Abstract:
Polarization Mode Dispersion (PMD) causes an optical pulse to be split into two arbitrarily oriented orthogonally polarized PMD pulses with a differential group delay therebetween in an optical transmission line. To compensate for the PMD induced distortion, a polarization controller in a PMD compensator selectively adjusts the aligning of the received arbitrarily oriented first and second principal states of polarization of the PMD pulses to match fixed orientations of predetermined first and second principal states of polarization of a beam splitter. First and second output paths of the beam splitter are coupled to first and second paths of a Mach-Zender interferometer arrangement. The Mach-Zender interferometer arrangement measures the PMD differential delay between the first and second principal states of polarization of the respective first and second PMD optical pulses, and selectively compensates for the measured differential delay. An optical combiner combines output signals from first and second paths of the interferometer arrangement to generate a combined output signal. A control arrangement selectively controls both the compensation for a measured differential delay in the interferometer arrangement in response to at least a portion of the combined output signal from the optical combiner, and selectively controls the adjustment of the aligning of the received arbitrarily oriented first and second polarizations in the polarization controller.

Description:
Field of the Invention  
         [0001]    The present invention relates to method and apparatus for compensating for Polarization Mode Dispersion (PMD) in high speed optical communication systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Polarization Mode Dispersion (PMD) compensation is becoming an urgent issue because it will be the most important limiting factor for high speed optical transmission systems (e.g., OC192 or OC768 systems). For standard single mode optical fibers, the transmission distance of, for example, an OC192 system is limited to 400-600 kilometers due to pulse distortion caused by PMD. Therefore, PMD compensation is very important in upgrading existing system capacity to or beyond the OC192 system level.  
           [0003]    Referring now to FIG. 1, there is shown an exemplary optical signal that is affected by PMD before PMD compensation is applied, and the same signal after PMD compensation. More particularly, PMD causes an optical pulse to be split into two orthogonally polarized pulses with a differential group delay between the two pulses as the optical pulse propagates down an optical fiber. This is shown by the two pulses indicated as “Before Compensation” in FIG. 1. Since receivers in most optical transmission systems are polarization independent, a detected signal will be distorted due to the differential group delay. PMD compensation is a technique which returns the two polarized pulses back into a single in-phase pulse, as is shown by the single pulse indicated as “After Compensation” in FIG. 1, before further transmission or processing in the high speed optical communication system.  
           [0004]    Current compensation schemes require a direct measurement of the PMD value, which is very complicated and slow. Since all compensation schemes are based on the assumption of the existence of principal states of polarization (PSP&#39;s), the implementation of PMD compensation requires endless polarization tracking of the PSP&#39;s. These compensation schemes are not compatible with other polarization control techniques that use scrambling since the data rate of the scrambling techniques are much higher than the response times of current PMD compensators. Other disadvantages of the current PMD compensators include, for example, complicated optical design, mechanically moving parts, high insertion loss, and high cost.  
           [0005]    It is desirable to provide a Polarization Mode Dispersion (PMD) compensator for use in high speed optical transmission systems, where the compensator has a simple optical design, a faster response time, and a low cost as compared with known PMD compensation arrangements.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is directed to method and apparatus for compensating for Polarization Mode Dispersion (PMD) in high speed optical communication systems. More particularly, the present invention relates to Polarization Mode Dispersion (PMD) compensators using an interferometer arrangement for use in high speed optical communication systems.  
           [0007]    Viewed from one aspect, the present invention is directed to a polarization mode dispersion (PMD) compensator for compensating for PMD occurring in an optical input transmission line. The PMD compensator comprises optical circuitry, and an interferometer arrangement. The optical circuitry receives from the optical input transmission line an input signal comprising first and second PMD generated associated optical pulses having first and second principal states of polarization, respectively, and directs the received first and second PMD optical pulses with their first and second principal states of polarization, respectively, onto respective first and second paths. The interferometer arrangement comprises first and second paths that are coupled to the first and second paths, respectively, of the optical circuitry for propagating the respective first and second PMD optical pulses. The interferometer arrangement measures a PMD differential delay between the first and second principal states of polarization of the respective first and second PMD optical pulses, and selectively compensates for the measured differential delay.  
           [0008]    Viewed from another aspect, the present invention is directed to a polarization mode dispersion (PMD) compensator for compensating for PMD occurring in an optical input transmission line. The PMD compensator comprising optical circuitry, an interferometer arrangement, an optical combiner, and a control arrangement. The optical circuitry receives from the optical input transmission line an input signal comprising first and second PMD generated optical pulses having arbitrary orientations of first and second principal states of polarization, respectively. The optical circuitry selectively adjusts the aligning of the received arbitrarily oriented first and second principal states of polarization to predetermined fixed first and second principal states of polarization required for directing the first and second PMD optical pulses onto respective first and second output paths. The interferometer arrangement comprises first and second paths that are coupled to the first and second output paths, respectively, of the optical circuitry for propagating the respective first and second PMD optical pulses. The interferometer arrangement measures a PMD differential delay between the first and second principal states of polarization of the respective first and second PMD optical pulses, and selectively compensates for the measured differential delay. The optical combiner optically combines the signals from the first and second paths of the interferometer arrangement to generate a combined output signal. The control arrangement selectively controls the compensation for a measured differential delay between the first and second polarization states in the interferometer arrangement in response to at least a portion of the combined output signal from the optical combiner, and for selectively adjusting the aligning of the received first and second polarizations to the required predetermined fixed first and second polarizations in the optical circuitry.  
           [0009]    Viewed from still another aspect, the present invention is directed to a method of compensating for polarization mode dispersion (PMD) produced in a transmission line. In a first step of the method, an input signal is received comprising first and second PMD generated optical pulses having arbitrary orientations of their first and second principal states of polarization, respectively. In a second step of the method, the arbitrarily oriented first and second principal states of polarization are adjusted in a polarization controller to match orientations of predetermined fixed first and second principal states of polarization of a polarization beam splitter. In a third step of the method, the first and second PMD optical pulses with the adjusted first and second principal states of polarization, respectively, are directed onto respective first and second output paths of the beam splitter. In a fourth step of the method, a PMD differential delay between the predetermined fixed first and second principal states of polarization of the first and second PMD optical pulses, respectively, is measured. In a fifth step of the method, the measured differential delay from the fourth step is selectively compensated for in an interferometer arrangement comprising first and second paths that are coupled to the first and second output paths, respectively, of the polarization beam splitter.  
           [0010]    The invention will be better understood from the following more detailed description taken with the accompanying drawings and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0011]    [0011]FIG. 1 shows how an exemplary optical signal is affected by Polarization Mode Dispersion (PMD), and the same signal after PMD compensation is applied;  
         [0012]    [0012]FIG. 2 is a block diagram of a Polarization Mode Dispersion compensator in accordance with a first embodiment of the present invention;  
         [0013]    [0013]FIG. 3 is a block diagram of a Polarization Mode Dispersion compensator in accordance with a second embodiment of the present invention; and  
         [0014]    [0014]FIG. 4 is a block diagram of a Polarization Mode Dispersion compensator in accordance with a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]    All components of the various embodiments of the present invention performing essentially the same function in the different embodiments have the same last two digits for their reference numbers.  
         [0016]    Referring now to FIG. 2, there is shown a block diagram of a Polarization Mode Dispersion (PMD) compensator  10  (shown within a dashed line rectangle) in accordance with a first embodiment of the present invention. The PMD compensator  10  comprises a polarization controller  20 , a polarization beam splitter (PBS)  22 , a Mach-Zender interferometer arrangement  30  (shown within a dashed line rectangle), and optical tap (OPT. TAP)  34 , a photodetector  36 , and a control device  38 . PMD occurs as an optical pulse propagates along the length of an optical transmission line  46  of a high speed optical transmission system. It results in the single pulse to be split into first and second orthogonally polarized pulses with a differential delay between them as is shown in FIG. 1. The amount of delay between the first and second orthogonally polarized pulses, after propagating the length of the optical transmission line  46 , is dependent on various factors such as transmission line length, frequency, temperature, etc. It is to be understood that any reference to an input signal to the arrangement  10  hereinafter is referring to the first and second orthogonally polarized PMD pulses that have a differential delay therebetween that occurred in the optical transmission line  46 .  
         [0017]    The polarization controller  20  is responsive to the input optical signal from the optical transmission line  46  for aligning the polarizations of the principal states of polarizations of the first and second input PMD pulses to two perpendicular axes of the polarization beam splitter  22 . The polarization beam splitter  22  is responsive to the output signal from the polarization controller  20  for diverting one of two principal states of polarization of the input signal onto a first path  24  and the other of the two principal states of polarization of the input signal onto a second path  25 . Therefore, the first PMD pulse of the input signal having the first polarization state is diverted to the first path  24  while a second PMD pulse of the input signal having the second orthogonal polarization state is diverted to the second path  25 . If the polarizations of the first and second PMD pulses were not aligned to the two perpendicular axes of the polarization beam splitter  22  by the polarization controller  20 , some of each of the first and second PMD pulses would be diverted into each of the paths  24  or  25  by the beam splitter  22  producing distortion.  
         [0018]    The Mach-Zender interferometer arrangement  30  includes a first delay element  40 , a ½ waveplate  42 , a second delay element  44 , and an optical combiner  32 . The first path  24  extends through the Mach-Zender interferometer arrangement  30  and includes the delay element  40  coupled therein which delays the first PMD pulse having the first polarization state by a fixed predetermined amount of time. The delay element  40  can comprise any suitable optical delay element known in the art as, for example, a length of an optical fiber or other optical delay element providing a fixed predetermined delay. The delayed output from the delay element  40  is coupled to a first input of the optical combiner  32 . The second path  25  extends through the Mach-Zender interferometer arrangement  30  and comprises a serial coupling of the ½ waveplate  42  and the second delay element  44 . The ½ waveplate  42  functions to rotate the polarization of the second PMD pulse of the input signal by 90 degrees so that the rotated polarization state is now aligned with the first polarization state of the first PMD signal in the first path  24  when the optical combiner  32  is a 50:50 coupler. If the optical combiner  32  is a polarization combiner, then the ½ waveplate  42  functions to align the polarization of the second PMD pulse to the proper polarization state of the polarization combiner. In this case, a polarizer (not shown) should be inserted in front of the photodetector  36  to allow proper interference between the two PMD pulses.  
         [0019]    The output from the ½ waveplate  42  is delayed in the second delay element  44  by a selective amount as determined by a control signal from the control device  38  to effect a synchronization of the second PMD pulse in the path  25  with the first PMD pulse in the path  24 . The delayed output from the second delay element  44  is coupled to a second input of the optical combiner  32 . In actuality, the Mach-Zender interferometer arrangement  30  is effectively used to measure an interference visibility between the two principal states of polarization, where a maximum interference visibility corresponds to a minimum delay between the two polarizations. Then, the optical delay is selectively adjusted between the two interfering paths  24  and  25  to maximize the interference visibility and enable the compensation for the PMD. It is to be understood that although the Mach-Zender interferometer provides special advantages as shown hereinabove, any other suitable interferometer arrangement which is able to measure a differential delay between two polarization states, and compensate for such differential delay can be used.  
         [0020]    The optical combiner  32  is, for example, a 50/50 optical combiner which combines the first and second PMD pulses into an output signal which is coupled to the optical tap  34 . An output signal of the optical combiner  32 , if processed properly in the Mach-Zender interferometer arrangement  30 , comprises an optical signal where both the first and second PMD pulses are synchronized and in phase. The optical tap  34  diverts a small portion (e.g., 5%) of the optical output signal from the optical combiner  32  to the photodetector  36  via an optical fiber  35  forming a part of a feedback path. The remaining portion of the output signal from the optical tap  34  provides the optical output signal from the PMD compensator  10  for propagation along a transmission line  48  to a remote receiver or processing device. The photodetector  36  converts the received optical signal into a corresponding electrical control signal which is coupled to the control device  38 . The control device  38  is responsive to the electrical control signal from the photodetector  36  to generate a control signal to the second delay element  44  to selectively alter the delay provided by the second delay element  44  in a direction that causes the second PMD pulse in the second path  25  to be synchronized with first PMD pulse in the path  24 . The control device  38  also provides a control signal to the polarization controller  20  to cause the polarization controller  20  to correctly align the polarization of the principal states of the input signal to the axes of the polarization beam splitter  22  if polarization of the principal states of the input signal are not already properly aligned. A combination of the polarization controller  20  and the PBS  22  may be denoted as an “optical circuit”. The control device  38  may be denoted as a “control arrangement”.  
         [0021]    Referring now to FIG. 3, there is shown a block diagram of a Polarization Mode Dispersion (PMD) compensator  100  (shown within a dashed line rectangle) in accordance with a second embodiment of the present invention. The PMD compensator  100  comprises a polarization controller  120 , a polarization beam splitter (PBS)  122 , a Mach-Zender interferometer arrangement  131  (shown within a dashed line rectangle), an optical tap (OPT. TAP)  134 , a photodetector  136 , a first control device  139 , and a second control device  152 . The polarization controller  120 , polarization beam splitter  122 , optical tap  134 , and photodetector  136  function in the same manner as described hereinbefore for the polarization controller  20 , polarization beam splitter  22 , optical tap  34 , and photodetector  36 , respectively, of the PMD compensator  10  of FIG. 2.  
         [0022]    The Mach-Zender interferometer arrangement  131  comprises a first delay element  140 , a ½ waveplate  142 , a second delay element  144 , an optical tap (OPT. TAP)  150 , and an optical combiner  132 . The difference between the Mach-Zender interferometer arrangement  131  and the Mach-Zender interferometer arrangement  30  of FIG. 2, is that the optical tap  150  is coupled in a first path  124  between the polarization beam splitter  122  and the first delay element  140 . The optical tap  150  diverts a small portion (e.g., 5%) of the first PMD pulse having the first polarization state from the polarization beam splitter  122  to the second control device  152  and the remainder (e.g., 95%) of the first PMD pulse to the first delay element  140 . The second control device  152  functions to convert the optical signal from the optical tap  150  into an electrical control signal to the polarization controller  120 . This electrical control signal is used by the polarization controller  120  to correctly align the principal states of polarization of the input signal from the transmission line  146  to the axes of the polarization beam splitter  122  if polarization of the principal states of polarization of the input signal are not already properly aligned.  
         [0023]    A second output path  125  from the polarization beam splitter  122  extends through the Mach-Zender interferometer arrangement  131  and comprises a serial coupling of the ½ waveplate  142  and the second delay element  144 . The first delay element  140 , ½ waveplate  142 , and second delay element  144  correspond in arrangement and function to the first delay element  40 , ½ waveplate  42 , and second delay element  44 , respectively, described for the Mach-Zender interferometer arrangement  30  of FIG. 2. The combined output signal from the Mach-Zender interferometer arrangement  131  is provided to the first control device  139  via the optical combiner  132 , the optical tap  134 , the optical feedback path  135 , the photodetector  136 , and the electrical feedback path  137  in the manner described for the corresponding elements in the PMD compensator  10  of FIG. 2. The first control device  139  is responsive to the control signal of the feedback path  137  for only altering the delay in the second delay element  144  and thereby reduce the differential group delay between the first and second PMD pulses in the first and second paths  124  and  125  in the Mach-Zender interferometer arrangement  131 . More particularly, the first control device  139  functions to automatically track the differential group delay changes while the second control device  152  functions to track the polarization fluctuations in the first and second PMD pulses in the input signal from transmission line  146 . An output signal from the PMD compensator  100  is provided to a transmission line  146  via the optical tap  134  in the manner described for the optical tap  34  of the PMD compensator  10  of FIG. 2. A combination of the polarization controller  120  and the PBS  122  may be denoted as an “optical circuit”. The control devices  139  and  152  may be denoted as a “control arrangement”.  
         [0024]    Referring now to FIG. 4, there is shown a block diagram of a Polarization Mode Dispersion (PMD) compensator  200  (shown within a dashed line rectangle) in accordance with a third embodiment of the present invention. The PMD compensator  200  comprises a polarization controller  220 , a polarization beam splitter (PBS)  222 , a Mach-Zender interferometer arrangement  231  (shown within a dashed line rectangle), a first photodetector  236 , a first control device  238 , an optical tap (OPT. TAP)  262 , a second photodetector  264 , and a second control device  266 . The polarization controller  220 , polarization beam splitter  222 , photodetector  236 , and first control device  238  function in the same manner as described hereinbefore for the polarization controller  20 , polarization beam splitter  22 , photodetector  36 , and control device  38 , respectively, of the PMD compensator  10  of FIG. 2.  
         [0025]    The Mach-Zender interferometer arrangement  231  comprises a first delay element  240 , a ½ waveplate  242 , a second delay element  244 , an optical tap (OPT. TAP)  260 , and an optical combiner  232 . The difference between the Mach-Zender interferometer arrangement  131  of FIG. 3 and the Mach-Zender interferometer arrangement  231 , is that in the interferometer arrangement  231  the optical tap  260  diverts a large portion (e.g., 95%) of the first PMD pulse having the first polarization state from the polarization beam splitter  222  to the optical tap  262 , and the remainder (e.g., 5%) of the first PMD pulse to the first delay element  240 . The optical tap  262  diverts a large portion (e.g., 95%) of the first PMD pulse having the first polarization state from the optical tap  262  to the output of the PMD compensator  200  via a transmission line  248 , and the remainder (e.g., 5%) of the first PMD pulse to the second photodetector  264 . The second photodetector  264  converts the optical signal from the optical tap  262  into an electrical control signal which is provided as an input to the second control device  266 . The second control device  266  is responsive to the electrical control signal from the second photodetector  264  for generating a control signal to the polarization controller  220 . The polarization controller uses this electrical control signal to coarsely align the principal states of polarization of the input signal from the transmission line  246  to the axes of the polarization beam splitter  222 .  
         [0026]    A second output path  225  from the polarization beam splitter  222  extends through the Mach-Zender interferometer arrangement  231  and comprises a serial coupling of the ½ waveplate  242  and the second delay element  244 . The first delay element  240 , ½ waveplate  242 , and second delay element  244  correspond in arrangement and function to the first delay element  40 , ½ waveplate  42 , and second delay element  44 , respectively, described for the Mach-Zender interferometer arrangement  30  of FIG. 2. The output signals from the Mach-Zender interferometer arrangement  231  are combined in the optical combiner  232  and provided to the first control device  238  via an optical feedback path  235 , the photodetector  236 , and an electrical feedback path  237  similar to that described for the corresponding elements having the same last two digits in the PMD compensator  10  of FIG. 2. The first control device  238  is responsive to the control signal from the feedback path  237  for generating an electrical control signal to the second delay element  244  for altering the delay therein to minimize the differential group delay between the first and second PMD pulses in the first and second paths  224  and  225  in the Mach-Zender interferometer arrangement  231 . The first control device  238  also generates an electrical control signal to the polarization controller  220  to cause a fine adjustment for aligning the principal states of polarization of the input signal to the axes of the polarization beam splitter  222 . More particularly, the second control device  266  functions to coarsely track polarization fluctuations in the first and second PMD pulses in the input signal from transmission line  246 . Concurrently, the first control device  238  functions to automatically track the differential group delay changes between the first and second PMD pulses for altering the delay in the second delay element  244  and thereby minimize the differential group delay between the first and second PMD pulses in the first and second paths  224  and  225  in the Mach-Zender interferometer arrangement  231 .  
         [0027]    The first control device  238  also uses this detected PMD delay to generate an electrical control signal to the polarization controller  220  in order cause the polarization controller  220  to fine tune the aligning of the principal states of polarization of the input signal to the axes of the polarization beam splitter  222 . In operation, the control signal from the second control device  266  has a higher priority that the control signal from the first control device  238 . Therefore, the polarization controller  220  always responds to a control signal from the second control device  266  to make a coarse adjustment before it responds to a concurrent control signal from the first control device  238  to make a fine adjustment. More particularly, the second control device  266  provides a control signal which is mostly sinusoidal and has a flat bottom near the optimum point of adjustment. Therefore, this control signal does not have sufficient resolution to permit the polarization controller  220  to further adjust and achieve maximum alignment of the two polarization states. This is where the control signal from the first control device  238  takes over to achieve that maximum alignment of the two polarization states with the axes of the polarization beam splitter  222 .  
         [0028]    The PMD compensator  200  differs from the PMD compensators  10  and  100  in that only one polarization state is selected and sent out on the transmission line  248  via the optical tap  262  without combining the other polarization component therewith. The power loss caused by discarding one polarization component can be compensated for by using an Erbium-doped fiber amplifier (not shown) in the transmission line  248 . The Mach-Zender interferometer arrangement  231  is used to detect the PMD value, while the PMD compensation is done by correctly selecting one polarization component and coarsely adjusting the polarization controller  220  therewith and fine tuning the adjusting of the polarization controller  220  based on the detected PMD value. Doing so drastically increases the compensating speed.  
         [0029]    The advantages of the present PMD compensators  10 ,  100 , and  200  are that each is a cost-effective first-order PMD compensator that has fewer optical components than that of prior art arrangement so as to enable the combining of the feedback control and PMD measurement within a single Mach-Zender interferometer arrangement  30 . A combination of the polarization controller  220  and the PBS  222  may be denoted as an “optical circuit”. The control devices  238  and  266  may be denoted as a “control arrangement”.  
         [0030]    It is to be appreciated and understood that the specific embodiments of the present invention described hereinabove are merely illustrative of the general principles of the invention. Various modifications may be made by those skilled in the art which are consistent with the principles set forth.