Abstract:
An optical transmitter includes a light source configured to generate direct current light; a multi-value modulator configured to modulate the direct current light; an opto-isolator configured to block output light of the multi-value modulator; a driving circuit configured to output a driving signal for driving the multi-value modulator; a bias control circuit configured to control a bias voltage applied to the multi-value modulator, according to an optical output power of the multi-value modulator; and a control circuit configured to control operations of the opto-isolator, the driving circuit and the bias control circuit, wherein the control circuit decreases an amplitude of the driving signal output from the driving circuit while synchronizing with the blocking of the output light of the multi-value modulator by the opto-isolator.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an optical transmitter and a method of controlling the optical transmitter. 
       BACKGROUND 
       [0002]    A transmission system in which multi-level modulation using optical phase information is applied has been used in order to handle a recent rapid increase in communication link capacity. As a receiving system of the transmission system in which the multi-level modulation is applied, there is a digital coherent signal system in which a signal is received through a digital signal processing of a coherent signal detected by using interference between signal light and local oscillation light. 
         [0003]    A Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) system in which multiplexing is performed by assigning two phase signals to each of two lights having different polarizations each other is currently used for multi-level modulation. Using the transmission system in which the DP-QPSK system is applied, it is achieved to transmit a signal with a transmission capacity of 100 Gbps per one wavelength. A transceiver using such a transmission system has been specified as disclosed in Non Patent Literature 1 (“Multisource Agreement for 100G Long-Haul DWDM Transmission Module—Electromechanical”, Optical Internetworking Forum, Jun. 8, 2010) and Non Patent Literature 2 (“Implementation Agreement for Integrated Polarization Multiplexed Quadrature Modulated Transmitters”, Optical Internetworking Forum, Mar. 12, 2010). 
         [0004]    In a typical transceiver specified in Non Patent Literatures 1 and 2, a CW light output from a transmission side light source is modulated with a transmission side electric signal using a multi-level modulator, and the modulated optical signal is transmitted to the outside as transmission light. At the same time, in such a transceiver, a light received from the outside and a local oscillation light output from a local oscillation light source are interfered using an interferometer and the interfered light is converted into a receiving side electric signal, with a photo diode. Herein, with the transceiver, in order to achieve further down-sizing and to reduce power consumption, a configuration in which the transmission side light source and the local oscillation light source are unified as one light source is considered. Such a configuration is an effective way of reducing power consumption and an installation size by unification of two wavelength controlling circuits, which were separated for transmission and receiving sides, respectively. 
         [0005]    For example, a semiconductor Mach-Zehnder modulator is used as the multi-level modulator. The Mach-Zehnder modulator has a configuration in which one optical waveguide branches off to two optical waveguides, at an input side thereof, and the two branching optical waveguides are joined into one optical waveguide, at an output side thereof. An electrode is formed on each of the two branching optical waveguides. A bias voltage is applied to the electrodes by a bias control circuit, and a pair of complimentary voltage signals, which phase are opposite to each other, is also applied to the electrodes as a modulation signal by a driving circuit. 
         [0006]    The bias voltage is set such that a phase difference between two lights passing through the two optical waveguide of the Mach-Zehnder modulator respectively is equal to π. In more detail, when the modulation signal is not input, the bias voltage is set such that a phase of light passing through one optical waveguide becomes 3π/2 and a phase of light passing through the other optical waveguide becomes π/2. 
         [0007]    In a state in which the bias voltage is set in this way, when the modulation signal is input as a pair of complementary voltage signals, the phase of the light passing through the one optical waveguide is changed from 2π to π as is 3π/2+x (herein, −π/2≦x≦π/2), and the phase of the light passing through the other optical waveguide is changed from 0 to π as is π/2−x. 
         [0008]    Further, a photo diode is put in at the output side of the Mach-Zehnder modulator. The photo diode receives a part of the transmission light branching from the Mach-Zehnder modulator as a monitoring light and detects an optical power of the transmission light. A bias control circuit controls the bias voltage to optimally maintain an operation point of the Mach-Zehnder modulator depending on the optical power detected by the photo diode. 
         [0009]    Further, Japanese Patent Laid-Open Publication No. 2010-204689 has been known as a literature in which a technology relating to the corresponding technical field is disclosed. 
       SUMMARY 
       [0010]    However, in Non Patent Literature 1, a transceiver is required to have a transmitter disable function for shutting off a transmission light according to an instruction of a host device. When one light source is used in common both for transmission and receiving sides, a dilemma of shutting off the output of the light source to perform the transmission disable function or simultaneously keeping the output of the light source not to stop the receiving side, however, arises. Thus, it is considered that a Variable Optical Attenuator (VOA) is equipped as an apparatus for blocking a light at an output side of a Mach-Zehnder modulator, and the transmission light is shut off by the VOA at the time of performing the transmitter disable function. 
         [0011]    However, when the VOA is used, a power consumption of the transceiver increases by an amount of electric power consumed for controlling the VOA. In order to compensate for the increase in the power consumption, it is considered that an operation of a driving circuit for driving a multi-level modulator is suspended at the time of performing the transmitter disable function. However, when the operation of the driving circuit is suspended, a problem occurs as described below. 
         [0012]    When the operation of the driving circuit is suspended at the time of performing the transmitter disable function, the output from the output side of the Mach-Zehnder modulator corresponds to light obtained by merging a light having a phase of 3π/2 passing through one optical waveguide and a light having a phase of π/2 passing through the other optical waveguide. Thus, in this case, the transmission light is not output from the Mach-Zehnder modulator since the two lights passing through the two respective optical waveguides cancel each other. 
         [0013]    In this way, when the transmission light from the output side of the Mach-Zehnder modulator is optically quenched by stopping the operation of the driving circuit, the monitoring light to the photo diode is also optically quenched. Accordingly, a bias control circuit cannot monitor a state of the modulator, especially, a phase difference between the two lights passing through the two respective optical waveguides. Therefore, an operation point of the Mach-Zehnder modulator drifts during a transmitter disable state, and immediately after the transmitter disable state is released, an optical signal transmitted from the transceiver temporarily deteriorates. 
         [0014]    In order to avoid such deterioration of the optical signal, it can be considered that the drifting of the operation point of the Mach-Zehnder modulator is compensated for by performing starting-up of the driving circuit and calibration of the bias control circuit until the VOA stops blocking the light output from the Mach-Zehnder modulator after the transmission disable state is released. However, when such a configuration is made up, there are problems in that power consumption increases while drift compensation is performed, and some time is required from starting-up until returning to a normal operation after releasing the transmitter disable state. 
         [0015]    In accordance with an aspect of the present invention, an optical transmitter is provided. The optical transmitter includes a light source configured to generate a continuous wave light; a multi-level modulator configured to modulate the continuous wave light; an optical shutter configured to block an output light of the multi-level modulator, a driving circuit configured to output a driving signal configured to drive the multi-level modulator; a bias control circuit configured to control a bias voltage applied to the multi-level modulator, according to an optical output power of the multi-level modulator; and a control circuit configured to control the optical shutter, the driving circuit and the bias control circuit, wherein the control circuit decreases an amplitude of the driving signal output from the driving circuit while synchronizing with the blocking of the output light of the multi-level modulator by the optical shutter. 
         [0016]    In accordance with such an optical transmitter, the amplitude of the driving signal configured to drive the multi-level modulator is decreased while synchronizing with the blocking of the optical output of the multi-level modulator by the optical shutter. Thus, a power consumption of the driving circuit decreases during the blocking operation of the optical shutter. At the same time, since light output from the multi-level modulator does not completely disappear, the bias control circuit can continuously control the bias voltage applied to the multi-level modulator in accordance with the optical output power of the multi-level modulator. Accordingly, a bias point of the multi-level modulator can be properly controlled while decreasing a power consumption of the optical transmitter. 
         [0017]    In the aforementioned optical transmitter, it is suitable that the multi-level modulator includes one or more semiconductor Mach-Zehnder modulators, the semiconductor Mach-Zehnder modulator includes a first optical waveguide and a second optical waveguide configured to pass two beams of light branching off from an input side of the semiconductor Mach-Zehnder modulator, a first electrode installed on the first optical waveguide, and a second electrode installed on the second optical waveguide, the bias control circuit applies individual bias voltages to the first and second electrodes, respectively, and the control circuit controls the operation of the bias control circuit to adjust the bias voltages such that a phase difference between light passing through the first optical waveguide and light passing through the second optical waveguide is kept to be π while synchronizing with the blocking of the output light of the multi-level modulator by the optical shutter. In this case, the bias voltage is adjusted such that the phase difference between the light passing through the first optical waveguide and the light passing through the second optical waveguide becomes π. At a time of a normal transmission operation of the optical transmitter, the phase difference between the light passing through the first optical waveguide and the light passing through the second optical waveguide becomes π. Accordingly, by the above configuration, the phase difference between the light passing through the first optical waveguide and the light passing through the second optical waveguide can be controlled to be a proper value when returning to a normal operation state. 
         [0018]    Further, it is also suitable that the multi-level modulator includes the two semiconductor Mach-Zehnder modulators and the control circuit controls the bias control circuit, with respect to each of the two semiconductor Mach-Zehnder modulators, in a time sharing system, to adjust the bias voltage such that a phase difference between light passing through the first optical waveguide and light passing through the second optical waveguide becomes π. In this case, the bias voltage is adjusted with respect to the two semiconductor Mach-Zehnder modulators in a time sharing system. Accordingly, the two semiconductor Mach-Zehnder modulators can use the one bias control circuit, the one driving circuit and the one control circuit together, and the downsizing and the reduction in the power consumption of the optical transmitter can be achieved. 
         [0019]    Further, it is also suitable that the driving circuit has a differential circuit configuration including an open collector output or an open drain output, and the driving signal is output to the multi-level modulator through current switching. In this case, since the driving circuit has the differential circuit configuration, a pair of complementary voltage signals is applied to the multi-level modulator as a driving signal, and the driving signal is output to the multi-level modulator through the current switching, so that the bias point of the driving signal can be optimized. 
         [0020]    Further, it is also suitable that the amplitude of the driving signal output from the driving circuit is decreased by decreasing the current defined by the current source included in the differential circuit. In this case, the amplitude of the driving signal can be easily decreased by decreasing the current defined by the current source included in the differential circuit. 
         [0021]    In accordance with an aspect of the present invention, a method of controlling an optical transmitter including a multi-level modulator, an optical shutter configured to block an output light of the multi-level modulator, a bias control circuit configured to control a bias voltage applied to the multi-level modulator according to an optical output power of the multi-level modulator is provided. The method performs a process wherein the multi-level modulator includes one or more semiconductor Mach-Zehnder modulators, the semiconductor Mach-Zehnder modulator includes a first optical waveguide and a second optical waveguide configured to pass two lights branching off from an input side of the semiconductor Mach-Zehnder modulator, a first electrode formed on the first optical waveguide, and a second electrode formed on the second optical waveguide, the bias control circuit applies individual bias voltages to the first and second electrodes, respectively, and while synchronizing with the blocking of the output light of the multi-level modulator by the optical shutter, an amplitude of a driving signal configured to drive the multi-level modulator is decreased, and the bias voltage is adjusted such that a phase difference between light passing through the first optical waveguide and light passing through the second optical waveguide becomes π. 
         [0022]    In accordance with such a method of controlling the optical transmitter, the amplitude of the driving signal configured to drive the multi-level modulator is decreased while synchronizing with the blocking of the optical output of the multi-level modulator by the optical shutter. Thus, a power consumption of the driving circuit is decreased at a time of the blocking operation of the optical shutter. At the same time, since light output from the multi-level modulator does not completely disappear, the bias control circuit can continuously control the bias voltage applied to the multi-level modulator in accordance with the optical output power of the multi-level modulator. Accordingly, a bias point of the multi-level modulator can be properly controlled while decreasing power consumption of the optical transmitter. Further, the bias voltage is adjusted such that the phase difference between the light passing through the first optical waveguide and the light passing through the second optical waveguide becomes π. At a time of a normal transmission operation of the optical transmitter, the phase difference between the light passing through the first optical waveguide and the light passing through the second optical waveguide becomes π. Accordingly, by the above configuration, the phase difference between the light passing through the first optical waveguide and the light passing through the second optical waveguide can be controlled to be a proper value when returning to a normal operation state. 
         [0023]    Further, in the method of controlling the optical transceiver, it is suitable that the multi-level modulator includes the two semiconductor Mach-Zehnder modulators and the bias voltage is adjusted with respect to each of the two semiconductor Mach-Zehnder modulators, in a time sharing system, such that a phase difference between light passing through the first optical waveguide and light passing through the second optical waveguide becomes π. In this case, the bias voltage is adjusted with respect to the two semiconductor Mach-Zehnder modulators in a time sharing system. Accordingly, the two semiconductor Mach-Zehnder modulators can use the one bias control circuit, the one driving circuit, the one control circuit and the control circuit configured to control the bias control or the driving circuit, together, and the downsizing and the reduction in the power consumption, of the optical transmitter, can be achieved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a block diagram illustrating a functional configuration of an optical transmitter according to an embodiment of the present invention; 
           [0025]      FIG. 2  schematically illustrates a configuration of a multi-level modulator according to an embodiment of the present invention; 
           [0026]      FIG. 3  illustrates a multi-level modulator and a configuration for controlling the multi-level modulator; 
           [0027]      FIG. 4  is a timing chart of a bias voltage and a driving voltage which are applied to a multi-level modulator; 
           [0028]      FIG. 5  is a circuit diagram illustrating an example of a driving circuit; 
           [0029]      FIG. 6  is a circuit diagram illustrating an example of an electric interconnection between an output port of a driving circuit and an electrode of a multi-level modulator; 
           [0030]      FIG. 7  is a flowchart illustrating a process in which an optical transmitter is switched from a normal operation state to a transmitter disable state; 
           [0031]      FIG. 8  is a flowchart illustrating a process in which an optical transmitter is switched from a transmitter disable state to a normal operation state; 
           [0032]      FIGS. 9A and 9B  are each vector diagrams illustrating a phase of output light of a Mach-Zehnder modulator; 
           [0033]      FIGS. 10A and 10B  respectively illustrate an optical output power curve of a Mach-Zehnder modulator and provide a timing chart of optical power for normal operation; 
           [0034]      FIGS. 11A and 11B  respectively illustrate an optical output power curve of Mach-Zehnder modulator and provide a timing chart of optical power while performing a transmitter disable function, in an optical transmitter according to the related art; 
           [0035]      FIGS. 12A and 12B  respectively illustrate an optical output power curve of a Mach-Zehnder modulator and provide a timing chart of optical power, while performing a transmitter disable function, in an optical transmitter according to the present invention; 
           [0036]      FIGS. 13A to 13D  together provide a timing chart of power consumption of each component during a general operation state and a transmitter disable state according to the present embodiment; 
           [0037]      FIG. 14  illustrates a modified example of a multi-level modulator and a configuration for controlling the multi-level modulator; 
           [0038]      FIG. 15  illustrates another modified example of a multi-level modulator and a configuration for controlling the multi-level modulator; and 
           [0039]      FIG. 16  illustrates yet another modified example of a multi-level modulator and a configuration for controlling the multi-level modulator. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. Meanwhile, in the description of the drawings, the same element or equivalent elements will be designated by the same reference numeral and a duplicate description thereof will be omitted. 
         [0041]      FIG. 1  is a block diagram illustrating a function configuration of an optical transceiver (optical transmitter and receiver) according to the present embodiment. As illustrated in  FIG. 1 , an optical transceiver  1  includes a light source  2 , a multi-level modulator  10 , a driving circuit  46 , a Variable Optical Attenuator (VOA)  45  and an interferometer  21 . The optical transceiver  1  is used in, for example, optical communication. 
         [0042]    A configuration for transmitting a signal light by the optical transceiver  1  will be described below. The light source  2  emits a continuous wave light. The multi-level modulator  10  modulates a part of the continuous wave light from the light source  2  based on a transmission side electric signal. The driving circuit  46  outputs a driving signal for driving the multi-level modulator  10  to the multi-level modulator  10  based on the transmission side electric signal from the outside of the optical transceiver  1 . The VOA  45  functions as an optical shutter for attenuating a modulated light output from the multi-level modulator  10 , for example, when a transmitter disable signal is received from a host device. After passing through the VOA  45 , the modulated light from the multi-level modulator  10  is output as a transmission light to the outside of the optical transceiver  1  through a transmission optical fiber connected to the optical transceiver  1 . 
         [0043]    Meanwhile, a configuration for receiving a signal light by the optical transceiver  1  will be described below. The interferometer  21  makes light, input as received light from the outside of the optical transceiver  1  through a receiving optical fiber connected to the optical transceiver  1 , and a part of the continuous wave light from the light source  2 , interfere with each other. A interfered light obtained by making the received light and the part of the continuous wave light interfere with each other is detected by a photo detector which is not illustrated, and is output as a reception side electric signal to the outside of the optical transceiver  1 . In this way, in the optical transceiver  1 , the light source  2  is used for both purposes to transmit a signal light (transmission light) to the outside and to receive a signal light from the outside. 
         [0044]    Next, a schematic configuration of the multi-level modulator  10  will be described with reference to  FIG. 2 .  FIG. 2  is a schematic configuration of the multi-level modulator  10 . The multi-level modulator  10  corresponds to a DP-QPSK modulator for generating a modulation signal (hereinafter, referred to as an “X side modulation signal”) of X polarization (first polarization) and a modulation signal (hereinafter, referred to as a “Y side modulation signal) of Y polarization (second polarization). The multi-level modulator  10  comprises an optical branching element  3 , a first QPSK modulator (multi-level modulator)  4 , a second QPSK modulator (multi-level modulator)  5 , a polarization rotation element  6 , a polarization composite element  7 , a first optical power monitor (photo diode)  11  and a second optical power monitor (photo diode)  12 , and modulates a continuous wave light from a laser light source  2 . 
         [0045]    The laser light source  2  corresponds to, for example, a laser diode, generates light having a predetermined frequency, and emits light to the optical branching element  3  through an optical waveguide wg 1 . This light corresponds to, for example, light having single-polarization. The optical branching element  3  is put in between the laser light source  2  and a couple of the first QPSK modulator  4  and the second QPSK modulator  5 , and divides the light emitted from the laser light source  2  into two beams of light (X side light and Y side light) having the same power. The optical branching element  3  branches off the X side light (a part of the light) to the first QPSK modulator  4  through an optical waveguide wg 2 , branches off the Y side light (the other part of the light) to the second QPSK modulator  5  through an optical waveguide wg 3 , and outputs the branched-off beams of light. 
         [0046]    The first QPSK modulator  4  performs QPSK modulation for the input X side light to generate an X side modulation signal. Further, the first QPSK modulator  4  outputs the X side modulation signal to the polarization rotation element  6  through an optical waveguide wg 4 . The second QPSK modulator  5  performs QPSK modulation for the input Y side light to generate a Y side modulation signal. Further, the second QPSK modulator  5  outputs the Y side modulation signal to the polarization composite element  7  through an optical waveguide wg 6 . 
         [0047]    Here, a configuration of the first QPSK modulator  4  will be described with respect to  FIG. 3 . As illustrated in  FIG. 3 , the first QPSK modulator  4  comprises a first Mach-Zehnder interference unit (semiconductor Mach-Zehnder modulator)  41 , a second Mach-Zehnder interference unit (semiconductor Mach-Zehnder modulator)  42 , an electrode  43 , an electrode  44 , a VOA (optical shutter)  45 , a driving circuit  46 , a bias control circuit  47 , a variable resistor element  48  and a control circuit  49 . 
         [0048]    In an input unit  4   a  of the first QPSK modulator  4 , the optical waveguide wg 2  branches off to an optical waveguide wg 21  and an optical waveguide wg 22 . The optical waveguide wg 21  extends from the input unit  4   a  to an input unit  41   a  of the first Mach-Zehnder interference unit  41 , and the optical waveguide wg 22  extends from the input unit  4   a  to an input unit  42   a  of the second Mach-Zehnder interference unit  42 . Further, an optical waveguide wg 23  extends from an output unit  41   b  of the first Mach-Zehnder inference unit  41  to an output unit  4   b , and an optical waveguide wg 24  extends from an output unit  42   b  of the second Mach-Zehnder interference unit  42  to the output unit  4   b . The optical waveguide wg 23  and the optical waveguide wg 24  are joined to the optical waveguide wg 4  at the output unit  4   b.    
         [0049]    The first Mach-Zehnder interference unit  41  generates an XI modulation signal by superposing a data signal D XI  to the X side light divided by the optical branching element  3 , and outputs the XI modulation signal. In the input unit  41   a  of the first Mach-Zehnder interference unit  41 , the optical waveguide wg 21  branches off to an optical waveguide (first optical waveguide) wg 21   a  and an optical waveguide (second optical waveguide) wg 21   b , and each of the optical waveguide wg 21   a  and the optical waveguide wg 21   b  extends from the input unit  41   a  to the output unit  41   b . Further, in the output unit  41   b , the optical waveguide wg 21   a  and the optical waveguide wg 21   b  are joined to the optical waveguide wg 23 . A first XI electrode (first electrode)  41   c  is formed on the optical waveguide wg 21   a , and a second XI electrode (second electrode)  41   d  is formed on the optical waveguide wg 21   b . The first XI electrode  41   c  and the second XI electrode  41   d  are arranged to be parallel to each other. Therefore, the pair of the first XI electrode  41   c  and the second XI electrode  41   d  function as a 2×2 optical directional coupler. 
         [0050]    In the input unit  41   a , the X side light passing through the optical waveguide wg 21  is output to the optical waveguide wg 21   a  and the optical waveguide wg 21   b . Further, in the first XI electrode  41   c , a bias voltage V XIB1  is applied by the bias control circuit  47 , and a driving voltage signal V XID1  is applied by the driving circuit  46  at the same time. In the second XI electrode  41   d , a bias voltage V XIB2  is applied by the bias control circuit  47 , and a driving voltage signal V XID2  is applied by the driving circuit  46  at the same time. The bias control circuit  47  adjusts the bias voltage V XIB1  and the bias voltage V XIB2  such that a phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  becomes π. The driving voltage signals V XID1  and V XID2  correspond to a voltage signal generated based on the data signal D XI . The voltage amplitude of the driving signal V XID1  is equal to the voltage amplitude of the driving voltage signal V XID2 , and the polarity of the driving voltage signal V XID1  is different from that of the driving voltage signal V XID2 . The driving voltage signal V XID1  is applied to the first XI electrode  41   c , so that the X side light passing through the optical waveguide wg 21   a  is modulated, and a first XI modulation signal is generated. The driving voltage signal V XID2  is applied to the second XI electrode  41   d , so that the X side light passing through the optical waveguide wg 21   b  is modulated, and a second XI modulation signal is generated. Further, in the output unit  41   b , an XI modulation signal is generated by coupling the first XI modulation signal and the second XI modulation signal, and is output to the optical waveguide wg 23 . 
         [0051]    The second Mach-Zehnder interference unit  42  generates an XQ modulation signal by superposing a data signal D XQ  to the X side light divided by the optical branching element  3 , and outputs the XQ modulation signal. In the input unit  42   a  of the second Mach-Zehnder interference unit  42 , the optical waveguide wg 22  branches off to an optical waveguide (first optical waveguide) wg 22   a  and an optical waveguide (second optical waveguide) wg 22   b , and each of the optical waveguide wg 22   a  and the optical waveguide wg 22   b  extends from the input unit  42   a  to the output unit  42   b . Further, in the output unit  42   b , the optical waveguide wg 22   a  and the optical waveguide wg 22   b  are joined to the optical waveguide wg 24 . A first XQ electrode (first electrode)  42   c  is formed on the optical waveguide wg 22   a , and a second XQ electrode (second electrode)  42   d  is formed on the optical waveguide wg 22   b . The first XQ electrode  42   c  and the second XQ electrode  42   d  are arranged to be parallel to each other. Therefore, the pair of the first XQ electrode  42   c  and the second XQ electrode  42   d  function as a 2×2 optical directional coupler. 
         [0052]    In the input unit  42   a , the X side light passing through the optical waveguide wg 22  is output to the optical waveguide wg 22   a  and the optical waveguide wg 22   b . Further, in the first XQ electrode  42   c , a bias voltage V XQB1  is applied by the bias control circuit  47 , and a driving signal V XQD1  is applied by the driving circuit  46  at the same time. In the second XQ electrode  42   d , a bias voltage V XQB2  is applied by the bias control circuit  47 , and a driving signal V XQD2  is applied by the driving circuit  46  at the same time. The bias control circuit  47  adjusts the bias voltages V XQB1  and V XQB2  such that a phase difference between the light passing through the optical waveguide wg 22   a  and the light passing through the optical waveguide wg 22   b  becomes π. The driving voltage signal V XQD1  and the driving voltage signal V XQD2  correspond to a voltage signal generated based on the data signal D XQ . The amplitude of the driving voltage signal V XQD1  is equal to the amplitude of the driving voltage signal V XQD2 , and the polarity of the driving voltage signal V XQD1  is different from that of the driving voltage signal V XQD2 . The driving voltage signal V XQD1  is applied to the first XQ electrode  42   c , so that the X side light passing through the optical waveguide wg 22   a  is modulated, and a first XQ modulation signal is generated. The driving voltage signal V XQD2  is applied to the second XQ electrode  42   d , so that the X side light passing through the optical waveguide wg 22   b  is modulated, and a second XQ modulation signal is generated. Further, in the output unit  42   b , an XQ modulation signal is generated by coupling the first XQ modulation signal and the second XQ modulation signal, and is output to the optical waveguide wg 24 . 
         [0053]    A first optical power monitor  11  corresponds to a monitor which detects an optical power of the X side modulation signal output from the first QPSK modulation unit  4 . The first optical power monitor  11  is placed close to the output unit  4   b  to optically couple the optical waveguide wg 23  and the optical waveguide wg 24 . Accordingly, the first optical power monitor  11  monitors an optical power of the light output from the optical waveguide wg 23  and the optical waveguide wg 24  to the optical waveguide wg 4 . The first optical power monitor  11  outputs a monitor current to the variable resistor element  48  as the monitored optical power. 
         [0054]    The VOA  45  is arranged on the optical waveguide wg 4 , and blocks an output light of the first QPSK modulator  4  under the control of the control circuit  49 . The VOA  45  regulates an optical power of the output light according to an applied voltage. 
         [0055]    As described above, the driving circuit  46  outputs the driving voltage signals V XID1 , V XID2 , V XQD1  and V XQD2  to the first XI electrode  41   c , the second XI electrode  41   d , the first XQ electrode  42   c , and the second XQ electrode  42   d , as driving signals for driving the first QPSK modulator  4 , respectively. 
         [0056]    As described above, the bias control circuit  47  outputs the bias voltages V XIB1 , V XIB2 , V XQB1  and V XQB2  to the first XI electrode  41   c , the second XI electrode  41   d , the first XQ electrode  42   c , and the second XQ electrode  42   d , as bias voltages provided to the first QPSK modulator  4 , respectively. 
         [0057]    One end of the variable resistor element  48  is connected to the first optical power monitor  11 , and the other end of the variable resistor element  48  is grounded. A resistance of the variable resistor element  48  is varied by the control circuit  49 . A potential of the one end of the variable resistor element  48  is proportional to the monitor current output from the first optical power monitor  11 . The bias control circuit  47  detects the optical power monitored by the first optical power monitor  11  by detecting the potential generated in the one end of the variable resistor element  48 . 
         [0058]    The control circuit  49  controls the VOA  45 , the driving circuit  46  and the bias control circuit  47 . In detail, the control circuit  49  receives a Tx_Disable signal corresponding to a transmitter disable signal from a host device installed on the outside of the optical transceiver  1 , for example, a personal computer, and blocks the light passing through the optical waveguide wg 4  with the VOA  45  according to the Tx_Disable signal. Further, as described below, the control circuit  49  controls amplitudes of the driving voltage signals V XID1 , V XID2 , V XQD1 , and V XQD2  output from the driving circuit  46 . Further, the control circuit  49  controls potential values and timings of the bias voltages V XIB1 , V XIB2 , V XQB1  and V XQB2  by controlling the bias control circuit  47 . Further, the control circuit  49  adjusts a resistance of the variable resistor element  48 . 
         [0059]    The second QPSK modulator  5  has a configuration approximately equal to that of the first QPSK modulator  4 . That is, the second QPSK modulator  5  has a first Mach-Zehnder interference unit (semiconductor Mach-Zehnder modulator)  51  and a second Mach-Zehnder interference unit (semiconductor Mach-Zehnder modulator)  52 . The first Mach-Zehnder interference unit  51  generates a YI modulation signal by superposing a data signal D YI  to the Y side light divided by the optical branching element  3 , and outputs the YI modulation signal. The second Mach-Zehnder interference unit  52  generates an YQ modulation signal by superposing a data signal D YQ  to the Y side light divided by the optical branching element  3 , and outputs the YQ modulation signal. 
         [0060]    Here, a change in the bias voltages V XIB1  and V XIB2  and the driving voltage signals V XID1  and V XID2  during a normal operation will be described with reference to  FIG. 4 .  FIG. 4  is a timing chart illustrating the change in the bias voltages V XIB1  and V XIB2  and the driving voltage signals V XID1  and V XID2  according to a time. A horizontal axis thereof denotes a time, and a vertical axis thereof denotes a voltage. Voltages V 0 , V π/2 , V π , V 3π/2  and V 2π  in the vertical axis correspond to the values obtained by shifting the phase of the light passing through the optical waveguide wg 21   a  or the optical waveguide wg 21   b , by angles of 0, π/2, π, 3π/2 and 2π, as compared with a referenced phase. 
         [0061]    As described above, the bias voltage V XIB1  and the bias voltage V XIB2  are values in which the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  becomes π. Thus, the bias voltage V XIB1  is set to be the voltage V π/2 , and the bias voltage V XIB2  is set to be the voltage V 3π/2 . 
         [0062]    Further, the driving voltage signals V XID1  and V XID2  are changed in a range from a voltage −V π/2  to a voltage V π/2  according to a time. As described above, the driving voltage signals V XID1  and V XID2  have the same absolute value but opposite polarity. Accordingly, from a time t0 to a time t1, a voltage V XIB1 +V XID1  applied to the first XI electrode  41   c  is the voltage V 0  obtained by subtracting the driving voltage V π/2  from the bias voltage V π/2 , and a voltage V XIB2 +V XID2  applied to the second XI electrode  41   d  is the voltage V 2π  obtained by adding the bias voltage V 3π/2  to the driving voltage V. From a time t2 to a time t3, the voltage V XIB1 +V XID1  applied to the first XI electrode  41   c  is the voltage Vπ obtained by adding the driving voltage V π/2  to the bias voltage V π/2 , and the voltage V XIB2 +V XID2  applied to the second XI electrode  41   d  is the voltage Vu obtained by subtracting the driving voltage V π/2  from the bias voltage V 3π/2 . From a time t4 to a time t5, the voltage V XIB1 +V XID1  and the voltage V XIB2 +V XID2  have the same voltage values from the time t0 to the time t1, and from a time t6 to a time t7, the voltage V XIB1 +V XID1  and the voltage V XIB2 +V XID2  have the same voltage values from the time t2 to the time t3. Thereafter, the voltage values are changed periodically. 
         [0063]    Next, a circuit configuration of the driving circuit  46  will be described with reference to  FIGS. 5 and 6 . The driving circuit  46  has a differential circuit configuration including an open collector output or an open drain output. 
         [0064]    For example, an output port of the driving circuit  46  includes a differential amplifier  46 A illustrated in  FIG. 5 . The differential amplifier  46 A includes transistors Tr 1 , Tr 2 , Tr 11 , Tr 12 , Tr 3 , Tr 4 , Tr 5  and Tr 6 , a capacitor C 1 , resistors R 11 , R 12 , R 13 , R 14 , R 15  and R 16 , and current sources I 1 , I 2 , I 11 , I 12  and I 4 . The differential amplifier  46 A corresponds to a differential amplifier which amplifies a pair of complementary input signals input to input ports In 1  and In 2 , and outputs a pair of complementary output signals from output ports Out 1  and Out 2 . 
         [0065]    In the differential amplifier  46 A, a base of the transistor Tr 1  is connected to the input port In 2 , a collector of the transistor Tr 1  is grounded, and an emitter of the transistor Tr 1  is connected to the current source I 1  and a base of the transistor Tr 11 . Further, a base of the transistor Tr 2  is connected to the input port In 1 , a collector of the transistor Tr 2  is grounded, and an emitter of the transistor Tr 2  is connected to the current source I 2  and a base of the transistor Tr 12 . Further, a collector of the transistor Tr 11  is grounded, and an emitter of the transistor Tr 11  is connected to the current source I 11  and a base of the transistor Tr 4 . Further, a collector of the transistor Tr 12  is grounded, and an emitter of the transistor Tr 11  is connected to the current source I 12  and a base of the transistor Tr 3 . These transistors Tr 1 , Tr 2 , Tr 11  and Tr 12  configure a two-stage emitter follower circuit. 
         [0066]    The transistor Tr 3  and the transistor Tr 4  configure paired differential transistors. The transistor Tr 5  and the transistor Tr 6  are connected to the transistor Tr 3  and the transistor Tr 4  in cascade, respectively. That is, the transistors Tr 5  and Tr 6  are a pair of cascode transistors connected to the transistors Tr 3  and Tr 4  in series, respectively. In more detail, a collector of the transistor Tr 3  is connected to an emitter of the cascode transistor Tr 5 , and a collector of the transistor Tr 4  is connected to an emitter of the cascode transistor Tr 6 . An emitter of the transistor Tr 3  is connected to the current source I 4  through the resistor R 11 , and an emitter of the transistor Tr 4  is connected to the current source I 4  through the resistor R 12 . 
         [0067]    A collector of the transistor Tr 5  is connected to the output port Out 1 , and a collector of the transistor Tr 6  is connected to the output port Out 2 . A base of the transistor Tr 5  and a base of the transistor Tr 6  are connected to a node N between the resistors R 13  and R 14 . One end of the capacitor C 1  is grounded, and the other end of the capacitor C 1  is connected to the node N. The resistors R 13  and R 14  configure a voltage divider for dividing a power supply voltage at the node N. A voltage of the node N determined by the resistors R 13  and R 14  sets a base bias of the transistors Tr 5  and Tr 6 . The transistors Tr 3  and Tr 4  may be InP-based n-type Double Heterojunction Bipolar Transistors (InP-DHBT). 
         [0068]    Further, one end of the resistor R 15  is connected to an emitter of the transistor Tr 5  and a collector of the transistor Tr 3 . Further, one end of resistor R 16  is connected to an emitter of the transistor Tr 6  and a collector of the transistor Tr 4 . These resistors R 15  and R 16  work as current sources for generating a current flowing through the emitters of the transistors Tr 5  and Tr 6 , regardless of switching states of the transistors Tr 3  and Tr 4 . 
         [0069]      FIG. 6  illustrates an example of a related circuit between the output ports Out 1  and Out 2  and the electrodes  41   c  and  41   d . As described below, the driving voltage signals from the driving circuit  46  are output to the electrodes  41   c  and  41   d  of the first QPSK modulator  4  through alternating current coupling. 
         [0070]    The output ports Out 1  and Out 2  are connected to a power supply Vcc through resistors R 21  and R 22 , respectively. That is, the driving circuit  46  has an open collector output configuration. Further, when the driving circuit  46  is configured not by a bipolar transistor but by a Metal-Oxide-Semiconductor (MOS) transistor, the driving circuit  46  may has an open drain output configuration. 
         [0071]    Further, the output ports Out 1  and Out 2  are connected to the electrodes  41   c  and  41   d  through capacitors C 21  and C 22 , respectively. The electrodes  41   c  and  41   d  are connected to the bias voltages V XIB1  and V XIB2  output from the bias control circuit  47  through inductors L 21  and L 22 , respectively. 
         [0072]    Here, the control circuit  49  decreases an amplitude of the driving voltage signal output from the driving circuit  46  by decreasing a current defined by the current source I 4 , of the differential circuit configured by the transistors Tr 3  and Tr 4  and the current source I 4  of the differential amplifier  46 A. The amplitude of the driving voltage signal output by the differential amplifier  46 A is determined based on the current defined by the current source I 4  and resistance of the resistors R 21  and R 22  as load resistors. Therefore, the amplitude of the driving voltage signal output by the driving circuit  46  is decreased by decreasing the current provided by the current source I 4 . 
         [0073]    Next, a process, in which the optical transceiver  1  according to the present embodiment is switched from the normal operation state to the transmitter disable state, will be described with reference to  FIG. 7 . 
         [0074]    First, the control circuit  49  detects assertion of the Tx_Disable signal from the hose device (step S 11 ). Next, the control circuit  49  stops an operation of the bias control circuit  47  (step S 12 ). Next, the control circuit  49  turns on an attenuation operation of the VOA  45 , and blocks an optical output from the first QPSK modulator  4  (or the second QPSK modulator  5 ) (step S 13 ). 
         [0075]    Next, the control circuit  49  decreases amplitudes of the driving voltage signals V XID1 , V XID2 , V XQD1  and V XQD2  output from the driving circuit  46  while blocking the optical output from the first QPSK modulator  4  (or the second QPSK modulator  5 ) by the VOA  45  (step S 14 ). As described above, the amplitudes of the driving voltage signals V XID1 , V XID2 , V XQD1  and V XQD2  are decreased, for example, by decreasing the current provided by the current source I 4  of the differential amplifier  46 A. 
         [0076]    Next, the control circuit  49  increases a resistance of the variable resistor element  48  (step S 15 ). When the amplitudes of the driving voltage signals V XID1 , V XID2 , V XQD1  and V XQD2  output from the driving circuit  46  is decreased, an optical power detected by the first optical power monitor  11  is decreased, so that an optical current from the first optical power monitor  11  is decreased. However, in this way, when the resistance of the variable resistor element  48  is increased, even when the optical current is being decreased, a monitoring sensitivity for the optical power is maintained by magnifying a potential at the one end of the variable resistor element  48 , so as to easily detect the optical power. 
         [0077]    Finally, the control circuit  49  resumes the operation of the bias control circuit  47  (step S 16 ). Then, the switch from the normal operation state to the transmitter disable state is completed. 
         [0078]    Next, a process, in which the optical transceiver  1  according to the present embodiment is switched from the transmitter disable state to the normal operation state, will be described with reference to  FIG. 8 . 
         [0079]    First, the control circuit  49  detects negation of the Tx_Disable signal from the host device (step S 21 ). Next, the control circuit  49  stops the operation of the bias control circuit  47  (step S 22 ). Further, the control circuit  49  increases the amplitudes of the driving voltage signals V XID1 , V XID2 , V XQD1 , and V XQD2  output from the driving circuit  46 , to values of the normal operation (step S 23 ). As described above, the increases in the amplitudes of the driving voltage signals V XID1 , V XID2 , V XQD1  and V XQD2  are performed, for example, by increasing the current provided by the current source I 4  of the differential amplifier  46 A, to a value of the normal operation. 
         [0080]    Next, the control circuit  49  decreases the resistance of the variable resistor element  48  (step S 24 ). By decreasing the resistance of the variable resistor element  48 , when the first QPSK modulator  4  starts the normal operation, even when the monitor current detected by the first optical power monitor  11  increases, the potential at the one end of the variable resistor element  48  is suppressed within a proper range, so that the optical power can be properly monitored. 
         [0081]    Next, the control circuit  49  turns off the attenuation operation of the VOA  45 , and passes the optical output from the first QPSK modulator  4  (or the second QPSK modulator  5 ) again (step S 25 ). 
         [0082]    Finally, the control circuit  49  resumes the operation of the bias control circuit  47  (step S 26 ). Then, the switching from the transmitter disable state to the normal operation state is completed. 
         [0083]    Next, a process of adjusting a phase difference between two lights passing through two optical waveguides of a Mach-Zehnder modulator, in a method of controlling the optical transceiver  1  according to the present embodiment, will be described with reference to  FIG. 9 . As in the optical transceiver  1  according to the present embodiment, when the first QPSK modulator  4  includes two Mach-Zehnder interference unit which are the first Mach-Zehnder interference unit  41  and the second Mach-Zehnder interference unit  42 , the first Mach-Zehnder interference unit  41  and the second Mach-Zehnder interference unit  42  may adjust the bias voltages V XIB1 , V XIB2 , V XQB1  and V XQB2  in a time sharing system such that a phase difference between the light passing through the optical waveguide wg 21   a  (or the optical waveguide wg 22   a ) and the light passing through the optical waveguide wg 21   b  (or the optical waveguide wg 22   b ) becomes π. When the optical waveguides are configured by semiconductor optical waveguides, if a large bias voltage is applied to the optical waveguides, light may not be transferred at all. When the first Mach-Zehnder interference unit  41  adjusts the bias voltages V XIB1  and V XIB2 , light is absorbed to the optical waveguide wg 24  by applying a large negative bias voltage to the electrode  44  installed in the optical waveguide wg 24  located at an output side of the second Mach-Zehnder interference unit  42 . Then, when the first Mach-Zehnder interference unit  41  adjusts the bias voltages, an optical output from the second Mach-Zehnder interference unit  42  becomes approximately zero. Likewise, when the second Mach-Zehnder interference unit  42  adjusts the bias voltages V XQB1  and V XQB2 , light is absorbed to the optical waveguide wg 23  by applying a large negative bias voltage to the electrode  43  installed in the optical waveguide wg 23  located at an output side of the first Mach-Zehnder interference unit  41 . Then, when the second Mach-Zehnder interference unit  42  adjusts the bias voltages, an optical output from the first Mach-Zehnder interference unit  41  becomes approximate zero. 
         [0084]    Otherwise, in another method according to the present invention, the second Mach-Zehnder interference unit  42  may maintain the normal operation state, and the first Mach-Zehnder interference unit  41  may adjust the bias voltages V XIB1  and V XIB2 . In this case, although an optical output is provided since the second Mach-Zehnder interference unit  42  performs the normal operation, it can be considered that offset light is provided when the first Mach-Zehnder interference unit  41  performs the adjustment, so that there is no problem when a relative phase difference is found. Further, even when the first time of the adjustment of the second Mach-Zehnder interference unit  42 , it can be considered that an offset light is only provided at the time of the adjustment of the second Mach-Zehnder interference unit  42 . Further, a switching of the adjustment of the first and second Mach-Zehnder interference units  41  and  42  is performed earlier than drifting of a driving point, so as to avoid an influence of light output by the other interference units. 
         [0085]      FIGS. 9A and 9B  are vector diagrams schematically illustrating phases of two lights passing through the optical waveguides wg 21   a  and wg 21   b .  FIG. 9A  illustrates a case where a phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  is π, and  FIG. 9B  illustrates a case where the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  is not π. 
         [0086]    Now, it may be assumed that the bias voltage V XIB1  is applied such that the phase of the light passing through the optical waveguide wg 21   a  becomes a phase value obtained by adding π/2 to a reference phase. At this time, the phase of the light passing through the optical waveguide wg 21   a  vibrates around the phase value obtained by adding π/2 to the reference phase. That is, the phase of the light passing through the optical waveguide wg 21   a  vibrates between vector a and vector b of  FIG. 9A . 
         [0087]    First, a case will be described where the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  is π. The phase of the light passing through the optical waveguide wg 21   b  has a phase value obtained by adding 3π/2 to the reference phase. At this time, the phase of the light passing through the optical waveguide wg 21   b  vibrates about the phase value obtained by adding 3π/2 to the reference phase. That is, the phase of the light passing through the optical waveguide wg 21   b  vibrates between a vector c and a vector d of  FIG. 9A . 
         [0088]    Here, a vector obtained by adding vector a to vector c refers to vector A, and a vector obtained by adding vector b to vector d refers to vector B. At this time, a size of a signal detected by the first optical power monitor  11  to monitor an optical output power is a difference between a real part of vector A and a real part of vector B. 
         [0089]    Next, a case will be described where the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  is not π. For convenience of description, the phase of the light passing through the optical waveguide wg 21   a  vibrates around the phase value obtained by adding π/2 to the reference phase. At this time, a phase difference of the light passing through the optical waveguide wg 21   b  vibrates around an angle deviating from the phase value obtained by adding 3π/2 to the reference phase. As illustrated in  FIG. 9B , the phase of the light passing through the optical waveguide wg 21   b  vibrates between vector cb and vector db. 
         [0090]    Here, a vector obtained by adding vector a to vector cb refers to vector Ab, and a vector obtained by adding vector b to vector db refers to vector Bb. At this time, a size of a signal detected by the first optical power monitor  11  to monitor an optical output power is a difference between a real part of vector Ab and a real part of vector Bb. 
         [0091]    As can be seen by comparing  FIG. 9A  with  FIG. 9B , a difference between a real part of vector A and a real part of vector B is larger than the difference between the real part of vector Ab and the real part of vector Bb. As described above, when the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  is π, the size of the signal detected by the first optical power monitor  11  is maximized. In other words, the control circuit  49  adjusts the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  to become π, by adequately adjusting the bias voltages V XIB1  and V XIB2  output from the bias control circuit  47 , such that the amplitude of the signal detected by the first optical power monitor  11  is maximized. 
         [0092]    In accordance with the aforementioned optical transceiver  1  according to the present embodiment, the amplitude of the driving signal for driving the first QPSK modulator  4  (or the second QPSK modulator  5 ) is decreased while blocking the optical output from the first QPSK modulator  4  (or the second QPSK modulator  5 ) by the VOA  45 . Thus, power consumption of the driving circuit is decreased during the blocking operation by the VOA  45 . At the same time, since the light input to the first optical power monitor  11  does not disappear completely, the bias control circuit  47  can continuously control the bias voltage applied to the first QPSK modulator  4  (or the second QPSK modulator  5 ), according to the optical output power of the first QPSK modulator  4  (or the second QPSK modulator  5 ). Accordingly, a bias point of the first QPSK modulator  4  (or the second QPSK modulator  5 ) can be adequately controlled while decreasing the power consumption of the optical transceiver  1 . 
         [0093]    Such an effect according to the present embodiment will be described with reference to  FIGS. 10A to 12B .  FIG. 10A  is a curve illustrating a relationship between the optical power at the output unit of the first Mach-Zehnder interference unit  41  and a potential difference between the driving signals applied to the two electrodes  41   c  and  41   d . A vertical axis thereof denotes an output power Pout, and a horizontal axis thereof denotes the potential difference between the driving voltage signals applied to the two electrodes  41   c  and  41   d . A voltage V π  denotes a difference value between the driving voltages where the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  becomes π. 
         [0094]    As described with reference to  FIG. 4 , During an approximately half time zone of the normal operation state, the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  is 2π. This state corresponds to point P 2π  in  FIG. 10A . During approximately another half time zone of the normal operation state, the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  is zero. This state corresponds to a point P 0  in  FIG. 10A . At a time of switching between the two states, the electric potential difference between the driving voltage signals applied to the electrodes  41   c  and  41   d  is varied between 0 and 2V π . 
         [0095]    In the aforementioned two states, since the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  is 2π or 0, the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  are strengthened with each other, so that the output light power becomes the maximum power Pout. Meanwhile, when the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  is not 2π or 0, the output light power is lower than the maximum power Pout. In particular, at a time when the potential difference between the driving voltage signals applied to the two electrodes  41   c  and  41   d  becomes Vπ, since the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  is π, the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  are weakened with each other, so that the output light power is decreased to theoretically become zero. The curve in  FIG. 10A  illustrates a relation between the potential difference between the driving voltage signals applied to the two electrodes  41   c  and  41   d  and the output power. 
         [0096]    During most of the normal operation, the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  is zero or 2π. Thus, as illustrated by points P 0  and P 2π  in  FIG. 10A , the output power is almost always Pmax, and only at a time of the switching between points P 0  and P 2π , the output light power is decreased. 
         [0097]    However, as described above, the electrodes  41   c  and  41   d  are biased by the bias control circuit such that the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  becomes π. Thus, when the driving circuit  46  is completely stopped, as illustrated in  FIG. 11A , the potential difference between the electrodes  41   c  and  41   d  becomes V π  which allows the phase difference between the light passing through the optical waveguide wg 21   a  and the light passing through the optical waveguide wg 21   b  to become π. In this state, as illustrated in  FIG. 10B , the output light power becomes almost zero, so that it is difficult to detect the optical power using the first optical power monitor  11 . Thus, the bias control circuit  47  cannot control the bias voltage by monitoring the output power, and operation points of the first Mach-Zehnder interference unit  41  and the second Mach-Zehnder interference unit  42  drifts. 
         [0098]    On the other hand, in the present embodiment, even when the VOA  45  starts an operation of blocking the output light, just the reduction of the amplitudes of the driving voltage signals for driving the first QPSK modulator  4  cannot prevent the driving circuit  46  from outputting the driving signal. Thus, the first Mach-Zehnder interference unit  41  and the second Mach-Zehnder interference unit  42  operate between point P A  and point P B  in  FIG. 12A . Accordingly, the optical power detected by the first optical power monitor  11  is lower than that of the normal operation, illustrated as a dotted line in  FIG. 12B . However, the optical power detected by the first optical power monitor  11  is a value having an amplitude illustrated as a solid line in  FIG. 12B . Thus, the bias control circuit  47  can continuously control the bias voltage applied to the first QPSK modulator  4  (or the second QPSK modulator  5 ) by monitoring the optical output power of the first QPSK modulator  4  (or the second QPSK modulator  5 ). 
         [0099]    Moreover, it is possible to reduce the power consumption of the optical transceiver  1 . As illustrated in  FIGS. 13A to 13D , after the control circuit  49  receives the Tx_Disable signal, power consumption P VOA  of the VOA  45  is increased as illustrated in  FIG. 13C . However, as illustrated in  FIG. 13B , an amount of a decrease in power consumption P d , of the driving circuit  46  is larger than an amount of the increase in the power consumption P VOA  of the VOA  45 . Thus, as illustrated in  FIG. 13A , power consumption P tx1  of the optical transceiver  1  (the power consumption P tx1  is increased and decreased equally with respect to the sum of the power consumption P driver  of the driving circuit  46  and the power consumption P VOA  of the VOA  45 ) is decreased after the Tx_Disable signal is received. Further, an optical power of the monitoring light of the first optical power monitor  11  is decreased as illustrated in  FIG. 13D , by decreasing the power consumption P driver  of the driving circuit  46  as illustrated in  FIG. 13B . However, even when the optical power of the monitoring light is decreased in this way, the bias control circuit  47  can control the bias voltage according to the optical power detected thereby. 
         [0100]    In this way, in accordance with the optical transceiver  1  according to the present embodiment, a bias point of the first QPSK modulator  4  (or the second QPSK modulator  5 ) can be adequately controlled while decreasing the power consumption of the optical transceiver  1 . 
         [0101]    Further, the bias voltage is adjusted such that the phase differences between two lights passing through the first optical waveguides wg 21   a  and wg 22   a  of the first Mach-Zehnder interference unit  41  and the second Mach-Zehnder interference unit  42  and two lights passing through the second optical waveguides wg 21   b  and wg 22  of the first Mach-Zehnder interference unit  41  and the second Mach-Zehnder interference unit  42  become π. Accordingly, the phase differences between two lights passing through the first optical waveguides wg 21   a  and wg 22   a  and two lights passing through the second optical waveguides wg 21   b  and wg 22   b  can be controlled to be a which is an adequate value when the optical transceiver returns to the normal operation state. 
         [0102]    Further, the bias voltages of the two Mach-Zehnder interference units which are the first Mach-Zehnder interference unit  41  and the second Mach-Zehnder interference unit  42  are adjusted in a time sharing system. Accordingly, the two Mach-Zehnder interference units can use the one bias control circuit  47 , the one driving circuit  46  or the one control circuit  49  together, and the downsizing and the reduction in power consumption of the optical transceiver  1  can be achieved. 
         [0103]    Further, since the driving circuit has a differential circuit configuration, the driving voltage signals can be provided to the first QPSK modulator  4  and the second QPSK modulator  5  as a pair of complementary voltage signals. Further, since the driving voltage signals are output to the first QPSK modulator  4  and the second QPSK modulator  5  through the current switching, the bias point of the driving voltage signals can be optimized. Furthermore, the amplitudes of the driving voltage signals can be easily decreased by decreasing the current provided by the current source I 4  included in the differential circuit of the driving circuit  46 . 
         [0104]    Further, the optical transceiver and the method of controlling the optical transceiver according to the present invention are not limited to the aforementioned embodiment. 
         [0105]    For example,  FIG. 14  illustrates a first QPSK modulator  4 A as one modified example of the first QPSK modulator  4  illustrated in  FIG. 3 . A bias control circuit  47 A and a control circuit  49 A instead of the bias control circuit  47  and the control circuit  49  are installed in the first QPSK modulator  4 A. In the first QPSK modulator  4 A, the bias control circuit  47 A instead of the control circuit  49 A controls a resistance of the variable resistor element  48 . 
         [0106]    Further,  FIG. 15  illustrates a first QPSK modulator  4 B as another modified example of the first QPSK modulator  4  illustrated in  FIG. 3 . A resistive element Rp for detecting a power consumption regarding an electric power line is added in the first QPSK modulator  4 B. A control circuit  49 B detects a voltage drop between opposite ends of the resistive element Rp to detect a power consumption generated by the VOA  45 . Further, while VOA  45  blocks the output light, the control circuit  49 B detects power consumption increased because of the blocking of the output light by the VOA  45 . The control circuit  49 B decreases the amplitudes of the driving voltage signals from the driving circuit  46  such that the power consumption of the driving circuit  46  is decreased to compensate the power consumption increased by the VOA  45 . 
         [0107]    Further,  FIG. 16  illustrates a first QPSK modulator  4 C as yet another modified example of the first QPSK modulator  4  illustrated in  FIG. 3 . The bias control circuit  47 A and a control circuit  49 C instead of the bias control circuit  47  and the control circuit  49  are installed in the first QPSK modulator  4 C. In the first QPSK modulator  4 C, the bias control circuit  47 A instead of the control circuit  49 C controls a resistance of the variable resistor element  48 . Further, a resistive element Rp for detecting a power consumption of an electric power line is installed in the first QPSK modulator  4 C. The control circuit  49 C detects a voltage drop between opposite ends of the resistive element Rp to detect a power consumption generated by the VOA  45 . Further, while VOA  45  blocks the output light, the control circuit  49 C detects power consumption increased because of the blocking of the output light by the VOA  45 . The control circuit  49 C decreases the amplitudes of the driving voltage signals from the driving circuit  46  such that the power consumption of the driving circuit  46  is decreased to compensate the power consumption increased by the VOA  45 . 
         [0108]    Further, the optical transceiver may include one or more semiconductor Mach-Zehnder modulators instead of the first QPSK modulator  4  and the second QPSK modulator  5  which are described above.