Abstract:
Since it is difficult to control correctly and optimally the DC biases of an IQ modulator driven with pre-equalized data, a method for controlling an optical transmitter according to an exemplary aspect of the invention includes the steps of (a) making direct current biases for driving children Mach-Zehnder modulators of an IQ modulator in the optical transmitter converge to values close to null driving points of the children Mach-Zehnder modulators, (b) driving the children Mach-Zehnder modulators with special driving data including a pair of training patterns between which there is a significant correlation, (c) scanning direct current biases for setting quadrature angle of the IQ modulator, (d) monitoring output of the IQ modulator during step (c), and (e) setting the direct current bias for setting quadrature angle on the basis of the driving data and monitored results in step (d).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to optical communications technologies, in particular, to an optical communication technology where pre-equalization, i.e. compensation of all or one part of the impairments occurring during transmission, is performed at an optical transmitter. 
       BACKGROUND ART 
       [0002]    Digital Signal Processing (DSP) techniques enable an optical transmitter to compensate the impairments affecting optical signal during transmission over a fiber by applying the inverse filter properties of the impairments. These techniques can be applied at two different stages of a transmission system including a transmitter and a receiver respectively transmitting and receiving a light wave signal through a medium, such as an optical fiber. 
         [0003]    In the first way, the implementation of the DSP technique is performed at the receiver. Notably, a coherent reception technology enables the receiver to get the information on both phase and amplitude of the received signal. This will allow the DSP to compensate for impairments occurring during transmission before the reception by utilizing appropriately calculated filters. An example of signal equalization in this configuration is illustrated in the non patent literature 1 (NPL1). Furthermore, the equalization based on DSP can be implemented in a signal processor as described in the non patent literature 2 (NPL2). 
         [0004]    In another way, the implementation of the DSP technique for equalization is performed at the transmitter. The equalization at the transmitter can be either called pre-distortion, pre-equalization, or pre-compensation depending on the sources. This will allow the DSP to compensate for impairments occurring during transmission after emission of the pre-equalized signal by utilizing appropriately calculated filters. The transmitter emits therefore signal that has been distorted in both amplitude and phase information according to the filters, in order to compensate for the signal with the impairment occurring during transmission in the medium. The non patent literature 3 (NPL3) discloses an example of pre-equalization technique, where both linear impairments like chromatic dispersion (CD) and non-linear impairments like Self Phase Modulation (SPM) are compensated in this manner. The pre-equalization with a DSP enables the transmitter to perform equalization in a more economic way than with a dedicated processor. Alternatively, pre-equalization enables to extend the compensating range of the receiver by adding the compensation range of the transmitter. 
         [0005]    In NPL3, the pre-equalized signal is modulated on the optical carrier with an optical IQ modulator (In phase—Quadrature phase modulator), sometimes called Cartesian modulator, vector modulator, Dual Parallel modulator, or nested modulator depending on the sources. In an IQ modulator, the electric signals drive two independent Mach-Zehnder devices, which can be called children Mach-Zehnder Modulators (MZM), or nested MZM depending on the sources. The children MZM modulate the phase and amplitude of the same optical carrier wave. The phase in one of their outputs is relatively delayed by 90 degrees before being recombined. The phase delay between the outputs of the children MZM can be called an angle of quadrature and is ideally 90 degrees, modulo 180 degrees. IQ Modulators enable a chirp-free modulation for the amplitude and phase information in the pre-equalized signal by accessing directly to the I component and the Q component of the light wave signal. 
         [0006]    However, it is known that there is a drift of DC (Direct Current) bias in IQ modulators due to variation of the temperature or ageing of the device. There are three types of applied biases, that is, the DC biases of each of the two children MZM and DC bias used to set the angle of quadrature. The drift of DC (Direct Current) bias causes a degradation of the transmitted signal, and therefore results in degradation of the received signal quality or in worst cases the impossibility to decode the received signal. This problem is likely to be revealed in the characterization tests of the modulator at the production stage or at the assembly stage of the transmitter in which the modulator is used. This problem can be solved by using Auto Bias. Control (ABC) circuits, which control the biases of the modulators and compensate for the DC bias change. In this manner, ABC technology can manage the drift of DC bias of IQ modulators, enabling correct modulation in optimal condition. 
         [0007]    An example of ABC circuits, which is able to control the DC biases of an IQ modulator driven with multilevel signals in order to generate QAM modulated optical signal, is disclosed in the patent literature 1 (PTL1). The ABC circuit of PTL1 is based on low frequency dither tones to control the DC biases of I and Q components in the children MZM as well as of the angle of quadrature. However, due to the properties of the monitor signal, the optimal DC biases of the children MZM have a periodicity of 2*Vpi, where Vpi is the voltage difference between the biases corresponding to constructive interferences and destructive interferences of the MZM. Similarly, the optimal quadrature angle has a periodicity of 180 degrees. Because of these periodicities, there is an uncertainty of 2*Vpi on the set of DC biases in the children MZM and an uncertainty of 180 degrees of the quadrature angle in the IQ modulator. 
         [0008]    Another example of ABC circuits, which is capable of controlling the three biases of an IQ modulator driven with pre-equalized data for pre-equalization of CD, is reported in the non patent literature 4 (NPL4). According to the design of ABC circuits including the example in NPL4, the optimal point for the DC biases of the children MZM is the point of minimum transmission at Vpi, where a phase difference of 180 degrees is created by the DC bias between two arms of the children MZM. The optimal bias point is periodic by 2*Vpi. The angle of quadrature is periodic by 180 degrees. According to these periodicities, there is an uncertainty of 2*Vpi on the set of DC biases in the children MZM and an uncertainty of 180 degrees of the quadrature angle in the IQ modulator. 
         [0009]    The output complex field representing the lightwave signal modulated by an IQ modulator can be expressed by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       E 
                       out 
                     
                      
                     
                       ( 
                       
                         t 
                         , 
                         
                           V 
                           
                             bias 
                             , 
                             I 
                           
                         
                         , 
                         
                           V 
                           
                             bias 
                             , 
                             Q 
                           
                         
                         , 
                         
                           ϕ 
                           IQ 
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           E 
                           0 
                         
                          
                         
                           ( 
                           t 
                           ) 
                         
                       
                       2 
                     
                      
                     
                         
                       
                         [ 
                         
                           
                             cos 
                              
                             
                               ( 
                               
                                 
                                   π 
                                   2 
                                 
                                 × 
                                 
                                   
                                     
                                       
                                         V 
                                         
                                           RF 
                                           , 
                                           I 
                                         
                                       
                                        
                                       
                                         ( 
                                         t 
                                         ) 
                                       
                                     
                                     + 
                                     
                                       V 
                                       
                                         bias 
                                         , 
                                         I 
                                       
                                     
                                   
                                   
                                     V 
                                     π 
                                   
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             
                                
                               
                                 jϕ 
                                 IQ 
                               
                             
                             × 
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   
                                     π 
                                     2 
                                   
                                   × 
                                   
                                     
                                       
                                         
                                           V 
                                           
                                             RF 
                                             , 
                                             Q 
                                           
                                         
                                          
                                         
                                           ( 
                                           t 
                                           ) 
                                         
                                       
                                       + 
                                       
                                         V 
                                         
                                           bias 
                                           , 
                                           Q 
                                         
                                       
                                     
                                     
                                       V 
                                       π 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where E out (t) represents the output complex field, E 0 (t) is proportional to the complex field of the input lightwave signal of the IQ modulator, V bias,I  represents the DC bias of the I child MZM in the IQ modulator, V bias,Q  represents the DC bias of the Q child MZM in the IQ modulator, φ IQ  is the angle of quadrature of the IQ modulator, V RF,I (t) represents a driving voltage of I child MZM in the IQ modulator, V RF,Q (t) represents a driving voltage of Q child MZM in the IQ modulator, and V π  represents a voltage difference between the biases corresponding to constructive interferences and destructive interferences in the children MZM. In this convention, the case V bias,I =V π  represents biasing at the null driving point of the child I Mach-Zehnder modulator. 
         [0010]    Considering the case of an IQ modulator, with optimal DC biases, the output complex field is as follows: 
         [0000]        E   out,optimal ( t )= E   out ( t,V   π   ,V   π ,90)  (2)
 
         [0000]    where the DC biases of the MZM children are set at Vpi and the angle of quadrature of the IQ modulator is set at 90 degrees. 
         [0011]    Referring to the ambiguity of the optimal DC biases set by an ABC circuit controlling the IQ modulator, the following cases are also optimal considering the DC biases of the IQ modulator: 
         [0000]        E   out ( t, 3× V   π   ,V   π ,90)=−    E   out,optimal ( t )  (Opposite of the complex conjugate of (2)),  (3)
 
         [0000]        E   out ( t,V   π ,3× V   π ,90)=    E   out,optimal ( t )  (Complex conjugate of (2)),  (4)
 
         [0000]        E   out ( t,V   π   ,V   π ,270)=    E   out,optimal ( t )  (Complex conjugate of (2)),  (5)
 
         [0012]    In the case of modulation of signal with QPSK or QAM format with the IQ modulator, depending on the DC bias set by the ABC circuit and considering the uncertainty of the set DC bias, the output field is susceptible to a reference output filed such as its opposite, its complex conjugate, or the opposite of its complex conjugate. The uncertainty of the state can be easily resolved at the receiver after symbol decision using training pattern or framing information. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL1: Japanese Patent Application Laid-Open Publication No. 2008-249848 
         PTL2: Japanese Patent Application Laid-Open Publication No. 2008-124893 
       
     
       Non Patent Literature 
       [0000]    
       
         NPL1: S. J. Savory, “Digital filters for coherent optical receivers”, Optics Express, Volume 16, No. 2, pages 804-817 (2008). 
         NPL2: E. Yamazaki et al., “Fast optical channel recovery in field demonstration of 100-Gbit/s Ethernet over OTN using real-time DSP”, Optics Express, Volume 19, No. 14, pages 13179-13184 (2011). 
         NPL3: K. Roberts et al., “Electronic Precompensation of Optical Nonlinearity”, IEEE Photonics Technology Letters, Volume 18, No. 2, pages 403-405 (2006). 
         NPL4: T. Yoshida et al., “A Study on Automatic Bias Control for Arbitrary Optical Signal Generation by Dual-Parallel Mach-Zehnder Modulator”, European Conference on Optical Communications 2010 (ECOC 2010, paper Tu.3.A.6). 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0019]    In the case of an IQ modulator driven by electrical data processed with pre-equalization function, the complex field emitted by the IQ modulator is equalized by the properties of the medium after the modulator. For instance, in the case of pre-equalization of chromatic dispersion “d” of a fiber transmission line, pre-equalization of “−d” can be applied in the frequency domain by the following filter characteristic: 
         [0000]    
       
         
           
             
               
                 
                   
                     H 
                      
                     
                       ( 
                       
                         ω 
                         , 
                         
                           - 
                           d 
                         
                       
                       ) 
                     
                   
                   = 
                   
                      
                     
                       
                         - 
                         j 
                       
                        
                       
                         
                           λ 
                           2 
                         
                         
                           4 
                            
                           
                               
                           
                            
                           π 
                            
                           
                               
                           
                            
                           c 
                         
                       
                        
                       
                         
                           d 
                            
                           
                             ( 
                             
                               ω 
                               - 
                               
                                 ω 
                                 0 
                               
                             
                             ) 
                           
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where ω represents a frequency, ω 0  represents the central frequency, d represents the value of the chromatic dispersion of the fiber transmission line to pre-equalize, c represents the velocity of light, and λ represents the wavelength of the lightwave signal. 
         [0020]    In this example, if the signal is emitted by the IQ modulator according to the conditions of (2), the chromatic dispersion in the fiber is equalized by the pre-equalized data processed in the transmitter, and then the residual chromatic dispersion after transmission becomes null. However, if the IQ modulator is set according to (3), (4), or (5), the emitted signal will no longer be equalized according to H(ω), but instead by the complex conjugate of H(ω), it will be equalized. Accordingly, the residual chromatic dispersion after transmission through the fiber will no longer become null, but instead becomes 2d which is twice the value of the chromatic dispersion in the fiber. In the latter cases of (3), (4), or (5), the quality of the received signal is lowered. Alternatively, if the chromatic dispersion that can be compensated by the receiver is lower than 2d, the signal reception is no longer possible. 
         [0021]    An example of a transmitter equipped with pre-equalization of chromatic dispersion is disclosed in the patent literature 2 (PTL2). The chromatic dispersion in the transmission fiber is pre-equalized at the transmitter. If the receiver is unable to receive correctly the data, the receiver requests the transmitter using the network control plane to change the conditions of driving for or the pre-equalization of the modulator in the transmitter. According to the patent literature 2 (PTL2), even though the uncertainties of the control of the IQ modulator cause an incorrect pre-equalization, those cases can be corrected. However, the solution proposed in the patent literature 2 (PTL2) requires a long time because the receiver has to determine if the signal can be received and pass the information to the transmitter through another channel of the network. Since short startup time of the transmitter embedded in transponders is desirable, the solution by the patent literature 2 (PTL2) needs to be improved. 
         [0022]    Furthermore, even if the receiver can compensate for twice the chromatic dispersion in the transmission line, the solution by the patent literature 2 (PTL2) is unable to detect the signals in the cases where the transmitter is not correctly set and the compensation of the receiver is not optimally performed. Those conditions limit the tolerance of the receiver to changing conditions which cause possible errors after network operation. 
         [0023]    Moreover, the use of control plane or monitoring channels to transmit orders from the receiver to the transmitter needs extra bandwidth in the network. Therefore, this is not an optimal solution in view of network bandwidth utilization. 
         [0024]    As mentioned above, there is a need for improvement of ABC circuits or of a system to remedy the uncertainties of ABC circuits to control an IQ modulator driven with pre-equalized data. 
         [0025]    An exemplary object of the invention is to provide an optical transmitter and method for controlling the same that include a fast, bandwidth effective control system for the DC biases of an IQ modulator driven with data generated by pre-equalization function. 
       Solution to Problem 
       [0026]    A method for controlling an optical transmitter according to an exemplary aspect of the invention includes the steps of (a) making direct current biases for driving children Mach-Zehnder modulators of an IQ modulator in the optical transmitter converge to values close to null driving points of the children Mach-Zehnder modulators, (b) driving the children Mach-Zehnder modulators with special driving data including a pair of training patterns between which there is a significant correlation, (c) scanning direct current biases for setting quadrature angle of the IQ modulator, (d) monitoring output of the IQ modulator during step (c), and (e) setting the direct current bias for setting quadrature angle on the basis of the driving data and monitored results in step (d). 
         [0027]    An optical transmitter according to an exemplary aspect of the invention includes an IQ modulator provided with children Mach-Zehnder modulators, an auto bias control circuit making direct current biases for driving the children Mach-Zehnder modulators converge to values close to null driving points of the children Mach-Zehnder modulators, a data selector selecting special driving data including a pair of training patterns between which there is a significant correlation, for driving the children Mach-Zehnder modulators, a scan circuit scanning direct current biases for setting quadrature angle of the IQ modulator, a monitor photo diode monitoring output of the IQ modulator during scanning direct current biases for setting the quadrature angle, and a control circuit setting the direct current bias for setting the quadrature angle on the basis of the driving data and results monitored by the monitor photo diode. 
       Advantageous Effects of Invention 
       [0028]    An exemplary advantage according to the invention provides an optical transmitter and method for controlling the same that can control an IQ modulator which emits a lightwave signal modulated with pre-equalized data so that the pre-equalized data matches the properties of the transmission medium, in spite of the uncertainties of the control biases of the IQ modulator. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0029]      FIG. 1  is a schematic representation of an optical transmitter according to the first exemplary embodiment. 
           [0030]      FIG. 2  is a flowchart showing a method for controlling the optical transmitter according to the first exemplary embodiment. 
           [0031]      FIG. 3  is the result of a simulation for the monitor signal of an IQ modulator according to the first exemplary embodiment, with the DC bias of I children MZM converged to Vpi. 
           [0032]      FIG. 4  is the result of a simulation for the monitor signal of an IQ modulator according to the first exemplary embodiment, with the DC bias of I children MZM converged to 3*Vpi. 
           [0033]      FIG. 5  is a schematic representation of an optical transmitter according to the second exemplary embodiment. 
           [0034]      FIG. 6  is a flowchart showing a method for controlling the optical transmitter according to the second exemplary embodiment. 
           [0035]      FIG. 7  is a schematic representation of an optical transmitter according to the third exemplary embodiment. 
           [0036]      FIG. 8  is a flowchart showing a method for controlling the optical transmitter according to the third exemplary embodiment. 
           [0037]      FIG. 9  is a schematic representation of an optical transponder according to the third exemplary embodiment. 
           [0038]      FIG. 10  is the simulation results of the output signal from the optical transponder according to the third exemplary embodiment. 
           [0039]      FIG. 11  is the simulation results of the output signal from a conventional optical transponder. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Exemplary Embodiment 
       [0040]      FIG. 1  is a schematic representation of an optical transmitter  100  including an IQ modulator  111  and an Auto Bias Control (ABC) circuit  130 . The optical transmitter  100  emits a lightwave signal  102  modulated by the IQ modulator  111  according to logical binary data stream  101 . A serializer/deserializer  120  transforms the logical binary data stream  101  into parallel lanes of binary data so that they can be processed by a Digital Signal Processing unit (DSP)  121 . The data processed by the DSP  121  are fed into a data selector  122 . The data selector  122  also receives a training pattern generated by a training pattern generator  144 . 
         [0041]    The training pattern generator  144  generates training patterns as driving data for two lanes with the condition that the correlation between two training patterns is non-null. That is to say, there is a significant correlation between a pair of training patterns. For instance, two training patterns can be equals to an arbitrary pattern. Alternatively, the training patterns can be chosen as constant. Alternatively, one of the training patterns can be arbitrarily chosen and the other one is the opposite of the first one. Alternatively, one of the training patterns can be arbitrarily chosen and the other one is proportional to the first one. 
         [0042]    According to the control signal emitted by a general control unit  140 , the output of the data selector  122  is generated from either the signal generated by the DSP  121  or by the training pattern generator  144 . Alternatively, the data selector  122  and the training pattern generator  144  can be integrated with the DSP  121 . The DSP  121  generates data by which to modulate continuous wave light from a laser  110  according to the binary data  101  and the modulation format of the optical transmitter  100 . The modulation format can be multilevel format such as Quadrature Phase Shift Keying (QPSK). Alternatively, the modulation format can be 16-Quadrature Amplitude Modulation (QAM), or higher order QAM such as 64QAM. The modulation can be done with Orthogonal Frequency Division Multiplexing (OFDM). The modulation format can be changed during the operation of the optical transmitter  100  or at its startup according to a command signal provided for the optical transmitter  100 . 
         [0043]    Furthermore, the DSP  121  generates data by which to apply pre-equalization. For instance, the DSP  121  generates data for pre-equalization of a determined chromatic dispersion value according to the filter characteristic expressed in ( 6 ). The DSP  121  can also generate data for pre-equalization of impairments due to non-linear distortions appearing in the fiber into, which the lightwave signal  102  is launched. 
         [0044]    The data output by the data selector  122  are fed into two Digital to Analog Converter (DAC)  123  and  124 . The DAC  123  and  124  generate analog signals according to the outputs of the data selector  122 . The analog signals generated by the DAC  123  and  124  are respectively amplified by driving amplifiers  125  and  126  so that each of their output voltage is suitable to drive the RF inputs of the IQ modulator  111 . 
         [0045]    A continuous wave signal is emitted by the laser  110  and fed into the input of the IQ modulator  111 . A child I Mach-Zehnder Modulator (MZM)  112  of the IQ modulator  111  is driven by the output of the driving amplifier  125 . A child Q MZM  113  of the IQ modulator  111  is driven by the output of the driving amplifier  126 . A phase adjuster  114  controls the angle of quadrature of the IQ modulator  111 . A low speed monitor Photo Diode (PD)  115  is integrated in the IQ modulator  111  and outputs a monitoring electrical signal proportional to the output of the IQ modulator  111 . Alternatively, the monitor Photo Diode (PD)  115  can be provided outside the IQ modulator  111  and generate a monitor electrical signal by means of receiving the lightwave signal emitted by the IQ modulator  111  at a tapped portion. 
         [0046]    The monitor signal generated by the monitor PD  115  is split by a divider  150 . One of the signals split by the divider  150  is provided for the ABC circuit  130  which controls the DC biases of the IQ modulator  111 . The ABC circuit  130  includes three control circuits  131 ,  132 , and  133  to control respectively the DC bias of the child I MZM  112 , the DC bias of the child Q MZM  113 , and the DC bias of the phase adjuster  114  for setting the angle of quadrature. The ABC circuit  130  optimizes the DC biases of the IQ modulator  111  according to the monitor signal generated by the monitor PD  115 . The ABC circuit  130  can be based on low frequency dither tones imprinted on the DC biases of the IQ modulator  111  and on the phase and amplitudes of the frequency components corresponding to the tone frequencies that are detected in the monitor signal generated by the monitor PD  115 . 
         [0047]    A general control unit  140  sets an ABC control unit  141  which can disable the control circuit  133  controlling the angle of quadrature of the IQ modulator  111 . The ABC control unit  141  is also able to set the control circuit  133  in order to generate a specific DC bias determined by the general control unit  140 . The general control unit  140  also controls a scan circuit  143  which generates several voltage values in order to set the angle of quadrature of the IQ modulator  111 . A switch unit  151  is controlled by the general control unit  140 . The output of the switch unit  151  is applied to the phase adjuster,  114  to set the quadrature angle of the IQ modulator  111 . The output of the switch unit  151  is either the DC bias generated by the control circuit  133  of the ABC circuit  130  or the DC voltage generated by the scan circuit  143 , depending on its state. 
         [0048]    When the switch unit  151  outputs the voltage generated by the scan circuit  143 , the general control unit  140  orders a monitor record unit  142  to record the output value of the monitor PD  115  which is split by the divider  150 , for each value of DC voltage generated by the scan circuit  143 . The general control unit  140  is able to compare the voltages recorded by the monitor record unit  142 , and to process DC voltages according to the compared values provided by the monitor record unit  142  and the voltages generated by the scan circuit  143  for each recorded monitor value. Moreover, the general control unit  140  can set the control circuit  133  to generate a voltage equal to the processed voltage through the ABC control unit  141 . 
         [0049]    Next, it will be explained the way that the ambiguity on the DC biases of the IQ modulator is resolved according to this exemplary embodiment.  FIG. 2  is a flowchart to control the optical transmitter  100  shown in  FIG. 1  according to this exemplary embodiment. The optical transmitter  100  operates as defined by consecutive steps S 601  to S 608  at the start of the transmitter. When these steps have been completed, the DC biases of the IQ modulator  111  of the optical transmitter  100  are correctly set and the optical transmitter  100  emits lightwave signal with pre-equalization feature. 
         [0050]    The DC biases of I and Q children MZM of the IQ modulator  111  have converged respectively according to the control circuits  131  and  132  to both Vpi (S 602 /YES). 
         [0051]    The data selector  122  selects the training data generated by the training pattern generator  144 . The switch unit  151  selects the output of the scan circuit  143  and the control circuit  133  is stopped (S 603 ). Only the angle of quadrature is controlled without resolving the ambiguity on the DC biases of the MZM children. 
         [0052]    The training patterns generated by the training pattern generator  144  are correlated and have a positive correlation. The data recorded by the monitor record unit  142 , while the scan circuit  143  generates DC biases enabling to scan a quadrature angle over a range of 360 degrees, is similar to the curve of  FIG. 3  (S 604 ). 
         [0053]      FIG. 3  is the result of a simulation for the monitor signal of an IQ modulator for different values of the quadrature angle using the IQ modulator according to this exemplary embodiment. The DC biases of I and Q children MZM have both converged to Vpi. In this configuration, the driving data for I and Q MZM are equal to an identical pattern. For the DC bias value Vmax corresponding to the quadrature angle equal to zero modulo 360 degrees, the monitor voltage is maximum. For the DC bias value Vmin corresponding to the quadrature angle equal to 180 degrees modulo 360 degrees, the monitor voltage is minimum. Accordingly, if Vmin&gt;Vmax, the quadrature angle for the DC bias (Vmin−Vmax)/2 is close to 90 degrees, modulo 360 degrees. 
         [0054]    By finding Vmax and Vmin to meet the condition of Vmin&gt;Vmax and by setting DC bias for the quadrature angle to (Vmin−Vmax)/2, it is ensured that the quadrature angle is set to 90 degrees and that the IQ modulator is set to the case corresponding to the equation (2) (S 605 , S 606 ). 
         [0055]    Alternatively, one of the DC biases of I and Q children MZM of the IQ modulator′ has converged to Vpi, whereas the other has converged to 3*Vpi according to the respective control circuits  131  and  132 . The training patterns generated by the training pattern generator  144  are correlated and have a positive correlation. The data recorded by the monitor record unit  142 , while the scan circuit  143  generates DC biases enabling to scan a quadrature angle over a range of 360 degrees, is similar to the curve of  FIG. 4 . 
         [0056]      FIG. 4  is the result of a simulation for the monitor signal of an IQ modulator for different values of the quadrature angle using the IQ Modulator according to this exemplary embodiment. The DC biases of I child MZM has converged to 3*Vpi and the DC bias of the Q child MZM has converged to Vpi. In this configuration, the driving data for I and Q MZM are equal to an identical pattern. For the DC bias value Vmin corresponding to the quadrature angle equal to zero modulo 360 degrees, the monitor voltage is minimum. For the DC bias value Vmax corresponding to the quadrature angle equal to 180 degrees modulo 360 degrees, the monitor voltage is maximum. Accordingly, if Vmin&lt;Vmax, the quadrature angle for the DC bias (Vmax−Vmin)/2 is close to 90 degrees, modulo 360 degrees. 
         [0057]    By finding Vmax and Vmin to meet the condition of Vmin&gt;Vmax and by setting DC bias for the quadrature angle to (Vmin−Vmax)/2, it is ensured that the quadrature angle is set to 270 degrees. By combination of the equations (3) and (5), the IQ modulator is set to the case corresponding to the equation (2). 
         [0058]    Alternatively, the training patterns can be chosen so that they have a negative but non-null correlation. In this case, by finding Vmax and Vmin to meet the condition of Vmin&lt;Vmax and by setting DC bias for the quadrature angle to (Vmax−Vmin)/2, it is ensured that the IQ modulator is set to the case corresponding to the equation (2). 
         [0059]    As mentioned above, according to the first exemplary embodiment, it becomes possible to optimally set the DC biases of the modulator and to optimally calculate the pre-equalized data. Moreover, it becomes possible to perform fast startups of optical transmitters, and the network including the optical transmitters does not need extra bandwidth. 
       Second Exemplary Embodiment 
       [0060]      FIG. 5  is a schematic representation of an optical transmitter  200  including an IQ modulator  211  and an Auto Bias Control (ABC) circuit  230 . The optical transmitter  200  emits a lightwave signal  202  modulated by the IQ modulator  211  according to the logical binary data stream  201 . A serializer/deserializer  220  is identical to the serializer/deserializer  120  shown in  FIG. 1 . A DSP  221  is identical to the DSP  121 . DAC  223  and  224 , as well as driving amplifiers  225  and  226  are identical to the DAC  123 ,  124  and the driving amplifiers  125 ,  126 , respectively. A laser  210 , an IQ modulator  211 , and an ABC circuit  230  are identical to the laser  110 , the IQ modulator  111 , and the ABC circuit  130 , respectively. A monitor record unit  242  and a scan circuit  243  are identical to the monitor record unit  142  and the scan circuit  143 , respectively. A divider  250  and a switch unit  251  are identical to the divider  150  and the switch unit  151 , respectively. 
         [0061]    A data selector  222  provides data processed by the DSP  221  for the DAC  223 . According to the control signal generated by a general control unit  240 , the signal provided for the DAC  224  by the data selector  222  is either the data processed by the DSP  221  or special data used for training. The special data is correlated to the data provided for the DAC  223 . The special data can be equal to the data provided for the DAC  223 . Alternatively, the special data can be equal to the opposite of the data provided for the DAC  223 . Alternatively, the special data can be proportional to the data provided for the DAC  223 . Alternatively, the special data can be proportional to the opposite of the data provided for the DAC  223 . 
         [0062]    The general control unit  240  sets an ABC control unit  241  which can disable the control circuit of the ABC circuit  230  controlling the quadrature angle of the IQ modulator  211 . The general control unit  240  also controls the scan circuit  243 . The switch unit  251  is controlled by the general control unit  240  in the same manner as the switch unit  151  is controlled by the general control unit  140 . 
         [0063]    When the switch unit  251  outputs the voltage generated by the scan circuit  243 , the general control unit  240  orders the monitor record unit  242  to record the value output by the monitor PD in the IQ modulator  211  and split by the divider  250 , for each value of DC voltage generated by the scan circuit  243 . The general control circuit  240  is able to read the DC bias generated by the ABC circuit  230  to control the quadrature angle of the IQ modulator  211  through the ABC control unit  241 . According to the read DC bias and to the voltages recorded by the monitor record unit  242  for each voltage generated by the scan circuit  243 , the general control unit  240  can change the setting of the DSP  221 . For instance, in the case where the DSP  221  pre-equalizes data for compensation of the chromatic dispersion, the general control unit  240  can change the sign of the compensated dispersion value. 
         [0064]    Next, it will be explained the way that the ambiguity on the DC biases of the IQ modulator is resolved according to this exemplary embodiment.  FIG. 6  is a flowchart to control the optical transmitter  200  shown in  FIG. 5  according to this exemplary embodiment. The optical transmitter  200  operates as defined by consecutive steps S 611  to  619  at the start of the transmitter. When these steps have been completed, the DC biases of the IQ modulator  211  of the optical transmitter  200  are correctly set and the optical transmitter  200  emits lightwave signal with pre-equalization feature. 
         [0065]    The DC biases of I and Q children MZM of the IQ modulator  211  have converged according to the ABC circuit  230  to both Vpi (S 612 /YES). The data driving I and Q children MZM of the IQ modulator  211  are chosen identical. The switch unit  251  selects the output of the scan circuit  243 . The control of the quadrature angle of the IQ modulator  211  by the ABC circuit  230  is stopped (S 613 ). Only the angle of quadrature is controlled without resolving the ambiguity on the DC biases of the MZM children. 
         [0066]    The data recorded by the monitor record unit  242 , while the scan circuit  243  generates DC biases enabling to scan a quadrature angle over a range of 360 degrees, is similar to the curve of  FIG. 3  (S 614 ). Then the switch unit  251  selects the output of the ABC circuit  230  and the control of the quadrature angle of the IQ modulator  211  by the ABC circuit  230  is enabled. 
         [0067]    The general control unit  240  reads the DC bias setting the quadrature angle Vconv and compares it to the processed values Vmax and Vmin. If the condition of Vmin&gt;Vconv&gt;Vmax is met, the general control unit  240  verifies that the angle of quadrature is set to 90 degrees modulo 360 degrees. If the condition of Vmin&lt;Vconv&lt;Vmax is met, the unit general control  240  verifies that the angle of quadrature is set to 270 degrees modulo 360 degrees. Accordingly, the general control unit  240  sets the pre-equalization of DSP  221  to reverse the characteristics of the pre-equalization filter (S 618 ). For instance, if the DSP is set to pre-equalize a chromatic dispersion “d”, the general control unit  240  resets the DSP  221  to compensate a value of “−d”. 
         [0068]    Other cases of convergence for the DC biases of I and Q MZM are resolved in the same manner. 
       Third Exemplary Embodiment 
       [0069]      FIG. 7  is a schematic representation of an optical transmitter  300  including an IQ modulator  311  and an Auto Bias Control (ABC) circuit  330 . The optical transmitter  300  emits a lightwave signal  302  modulated by the IQ modulator  311  according to a logical binary data stream  301 . A serializer/deserializer  320  is identical to the serializer/deserializer  220  shown in  FIG. 5 . A DSP  321  and a data selector  322  are identical to the DSP  121  and the data selector  122  shown in  FIG. 1 , respectively. DAC  323  and  324  as well as driving amplifiers  325  and  326  are identical to the DAC  223 ,  224  and the driving amplifiers  225 ,  226 , respectively. 
         [0070]    A laser  310 , IQ modulator  311 , and ABC circuit  330  are identical to the laser  210 , the IQ modulator  211 , and the ABC circuit  230 , respectively. An ABC control unit  341 , a monitor record unit  342 , a scan circuit  343 , and a training pattern generator  344  are identical to the ABC control unit  241 , the monitor record unit  242 , the scan circuit  243 , and the training pattern generator  144 , respectively. A divider  350  and a switch unit  351  are identical to the divider  250  and the switch unit  251 , respectively. 
         [0071]    A general control unit  340  sets the ABC control unit  341 , the monitor record unit  342 , the scan circuit  343 , and the switch unit  351  in the same manner as the general control unit  240  controls the ABC control unit  241 , the monitor record unit  242 , the scan circuit  243 , and the switch unit  251 , respectively. According to the read DC bias and to the voltages recorded by the monitor record unit  342  for each voltage generated by the scan circuit  343 , the general control unit  340  can change the setting of a data switch  329  so that it either passes the data generated by the DSP  321  and selected by the data selector  322  directly to the DAC  323  and  324  or interchanges the data between the DAC  323  and  324 . 
         [0072]    Next, it will be explained the way that the ambiguity on the DC biases of the IQ modulator is resolved according to this exemplary embodiment.  FIG. 8  is a flowchart to control the optical transmitter  300  shown in  FIG. 7 . The optical transmitter  300  operates as defined by the consecutive steps S 621  to S 629  at the start of the transmitter. When these steps have been completed, the DC biases of the IQ modulator  311  in the optical transmitter  300  are correctly set and the optical transmitter  300  emits lightwave signal with pre-equalization feature. 
         [0073]    The DC biases of I and Q children MZM in the IQ modulator  311  have converged to both Vpi according to the ABC circuit  330  (S 622 /YES). The data selector  322  selects the training data generated by the training pattern generator  344 . The switch unit  351  selects the output of the scan circuit  343 . And control of the quadrature angle of the IQ modulator  311  by the ABC circuit  330  is stopped (S 623 ). Only the angle of quadrature is controlled without resolving the ambiguity on the DC biases of the MZM children. 
         [0074]    The data recorded by the monitor record unit  342 , while the scan circuit  343  generates DC biases enabling to scan a quadrature angle over a range of 360 degrees, is similar to the curve of  FIG. 3  (S 624 ). Then the switch unit  351  selects the output of the ABC circuit  330  and the control of the quadrature angle of the IQ modulator  311  by the ABC circuit  330  is enabled. 
         [0075]    The general control unit  340  reads the DC bias setting the quadrature angle Vconv and compares it to the processed values Vmax and Vmin. If the condition of Vmin&gt;Vconv&gt;Vmax is met, the general control unit  340  verifies that the quadrature angle is set to 90 degrees modulo 360 degrees (S 625 ). If the condition of Vmin&lt;Vconv&lt;Vmax is met, the general control unit  340  verifies that the quadrature angle is set to 270 degrees modulo 360 degrees. Accordingly, the general control unit  340  sets the data switch  329  in order to interchange the driving data provided for the DAC  323  and  324  (S 628 ). Other cases of convergence for the DC biases of I and Q MZM are resolved in the same manner. 
         [0076]      FIG. 9  is a schematic representation of an optical transponder  400  including an optical transmitter  405  and an optical receiver  406 . The optical transmitter  405  emits a lightwave signal  402  modulated by a polarization multiplexed IQ modulator  411  according to the logical binary data stream  401 . The optical receiver  406  generates a binary data stream  404  according to a received lightwave signal  403 . 
         [0077]    The optical transmitter  405  includes the polarization multiplexed IQ modulator  411 . The polarization multiplexed IQ modulator  411  is equivalent to two IQ modulators identical to the IQ modulator  111  for each polarization, and has a polarization multiplexing function for the modulated lightwave for each of the polarization. 
         [0078]    A serializer/deserializer  420 , DSP  421 , and a data selector  422  are equivalent to the serializer/deserializer  120 , DSP  121 , and the data selector  122 , respectively. They treat twice information included by the polarization multiplexed lightwave signal  402 . DAC  423 ,  424 ,  427 , and  428  are identical to the DAC  123 . Driving amplifiers  424 ,  425 ,  429 , and  430  are identical to the driving amplifier  125 . A laser  410  and a training pattern generator  444  are identical to the laser  110  and the training pattern generator  144 , respectively. Ambiguity free ABC circuits  440  and  441  are identical. They include equivalent functions and circuits to the ABC circuit  130 , the general control unit  140 , the ABC control unit  141 , the scan circuit  143 , the monitor record unit  142 , the divider  150 , and switch unit  151 . 
         [0079]    The receiver  406  is equipped with an optical front end  451 . The optical front end  451  includes an optical hybrid with polarization multiplexing, balanced photo detectors, and trans-impedance amplifiers. The optical front end  451  receives the received lightwave signal  403  and mixes it with the continuous lightwave emitted by a local oscillator  450  for coherent reception. The four differential outputs from the optical front end  451  are connected to a receiver chip  452 , which integrates high speed Analog to Digital Converters (ADC), DSP, and serializer/deserializer. The DSP integrated in the receiver chip  452  has function of electrical filtering, chromatic dispersion compensation, adaptive equalization, polarization de-multiplexing, carrier phase estimation, and decision. The signal recovered and re-serialized by the DSP in the receiver chip  452  is output as the binary data stream  404 . 
         [0080]      FIG. 10  is the simulation results of the output signal of the X polarization of the optical transponder  400  shown in  FIG. 9 . The modulation format is chosen as Polarization Multiplexed QPSK (PM-QPSK) with a baud rate of 12.5 GBaud. The pre-equalization in the optical transmitter  405  is set to pre-equalize a chromatic dispersion of 10,000 ps/nm. The constellation  700  is measured after transmission for 500 km through a fiber with chromatic dispersion of 20 ps/nm/km. The residual dispersion is null at this point. The constellation  700  has clearly four symbols  701 ,  702 ,  703 , and  704  of the QPSK signal. Such signal can be recovered without error by an optical receiver similar to the optical receiver  406 . 
         [0081]    In the same conditions and by using a conventional method, due to the ambiguity of the DC biases of the IQ modulators controlled by ABC circuit, the emitted signal is susceptible to the conditions of equations (3), (4), or (5). As shown in  FIG. 11 , in the case of signal emitted by an optical transponder similar to the optical transponder  400  with a conventional method, the related constellation  710  represents the signal when the IQ modulator obeys the condition of equation (3). The residual chromatic dispersion is 20,000 ps/nm after transmission for, 500 km. This signal exceeds the capacity for compensating by the DSP in the receiver chip  452 . Therefore, by using a conventional method, the reception of the signal is not possible in this configuration. 
         [0082]    As mentioned above, according to these exemplary embodiments, it becomes possible to control an IQ modulator which emits a modulated lightwave signal featured by pre-equalization, which a receiver can receive optimally. The controlled IQ modulator has DC biases set according to the pre-equalization settings. 
         [0083]    While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims. 
       INDUSTRIAL APPLICABILITY 
       [0084]    This invention can be applied to an optical communication system which utilizes pre-equalization technique. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100 ,  200 ,  300 ,  405  optical transmitter 
           101 ,  201 ,  301 ,  401  logical binary data stream 
           102 ,  202 ,  302 ,  402  lightwave signal 
           110 ,  210 ,  310 ,  410  laser 
           111 ,  211 ,  311 . IQ modulator 
           112  child I MZM 
           113  child Q MZM 
           114  phase adjuster 
           115  monitor PD 
           120 ,  220 ,  320 ,  420  serializer/deserializer 
           121 ,  221 ,  321 ,  421  DSP 
           122 ,  222 ,  322 ,  422  data selector 
           123 ,  124 ,  223 ,  224 ,  323 ,  324 ,  423 ,  424 ,  427 ,  428  DAC 
           125 ,  126 ,  225 ,  226 ,  325 ,  326 ,  424 ,  425 ,  429 ,  430  driving amplifier 
           130 ,  230 ,  330  ABC circuit 
           131 ,  132 ,  133  control circuit 
           140 ,  240 ,  340  general control unit 
           141 ,  241 ,  341  ABC control unit 
           142 ,  242 ,  342  monitor record unit 
           143 ,  243 ,  343  scan circuit 
           144 ,  344 ,  444  training pattern generator 
           150 ,  250 ,  350  divider. 
           151 ,  251 ,  351  switch unit 
           329  data switch 
           400  optical transponder 
           403  received lightwave signal 
           404  binary data stream 
           406  optical receiver 
           411  polarization multiplexed IQ modulator 
           440 ,  441  ambiguity free ABC circuit 
           450  local oscillator 
           451  optical front end 
           452  receiver chip 
           700  constellation 
           701 ,  702 ,  703 ,  704  symbol 
           710  related constellation