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A phase shift unit provides a prescribed phase difference (π/2, for example) between a pair of optical signals transmitted via a pair of arms constituting a data modulation unit. A low-frequency signal f0 is superimposed on one of the optical signals. A signal of which phase is shifted by 

A phase shift unit provides a prescribed phase difference (π/2, for example) between a pair of optical signals transmitted via a pair of arms constituting a data modulation unit. A low-frequency signal f0 is superimposed on one of the optical signals. A signal of which phase is shifted by π/2 from the low-frequency signal f0 is superimposed on the other optical signal. A pair of the optical signals is coupled, and a part of which is converted into an electrical signal by a photodiode. 2f0 component contained in the electrical signal is extracted. Bias voltage provided to the phase shift unit is controlled by feedback control so that the 2f0 component becomes the minimum.

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What is claimed is: 1. An optical transmitting apparatus for transmitting an optical signal modulated corresponding to a data signal, comprising: a phase shifter to control a phase of at least one of a first optical signal and a second optical signal, acquired by splitting by an optical input, so t

What is claimed is: 1. An optical transmitting apparatus for transmitting an optical signal modulated corresponding to a data signal, comprising: a phase shifter to control a phase of at least one of a first optical signal and a second optical signal, acquired by splitting by an optical input, so that the first and the second optical signals have a predetermined phase difference on an optical waveguide; a data modulator to modulate the first and the second optical signals by using the data signal on the optical waveguide; a first generator to generate a first low-frequency signal; a second generator to generate a second low-frequency signal from the first low-frequency signal; a superimposer to superimpose the first and second low-frequency signals on the first and the second optical signals, respectively; a monitor to monitor at least one of maximum power, minimum power and phase of the low-frequency signal or a higher harmonic signal of the low-frequency signal, superimposed on a modulated optical signal acquired by coupling the first and second optical signals modulated by the data modulator; and a controller to control the phase shift unit using an output of the monitor unit by maximizing a frequency component with a same frequency as the low-frequency signal or minimizing a frequency component with a double frequency of the low-frequency signal. 2. An optical communication system, comprising: the optical transmitting apparatus according to claim 1; and an optical receiving apparatus receiving an optical signal transmitted from the optical transmitting apparatus. 3. The system of claim 2, wherein the optical receiving apparatus comprises: an interferometer comprising a first arm for delaying first split light of optical input by a symbol time period and a second arm for shifting a phase of the second split light of the optical input by a prescribed amount; a photodetector circuit for converting an optical signal output from the interferometer into an electrical signal; a calculation circuit for generating a squared signal or an absolute value signal of the electrical signal; a filter, connected to the calculation circuit, for transmitting at least a part of frequency component except for the frequency, which is a integral multiple of a symbol frequency; and a control unit for controlling the amount of the phase shift in the second arm based on the output from the filter. 4. The system according to claim 3, wherein the photodetector circuit comprises a photodiode for generating current corresponding to an optical signal output from the interferometer and a converter for converting the current generated by the photodiode into a voltage signal, and wherein the calculation circuit generates a squared signal or an absolute value signal of an electrical signal output from the converter. 5. The system according to claim 3, wherein the photodetector circuit comprises a photodiode for generating current corresponding to an optical signal output from the interferometer, a first converter for converting a first part of the current generated by the photodiode into a voltage signal, and a second converter for converting a second part of the current generated by the photodiode into a voltage signal, and wherein transmission data is recovered from an output signal of the first converter, and wherein the calculation circuit generates a squared signal or an absolute value signal of an electrical signal output from the second converter. 6. The system according to claim 3, wherein the photodetector circuit comprises a photodiode for generating current corresponding to an optical signal output from the interferometer and a load resistance connected to the photodiode, and the calculation circuit generates a squared signal or an absolute value signal of an electrical signal obtained as voltage across the load resistance. 7. The system according to claim 3, wherein the photodetector circuit comprises a pair of photodiodes for generating current corresponding to a pair of optical signals from the interferometer and a pair of load resistances connected to each of the pair of the photodiodes, and the calculation circuit generates a squared signal or an absolute value signal of an electrical signal obtained as voltage across either one of the pair of the load resistances. 8. The system according to claim 2, wherein the optical receiving apparatus comprises: an interferometer comprising a first arm for delaying first split light of optical input by a symbol time period and a second arm for shifting a phase of the second split light of the optical input by a prescribed amount; a low-frequency signal generator unit for providing a low-frequency signal to the second arm; a photodetector circuit for converting an optical signal output from the interferometer into an electrical signal; and a control unit for controlling the amount of the phase shift in the second arm based on at least one of the phase or the power of the low-frequency signal or a higher harmonic signal of the low-frequency signal extracted from the electrical signal. 9. The system according to claim 2, wherein the optical receiving apparatus comprises: an interferometer comprising a first arm for delaying first split light of optical input by a symbol time period and a second arm for shifting a phase of the second split light of the optical input by a prescribed amount; a photodetector circuit for converting an optical signal output from the interferometer into an electrical signal; a sampling unit for sampling the electrical signal in a period of a symbol period or an integral multiple of the symbol period; and a control unit for controlling the amount of the phase shift in the second arm based on distribution of sampling values acquired by the sampling unit. 10. The system according to claim 2, wherein the optical receiving apparatus comprises: an interferometer comprising a first arm for delaying first split beam of an optical input by a symbol time and a second arm for shifting a phase of a second split beam of the optical input by a prescribed amount; a photodetector circuit for converting an optical signal output from the interferometer into an electrical signal; a limiter circuit, when an amplitude of the electrical signal exceeds a threshold level, for limiting an amplitude of the electrical signal; and a control unit for controlling an amount of phase shift of the second arm according to an average power of an output signal of the limiter circuit. 11. The system according to claim 10, wherein the limiter circuit is an amplifier for performing linear amplification when an amplitude of an input signal is less than the threshold level, and a gain of the amplifier is saturated when the amplitude of the input signal exceeds the threshold level, and wherein the threshold level of the amplifier matches or approximately matches an amplitude of an output signal of the photodetector circuit when the amount of phase shift is adjusted at an optimal value. 12. The system according to claim 2, wherein the optical receiving apparatus comprises: an interferometer comprising a first arm for transmitting first split light of an optical input and a second arm for delaying second split light of the optical input by one bit; a photodetector circuit for converting an optical signal output from the interferometer into an electrical signal; a calculation circuit for generating a squared signal or an absolute value signal of the electrical signal; and a control unit for controlling delay time in the second arm based on the output of the calculation circuit. 13. The system according to claim 2, wherein the optical receiving apparatus comprises: an interferometer comprising a first arm for transmitting first split light of an optical input and a second arm for delaying second split light of the optical input by one bit; a low-frequency signal generator unit for providing a low-frequency signal to the second arm; a photodetector circuit for converting an optical signal output from the interferometer into an electrical signal; and a control unit for controlling delay time in the second arm based on power of the low-frequency signal or a higher harmonic signal of the low-frequency signal extracted from the electrical signal. 14. The optical transmitting apparatus according to claim 1, wherein a phase of the second low-frequency signal is different by a predetermined value from a phase of the first low-frequency signal. 15. The optical transmitting apparatus according to claim 14, wherein the phase of the first low-frequency signal and the phase of the second low-frequency signal are shifted to each other by nπ/2 where n is a natural number other than 0 and a multiple of 4. 16. An optical transmitting apparatus comprising a phase modulator and a driving signal generator to drive the phase modulator, wherein the phase modulator comprises a phase shift unit which provides a proper phase difference between a pair of split optical signals on an optical waveguide, a data modulator which performs a phase modulation of the optical signals on the split optical waveguide and an electrode for superimposing a low-frequency signal, and wherein said optical transmitting apparatus further comprises: a low-frequency signal superimposer to generate low-frequency signals with a proper phase difference and to provide the low-frequency signals to the electrode on the split optical waveguide; a monitor to monitor at least one of maximum power, minimum power and phase of a low-frequency signal or a higher harmonic signal of the low-frequency signal superimposed on the optical signal after coupling of the split optical waveguide; and a phase difference controller to control the phase shift unit so as to obtain a proper phase difference using the output of the monitor unit by maximizing a frequency component with a same frequency as the low-frequency signal or minimizing a frequency component with a double frequency of the low-frequency signal. 17. The optical transmitting apparatus according to claim 16, wherein the phase shift unit is configured in a former stage or a later stage of the data modulation unit. 18. The optical transmitting apparatus according to claim 16, wherein the monitor comprises a synchronous detector to extract and to synchronously detect a signal with twice of the frequency of the low-frequency signal from a photodetector or a peak power detector to extract a signal with the same frequency as the low-frequency signal from the photodetector to detect peak power. 19. The optical transmitting apparatus according to claim 18, wherein the monitor comprises the synchronous detector and the peak power detector. 20. The optical transmitting apparatus according to the claim 16, wherein the low-frequency signal superimposer comprises a phase shifter and the phase shifter adjusts the phase difference between the low-frequency signals at nπ/2 where n is a natural number other than 0 and multiples of 4. 21. An optical transmitting apparatus comprising a phase modulator and a driving signal generator to drive the phase modulator, wherein the phase modulator comprises a phase shifter which provides a proper phase difference between a pair of split optical signals on an optical waveguide, a data modulator with a data input unit on the split optical waveguide and an electrode, provided on a different optical waveguide from the optical waveguide where the phase shifter is configured, for superimposing a low-frequency signal, and wherein said optical transmitting apparatus further comprises: a low-frequency signal superimposer to generate low-frequency signals with a proper phase difference and to provide the low-frequency signals to the electrode and on a bias input terminal of the phase shift unit; a monitor to monitor at least one of maximum power, minimum power and phase of a low-frequency signal or a higher harmonic signal of the low-frequency signal superimposed on the optical signal after coupling of the split optical waveguide; and a phase difference controller to control the phase shift unit so as to obtain a proper phase difference using the output of the monitor unit by maximizing a frequency component with a same frequency as the low-frequency signal or minimizing a frequency component with a double frequency of the low-frequency signal. 22. The optical transmitting apparatus according to claim 21, wherein the phase shifter is configured in a former stage or a later stage of the data modulator. 23. An optical transmitting apparatus comprising a phase modulator and a driving signal generator to drive the phase modulator, wherein the phase modulator comprises a phase shifter which provides a proper phase difference between a pair of split optical signals on an optical waveguide and data modulator with a data input unit on a split optical waveguide, and wherein said optical transmitting apparatus further comprises: a low-frequency signal superimposer to generate low-frequency signals with a proper phase difference and to provide the low-frequency signal to the data input unit of the data modulator; a monitor to monitor at least one of maximum power, minimum power and phase of a low-frequency signal or a higher harmonic signal of the low-frequency signal superimposed on the optical signal after coupling of the split optical waveguide; and a phase difference controller to control the phase shift unit so as to obtain a proper phase difference using the output of the monitor unit by maximizing a frequency component with a same frequency as the low-frequency signal or minimizing a frequency component with a double frequency of the low-frequency signal. 24. The optical transmitting apparatus according to claim 23, wherein the phase shifter is configured in a former stage or in a later stage of the data modulator. 25. The optical transmitting apparatus according to claim 23, wherein the monitor comprises synchronous detector to extract and synchronously detecting a signal component with a frequency twice of the low-frequency signal from a photodetector. 26. An optical transmitting apparatus comprising a phase modulator and a driving signal generator to drive the phase modulator, wherein the phase modulator comprises a phase shifter which provides a proper phase difference between a pair of split optical signals on an optical waveguide and data modulator with a data input unit and a bias input unit on a split optical waveguide, and wherein said optical transmitting apparatus further comprises: a low-frequency signal superimposer to generate low-frequency signals with a proper phase difference and to provide the low-frequency signal to the bias input unit of the data modulator; a monitor to monitor at least one of maximum power, minimum power and phase of a low-frequency signal or a higher harmonic signal of the low-frequency signal superimposed on the optical signal after coupling of the split optical waveguide; and a phase difference controller to control the phase shifter so as to obtain a proper phase difference using the output of the monitor by maximizing a frequency component with a same frequency as the low-frequency signal or minimizing a frequency component with a double frequency of the low-frequency signal. 27. An optical transmitting apparatus comprising a phase modulator and a driving signal generator to drive the phase modulator, wherein the phase modulator comprises a phase shifter which provides a proper phase difference between a pair of split optical signals on an optical waveguide, a data modulator with a data input unit on a split optical waveguide, and an electrode, which is configured in a former stage of the data modulator, to superimpose a low-frequency signal, and wherein said optical transmitting apparatus further comprises: a low-frequency signal superimposer to generate low-frequency signals with a proper phase difference and to provide the low-frequency signals to the electrode; a monitor to monitor at least one of maximum power, minimum power and phase of a low-frequency signal or a higher harmonic signal of the low-frequency signal superimposed on the optical signal after coupling of the split optical waveguide; and a phase difference controller to control the phase shift unit so as to obtain a proper phase difference using the output of the monitor unit by maximizing a frequency component with a same frequency as the low-frequency signal or minimizing a frequency component with a double frequency of the low-frequency signal. 28. The optical transmitting apparatus according to claim 27, wherein the phase shifter is configured in a former stage of the electrode or a later stage of the data modulator. 29. The optical transmitting apparatus according to claim 27, wherein the monitor comprises a synchronous detector to extract and to synchronously detect a signal with twice of the frequency of the low-frequency signal from a photodetector or a peak power detector to extract a signal with the same frequency as the low-frequency signal from the photodetector to detect peak power. 30. An optical transmitting apparatus comprising a phase modulator and a driving signal generator to drive the phase modulator, wherein the phase modulator comprises a phase shifter which provides a proper phase difference between a pair of split optical signals on an optical waveguide, a data modulator with a data input unit on a split optical waveguide, and an electrode, which is configured in a former stage or later stage of the data modulator to superimpose a low-frequency signal, and wherein said optical transmitting apparatus further comprises: a low-frequency signal superimposer to generate a low-frequency signal and to provide the low-frequency signal to the electrode configured in an optical waveguide, which is the same as the optical waveguide where the phase shifter or a bias input terminal of the phase shifter is configured or on the electrode configured on an optical waveguide, which is different from the optical waveguide where the phase shifter is configured; a monitor to monitor at least one of maximum power, minimum power and phase of a low-frequency signal or a higher harmonic signal of the low-frequency signal superimposed on the optical signal after coupling of the split optical waveguide; and a phase difference controller to control the phase shifter so as to obtain a proper phase difference using the output of the monitor by minimizing a frequency component with a double frequency of the low-frequency signal. 31. An optical transmitting apparatus comprising a phase modulator, a driving signal generator to drive the phase modulator and an intensity modulator for modulating an optical output signal from the phase modulator, wherein the phase modulator comprises a phase shifter which provides a proper phase difference between a pair of split optical signals on an optical waveguide, a data modulator with a data input unit on a split optical waveguide, and an electrode, which is configured in a later stage of the data modulation unit, to superimpose a low-frequency signal, and wherein said optical transmitting apparatus further comprises: a monitor to monitor any of the maximum power of the low-frequency signal, the minimum power of a higher harmonic signal with a frequency of twice of the frequency of the low-frequency signal, or the phase of the higher harmonic signal, by extracting the low-frequency signal after coupling of the split optical waveguide; a phase shifter controller to provide the low-frequency signals with a proper phase difference to the electrode and to control the phase shifter by bias control so that the proper phase difference can be obtained based on the output from the monitor; first and second automatic bias controllers to add the low-frequency signals on each arm of the data modulation unit and to control the data modulator by bias control based on the output of the monitor; a third automatic bias controller to add the low-frequency signal on the intensity modulator and to control the intensity modulator by bias control based on the output from the monitor; and a switch controller comprising a switch to perform the controls in the monitoring in the monitor, the phase shifter controller and the first through the third automatic controller by time division. 32. An optical transmitting apparatus comprising a phase modulator, a driving signal generator to drive the phase modulator and an intensity modulator for modulating an optical output signal from the phase modulator, wherein the phase modulator comprises a phase shifter which provides a proper phase difference between a pair of split optical signals on an optical waveguide, a data modulator with a data inputter on a split optical waveguide, and an electrode, which is configured in a later stage of the data modulator, to superimpose a low-frequency signal, and wherein said optical transmitting apparatus further comprises: a monitor to monitor at least one of maximum power, minimum power and phase of a low-frequency signal or a higher harmonic signal of the low-frequency signal superimposed on the optical signal after coupling of the split optical waveguide; a phase shifter controller to add a first low-frequency signals with a proper phase difference to the electrode, and to control the phase shifter by bias control so as to obtain a proper phase difference based on the output from the monitor; first and second automatic bias controllers to add second and third low-frequency signals on each arm of the data modulator and to control the data modulator by bias control based on the output from the monitor; a third automatic bias controller to add a fourth low-frequency signal on the intensity modulator and to control the intensity modulator by bias control based on the output from the monitor; and a collective controller to cause controls in the monitor operation in the monitor, the phase shifter controller , and the first through third automatic bias controllers in parallel. 33. An optical transmitting apparatus comprising a phase modulator to perform phase modulation according to input data signal, an intensity modulator to perform intensity modulation on an optical output signal from the phase modulator, and a driving signal generator to drive the phase modulator and the intensity modulator, comprising: a monitor to monitor at least one of maximum power, minimum power and phase of a low-frequency signal or a higher harmonic signal of the low-frequency signal superimposed on the optical signal after coupling of the split optical waveguide; an automatic bias controller to add a low-frequency signal on the phase modulator and the intensity modulator and to control the phase modulator and the intensity modulator by bias control using the output from the monitor; and a controller to cause the bias control in the monitor operation of the monitor unit and the automatic bias control unit by time division. 34. An optical transmitting apparatus for transmitting an optical signal modulated corresponding to a data signal, comprising: a phase shifter to control a phase of at least one of a first optical signal and a second optical signal, acquired by splitting an optical input, so that the first and the second optical signals have a predetermined phase difference on an optical waveguide; a data modulator to modulate the phases of the first and the second optical signals by using the data signal on the optical waveguide; a monitor to monitor average optical power of a modulated optical signal acquired by coupling the first and the second optical signals modulated by the data modulator; and a controller to control the phase shifter using an output of the monitor, wherein the data modulator is a Mach-Zehnder modulator and comprises a phase adder to add a prescribed phase to a phase determined according to the data signal, and the phase adder is realized by providing a phase modulation region to form an electrode for applying voltage to one waveguide of the Mach-Zehnder modulator so that the electrode reaches the coupled waveguide in the output side of the Mach-Zehnder modulator. 35. An optical transmitting apparatus for transmitting an optical signal modulated corresponding to a data signal, comprising: a phase shifter to control a phase of at least one of a first optical signal and a second optical signal, acquired by splitting an optical input, so that the first and the second optical signals have a predetermined phase difference on an optical waveguide; a data modulator to modulate the phases of the first and the second optical signals by using the data signal on the optical waveguide; a monitor to monitor average optical power of a modulated optical signal acquired by coupling the first and the second optical signals modulated by the data modulator; and a controller to control the phase shifter based on an output of the monitor, wherein the data modulator is a Mach-Zehnder modulator and comprises a phase adder to add a prescribed phase to a phase determined according to the data signal, and the phase adder is an attenuation element for causing difference in amplitudes of a pair of data signals provided for the Mach-Zehnder modulator from each other. 36. An optical transmitting apparatus for transmitting an optical signal modulated corresponding to a data signal, comprising: a phase shifter to control a phase of at least one of a first optical signal and a second optical signal, acquired by splitting an optical input, so that the first and the second optical signals have a predetermined phase difference on an optical waveguide; a data modulator to modulate the phases of the first and the second optical signals by using the data signal on the optical waveguide; a monitor to monitor average optical power of a modulated optical signal acquired by coupling the first and the second optical signals modulated by the data modulator; and a controller to control the phase shifter based on an output of the monitor, wherein the data modulator is a Mach-Zehnder modulator and comprises a phase adder to add a prescribed phase to a phase determined according to the data signal, and the phase adder is a delay element to cause difference in timings of a pair of data signals provided for the Mach-Zehnder modulator from each other. 37. An optical transmitting apparatus for transmitting an optical signal modulated corresponding to a data signal, comprising: a mark rate unbalancer to unbalance mark rate of the data signal in such a way that an appearance rate of each symbol value becomes unbalanced; a phase shifter to control a phase of at least one of a first optical signal and a second optical signal, acquired by splitting an optical input, so that the first and the second optical signals have a predetermined phase difference on an optical waveguide; a data modulator to modulate the phase of the first and the second optical signals by using a data signal with its mark rate adjusted on the optical waveguide; a monitor to monitor average optical power of a modulated optical signal acquired by coupling the first and the second optical signals modulated by the data modulator; and a controller to control the phase shifter based on the output of the monitor.