CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-088622, filed on Mar. 29, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    The present invention relates to an optical modulator using a Mach Zehnder modulator and a light-modulation switching method. 
         [0003]    Recently, in association with a rapid increase in amount of information, large capacity and long distance of an optical communication system is required. For example, an optical amplification repeating system of 40 Gb/s has been put into practical use. In the future, the larger capacity and the long distance will be further required. A TDM (Time Division Multiplexing) system and a WDM (Wavelength Division Multiplexing) system have been researched and developed. 
         [0004]    With respect to an electro-optical modulator circuit in an optical communication system, an intensity modulation (direct modulation) is the most simple. In this modulation, light emission/quenching is controlled by directly turning on/off current flowing to a semiconductor laser in response to “0” and “1” of a data signal. However, the laser is directly turned on/off and the property of a semiconductor then causes chirping in an optical signal. 
         [0005]    As a bit rate is higher, the chirping affects a harmful influence to transmission characteristics. Because an optical fiber has a property of wavelength division that signal light having different wavelengths has different propagation rates. The direct modulation causes the chirping, then the propagation rate is delayed and waveforms deteriorate during transmission through the optical fiber and the long-distance transmission and the fast transmission are not possible. 
         [0006]    Therefore, in transmission at fast rates of 2.5 Gb/s and 10 Gb/s, such an external modulation is performed that a laser diode continuously emits light and an external modulator turns on/off continuous light generated from the laser diode on the basis of “0” and “1” of the data signal. As the external modulator, a Mach Zehnder (MZ) optical modulator is mainly used. 
         [0007]      FIG. 20  is a block diagram showing the structure of a conventional optical modulator. Referring to  FIG. 20 , a conventional optical modulator  2000  comprises: a light source  2010 ; a MZ modulator  2020 ; and a MZ modulator  2030 . The MZ modulator  2020  is a DQPSK (Differential Quadrature Phase Shift Keying) modulator that modulates carrier light output from the light source  2010  to a differential quadrature phase thereof. 
         [0008]    The MZ modulator  2020  comprises two MZ modulators (I arm  2020 A and Q arm  2020 B), and performs the DQPSK by interference of the signal light phase-modulated by the MZ modulators with the phase difference of π/2. A bias supply unit  2021  supplies, to the MZ modulator  2020 , a bias voltage corresponding to the DQPSK signal modulated by the MZ modulator  2020 . 
         [0009]    The MZ modulator  2030  is an RZ modulator that converts the signal light subjected to the DQPSK modulated by the MZ modulator  2020  into RZ (Return to Zero) pulses. A bias control unit  2031  supplies a bias voltage corresponding to the signal light subjected to the RZ-DQPSK modulated by the MZ modulator  2030  to MZ modulator  2030 . 
         [0010]    In addition to the RZ-DQPSK, external modulators using various-modulations are used in accordance with transmission conditions, e.g., an NRZ (Non Return to Zero) intensity modulation, CZRZ-DQPSK (Carrier Suppressed RZ-DQPSK) modulation, and Duobinary (Alternate mark inversion) modulation (refer to Japanese Laid-open Patent Publication No. 2000-162563 and Japanese Laid-open Patent Publication No. H3-251815). 
         [0011]    For example, the RZ-DQPSK is advantageous to the long-distance transmission because of high proof strength of Polarization Mode Dispersion (PMD) and high Optical Signal Noise Ratio (OSNR) in oncoming transmission and reception. However, the spectrum of the signal light is wide. 
         [0012]    Therefore, in the case of a small interval between wavelengths in a WDM transmission system comprising a repeater including a wavelength division multiplexing device, the signal is cut-off by the wavelength division multiplexing device, thereby increasing the penalty. Accordingly, the DQPSK or DPSK can be used in a short WDM-transmission path with a small interval between wavelengths and a transmission path through which a nonlinear optical effect is frequently caused. 
         [0013]    However, with the above-mentioned conventional arts, the modulation is fixed depending on the type of modulator and the initial setting. Therefore, even if changing the transmission conditions of the optical communication system, such as the interval between the wavelengths in the WDM and the number of steps of the repeater, the modulation is not switched corresponding to the changed transmission condition. As a consequence, there is a program that transmission characteristics deteriorate depending on the transmission condition. 
         [0014]    Further, if one optical communication system uses modulations varied depending on optical communication devices, the modulation needs to be matched to the optical communication device as the communication destination. However, the conventional arts cannot switch the modulation to that matching to the optical communication device as the communication destination. Therefore, there is a problem that the optical transmission is not possible between the optical communication devices using different modulations. 
         [0015]    On the other hand, it is considered that a plurality of modulators corresponding to the modulations are arranged to switch a plurality of modulations. However, the arrangement of a plurality of modulators causes a problem of a large scale, a complicated structure, and an increase in costs of the device. Further, upon switching the modulation by manually switching the modulator, the switching of the modulator is troublesome. Therefore, there is a problem that it is not possible to flexibly cope with the optical communication system in which the transmission condition frequently changes. 
       SUMMARY 
       [0016]    It is an object of an aspect of present invention is to provide an optical modulation device which is available to modulate with plurality of modulation format. 
         [0017]    According to an aspect of an embodiment, an optical modulation device includes a Mach-Zehnder modulator and a controller. The Mach-Zehnder modulator is supplied a drive signal and a bias voltage. The Mach-Zehnder modulator modulates inputted light on the bases of the drive signal and the bias voltage. The drive signal selectively is superimposes a predetermined frequency signal. The bias voltage is selectively superimposed the predetermined frequency signal. The controller selects a superimposing target which is the drive signal or the bias voltage so as to change modulation formats. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a block diagram showing an optical modulator according to the first embodiment. 
           [0019]      FIG. 2  is a diagram showing input/output characteristics (of DPSK) of a MZ modulator. 
           [0020]      FIG. 3  is a diagram showing input/output characteristics (of NRZ intensity modulation) of the MZ modulator. 
           [0021]      FIG. 4  is a graph showing synchronous detecting characteristics of a bias control unit. 
           [0022]      FIG. 5  is a diagram showing bias points of the MZ modulator (in DPSK and NRZ intensity modulation). 
           [0023]      FIG. 6  is a flowchart showing an example of an operation of an optical modulator according to the first embodiment. 
           [0024]      FIG. 7  is a block diagram showing the structure of an optical modulator according to the second embodiment. 
           [0025]      FIG. 8  is a diagram showing input/output characteristics (of DQPSK) of a MZ modulator. 
           [0026]      FIG. 9  is a diagram showing bias points of the MZ modulator (in RZ-DQPSK and DQPSK). 
           [0027]      FIG. 10A  is a block diagram showing the structure of an optical modulator according to the third embodiment. 
           [0028]      FIG. 10B  is a diagram showing various RZ modulations with the MZ modulator. 
           [0029]      FIG. 11  is a diagram showing bias points of the MZ modulator (in RZ-DQPSK and CZ-DQPSK). 
           [0030]      FIG. 12  is a block diagram showing the structure of an optical modulator according to the fourth embodiment. 
           [0031]      FIG. 13  is a diagram showing a bias point of a Q arm (in DQPSK and DPSK). 
           [0032]      FIG. 14  is a block diagram showing the structure of an optical modulator according to the fifth embodiment. 
           [0033]      FIG. 15  is a diagram showing bias points of an I arm (in RZ-DQPSK and NRZ intensity modulation). 
           [0034]      FIG. 16  is a diagram showing a bias point of a Q arm (in RZ-DQPSK and NRZ intensity modulation). 
           [0035]      FIG. 17A  is a block diagram showing an optical modulator according to the sixth embodiment. 
           [0036]      FIG. 17B  is a diagram showing the switching of a duobinary modulation and an AMI modulation. 
           [0037]      FIG. 18  is a diagram showing bias point of the MZ modulator (in RZ-DQPSK, duobinary modulation and AMI modulation). 
           [0038]      FIG. 19  is a block diagram showing the structure of an optical communication system according to the seventh embodiment. 
           [0039]      FIG. 20  is a block diagram showing the structure of a conventional optical modulator. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]    Hereinbelow, a detailed description will be given of an optical modulator device and an optical modulator device switching method according to embodiments with reference to the attached drawings. 
       First Embodiment 
       [0041]      FIG. 1  is a block diagram showing the structure of an optical modulator according to the first embodiment. In  FIG. 1 , a solid arrow shows a light flow and a dotted arrow shows an electrical flow (similarly in the following block diagrams). An optical modulation device  100  according to the first embodiment is an optical modulator that can switch the DPSK and the NRZ intensity modulation format in accordance with modulation switching information. 
         [0042]    Referring to  FIG. 1 , the optical modulation device  100  comprises: a light source  110 ; a Mach-Zehnder modulator (MZ modulator)  120 ; a driving unit (driving circuit)  130 ; an oscillator  140 ; a branch unit  151 ; a light receiving unit  152  (PD); an amplifying unit  153 ; a bias control unit  160 ; a switch  170 ; a modulation switching unit (controller)  180 ; and a bias-point holding unit  190 . The light source  110  generates continuous light as carrier light and outputs the generated continuous light to the MZ modulator  120 . 
         [0043]    The MZ modulator  120  performs NRZ intensity modulation of the continuous light output from the light source  110  on the basis of a drive signal output from the driving unit  130 . Specifically, the MZ modulator  120  contains a basic material having an electro-optical effect, such as LiNbO 3 , and comprises: a branch unit  121 ; an optical waveguide  122   a ; an optical waveguide  122   b ; and a coupling unit  125 . 
         [0044]    The branch unit  121  branches the continuous light output from the light source  110 , and outputs one branched piece of the continuous light to the optical waveguide  122   a , and further outputs the other branched piece of the continuous light to the optical waveguide  122   b . The optical waveguide  122   a  comprises a phase modulating section  123   a . The phase modulating section  123   a  performs binary phase-modulation of the light passing through the optical waveguide  122   a  in accordance with the drive signal output from the driving unit  130 . 
         [0045]    The optical waveguide  122   b  comprises: a phase modulating section  123   b  and a phase modulating section  124 . The phase modulating section  123   b  performs binary phase-modulation of the light passing through the optical waveguide  122   b  in accordance with the drive signal output from the driving unit  130 . The phase modulating section  124  performs phase modulation of light passing through the optical waveguide  122   b  in accordance with the bias voltage supplied from a bias supply unit  162 . 
         [0046]    The coupling unit  125  couples (performs interference of) the light passing through the optical waveguide  122   a  and the light passing through the optical waveguide  122   b  and outputs the coupled light to the branch unit  151 . The driving unit  130  inputs a data signal (DATA), and outputs the input data signal as a drive signal to the MZ modulator  120 . Further, the driving unit  130  changes, to 2Vπ or Vπ, a voltage of the drive signal to be output to the MZ modulator  120  under the control of the modulation switching unit  180 . 
         [0047]    The driving unit  130  performs push-pull modulation for always outputting drive signals with inverse signs to, e.g., the phase modulating section  123   a  and the phase modulating section  123   b  in the MZ modulator  120 . Herein, an output from one driver is differential-operated, thereby driving the modulator. However, two drivers may be used, thereby driving the modulator. Further, upon outputting a low-frequency signal from the switch  170 , the driving unit  130  superimposes the low-frequency signal to the drive signal to be output to the MZ modulator  120 . 
         [0048]    The oscillator  140  oscillates a low-frequency signal of a frequency f 0  (predetermined frequency). The low-frequency signal oscillated by the oscillator  140  is a signal with a frequency much lower than the frequency of the drive signal output by the driving unit  130 , e.g., a signal of a frequency 1 kHz. The oscillator  140  outputs the oscillated low-frequency signal to the switch  170 . Further, the oscillator  140  outputs the oscillated low-frequency signal to a phase comparator  161  of the bias control unit  160 . 
         [0049]    The branch unit  151  branches a part of the signal light output from the MZ modulator  120 , and outputs the branched light to the light receiving unit  152 . The light receiving unit  152  receives the signal light output from the branch unit  151 , and converts the received light into an electrical signal. The light receiving unit  152  outputs the converted electrical signal to the amplifying unit  153 . The light receiving unit  152  is, e.g., a PD (Photo Diode). The amplifying unit  153  amplifies the electrical signal output from the light receiving unit  152 , and outputs the amplified signal to the bias control unit  160 . 
         [0050]    The bias control unit  160  supplies, to the MZ modulator  120 , a bias voltage corresponding to the component of the frequency f 0  included in the signal light modulated by the MZ modulator  120 . The bias control unit  160  comprises the phase comparator  161  and the bias supply unit  162 . The phase comparator  161  extracts the component of the frequency f 0  included in the electrical signal output from the amplifying unit  153  with synchronous detection based on the low-frequency signal output from the oscillator  140 . 
         [0051]    For example, the phase comparator  161  is a multiplying circuit that multiplies the electrical signal output from the amplifying unit  153  and the low-frequency signal output from the oscillator  140 . In this case, the phase comparator  161  outputs, to the bias supply unit  162 , a DC voltage corresponding to the intensity and phase of the frequency f 0 , as the multiplying result of the electrical signal and the low-frequency signal. 
         [0052]    The bias supply unit  162  supplies the bias voltage to the MZ modulator  120 . Further, the bias supply unit  162  controls the bias voltage to be supplied to the MZ modulator  120  so as to minimize the intensity of the component of the frequency f 0  output from the phase comparator  161 . Further, the bias supply unit  162  outputs control information on the bias point corresponding to the modulation to the bias-point holding unit  190 . 
         [0053]    The control information on the bias point corresponding to the modulation is, e.g., information on the direction for controlling the bias voltage supplied to the MZ modulator  120  upon switching the modulation. Further, the bias supply unit  162  controls the bias voltage supplied to the MZ modulator  120  on the basis of the control information on the bias point and the component of the frequency f 0  upon outputting the control information on the bias point from the bias-point holding unit  190 . 
         [0054]    Further, the bias supply unit  162  superimposes the low-frequency signal to the bias voltage to be supplied to the MZ modulator  120 , upon outputting the low-frequency signal from the switch  170 . The switch  170  switches, under the control from the modulation switching unit  180 , a first path  171  for outputting the low-frequency signal output from the oscillator  140  to the driving unit  130  and a second path  172  for outputting the low-frequency signal output from the oscillator  140  to the bias supply unit  162  in the bias control unit  160 . 
         [0055]    The modulation switching unit  180  (controller) obtains the modulation switching information (modulation format information changing information) by an input from a user. The modulation switching unit  180  controls the voltage of the drive signal output from the driving unit  130  and the paths in the switch  170  in accordance with the obtained modulation switching information. Further, the modulation switching unit  180  outputs control information on the switching to the modulation matching the obtained modulation switching information to the bias-point holding unit  190 . 
         [0056]    Specifically, upon obtaining the modulation switching information indicating the switching from the NRZ intensity modulation to the DPSK, the modulation switching unit  180  controls, to 2Vπ, the voltage of the drive signal to be output from the driving unit  130  to the MZ modulator  120 , and switches the switch  170  to the second path  172 . As a consequence, the MZ modulator  120  is operated as a DPSK modulator. 
         [0057]    Further, upon obtaining the modulation switching information indicating the switching from the DPSK to the NRZ intensity modulation, the modulation switching unit  180  controls, to Vπ, the voltage of the drive signal to be output from the driving unit  130  to the MZ modulator  120 , and switches the switch  170  to the first path  171 . As a consequence, the MZ modulator  120  is operated as an NRZ intensity modulation. 
         [0058]    Furthermore, upon obtaining the modulation switching information indicating the switching from the NRZ intensity modulation to the DPSK, the modulation switching unit  180  outputs the control information on the switching to the DPSK to the bias-point holding unit  190 . In addition, upon obtaining the modulation switching information indicating the switching from the DPSK to the NRZ intensity modulation, the modulation switching unit  180  outputs the control information on the switching to the NRZ intensity modulation to the bias-point holding unit  190 . 
         [0059]    The bias-point holding unit  190  holds the control information on the bias-point corresponding to the modulation output from the bias supply unit  162  in the bias control unit  160 . Further, the bias-point holding unit  190  outputs, to the bias supply unit  162  in the bias control unit  160 , the control information on the bias point in accordance with the modulation corresponding to the switching control information output from the modulation switching unit  180 . 
         [0060]      FIG. 2  is a diagram showing input/output characteristics (of DPSK format) of the MZ modulator. Referring to  FIG. 2 , reference numeral  210  ( 210   a  to  210   c ) denotes input/output characteristics of the MZ modulator  120 . In the input/output characteristics  210 , the abscissa denotes the voltage of the drive signal to be output from the driving unit  130  to the MZ modulator  120 , and the ordinate denotes the optical intensity of the signal light to be output from the MZ modulator  120 . 
         [0061]    As shown by the input/output characteristics  210 , the optical intensity of the signal light to be output from the MZ modulator  120  is periodic to the voltage of the drive signal. In the input/output characteristics  210 , Vπ is a voltage (half-wavelength voltage) for changing the optical intensity of the signal light to be output from the MZ modulator  120  by a half period, and 2Vπ is a voltage for changing the optical intensity of the signal light by one period. 
         [0062]    In the MZ modulator  120 , a substrate itself is polarized by the change in temperature of the basic material LiNbO 3 , applying an electrical field for a long time, and the change in ages, and charges remain on the substrate surface. Therefore, the bias voltage changes between the optical waveguide  122   a  and the optical waveguide  122   b  in the MZ modulator  120 , thereby also changing the input/output characteristics  210  of the MZ modulator  120 . 
         [0063]    The input/output characteristics  210   a  represent ideal input/output characteristics of the MZ modulator  120 . The input/output characteristics  210   b  and input/output characteristics  210   c  represent input/output characteristics changed (obtained by drifting an operation point) from the ideal input/output characteristics  210   a  of the MZ modulator  120 . The input/output characteristics  210   b  specifically represent input/output characteristics obtained by drifting the operation point toward the negative direction. The input/output characteristics  210   c  specifically represent input/output characteristics obtained by drifting the operation point toward the positive direction. 
         [0064]    Reference numeral  220  denotes a voltage applied to the MZ modulator  120  as a result of outputting the drive signal and the bias voltage to the MZ modulator  120 . In the applied voltage  220 , the abscissa denotes a voltage applied to the MZ modulator  120 , corresponding to the abscissa of the input/output characteristics  210  of the MZ modulator  120 , and the ordinate denotes time. 
         [0065]    Upon obtaining the modulation switching information indicating the switching to the DPSK format, the modulation switching unit  180  controls, to 2Vπ, the voltage of the drive signal as mentioned above, and switches the switch  170  to the second path  172 , thereby superimposing the low-frequency signal of the frequency f 0  to the bias voltage. Reference numeral  221  denotes the voltage of the drive signal (2Vπ) output from the driving unit  130 . 
         [0066]    Reference numeral  222  denotes a central value (bias point) of the voltage applied to the MZ modulator  120 . The bias point  222  is controlled by a bias voltage output from the bias supply unit  162 . In the case of the DPSK format, the bias point  222  is set to a voltage as a valley of the input/output characteristics  210  (quenching state). 
         [0067]    The bias point  222  is set to the valley of the input/output characteristics  210 , and the voltage of the drive signal  221  is 2Vπ. Accordingly, when the drive signal indicates both “0” and “1”, the intensity of the signal light is “1” (light emission state). Herein, when the drive signal indicates “0” (interference at the phase difference of 0) and “1” (interference at the phase difference of 2π), the phases of the signal light output from the MZ modulator  120  are different from each other by π. 
         [0068]    Therefore, the signal light output from the MZ modulator  120  becomes DPSK signal light with the phase of 0 or π depending on the drive signal indicating “0” or “1”. Further, since the low-frequency signal of the frequency f 0  is superimposed to a bias electrode, the voltage  220  applied to the MZ modulator  120  changes by the frequency f 0 . 
         [0069]    Reference numeral  230  ( 230   a  to  230   c ) denotes signal light output from the MZ modulator  120  in the case of the DPSK. In the signal light  230 , the abscissa denotes time, and the ordinate denotes the intensity of the signal light output from the MZ modulator  120 . The signal light  230   a ,  230   b , and  230   c  denotes signal light when input/output characteristics of the MZ modulator  120  are respectively the input/output characteristics  210   a ,  210   b , and  210   c.    
         [0070]    When the input/output characteristics  210  of the MZ modulator  120  are the input/output characteristics  210   a , the voltage  220  applied to the MZ modulator  120  passes a peak portion (minimum or maximum light-intensity) of the input/output characteristics  210   a  each time when the applied voltage  220  changes by the frequency f 0 . Therefore, the change of the frequency f 0  of the applied voltage  220  is output as a change in frequency f 0 ×2 of the intensity of the signal light  230   a , and the signal light  230   a  does not include the component of the frequency f 0 . 
         [0071]    When the input/output characteristics  210  of the MZ modulator  120  are the input/output characteristics  210   b , the voltage  220  applied to the MZ modulator  120  changes by the frequency f 0  on the high-voltage side, rather than the peak portion of the input/output characteristics  210   b . Therefore, the change of the frequency f 0  of the applied voltage  220  is output as a change in frequency f 0  of the intensity of the signal light  230   b , and the signal light  230   b  includes the component of the frequency f 0 . 
         [0072]    When the input/output characteristics  210  of the MZ modulator  120  are the input/output characteristics  210   c , the voltage  220  applied to the MZ modulator  120  changes by the frequency f 0  on the low-voltage side, rather than the peak portion of the input/output characteristics  210   c . Therefore, the change in frequency f 0  of the applied voltage  220  is output as a change in frequency f 0  of the intensity of the signal light  230   c , and the signal light  230   c  includes the component of the frequency f 0 . 
         [0073]    When the signal light output from the MZ modulator  120  does not include the component of the frequency f 0 , it is determined that the input/output characteristics  210  are the input/output characteristics  210   a . In this case, the bias supply unit  162  in the bias control unit  160  keeps the bias voltage to be supplied to the MZ modulator  120 . 
         [0074]    Further, when the input/output characteristics  210  of the MZ modulator  120  are the input/output characteristics  210   b  and the input/output characteristics  210   c , the phases of the component of the frequency f 0  of the intensity of the signal light inverse each other. Therefore, when the signal light includes the component of the frequency f 0 , it is determined by using the phase of the component of the frequency f 0  whether the input/output characteristics  210  are the input/output characteristics  210   b  or the input/output characteristics  210   c.    
         [0075]    When the input/output characteristics  210  are the input/output characteristics  210   b , the bias supply unit  162  in the bias control unit  160  shifts the bias point  222  to the high-voltage side by increasing the bias voltage to be supplied to the MZ modulator  120 . When the input/output characteristics  210  are the input/output characteristics  210   c , the bias supply unit  162  in the bias control unit  160  shifts the bias point  222  to the low-voltage side by reducing the bias voltage to be supplied to the MZ modulator  120 . 
         [0076]      FIG. 3  is a diagram showing the input/output characteristics (of NRZ intensity modulation format) of the MZ modulator. Referring to  FIG. 3 , the same portions as those shown in  FIG. 2  are designated by the same reference numerals and a description thereof is omitted. Upon obtaining the modulation switching information indicating the switching to the NRZ intensity modulation, the modulation switching unit  180  controls, to Vπ, the voltage of the drive signal as mentioned above, and switches the switch  170  to the first path, thereby superimposing the low-frequency signal of the frequency f 0  to the drive signal. 
         [0077]    Reference numeral  321  denotes the voltage of the drive signal (Vπ) output from the driving unit  130 . In the NRZ intensity modulation, the bias point  222  is set to a voltage as the center between peak portions of the input/output characteristics  210 . Herein, the bias point  222  is set to the center between the peak portions of the input/output characteristics  210 , serving as a voltage having a positive differential value of the input/output characteristics  210 . 
         [0078]    Since the bias point  222  is set to the center between a valley (quenching state) and a peak (light emission state) of the input/output characteristics  210  of the input/output characteristics  210  and the voltage of the drive signal  221  is Vπ, when the drive signal indicates “0”, the intensity of the signal light is “0” (quenching state). Further, when the drive signal indicates “1”, the intensity of the signal light is “1” (light emission state). 
         [0079]    Therefore, depending on as whether the drive signal indicates “0” or “1”, the signal light output from the MZ modulator  120  becomes signal light in binary NRZ intensity modulation having the intensity of “0” or “1”. Further, the low-frequency signal of the frequency f 0  is superimposed to the drive signal. Thus, similarly to the case in which the driving unit  130  superimposes the low-frequency signal of the frequency f 0  to the drive signal, the voltage  220  applied to the MZ modulator  120  always changes by the frequency f 0 . 
         [0080]    Reference numeral  330  ( 330   a  to  330   c ) denotes signal light output from the MZ modulator  120  in the NRZ intensity modulation. The signal light  330   a ,  330   b , and  330   c  denotes signal light when the input/output characteristics  210  of the MZ modulator  120  are respectively the input/output characteristics  210   a ,  210   b , and  210   c.    
         [0081]    Similarly to the DPSK (refer to reference numeral  230  in  FIG. 2 ), when the input/output characteristics  210  are the input/output characteristics  210   a , the signal light  330   a  does not include the component of the frequency f 0 . When the input/output characteristics  210  are the input/output characteristics  210   b  or  210   c , the signal light  330   b  includes the component of the frequency f 0 . Further, depending on as whether the input/output characteristics  210  are the input/output characteristics  210   b  or the input/output characteristics  210   c , the phase of the component of the frequency f 0  of the intensity of the signal light inverses. 
         [0082]      FIG. 4  is a graph showing synchronous detecting characteristics of the bias control unit. Referring to  FIG. 4 , the abscissa denotes the difference between the bias voltage to be supplied to the MZ modulator  120  by the bias supply unit  162  and the best bias voltage when the input/output characteristics  210  become the input/output characteristics  210   a , and the ordinate denotes synchronous detecting characteristics output from the phase comparator  161 . 
         [0083]    Referring to  FIG. 4 , when the difference between the bias voltage to be supplied by the bias supply unit  162  and the best bias voltage is 0 and the input/output characteristics  210  thus become the input/output characteristics  210   a , the signal light (the signal light  230   a  and  330   a ) output from the MZ modulator  120  does not include the component of the frequency f 0 . Therefore, the synchronous detecting characteristics output from the phase comparator  161  are 0. 
         [0084]    Further, when the input/output characteristics  210  are the input/output characteristics  210   b  (−Δ%) since the bias voltage to be supplied by the bias supply unit  162  is much lower, the signal light (the signal light  230   b  and  330   b ) output from the MZ modulator  120  includes the component of the frequency f 0 , and the phase of the component of the frequency f 0  is inverse to a phase of the low-frequency signal output from the oscillator  140 . Therefore, the synchronous detecting characteristics output from the phase comparator  161  have a negative value. 
         [0085]    Further, when the input/output characteristics  210  are the input/output characteristics  210   c  (+Δ%) since the bias voltage to be supplied by the bias supply unit  162  is much higher, the signal light (the signal light  230   c  and  330   c ) output from the MZ modulator  120  includes the component of the frequency f 0 , and the phase of the component of the frequency f 0  has the same phase as that of the low-frequency signal output from the oscillator  140 . Therefore, the synchronous detecting characteristics output from the phase comparator  161  have a positive value. 
         [0086]    The bias supply unit  162  in the bias control unit  160  controls the bias voltage to be supplied to the MZ modulator  120 , depending on the synchronous detecting characteristics output from the phase comparator  161 . Specifically, when the synchronous detecting characteristics are 0, the bias supply unit  162  keeps the bias voltage to be supplied to the MZ modulator  120 . 
         [0087]    Further, when the synchronous detecting characteristics have the negative value, the bias supply unit  162  increases the bias voltage to be supplied to the MZ modulator  120 , thereby shifting the bias point  222  to the high-voltage side. As a consequence, the input/output characteristics  210  of the MZ modulator  120  are controlled from the input/output characteristics  210   b  to the input/output characteristics  210   a.    
         [0088]    Furthermore, when the synchronous detecting characteristics have the positive value, the bias supply unit  162  reduces the bias voltage to be supplied to the MZ modulator  120 , thereby shifting the bias point  222  to the low-voltage side. As a consequence, the input/output characteristics  210  of the MZ modulator  120  are controlled from the input/output characteristics  210   c  to the input/output characteristics  210   a.    
         [0089]      FIG. 5  is a diagram showing bias points of the MZ modulator (in DPSK format and NRZ intensity modulation format). Referring to  FIG. 5 , in the DPSK, the bias point  222  is set to a voltage as a valley (quenching state) of the input/output characteristics  210 . In the NRZ intensity modulation, the bias point  222  is at the center between the valley (quenching state) and the peak (light emission state) of the input/output characteristics  210 , and is set to a value having a differential value of the input/output characteristics  210  as the positive value. 
         [0090]    Upon switching the NRZ intensity modulation to the DPSK, the control information on the bias point corresponding to the modulation held by the bias-point holding unit  190  includes information for controlling the bias voltage to be supplied to the MZ modulator  120  in the negative direction. Further, upon switching the DPSK to the NRZ intensity modulation, the control information on the bias point corresponding to the modulation held by the bias-point holding unit  190  includes information for controlling the bias voltage to be supplied to the MZ modulator  120  in the positive direction. 
         [0091]    The control information on the bias point corresponding to the modulation held by the bias-point holding unit  190  may be information for every modulation of the bias voltage value when the bias point  222  is the best one. The information for every modulation of the bias voltage value indicates a value of the bias voltage to be supplied to the MZ modulator  120  upon switching the modulation to the DPSK and upon switching the modulation to the NRZ intensity modulation. 
         [0092]    Further, the information for every modulation of the bias voltage value includes information indicating that the bias voltage to be supplied to the MZ modulator  120  is reduced by Vπ/4 in the switching from the NRZ intensity modulation to the DPSK, and information indicating that the bias voltage to be supplied to the MZ modulator  120  is increased by Vπ/4 in the switching from the DPSK to the NRZ intensity modulation. 
         [0093]    When the control information on the bias point corresponding to the modulation is the information for every modulation of the bias voltage value, the bias supply unit  162  controls the bias voltage on the basis of the information for every modulation of the bias voltage value, thereby setting the bias point  222  corresponding to the switching of the modulation. Thereafter, the bias supply unit  162  controls the bias voltage on the basis of the component of the frequency f 0  output from the phase comparator  161 , thereby setting the bias point  222  corresponding to the change in input/output characteristics  210  of the MZ modulator  120 . 
         [0094]    In the NRZ intensity modulation, the bias point  222  is set to a voltage so that the differential value of the input/output characteristics  210  is positive, as mentioned above. Alternatively, in the NRZ intensity modulation, the bias point  222  may be set to a voltage so that the differential value of the input/output characteristics  210  is negative, as shown by reference numeral  501 . 
         [0095]    In this case, the change in intensity of the signal light inverses in response to the drive signal. Specifically, when the drive signal indicates “0”, the intensity of the signal light is “1” (light emission state). Further, when the drive signal indicates “1”, the intensity of the signal light is “0” (quenching state). In this case, the signal light output from the MZ modulator  120  is binary signal light of the NRZ intensity modulation. 
         [0096]      FIG. 6  is a flowchart showing an example of the operation of the optical modulator according to the first embodiment. Referring to  FIG. 6 , the modulation switching unit  180  is in a standby mode until obtaining the modulation switching information (in a loop of No in step S 601 ). After obtaining the modulation switching information (Yes in step S 601 ), it is determined whether or not the obtained modulation switching information is modulation switching information indicating the switching to the DPSK (in step S 602 ). 
         [0097]    When it is determined in step S 602  that the obtained modulation switching information is the modulation switching information indicating the switching to the DPSK (Yes in step S 602 ), the modulation switching unit  180  controls, to 2Vπ, the voltage of the drive signal output from the driving unit  130  to the MZ modulator  120 , and switches the switch  170  to the second path  172  (in step S 603 ). Then, the processing shifts to step S 605 . 
         [0098]    When it is determined in step S 602  that the obtained modulation switching information is not the modulation switching information indicating the switching to the DPSK (No in step S 602 ), the modulation switching unit  180  determines that the obtained modulation switching information is the modulation switching information indicating the switching to the NRZ intensity modulation, controls, to Vπ, the voltage of the drive signal output from the driving unit  130  to the MZ modulator  120 , and switches the switch  170  to the first path  171  (in step S 604 ). Then, the processing shifts to step S 605 . 
         [0099]    Subsequently, the modulation switching unit  180  outputs, to the bias-point holding unit  190 , the control information on the switching to the modulation corresponding to the modulation switching information determined in step S 602 . Further, the bias-point holding unit  190  determines whether or not to hold the control information on the bias point corresponding to the modulation indicated by the switching control information output from the modulation switching unit  180  (in step S 605 ). 
         [0100]    When it is determined in step S 605  that the control information on the bias point corresponding to the modulation is not held (No in step S 605 ), the bias-point holding unit  190  outputs, to the bias supply unit  162 , information indicating that the control information on the bias point is not held. Further, the bias supply unit  162  controls the bias voltage on the basis of the component of the frequency f 0  output from the phase comparator  161  (in step S 606 ). 
         [0101]    The bias supply unit  162  repeats step S 606  until the intensity of the component of the frequency f 0  output from the phase comparator  161  is minimum (No in step S 607  and in a loop of step S 606 ). When the intensity of the component of the frequency f 0  output from the phase comparator  161  is minimum, (Yes in step S 607 ), the processing shifts to step S 610 . 
         [0102]    When it is determined in step S 605  that the control information on the bias point corresponding to the modulation is held (Yes in step S 605 ), the bias-point holding unit  190  outputs the control information on the bias point to the bias supply unit  162 . Further, the bias supply unit  162  controls the bias voltage on the basis of the control information on the bias point output from the bias-point holding unit  190  and the component of the frequency f 0  output from the phase comparator  161  (in step S 608 ). 
         [0103]    The bias supply unit  162  repeats step S 608  until the intensity of the component of the frequency f 0  output from the phase comparator  161  is minimum (No in step S 609  and in a loop of step S 608 ). When the intensity of the component of the frequency f 0  output from the phase comparator  161  is minimum (Yes in step S 609 ), the processing shifts to step S 610 . 
         [0104]    Subsequently, the bias supply unit  162  outputs, to the bias-point holding unit  190 , the control information on the bias point corresponding to the modulation. Further, the bias-point holding unit  190  holds the control information on the bias point output from the bias supply unit  162  (in step S 610 ). Subsequently, it is determined whether or not an end condition of a series of operations is satisfied (in step S 611 ). When the end condition is not satisfied (No in step S 611 ), the processing shifts to step S 601 . When the end condition is satisfied (Yes in step S 611 ), a series of operations end. 
         [0105]    With the optical modulation device  100  according to the first embodiment as mentioned above, the drive signal and the path of the switch for the MZ modulator  120  are switched, thereby switching the DPSK and the NRZ intensity modulation. Therefore, even if changing the transmission condition of the optical communication system, the modulation can be switched to a modulation by which transmission characteristics do not deteriorate against the changed transmission condition. 
         [0106]    Further, with the optical modulation device  100  according to the first embodiment, one MZ modulator  120  can switch the DPSK and the NRZ intensity modulation. Therefore, it is possible to decrease the size of the apparatus, simplify the apparatus, and reduce costs without arranging a plurality of modulators corresponding to modulations in order to change the modulation. 
         [0107]    Furthermore, with the optical modulation device  100  according to the first embodiment, the modulation can be matched to the optical communication apparatus as the communication destination. Therefore, the optical transmission is possible with the optical communication apparatus using different modulations. Moreover, the modulation switching information is input, thereby automatically and immediately the modulation. Therefore, it is possible to flexibly cope with the optical communication system in which the transmission condition frequently changes. 
         [0108]    In addition, with the optical modulation device  100  according to the first embodiment, the bias-point holding unit  190  holds the control information on the bias point corresponding to the modulation, thereby efficiently controlling the bias voltage with the bias supply unit  162  in the switching of the modulation. Therefore, it is possible to reduce the time from the switching of the modulation to a stable state of transmission characteristics of the signal light. 
       Second Embodiment 
       [0109]      FIG. 7  is a block diagram showing the structure of an optical modulator according to the second embodiment. Referring to  FIG. 7 , the same component as that shown in  FIG. 1  is designated by the same reference numeral and a description thereof is omitted. An optical modulation device  100  according to the second embodiment can switch RZ modulation format (e.g., duty ratio is 50%) and non-modulation format in accordance with the modulation switching information. As an example, a description will be given of the optical modulation device  100  that can switch RZ-DQPSK and DQPSK. 
         [0110]    Referring to  FIG. 7 , the optical modulation device  100  according to the second embodiment comprises: a MZ modulator  700 ; a driving unit  740 A; a driving unit  740 B; a branch unit  750 ; a light receiving unit  760 ; and a bias supply unit  770 , in addition to the structure of the optical modulation device  100  according to the first embodiment. The MZ modulator  700  is a DQPSK modulator that performs differential quadrature phase shift keying. Although the driving unit performs differential operation of one driver, the modulator may be driven by two drivers. 
         [0111]    The light source  110  generates continuous light, and outputs the generated light to the MZ modulator  700 . The MZ modulator  700  comprises: a branch unit  710 ; an I arm  720 A (second MZ modulator); a Q arm  720 B (third MZ modulator); and a coupling unit  730 . The branch unit  710  branches the continuous light output from the light source  110 , and outputs one branched piece of the continuous light to the I arm  720 A and further outputs the other piece of continuous light to the Q arm  720 B. 
         [0112]    The I arm  720 A performs binary phase modulation of light passing through the I arm  720 A in accordance with the drive signal output from the driving unit  740 A. The I arm  720 A comprises: a branch unit  721 A; an optical waveguide  722 Aa; an optical waveguide  722 Ab; and a coupling unit  723 A. The branch unit  721 A branches the continuous light output from the branch unit  710 , and outputs one branched piece of the continuous light to the optical waveguide  722 Aa and further outputs the other piece of the continuous light to the optical waveguide  722 Ab. 
         [0113]    The optical waveguide  722 Aa comprises a phase modulating section  724 Aa. The phase modulating section  724 Aa performs phase modulation of light passing through the optical waveguide  722 Aa in accordance with the drive signal output from the driving unit  740 A. The optical waveguide  722 Ab comprises: a phase modulating section  724 Ab; and a phase modulating section  725 A. The phase modulating section  724 Ab performs phase modulation of light passing through the optical waveguide  722 Ab in accordance with the drive signal output from the driving unit  740 A. 
         [0114]    The phase modulating section  725 A performs phase modulation of light passing through the optical waveguide  722 Ab in accordance with the bias voltage supplied from the bias supply unit  770 . The coupling unit  723 A couples the light passing through the optical waveguide  722 Aa and the light passing through the optical waveguide  722 Ab, and outputs the coupled light to the coupling unit  730 . The signal light output from the coupling unit  723 A through the I arm  720 A becomes a binary (0, π) differential phase modulation signal. 
         [0115]    The Q arm  720 B performs binary phase modulation of the light passing through the Q arm  720 B in accordance with the drive signal output from the driving unit  740 B. The Q arm  720 B comprises: a branch unit  721 B; an optical waveguide  722 Ba; an optical waveguide  722 Bb; a coupling unit  723 B; and a π/2 delay unit  726 . The branch unit  721 B branches the continuous light output from the branch unit  710 , and output one branched piece of the continuous light to the optical waveguide  722 Ba and further outputs the other branched piece of the continuous light to the optical waveguide  722 Bb. 
         [0116]    The optical waveguide  722 Ba comprises a phase modulating section  724 Ba. The phase modulating section  724 Ba performs phase modulation of the light passing through the optical waveguide  722 Ba in accordance with the drive signal output from the driving unit  740 B. The optical waveguide  722 Bb comprises: a phase modulating section  724 Bb; and a phase modulating section  725 B. The phase modulating section  724 Bb performs phase modulation of the light passing through the optical waveguide  722 Bb in accordance with the drive signal output from the driving unit  740 B. 
         [0117]    The phase modulating section  725 B performs phase modulation of the light passing through the optical waveguide  722 Bb in accordance with the bias voltage supplied from the bias supply unit  770 . The coupling unit  723 B couples (interferes) the light passing through the optical waveguide  722 Ba and the light passing through the optical waveguide  722 Bb, and outputs the coupled light to the π/2 delay unit  726 . The π/2 delay unit  726  delays, by π/2, the light output from the coupling unit  723 B on the basis of the bias voltage supplied from the bias supply unit  770 , and outputs the delayed signal to the coupling unit  730 . 
         [0118]    The signal light output from the π/2 delay unit  726 , passing through the Q arm  720 B, becomes a binary (π/2, 3π/2) differential phase modulation signal, with the phase shifted by π/2 from the phase of the signal light passing through the I arm  720 A. The coupling unit  730  couples (interferes) the light passing through the I arm  720 A and the light passing through the Q arm  720 B, and outputs the coupled light to the branch unit  750 . The signal light output from the coupling unit  730  becomes quadrature (0, π/2, π, 3/2π) signal light subjected to the DQPSK. 
         [0119]    The driving unit  740 A inputs a data signal (DATA_A), and outputs the input data signal as the drive signal to the I arm  720 A in the MZ modulator  700 . Further, the driving unit  740 A controls, to 2Vπ, the voltage of the drive signal to be output to the I arm  720 A. For example, the driving unit  740 A performs push-pull modulation for always outputting the drive signals (voltage Vπ) of inverse signs, of the phase modulating section  724 Aa and the phase modulating section  724 Ab in the I arm  720 A. 
         [0120]    The driving unit  740 B inputs a data signal (DATA_B), and outputs the input data signal as the drive signal to the Q arm  720 B in the MZ modulator  700 . Further, the driving unit  740 B controls, to 2Vπ, the voltage of the drive signal to be output to the Q arm  720 B. For example, the driving unit  740 B performs push-pull modulation for always outputting the drive signals (voltage Vπ) of inverse signs, of the phase modulating section  724 Ba and the phase modulating section  724 Bb in the Q arm  720 B. 
         [0121]    The branch unit  750  branches the signal light subjected to the DQPSK output from the MZ modulator  700 , and outputs one branched piece of the signal light subjected to the DQPSK to the MZ modulator  120  and further outputs the other branched piece of the signal light subjected to the DQPSK to the light receiving unit  760 . The light receiving unit  760  converts the DQPSK signal output from the branch unit  750  into the electrical signal. 
         [0122]    The bias supply unit  770  supplies the bias voltage to the MZ modulator  700 . Although not shown, the MZ modulator  700  may superimpose the low-frequency signal to the drive signal and may perform synchronous detection of the DQPSK signal output from the MZ modulator  700 . 
         [0123]    In this case, with the same structure as that of the bias control unit  160  as mentioned above, the bias supply unit  770  supplies, to the phase modulating section  725 A, the phase modulating section  725 B, and the π/2 delay unit  726  in the MZ modulator  700 , the bias voltage corresponding to the component of the frequency f 0  included in the electrical signal converted by the light receiving unit  760 . 
         [0124]    The MZ modulator  120  performs RZ modulation of the signal light subjected to the DQPSK output from the branch unit  750 . Further, the MZ modulator  120  switches the RZ modulation/non-modulation in accordance with the modulation switching information. The driving unit  130  inputs a clock signal (CLOCK), and outputs the input clock signal as the drive signal to the MZ modulator  120 . 
         [0125]    Further, the driving unit  130  controls, to Vπ or OFF, the voltage of the drive signal to be output to the MZ modulator  120  under the control of the modulation switching unit  180 . For example, the driving unit  130  performs push-pull modulation for outputting clock signals (with a voltage Vπ/2) with inverse phases, of non-inverse clock signals to the phase modulating section  123   a  and the phase modulating section  123   b  in the MZ modulator  120 . 
         [0126]    Upon obtaining the modulation switching information indicating the switching from the DQPSK to the RZ-DQPSK, the modulation switching unit  180  controls, to Vπ, the voltage of the drive signal to be output from the driving unit  130  to the MZ modulator  120 , and switches the switch  170  to the first path  171 . As a consequence, the MZ modulator  120  is operated as an RZ modulator. Therefore, the signal light output from the MZ modulator  120  becomes the signal light subjected to the RZ-DQPSK. 
         [0127]    Further, upon obtaining the modulation switching information indicating the switching from the RZ-DQPSK to the DQPSK, the modulation switching unit  180  controls, to OFF, the voltage of the drive signal to be output from the driving unit  130  to the MZ modulator  120 , and switches the switch  170  to the second path  172 . As a consequence, the function of the MZ modulator  120  is reset as the RZ modulator. Therefore, the signal light output from the MZ modulator  120  becomes the signal light subjected to the DQPSK. 
         [0128]    Furthermore, upon obtaining the modulation switching information indicating the switching from the DQPSK to the RZ-DQPSK, the modulation switching unit  180  outputs the control information on the switching to the RZ-DQPSK to the bias-point holding unit  190 . In addition, obtaining the modulation switching information indicating the switching from the RZ-DQPSK to the DQPSK, the modulation switching unit  180  outputs the control information on the switching to the DQPSK to the bias-point holding unit  190 . 
         [0129]      FIG. 8  is a diagram showing input/output characteristics (of DQPSK format) of the MZ modulator. Referring to  FIG. 8 , the same components as those shown in  FIG. 2  are designated by the same reference numerals, and a description thereof is omitted. In the DQPSK, the modulation switching unit  180  controls the voltage of the drive signal to OFF, switches the switch  170  to the second path  172 , and superimposes the low-frequency signal of the frequency f 0  to the bias voltage. 
         [0130]    Further, in the DQPSK, the bias point  222  is set to a voltage as a peak (light emission state) of the input/output characteristics  210 . Since the bias point  222  is set to the voltage as the peak of the input/output characteristics  210  and the voltage of the drive signal is OFF, the intensity of the signal light is always “1” (light emission state). 
         [0131]    Therefore, the signal light subjected to the DQPSK output to the MZ modulator  120  is not modulated by the MZ modulator  120 , and is still output as the signal light subjected to the DQPSK. Further, since the low-frequency signal of the frequency f 0  is superimposed to the bias voltage, the voltage applied to the MZ modulator  120  is always changed by the frequency f 0 , similarly to the case in which the driving unit  130  superimposes the low-frequency signal of the frequency f 0  to the drive signal. 
         [0132]    In the DQPSK, reference numeral  830  ( 830   a  to  830   c ) denotes signal light output from the MZ modulator  120 . The signal light  830   a ,  830   b , and  830   c  denotes signal light when the input/output characteristics  210  of the MZ modulator  120  are individually the input/output characteristics  210   a ,  210   b , and  210   c.    
         [0133]    Similarly to the DPSK (refer to reference numeral  230  in  FIG. 2 ), when the input/output characteristics  210  are the input/output characteristics  210   a , the signal light  830   a  does not include the component of the frequency f 0 . When the input/output characteristics  210  are the input/output characteristics  210   b , the signal light  830   b  includes the component of the frequency f 0 . When the input/output characteristics  210  are the input/output characteristics  210   c , the signal light  830   c  includes the component of the frequency f 0 . 
         [0134]    The input/output characteristics (of RZ-DQPSK format) of the MZ modulator  120  are the same as the input/output characteristics (of NRZ intensity modulation format) of the MZ modulator  120  shown in  FIG. 3  and therefore are not shown. In the RZ-DQPSK, the driving unit  130  outputs the clock signal as the drive signal to the MZ modulator  120 . Hence, the signal light subjected to the DQPSK output to the MZ modulator  120  is RZ-modulated in accordance with the clock signal and is output as signal light subjected to the RZ-DQPSK. 
         [0135]      FIG. 9  is a diagram showing bias points of the MZ modulator (in RZ-DQPSK format and DQPSK format). Referring to  FIG. 9 , in the DQPSK, the bias point  222  of the MZ modulator  120  is set to a voltage as a peak of the input/output characteristics  210  (light emission state). In the RZ-DQPSK, the bias point  222  is at the center between the valley (quenching state) and the peak (light emission state) of the input/output characteristics  210 , and is set to a voltage so that the differential value of the input/output characteristics  210  is positive. 
         [0136]    The bias-point holding unit  190  may hold information on voltage values of the bias point  222  for every modulation. When the bias supply unit  162  switches the RZ-DQPSK to the DPSK on the basis of the information, the bias voltage is increased by Vπ/4 and the bias voltage is thereafter controlled on the basis of the component of the frequency f 0  output from the phase comparator  161 . 
         [0137]    Although the RZ-DQPSK and the DQPSK can be switched as a structure for switching the RZ modulation and non-modulation by the optical modulation device  100 , the structure for switching the RZ modulation and the non-modulation is not limited to this. In place of the MZ modulator  700 , when an NRZ intensity modulator is provided, the optical modulation device  100  can switch the RZ intensity modulation and the NRZ intensity modulation. 
         [0138]    Further, in place of the MZ modulator  700 , when a phase modulator for binary phase modulation is provided, the optical modulation device  100  can switch the RZ-DPSK and the DPSK. In addition, in place of the MZ modulator  700 , various modulators can be provided. 
         [0139]    Furthermore, the MZ modulator  700  is arranged at the before-stage of the MZ modulator  120 . However, the MZ modulator  700  may be arranged at the after-stage of the MZ modulator  120 . In this case, the MZ modulator  120  RZ-modulates the continuous light output from the light source  110 . Moreover, the MZ modulator  700  performs the RZ-DQPSK of RZ pulses obtained by RZ-modulation with the MZ modulator  120 . 
         [0140]    As mentioned above, with the optical modulation device  100  according to the second embodiment, the drive signal and the path of the switch for the MZ modulator  120  are switched, thereby switching the RZ modulation and the non-modulation. Therefore, even if changing the transmission condition of the optical communication system, the modulation can be switched to modulation in which transmission characteristics do not deteriorate against the changed transmission condition. 
         [0141]    In addition, with the optical modulation device  100  according to the second embodiment, similarly to the optical modulation device  100  according to the first embodiment, the size of the apparatus is reduced, the apparatus is simplified, and costs are reduced. Moreover, it is possible to flexibly cope with the optical communication system in which the transmission condition frequently changes and also to reduce the time from changing the modulation to stabilizing the transmission characteristics of the signal light. 
       Third Embodiment 
       [0142]      FIG. 10A  is a block diagram showing the structure of a optical modulator according to the third embodiment. Referring to  FIG. 10A , the same components as those shown in  FIG. 7  are designated by the same reference numerals and a description thereof is omitted. An optical modulation device  100  according to the third embodiment can switch the RZ modulation format and CSRZ modulation format in accordance with the modulation switching information. As an example, a description will be given of the structure in which the optical modulation device  100  can switch the RZ-DQPSK and the CSRZ-DQPSK. 
         [0143]    The MZ modulator  120  performs the RZ modulation or the CSRZ modulation of the signal light subjected to the DQPSK output from the branch unit  750 . Further, the MZ modulator  120  switches the RZ modulation/CSRZ modulation in accordance with the modulation switching information. Referring to  FIG. 10A , the optical modulation device  100  according to the third embodiment comprises a frequency converting unit  1010  in addition to the optical modulation device  100  according to the second embodiment. 
         [0144]    The frequency converting unit  1010  inputs the clock signal (CLOCK), converts a frequency of the input clock signal into a frequency Br/2 as half of a frequency Br corresponding to the RZ modulation. The frequency converting unit  1010  switches the frequency of the clock signal to the frequency Br or Br/2 under the control of the modulation switching unit  180 . The frequency converting unit  1010  outputs, to the driving unit  130 , the clock signal whose frequency is converted. 
         [0145]    The driving unit  130  outputs, to the MZ modulator  120 , the clock signal output from the frequency converting unit  1010  as the drive signal. Further, the driving unit  130  controls, to Vπ or 2Vπ, the voltage of the drive signal output to the MZ modulator  120  under the control of the modulation switching unit  180 . 
         [0146]    Upon obtaining the modulation switching information indicating the switching from the CSRZ-DQPSK to the RZ-DQPSK, the modulation switching unit  180  controls, to Vπ, the voltage of the drive signal output from the driving unit  130  to the MZ modulator  120 , switches the switch  170  to the first path  171 , and further switches the frequency of the clock signal to Br. As a consequence, the MZ modulator  120  is operated as an RZ modulator. Therefore, the signal light output from the MZ modulator  120  becomes the signal light subjected to the RZ-DQPSK. 
         [0147]    Further, upon obtaining the modulation switching information indicating the switching from the RZ-DQPSK to the CSRZ-DQPSK, the modulation switching unit  180  controls, to 2Vπ, the voltage of the drive signal output from the driving unit  130  to the MZ modulator  120 , switches the switch  170  to the second path  172 , and further switches the frequency of the clock signal to Br/2. As a consequence, the MZ modulator  120  is operated as a CSRZ modulator. Therefore, the signal light output from the MZ modulator  120  becomes the signal light subjected to the CSRZ-DQPSK. 
         [0148]    Furthermore, upon obtaining the modulation switching information indicating the switching from the CSRZ-DQPSK to the RZ-DQPSK, the modulation switching unit  180  outputs control information on the switching to the RZ-DQPSK to the bias-point holding unit  190 . In addition, upon obtaining the modulation switching information indicating the switching from the RZ-DQPSK to the CSRZ-DQPSK, the modulation switching unit  180  outputs control information on the switching to the CSRZ-DQPSK to the bias-point holding unit  190 . 
         [0149]      FIG. 10B  is a diagram showing various RZ modulations with the MZ modulator. In  FIG. 10A , the optical modulation device  100  can switch 50%-RZ modulation (duty ratio 50%) and the CSRZ modulation. However, the optical modulation device  100  can switch 33%-RZ modulation. 
         [0150]    Referring to  FIG. 10B , a light source  1051  (Laser) corresponds to the above-mentioned light source  110 . A MZ modulator  1052  (MZM) corresponds to the above-mentioned MZ modulator  700 . A MZ modulator  1053  (MZM) corresponds to the above-mentioned MZ modulator  120  (specifically, refer to JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 1, JANUARY 2005). 
         [0151]    The 33%-RZ modulation is an RZ modulation having the duty ratio of approximately 33%. Further, the CSRZ modulation is an RZ modulation having the duty ratio of approximately 67%. Referring to A of  FIG. 10B , the signal light subjected to the RZ modulation has intensities differing depending on the duty ratios. For example, the signal light subjected to the CSRZ modulation has low strength and the signal light subjected to the 33%-RZ modulation has high strength. 
         [0152]    As mentioned above, upon obtaining the modulation switching information indicating the switching to 50%-RZ modulation, the modulation switching unit  180  controls, to Vπ, a voltage of the drive signal output from the driving unit  130  to the MZ modulator  120 , switches the switch  170  to the first path  171 , and further switches the frequency of the clock signal to Br. 
         [0153]    On the other hand, upon obtaining the modulation switching information indicating the switching to 33%-RZ modulation, the modulation switching unit  180  controls, to Vπ, the voltage of the drive signal output from the driving unit  130  to the MZ modulator  120 , switches the switch  170  to the second path  172 , and further switches the frequency of the clock signal to Br. 
         [0154]      FIG. 11  is a diagram showing bias points of the MZ modulator (in RZ-DQPSK format and CZ-DQPSK format). Referring to  FIG. 11 , in 50% RZ-DQPSK, the bias point  222  of the MZ modulator  120  is at the center of the valley (quenching state) and the peak (light emission state) of the input/output characteristics  210 , and is set to a voltage so that the differential value of the input/output characteristics  210  is positive. In the CSRZ-DQPSK, the bias point  222  is set to a voltage having the valley (quenching state) of the input/output characteristics  210 . In the 33% RZ-DQPSK, the bias point  222  is set to a voltage having the peak (light emission state) of the input/output characteristics  210 . 
         [0155]    Although the optical modulation device  100  can switch the RZ-DQPSK and the CSRZ-DQPSK as the structure for switching the RZ modulation and the CSRZ modulation, the example of the structure for switching the RZ modulation and the CSRZ modulation is not limited to this. For example, in place of the MZ modulator  700 , when an NRZ intensity modulator is provided, the optical modulation device  100  can switch the RZ intensity modulation and the CSRZ intensity modulation. 
         [0156]    Further, in place of the MZ modulator  700 , when a phase modulator for the binary phase modulation is provided, the optical modulation device  100  can switch the RZ-DPSK and the DSRZ-DPSK. In addition, in place of the MZ modulator  700 , various modulators can be provided. 
         [0157]    As mentioned above, in the optical modulation device  100  according to the third embodiment, the drive signal and the path of the switch of the MZ modulator  120  are switched, thereby switching the RZ modulation and the CSRZ modulation. Therefore, even if changing the transmission condition of the optical communication system, the modulation can be changed to a modulation in which the transmission characteristics do not deteriorate against the changed transmission condition. 
         [0158]    Further, with the optical modulation device  100  according to the third embodiment, similarly to the optical modulation device  100  according to the first embodiment, the size of the apparatus is reduced, the apparatus is simplified, and costs are reduced. Moreover, it is possible to flexibly cope with the optical communication system in which the transmission condition frequently changes and also to reduce the time from changing the modulation to stabilizing the transmission characteristics of the signal light. 
       Fourth Embodiment 
       [0159]      FIG. 12  is a block diagram showing the structure of an optical modulator according to the fourth embodiment. Referring to  FIG. 12 , the same components as those shown in  FIG. 7  are designated by the same reference numerals and a description thereof is omitted. An optical modulation device  100  according to the fourth embodiment can switch the DQPSK format and the DPSK format in accordance with the modulation switching information. As an example, a description will be given of an operation for switching the RZ-DQPSK and the DPSK by the optical modulation device  100 . 
         [0160]    Referring to  FIG. 12 , the optical modulation device  100  according to the fourth embodiment comprises a data processing unit  1210  (data generating means) in addition to the structure of the optical modulation device  100  according to the second embodiment. The MZ modulator  700  performs the DQPSK or the DPSK of the continuous light output from the light source  110 . Further, the MZ modulator  120  switches the DQPSK/DPSK in accordance with the modulation switching information. 
         [0161]    The data processing unit  1210  generates quadrature differential sign data (DATA_A and DATA_B) on the basis of the transmission data under the control of the modulation switching unit  180 . In this case, the data processing unit  1210  outputs the DATA_A to the driving unit  740 A, and further outputs the DATA_B to the driving unit  740 B. 
         [0162]    Further, the data processing unit  1210  generates binary differential sign data (DATA_A&amp;B) on the basis of the transmission data under the control of the modulation switching unit  180 . In this case, the data processing unit  1210  outputs the DATA_A&amp;B to the driving unit  740 A and does not output the data to the driving unit  740 B (OFF). 
         [0163]    The driving unit  740 A outputs, to the MZ modulator  700 , the DATA_A or DATA_A&amp;B output from the data processing unit  1210 , as the drive signal. The driving unit  740 B outputs, to the MZ modulator  700 , the DATA_B output from the data processing unit  1210 , as the drive signal. The driving unit  740 B further controls, to 2Vπ or OFF, the voltage of the drive signal to be output to the MZ modulator  700  under the control of the modulation switching unit  180 . 
         [0164]    Upon obtaining the modulation switching information indicating the switching from the DPSK to the RZ-DQPSK, the modulation switching unit  180  controls such an operation that the data processing unit  1210  generates the quadrature differential sign data (DATA_A and DATA_B), and further controls, to 2Vπ, the voltage of the drive signal to be output from the driving unit  740 B to the MZ modulator  700 . As a consequence, the MZ modulator  700  is operated as a DQPSK modulator. 
         [0165]    Further, the modulation switching unit  180  controls, to Vπ, the voltage of the drive signal to be output from the driving unit  130  to the MZ modulator  120  in this case, and switches the switch  170  to the first path  171 . As a consequence, the MZ modulator  120  is operated as an RZ modulator. Therefore, the signal light output from the MZ modulator  120  becomes the signal light subjected to the RZ-DQPSK. 
         [0166]    Further, upon obtaining the modulation switching information indicating the switching from the DQPSK to the DPSK, the modulation switching unit  180  controls an operation so that the data processing unit  1210  generates binary differential sign data (DATA_A&amp;B), and further controls, to OFF, the voltage of the drive signal to be output from the driving unit  740 B to the MZ modulator  700 . 
         [0167]    In this case, since the voltage of the drive signal output from the driving unit  740 B is OFF, the MZ modulator  700  does not perform the binary phase modulation using the Q arm  720 B, but performs the binary phase modulation using the I arm  720 A. As a consequence, the MZ modulator  700  is operated as a DPSK modulator. 
         [0168]    When the operation is performed so that the data processing unit  1210  generates the binary differential sign data (DATA_A&amp;B), the data is not transmitted from the data processing unit  1210  to the driving unit  740 B. Therefore, it is possible to omit the control operation for switching-OFF the voltage of the drive signal to be output from the driving unit  740 B to the MZ modulator  700 . 
         [0169]    Further, the modulation switching unit  180  controls, to OFF, the voltage of the drive signal to be output from the driving unit  130  to the MZ modulator  120  in this case, and switches the switch  170  to the second path  172 . As a consequence, the function of the MZ modulator  120  is reset as the RZ modulator. Therefore, the signal light output from the MZ modulator  120  becomes the signal light subjected to the DQPSK. 
         [0170]    Upon obtaining the modulation switching information indicating the switching form DPSK to the RZ-DQPSK, the modulation switching unit  180  outputs control information on the switching to the RZ-DQPSK to the bias-point holding unit  190 . Further, upon obtaining the modulation switching information on the switching from the RZ-DQPSK to the DPSK, the modulation switching unit  180  outputs control information on the switching to the DPSK to the bias-point holding unit  190 . 
         [0171]      FIG. 13  is a diagram showing a bias point of the Q arm (in DQPSK format and DPSK format). Referring to  FIG. 13 , in both the DQPSK and the DPSK, the bias point  222  of the Q arm  720 B in the MZ modulator  700  is set to a voltage having the valley (quenching state) of the input/output characteristics  210 . 
         [0172]    Although the optical modulation device  100  switches the RZ-DQPSK and the DPSK as the example in which the optical modulation device  100  switches the DQPSK and the DPSK, the present invention is not limited to this. For example, the optical modulation device  100  can switch the DQPSK and the RZ-DPSK. 
         [0173]    Upon obtaining the modulation switching information indicating the switching from the RZ-DPSK to the DQPSK, the modulation switching unit  180  allows the MZ modulator  700  to be operated as a DQPSK modulator under the above-mentioned control, and the function of the MZ modulator  120  is reset as the RZ modulator. As a consequence, the signal light output from the MZ modulator  120  becomes the signal light subjected to the DQPSK. 
         [0174]    Further, upon obtaining the modulation switching information indicating the switching from the DQPSK to the RZ-DPSK, the modulation switching unit  180  allows the MZ modulator  700  to be operated as a DPSK modulator under the above-mentioned control, and the function of the MZ modulator  120  is reset as the RZ modulation. As a consequence, the signal light output from the MZ modulator  120  becomes RZ-DPSK signal light. 
         [0175]    Although the optical modulation device  100  switches the RZ-DQPSK and the DPSK as the example in which the optical modulation device  100  switches the DQPSK and the DPSK, the present invention is not limited to the switching of the DQPSK and the DPSK. 
         [0176]    For example, in place of the MZ modulator  120 , a multivalued intensity modulator is provided and the optical modulation device  100  can then switch multivalued modulation such as an octonary QAM (Quadrature Amplitude Modulation) and sixteen-valued QAM. Further, the MZ modulator  120  is not provided, and the optical modulation device  100  can thus switch the DQPSK and the DPSK. 
         [0177]    As mentioned above, the optical modulation device  100  according to the fourth embodiment switches the drive signal for the MZ modulator  700 , thereby switching the DQPSK and the DPSK. Therefore, even if changing the transmission condition of the optical communication system, it is possible to switch the modulation to a modulation by which the transmission characteristics do not deteriorate against the changed transmission condition. 
         [0178]    Further, with the optical modulation device  100  according to the fourth embodiment, similarly to the optical modulation device  100  according to the first embodiment, the size of the apparatus is decreased, the apparatus is simplified, and costs are reduced. Moreover, it is possible to flexibly cope with the optical communication system in which the transmission condition frequently changes and also to reduce the time from changing the modulation to stabilizing the transmission characteristics of the signal light. 
       Fifth Embodiment 
       [0179]      FIG. 14  is a block diagram showing the structure of a optical modulator according to the fifth embodiment. Referring to  FIG. 14 , the same components as those shown in  FIG. 12  are designated by the same reference numerals and a description thereof is omitted. A optical modulation device  100  according to the fifth embodiment can switch the DQPSK format and the NRZ intensity modulation format in accordance with the modulation switching information. 
         [0180]    The MZ modulator  700  performs the DQPSK or the NRZ intensity modulation of the continuous light output from the light source  110 . Further, the MZ modulator  700  switches the DQPSK/NRZ intensity modulation in accordance with the modulation switching information. The driving unit  740 A controls, to 2Vπ or Vπ, the voltage of the drive signal to be output to the MZ modulator  700  under the control of the modulation switching unit  180 . 
         [0181]    Upon obtaining the modulation switching information indicating the switching from the DPSK to the DQPSK, the modulation switching unit  180  controls an operation that the data processing unit  1210  generates quadrature differential sign data (DATA_A and DATA_B), and further controls, to 2Vπ, the voltage of the drive signal to be output from the driving unit  740 A and the driving unit  740 B to the MZ modulator  700 . As a consequence, the MZ modulator  700  is operated as a DQPSK modulator. 
         [0182]    Upon obtaining the modulation switching information indicating the switching from the DQPSK to the NRZ intensity modulation, the modulation switching unit  180  controls an operation so that the data processing unit  1210  generates binary differential sign data (DATA_A&amp;B), further controls, to 2Vπ, the voltage of the drive signal to be output from the driving unit  740 A to the MZ modulator  700 , and furthermore controls, to OFF, the voltage of the drive signal to be output from the driving unit  740 B to the MZ modulator  700 . 
         [0183]    In this case, since the voltage of the drive signal to be output from the driving unit  740 B is OFF, the Q arm  720 B does not perform the binary phase modulation in the MZ modulator  700 . Further, since the voltage of the drive signal to be output from the driving unit  740 A is Vπ, the I arm  720 A performs the binary NRZ intensity modulation. As a consequence, the MZ modulator  700  is operated as an NRZ intensity modulator. 
         [0184]    The operation is controlled that the data processing unit  1210  generates the binary differential sign data (DATA_A&amp;B) and then the data is not transmitted from the data processing unit  1210  to the driving unit  740 B. Therefore, it is possible to omit the control operation for switching-OFF the voltage of the drive signal to be output from the driving unit  740 B to the MZ modulator  700 . 
         [0185]    Further, upon obtaining the modulation switching information indicating the switching from the NRZ intensity modulation to the DQPSK, the modulation switching unit  180  outputs control information on the switching to the DQPSK to the bias-point holding unit  190 . Furthermore, upon obtaining the modulation switching information indicating the switching from the DQPSK to the NRZ intensity modulation, the modulation switching unit  180  outputs control information on the switching to the NRZ intensity modulation to the bias-point holding unit  190 . 
         [0186]      FIG. 15  is a diagram showing bias points of the I arm (in RZ-DQPSK format and NRZ intensity modulation format). Referring to  FIG. 15 , in the RZ-DQPSK, the bias point  222  of the I arm  720 A is set to a voltage having the valley (quenching state) of the input/output characteristics  210  in the MZ modulator  700 . In the NRZ intensity modulation, the bias point  222  of the I arm  720 A is at the center between the valley (quenching state) and the peak (light emission state) of the input/output characteristics  210 , and is set to a voltage for setting the differential value of the input/output characteristics  210  to the positive. 
         [0187]      FIG. 16  is a diagram showing the bias point of the Q arm (in RZ-DQPSK format and NRZ intensity modulation format). Referring to  FIG. 16 , in both the RZ-DQPSK and the NRZ intensity modulation, the bias point  222  of the Q arm  720 B in the MZ modulator  700  is set to a voltage having the valley (quenching state) of the input/output characteristics  210 . 
         [0188]    The description is given of the example in which the optical modulation device  100  switches the RZ-DQPSK and the NRZ intensity modulation as the example for switching the DQPSK and the NRZ intensity modulation. However, the operation for switching of the DQPSK and the DPSK is not limited to this. For example, the optical modulation device  100  can switch the DQPSK and the RZ intensity modulation. 
         [0189]    Upon obtaining the modulation switching information indicating the switching from the RZ intensity modulation to the DQPSK, the modulation switching unit  180  operates the MZ modulator  700  as a DQPSK modulator under the above-mentioned control, and the function of the MZ modulator  120  is reset as the RZ modulator. As a consequence, the signal light output from the MZ modulator  120  becomes the signal light subjected to the DQPSK. 
         [0190]    Further, upon obtaining the modulation switching information indicating the switching from the DQPSK to the RZ intensity modulation, the modulation switching unit  180  operates the MZ modulator  700  as an NRZ intensity modulator and the MZ modulator  120  as an RZ modulator under the above-mentioned control. As a consequence, the signal light output from the MZ modulator  120  becomes the signal light subjected to the RZ intensity modulation. 
         [0191]    The description is given of the example in which the optical modulation device  100  can switch the RZ-DQPSK and the NRZ intensity modulation as the example for switching the DQPSK and the NRZ intensity modulation. The example for switching the DQPSK and the DPSK is not limited to this. For example, the MZ modulator  120  is not provided, thereby switching the DQPSK and the NRZ intensity modulation by the optical modulation device  100 . 
         [0192]    As mentioned above, the optical modulation device  100  according to the fifth embodiment controls the data generation of the data processing unit  1210  and the drive signal for the MZ modulator  700 , thereby switching the DQPSK and the NRZ intensity modulation. Therefore, even if changing the transmission condition of the optical communication system, the modulation can be switched to a modulation in which transmission characteristics do not deteriorate against the changed transmission condition. 
         [0193]    Further, with the optical modulation device  100  according to the fifth embodiment, similarly to the optical modulation device  100  according to the first embodiment, it is possible to decrease the size of the apparatus, simplify the apparatus, and reduce costs without providing a plurality of modulators corresponding to modulations in order to change the modulation. Furthermore, it is possible to flexibly cope with the optical communication system in which the transmission condition frequently changes and also to reduce the time from changing the modulation to stabilizing the transmission characteristics of the signal light. 
       Sixth Embodiment 
       [0194]      FIG. 17A  is a block diagram showing the structure of an optical modulator according to the sixth embodiment. Referring to  FIG. 17A , the same components as those shown in  FIGS. 12 and 14  are designated by the same reference numerals, and a description thereof is omitted. An optical modulation device  100  according to the sixth embodiment can switch the DQPSK format and the duobinary modulation format in accordance with the modulation switching information. 
         [0195]    A description will be given of the structure in which the optical modulation device  100  can switch the RZ-DQPSK and the duobinary modulation as an example. The MZ modulator  700  performs the DQPSK or the duobinary modulation of the continuous light output from the light source  110 . Further, the MZ modulator  700  switches the DQPSK/duobinary modulation in accordance with the modulation switching information. 
         [0196]    Referring to  FIG. 17A , the optical modulation device  100  according to the sixth embodiment comprises a delay unit  1710 A and a delay unit  1710 B in addition to the optical modulation device  100  according to the fourth embodiment. Further, the optical modulator according to the sixth embodiment comprises a delay unit  1720  (first-phase control means) in addition to the π/2 delay unit  726  in the optical modulation device  100  according to the fourth embodiment. 
         [0197]    The data processing unit  1210  generates quadrature differential sign data (DATA_A and DATA_B) on the basis of transmission data under the control of the modulation switching unit  180 . In this case, the data processing unit  1210  outputs the DATA_A to the delay unit  1710 A and further outputs the DATA_B to the delay unit  1710 B. 
         [0198]    Further, the data processing unit  1210  generates binary differential sign data (DATA_A) on the basis of the transmission data under the control of the modulation switching unit  180 . In this case, the data processing unit  1210  outputs the DATA_A to the delay unit  1710 A and the delay unit  1710 B. The delay unit  1710 A and the delay unit  1710 B (second-phase control means) control the phase difference between the DATA_A output from the delay unit  1710 A and the DATA_A output from the delay unit  1710 B under the control of the modulation switching unit  180 . 
         [0199]    Specifically, the delay unit  1710 A delays the DATA_A output from the data processing unit  1210  under the control of the modulation switching unit  180 , and outputs the delay data to the driving unit  740 A. The delay unit  1710 B delays the DATA_B or DATA_A output from the data processing unit  1210  under the control of the modulation switching unit  180 , and outputs the delay data to the driving unit  740 B. 
         [0200]    Further, the delay unit  1710 A and the delay unit  1710 B control the amount of delay under the control of the modulation switching unit  180  so that the DATA_B output from the delay unit  1710 B has the same phase as the phase of the DATA_A output from the delay unit  1710 A or so that the DATA_B is delayed from the DATA_A by one bit. 
         [0201]    The driving unit  740 A outputs the DATA_A output from the delay unit  1710 A, as the drive signal, to the MZ modulator  120 . The driving unit  740 B outputs the DATA_B or DATA_A output from the delay unit  1710 B, as the drive signal, to the MZ modulator  120 . 
         [0202]    The delay unit  1720  controls the phase difference between the I arm  740 A and the I arm  740 B. Specifically, the delay unit  1720  delays, by π/2 or nπ (where n is integer), the phase of the light output from the coupling unit  723 B under the control of the modulation switching unit  180 , and outputs the delay phase to the coupling unit  730 . The driving unit  130  controls, to Vπ or OFF, the voltage of the drive signal to be output to the MZ modulator  120  under the control of the modulation switching unit  180 . 
         [0203]    Upon obtaining the modulation switching information indicating the switching from the duobinary modulation to the RZ-DQPSK, the modulation switching unit  180  controls an operation so that the data processing unit  1210  generates quadrature differential sign data (DATA_A and DATA_B), and further controls the amount of delay of the delay unit  1720  to π/2. 
         [0204]    In this case, the modulation switching unit  180  controls the DATA_A to be output from the delay unit  1710 B to have the same phase as the phase of the DATA_A to be output from the delay unit  1710 A. As a consequence, the MZ modulator  700  is operated as a DQPSK modulator. Therefore, the signal light output from the MZ modulator  700  becomes the signal light subjected to the DQPSK. 
         [0205]    In this case, the modulation switching unit  180  controls, to Vπ, the voltage of the drive signal output from the driving unit  130  to the MZ modulator  120 , and switches the switch  170  to the first path  171 . As a consequence, the MZ modulator  120  is operated as an RZ modulator. Therefore, the signal light output from the MZ modulator  120  becomes the signal light subjected to the RZ-DQPSK. 
         [0206]    Upon obtaining the modulation switching information indicating the switching from the RZ-DQPSK to the duobinary modulation, the modulation switching unit  180  controls, to nπ, the amount of delay of the delay unit  1720  so that the data processing unit  1210  generates binary differential sign data (DATA_A). 
         [0207]    In this case, the modulation switching unit  180  controls such an operation that the DATA_A output from the delay unit  1710 B is delayed from the DATA_A output from the delay unit  1710 A by one bit. As a consequence, the MZ modulator  700  is operated as a duobinary modulator. Therefore, the signal light output from the MZ modulator  700  becomes duobinary signal light. 
         [0208]    Further, the modulation switching unit  180  controls, to OFF, the voltage of the drive signal output from the driving unit  130  to the MZ modulator  120 , and switches the switch  170  to the second path  172 . As a consequence, the function of the MZ modulator  120  is reset as the RZ modulator. Therefore, the signal light output from the MZ modulator  120  becomes the duobinary signal light. 
         [0209]    Upon obtaining the modulation switching information indicating the switching from the duobinary modulation to the RZ-DQPSK, the modulation switching unit  180  outputs control information on the switching to the RZ-DQPSK to the bias-point holding unit  190 . Further, upon obtaining the modulation switching information indicating the switching from the RZ-DQPSK to the duobinary modulation, the modulation switching unit  180  outputs control information on the switching to the duobinary modulation to the bias-point holding unit  190 . 
         [0210]      FIG. 17B  is a diagram showing the switching of duobinary modulation and AMI modulation. The description is given of the structure in which the optical modulation device  100  can switch the DQPSK and the duobinary modulation with reference to  FIG. 17A , the optical modulation device  100  can switch the DQPSK and AMI (Alternate Mark Inversion) modulation. 
         [0211]    Referring to  FIG. 17B , a waveform  1751  denotes a waveform of the signal light subjected to the RZ-duobinary modulation. A waveform  1752  denotes a waveform of signal light subjected to the RZ-AMI modulation (specifically, refer to JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 1, JANUARY 2005). 
         [0212]    Upon obtaining the modulation switching information indicating the switching to the duobinary modulation or AMI modulation, the modulation switching unit  180  controls an operation that the data processing unit  1210  generates binary differential sign data (DATA_A), further controls, to nπ, the amount of delay of the delay unit  1720 , and furthermore controls an operation that the DATA_A output from the delay unit  1710 B is delayed from the DATA_A output from the delay unit  1710 A. 
         [0213]    The coupling unit  730  in the MZ modulator  700  has another output path  1760  in addition to the output path connected to the MZ modulator  120 , and outputs the signal light subjected to the duobinary modulation and the signal light subjected to the AMI modulation from the two output paths of coupling unit  730 . The structure shown in  FIG. 17A  sets the output of the signal light subjected to the duobinary modulation from the path connected to the MZ modulator  120 . 
         [0214]    An Even/odd of n in the amount nπ of delay of the delay unit  1720  is switched, thereby switching the output path of the signal light subjected to the duobinary modulation and the AMI modulation. As a consequence, the optical modulation device  100  switches even/odd of n in the amount nπ of delay of the delay unit  1720 , thereby switching the duobinary modulation and the AMI modulation. Therefore, the optical modulation device  100  sets even/odd of n so as to obtain the AMI modulation, thereby switching the DQPSK modulation and the AMI modulation. 
         [0215]      FIG. 18  is a diagram showing the bias points of the MZ modulator (in RZ-DQPSK format, duobinary modulation format and AMI modulation format). Referring to  FIG. 18 , in the RZ-DQPSK modulation, the bias point  222  of the MZ modulator  120  is at the center between the valley (quenching state) and the peak (light emission state) of the input/output characteristics  210 , and is set to a voltage having the differential value of the input/output characteristics  210  that is positive. In the duobinary modulation, the bias point  222  of the MZ modulator  120  is set to a voltage having the peak (light emission state) of the input/output characteristics  210 . 
         [0216]    As an example in which the optical modulation device  100  switches the DQPSK and the duobinary modulation, the description is given of the operation for switching the RZ-DQPSK and the duobinary modulation by the optical modulation device  100 . However, the operation for switching the DQPSK and the duobinary modulation is not limited to this. For example, the optical modulation device  100  can switch the DQPSK and the RZ-duobinary modulation. 
         [0217]    Upon obtaining the modulation switching information indicating the switching from the RZ intensity modulation to the DQPSK, the modulation switching unit  180  operates the MZ modulator  700  as a DQPSK modulator under the above-mentioned control, and the function of the MZ modulator  120  is reset as the RZ modulator. As a consequence, the signal light output from the MZ modulator  120  becomes the signal light subjected to the DQPSK. 
         [0218]    Further, upon obtaining the modulation switching information indicating the switching from the DQPSK to the RZ intensity modulation, the modulation switching unit  180  operates the MZ modulator  700  as an NRZ intensity modulator under the above-mentioned control, and further operates the MZ modulator  120  as an RZ modulator. As a consequence, the signal light output from the MZ modulator  120  becomes the signal light subjected to the RZ intensity modulation. 
         [0219]    As an example in which the optical modulation device  100  switches the DQPSK and the duobinary modulation, the description is given of the operation for switching the RZ-DQPSK and the duobinary modulation by the optical modulation device  100 . However, the operation for switching the DQPSK and the duobinary modulation is not limited to this. For example, the MZ modulator  120  is not provided and the optical modulation device  100  can thus switch the DQPSK and the duobinary modulation. 
         [0220]    The optical modulation device  100  according to the sixth embodiment controls the data generation of the data processing unit  1210 , the amount of delay of the delay unit  1720 , the phase difference of the data, the drive signal for the MZ modulator  700 , thereby switching the DQPSK and the duobinary modulation. Therefore, even if changing the transmission condition of the optical communication system, it is possible to switch the modulation to a modulation by which the transmission characteristics do not deteriorate against the transmission condition. 
         [0221]    In addition, with the optical modulation device  100  according to the sixth embodiment, similarly to the optical modulation device  100  according to the first embodiment, the size of the apparatus is reduced, the apparatus is simplified, and costs are reduced. Moreover, it is possible to flexibly cope with the optical communication system in which the transmission condition frequently changes and also to reduce the time from changing the modulation to stabilizing the transmission characteristics of the signal light. 
       Seventh Embodiment 
       [0222]      FIG. 19  is a block diagram showing the structure of an optical communication system according to the seventh embodiment. Referring to  FIG. 19 , the same components as those shown in  FIG. 1  are designated by the same reference numerals, and a description thereof is omitted. As shown in  FIG. 19 , an optical communication system  1900  according to the seventh embodiment comprises: a light transmitting device  1910 ; a light repeater  1920 ; and a light receiving device  1930 . 
         [0223]    The light transmitting device  1910  comprises: the optical modulation device  100  according to the first embodiment; a receiving unit  1911 ; and a determining unit  1912 . The light transmitting device  1910  transmits, via the light repeater  1920 , the signal light that is modulated by the optical modulation device  100  and is output from the branch unit  151  to the light receiving device  1930 . 
         [0224]    The light repeater  1920  repeats the signal light transmitted from the light transmitting device  1910  to the light receiving device  1930 . Specifically, the light repeater  1920  comprises: an amplifying unit  1921 ; a multiplexing and splitting unit  1922 : a multiplexing unit  1923 ; an amplifying unit  1924 ; and a monitoring unit  1925 . The amplifying unit  1921  amplifies the signal light transmitted from the light transmitting device  1910  and the amplified light to the multiplexing and splitting unit  1922 . 
         [0225]    The multiplexing and splitting unit  1922  multiplexes and splits the signal light output from the amplifying unit  1921 . The multiplexing unit  1923  multiplexes the signal light transmitted from the light transmitting device  1910  to another signal light, and outputs the resultant light to the amplifying unit  1924 . The amplifying unit  1924  transmits the signal light output from the multiplexing unit  1923  to the light receiving device  1930 . The monitoring unit  1925  monitors the signal light output from the light transmitting device  1910 , and transmits information on the result of monitoring the signal light to the light transmitting device  1910 . 
         [0226]    The light receiving device  1930  receives the signal light transmitted from the light repeater  1920 . Further, the light receiving device  1930  monitors the received signal light and transmits information on the monitoring result to the light transmitting device  1910 . Furthermore, the light receiving device  1930  may transmit a request for transmitting the signal light to the light transmitting device  1910 . 
         [0227]    The receiving unit  1911  in the light transmitting device  1910  receives the information transmitted from the light repeater  1920  or the light receiving device  1930 , and outputs the received information to the determining unit  1912 . The determining unit  1912  determines the modulation on the basis of the information output from the receiving unit  1911 . For example, the determining unit  1912  determines the modulation by which the transmission characteristics are best on the basis of the information on the result of monitoring the signal light transmitted from the light repeater  1920  or the light receiving device  1930 . 
         [0228]    For example, the determining unit  1912  collects information on a transmission path including the transmission distance from the light transmitting device  1910  to the light receiving device  1930 , the interval between wavelengths of WDM, the number of steps of the repeater, or a transmission band of an optical filter on the basis of the request for transmitting the signal light transmitted from the light receiving device  1930 , and determines the modulation by which the transmission characteristics become the best ones on the basis of the collected information on the transmission path. 
         [0229]    Further, the light repeater  1920  or the light receiving device  1930  may transmit the information on the transmission path, and the determining unit  1912  may determine the modulation having the best transmission characteristics on the basis of the information on the transmission path transmitted from the light repeater  1920  or the light receiving device  1930 . The determining unit  1912  outputs the modulation switching information indicating the switching to the determined modulation to the modulation switching unit  180 . The modulation switching unit  180  obtains the modulation switching information output from the determining unit  1912 , and switches the modulation in accordance with the obtained modulation switching information. 
         [0230]    Although the optical modulation device  100  according to the first embodiment is applied to the optical communication system  1900 , the optical modulation device  100  according to the embodiments can be applied to the optical communication system  1900 . Further, the optical communication system  1900  comprises the light repeater  1920  and the light receiving device  1930  as mentioned above. However, the optical communication system  1900  may comprise one of the light repeater  1920  and the light receiving device  1930 . 
         [0231]    The optical communication system  1900  according to the seventh embodiment has the advantages of the optical modulation device  100  according to the above embodiments. Further, the best modulation is automatically determined on the basis of the information sent from the light repeater  1920  or the light receiving device  1930 , and the modulation can be switched to the determined modulation. 
         [0232]    As mentioned above, with the optical modulator and the light-modulation switching method according to the present invention, the drive signal for the MZ modulator is switched, thereby switching the modulation. Therefore, even if changing the transmission condition of the optical communication system, the modulation can be switched to a modulation by which the transmission characteristics do not deteriorate against the changed transmission condition. 
         [0233]    Further, with the optical modulator and the light-modulation switching method according to the present invention, one MZ modulator can switch the modulation. Therefore, a plurality of modulators corresponding to the modulations do not need to be provided so as to switch the modulations, the size of the apparatus is decreased, the apparatus is simplified, and costs are reduced. 
         [0234]    Furthermore, with the optical modulator and the light-modulation switching method according to the present invention, the modulation is matched to that of the optical communication device as the communication destination. Therefore, the optical transmission is possible between the optical communication devices using different modulations. In addition, the modulation switching information is obtained, thereby automatically and immediately switching the modulation. Moreover, it is possible to flexibly cope with the optical communication system in which the transmission condition frequently changes. 
         [0235]    In addition, with the optical modulator and the light-modulation switching method according to the present invention, the control information on the bias point corresponding to the modulation is held, thereby efficiently controlling the bias voltage upon switching the modulation. Therefore, it is possible to reduce the time from changing the modulation to stabilizing the transmission characteristics of the signal light. 
       INDUSTRIAL APPLICABILITY 
       [0236]    As mentioned above, the optical modulator and the light-modulation switching method according to the present invention are advantageous for switching the modulation. In particular, the optical modulator and the light-modulation switching method according to the present invention are suitable to the case of switching the modulation in accordance with the transmission condition and the optical communication device as the communication destination. 
       ADVANTAGES 
       [0237]    Advantageously, the modulation can be flexibly switched without arranging a plurality of modulators corresponding to the modulations according to the present invention.
According to an aspect of an embodiment, an optical modulation device includes a Mach-Zehnder modulator and a controller. The Mach-Zehnder modulator is supplied a drive signal and a bias voltage. The Mach-Zehnder modulator modulates inputted light on the bases of the drive signal and the bias voltage. The drive signal selectively is superimposes a predetermined frequency signal. The bias voltage selectively is superimposes the predetermined frequency signal. The controller selects a superimposing target which is the drive signal or the bias voltage so as to change modulation formats.