Abstract:
The method and system are disclosed for automatic feedback control of integrated optical quadrature modulator for generation of optical quaternary phase-shift-keyed signal in coherent optical communications. The method comprises the steps of detecting at least a part of an output optical signal from the QPSK modulator, extracting of a particular portion of the output signal in frequency domain, and processing the signal in frequency domain to optimize the transmission of an optical link. The system and method of optical communications in fiber or free space are disclosed that implement the quadrature data modulator with automatic feedback control.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims is a continuation of U.S. Ser. No. 11/679,376 filed Feb. 27, 2007, which is a continuation-in-part of U.S. Ser. No. 10/613,772 filed Jul. 2, 2003 and also a continuation-in-part of U.S. patent applications Ser. No. 10/669,130 filed on Sep. 22, 2003 and Ser. No. 10/672,372 filed on Feb. 7, 200, all of them incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    This invention relates generally to optical communications especially coherent communication with quaternary phase shift keying (QPSK) modulation format. Quadrature modulators are used in these systems for QPSK data encoding. The present invention relates to methods and systems of control for integrated quadrature modulators. 
       BACKGROUND OF THE INVENTION 
       [0003]    Multi-level phase-shift-keying (PSK) offers high spectral efficiency transmission in coherent optical communication systems. Quaternary PSK (QPSK) format, in particular, has recently received much attention. An optical QPSK signal can be generated, e.g., by an integrated LiNbO 3  quadrature modulator (QM) with two parallel Mach-Zehnder modulators (MZMs) nested in a MZ interferometer. Each MZM is driven to produce a binary PSK (BPSK) signal. An optical QPSK signal is produced when two MZMs are biased at their null transmission points and the MZ interferometer is biased at the quadrature phase (π/2).  FIG. 1  shows a schematic of a quadrature modulator  1  known in the prior art. The principle of its operation is as follows. Input optical beam  2  is splitted into two arms of the MZ interferometer by a splitter  3 . Two Mach-Zehnder modulators  4  and  5  are placed in parallel; each MZM being located in each arm of the MZ interferometer. The biases of the MZMs are controlled by control signals  7  and  8  and driven by RF data signals  8  and  9 . The Phase port of the QM  10  controls relative phase shift between the arms of the MZ interferometer. 
         [0004]    In modern communication systems operating at a speed exceeding 10 Gbits/s, a precise stabilization of QPSK modulators is required. There is a need for an automatic feedback control loop that searches for these biases and phase operating points of the QM at initial startup and maintains them during operation. 
       SUMMARY OF THE INVENTION 
       [0005]    The method and system are disclosed for an automatic feedback control of integrated quadrature modulator for generation of optical quaternary phase-shift-keyed signal in coherent optical communications. 
         [0006]    The method comprises the steps of detecting at least a part of output signal from the modulator; extracting of a particular portion of the output signal in RF frequency domain; and minimizing the output signal in RF frequency domain by dithering a voltage applied to a phase shifter of the QPSK modulator. Additionally the method includes detecting the output signal power and minimizing this output signal power by dithering a voltage applied to a first and a second bias of the QPSK modulator. 
         [0007]    Alternative method includes detecting the output signal power and maximizing this output signal power by dithering a voltage applied to a first and a second bias of the QPSK modulator. 
         [0008]    The control loop algorithm uses a steepest decent algorithm to search for optimal operating points of the quadrature modulator via dithering of its biases and phase. The criteria, for the dithering are based on minimization of the RF signal voltage and maximization or minimization of the optical average power of the output signal. 
         [0009]    An optical communications system is proposed that incorporates QPSK modulator for data encoding with the feedback loop control of the modulator to improve transmission performance. In the preferred embodiment the communication system includes an integrated coherent receiver based on 90-degrees optical hybrid 
         [0010]    Yet another object of the present invention is an optical communications system operating in two polarization states of light The system incorporates two QPSK modulators each having its feedback loop control. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows a schematic of a quadrature modulator with two parallel MZMs nested in a MZ interferometer with a phase bias (Prior Art). 
           [0012]      FIG. 2  shows a schematic diagram of control unit for quadrature modulator in optical communications. 
           [0013]      FIG. 3  shows simulation results for (a) average power versus bias voltage of MZM and (b) integrated RF spectral power versus Δφ IQ  for NRZ drive signal swing of 0.75 and 1.2V π . 
           [0014]      FIG. 4  shows constellation plots of the QM optical output at startup (a) and after 50 iterations of the control loop (b). Plot of deviations of the two biases and phase from then optimal points (π and π/2) versus iteration number are shown in (c). 
           [0015]      FIG. 5  shows BER versus received optical power of the differentially detected QPSK signal with automatic control loop or with manual adjustment of the QM. Inset shows eye diagram (top) of the differentially detected 12.5 GSmp/s QPSK signal and the directly detected output from the QM (bottom), Horizontal scale: 20 ps/div. 
           [0016]      FIG. 6  shows block diagram of an optical communication system that includes quadrature modulator having a control unit according to  FIG. 2 . 
           [0017]      FIG. 7  shows a block diagram of a coherent optical receiver for the optical communications system of  FIG. 6 . 
           [0018]      FIG. 8  shows a block diagram of a coherent optical receiver operating in two polarization states of light. 
           [0019]      FIG. 9  shows block diagram of an optical communication system operating in two polarization states of light. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    A schematic diagram for a feedback control loop for the quadrature modulator is shown in  FIG. 2 . A light source  12  is launched into a quadrature modulator  1 . In the preferred embodiment the light source is a CW or pulsed laser. The quadrature modulator is driven by two high-speed (&gt;10 Gbits/s) non-return-to-zero (NRZ) binary data streams through RF ports  8  and  9 . The optical output of the quadrature modulator is divided into two paths using a tap  13 . The optical beam  14  from the tapped output is impinged to a low-speed (e.g., 750 MHz) photodetector  15 . In the preferred embodiment the photodetector  15  is a single photon absorption photodetector. The electrical signal  16  from the photodetector  15  is divided into two by splitter  17  with one path connected to a DC block  18  to reject dc components of the electrical signal. This is followed by a RF spectral power detector (e.g., Schottky diode)  19  to extract the low-frequency RF spectral power (V RF ). The signal is then digitized using an analog-to-digital converter (ADC)  20  connected to a digital signal processing (DSP) unit  21 . The DSP unit contains a code that executes the control loop algorithm. The second path from the photodetector output is directly connected to another ADC  22  that provides monitoring of the optical average power. Outputs of the DSP unit are converted in analog signals by digital-to-analog converters  23 ,  24 ,  25  and directed to the two bias ports  6  and  7  and the phase port  10  of the quadrature modulator  1 . 
         [0021]    The principle of feedback loop operation becomes clear from the following detailed description of its operation.  FIG. 1  shows a schematic of a QM with two push-pull type MZMs with RF and DC bias electrodes nested in a MZ interferometer with a phase electrode for quadrature bias. Consider a single MZM, the directly detected optical output power is P o (t)=(kP i 2){1+cos [π(V s (t)+V B )/V π  }, where V s (t) is the NRZ drive signal with a peak-to-peak voltage swing V pp , V B  is the bias voltage, V π  is the half-wave voltage, P t  is the input optical power, and k accounts for the insertion loss of the MZM. To generate optical BPSK signal, the MZM bias is set to the null transmission with V B =±V π , ±3V π , . . . , and V s  varies between ±V π . The output average power over a period of time T is 
         [0000]    
       
         
           
             
               〈 
               
                 
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                   o 
                 
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                   ( 
                   t 
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               〉 
             
             = 
             
               
                 
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                   ( 
                   
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         [0022]    Taking the derivative of the above with respect to V B  and equating to zero gives 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0023]    The above is satisfied if T s =mV π  and V B =nV π (m,n=0,±1,±2, . . . ). Taking the second derivative of          P o (t)          with respect to V B  gives 
         [0000]    
       
         
           
             
               
                 
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         [0024]    Therefore, the conditions for extrema of the average optical power are 
         [0000]    
       
         
           
             
               
                 
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         [0025]    For null transmission of the MZM, V B =≧V π , ±3V π , . . . , so that the above can be written as follows 
         [0000]    
       
         
           
             
               
                 
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                     π 
                   
                   . 
                 
               
             
           
         
       
     
         [0026]    Therefore, in order to maintain null transmission of the MZM for BPSK operation the average power of the MZM output should be maximized for V π &lt;V pp ≦2V π  or minimized for 0&lt;V pp &lt;V π . 
         [0027]      FIG. 3   a  shows a simulated output optical average power of the QM versus V B  for V pp  of 075 and 1.2V π . The simulation uses a 12.5 Gb/s NRZ pseudo-random binary sequence (PRBS) with a word length of 2 11 −1 with realistic waveforms (finite rise and fall times and ringings) driving the two MZMs of the QM biased to quadrature phase. The two NRZ signals are complementary with a 2-symbol relative time delay. Gaussian noise was added to the chive signal and to the input optical field to check the robustness of the response. As can be seen, the simulation result is consistent with the dependence of the average power on the MZM bias analyzed above. 
         [0028]    Consider now the phase bias of the MZ interferometer of the QM where the phase shift between the two BPSK signals (I and Q) is Δφ IQ . It can be shown that the directly detected output power of the QM is given by 
         [0000]        P   QM =( kP   i /4){1−cos(π V   I   /V   π )/2−cos(π V   Q   /V   π )/2+2 sin[πV I /(2 V   π )] sin [π V   Q /(2 V   π )] cos(Δφ IQ )},
 
         [0000]    where V I  and V Q  are the NRZ binary data signals applied to the two Minis biased at their null transmission points (V B =V π ). Assuming V I  and V Q  varies between ±V π , the detected output can thus be simplified as follows 
         [0000]    
       
         
           
             
               P 
               QM 
             
             = 
             
               { 
               
                 
                   
                     
                       
                         k 
                          
                         
                             
                         
                          
                         
                           
                             
                               P 
                               i 
                             
                              
                             
                               [ 
                               
                                 1 
                                 + 
                                 
                                   cos 
                                    
                                   
                                     ( 
                                     
                                       Δφ 
                                       IQ 
                                     
                                     ) 
                                   
                                 
                               
                               ] 
                             
                           
                           / 
                           2 
                         
                       
                       , 
                     
                   
                   
                     
                       
                         
                           for 
                            
                           
                               
                           
                            
                           
                             V 
                             I 
                           
                         
                         = 
                         
                           
                             V 
                             Q 
                           
                           = 
                           
                             ± 
                             
                               V 
                               π 
                             
                           
                         
                       
                       , 
                     
                   
                 
                 
                   
                     
                       
                         k 
                          
                         
                             
                         
                          
                         
                           
                             
                               P 
                               i 
                             
                              
                             
                               [ 
                               
                                 1 
                                 - 
                                 
                                   cos 
                                    
                                   
                                     ( 
                                     
                                       Δφ 
                                       IQ 
                                     
                                     ) 
                                   
                                 
                               
                               ] 
                             
                           
                           / 
                           2 
                         
                       
                       , 
                     
                   
                   
                     
                       
                         for 
                          
                         
                             
                         
                          
                         
                           V 
                           I 
                         
                       
                       = 
                       
                         
                           
                             ± 
                             
                               V 
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                            
                           
                               
                           
                            
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                            
                           
                               
                           
                            
                           
                             V 
                             Q 
                           
                         
                         = 
                         
                           ∓ 
                           
                             
                               V 
                               π 
                             
                             . 
                           
                         
                       
                     
                   
                 
               
             
           
         
       
     
         [0029]    It is clear that data-like binary pattern will appear at the output of the QM if the MZ interferometer is not in quadrature (Δφ IQ ≠π/2). The RF spectrum of P QM  contains low-frequency components due to this data pattern. Therefore, a minimum integrated RF spectral power of P QM  should be an indication that Δφ IQ  is close to π/2.  FIG. 3   b  shows the simulated integrated RF spectral power of P QM  (V RF ) for V pp  of 0.75 and 1.2V π  versus Δφ IQ  using similar NRZ drive signals with Gaussian noise as in  FIG. 3   a.  The results are in agreement with the analysis. Note that the dependence of V RF  on Δφ IQ  is not affected by V pp . Based on the analysis and results shown in  FIG. 3 , a QM control loop algorithm and model was developed. 
         [0030]    The control loop uses a steepest decent algorithm to search for optimal operating points of the QM via dithering of its biases and phase. The dithering is performed continuously while monitoring the two feedback signals: V RF  and the average optical power. The criteria for the dithering is based on minimization of the signal V RF  and maximizes or minimizes the optical average power if the peak-to-peak NRZ drive signal is above or below the half-wave voltage (V π ) of the quadrature modulator as described earlier. 
         [0031]      FIG. 4  shows typical simulation results of the control loop with Gaussian noise added to the drive signals and to the input optical field as before. One can see that the control loop is quite robust even in the presence of significant amount of amplitude and phase noise. Convergence to optimal operating points was observed for many random initial biases and phases of the QM tested. 
       EXAMPLE 1 
     QM Control Loop Experiment 
       [0032]    An experiment on closed-loop control of the QM was conducted to investigate its performance for generation of a 12.5-GSym/s optical QPSK signal. A packaged LiNbO 3  QM was driven by two 12.5 Gb/s NRZ PRBS (word length: 2 15 −1) signals. The two NRZ signals are complementary with a 2-symbol relative time delay. The NRZ drive voltage swing applied to the QM was V PP ˜1.2V π . The output of the QM was tapped off and directed to a 750-MHz photodetector where its output was divided into two with one path connected to a Schottky diode detector to extract the low-frequency RF spectral power (V RF ). The signal was amplified and directed to a commercial off-the-shelf (COTS) analog-to-digital converter (ADC) connected to a desktop computer (PC) running a code based on the control loop algorithm described earlier. The second path was amplified and directly connected to the ADC that provides monitoring of the optical average power. Outputs of a COTS digital-to-analog converter connected to the PC are directed to the two MZM bias ports and the phase port of the QM. This completes the QM feedback control loop. 
         [0033]    The 12.5 GSym/s optical QPSK signal was directed to a receiver with an optical pre-amp and a band-pass filter. Differential detection of the 12.5 GSym/s QPSK signal was employed using a fiber-based asymmetric Mach-Zehnder (AMZ) interferometer with a one-symbol differential delay (80 ps). The two outputs of the AMZ demodulator were directed to a 15-GHz balanced photoreceiver. The differential phase shift of the AMZ was adjusted to approximately ±π/4 to obtain maximum eye opening.  FIG. 5  shows BER measurements of the differentially detected QPSK signal using the automatic control loop. Measurements using manual adjustment of the DC biases and phase of the QM by minimizing the BER are also shown. A power penalty of about 1 dB at 10 −9  BER was observed for the control loop. This is attributed to the dithering and the relatively flat responses of V RV  and the average power near their optimal points as can be seen in  FIG. 2 . Nevertheless, the control loop concept was demonstrated and validated using COTS components. The QM control loop was operated continuously for about 20 hours with no degradation in performance. The control loop is expected to work for higher symbol rates since no high-speed components are required in the loop. The control loop also works for RZ format of the QPSK signal. Generation and detection of 12.5 GSym/s RZ-DQPSK has using the QM control loop was conducted and its performance was verified. 
         [0034]    Coherent communications system with quadrature modulator having a control unit is another object of the present invention. 
         [0035]    The block diagram of a coherent communications system according to the present invention is shown in  FIG. 6 . Optical transmitter comprises a light source  12  and a quadrature modulator  1  with a control unit  26  that allows optimizing the data transmission performance. Encoded optical signal is transmitted over the transmission link  28  to the coherent optical receiver  29  where the data is decoded by mixing the transmitted optical signal with a signal from a local oscillator  30 . 
         [0036]    In the preferred embodiment the coherent optical receiver is an integrated receiver based on 90-degrees optical hybrid as disclosed in co-pending U.S. patent applications Ser. No. 10/669,130 filed on Sep. 22, 2003 and Ser. No. 10/672,372 filed on Feb. 7, 2007 by the same inventors, incorporated herein by references. 
         [0037]      FIG. 7  illustrates a coherent receiver  29  of the preferred embodiment. It includes an optical interface  31  and a receiving unit  32 . The interface includes a first device input  33  and a second device input  34 ; first  35 , second  36 , third  37  and fourth  38  couplers (mixers); a first phase shifter  39  and a second phase shifter  40 , and first  41 , second  42 , third  43 , and fourth  44  outputs. The optical interface further includes two crossing waveguides  45  and  46 , which cross each other. The receiving unit  32  includes four photodetectors  47 ,  48 ,  49 , and  50  having outputs  51 ,  52 ,  53 , and  54  respectively. The receiver further includes data digital signal processing unit  55 . 
         [0038]    The first  33  and the second  34  device inputs both are connected, respectively, to the first coupler  35  and the second coupler  36 . One output of the first coupler  35  is connected to one input, of the third coupler  37  while another output of the first coupler  35  is connected to the one input of the fourth coupler  38  by a first crossing waveguide  45 . An output of the second coupler  36  is connected to another input of the fourth coupler  38  while another output of the second coupler  36  is connected to another input of the third coupler  37  by a second crossing waveguide  46 . The optical interface also includes at least one phase shifter positioned between two locations. The first location is one of the outputs of the first or second coupler. The other location is one of the inputs of the third or fourth couplers, which corresponds (connected by a crossing waveguide) to the first location. The first and second outputs of the third coupler  37  produce the first  41  and the second  42  device outputs, respectively. The first and second outputs of the fourth coupler  38  produce the third  43  and the fourth  44  device outputs, respectively. 
         [0039]    Signals coming out of the outputs  41 ,  42 ,  43 , and  44  impinge photodetectors  47 ,  48 ,  49 , and  50 , respectively. It is preferred that the photodetectors are PIN photodiodes. The photodiodes are located at equal distance apart. The distance between the neighbor photodiodes can be from 0.01 to 1 mm. In the preferred embodiment the distance is from 0.1 to 0.2 mm. The array of the photodiodes is fabricated on top of a single substrate. InGaAs photodiodes produced by OSI Optoelectronics, Inc. (Hawthorne, Calif.) are examples of such photodiodes. In the preferred embodiment the substrate is made of alumina. 
         [0040]    In another embodiment an optical signal in two polarization states is transmitted over the communications link and by a two polarization coherent detector. One embodiment of a coherent optical receiver  59  operating in two polarizations is shown in  FIG. 8 . It includes an optical interface  60  and a set of photodiodes  61 . The interface includes a first device input  62 , a second device input  63 , a third device input  64 , a fourth device input  65 , a fifth device input  66 ; a polarization beam splitter  67 , first  68 , second  69 , third  70 , fourth  71 , fifth  72 , sixth  73 , seventh  74 , and eighth  75  couplers (mixers); a first phase shifter  76 , a second phase shifter  77 , a third phase shifter  78 , and a fourth phase shifter  79 , first  80 , second  81 , third  82 , fourth  63 , fifth  84 , sixth  85 , seventh  86 , eighth  87 , ninth  88 , and tenth  89  device outputs. The device further includes two sets of crossing waveguides ( 90  and  91 ) and ( 92  and  93 ). The receiver may optionally include two alignment waveguides  94  and  95  are located on opposite sides of the optical interface  60 . 
         [0041]    Signals coming out of the ten outputs  80 ,  81 ,  82 ,  83 ,  84 ,  85 ,  86 ,  87 ,  88 ,  89  impinge photodetectors  96 ,  97 ,  98 ,  99 ,  100 , 101 ,  102 ,  103 ,  104 ,  105 , respectively. It is preferred that the photodetectors are PIN photodiodes. Similarly to the device in  FIG. 7 , the photodiodes are located at equal distance apart. The distance between the neighbor photodiodes can be from 0.01 to 1 mm. In the preferred embodiment the distance is from 0.1 to 0.2 mm. During the fabrication the optical interface  60  alignment relatively the photodetector unit  61  is performed by light passing through waveguides  94  and  95  and positioning the unit  61  to maximize the current from photodiodes  96  and  105 . The accuracy of alignment is at least 1 micron. In the preferred embodiment the accuracy is about 0.1 micron. 
         [0042]    An optical communication link shown in  FIG. 9  illustrates light transmission in two polarizations according to the present invention. Two-polarization transmitter  106  includes the light source  12  and two (PSK modulators  1   a  and  1   b,  each being controlled by its control unit  26   a  and  26   b  respectively. Output beam from the light source  12  is split into two beams by polarization beam splitter  107 . Each of modulators  1   a  and  1   b  operates with the light of one polarization state. Output beam from the modulators  1   a  and  1   b  being combined by a polarization beam combiner  108  is transmitted over the communications link  28  towards the coherent optical receiver  59  operating in two light polarization states The transmitted signal impinges the input  64  of the receiver  59 . In the receiver  59  the transmitted signal is mixed with two signals from a local oscillator  109  operating in two light polarization states. Local oscillator beams  110  and  110  have orthogonal polarization states and impinge the receiver  59  via the inputs  63  and  65  shown in  FIG. 8 . 
         [0043]    The elements in the optical receivers  31  and  60  can each be formed as part of a single planar chip made of an electro-optical material. In various embodiments, the chip is a monolithic piece of a wafer that can be made of semiconductor or ferroelectric materials including but not limited to LiNbO 3 , and the like. In various embodiments, different effects relative to the output of the chip of the present invention are possible., including but not limited to, (i) thermo-optical, (ii) electro-optical, (iii) electro-absorption, and the like. The electro-optical material, which can be LiNbO 3 , can be cut at X, Y, or Z planes. The device of the present invention can utilize a variety of different processes in its creation, including but not limited to, metal in-diffusion and/or (annealed) protonic-exchange technology, wet etching, reactive ion (beam) etching, plasma etching, and the like. 
         [0044]    Integration of components in a single chip, such as LiNbO 3  and the like, can, among other things, reduce cost, improve performance, and provide better stability and control. The optical interfaces  31  and  60  of the present invention, when integrated on a single chip and/or in single package, can be used for various applications, including those that require simultaneous measurement of phase and amplitude of the optical field. In the preferred embodiment the receiving units  32  and  61  include the balanced receivers and optionally Trans-Impedance Amplifiers (TIAs), all formed as a part of a single integrated package. 
         [0045]    Alternatively the integrated device chip can be made of the semiconductor material selected from Si and InP. 
         [0046]    The description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The application of the disclosed quadrature modulator is not limited to optical communications either free-space, fiber or waveguide. The present invention is related to any other possible applications of QPSK modulation technique.