Patent Publication Number: US-11397363-B2

Title: Automatic bias control circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 U.S. National Phase of International Application No. PCT/JP2019/019280 filed on May 15, 2019, which claims priority to Japanese Application No. 2018-163770 filed on Aug. 31, 2018. The entire disclosures of the above applications are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an automatic bias control circuit. This application claims priority to Japanese Patent Application No. 2018-163770 filed in Japan on Aug. 31, 2018, of which the contents are incorporated herein. 
     BACKGROUND ART 
     Currently, in a high speed and capacity optical communication system, optical quadrature amplitude modulation (QAM) signals in signal formats with high spectral efficiency, are widely employed. In generating such an optical QAM signal, the light intensity and the optical phase need to be modulated depending on a given data sequence. In order to achieve such modulation, a configuration is typically employed in which a transmitter in which a CW light source that generates continuous light (CW light) and an in-phase/quadrature (IQ) optical modulator are combined is employed, and the IQ optical modulator is driven with a plurality of electrical drive signals corresponding to a data sequence. 
     The IQ optical modulator is also capable of generating an optical signal in a format other than the format of an optical QAM signal, and provides adjustment methods different depending on a signal format. The present application relates to an optical transmitter for generating an optical QAM signal. Thus, unless otherwise specified below, description about an adjustment method for the IQ optical modulator is on the assumption of generation of an optical QAM signal. 
       FIG. 12  is a diagram illustrating a typical example of an IQ optical modulator M and its peripheral circuits included in an optical QAM signal generation optical transmitter. The IQ optical modulator M is an optical modulator using nested Mach-Zehnder interferometers (MZIs). CW light entering the IQ optical modulator M is branched by an optical branch unit  200  inside the IQ optical modulator M, and the branched CW light beams enter each of the MZIs. These MZIs are Mach-Zehnder (MZ) type optical modulators, and these optical modulators are called herein an MZ optical modulator  2   a  for an in-phase component and an MZ optical modulator  2   b  for a quadrature component. Hereinafter, the MZ optical modulator  2   a  for an in-phase component is simply referred to as “I-component MZ optical modulator  2   a , and the MZ optical modulator  2   b  for a quadrature component is referred to as “Q-component MZ optical modulator  2   b”.    
     A branch unit  21   a  of the I-component MZ optical modulator  2   a  receives one of the CW light beams branched by the optical branch unit  200  and branches the received CW light beam into two branches. An optical phase difference adjustment is applied to the optical signals branched into these two branches. A multiplexing unit  22   a  multiplexes the optical signals in these two branches adjusted in optical phase difference, and outputs the multiplexed signal to an optical multiplexing and demultiplexing unit  201 . A branch unit  21   b  of the Q-component MZ optical modulator  2   b  receives the other of the CW light beams branched by the optical branch unit  200  and branches the received CW light beam into two branches. An optical phase difference adjustment is applied to the optical signals branched into these two branches. A multiplexing unit  22   b  multiplexes the optical signals in these two branches adjusted in optical phase difference, and outputs the multiplexed signal to the optical multiplexing and demultiplexing unit  201 . Outputs from both of the I-component MZ optical modulator  2   a  and the Q-component MZ optical modulator  2   b  are multiplexed by the optical multiplexing and demultiplexing unit  201  so that a third MZI including the two MZIs is configured. The third MZI is herein called a parent MZI. 
     The output of the optical multiplexing and demultiplexing unit  201  is branched into two ports, that is, a signal output port  301  and a monitor port  302 . Optical electric fields output from these two ports are closely related but not exactly the same. A difference therebetween will be described later. The optical QAM signal passing through the signal output port  301  is output from the IQ optical modulator M and sent to a transmission path. Modulated light output from the monitor port  302  is input into an IQ modulator-incorporated photodetector  300 . The IQ modulator-incorporated photodetector  300  converts the input modulated light into electricity and outputs a monitor signal. 
     In the IQ optical modulator M, light is modulated with two types, that is, a data signal Data_I for an in-phase component and a data signal Data_Q for a quadrature component. A manner in which a modulation signal is applied slightly differs depending on a type of modulator, but here, for the sake of description, a lithium niobate (LnNbO3; Ln) type optical modulator is used. The Data_I is amplified by a differential amplifier  3   a . Output voltages ±V data_I  of the differential amplifier  3   a  are applied to a first I-component modulation unit  6   a  and a second I-component modulation unit  6   b , respectively, to modulate optical path lengths of two arms of the I-component MZ optical modulator  2   a  in a push-pull fashion. Similarly, the Data_Q is amplified by a differential amplifier  3   b , and output voltages ±V data_Q  of the differential amplifier  3   b  are applied to a first Q-component modulation unit  6   c  and a second Q-component modulation unit  6   d , respectively, to modulate optical path lengths of two arms of the Q-component MZ optical modulator  2   b  in a push-pull fashion. 
     If the ±V data_I  and the ±V data_Q  are intensity modulation signals with n-values, modulated light output from the IQ optical modulator M is a QAM signal with an n 2 -value. However, in order to perform correct modulation, it is necessary to precisely adjust the optical path length of each MZI so that the two light beams interfering in each MZI have a correct optical phase difference. Generally, three types of bias voltages are used for this adjustment. In the present application, Bias_I denotes a bias voltage for controlling the optical path length of the I-component MZ optical modulator  2   a , Bias_Q denotes a bias voltage for controlling the optical path length of the Q-component MZ optical modulator  2   b , and Bias_Ph denotes a bias voltage for controlling the optical path length of the parent MZI. 
     In order that the bias voltage is reflected in the optical path length, for example, a Pockels effect may be effected within each MZI, or each of the bias voltages may be applied to a heater arranged closely to an optical waveguide to thermally expand the optical waveguide. 
     The Bias_I and the Bias_Q are adjusted so that the I-component MZ optical modulator  2   a  and the Q-component MZ optical modulator  2   b  are biased to a null point. That is, the optical path length is adjusted so that optical outputs of the I-component MZ optical modulator  2   a  and the Q-component MZ optical modulator  2   b  are extinguished at a moment when the V data_I =V data_Q =0 is established. 
     In an example of  FIG. 12 , Bias_I generated and output by a Bias_I voltage generator  7   a  adjusts the optical path length of the I-component MZ optical modulator  2   a  via a Bias_I phase adjusting means  8   a  and biases the I-component MZ optical modulator  2   a  to the null point. Output of Bias_Q generated and output by a Bias_Q voltage generator  7   b  adjusts the optical path length of the Q-component MZ optical modulator  2   b  via a Bias_Q phase adjusting means  8   b , and biases the Q-component MZ optical modulator  2   b  to the null point. In this example, the optical path lengths of one of the two arms of the I-component MZ optical modulator  2   a  and one of the two arms of the Q-component MZ optical modulator  2   b  are controlled, but configuration may be that the optical path lengths of all the four arms are controlled to control two sets of optical path lengths in a push-pull fashion. 
     The adjustment of the optical path length of the parent MZI is selected so that optical phases of modulated light output from each of the MZ optical modulators are orthogonal. That is, at a moment when neither the V data_I  nor the V data_Q  is not 0, the optical path length of the parent MZI is selected so that an optical phase difference between output light of the I-component MZ optical modulator  2   a  and output light of the Q-component MZ optical modulator  2   b , which are observed in the signal output port  301 , is ±π/2. Either a plus sign or a minus sign may be selected with an exception of a special exception, and this exception will be described in a second embodiment. 
     Bias_Ph generated and output by a Bias_Ph voltage generator  7   c  adjusts the optical path length of the parent MZI via a Bias_Ph phase adjusting means  8   c  to maintain the orthogonality. 
     In a real optical modulator, an optimum value of each bias voltage is not uniquely obtained and changes over time due to a temperature variation or other reasons. This phenomenon is called a bias drift. If the bias drift is left unprocessed, an optical signal is deteriorated to such an extent that the optical signal cannot be demodulated, and thus, auto bias control (ABC) (automatic bias voltage control) in an in-service state is essential. It is known that a bias drift of a semiconductor type optical modulator is very small as compared to that of an LN type optical modulator, but an optimum value of a bias depends on a wavelength of CW light to some extent, and thus, when a wavelength channel is changed, it may be necessary to promptly reselect an optimum bias voltage by using the ABC. 
     A plurality of solutions for performing the ABC in the IQ optical modulator are proposed in the past (see, NPLs 1 to 4, for example). In any of the technologies, modulated light modulated by an IQ optical modulator is extracted from a monitor port, the output light from the monitor port is electrically converted by a photodetector to obtain a monitor signal, and information on the modulated light obtained from the monitor signal is referred to realize the ABC. Generally, the information on the modulated light includes a light intensity of the modulated light or a low-speed dither signal component superimposed on the light intensity. Note that the light intensity as used herein indicates an average intensity of modulated light obtained in an average time sufficiently longer than a symbol cycle of the optical QAM signal but sufficiently shorter than a cycle of a dither signal. 
     If a band of the photodetector is close to a baud rate of the QAM signal, not only the light intensity of the modulated light but also high-speed modulation components derived from the modulation signals Data_I and Data_Q are extracted from the monitor signal, a peak value or an effective value (a root mean square: RMS) of the high-speed modulation component or a parameter equivalent thereto is referred to realize the ABC. 
     Here, there are two approaches to realize a monitor port in which modulated light is monitored.  FIG. 13  and  FIG. 14  are diagrams illustrating these two approaches.  FIG. 13  illustrates an approach identical in configuration to that illustrated in  FIG. 12 , where one of two output ports provided in the optical multiplexing and demultiplexing unit  201  is used as the monitor port  302 .  FIG. 14  illustrates the other approach. In this approach, instead of the optical multiplexing and demultiplexing unit  201 , an optical multiplexing unit  202  and an optical branch unit  203  are employed, the optical branch unit  203  branches output light multiplexed by the optical multiplexing unit  202 , and one of the branched light beams is used as the monitor port. 
     The second approach illustrated in  FIG. 14  has a disadvantage that the light intensity output from the IQ optical modulator M is lost due to an optical loss of the optical branch unit  203 . An optical electric field of the output from the optical multiplexing unit  202  is defined as E OUT , and optical electric fields of the two outputs from the optical branch unit  203  are defined as k 1 ×E OUT  and k 2 ×E OUT , respectively, where absolute values of k 1  and k 2  are always less than 1. The absolute value of k 1  needs to be close to 1 in order to increase the light intensity of a transmission signal, and the absolute value of k 2  approaches 0 on the basis of the law of conservation of energy. Thus, the light intensity output from the monitor port decreases, and a signal-to-noise (SN) ratio of a signal received by an ABC circuit deteriorates. This increases a control error of the ABC. In addition, in the approach illustrated in  FIG. 14 , the optical branch unit  203  needs to be provided additionally. Thus, the approach illustrated in  FIG. 14  has another disadvantage that a circuit size increases compared to the approach in  FIG. 13 . 
     Thus, it is desirable to perform the ABC by using the optical multiplexing and demultiplexing unit  201  and the monitor port  302  illustrated in  FIG. 12  and  FIG. 13 . However, as described above, it should be noted that the optical electric field E MON  output from the monitor port  302  is not the same as the optical electric field E OUT  output from the signal output port  301 . This difference theoretically occurs, and also occurs even in a case of an ideal MZI where manufacturing errors are ignored. The difference between these optical electric fields and the presence or absence of an influence on the ABC will be specifically described below. 
     It is assumed that the IQ optical modulator M generates a quaternary quadrature amplitude modulation (QAM) signal, that is, a quadrature phase shift keying (QPSK). In this case, the V data_I  and the V data_Q  in  FIG. 12  are binary drive signals. Here, it is assumed that the IQ optical modulator M and a drive signal waveform are ideal and a manufacturing error or a waveform distortion does not exist. Each bias is set to an optimum value that satisfies the above condition. That is, the I-component MZ optical modulator  2   a  and the Q-component MZ optical modulator  2   b  are biased to the null point by Bias_I and Bias_Q, respectively, and the optical path length of the parent MZI maintains the above orthogonality by Bias_Ph. 
     At this time, the optical electric field E OUT  output from the signal output port  301  is a vector composition of an optical electric field EI derived from the output of the I-component MZ optical modulator  2   a  and an optical electric field EQ derived from the output of the Q-component MZ optical modulator  2   b . The I-component MZ optical modulator  2   a  is modulated by ±V data_I  in a push-pull fashion, and the Q-component MZ optical modulator  2   b  is modulated by ±V data_Q  in a push-pull fashion, respectively. Thus, both of the optical electric field EI and the optical electric field EQ have two values including a positive value and a negative value. Thus, a constellation having four symbols is drawn as a result of the vector compositions.  FIG. 15( a )  is a schematic diagram illustrating the constellation in which these four symbols of the optical electric field E OUT  are drawn. If an optical phase difference between the optical electric field EI and the optical electric field EQ observed at the signal output port  301  is defined as θ OUT , θ OUT =±/2 (hereinafter, the unit of phase is radian) is established in this example. 
     On the other hand, the optical electric field E MON  output from the monitor port  302  is also a vector composition of the optical electric field EI and the optical electric field EQ, but the optical phase difference θ MON  between the optical electric field EI and the optical electric field EQo is θ OUT +π=−π/2.  FIG. 15( b )  is a schematic diagram illustrating a constellation in which four symbols of the optical electric field E MON  output from the monitor port  302  are drawn. In  FIG. 15( b ) , the symbols corresponding to those in  FIG. 15( a )  use the same symbol. The constellations illustrated in  FIGS. 15( a ) and 15( b )  differ in symbol arrangement, but the shapes of the symbols are identical. Thus, the light intensity output from the signal output port  301  and the light intensity output from the monitor port  302  are the same (difference in optical loss due to manufacturing errors and the like are ignored). For the same reason, the peak value or a root mean square (RMS) value derived from the high-speed modulation component measured in the monitor port  302  is the same as that measured in the signal output port  301 . 
     In  FIGS. 15( a ) and 15( b ) , bias settings to the IQ optical modulator M are all optimal, where the modulated light has the following three properties: Firstly, the constellation shape either in the signal output port  301  or in the monitor port  302  is four-fold symmetrical. Secondly, the light intensity is an extreme value either in the monitor port  302  or in the signal output port  301 . Whether the extreme value is a maximum or a minimum depends on a multi-level number and an amplitude of the drive waveform. The details are described in NPL 1, but the details are complicated, and thus, such a description is omitted herein. Unless otherwise indicated in the following description, the description proceeds on the assumption that the light intensity is minimized if each bias is optimized. Thirdly, either in the monitor port  302  or in the signal output port  301 , a peak value or an RMS value from the high-speed modulation is minimized. 
     The constellation illustrated in  FIG. 15( a )  and the constellation illustrated in  FIG. 15( b )  have a phase conjugate relationship, and with the exception of the special case mentioned above, the both constellations have the optimum signal quality, and thus, there is no problem even if the transmission is performed by the optical electric field E MON  instead of the optical electric field E OUT . 
     Here, if any one of the three types of biases drifts, any of the three properties described above will not be satisfied. Thus, it is possible to achieve the ABC by monitoring the output of the monitor port  302  and adjusting each bias so that the above-mentioned three types of properties are satisfied, and to maintain a signal quality of an optical QAM to be transmitted at the optimum level. 
     As a specific example, a case is assumed in which both of two of the three types of bias, Bias_Q and Bias_Ph, are bias-drifted, and only Bias_I is maintained at an optimum value.  FIG. 16( a )  is a schematic diagram illustrating a constellation in which symbols of the optical electric field E OUT  output from the signal output port  301  in the case are drawn, and  FIG. 16( b )  is a schematic diagram illustrating a constellation in which symbols of the optical electric field E MON  output from the monitor port  302  in the case are drawn. These two constellations have different symbol arrangements, and the orthogonality between the optical electric field EI and the optical electric field EQ is not maintained. However, a distance of each of the symbols from the origin in  FIG. 16( a )  is the same as a distance of each of the symbols from the origin in a case where the “star” is replaced with “triangle”, and “round” is replaced with “square” in  FIG. 16( b ) . Thus, the light intensity output from the signal output port  301  and the light intensity output from the monitor port  302  are the same. For the same reason, the peak value or the RMS value derived from the high-speed modulation component measured in the monitor port  302  is the same as that measured in the signal output port  301 . However, an observation time needs to be sufficiently long so as to hide a pattern dependency. 
     Thus, if the modulated light output from the monitor port  302  is monitored and ABC is performed so that the three types of biases are sequentially close to the optimum values, it is possible to maintain the signal quality of the optical QAM signal output from the signal output port  301 . A more general bias adjustment procedure is described, for example, in PTL 1. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: JP 5671130 B1 
       
    
     Non Patent Literature 
     
         
         NPL 1: H. Kawakami, and three other authors, “Auto bias control technique for optical 16-QAM transmitter with asymmetric bias dithering”, Optics Express, 2011, Vol. 19, No. 26, B308-8312 
         NPL 2: H. Kawakami, and four other authors, “Auto bias control and bias hold circuit for IQ-modulator in flexible optical QAM transmitter with Nyquist filtering”, Optics Express, 2014, Vol. 22, No. 23, p. 28163-28168 
         NPL 3: S. Pak, and another author, “Bias Control for Optical OFDM Transmitters”, IEEE Photonics Technology Letters, 2010, Vol. 22, No. 14, p. 1030-1032 
         NPL 4: H. Kawakami, and two other authors, “Drive-amplitude-independent Auto Bias Control Circuit for QAM Signals and Its Demonstration with an InP-based IQ Modulator”, ECOC2016; 42nd European Conference and Exhibition on Optical Communications, 2016, W.4.P1.SC4, p. 815-817 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     The description so far assumes that the optical IQ modulator and the drive signal waveform are ideal. However, due to imperfection such as a manufacturing error of the MZI included in the optical IQ modulator and a distortion of the modulator drive signal, the constellation may be distorted to lose the symmetry. In this case, in particular, in a case of a QAM signal with a large multi-level number, the modulated light output from the signal output port  301  and the modulated light output from the monitor port  302  may differ greatly. Such a case will be described below by using 16-QAM. 
       FIG. 17  illustrates constellations obtained when the optical losses of the two arms configuring the MZI is imbalanced, in a simulated manner, for each of the I-component MZ optical modulator  2   a  and the Q-component MZ optical modulator  2   b . It is assumed that, for ease of understanding, the optical loss has an extremely large imbalance in the simulation. In such a case, even if the V data_I  and the V data_Q  in  FIG. 12  are both 0, it is not possible to completely extinguish the output light beams of the I-component MZ optical modulator  2   a  and the Q-component MZ optical modulator  2   b , and thus, it is not possible to bias the I-component MZ optical modulator  2   a  and the Q-component MZ optical modulator  2   b  to the null point in a sense as described in the Background Art. 
       FIGS. 17( a ) and 17( b )  illustrate constellations obtained when the Bias_I and the Bias_Q are set so that the intensities of the output light beams from the I-component MZ optical modulator  2   a  and the Q-component MZ optical modulator  2   b  are minimum when the V data_I =V data_Q =0 is established.  FIG. 17( a )  corresponds to the optical electric field E OUT  output from the signal output port  301 , and  FIG. 17( b )  corresponds to the optical electric field E MON  output from the monitor port  302 . 
     As a result of a mutual interference of the not perfectly extinguished light with the QAM signal, the constellation bends like a bow, and no matter how the Bias_Ph is selected, the constellation is not square. The two constellations illustrated in  FIGS. 17( a ) and 17( b )  have a mirror image relationship. Thus, the output light intensity and the RMS value from the high-speed modulation component in the monitor port  302  are the same as those in the signal output port  301 .  FIG. 17( b )  illustrates an output light intensity together with the RMS value from the high-speed modulation component obtained by the simulation. The units are arbitrary units (arb.). These values are not minimum values. Further, in the both constellations, some of the symbols contact axes of the optical electric field EI and the optical electric field EQ, and thus, the signal quality is poor. 
     Next, a case is assumed in which under a condition that the same imbalance IQ optical modulator as that of  FIG. 17  is used, the output light in the monitor port  302  is referred to, and the ABC is performed by using the conventional technologies described in NPLs 1 to 4. At this time, each bias is controlled so that the output light intensity or the RMS value in the monitor port  302  is minimized and that the symbol arrangement in the constellation is close to four-fold symmetry. 
       FIGS. 18( a ) and 18( b )  illustrate a constellation of the optical electric field E OUT  of the signal output port  301  and a constellation of the optical electric field E MON  of the monitor port  302 , obtained by simulation. 
     A target on which the ABC is performed is the output in the monitor port  302 , and thus, the constellation of  FIG. 18( b )  is greatly improved as compared with that of  FIG. 17( b ) . The output light intensity and the RMS value from the high-speed modulation component are 0.490 and 0.318 (arbitrary unit), respectively, which are smaller than those in  FIG. 17( b ) . As a result, a center of the constellation substantially matches a center of a phase space. A value different in Bias_Ph from that in  FIG. 17( b )  is selected in the process of suppressing a bowing distortion of the constellation and the constellation rotates, but the rotation is removed in the process of demodulation, and thus, deterioration of a signal quality does not occur. 
     However, for the output in the signal output port  301  to be used as a transmission signal, a symbol in a second row from the bottom and in a second column from the right shifts to a quadrant different from a position where the symbol should be present, as illustrated in  FIG. 18( a ) , and as a result, the signal quality deteriorates so greatly that the signal can not demodulated. Correspondingly, the output light intensity and the RMS value from the high-speed modulation component also greatly increase to 0.975 and 0.723 (arbitrary unit), respectively. 
     As described above, under a condition that an imperfection of the IQ optical modulator or the drive signal cannot be ignored, if the conventional ABC is performed by using the monitor port of a type illustrated in  FIG. 13 , there is a problem that the signal quality of a transmission signal is deteriorated. 
     In view of the above circumstances, an object of the present invention is to provide an automatic bias control circuit capable of suppressing deterioration of a signal quality of a transmission signal output from an optical transmitter, under a condition that it is not possible to ignore an imperfection of the optical transmitter using an MZI. 
     Means for Solving the Problem 
     One aspect of the present invention is an automatic bias control circuit for controlling a bias voltage or a bias power applied to an in-phase/quadrature (IQ) optical modulator, in which the IQ optical modulator includes an optical branch unit configured to branch continuous light into two being in-phase component light and quadrature component light, an in-phase component MZ optical modulator being a Mach-Zehnder interferometer, the in-phase component MZ optical modulator configured to modulate the in-phase component light obtained by branching the continuous light by the optical branch unit, a quadrature component MZ optical modulator being a Mach-Zehnder interferometer, the quadrature component MZ optical modulator configured to modulate the quadrature component light obtained by branching the continuous light by the optical branch unit, and an optical multiplexing and demultiplexing unit configured to branch an optical quadrature amplitude modulation (QAM) signal obtained by multiplexing modulated light output from the in-phase component MZ optical modulator and modulated light output from the quadrature component MZ optical modulator after a phase adjustment unit adjusts an optical phase between the modulated light output from the in-phase component MZ optical modulator and the modulated light output from the quadrature component MZ optical modulator, and output the branched signals from each of a signal output port and a monitor port, the automatic bias control circuit includes an in-phase component bias power source configured to generate a voltage or a current applied to the in-phase component MZ optical modulator to bias the in-phase component MZ optical modulator and the quadrature component MZ optical modulator to an area near a null point, a quadrature component bias power source configured to generate a voltage or a current applied to the quadrature component MZ optical modulator to bias the in-phase component MZ optical modulator and the quadrature component MZ optical modulator to an area near a null point, a phase adjustment bias power source configured to generate a voltage or a current to determine a change amount of the optical phase applied by the phase adjustment unit, a monitor unit configured to monitor the optical QAM signal output from the monitor port, and a control unit configured to control, based on a monitor result from the monitor unit, a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source, and 
     the control unit performs a first-stage process of controlling a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source so that a signal quality of the optical QAM signal obtained from the monitor result approaches a target quality, in startup sequences of the IQ optical modulator, and a second-stage process of obtaining a voltage or a current by changing a voltage or a current output from the phase adjustment bias power source by a predetermined change amount ΔBias_Ph, after a completion of the first-stage process. 
     One aspect of the present invention is the automatic bias control circuit in which the monitor unit monitors at least one of an average intensity, a peak intensity, or an RMS value of the optical QAM signal output from the monitor port, and the control unit controls, in the first-stage process, a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source so that the monitor result approaches a maximum or a minimum. 
     One aspect of the present invention is the automatic bias control circuit further including a dithering unit configured to apply dithering to at least one of an output of the in-phase component bias power source, an output of the quadrature component bias power source, an output of the phase adjustment bias power source, a modulation efficiency of the in-phase component MZ optical modulator, or a modulation efficiency of the quadrature component MZ optical modulator, in which the monitor unit monitors at least one of an average intensity, a peak intensity, or an RMS value of the optical QAM signal output from the monitor port, and the control unit synchronously detects, in the first-stage process, a dither component with a frequency fd or a higher harmonic wave of the dither component superimposed on the monitor result obtained when a dithering at a constant frequency fd is applied by the dithering unit, and controls a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source so that an absolute value of the synchronous detection result approaches a maximum or 0. 
     One aspect of the present invention is the automatic bias control circuit in which the change amount ΔBias_Ph is an amount by which an optical phase is changed, by the phase adjustment unit, by π radian. 
     One aspect of the present invention is the automatic bias control circuit in which the monitor unit monitors at least one of an average intensity, a peak intensity, or an RMS value of the optical QAM signal output from the monitor port, and the control unit performs, in the startup sequences, a third-stage process of recording, as a new target value, a result obtained by monitoring the optical QAM signal by the monitor unit, into a memory, after a completion of the second-stage process, regularly compares the monitor result with the target value stored in the memory after a completion of the third-stage process, and controls, if a deviation is detected as a result of the comparison, a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source so that the monitor result approaches the target value. 
     One aspect of the present invention is the automatic bias control circuit further including a dithering unit configured to apply dithering to at least one of an output of the in-phase component bias power source, an output of the quadrature component bias power source, an output of the phase adjustment bias power source, a modulation efficiency of the in-phase component MZ optical modulator, or a modulation efficiency of the quadrature component MZ optical modulator, in which the monitor unit monitors at least one of an average intensity, a peak intensity, or an RMS value of the optical QAM signal output from the monitor port, and the control unit synchronously detects, in the startup sequences and after a completion of the second-stage process, a dither component with a frequency fd or a higher harmonic wave of the dither component superimposed on the monitor result obtained when a dithering at a constant frequency fd is applied by the dithering unit, performs a third-stage process of recording, as a new target value, the synchronous detection result, into a memory, regularly synchronously detects, after a completion of the third-stage process, a dither component with a frequency fd or a higher harmonic wave of the dither component superimposed on the monitor result obtained when a dithering at a constant frequency fd is applied by the dithering unit, compares the synchronous detection result with the target value recorded in the memory, and controls, if a deviation is detected as a result of the comparison, a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source so that the synchronous detection result approaches the target value. 
     One aspect of the present invention is the automatic bias control circuit in which the control unit controls, in time division, a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source. 
     One aspect of the present invention is the automatic bias control circuit in which the memory is a non-volatile memory, and the control unit finally uses the target value recorded in the memory if the third-stage process is not performed in a second or later startup sequence of the startup sequences. 
     One aspect of the present invention is the automatic bias control circuit further including a demodulation unit configured to demodulate the optical QAM signal output from the monitor port, in which the control unit adjusts a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source, or a peripheral circuit used when a drive signal is applied to the IQ optical modulator so that a signal quality of the optical QAM signal demodulated by the demodulation unit improves. 
     One aspect of the present invention is the automatic bias control circuit further including a switch unit configured to select and input, into the demodulation unit, one of the optical QAM signal output from the monitor port and an optical transmission signal sent through an optical transmission path. 
     One aspect of the present invention is the automatic bias control circuit in which an optical transmission path from the monitor port to the demodulation unit is polarization-maintaining. 
     One aspect of the present invention is the automatic bias control circuit further including a wavelength change unit configured to change a wavelength of the optical QAM signal output from the monitor port and input the optical QAM signal with the changed wavelength into the demodulation unit. 
     Effects of the Invention 
     According to the present invention, it is possible to suppress deterioration of a signal quality of a transmission signal output from an optical transmitter even under a condition that it is not possible to ignore an error of an ABC monitor port derived from an imperfection of the optical transmitter using an MZI. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a table showing a basic effect of an error compensation method according to an embodiment of the present invention. 
         FIG. 2  is a block diagram illustrating an example of a configuration of an optical transmitter according to a first embodiment. 
         FIG. 3  is a flowchart illustrating processing of a control processor according to the first embodiment. 
         FIG. 4  is a block diagram illustrating an example of a configuration of an optical transmitter according to a second embodiment. 
         FIG. 5  is a flowchart illustrating processing of a control processor according to the second embodiment. 
         FIG. 6  is graphs illustrating a principle confirmation experiment result according to the second embodiment. 
         FIG. 7  is graphs illustrating a principle confirmation experimental result according to the second embodiment. 
         FIG. 8  is a graph illustrating a principle confirmation experimental result according to the second embodiment. 
         FIG. 9  is a block diagram illustrating an example of a configuration of an optical transmitter according to a third embodiment. 
         FIG. 10  is a block diagram illustrating another example of a configuration of the optical transmitter according to the third embodiment. 
         FIG. 11  is a block diagram illustrating another example of a configuration of the optical transmitter according to the third embodiment. 
         FIG. 12  is a diagram illustrating an optical QAM signal generation optical transmitter according to a conventional technology. 
         FIG. 13  is a diagram illustrating an example in which a monitor port for modulated light is realized. 
         FIG. 14  is a diagram illustrating an example in which a monitor port for modulated light is realized. 
         FIG. 15  is a schematic diagram illustrating constellations of optical electric fields of a signal output port and a monitor port. 
         FIG. 16  is a schematic diagram illustrating constellations of optical electric fields of a signal output port and a monitor port. 
         FIG. 17  is a diagram illustrating constellations of optical electric fields of a signal output port and a monitor port. 
         FIG. 18  is a diagram illustrating constellations of optical electric fields of a signal output port and a monitor port. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present embodiments relate to an error compensation of an optical monitoring circuit for an IQ optical modulator used in an optical QAM signal generation optical transmitter. In particular, the present embodiments are suitable for an error compensation of an optical monitoring circuit for use in automatic bias control of an IQ optical modulator. 
       FIG. 1  shows a basic effect of an error compensation method according to the present embodiment. The error compensation method includes a plurality of steps. In the present embodiment, modulated light generated by the IQ optical modulator M illustrated in  FIG. 12  is monitored according to an approach illustrated in  FIGS. 2 and 3 . A target to be monitored is modulated light output from the monitor port  302  illustrated in  FIG. 13 . 
     In a first step, based on a monitor result of modulated light output from the monitor port  302 , ABC according to a conventional technology is performed in the present embodiment. A resulting constellation is the same as that in  FIGS. 18( a ) and 18( b ) , and such a constellation is again provided in the left column of  FIG. 1 . In the left column of  FIG. 1 , a light intensity of modulated light output from each of output ports and an RMS value derived from a high-speed modulation component obtained by simulation are shown together in arbitrary units (arb.). The upper row of  FIG. 1  corresponds to the optical electric field E OUT  from the signal output port  301 . The lower row of  FIG. 1  corresponds to the optical electric field E MON  output from the monitor port  302 . The intensity of light output from the monitor port  302  and the RMS value are minimum values. The light intensity is 0.490 and the RMS is 0.318. 
     Next, in a second step, the ABC is suspended in the present embodiment and Bias_Ph is increased or decreased by Vπ_bias. Here, Vπ_bias is a half-wavelength voltage in a bias electrode used as the Bias_Ph phase adjusting means  8   c . The half-wavelength voltage indicates an increment in voltage required to increase a difference in optical path length between two arms by half the wavelength in an MZ optical modulator. The optical path length depends on both the voltage of the modulation signal (±V data_I , ±V data_Q  in  FIG. 12 ) and each bias voltage, and thus, the half-wavelength voltage may be defined in a plurality of ways. However, in the present embodiment, Vπ_bias, which is a half-wavelength voltage in Bias_Ph, indicates a change amount of Bias_Ph required to increase or decrease by half the wavelength (in other words, to change an optical phase difference θ OUT  by ±π) a difference in optical path length of the parent MZI. 
     Note that if the optical path length is changed by heat generation of a heater, the term half-wavelength power may be used instead of the half-wavelength voltage. 
     It should be noted here that Vπ_bias has an extremely small drift and may be regarded as almost constant. The Bias_Ph giving θ OUT =π/2 may be defined as Bias_Ph 1 , and Bias_Ph giving θ OUT =−π/2 may be defined as Bias_Ph 2 . As described above, Bias_Ph 1  and Bias_Ph 2  drift over time, but drift amounts of Bias_Ph 1  and Bias_Ph 2  are almost the same due to periodicity of the MZI. Thus, the drift of Vπ_bias=(Bias_Ph 1 −Bias_Ph 2 ) may be substantially ignored and may be regarded as a constant. 
     A description will be provided with reference to  FIG. 1  again. At an end of the second step, the optical electric fields of the signal output port  301  and the monitor port  302  are switched as shown in the right column of  FIG. 1 . The constellation of the signal output port  301  is optimized. An optimized optical QAM signal is transmitted to the transmission path. On the other hand, the intensity of light output from the monitor port  302  and the RMS are not minimum anymore. The light intensity increases to 0.975 and the RMS to 0.723. 
     In a third step, an ABC circuit connected to the monitor port  302  is operated again in the present embodiment. In the third step, the ABC circuit controls each bias to maintain the light intensity at 0.975 and RMS at 0.723, which are values at the end of the second step, rather than to minimize the light intensity and the RMS. As a result, the transmission signal remains optimal regardless of an error in the monitor port  302 . 
     Note that in the first step and the second step, the optical phase difference θ OUT  changes by π, however, this does not cause a problem in a normal optical QAM signal. This does not apply to the above special exception, but a solution for coping with this problem will be described in a second embodiment. 
     An optical transmitter according to each embodiment of the present invention will be described below. In the embodiments described below, the same reference signs may be assigned to the same components as the components of the optical transmitter illustrated in  FIG. 12  or components in other embodiments, and duplicated descriptions thereof may be omitted. 
     First Embodiment 
       FIG. 2  is a block diagram illustrating an example of a configuration of an optical transmitter  100  according to a first embodiment. In  FIG. 2 , portions identical to those of the optical QAM signal generation optical transmitter according to the conventional technology illustrated in  FIG. 12  are given the same reference signs. The optical transmitter  100  includes an IQ optical modulator M, differential amplifiers  3   a  and  3   b , and a bias control circuit  40 . The IQ optical modulator M and the differential amplifiers  3   a  and  3   b  are configured in much the same way as the IQ optical modulator M and the differential amplifiers  3   a  and  3   b  in the optical QAM signal generation optical transmitter illustrated in  FIG. 12 . 
     The bias control circuit  40  is an ABC circuit of the IQ optical modulator M. The bias control circuit  40  includes the Bias_I voltage generator  7   a , the Bias_Q voltage generator  7   b , the Bias_Ph voltage generator  7   c , a distributor  400 , a low-pass filter  401 , a first analog-to-digital converter (ADC)  402 , an RMS measurement circuit  403 , a second ADC  404 , a control processor  405 , a first non-volatile memory  406 , and a second non-volatile memory  407 . The first non-volatile memory  406  and the second non-volatile memory  407  may be physically different non-volatile memories and may be separate storage regions on the same non-volatile memory. 
     The distributor  400  branches, into two, an electrical monitor signal output from an IQ modulator-incorporated photodetector  300 . One of the signals branched by the distributor  400  is input to the first analog-to-digital converter (ADC)  402  through the low-pass filter  401  that attenuates a high-frequency band. The first ADC  402  converts an analog signal into a digital signal and outputs the digital signal. The output from the first ADC  402  is digital data representative of the intensity of light output from the monitor port  302 . The other of the signals branched by the distributor  400  is input to the RMS measurement circuit  403 . The RMS measurement circuit  403  measures the RMS of an analog signal input from the distributor  400  and then outputs the signal to the second ADC  404 . The second ADC  404  converts the analog signal input from the RMS measurement circuit  403  into a digital signal and outputs the digital signal. The output of the second ADC  404  is digital data representative of an RMS value of a high-speed modulation component derived from the Data_I and the Data_Q. Note that in  FIG. 2 , the digital data is represented by a dot-dash line. 
     The digital data items output from the first ADC  402  and the second ADC  404  are each input to the control processor  405 . The control processor  405  performs processing illustrated in a flowchart illustrated in  FIG. 3 . Through the processing, the control processor  405  sends a feedback signal to the Bias_I voltage generator  7   a , the Bias_Q voltage generator  7   b , and the Bias_Ph voltage generator  7   c , and modifies each bias voltage for optimization. The first non-volatile memory  406  stores a value of the light intensity as a control target. The second non-volatile memory  407  stores a value of the RMS as a control target. The first non-volatile memory  406  is described as a first memory and the second non-volatile memory  407  is described as a second memory, below. The first memory and the second memory may be provided in the control processor  405 . 
       FIG. 3  is a flowchart illustrating processing of the control processor  405 . A correction procedure in the present embodiment cannot be performed in an in-service state. Thus, the correction procedure needs to be ended during a startup sequence. 
     Immediately after the startup sequence begins, random signals for training are used for the Data_I and the Data_Q (step S 101 ). The IQ optical modulator M outputs an optical QAM signal and a monitor signal generated based on these random signals. The monitor signal is converted to an electrical signal by the IQ modulator-incorporated photodetector  300  and then is branched by the distributor  400 . One of the branched monitor signals is converted to a digital signal by the first ADC  402  after a high frequency component of the monitor signal is removed by the low-pass filter  401 , and the digital signal is input to the control processor  405 . An RMS for the other of the branched monitor signals is measured by the RMS measurement circuit  403 , and then the other of the branched monitor signals is converted into a digital signal representing an RMS value by the second ADC  404 , and the digital signal is input to the control processor  405 . As a result of the digital signals being input, the control processor  405  obtains the light intensity and the RMS of the monitor signal. Here, as defined in the Background Art, the light intensity is the average intensity of modulated light averaged over a time sufficiently longer than a symbol cycle of the optical QAM signal. Note that the bias control circuit  40  may be configured to acquire the peak intensity instead of the RMS value. If the peak intensity is obtained, the bias control circuit  40  measures the monitor signal which does not pass through the low-pass filter. 
     The control processor  405  adjusts the Bias_I and the Bias_Q so that the light intensity is reduced (steps S 102  and S 103 ), and the Bias_Ph so that the RMS value or the peak intensity are minimized (step S 104 ). The control processor  405  cyclically repeats the adjustment processing of steps S 102  to step S 104  until the control processor  405  determines that each of the Bias_I, the Bias_Q, and the Bias_Ph is converged (step S 105 : NO). 
     In determining that each of the biases is converged (step S 105 : YES), the control processor  405  changes the Bias_Ph by Vπ_bias (step S 106 ). At this stage, the control processor  405  records the current light intensity as a target value into the first memory (step S 107 ) and records the current RMS value or the peak intensity as a target value into the second memory (step S 108 ). With these steps, the startup sequence ends, and the processing moves to in-service processing. 
     When the in-service processing starts, instead of the random signals for training, data signals for a transmission service are used as the Data_I and the Data_Q (step S 109 ). The control processor  405  determines whether the monitor results of the monitor signal match the corresponding target values recorded in the first and second memories, and if a determination result indicates a mismatch (deviation by a predetermined degree or above), the control processor  405  repeats processing of controlling the Bias_I voltage generator  7   a , the Bias_Q voltage generator  7   b , and the Bias_Ph voltage generator  7   c  so that the monitor results return again to the target values. Thus, the control processor  405  obtains the light intensity and the RMS value or the peak intensity of the monitor signal, as in the above example. The control processor  405  adjusts the Bias_I so that the light intensity approaches the target value recorded in the first memory (step S 110 ) and also adjusts the Bias_Q (step S 111 ). Further, the control processor  405  adjusts the Bias_Ph so that the RMS value approaches the target value recorded in the second memory. The control processor  405  repeats regularly, for example, cyclically this adjustment operation in steps S 110  to S 112 . 
     Second Embodiment 
       FIG. 4  is a block diagram illustrating an example of a configuration of an optical transmitter  101  according to a second embodiment. In  FIG. 4 , the same components as those of the optical transmitter  100  according to the first embodiment illustrated in  FIG. 2  are denoted by the same reference signs and descriptions thereof are omitted. The optical transmitter  101  illustrated in  FIG. 4  is different from the optical transmitter  100  illustrated in  FIG. 2  in that the former includes a bias control circuit  50  instead of the bias control circuit  40 . The bias control circuit  50  is different from the bias control circuit  40  according to the first embodiment in that the former includes a synchronous detection circuit  503  that performs a dithering by an oscillator (dither generator or dithering unit)  500  that outputs a periodic signal with a frequency fd and synchronously detects a dither signal superimposed on a monitor output. 
     The bias control circuit  50  includes the Bias_I voltage generator  7   a , the Bias_Q voltage generator  7   b , the Bias_Ph voltage generator  7   c , the oscillator  500 , a multiplexer  501 , a multiplexer  502 , the synchronous detection circuit  503 , a control processor  504 , a first non-volatile memory  505 , a second non-volatile memory  506 , and a third non-volatile memory  507 . In the present embodiment, the first non-volatile memory  505  is also described as a first memory, the second non-volatile memory  506  is also described as a second memory, and the third non-volatile memory  507  is also described as a third memory. The first memory, the second memory, and the third memory may be physically different non-volatile memories and may be separate storage regions on the same non-volatile memory. The first memory, the second memory, and the third memory may be provided in the control processor  504 . 
     The oscillator (dither generator)  500  has three outputs all having a frequency fd. The dither signals, which are the three outputs, may be individually turned on and off in accordance with instructions from the control processor  504 . One of the outputs from the oscillator  500  passes through the multiplexer  501  to be used to dither the Bias_I, another of the outputs passes through the multiplexer  502  to be used to dither the Bias_Q, and the other of the outputs is used as a reference clock (Ref) of the synchronous detection circuit  503 . 
     The multiplexer  501  multiplexes the dither signal with the frequency fd output from the oscillator  500  on the Bias_I output from the Bias_I voltage generator  7   a , and applies the dithering at the frequency fd to the Bias_I. The multiplexer  502  multiplexes the dither signal with the frequency fd output from the oscillator  500  on the Bias_Q output from the Bias_Q voltage generator  7   b , and applies the dithering at the frequency fd to the Bias_Q. 
     The synchronous detection circuit  503  receives an electrical monitor signal output from the IQ modulator-incorporated photodetector  300 . The synchronous detection circuit  503  uses the reference clock at the frequency fd received from the oscillator  500  to perform a synchronous detection on the monitor signal, and inputs a synchronous detection result, as digital data, into the control processor  504 . The synchronous detection circuit  503  synchronously detects a dither component to be superimposed on the monitor result of the average intensity, the peak intensity, or the RMS value of the monitor signal, or a higher harmonic wave of the dither component. According to the flowchart illustrated in  FIG. 5 , the control processor  504  sends a feedback signal to the Bias_I voltage generator  7   a , the Bias_Q voltage generator  7   b , and the Bias_Ph voltage generator  7   c  to modify each of the bias voltages for optimization. 
       FIG. 5  is a flowchart illustrating processing of the control processor  504 . Similarly to the first embodiment, the correction procedure needs to be ended during the startup sequence. Thus, Immediately after the startup sequence begins, random signals for training are used for the Data_I and the Data_Q (step S 201 ). The control processor  504  sends an instruction to the oscillator  500  to dither only the Bias_I at the frequency fd (step S 202 ). Next, the control processor  504  controls the Bias_I so that the synchronous detection result obtained by the synchronous detection circuit  503  is 0 (step S 203 ). 
     Here, there are two types of zero points of the synchronous detection. That is, there are two cases where a positive value is returned and where a negative value is returned when the synchronous detection result is differentiated by Bias_I. The cases corresponds one-to-one to a case where a light output intensity in the monitor port  302  is minimized and a case where the light output intensity is maximized. Up to this point, description focuses on the control to minimize the light intensity. However, as described in NPL 1, if a multi-level number and an amplitude of a drive waveform satisfy a specific condition, the light intensity is maximum rather than minimum at an optimal bias value. Thus, of two different types of zero points of the synchronous detection, it is necessary to select a zero point corresponding to the optimal bias. This selection may be realized by appropriately selecting a loop gain of a feedback loop as described in NPL 1. 
     Next, the control processor  504  sends an instruction to the oscillator  500  to dither only the Bias_Q at the frequency fd (step S 204 ). Next, the control processor  504  controls the Bias_Q so that the synchronous detection result obtained by the synchronous detection circuit  503  is 0 (step S 205 ). The zero point may be selected in much the same way as in the case of the Bias_I. 
     Next, the control processor  504  sends an instruction to the oscillator  500  to dither both the Bias_I and the Bias_Q at the frequency fd (step S 206 ). Here, the frequency of the dither signal output to the multiplexer  501  to dither the Bias_I and the frequency of the dither signal output to the multiplexer  502  to dither Bias_Q are both fd. However, these dither signals are set so that the phases differ by π/2. The control processor  504  controls the Bias_Ph so that the synchronous detection result of a second-order harmonic (2fd) obtained by the synchronous detection circuit  503  is 0 (step S 207 ). 
     Here, there are two types of zero points of synchronous detection of the second-order harmonic. That is, there are two cases where a positive value is returned and where a negative value is returned when the synchronous detection result of the second-order harmonic is differentiated by Bias_Ph. As described in NPL 2, the cases correspond one-to-one to a case where the optical phase difference θ MON  is +π/2 and a case where the optical phase difference θ MON  is −π/2. In a normal optical QAM signal, either sign may be selected without any problem, but in performing a pre-equivalence with a fine adjustment of an optical phase such as a pre-chromatic dispersion, it is necessary to select an appropriate sign. If an appropriate sign is desirably selected so that the optical phase difference θ OUT  be +π/2 in the in-service state, the zero point where the optical phase difference θ MON  is +π/2 is selected instead of the optical phase difference θ OUT  in step S 207 . 
     The control processor  504  cyclically repeats the adjustment processing of steps S 202  to step S 207  until the control processor  504  determines that each bias of the Bias_I, the Bias_Q, and the Bias_Ph is converged (step S 208 : NO). In determining that each of the biases is converged (step S 208 : YES), the control processor  504  changes the Bias_Ph by Vπ_bias (step S 209 ). Here, the optical electric field E OUT  and the optical electric field E MON  are switched as illustrated in  FIG. 1 , and thus, the optical phase difference θ OUT  is +π/2. If −π/2 is desirably selected for the optical phase difference θ OUT  in the in-service state, it is only required that a zero point different from the zero point described above is selected in step S 207 . 
     When step S 209  ends, the control processor  504  sends an instruction to the oscillator  500  to dither only the Bias_I at the frequency fd (step S 210 ), and records the synchronous detection result obtained by the synchronous detection circuit  503  into the first memory (step S 211 ). Next, the control processor  504  sends an instruction to the oscillator  500  to dither only the Bias_Q at the frequency fd (step S 212 ). The control processor  504  records the synchronous detection result obtained by the synchronous detection circuit  503  into the second memory (step S 213 ). Next, the control processor  504  sends an instruction to the oscillator  500  to dither both the Bias_I and the Bias_Q with a phase relationship in much the same way as in step S 206  (step S 214 ). The control processor  504  records the synchronous detection result of the second-order harmonic (2fd) obtained by the synchronous detection circuit  503  into the third memory (step S 215 ). 
     With these steps, the startup sequence ends. Next, processing in the in-service state starts. 
     When the in-service processing starts, instead of the random signals for training, data signals for a transmission service are used for the Data_I and the Data_Q (step S 216 ). Next, the control processor  504  sends an instruction to the oscillator  500  to dither only the Bias_I at the frequency fd (step S 217 ). The control processor  504  determines whether the synchronous detection results of the monitor signal match the corresponding target values recorded in the first to third memories. In determining a mismatch (deviation by a predetermined degree or above), the control processor  504  repeats the processing of controlling the Bias_I voltage generator  7   a , the Bias_Q voltage generator  7   b , and the Bias_Ph voltage generator  7   c  so that the synchronous detection results return again to the target values. Thus, the control processor  504  subtracts the target value recorded in the first memory from the synchronous detection results obtained by the synchronous detection circuit  503  to calculate an error signal. The control processor  504  controls the Bias_I so that the calculated error signal is close to 0 (step S 218 ). There are two different zero points at this time, but the control processor  504  selects a zero point in the same type as that selected in step S 203 . 
     Next, the processor  504  performs a process in which the notations “I” and “first memory” in steps S 217  and S 218  are replaced with the notations “Q” and “second memory”, and controls the Bias_Q (steps S 219  and S 220 ). That is, the control processor  504  sends an instruction to the oscillator  500  to dither only the Bias_Q at the frequency fd. The control processor  504  subtracts the target value recorded in the second memory from the synchronous detection results obtained by the synchronous detection circuit  503  to calculate an error signal. The control processor  504  controls the Bias_Q so that the calculated error signal is close to 0. At this time, the control processor  504  selects a zero point in the same type as that selected in step S 205 . 
     Next, the control processor  504  sends an instruction to the oscillator  500  to dither both the Bias_I and the Bias_Q with a phase relationship in much the same way as in step S 206  (step S 221 ). The control processor  504  controls the Bias_Ph so that a value obtained by subtracting the target value recorded in the third memory from the synchronous detection result of the second-order harmonic obtained by the synchronous detection circuit  503  is 0 (step S 222 ). There are also two different zero points at this time, but the control processor  504  selects a zero point different in type from that selected in step S 207 . This is because the Bias_Ph is changed by Vπ_bias in step S 209 . After these steps, the control processor  504  repeats regularly, for example, cyclically steps S 217  to S 222 . 
       FIGS. 6 and 7  illustrate principle confirmation experiment results of the second embodiment. 
     Two graphs in the left column of  FIG. 6  indicate the light intensity in the signal output port  301  immediately before the process of step S 209  in  FIG. 5  ( FIG. 6( a ) ) and results obtained by synchronously detecting the output of the IQ modulator-incorporated photodetector  300  in a state where the dithering is applied to the Bias_I ( FIG. 6( b ) ). 
     The converged values of the biases immediately before the process in step S 209  were Bias_I=6.20, Bias_Q=8.06, and Bias_Ph=6.12 (all arbitrary units). On the other hand, in  FIG. 6 , only the Bias_I is swept near 6.20 for an operation confirmation. The synchronous detection result was 0 at Bias_I=6.20 while the output light intensity in the signal output port  301  was minimal at Bias_I=6.1. If the ideal IQ optical modulator M was selected, the both values should match. 
     Next, results obtained when the same tests were conducted immediately after the process of step S 209  are provided in two graphs in the right column of  FIG. 6 .  FIG. 6( c )  indicates a result obtained by synchronously detecting the light intensity in the signal output port  301 , and  FIG. 6( d )  indicates results obtained by synchronously detecting the output of the IQ modulator-incorporated photodetector  300  in a state where the dithering is applied to the Bias_I. Bias_Ph was set to 11.09. This value was obtained when Bias_Ph was increased from 6.12 by Vπ_bias. The synchronous detection result is plotted at 0 when Bias_I=6.12. On the other hand, the output light intensity in the signal output port  301  was minimal at Bias_I=6.20. It is seen from the results that the switching of the constellations illustrated in  FIG. 1  actually occurs. 
     Here, it is considered that the process of step S 211  in  FIG. 5  is performed. In  FIG. 6 , Bias_I was swept for an operation confirmation. In the process of step S 211  of  FIG. 5 , the converged value immediately before the process of step S 209  is used, and thus, the control processor  504  records the synchronous detection result at Bias_I=6.20 into the first memory. As illustrated in  FIG. 6( d ) , the value is 4×10 −4 . When the process of step S 218  in  FIG. 5  is performed, the control processor  504  subtracts 4×10 −4  from the synchronous detection result and adjusts Bias_I so that the result is 0. The control processor  504  performs a similar process on Bias_Q. 
     Here again, the experiment returned to bias values (Bias_I=6.20, Bias_Q=8.06, and Bias_Ph=6.12) immediately before the process of step S 209 , and only the Bias_I was swept near 6.12 for an operation confirmation.  FIG. 7  are graphs indicating results of the operation confirmation.  FIG. 7( a )  indicates results obtained by measuring the synchronous detection of the second-order harmonic described in step S 206  and step S 207  in  FIG. 5 . In  FIG. 7( a ) , the result is 0 at Bias_Ph=6.12. Next, as a result of the process in step S 209 , the Bias_Ph increases from 6.12 by Vπ_bias to 11.09. 
     Again, only the Bias_Ph was swept near 11.09 for an operation confirmation.  FIG. 7( b )  indicates results obtained by measuring the synchronous detection of the second-order harmonic described in steps S 214  and S 215  of  FIG. 5 . The process in step S 215  of  FIG. 5  is performed with the Bias_Ph immediately after the process in step S 209 , and thus, the control processor  504  records the synchronous detection result of the second-order harmonic at Bias_Ph=11.09, into the third memory. As illustrated in  FIG. 7( b ) , the value is 3.7×10 −6 . When the process of step S 222  in  FIG. 5  is performed, the control processor  504  subtracts 3.7×10 −6  from the synchronous detection result of the second-order harmonic and adjusts the Bias_Ph so that the subtraction result is 0. 
     It should be noted that in the synchronous detection result of the second-order harmonic illustrated in  FIG. 7 , a slop near the zero point changes before and after the process in step S 209  of  FIG. 5 . Thus, it is necessary to invert a sign of a feedback gain of the Bias_Ph control in steps S 207  and S 222  of  FIG. 5 . That is, in step S 207  of  FIG. 5 , the control processor  504  decreases the Bias_Ph if the synchronous detection result of the second-order harmonic is positive, and increases the Bias_Ph if the result is negative (see the left side of  FIG. 7 ). However, the slope is in the opposite direction in step S 222  of  FIG. 5 , and thus, the control processor  504  increases the Bias_Ph if the result obtained by subtracting the value stored in the third memory from the synchronous detection result of the second-order harmonic is positive and decreases the Bias_Ph if the result is negative (see the right side of  FIG. 7 ). 
     These two slopes correspond to the sign of the optical phase difference θ OUT =±π/2 as described above. Thus, if it is necessary to select the sign in the in-service state for pre-equivalence and the like, the slope and a feedback gain are determined so that the slope after the process of step S 209  corresponds to the desired sign. 
     It is empirically found that the correction amount recorded in the third memory is very small compared to the correction amounts recorded in the first memory and the second memory. Thus, a control program may be simplified by omitting the process of “subtracting the target value recorded in the third memory” in step S 222  of  FIG. 5 . 
     With these processes, the light output in the signal output port  301  is optimized. To confirm the optimization, the light output in the signal output port  301  was demodulated to measure a Q value.  FIG. 8  is a graph indicating results obtained by measuring the Q value as a principle confirmation experiment result. A signal format is 16-QAM. A round symbol is obtained in a case where the demodulation was performed by using a bias value selected immediately before step S 209 , and a triangular symbol is obtained in a case where the demodulation was performed by using the bias value selected immediately after step S 209  (increased by Vπ_bias only in the Bias_Ph). Six measurements were conducted for each of the bias values in consideration of variations in Q-value measurements. It is indicated that the use of the present embodiment significantly improves the Q value. 
     Variation of Second Embodiment 
     In the second embodiment, in controlling the Bias_I and the Bias_Q, the bias control circuit  50  applies the dithering to the Bias_I and the Bias_Q to perform control so that the synchronous detection is 0 during the startup sequence period, and so that the value obtained by subtracting the values recorded in the first memory and the second memory from the synchronous detection is 0 at an end of the startup sequence period. In other words, the bias control circuit  50  performs control so that the value of the synchronous detection is the same as the values recorded in the first memory and the second memory at the end of the startup sequence period. 
     However, a method of controlling the Bias_I and the Bias_Q is not limited to the above procedure, and may apply the drive-amplitude independent control method described in NPL 4. In this method, modulation efficiencies of the I-component MZ optical modulator  2   a  and the Q-component MZ optical modulator  2   b  are dithered. This can be achieved by applying the dithering to the gain of the differential amplifiers  3   a  and  3   b  in  FIG. 4 . If the I-component MZ optical modulator  2   a  and the Q-component MZ optical modulator  2   b  are semiconductor modulators, the dithering may be applied to the bias voltage to be applied to the V data_I  and the V data_Q . As described in NPL 4, at a time when the Bias_I and the Bias_Q reach optimal values, the amplitude of the dither component superimposed on the output intensity of the IQ optical modulator is not 0 but reaches maximum. 
     If this method is used, the steps S 202 , S 204 , S 210 , S 212 , S 217 , and S 219  in  FIG. 5  are changed, and the modulation efficiency of the I-component MZ optical modulator  2   a  and the modulation efficiency of the Q-component MZ optical modulator  2   b  are subject to be dithered. Further, step S 203  and step S 206  in  FIG. 5  are changed, and the control processor  504  adjusts the Bias_I and the Bias_Q so that the dither amplitude is not 0 but maximized when the output from the monitor port  302  is subject to the synchronous detection. 
     In this method, the dithering is applied to a gain adjustment terminal of the differential amplifier, for example, and as a result, the circuit is somewhat complex. However, there is an advantage in this method in that it is not necessary to consider the problem that the light intensity in the optimum bias turns to the maximum instead of the minimum once the multi-level number and the amplitude of the drive waveform satisfy a specific condition. 
     Variation of First and Second Embodiments 
     In the embodiments heretofore, the description is provided about the error derived from the imperfections of the I-component MZ optical modulator  2   a  and the Q-component MZ optical modulator  2   b  and is provided on the assumption that the configuration of the optical multiplexing and demultiplexing unit  201  is ideal. However, if there is the imperfection in the optical multiplexing and demultiplexing unit  201 , the correspondence relationship of θ MON =θ OUT +π may be slightly corrupted. If this error cannot be ignored, the change amount ΔBias_Ph of the Bias_Ph required to cause a phase change equal to θ OUT −θ MON  may be evaluated in advance, and ΔBias_Ph may be used instead of Vπ_bias in each embodiment. 
     Further, in the embodiments described heretofore, no mention is made of a convergence speed of a loop (step S 102  to step S 104  in  FIG. 3  or step S 202  to step S 207  in  FIG. 5 ) in the startup sequence. The technology described in PTL 1 may be used in combination to more reliably and quickly converge the loop. 
     In the embodiments heretofore, an operation of writing the numerical value for error correction into the first memory to the third memory is performed for each startup sequence. However, depending on a usage environment of the optical transmitter  101 , these error correction values do not greatly change, and thus, may be considered constant values. In such a case, the control processors  405  and  504  may not perform the write operation into the first memory to the third memory each time and may use the value recorded in the previous startup sequence as-is when the write operation is omitted in a second or later startup sequence. 
     In the embodiments heretofore, the description is provided on the assumption that the IQ optical modulator M is used for a single polarization. However, there is a commercially available IQ optical modulator of the type in which two of the configurations illustrated in  FIG. 12  are arranged in parallel, and outputs from these configurations are polarization-multiplexed and sent to the transmission path. In this type of IQ optical modulator, two IQ modulator-incorporated photodetectors  300  are normally also arranged for each polarization, and thus, it is possible to employ the procedure of the present embodiment for each polarization. 
     Further, in the embodiments described heretofore, although the differential output type is employed for the modulator drive amplifier, some of commercially available IQ optical modulators include an IQ optical modulator driven by a single-phase output amplifier. In such an IQ optical modulator, the electric field inside the optical modulator is designed so that the two arms of the MZ optical modulator are driven in a push-pull fashion, and thus, the present embodiment is applicable. Further, in the embodiments described heretofore, the case is described as an example where the control processors  405  and  504  control the bias voltage for controlling the optical path lengths of the I-component MZ optical modulator  2   a  and the Q-component MZ optical modulator  2   b , and the optical path length of the parent MZI, but the electric current applied to control these optical path lengths may be controlled. 
     Further, in the embodiments described heretofore, near the end of the startup sequence, the Bias_Ph is changed by Vπ_bias or ΔBias_Ph. The MZI has periodicity, and thus, it is possible to realize this change by increasing the voltage or the current of the Bias_Ph and it is possible to realize this change by decreasing the voltage or the current of the Bias_Ph. A direction of change may be selected such that the voltage or the current of the Bias_Ph approaches 0. Such a selection can suppress power consumption in the in-service state. 
     Further, in the embodiments described heretofore, a 2×2 optical coupler including two input ports and two output ports is employed for the optical multiplexing and demultiplexing unit  201 . Instead of such a 2×2 optical coupler, a 1×2 optical coupler including two input ports and one output port may be employed for the optical multiplexing and demultiplexing unit  201 . In this case, the modulated light output from the I-component MZ optical modulator  2   a  is input into one of the input ports of the 1×2 optical coupler, and the modulated light output from the Q-component MZ optical modulator  2   b  and passing through the Bias_Ph phase adjusting means  8   c  is input into the other of the input ports of the 1×2 optical coupler. On the other hand, one output port of the 1×2 optical coupler is optically coupled to the signal output port  301 . The IQ modulator-incorporated photodetector  300  receives leaked light that does not enter and comes out of the optical waveguide configuring the signal output port  301 , of the light output from the output port of the 1×2 optical coupler. If the optical electric field propagating through the signal output port  301  is newly defined as E OUT , and the optical electric field of the leaked light received by the IQ modulator-incorporated photodetector  300  is newly defined as E MON , the argument similar to that in the embodiments described heretofore holds. 
     Third Embodiment 
     In the first and second embodiments described heretofore, the bias control circuit performs the ABC by monitoring the average intensity, the peak intensity, or the RMS value of the optical QAM signal by using an IQ modulator-incorporated photodetector. However, in general, the band of the IQ modulator-incorporated photodetector is narrow, and thus, the IQ modulator-incorporated photodetector is not capable of demodulating the optical QAM signal. Thus, the bias control circuit in the first and second embodiments, which can perform the ABC, cannot easily perform more sophisticated control on the IQ optical modulator. 
     In an example, a case is provided in which the gains of the differential amplifier  3   a  that amplifies the Data_I and the differential amplifier  3   b  that amplifies the Data_Q are not the same across the ages. In this case, even though the bias is optimally maintained by the ABC, the constellation is degraded into a rectangular shape, not into a square shape. An advanced control in which such an IQ imbalance is detected and fed back to the gain of the differential amplifiers to correct the constellation into a square shape, is very difficult as long as the optical QAM signal is monitored by using the IQ modulator-incorporated photodetector. 
     To solve such problems, it is possible to employ a configuration where a more advanced control is performed on the IQ optical modulator if the optical receiver demodulates the optical electric field E MON  output from the monitor port and checks the shape of the constellation of the demodulated optical QAM signal. A specific example of such a configuration is illustrated in  FIG. 9 . 
       FIG. 9  is a block diagram illustrating an example of a configuration of an optical transmitter  102  according to a third embodiment. In  FIG. 9 , the same components as those of the optical transmitter  101  according to the second embodiment illustrated in  FIG. 4  are denoted by the same reference signs and descriptions thereof are omitted. A large part of the configuration of the optical transmitter  102  is common to that of the optical transmitter  101  in the second embodiment. The optical transmitter  101  illustrated in  FIG. 9  differs from the optical transmitter  101  illustrated in  FIG. 4  in that the former further includes an optical branch unit  1000 , an optical receiver  1001 , and a control processor  1002 . 
     The optical branch unit  1000  is disposed between the monitor port  302  and the IQ modulator-incorporated photodetector  300 . The optical branch unit  1000  branches the modulated light output from the monitor port  302  into two. The optical branch unit  1000  outputs one of the branched modulated light beams to the IQ modulator-incorporated photodetector  300  and outputs the other of the branched modulated light beams to the optical receiver  1001 . The optical receiver  1001  demodulates the modulated light received from the optical branch unit  1000  and outputs information on the demodulated optical QAM signal to the control processor  1002 . It should be noted that unlike the optical multiplexing and demultiplexing unit  201 , the optical branch unit  1000  includes only one input port, and thus, the two modulated light beams branched by the optical branch unit  1000  are substantially equivalent. That is, the modulated light received by the IQ modulator-incorporated photodetector  300  and the modulated light received by the optical receiver  1001  are the same, and there is no difference between these modulated light beams as in the case of the E OUT  and the E MON . 
     Inside the bias control circuit  50  of the present embodiment, similarly to the optical transmitter  101  according to the other embodiments and illustrated in  FIG. 4 , the control processor  504  is included. Here, a differences between the processing performed by the control processor  504  and the processing performed by the control processor  1002  will be described. Before the control processor  1002  starts the operation, firstly, the control processor  504  executes the processing of step S 201  to step S 208  in the flowchart illustrated in  FIG. 5 . If the bias is significantly detuned at step S 201 , it is difficult or impossible to demodulate the optical QAM signal generated from the random signals for training. However, the control processor  504  determines YES in determination processing in step S 208 , and the bias is optimized at a time when the loop processing is ended. Thus, the optical receiver  1001  is capable of demodulating the optical QAM signal. 
     The control processor  504  determines YES in the determination processing of step S 208  and suspends the operation once the loop processing is ended, and transfers the control to the control processor  1002 . The control processor  1002  analyzes the optical QAM signal demodulated by the optical receiver  1001 , and fine-adjusts the gains of the differential amplifier  3   a  and the differential amplifier  3   b  so that the shape of the constellation obtained by the analysis is optimal. Alternatively, the control processor  1002  may calculate frequency characteristics of the differential amplifier  3   a  and the differential amplifier  3   b  by mutually comparing a spectral analysis of the Data_I and the Data_Q with a spectral analysis of the demodulation signal, and apply pre-emphasis to the Data_I and the Data_Q so that a flatter frequency characteristic is obtained. Alternatively, in order to more precisely match each bias to the optimum point, the control processor  1002  may analyze the optical QAM signal demodulated by the optical receiver  1001  and perform a second bias adjustment operation so that the shape of the constellation obtained by the analysis is optimal. Unlike the control processor  504 , the control processor  1002  receives information on the optical electric field rather than the light intensity of the modulated light, and thus, is capable of analyzing the shape of the constellation. The constellation is square if each bias is optimal as described above, and thus, the control processor  1002  utilizes this property to adjust the Bias_I, the Bias_Q, and the Bias_Ph so that the shape of the constellation is optimal. 
     It should be noted, however, that the optimization operation by the control processor  1002  described above is an optimization for the optical QAM signal output from the monitor port  302 . Thus, the optical QAM signal output from the signal output port  301  is not necessarily optimized. For example, as shown in the first step of  FIG. 1 , the optical QAM signal may be rather degraded. 
     To solve this problem, the optical transmitter  102  returns the control back to the control processor  504 , and performs processing of step S 209  and the subsequent processing in  FIG. 5 . Thus, it is possible to optimize the optical QAM signal output from the signal output port  301  according to the procedure described above. 
     Variation of Third Embodiment 
     In  FIG. 9 , the IQ optical modulator M and the optical receiver  1001  are illustrated as independent components. However, many of commercial optical transceivers have a configuration in which a transmitter and a receiver are accommodated in the same housing. Thus, it is more desirable to utilize the receiver accommodated within the same optical transceiver as the optical receiver  1001 . A specific example of such a configuration is illustrated in  FIG. 10 . 
       FIG. 10  is a block diagram illustrating an example of a configuration of an optical transmitter  103 . In  FIG. 10 , the same components as those of the optical transmitter  102  according to the third embodiment illustrated in  FIG. 9  are denoted by the same reference signs and descriptions thereof are omitted. The optical transmitter  103  illustrated in  FIG. 10  differs from the optical transmitter  102  illustrated in  FIG. 9  in that the former further includes an optical switch  1003 . The optical switch  1003  selects either the optical QAM signal sent through the optical transmission path or the optical QAM signal sent from the monitor port  302 , and inputs the selected optical QAM signal to the optical receiver  1001 . 
     In starting up the optical transceiver, the optical switch  1003  selects the optical QAM signal from the monitor port  302  and inputs the selected optical QAM signal to the optical receiver  1001 . The optical transmitter  103  optimizes the IQ optical modulator M according to the procedure described in the third embodiment. At the end of the optimization, the optical switch  1003  selects the optical QAM signal through the optical transmission path and inputs the selected optical QAM signal to the optical receiver  1001 . Similarly to a receiver in a general optical transceiver, the optical receiver  1001  demodulates optical QAM signals transmitted from another optical transceiver in a remote location. 
     The optical transmitter  103  in a configuration illustrated in  FIG. 10  is not capable of simultaneously performing the optimization of the IQ optical modulator M and the demodulation of optical signals transmitted from the other optical transceiver. Thus, there is a problem in that it is not possible to monitor the IQ optical modulator M and optimize the state of the IQ optical modulator M in the in-service state. An example of a configuration for resolving this problem is illustrated in  FIG. 11 . 
       FIG. 11  is a block diagram illustrating an example of a configuration of an optical transmitter  104 . In  FIG. 11 , the same parts as those of the optical transmitter  102  illustrated in  FIG. 9  are denoted by the same reference signs and descriptions thereof are omitted. The optical transmitter  104  illustrated in  FIG. 11  differs from the optical transmitter  102  illustrated in  FIG. 9  in that the former further includes a wavelength shifter  1004  and an optical multiplexing unit  1005 . 
     The wavelength shifter  1004  changes a carrier wavelength of an optical QAM signal output from the monitor port  302  and outputs the optical QAM signal to the optical multiplexing unit  1005 . The optical multiplexing unit  1005  multiplexes the optical QAM signal output from the wavelength shifter  1004  and an optical QAM signal sent through the optical transmission path, and inputs the resultant signals to the optical receiver  1001 . 
     The optical receiver  1001  collectively receives the optical QAM signal sent through the optical transmission path and the optical QAM signal sent from the monitor port  302 , and outputs the both signals to the control processor  1002 . The carrier wavelengths of the two optical QAM signals collectively received by the optical receiver  1001  are different from each other, and thus, the control processor  1002  is capable of separating the two optical QAM signals. 
     As a specific example, the optical receiver  1001  performs digital coherent demodulation. Here, f 1  denotes a difference frequency between a wavelength of local oscillation light built in the optical receiver  1001  and the carrier wavelength of the optical QAM signal sent through the optical transmission path, and f 2  denotes a difference frequency between a wavelength of local oscillation light built in the optical receiver  1001  and the carrier wavelength of the optical QAM signal sent from the monitor port  302 . If the difference frequency f 1  and the difference frequency f 2  are sufficiently separated, the optical receiver  1001  that performs the digital coherent demodulation is capable of separating, by filtering processing, the optical QAM signal sent through the optical transmission path and the optical QAM signal sent from the monitor port  302  to demodulate the signals. With such a configuration, it is possible to simultaneously realize two roles described below with the single optical receiver  1001 . The first role is a role as a receiver that demodulates a signal sent through the optical transmission path. The second role is a role as a controller that monitors the output light of the IQ optical modulator M to best maintain a signal quality of the output light. 
     It is more desirable that a path of light from the monitor port  302  to the optical receiver  1001  has polarization maintaining characteristics. With such a configuration, two effects described below are achieved. The first effect is that if the output light of the IQ optical modulator M is a single polarization, it is easy to increase an interference efficiency between the local oscillation light and the signal light. The second effect is that if the output light of the IQ optical modulator M is polarization-multiplexed, it is possible to simplify polarization separation processing during the demodulation. 
     As described above, an automatic bias control circuit (the bias control circuits  40 ,  50 , for example) according to the embodiments controls a bias voltage or a bias power applied to an IQ optical modulator. The IQ optical modulator includes an optical branch unit (the optical branch unit  200 , for example, it may also be referred to as “optical branch”), an in-phase component MZ optical modulator (the I-component MZ optical modulator  2   a , for example), a quadrature component MZ optical modulator (the Q-component MZ optical modulator  2   b , for example), and an optical multiplexing and demultiplexing unit (the optical multiplexing and demultiplexing unit  201 , for example, it may also be referred to as “multiplexer and demultiplexer”). The optical branch unit branches continuous light into two, that is, in-phase component light and quadrature component light. The in-phase component MZ optical modulator is a Mach-Zehnder interferometer that modulates the in-phase component light obtained by branching the continuous light by the optical branch unit. The quadrature component MZ optical modulator is a Mach-Zehnder interferometer that modulates the quadrature component light obtained by branching the continuous light by the optical branch unit. The optical multiplexing and demultiplexing unit branches an optical QAM signal described below and outputs the signals from each of a signal output port and a monitor port. The optical QAM signal is an optical QAM signal obtained by adjusting an optical phase between the two modulated light beams by a phase adjustment unit (phase adjuster), and then multiplexing the light beams. The two modulated light beams are modulated light output from the in-phase component MZ optical modulator and modulated light output from the quadrature component MZ optical modulator. 
     The automatic bias control circuit includes an in-phase component bias power source (the Bias_I voltage generator  7   a , for example), a quadrature component bias power source (the Bias_Q voltage generator  7   b , for example), a phase adjustment bias power source (the Bias_Ph voltage generator  7   c , for example), a monitor unit (which may include the distributor  400 , the low-pass filter  401 , the first ADC  402 , the RMS measurement circuit  403 , the second ADC  404 , the control processor  405 , and the synchronous detection circuit  503 , for example, and may also be referred to as “monitor”), and a control unit (which may include control processors  405  and  504 , for example, and may also be referred to as “controller”). The in-phase component bias power source generates a voltage or a current applied to the in-phase component MZ optical modulator to bias the in-phase component MZ optical modulator and the quadrature component MZ optical modulator to an area near a null point. The quadrature component bias power source generates a voltage or a current applied to the quadrature component MZ optical modulator to bias the in-phase component MZ optical modulator and the quadrature component MZ optical modulator to an area near a null point. The phase adjustment bias power source generates a voltage or a current for the phase adjustment unit to determine a change amount of an optical phase applied to between the modulated light beams output from the in-phase component MZ optical modulator and the quadrature component MZ optical modulator. The monitor unit monitors an optical QAM signal output from the monitor port. The control unit controls, based on a monitor result from the monitor unit, a voltage or a current generated by each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source. The control unit performs two processes described below in a startup sequence of the IQ optical modulator. The first process is a first-stage process of controlling a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source so that a signal quality of the optical QAM signal obtained from the monitor result approaches a target quality, for example, the best quality. The second process is a second-stage process of obtaining a voltage or a current by changing a voltage or a current output from the phase adjustment bias power source by a predetermined change amount ΔBias_Ph, after a completion of the first-stage process. For example, the change amount ΔBias_Ph is an amount by which an optical phase is changed, by the phase adjustment unit, by π radian. The control unit may control, in time division (time sharing), a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source. 
     The monitor unit monitors, for example, at least one of an average intensity, a peak intensity, or an RMS value of the optical QAM signal. In the first-stage process, the control unit controls a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source so that the monitor result approaches a maximum or a minimum. The control unit may perform, in the startup sequence, a third-stage process of recording, as a new target value, a result obtained by monitoring the optical QAM signal by the monitor unit, into a memory, after a completion of the second-stage process. The control unit regularly compares the monitor result with the target value stored in the memory after a completion of the third-stage process. The control unit controls, if a deviation is detected as a result of the comparison, a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source so that the monitor result approaches the target value. 
     The automatic bias control circuit may further include a dithering unit (dither generator) that applies dithering to at least one of an output of the in-phase component bias power source, an output of the quadrature component bias power source, an output of the phase adjustment bias power source, a modulation efficiency of the in-phase component MZ optical modulator, or a modulation efficiency of the quadrature component MZ optical modulator. The monitor unit monitors at least one of an average intensity, a peak intensity, or an RMS value of the optical QAM signal output from the monitor port. Thereafter, the control unit synchronously detects, in the first-stage process, a dither component with a frequency fd or a higher harmonic wave of the dither component superimposed on the monitor result obtained when a dithering at a constant frequency fd is applied by the dithering unit, and controls a voltage or a current generated from each of three power sources described below so that an absolute value of the synchronous detection result approaches a maximum or 0. The three power sources are the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source. The control unit may synchronously detect, in the startup sequence and after a completion of the second-stage process, a dither component with a frequency fd or a higher harmonic wave of the dither component superimposed on the monitor result obtained when a dithering at a constant frequency fd is applied by the dithering unit, and perform a third-stage process of recording the synchronous detection result, as a new target value, into a memory. The control unit regularly synchronously detects, after a completion of the third-stage process, a dither component with a frequency fd or a higher harmonic wave of the dither component superimposed on the monitor result obtained when a dithering at a constant frequency fd is applied by the dithering unit, and compares the synchronous detection result with the target value recorded in the memory to control, if a deviation is detected as a result of the comparison, a voltage or a current generated from each of three power sources described below so that the synchronous detection result approaches the target value. The three power sources are the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source. 
     The automatic bias control circuit may further include a demodulation unit (for example, the optical receiver  1001 , which may also be referred to as “demodulator”) that demodulates the optical QAM signal output from the monitor port. The control unit (for example, the control processor  1002 ) performs adjustment described below so that a signal quality of the optical QAM signal demodulated by the demodulation unit improves. The adjustment is a fine adjustment applied to a voltage or a current generated from each of the in-phase component bias power source, the quadrature component bias power source, and the phase adjustment bias power source, or the peripheral circuit used when a drive signal is applied to the IQ modulator. The peripheral circuit is, for example, the differential amplifier  3   a , the differential amplifier  3   b , or a circuit that applies pre-emphasis to a drive signal. The drive signal is, for example, the Data_I and the Data_Q. 
     The automatic bias control circuit may further include a switch unit (the optical switch  1003 ) that selects and inputs, into the demodulation unit, one of an optical QAM signal output from the monitor port and an optical transmission signal sent through an optical transmission path. 
     The automatic bias control circuit may further include a wavelength change unit (the wavelength shifter  1004 , for example) that changes a wavelength of an optical QAM signal output from the monitor port and inputs the optical QAM signal with the changed wavelength into the demodulation unit. 
     The embodiments of the present invention have been described above in detail with reference to the drawings. However, specific configurations are not limited to those embodiments, and include any design or the like within the scope not departing from the gist of the present invention. 
     INDUSTRIAL APPLICABILITY 
     The present invention may be applied to bias control of an optical modulator using nested Mach-Zehnder interferometers. 
     REFERENCE SIGNS LIST 
     
         
         M IQ optical modulator 
           2   a  MZ optical modulator 
           2   b  MZ optical modulator 
           3   a  Differential amplifier 
           3   b  Differential amplifier 
           6   a  First I-component modulation unit 
           6   b  Second I-component modulation unit 
           6   c  First Q-component modulation unit 
           6   d  Second Q-component modulation unit 
           7   a  Bias_I voltage generator 
           7   b  Bias_Q voltage generator 
           7   c  Bias_Ph voltage generator 
           8   a  Bias_I phase adjusting means 
           8   b  Bias_Q phase adjusting means 
           8   c  Bias_Ph phase adjusting means 
           40  Bias control circuit 
           50  Bias control circuit 
           100  Optical transmitter 
           101  Optical transmitter 
           102  Optical transmitter 
           103  Optical transmitter 
           104  Optical transmitter 
           200  Optical branch unit 
           201  Optical multiplexing and demultiplexing unit 
           202  Optical multiplexing unit 
           203  Optical branch unit 
           300  IQ modulator-incorporated photodetector 
           301  Signal output port 
           302  Monitor port 
           400  Distributor 
           401  Low-pass filter 
           402  First analog-to-digital converter 
           403  RMS measurement circuit 
           404  Second analog-to-digital converter 
           405  Control processor 
           406  First non-volatile memory 
           407  Second non-volatile memory 
           501  Multiplexer 
           502  Multiplexer 
           503  Synchronous detection circuit 
           504  Control processor 
           505  First non-volatile memory 
           506  Second non-volatile memory 
           507  Third non-volatile memory 
           1000  Optical branch unit 
           1001  Optical receiver 
           1002  Control processor 
           1003  Optical switch 
           1004  Wavelength shifter 
           1005  Optical multiplexing unit