Abstract:
An apparatus and method for detecting a PMUX multilevel DPSK signal having at least two polarization components with equal symbol periods, which comprises utilizing two polarization-independent Optical Delay Interferometers (ODIs), detecting the four outputs of the two ODIs with two balanced detectors, and digitizing the two detected electronic signals at a sampling rate of twice the symbol rate of the said polarization component signals.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to high data rate communications signal detection, and more particularly, an efficient means of detecting a polarization-multiplexed multilevel signal. 
       BACKGROUND OF THE INVENTION 
       [0002]    As advances in technology require communication systems to facilitate increasingly higher data rates, spectrally efficient modulation methods must be utilized to support them. Polarization-multiplexing is a means of providing an additional layer to an existing modulation scheme, onto which further information can be imparted. Examples of such modulation methods include, but are not limited to, Polarization-Multiplexed Differential Quadrature Phase Shift Keying (PMUX-DQPSK), and Polarization-Multiplexed Differential 8-ary Phase Shift Keying (PMUX-D8PSK). To receive a PMUX signal, polarization demultiplexing is needed. In most of proof-of-concept demonstrations, polarization demultiplexing is performed by a polarization beam splitter (PBS) following a manually adjusted polarization controller. In practical systems, automatic polarization demultiplexing, without any manual intervention, is required. An Optical polarization stabilizer could be used before the PBS to realize polarization demultiplexing, but the cost and complexity associated with the polarization stabilizer are potential deficiencies. As the usefulness of polarization-multiplexing modulation techniques is continually demonstrated, the need for a means of efficiently detecting such PMUX signals becomes apparent. 
       SUMMARY OF THE INVENTION 
       [0003]    Various deficiencies of the prior art are addressed by an apparatus and method for detecting a Polarization Multiplexed Differential m-ary Phase Shift Keyed (PMUX-DmPSK) signal. 
         [0004]    Specifically, an apparatus according to one embodiment of the invention comprises two polarization-independent optical delay interferometers (ODIs), which have substantially the same delay but differ in phase offset by about 90 degrees (or /2), for detecting a PMUX-mDPSK signal having first and second polarization components, outputting a first pair of “in-phase” signals for both the first and second polarization components, and outputting a second pair of “quadrature-phase” signals for both the first and second polarization components; a first balanced detector for receiving the in-phase signals for the first and second polarization components, and providing thereby a first electronic signal; a second balanced detector for receiving the quadrature-phase signals for the first and second polarization components, and providing thereby a second electronic signal; a first digitizer operating at twice the symbol rate of the PMUX-DmPSK signal for receiving the first electronic signal, and providing thereby a first digitized output; a second digitizer operating at twice the symbol rate of the PMUX-DmPSK signal for receiving the second electronic signal, and providing thereby a second digitized output; a first deinterleaver for receiving the first digitized output, and providing thereby separate outputs for the in-phase digital representations for the first and second polarization components; and a second deinterleaver for receiving the second digitized output, and providing thereby separate outputs for the quadrature-phase digital representations for the first and second polarization components. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
           [0006]      FIG. 1  depicts a detector for receiving a polarization-multiplexed differential quadrature phase-shift keying (PMUX-DQPSK) signal; 
           [0007]      FIG. 2  depicts a detector for receiving a PMUX-DQPSK signal, with further digital signal processing to enhance receiver sensitivity and mitigate transmission impairments such as nonlinear phase noise; and 
           [0008]      FIG. 3  depicts a detector for receiving a PMUX-D8PSK signal. 
       
    
    
       [0009]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the various figures. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    The invention will be primarily described within the context of a polarization-multiplexed multilevel differential phase shift keyed signal detector for detecting polarization-multiplexed DQPSK and D8PSK signals, however, those skilled in the art and informed by the teachings herein will realize that the invention is also applicable to detecting other polarization multiplexed differential m-ary phase shift keyed (PMUX-DmPSK) signal types. 
         [0011]      FIG. 1  depicts a detector  100  for receiving a polarization-multiplexed differential quadrature phase-shift keyed PMUX-DQPSK signal  110 , per an embodiment of the invention. Modulated upon PMUX-DQPSK signal  110 , is pre-coded data from four original “in-phase” (“I”) and “quadrature-phase” (“Q”) data tributaries of two polarization components, I x , I y , Q x , and Q y . PMUX-DQPSK signal  110  comprises two polarization components, orthogonally oriented with respect to each other, including, for example, an ‘x’ polarized signal component E x    110   x , and a ‘y’ polarized signal component E y    110   y . E x    110   x , and E y    110   y  both have an identical symbol period T s . In one embodiment PMUX-DQPSK  110  signal utilizes a return-to-zero (RZ) format, and  110   x , and E y    110   y  are offset in time from each other by substantially T s /2. 
         [0012]    PMUX-DQPSK signal  110  is received by polarization independent orthogonal delay interferometer pair (ODIP) circuit  120 . Internally, ODIP circuit  120  is depicted as power dividing PMUX-DQPSK signal  110 , and comprising two separate branches; namely, OD I    122   I  for receiving and recovering the ‘I’ components of PMUX-DQPSK signal  110 , and ODI Q    122   Q  for receiving and recovering its ‘Q’ components. ODI I    122   I  and ODI Q    122   Q  both have delay paths tuned to a delay value of substantially T s , but differ in phase offset by substantially 90 degrees (or /2). In this manner, the four outputs of ODI circuit  120  correspond to the constructive and destructive interferences of the ‘I’ and ‘Q’ components of PMUX-DQPSK signal  110 . 
         [0013]    The outputs of ODI I    122   I  and ODI Q    122   Q  are fed respectively to the inputs of two balanced detectors  124 , and  124   Q , which generate corresponding electronic signals. In this embodiment, ODIP circuit  120  and balanced detectors  124   I  and  124   Q  collectively comprise a detection stage for detector  100 . The outputs of balanced detectors  124   I  and  124   Q  are received respectively by a digitizing stage, comprising digitizers  130   I  and  130   Q . Both digitizers  130   I  and  130   Q  operate at a sample rate (f s ) of twice the symbol rate, or 2/T s . Binary digitizer  130 , thereby contemporaneously digitizes the ‘I’ portions of both polarization components E x    110   x  and E y    110   y  to produce a single binary output, while binary digitizer  130   Q  does the same for the ‘Q’ portions of both polarization components to produce a single binary output. Following combined-polarization digitization, the two recovered ‘I’ and ‘Q’ binary outputs are passed respectively to two 1:2 electronic demultiplexers, or deinterleavers (D-INTs)  140   I  and  140   Q , which represent a deinterleaving stage to separate the digital binary representation of the ‘I’ data symbols modulated on the ‘x’ polarized component of PMUX-DQPSK signal  110  (E x    110   x ), from that modulated on the ‘y’ polarized component (E y    110   y ), and in the same manner, the digital binary representation of the ‘Q’ data symbols modulated on E x    110   x , from that on E y    110   y . 
         [0014]    In another embodiment of the invention, shown as detector  200  in  FIG. 2 , binary digitizers  130   I  and  130   Q  are replaced by multilevel analog-to-digital converters (ADCs)  210   I  and  210   Q , each having a sampling rate of two times the symbol rate, 2/T s , and providing a digital multi-level (instead of binary) output, e.g. with a resolution of about 5 bits (or 32 levels). The digital multilevel ‘I’ and ‘Q’ outputs are then passed respectively to D-INT  220   I  and D-INT  220   Q , which like D-INTs  220   I  and  220   Q  (of  FIG. 1 ), respectively separate the digital multilevel representation of ‘I’ data symbols modulated on E x    110   x  from that on E y    110   y , and the digital multilevel representation of ‘Q’ data symbols contained on E x    110   x , from that on E y    110   y , But, D-INTs  220   I  and  220   Q  are additionally configured to support the multi-level resolution provided by ADCs  210   I  and  210   Q . The outputs of D-INTs  140   I  and  140   Q , representing the digital multilevel representations of the data from the original four ‘I’ and ‘Q’ data tributaries of two polarization components, I x , I y , Q x , and Q y , are passed to a Digital Signal Processor (DSP)  230 , configured to enhance the functionality of detector  200 , receiver sensitivity and/or mitigate transmission impairments such as nonlinear phase noise, before recovering the original four data tributaries in the binary format. Receiver sensitivity enhancement, as an example, can be achieved through data-aided multi-symbol phase estimation, as described in X. Liu, “Generalized data-aided multi-symbol phase estimation for improving receiver sensitivity in direct-detection optical m-ary DPSK,” Opt. Express, vol. 15, 2927-2939, 2007, incorporated herein by reference in its entirety. Nonlinear phase noise mitigation can be achieved through post nonlinear phase noise compensation similar to that described in K.-P. Ho and J. M. Kahn, “Electronic compensation technique to mitigate nonlinear phase noise,” J. Lightwave Technology, vol. 22, pp. 779-783, 2004, incorporated herein by reference in its entirety. 
         [0015]    In another embodiment of the invention, shown as detector  300  in  FIG. 3 , the invention detects PMUX-D8PSK signal  310 , generated from six original data tributaries I x , I y , Q x , Q y , T x , and T y . PMUX-D8PSK signal  310  comprises two polarization components, orthogonally oriented with respect to each other, including, for example, an ‘x’ polarized signal component E x    310   x , and a ‘y’ polarized signal component E y    310   y . E x    310   x , and E y    310   y  both have an identical symbol period T s . ADCs  210   I  and  210   Q . each has a sampling rate of two times the symbol rate, 2/T s , and provides a digital multi-level output. The digital multilevel ‘I’ and ‘Q’ outputs are then passed respectively to D-INT  220   I  and D-INT  220   Q , which respectively separate the digital multilevel representation of ‘I’ data symbols modulated on E x    310   x  from that on E y    310   y , and the digital multilevel representation of ‘Q’ data symbols contained on E x    310   x , from that on E y    310   y . The four outputs of D-INTs  220   I  and  220   Q  represent the digital multilevel representations of the data from four of the six original data tributaries of two polarization components, I x , I y , Q x , and Q y . The digital multilevel representations of the remaining two original data tributaries, T x  and T y , are obtained through signal processing in DSP  330  using the available digital multilevel representations of I x , I y , Q x , and Q y , similar to that also described in X. Liu, “Generalized data-aided multi-symbol phase estimation for improving receiver sensitivity in direct-detection optical m-ary DPSK,” Opt. Express, vol. 15, 2927-2939, 2007, previously incorporated by reference. DSP  330  can also optionally perform receiver sensitivity enhancement and/or mitigation of transmission impairments such as nonlinear phase noise. Finally, DSP  330  recovers the original six data tributaries of the PMUX-8DPSK signal in the binary format. 
         [0016]    The various processes described above as apparatus functionality may also be construed as a methodology for recovering the in-phase and quadrature-phase components for each polarization of a multi-level polarization-multiplexed DmPSK signal. Thus, for example, one embodiment of the invention comprises contemporaneously detecting in-phase and quadrature-phase components for both polarizations of a polarization-multiplexed DmPSK signal to produce an in-phase components signal and a quadrature-phase components signal; digitizing each of the in-phase and quadrature-phase components signals at a sample rate of at least twice the symbol rate; and deinterleaving the digitized in-phase and quadrature-phase components signals to provide, respectively, the in-phase components for each polarization and the quadrature-phase components for each polarization. 
         [0017]    It will be appreciated by those skilled in the art, and informed by the teachings of the present invention, that the invention may be configured to support any PMUX multilevel DPSK modulation formats and additional PMUX modulation formats, beyond those mentioned above. Hence, while the foregoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.