is directed to a photo diode. The photo diode converts the light energy to an electrical current, which is subsequently decoded into the received bit stream. 
         [0004]    As transmission rates increase above 10 Giga bits per second (Gbps), more complex modulation formats are being developed. These modulation formats generally result in superior performance over other modulation formats such as on-off keying. 
         [0005]    At transmission rates of 40 Gbps and higher, phase modulation (PM) is a very popular modulation format. Phase modulation represents a signal as variations of phase of its carrier wave. Phase modulation employs an optical carrier, which for wavelength division multiplexing (WDM) systems is in the range of 190 Tera Hertz (THz). This optical carrier is phase modulated to represent the logical “ones” and “zeros” of the bit stream. Most phase modulated formats are differentially encoded, such that the phase difference between any two successive bit intervals represents a logical “one” or a logical “zero” to the receiver. 
         [0006]    Differential phase shift keying (DPSK) is another modulation technique that represents the digital data as variations in the phase of its carrier wave. In differential phase shift keying, the receiver decodes a logical “one” bit if the difference between the phase in the current bit interval differs from the phase of the previous bit interval by one hundred and eighty degrees. If the phase between the current and last interval does not change, the receiver decodes the phase modulated optical carrier as a logical “zero.” 
       SUMMARY 
       [0007]    A method or corresponding apparatus in an example embodiment of the present invention compensates for polarization state sensitivity of a receiver by determining a performance metric. The performance metric relates to an error rate due to transmission and reception of a modulated optical signal in a medium introducing differential group delay. Based on the performance metric, a control vector is determined and used to control a polarization state of the modulated optical signal. The control vector is applied to a polarization effecting device, which, in turn, compensates for the differential group delay and the polarization state sensitivity of the receiver. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    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. 
           [0009]      FIG. 1  is a schematic diagram that illustrates an example embodiment of the present invention employing a polarization state sensitivity compensation module; 
           [0010]      FIGS. 2A and 2B  are plots that illustrate bit error rate (BER) performance as a function of additive optical noise at various differential group delay (DGD) values; 
           [0011]      FIG. 3  is a plot that illustrates tolerable noise versus differential group delay with and without state of polarization (SOP); 
           [0012]      FIG. 4  illustrates an example of an optical communications network with a receiver that employs a polarization state sensitivity compensation module according to an example embodiment of the present invention; 
           [0013]      FIG. 5  is a flow diagram of an example embodiment of the present invention; and 
           [0014]      FIG. 6  is a block diagram of an example embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    A description of example embodiments of the invention follows. 
         [0016]    An example embodiment of the present invention relates to compensating for polarization state sensitivity of a receiver. A method or corresponding apparatus in this example embodiment compensates for the polarization state sensitivity of a receiver by determining a performance metric. The performance metric relates to an error rate due to transmission and reception of a modulated optical signal in a medium introducing differential group delay. Based on the performance metric, a control vector is determined to control a polarization state of the modulated optical signal. The control vector is applied to a polarization effecting device to compensate for the differential group delay and the polarization state sensitivity of the receiver. 
         [0017]    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 due to transmission and reception of a modulated optical signal in a medium introducing differential group delay. The performance metric is used to determine a control vector to control a polarization state of the modulated optical signal. The control vector is applied to a polarization effecting device to compensate for the differential group delay and the polarization state sensitivity of the receiver. 
         [0018]    In the view of the foregoing, the following description illustrates example embodiments and features that may be incorporated into a system for compensation of polarization state sensitivity of a receiver, where the term “system” may be interpreted as a system, subsystem, device, apparatus, method, or any combination thereof. 
         [0019]    The system may determine the control vector based on the performance metric and polarization state of the modulated optical signal. The system may determine the control vector based on optical power of a demodulated representation of the modulated optical signal. The system may determine the control vector based on the performance metric during startup and, following startup, the system may determine the control vector based on the performance metric and the polarization state. The system may determine the control vector based on two polarization modes. The system may determine the control vector by applying dither control by including dither parameters in the control vector. 
         [0020]    The system may determine the performance metric as a function of bit error rate. The system may determine the performance metric as a function of at least one of the following: eye opening, eye height, eye width, or Q-Factor. 
         [0021]    The system may determine the error rate from a forward error detection and optionally from an error correction function. 
         [0022]    The system may perform polarization beam splitting of the modulated optical signal to measure the polarization state independent of the performance metric. 
         [0023]      FIG. 1  illustrates an example embodiment  100  of the present invention employing a receiver  110  with polarization state sensitivity compensation. The receiver  110  may be a differential phase shift keying (DPSK) receiver, and the example embodiment may further include a polarization control system  120 . 
         [0024]    In this example embodiment, an optical transmitter node (not shown) transmits an optical signal  130  through a transmission path  150 , which may include a fiber or free space. The optical signal  130  propagates on the optical path  150  (e.g., an optical fiber) in two different polarization modes  101 ,  102  about a time axis  103 , such that the modulated bit stream exists on two separate electromagnetic waves that are orthogonal to each other. Due to the two different paths of propagation, the optical signal  130  is affected by polarization mode dispersion. At the top of  FIG. 1 , arrows represent the two polarization modes  101 ,  102  about the time axis  103  at various points in the example embodiment  100 . 
         [0025]    Initially, the two modes  101 ,  102  are split along the time axis  103  with the horizontal mode  102  delayed with respect to the vertical mode  101 . Additionally, the two modes  101  and  102  always remain orthogonal to each other along the time axis  103 . 
         [0026]    A polarization effecting device  151  can rotate the polarization of the optical signal  130 , under control of a control system  120 , to any particular orientation. The polarization effecting device  151  may be implemented using one or more fiber stretchers. The control system  120  rotates the polarization of the optical signal  130  so that one of the polarization modes (e.g., horizontal mode  102 ) is aligned with the vertical polarizer  152   a  along an optical path  150  and the other (e.g., vertical mode  101 ) is aligned to the horizontal polarizer  152   b . A polarization beam splitter (not shown) may be employed to separate the horizontal mode  102  from the vertical mode  101  and send the horizontal mode  102  to the optical receiver  110 . It should be understood that in place of the polarization beam splitter, the example embodiment  100  may alternatively employ a filter or other optical element(s) used to separate the polarization modes as described herein. 
         [0027]    The polarization effecting device  151  in this example embodiment may be used to maximize the optical power entering the optical receiver  110 . Unlike traditional polarization compensators, the polarization effecting device  151  of this example embodiment is not controlled based on polarization alignment with a polarization beam. This simplifies control and reduces both costs and computational complexity. 
         [0028]    The optical signal  120  propagates through the optical path  150  and passes through a Delay Line Interferometer (DLI)  140 , which acts as an optical demodulator. The delay line interferometer  140  includes output ports that are connected to a pair of photo diodes  143 , 148 . The photo diodes  143 , 148  convert the optical signal to an electrical signal through a chain of electronics  155 , which ultimately results in the receive bit stream  112 . 
         [0029]    The differential phase comparison or demodulation is performed in the delay line interferometer  140  component. This task may also be implemented in a similar device known as a Mach-Zehnder interferometer (not shown). Delay line interferometer  140  components are free space interferometers. Mach-Zehnder interferometers work on the same interferometric principle but are typically constructed from silica. The delay line interferometer  140  splits the input signal  130  and delays one version of the input signal  130  by the bit period. The delay line interferometer  140  outputs the sum of the current signal plus the delayed signal on its constructive port  143 . The delay line interferometer  140  also outputs the difference between the current signal and delayed signal on its destructive port  148 . 
         [0030]    The delay line interferometer  140  works on the principle of reflection or refraction and also interference. The effect of reflection and refraction is inherently sensitive to the state of polarization of the optical signal  130  entering the delay line interferometer  140 . Light propagating in the optical path  150  (e.g. optical fiber) travels as two orthogonal electromagnetic waves. The state of polarization of the optical signal  130  is dependent on the orientation of the waves around the axis of the optical path  150  (e.g., optical fiber) and the relative phase between the two traveling waves. 
         [0031]    The performance of the receiver  110  of this example embodiment may vary depending on the state of polarization. Normally, the polarization at the end of the transmission fiber  199  is in a random state and changes over time. If the state of polarization happens to be in a bad state (e.g., polarization misalignment), then performance can worsen to the point of link failure. 
         [0032]    The control system  120  is responsible for optimizing and stabilizing the state of polarization. The control system  120  may be analog (e.g., proportional, integral, or differential) or digital (e.g., state space), which may be in form of hardware, firmware, or software. If software, the software may be implemented in any software language consistent with the teachings herein and may be stored on any computer readable medium known or later developed in the art. The software, typically, in form of instructions, can be coded and executed by a processor in a manner understood in the art. 
         [0033]    The polarization effecting device  151  may take any arbitrary input state of polarization and transform it to any desired output state of polarization. In order to optimize the state of polarization, a controller  120  determines a state of polarization of the polarization effecting device  151  (i.e., the state of polarization of the delay line interferometer  140  input) that results in the best performance. This is performed as a feedback control loop, where the feedback loop may be configured to maximize some performance measure  170 , such as the power of the delay line interferometer  172 , eye opening  174 , slicer error  176 , and/or the number of bit errors  178 . When the performance measure  170  is the number of bit errors  178 , the example embodiment  100  minimizes the bit error rate, where the number of bit errors  178  is known from a forward error detection block  160  or optionally from a forward error correction block  165 . 
         [0034]    In order to obtain the state of polarization of the polarization effecting device  151  that results in the best performance, the example embodiment  100  may minimize a slicer error  176 . This is similar to maximizing an eye opening measure  174 . The example embodiment may employ the optical power  172  of the constructive and destructive output ports  143 ,  148  of the delay line interferometer  140  to find a good power balance. 
         [0035]    It should be noted that the optimal state of polarization is not static. The optimal state may change slowly in time due to changes in the delay line interferometer  140  and interconnect between the polarization effecting device  151  and the delay line interferometer  140 . These changes can be due to temperature, vibration, or other possible physical causes. Thus, the optimization control loop remains active for good polarization stabilization performance of the embodiment  100  but the optimization control loop can be run with a low bandwidth. 
         [0036]    Once the optimal point is identified in the control system  120 , the polarization effecting device  151  is actively controlled to maintain the desired state of polarization while the input state of polarization from the fiber varies over time. An optional polarimeter  180  may be employed to solve this polarization stabilization control function. The polarimeter  180  measures the Stokes vector of the light exiting the polarization effecting device  151 . The polarimeter  180  outputs three numeric values that uniquely identify the state of polarization. In this case, the control system  120  drives the polarization effecting device  151  to maintain the optimal state of polarization as represented by the Stokes vector. 
         [0037]    It is possible to eliminate the polarimeter  180  device and use a dither-like controller (not shown), which maximizes one of the example performance measure  170  variables (i.e., the power of the delay line interferometer  172 , eye opening  174 , slicer error  176 , and/or the number of bit errors  178 ). 
         [0038]    The example embodiment may be applied to any differential Phase Shift Keying (PSK) signal that uses a polarization sensitive phase demodulator (interferometer). 
         [0039]      FIG. 2A  demonstrates the bit error rate  201  (BER) performance as a function of additive optical noise at various differential group delay (DGD) values. In this example, the performance of a forty three Gigabits per second (Gbps) differential phase shift keying (DPSK) receiver in presence of optical noise and differential group delay (i.e., polarization mode dispersion) is measured. The example of  FIG. 2A  illustrates the bit error rate  201 , when the state of polarization is optimized and stabilized according to an example embodiment of the present invention. As shown, the performance or the optical signal to noise ratio  202  (OSNR) of the receiver decreases as the group differential delay increases. 
         [0040]      FIG. 2B  demonstrates the bit error rate  201  (BER) performance as a function of additive optical noise at various differential group delay (DGD) values. In this example, the performance of a forty three Gigabits per second (Gbps) differential phase shift keying (DPSK) receiver in presence of optical noise and differential group delay (i.e., polarization mode dispersion) is measured. The example of  FIG. 2B  illustrates the bit error rate  201  when the state of polarization is left uncontrolled. As shown, similar to the example of  FIG. 2A , the performance or the optical signal to noise ratio  202  (OSNR) of the receiver decreases as the group differential delay increases. However, comparing plots  210 ,  220 ,  230 ,  240 ,  250  of  FIG. 2A  against their corresponding plots  260 ,  270 ,  280 ,  290 ,  295  in  FIG. 2B , it can be seen that the performance  202  degrades rapidly with increasing differential group delay if the state of polarization is left uncontrolled. Thus, the example embodiment employed in  FIG. 2A  solves the significant problem of performance degradation by controlling and stabilizing the state of polarization. 
         [0041]      FIG. 3  is a plot  300  that illustrates tolerable noise versus differential group delay  302  with controlling the state of polarization  310  and without controlling the state of polarization  320 . As shown on  FIG. 3 , the performance  301  degrades rapidly with increasing differential group delay  302  if the state of polarization is left uncontrolled  320 . However, this problem of performance  301  degradation is solved when an example embodiment of the present invention is employed to control and stabilize the state of polarization  310 . 
         [0042]      FIG. 4  illustrates an example of an optical communication network  400  with a receiver  410  that employs a polarization state sensitivity compensation module (not shown) according to an example embodiment of the present invention. The receiver  410  may be a differential phase shift keying (DPSK) receiver. 
         [0043]    In this example embodiment, an optical transmitter node  420  transmits a modulated optical signal  430  through a transmission medium  450  introducing differential group delay, which may include a fiber or free space. 
         [0044]    In order to compensate for the polarization state sensitivity, the optical communications network  400  employs the receiver  410  with the polarization state sensitivity compensation module according to an example embodiment of the invention. The polarization state sensitivity compensation module employs an error rate due to transmission and reception of the modulated optical signal  430  in the medium  450  introducing differential group delay to determine a performance metric. Based on the performance metric, the polarization state sensitivity compensation module determines a control vector that is used to control the polarization state of the modulated optical signal. The polarization state sensitivity compensation module applies the determined control vector to a polarization effecting device to compensate for the differential group delay and the polarization state sensitivity of the receiver. 
         [0045]      FIG. 5  is a flow diagram of an example embodiment that compensates for polarization state sensitivity of a receiver (not shown). The example embodiment  500  compensates for polarization state sensitivity and differential group delay of a receiver by determining  580  the performance metric  585 . The performance metric  585  is calculated as a function of error rate  510  due to transmission and reception of a modulated optical signal in a medium introducing differential group delay  520 . 
         [0046]    The performance metric  585  is used to determine  560  a control vector  562 . The control vector  562  is arranged to control a polarization state of the modulated optical signal based on the performance metric  585 . The control vector  562  is applied  540  to a polarization effecting device (not shown) to compensate for differential group delay and the polarization state sensitivity of the receiver. 
         [0047]      FIG. 6  is a block diagram of an example embodiment of the polarization state sensitivity compensation module  600 . The example embodiment  600  includes a first determination module that compensates for polarization state sensitivity and differential group delay of a receiver (not shown) by determining a performance metric  685 . The performance metric  685  is calculated as a function of error rate  610  due to transmission and reception of a modulated optical signal in a medium introducing differential group delay. 
         [0048]    A second determination module  660  determines a control vector  662  as a function of the performance metric  685 . An optional dither source  670  may be also employed. The dither source may be internal or external to the second determination module  660 . The second determination module  660  may determine the control vector  662  as a function of the dither signal  672  applied by the dither source as dither parameters in the control vector  662 . The control vector  662  is arranged to control a polarization state of the modulated optical signal based on the performance metric  685 . An application module  640  applies the control vector  662  to a polarization effecting device (not shown) to compensate for differential group delay and the polarization state sensitivity of the receiver. 
         [0049]    It should be understood that procedures, such as those illustrated by flow diagram or block diagram herein or otherwise described herein, may be implemented in the form of hardware, firmware, or software. If implemented in software, the software may be implemented in any software language consistent with the teachings herein and may be stored on any computer readable medium known or later developed in the art. The software, typically, in form of instructions, can be coded and executed by a processor in a manner understood in the art. 
         [0050]    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.