Patent Publication Number: US-9413467-B2

Title: Optical transmitter, optical transmission/reception system, and drive circuit

Description:
TECHNICAL FIELD 
     The present invention relates to an optical transmitter, an optical transmission/reception system, and a drive circuit, and more particularly, to an optical transmitter, an optical transmission/reception system, and a drive circuit, which perform multilevel modulation. 
     BACKGROUND ART 
     With an explosive increase in demand of a broadband multimedia communication service such as the Internet or a high-definition digital TV broadcast, a dense wavelength-division multiplexing optical fiber communication system, which is suitable for a long-distance and large-capacity transmission and is highly reliable, has been introduced in trunk line networks and metropolitan area networks. In access networks, an optical fiber access service spreads rapidly. In such an optical fiber communication system, cost reduction for laying optical fibers as optical transmission lines and improvement of spectral efficiency per optical fiber are important. Therefore, a wavelength-division multiplexing technology which multiplexes multiple optical signals having different wavelengths is widely used. 
     In an optical transmitter for such a high-capacity wavelength-division multiplexing communication system, an optical modulator is required. In the optical modulator, high speed operation with small wavelength dependence is indispensable. Further, an unwanted optical phase modulation component which degrades the waveform of the received optical signal after long-distance transmission (in the case of using optical intensity modulation as a modulation method), or an optical intensity modulation component (in the case of using optical phase modulation as a modulation method) should be suppressed as small as possible. A Mach-Zehnder (MZ) optical intensity modulator in which waveguide-type optical phase modulators are embedded into an optical waveguide-type MZ interferometer is suitable for such a use. 
     To increase the transmission capacity per wavelength channel, a multilevel optical modulation signal system having a smaller optical modulation spectrum bandwidth than a typical binary optical intensity modulation system is advantageous in terms of the spectral efficiency, wavelength dispersion of an optical fiber, and resistance to polarization mode dispersion, each of which poses a problem. This multilevel optical modulation signal system is considered to become mainstream particularly in optical fiber communication systems in trunk line networks exceeding 40 Gb/s, the demand for which is expected to increase in the future. For such use, a monolithically integrated multilevel IQ optical modulator in which two MZ optical intensity modulators described above and an optical multiplexer/demultiplexer are used in combination has recently been developed. 
     In high speed optical modulation by using this optical modulator, especially in the high-frequency region in which the frequency of a modulation electric signal is over 1 GHz, the propagating wavelength of the modulation electric signal becomes not negligibly short compared with the length of an electrode formed in an optical phase modulator region in the optical modulator. Therefore, voltage distribution of the electrode serving as means for applying an electric field to the optical phase modulator is no longer regarded as uniform in an optical signal propagation axis direction. To estimate optical modulation characteristics exactly, it is required to treat the electrode as a distributed constant line and treat the modulation electric signal propagating through the optical phase modulator region as a traveling-wave, respectively. In that case, in order to increase the effective interaction length with the modulated optical signal and the modulation electric signal, a so-called traveling-wave type electrode which is devised to make a phase velocity v o  of the modulated optical signal and a phase velocity v m  of the modulation electric signal as close to each other as possible (phase velocity matching) is required. 
     An optical modulator module having a segmented electrode structure to realize the traveling-wave type electrode and the multilevel optical modulation signal system has already been proposed (Patent Literature 1 to 4). An optical modulator module capable of performing multilevel control of a phase variation of a modulated optical signal in each segmented electrode has also been proposed. This optical modulator module is a compact, broad-band, and low-drive-voltage optical modulator module capable of generating any multilevel optical modulation signal, while maintaining phase velocity matching and impedance matching, which are required for a traveling-wave structure operation, by inputting a digital signal. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. H07-13112 
     Patent Literature 2: Japanese Unexamined Patent Application Publication No. H05-289033 
     Patent Literature 3: Japanese Unexamined Patent Application Publication No. H05-257102 
     Patent Literature 4: International Patent Publication No. WO 2011/043079 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the present inventor has found that the above-mentioned optical modulator module has the following problem. In theory, in the segmented electrode structure, value multiplexing corresponding to the number of segmented electrodes can be achieved by increasing the number of segmented electrodes. However, the number of segmented electrodes mountable on the optical modulator module to be actually prepared is limited depending on the size of the optical modulator. Accordingly, the number of levels of the multilevel modulation is limited in practice. 
     In this regard, the signal to be applied to each segmented electrode can be multileveled. A driving signal may be supplied to each segmented electrode by a multilevel D/A converter that outputs an analog signal according to an input digital signal. However, this technique requires a number of multilevel D/A converters corresponding to the number of segmented electrodes. In general, when a large number of multilevel D/A converters having a large circuit size are mounted on an optical transmitter, the size of the optical transmitter itself increases, which leads to an increase in cost. 
     The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an optical transmitter, an optical transmission/reception system, and a drive circuit, which are capable of higher-order multilevel modulation with a small-scale circuit configuration. 
     Solution to Problem 
     An optical transmitter according to an exemplary aspect of the present invention includes: an optical modulator including an optical transmission line through which an optical signal propagates, a plurality of phase modulation regions being formed on the optical transmission line; and a drive circuit that outputs a driving signal to each of the plurality of phase modulation regions according to an input digital signal. The drive circuit includes: a bit splitting unit that splits the input digital signal into upper bits and lower bits; a lower-bit drive unit that outputs, as a driving signal, a value obtained by performing D/A conversion on the lower bits, to a first phase modulation region of the plurality of modulation regions; and an upper-bit drive unit that outputs, to a phase modulation region different from the first phase modulation region, a value greater than a maximum value of the driving signal output from the lower-bit drive unit, or a minimum value of the driving signal output from the lower-bit drive unit, as a driving signal, according to a value of the upper bits. 
     An optical transmission/reception system according to another exemplary aspect of the present invention includes: an optical transmitter that sends an optical signal; a transmission line through which the optical signal propagates; and an optical receiver that receives the optical signal via the transmission line. The optical transmitter includes: an optical modulator including an optical transmission line through which an optical signal propagates, a plurality of phase modulation regions being formed on the optical transmission line; and a drive circuit that outputs a driving signal to each of the plurality of phase modulation regions according to an input digital signal. The drive circuit includes: a bit splitting unit that splits the input digital signal into upper bits and lower bits; a lower-bit drive unit that outputs, as a driving signal, a value obtained by performing D/A conversion on the lower bits, to a first phase modulation region of the plurality of modulation regions; and an upper-bit drive unit that outputs, to a phase modulation region different from the first phase modulation region, a value greater than a maximum value of the driving signal output from the lower-bit drive unit, or a minimum value of the driving signal output from the lower-bit drive unit, as a driving signal, according to a value of the upper bits. 
     A drive circuit according to still another exemplary aspect of the present invention includes: a bit splitting unit that splits an input digital signal into upper bits and lower bits; a lower-bit drive unit that outputs, as a driving signal, a value obtained by performing D/A conversion on the lower bits, to a first phase modulation region of a plurality of modulation regions formed on an optical transmission line through which an optical signal propagates, the optical transmission line being formed in an optical modulator; and an upper-bit drive unit that outputs, to a phase modulation region different from the first phase modulation region, a value greater than a maximum value of the driving signal output from the lower-bit drive unit, or a minimum value of the driving signal output from the lower-bit drive unit, as a driving signal, according to a value of the upper bits. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide an optical transmitter, an optical transmission/reception system, and a drive circuit, which are capable of higher-order multilevel modulation with a small-scale circuit configuration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram schematically showing a configuration of a multilevel optical transmitter  500  having a typical segmented electrode structure; 
         FIG. 2A  is a diagram schematically showing a configuration of an optical multiplexer/demultiplexer  513 : 
         FIG. 2B  is a diagram schematically showing a configuration of an optical multiplexer/demultiplexer  514 ; 
         FIG. 3  is an operation table showing operations of the optical transmitter  500 ; 
         FIG. 4  is a diagram schematically showing a mode in which light propagates in the optical transmitter  500 ; 
         FIG. 5A  is a constellation diagram showing light L 1  and light L 2  which are not subjected to phase modulation by phase modulation regions PM 51 _ 1  to PM 51 _ 4  and phase modulation regions PM 52 _ 1  to PM 52 _ 4 ; 
         FIG. 5B  is a constellation diagram showing the light L 1  and light L 2  when the binary code of an input digital signal is “0000” in the optical transmitter  500 ; 
         FIG. 5C  is a constellation diagram showing the light L 1  and light L 2  in the optical transmitter  500 ; 
         FIG. 5D  is a constellation diagram showing the light intensity of output light OUT obtained by multiplexing the light L 1  and light L 2  in the optical transmitter  500 : 
         FIG. 6  is a block diagram schematically showing a configuration of an optical transmitter  100  according to a first exemplary embodiment; 
         FIG. 7  is an operation table showing operations of the optical transmitter  100  according to the first exemplary embodiment; 
         FIG. 8  is a block diagram schematically showing a configuration of an optical transmitter  200  according to a second exemplary embodiment; 
         FIG. 9  is a block diagram schematically showing a configuration of an optical transmitter  300  according to a third exemplary embodiment; 
         FIG. 10  is an operation table showing operations of the optical transmitter  300  according to the third exemplary embodiment; and 
         FIG. 11  is a block diagram schematically showing a configuration of an optical transmission/reception system  400  according to a fourth exemplary embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Exemplary embodiments of the present invention will be described below with reference to the drawings. In the drawings, the same elements are denoted by the same reference numerals, and a redundant explanation is omitted as needed. 
     As a prerequisite for understanding the configuration and operation of each optical transmitter according to exemplary embodiments described below, a multilevel optical transmitter  500  having a typical segmented electrode structure will be described. The optical transmitter  500  is a multilevel modulation optical transmitter. In this case, however, to simplify the explanation, the optical transmitter  500  will be described as a 4-bit optical transmitter.  FIG. 1  is a block diagram schematically showing the configuration of the multilevel optical transmitter  500  having a typical segmented electrode structure. The optical transmitter  500  includes an optical modulator  51 , a decoder  52 , and a drive circuit  53 . 
     The optical modulator  51  outputs output light OUT which is obtained by modulating input light IN. The optical modulator  51  includes optical waveguides  511  and  512 , optical multiplexers/demultiplexers  513  and  514 , and phase modulation regions PM 51 _ 1  to PM 51 _ 4  and PM 52 _ 1  to PM 52 _ 4 . The optical waveguides  511  and  512  are arranged in parallel. 
     The optical multiplexer/demultiplexer  153  is disposed at the optical signal input (input light IN) side of the optical waveguides  511  and  512 . At the input side of the optical multiplexer/demultiplexer  513 , the input light IN is input to an input port P 1 , and an input port P 2  has no input. At the output side of the optical multiplexer/demultiplexer  513 , the optical waveguide  511  is connected to an output port P 3  and the optical waveguide  512  is connected to an output port P 4 . 
       FIG. 2A  is a diagram schematically showing the configuration of the optical multiplexer/demultiplexer  513 . In the optical multiplexer/demultiplexer  513 , the light which has entered the input port P propagates to the output ports P 3  and P 4 ; however, the light propagating from the input port P 1  to the output port P 4  has a phase that is delayed by 90° relative to the light propagating from the input port P 1  to the output port P 3 . The light which has entered the input port P 2  propagates to the output ports P 3  and P 4 ; however, the light propagating from the input port P 2  to the output port P 3  has a phase that is delayed by 90° relative to the light propagating from the input port P 2  to the output port P 4 . 
     The optical multiplexer/demultiplexer  514  is disposed at the optical signal output (output light OUT) side of the optical waveguides  511  and  512 . At the input side of the optical multiplexer/demultiplexer  514 , the optical waveguide  511  is connected to an input port P 5  and the optical waveguide  512  is connected to an input port P 6 . At the output side of the optical multiplexer/demultiplexer  514 , the output light OUT is output from an output port P 7 . 
       FIG. 2B  is a diagram schematically showing the configuration of the optical multiplexer/demultiplexer  514 . The optical multiplexer/demultiplexer  514  has a configuration similar to that of the optical multiplexer/demultiplexer  513 . The input ports P 5  and P 6  respectively correspond to the input ports P 1  and P 2  of the optical multiplexer/demultiplexer  513 . The output ports P 7  and P 8  respectively correspond to the output ports P 3  and P 4  of the optical multiplexer/demultiplexer  513 . The light which has entered the input port P 5  propagates to the output ports P 7  and P 8 ; however, the light propagating from the input port P 5  to the output port P 8  has a phase that is delayed by 90° relative to the light propagating from the input port P 5  to the output port P 7 . The light which has entered the input port P 6  propagates to the output ports P 7  and P 8 ; however, the light propagating from the input port P 6  to the output port P 7  has a phase that is delayed by 90° relative to the light propagating from the input port P 6  to the output port P 8 . 
     The phase modulation regions PM 51 _ 1  to PM 51 _ 4  are arranged on the optical waveguide  511  between the optical multiplexer/demultiplexer  513  and the optical multiplexer/demultiplexer  514 . The phase modulation regions PM 52 _ 1  to PM 52 _ 4  are arranged on the optical waveguide  512  between the optical multiplexer/demultiplexer  513  and the optical multiplexer/demultiplexer  514 . 
     The term “phase modulation region” used herein refers to a region including an electrode formed on the optical waveguide. When an electric signal, such as a voltage signal, is applied to the electrode, the effective refractive index of the optical waveguide under the electrode changes. As a result, the substantial optical path length of the optical waveguide of the phase modulation region can be changed. This allows the phase modulation region to change the phase of the optical signal propagating through the optical waveguide. Further, the optical signal can be modulated by applying a phase difference between the optical signals propagating through the two optical waveguides  511  and  512 . That is, the optical modulator  51  forms a multilevel Mach-Zehnder optical modulator having two arms and an electrode segmented structure. 
     The decoder  52  decodes 4-bit input digital signals D[3:0], and outputs, for example, multi-bit signals D 1  to D 4 , to the drive circuit  53 . 
     The drive circuit  53  includes five-value D/A converters DAC 51  to DAC 54 . The D/A converters DAC 51  to DAC 54  are respectively supplied with the signals D 1  to D 4 . The D/A converters DAC 51  to DAC 54  output a pair of differential output signals according to the signals D 1  to D 4 , respectively. At this time, the positive-phase output signals of the differential output signals output from the D/A converters DAC 51  to DAC 54  are respectively output to the phase modulation regions PM 51 _ 1  to PM 51 _ 4 . The negative-phase output signals of the differential output signals output from the D/A converters DAC 51  to DAC 54  are respectively output to the phase modulation regions PM 52 _ 1  to PM 52 _ 4 . 
     The differential output signals output from the D/A converters DAC 51  to DAC 54  will now be described. As mentioned above, the D/A converter DAC 51  is a D/A converter which outputs five values (0, 1, 2, 3, and 4). Specifically, the DAC 51  increases the value of the positive-phase output signal in the order of “0”→“1”→“2”→“3”→“4” in accordance with an increase in the value of the signal D 1 . 
     On the other hand, the DAC 51  outputs an inverted signal of the positive-phase output signal as the negative-phase output signal. Specifically, the DAC 51  increases the value of the negative-phase output signal in the order of “4”→“3”→“2”→“1”→“0” in accordance with an increase in the value of the signal D 1 . It can also be understood that the value of the negative-phase output signal is determined so that the sum of the values of the positive-phase output signal and the negative-phase output signal becomes equal to the maximum value “4” of the five output values. 
       FIG. 3  is an operation table showing operations of the optical transmitter  500 . As the value of the input digital signals D[3:0] increases in the order of “0000”→“0001”→“0010”→“0011”→“0100”, the D/A converter DAC 51  increases the value of the positive-phase output signal in the order of “0”→“1”→“2”→“3”→“4” and decreases the value of the negative-phase output signal in the order of “4”→“3”→“2”→“1”→“0”. In this case, however, when the value of the input digital signals D[3:0] is equal to or greater than “0101”, the value of the positive-phase output signal of the D/A converter DAC 51  is “4” and the value of the negative-phase output signal is “0”. 
     As the value of the input digital signals D[3:0] increases in the order of “0100”→“0101”→“0110”→“0111”→“1000”, the D/A converter DAC 52  increases the value of the positive-phase output signal in the order of “0”→“1”→“2”→“3”→“4” and decreases the value of the negative-phase output signal in the order of “4”→“3”→“2”→“1”→“0”. In this case, however, when the value of the input digital signals D[3:0] is equal to or smaller than “0011”, the value of the positive-phase output signal of the D/A converter DAC 52  is “0” and the value of the negative-phase output signal is “4”. When the value of the input digital signals D[3:0] is equal to or greater than “1001”, the value of the positive-phase output signal of the D/A converter DAC 52  is “4” and the value of the negative-phase output signal is “0”. 
     As the value of the input digital signals D[3:0] increases in the order of “1000”→“1001”→“1010”→“1011”→“1100”, the D/A converter DAC 53  increases the value of the positive-phase output signal in the order of “0”→“1”→“2”→“3”→“4” and decreases the value of the negative-phase output signal in the order of “4”→“3”→“2”→“1”→“0”. In this case, however, when the value of the input digital signals D[3:0] is equal to or smaller than “0111”, the value of the positive-phase output signal of the D/A converter DAC 53  is “0” and the value of the negative-phase output signal is “4”. When the value of the input digital signals D[3:0] is equal to or greater than “1101”, the value of the positive-phase output signal of the D/A converter DAC 53  is “4” and the value of the negative-phase output signal is “0”. 
     As the value of the input digital signals D[3:0] increases in the order of “1100”→“1101”→“1110”→“1111”, the D/A converter DAC 54  increases the value of the positive-phase output signal in the order of “0”→“1”→“2”→“3” and decreases the value of the negative-phase output signal in the order of “4”→“3”→“2”→“1”. In this case, however, when the value of the input digital signals D[3:0] is equal to or smaller than “1011”, the value of the positive-phase output signal of the D/A converter DAC 51  is “0” and the value of the negative-phase output signal is “4”. 
     The phase modulation operation of the optical transmitter  500  will now be described.  FIG. 4  is a diagram schematically showing a mode in which light propagates in the optical transmitter  500 . In this example, as shown in  FIG. 1 , the input light IN is input to the input port P 1  of the optical multiplexer/demultiplexer  513 . Accordingly, the light output from the output port P 4  has a phase that is delayed by 90° relative to the light output from the output port P 3 . After that, the light output from the output port P 3  passes through the phase modulation regions PM 51 _ 1  to PM 51 _ 4  and reaches the input port P 5  of the optical multiplexer/demultiplexer  514 . The light which has reached the input port P 5  directly reaches the output port P 7 . On the other hand, the light output from the output port P 4  passes through the phase modulation regions PM 52 _ 1  to PM 52 _ 4  and reaches the input port P 6  of the optical multiplexer/demultiplexer  514 . The light which has reached the input port P 6  has a phase that is further delayed by 90°, and reaches the output port P 7 . 
     In other words, light L 2  which reaches the output port P 7  from the input port P 6  has a phase that is delayed by 180° relative to light L 1  which reaches the output port P 7  from the input port P 5 , even when the phase modulation regions PM 51 _ 1  to PM 51 _ 4  and the phase modulation regions PM 52 _ 1  to PM 52 _ 4  do not perform any phase modulation. 
       FIG. 5A  is a constellation diagram showing the light L and light L 2  which are not subjected to phase modulation by the phase modulations regions PM 51 _ 1  to PM 51 _ 4  and the phase modulation regions PM 52 _ 1  to PM 52 _ 4 . As described above, the light L 2  which reaches the output port P 7  from the input port P 6  is delayed by 180° relative to the light L which reaches the output port P 7  from the input port P 5 . 
     Meanwhile, in the optical transmitter  500 , the positive-phase output signal is input to each of the phase modulation regions PM 51 _ 1  to PM 51 _ 4 , and the negative-phase output signal is input to each of the phase modulation regions PM 52 _ 1  to PM 52 _ 4 . Accordingly, the phase delay of the light L 2  which reaches the output port P 7  from the input port P 6  is compensated.  FIG. 5B  is a constellation diagram showing the light L 1  and light L 2  when the binary code of the input digital signals D[3:0] is “0000” in the optical transmitter  500 . For example, when the binary code of the input digital signals D[3:0] is “0000”, the positive-phase output signal indicating “0” is input to each of the phase modulation regions PM 51 _ 1  to PM 51 _ 4 , and the negative-phase output signal indicating “4” is input to each of the phase modulation regions PM 52 _ 1  to PM 52 _ 4 . Accordingly, the phase of the light passing through the phase modulation regions PM 52 _ 1  to PM 52 _ 4  is further delayed by 180°. 
     That is, the phase delay of 180° generated due to the phase modulation regions PM 52 _ 1  to PM 52 _ 4 , as well as the original phase delay of 180°, is added to the light L 2  which reaches the output port P 7  from the input port P 6 . Thus, a phase delay of 360° is generated in the light L 2  which reaches the output port P 7  from the input port P 6 , so that the phase delay with respect to the light L 1 , which reaches the output port P 7  from the input port P 5 , is substantially eliminated. Furthermore, the value of the negative-phase output signal is decreased as the binary code of the input digital signals D[3:0] increases and the value of each of the positive-phase output signals output from the DAC 51  to DAC 54  increases. 
       FIG. 5C  is a constellation diagram showing the light L 1  and light L 2  in the optical transmitter  500 . As shown in  FIG. 5C , when the differential output signals are used, the optical phases of the light L 1  and the light L 2  change symmetrically with respect to an Re axis, while the phase delay of the light L 2 , which reaches the output port P 4  from the input port P 1  and reaches the output port P 7  from the input port P 6 , is compensated according to a change in the input digital signals D[3:0], thereby achieving an optical D/A conversion in the optical transmitter. With this configuration, the amount of phase modulation of the light L 1  can be changed in 16 levels, i.e., 0 to 15 Δθ, and the amount of phase modulation of the light L 2  can be changed in levels, i.e., 0 to −15 Δθ, according to the value of the input digital signals D[3:0], as shown in the operation table of  FIG. 3 . 
     To facilitate understanding of the drawings,  FIGS. 5B and 5C  illustrate the positions of the light L 1  and light L 2  so as not to coincide with each other when the binary code of the input digital signals D[3:0] is “0000” or “1111”. In other words, when the binary code of the input digital signals D[3:0] is “0000” or “1111”, the positions of the light L 1  and light L 2  may coincide with each other. The case where the amount of variation in the phase that is modulated in the phase modulation regions varies in the range of 0 to 180 degrees according to the input digital signal has been described above, but the amount of phase variation is not limited to this range. 
     The configuration described above allows the optical transmitter to function as a 4-bit optical transmitter. However, if the levels of the phases of the light L 1  and light L 2 , which are subjected to phase modulation by the drive circuit  53 , are at regular intervals, the following problem arises.  FIG. 5D  is a constellation diagram showing the light intensity of the output light OUT obtained by multiplexing the light L 1  and light L 2  in the optical transmitter  500 . As shown in  FIG. 5D , when the phase of the optical signal is shifted at regular intervals, the interval between the levels of the light intensity of the output light is not uniform, which makes it difficult to ensure the linearity of the signal intensity of the output light with respect to the input digital signal. 
     First Exemplary Embodiment 
     First, an optical transmitter  100  according to a first exemplary embodiment of the present invention will be described. The optical transmitter  100  is a multilevel modulation optical transmitter. In this case, however, to simplify the explanation, the optical transmitter  100  will be described as a 4-bit optical transmitter.  FIG. 6  is a block diagram schematically showing the configuration of the optical transmitter  100  according to the first exemplary embodiment. The optical transmitter  100  includes an optical modulator  11  and a drive circuit  12 . 
     The optical modulator  11  outputs output light OUT which is obtained by modulating input light IN. The optical modulator  11  includes optical waveguides  111  and  112 , optical multiplexers/demultiplexers  113  and  114 , and phase modulation regions PM 11 _ 1  to PM 11 _ 4  and PM 12 _ 1  to PM 12 _ 4 . The optical waveguides  111  and  112  are arranged in parallel. 
     The optical multiplexer/demultiplexer  113  is disposed at the optical signal input (input light IN) side of the optical waveguides  111  and  112 . The optical multiplexer/demultiplexer  113  has a configuration similar to that of the optical multiplexer/demultiplexer  513  described above. At the input side of the optical multiplexer/demultiplexer  113 , the input light IN is input to the input port P 1 , and the input port P 2  has no input. At the output side of the optical multiplexer/demultiplexer  113 , the optical waveguide Ill is connected to the output port P 3  and the optical waveguide  112  is connected to the output port P 4 . 
     The optical multiplexer/demultiplexer  114  is disposed at the optical signal output (output light OUT) side of the optical waveguides  111  and  112 . The optical multiplexer/demultiplexer  114  has a configuration similar to that of the optical multiplexer/demultiplexer  514  described above. At the input side of the optical multiplexer/demultiplexer  114 , the optical waveguide  111  is connected to the input port P 5 , and the optical waveguide  112  is connected to the input port P 6 . At the output side of the optical multiplexer/demultiplexer  114 , the output light OUT is output from the output port P 7 . 
     The phase modulation regions PM 11 _ 1  to PM 11 _ 4  are arranged on the optical waveguide  111  between the optical multiplexer/demultiplexer  113  and the optical multiplexer/demultiplexer  114 . The phase modulation regions PM 12 _ 1  to PM 12 _ 4  are arranged on the optical waveguide  112  between the optical multiplexer/demultiplexer  113  and the optical multiplexer/demultiplexer  114 . 
     The term “phase modulation region” used herein refers to a region including an electrode formed on the optical waveguide. When an electric signal, such as a voltage signal, is applied to the electrode, the effective refractive index of the optical waveguide under the electrode changes. As a result, the substantial optical path length of the optical waveguide of the phase modulation region can be changed. Accordingly, the phase modulation region can change the phase of the optical signal propagating through the optical waveguide. Further, the optical signal can be modulated by applying a phase difference between the optical signals propagating through the two optical waveguides  111  and  112 . That is, the optical modulator  11  forms a multilevel Mach-Zehnder optical modulator having two arms and an electrode segmented structure. 
     The drive circuit  12  includes a lower-bit drive unit  121 , an upper-bit drive unit  122 , and a bit splitting unit  123 . The bit splitting unit  123  splits the 4-bit input digital signals D[3:0], which are supplied to the drive circuit  12 , into upper bits and lower bits. In this case, the bit splitting unit  123  splits the input digital signals D[3:0] into two upper bits D[3:2] and two lower bits D[1:0]. 
       FIG. 7  is an operation table showing operations of the optical transmitter  100  according to the first exemplary embodiment. The lower-bit drive unit  121  is supplied with the lower bits D[1:0]. The lower-bit drive unit  121  outputs a pair of differential output signals according to the value of the lower bits D[0:1]. At this time, the positive-phase output signal of the differential output signals output from the lower-bit drive unit  121  is output to the phase modulation region PM 11 _ 1 . The negative-phase output signal of the differential output signals output from the lower-bit drive unit  121  is output to the phase modulation region PM 12 _ 1 . 
     Specifically, the lower-bit drive unit  121  outputs four values (0, 1, 2, and 3) according to the lower bits D[1:0]. The lower-bit drive unit  121  increases the value of the positive-phase output signal in the order of “0”→“1”→“2”→“3” in accordance with an increase in the value of the lower bits D[1:0]. 
     On the other hand, the lower-bit drive unit  121  outputs an inverted signal of the positive-phase output signal as the negative-phase output signal. Specifically, the lower-bit drive unit  121  decreases the value of the negative-phase output signal in the order of “3”→“2”→“1”→“0” in accordance with an increase in the value of the lower bits D[1:0]. It can also be understood that the value of the negative-phase output signal is determined so that the sum of the values of the positive-phase output signal and the negative-phase output signal becomes equal to the maximum value “3” of the four output values. 
     The upper-bit drive unit  122  is supplied with the upper bits D[3:2]. The upper-bit drive unit  122  outputs three pairs of differential output signals according to the value of the upper bits D[3:2]. At this time, the positive-phase output signals of the differential output signals output from the upper-bit drive unit  122  are respectively output to the phase modulation regions PM 11 _ 2  to PM 11 _ 4 . The negative-phase output signals of the differential output signals output from the upper-bit drive unit  122  are respectively output to the phase modulation regions PM 12 _ 2  to PM 12 _ 4 . In the upper-bit drive unit  122 , the positive-phase output signal and the negative-phase output signal take only the value of “0” or “4”. That is, when the positive-phase output signal indicates “0”, the negative-phase output signal indicates “4”, and when the positive-phase output signal indicates “4”, the negative-phase output signal indicates “0”. 
     Specifically, when a most significant bit D[3] and a most significant bit D[2] of the upper bits D[3:2] are “0”, the upper-bit drive unit  122  outputs “0” as the positive-phase output signals to the phase modulation regions PM 11 _ 2  to PM 11 _ 4  and outputs “4” as the negative-phase output signals to the phase modulation regions PM 12 _ 2  to PM 12 _ 4 . 
     When the most significant bit D[ 3 ] of the upper bits D[3:2] is “0” and the most significant bit D[2] thereof is “1”, the upper-bit drive unit  122  outputs “4” as the positive-phase output signal to the phase modulation region PM 11 _ 2  and outputs “0” as the positive-phase output signals to the phase modulation regions PM 11 _ 3  and PM 11 _ 4 . Further, the upper-bit drive unit  122  outputs “0” as the negative-phase output signal to the phase modulation region PM 12 _ 2  and outputs “4” as the negative-phase output signals to the phase modulation regions PM 12 _ 3  and PM 12 _ 4 . 
     When the most significant bit D[ 3 ] of the upper bits D[3:2] is “1” and the most significant bit D[2] thereof is “0”, the upper-bit drive unit  122  outputs “4” as the positive-phase output signals to phase modulation regions PM 11 _ 2  and PM 11 _ 3  and outputs “0” as the positive-phase output signal to the phase modulation region PM 11 _ 4 . The upper-bit drive unit  122  outputs “0” as the negative-phase output signals to the phase modulation regions PM 12 _ 2  and PM 12 _ 3  and outputs “4” as the negative-phase output signal to the phase modulation region PM 12 _ 4 . 
     When the most significant bit D[3] and the most significant bit D[2] of the upper bits D[3:2] are “1”, the upper-bit drive unit  122  outputs “4” as the positive-phase output signals to the phase modulation regions PM 11 _ 2  to PM 11 _ 4  and outputs “0” as the negative-phase output signals to the phase modulation regions PM 12 _ 2  to PM 12 _ 4 . 
     That is, the upper-bit drive unit  122  performs a rough control according to the upper bits, whereas the lower-bit drive unit  121  performs a fine control according to the values of the lower bits. 
     In the optical signals propagating through the same waveguide, the phase modulations induced by the divided phase modulation regions are added. Accordingly, the optical transmitter  100  is driven for the lower bits and the upper bits separately, thereby achieving an optical transmitter capable of large-scale multilevel modulation with a small number of divisions. 
     In this configuration, a binary driver can be used for multilevel modulation, instead of a multilevel DAC. Therefore, the circuit size of the drive circuit can be reduced as compared with the drive circuit configured using only the multilevel DAC. This results in downsizing of the optical transmitter itself. 
     When the number of levels of the multilevel modulation is large, it is difficult to add signals such as, especially, electric signals, at a high speed. However, in this configuration, an electric signal is converted into an optical phase and variations of the phase are added, thereby making it possible to perform a high-speed addition operation. Consequently, it is possible to provide an optical transmitter which can be suitably used for high-speed optical communication. 
     Second Exemplary Embodiment 
     Next, an optical transmitter  200  according to a second exemplary embodiment of the present invention will be described. The optical transmitter  200  is a specific example of the optical transmitter  100  according to the first exemplary embodiment.  FIG. 8  is a block diagram schematically showing the configuration of the optical transmitter  200  according to the second exemplary embodiment. 
     The lower-bit drive unit  121  includes a four-value D/A converter DAC 1  which is supplied with the lower bits D[1:0]. As shown in  FIG. 7 , the D/A converter DAC 1  outputs a positive-phase output signal to the phase modulation region PM 11 _ 1  and outputs a negative-phase output signal to the phase modulation region PM 12 _ 1  according to the lower bits D[1:0]. 
     The upper-bit drive unit  122  includes a decoding unit  21  and binary drivers DRV 1  to DRV 3 . The decoding unit  21  converts the upper bits D[3:2] from a binary code to a thermometer code. The decoding unit  21  sequentially drives the drivers DRV 1  to DRV 3  in accordance with an increase in the thermometer code. The drivers DRV 1  to DRV 3  output differential output signals according to the value of the upper bits D[3:2]. At this time, the positive-phase output signals output from the drivers DRV 1  to DRV 3  are respectively output to the phase modulation regions PM 11 _ 2  to PM 11 _ 4 , and the negative-phase output signals are respectively output to the phase modulation regions PM 12 _ 2  to PM 12 _ 4 . The positive-phase output signal and negative-phase output signal output from each of the drivers DRV 1  to DRV 3  take only the value of “0” or “4”, as in the first exemplary embodiment. That is, when the positive-phase output signal indicates “0”, the negative-phase output signal indicates “4”, and when the positive-phase output signal indicates “4”, the negative-phase output signal indicates “0”. 
     Specifically, when the most significant bit D[3] and the most significant bit D[2] of the upper bits D[3:2] are “0”, the drivers DRV 1  to DRV 3  output “0” as the positive-phase output signals to the phase modulation regions PM 11 _ 2  to PM 11 _ 4 , respectively. The drivers DRV 1  to DRV 3  output “4” as the negative-phase output signals to the phase modulation regions PM 12 _ 2  to PM 12 _ 4 , respectively. 
     When the most significant bit D[3] of the upper bits D[3:2] is “0” and the most significant bit D[2] thereof is “1”, the driver DRV 1  outputs “4” as the positive-phase output signal to the phase modulation region PM 11 _ 2 , and outputs “0” as the negative-phase output signal to the phase modulation region PM 12 _ 2 . The drivers DRV 2  and DRV 3  output “0” as the positive-phase output signals to the phase modulation regions PM 11 _ 3  and PM 11 _ 4 , respectively, and output “4” as the negative-phase output signals to the phase modulation regions PM 12 _ 3  and PM 12 _ 4 , respectively. 
     When the most significant bit D[3] of the upper bits D[3:2] is “1” and the most significant bit D[2] thereof is “0”, the drivers DRV 1  and DRV 2  output “4” as the positive-phase output signals to the phase modulation regions PM 11 _ 2  and PM 11 _ 3 , respectively, and output “0” as the negative-phase output signals to the phase modulation regions PM 12 _ 2  and PM 12 _ 3 , respectively. The driver DRV 3  outputs “0” as the positive-phase output signal to the phase modulation region PM 11 _ 4  and outputs “4” as the negative-phase output signal to the phase modulation region PM 12 _ 4 . 
     When the most significant bit D[3] and the most significant bit D[2] of the upper bits D[3:2] are “1”, the drivers DRV 1  to DRV 3  output “4” as the positive-phase output signals to the phase modulation regions PM 11 _ 2  to PM 11 _ 4 , respectively. The drivers DRV 1  to DRV 3  output “0” as the negative-phase output signals to the phase modulation regions PM 12 _ 2  to PM 12 _ 4 , respectively. 
     Therefore, according to this configuration, the optical transmitter capable of performing an operation similar to that of the optical transmitter  100  according to the first exemplary embodiment can be specifically achieved. 
     While the 4-bit optical transmitter has been described in this exemplary embodiment, this configuration can be understood by generalizing the configuration as follows. Assuming that the upper bits are m (m is an integer equal to or greater than 1) bits and the lower bits are n (n is an integer equal to or greater than 2) bits, the input digital signal is represented by (m+n) bits. Accordingly, the D/A converter DAC 1  of the lower-bit drive unit outputs 2 n -level signals (“0” to “2 n −1”) which are obtained by performing D/A conversion on an n-bit signal. 
     The upper-bit drive unit includes (2 m −1) drivers. The (2 n −1) drivers output, to different phase modulation regions, values greater than a maximum value of a driving signal, which is output from the lower-bit drive unit, according to the value of an m-bit signal. Specifically, the (2 m −1) drivers output “0” when the value of the m bits is 0. Among the (2 m −1) drivers, the number of drivers that output “2 n ”, which is greater by 1 than the maximum value “2 n −1” of the driving signal output from the lower-bit drive unit, is increased by 1 as the value of the m bits is increased by 1. 
     Third Exemplary Embodiment 
     Next, an optical transmitter  300  according to a third exemplary embodiment of the present invention will be described. The optical transmitter  300  is a modified example of the optical transmitter  100  according to the first exemplary embodiment and the optical transmitter  200  according to the second exemplary embodiment.  FIG. 9  is a block diagram schematically showing the configuration of the optical transmitter  300  according to the third exemplary embodiment. The optical transmitter  300  includes an optical modulator  31  and a drive circuit  32 . The optical modulator  31  and the drive circuit  32  respectively correspond to the optical modulator  11  and the drive circuit  12  of the optical transmitters  100  and  200 . The optical transmitter  300  is configured as a 5-bit optical transmitter. 
     The optical modulator  31  includes the optical waveguides  111  and  112 , the optical multiplexers/demultiplexers  113  and  114 , and phase modulation regions PM 31 _ 1  to PM 31 _ 3  and PM 32 _ 1  to  32 _ 3 . The phase modulation regions PM 31 _ 1  to PM 31 _ 3  are arranged on the optical waveguide  11  between the optical multiplexer/demultiplexer  113  and the optical multiplexer/demultiplexer  114 . The phase modulation regions PM 32 _ 1  to PM 32 _ 3  are arranged on the optical waveguide  112  between the optical multiplexer/demultiplexer  113  and the optical multiplexer/demultiplexer  114 . The other components of the optical modulator  31  are similar to those of the optical modulator  11 , and so the description thereof is omitted. 
     The drive circuit  32  includes the lower-bit drive unit  121 , an upper-bit drive unit  322 , and a bit splitting unit  323 . The bit splitting unit  323  splits 5-bit input digital signals D[4:0], which are supplied to the drive circuit  32 , into upper bits and lower bits. In this case, the bit splitting unit  323  splits the input digital signals D[4:0] into three upper bits D[4:2] and two lower bits D[1:0]. 
     The lower-bit drive unit  121  is similar to that of the second exemplary embodiment, and so the description thereof is omitted. 
     The upper-bit drive unit  322  is supplied with the upper bits D[4:2]. The upper-bit drive unit  322  includes a bit splitting unit  324 , a four-value D/A converter DAC 2 , and a binary driver DRV 4 . The bit splitting unit  324  splits the upper bits D[4:2] into a most significant bit D[4] and lower bits D[3:2]. The lower bits D[3:2] are supplied to the D/A converter DAC 2 , and the most significant bit D[4] is supplied to the driver DRV 4 . 
       FIG. 10  is an operation table showing operations of the optical transmitter  300 . The lower-bit drive unit  121  repeatedly outputs values in the order of “0”→“1”→“2”→“3”→“0” . . . in accordance with an increase in the value of the input digital signal, as in the first and second exemplary embodiments. 
     The D/A converter DAC 2  of the upper-bit drive unit  322  outputs a positive-phase output signal to the phase modulation region PM 31 _ 2  and outputs a negative-phase output signal to the phase modulation region PM 32 _ 2 , according to the value of the lower bits D[3:2]. The positive-phase output signal and negative-phase output signal output from the D/A converter DAC 2  take one of the values “0”, “4”, “8”, and “12”. Specifically, when the positive-phase output signal indicates “0”, the negative-phase output signal takes “12”; when the positive-phase output signal indicates “4”, the negative-phase output signal takes “8”; when the positive-phase output signal indicates “8”, the negative-phase output signal takes “4”; and when the positive-phase output signal indicates “12”, the negative-phase output signal takes “0”. It can also be understood that the value of the negative-phase output signal is determined so that the sum of the positive-phase output signal and the negative-phase output signal becomes equal to the maximum value “12”. 
     When the most significant bit D[4] is “0”, the driver DRV 4  of the upper-bit drive unit  322  outputs “0” as the positive-phase output signal to the phase modulation region PM 31 _ 3 , and outputs “16” as the negative-phase output signal to the phase modulation region PM 32 _ 3 . On the other hand, when the most significant bit D[4] is “1”, the driver DRV 4  outputs “16” as the positive-phase output signal to the phase modulation region PM 31 _ 3 , and outputs “0” as the negative-phase output signal to the phase modulation region PM 32 _ 3 . 
     That is, as in the first and second embodiments, the lower-bit drive unit  121  (D/A converter DAC 1 ) repeatedly outputs values in the order of “0”→“1”→“2”→“3”→“0” . . . in accordance with an increase in the value of the input digital signal. Thus, the lower-bit drive unit  121  performs a fine control according to the values of the lower bits. 
     In the upper-bit drive unit  322 , the driver DRV 4  performs a first rough control according to the most significant bit. Further, the D/A converter DAC 2  performs a second rough control which is finer than the first rough control. 
     In other words, in the optical transmitter  300 , the upper-bit drive unit is capable of performing a rough control according to the upper bits and the lower-bit drive unit is capable of performing a fine control according to the values of lower bits, as in the optical transmitters  100  and  200 . 
     In the optical transmitter  300 , the number of phase modulation regions, i.e., segmented electrodes, can be reduced as compared with the optical transmitters  100  and  200 . This is advantageous in downsizing the optical transmitter. Moreover, the optical transmitter can perform a modulation at more multiple levels than that of the optical transmitters  100  and  200 , even though the number of phase modulation regions (segmented electrodes) is reduced. Consequently, a compact optical transmitter capable of performing a modulation at more multiple levels can be achieved. 
     While the 4-bit optical transmitter has been described in this exemplary embodiment, this configuration can be understood by generalizing the configuration as follows. Assuming that the upper bits are m (m is an integer equal to or greater than 1) bits and the lower bits are n (n is an integer equal to or greater than 2) bits, the input digital signal is represented by (m+n) bits. Accordingly, the D/A converter DAC 1  of the lower-bit drive unit outputs 2 n -level signals (“0” to “2 n −1”) which are obtained by performing D/A conversion on an n-bit signal. 
     The upper-bit drive unit includes one driver. The one driver outputs a value greater than a maximum value of a driving signal, which is output from the lower-bit drive unit, according to the value of the m-bit signal. Specifically, when the most significant bit of the upper bits (m-bit signal) is “0”, the driver outputs “0”, and when the most significant bit is “1”, the driver outputs “2 (n+m−1) ”. 
     The upper-bit drive unit includes one D/A converter. The one D/A converter outputs a value obtained by multiplying a value, which is obtained by performing D/A conversion on a value indicated by a bit of the upper bits (m-bit signal) other than the most significant bit, by “2 n ”. 
     Fourth Exemplary Embodiment 
     Next, an optical transmission/reception system  400  according to a fourth exemplary embodiment of the present invention will be described. The optical transmission/reception system  400  is an optical transmission/reception system using one of the above-described optical transmitters  100 ,  200 , and  300 . An example in which the optical transmission/reception system  400  includes the optical transmitter  100  will now be described.  FIG. 11  is a block diagram schematically showing the configuration of the optical transmission/reception system according to the fourth exemplary embodiment. 
     The optical transmission/reception system  400  includes the optical transmitter  100 , an optical receiver  401 , an optical transmission line  402 , and optical amplifiers  403 . 
     The optical transmitter  100  outputs, as an optical signal, a QPSK optical signal which is obtained by performing, for example, quadrature phase shift keying (hereinafter referred to as “QPSK”). 
     The optical transmitter  100  and the optical receiver  401  are optically connected via the optical transmission line  402 , and the QPSK optical signal propagates therethrough. The optical amplifiers are disposed on the optical transmission line  402 , and amplify the QPSK optical signal propagating through the optical transmission line  402 . The optical receiver  401  demodulates the QPSK optical signal into an electric signal. 
     The configuration described above allows the optical transmission/reception system  400  to transmit the optical signal by using the optical transmitter  100 . The optical transmitter  100  can be replaced by the optical transmitter  200  or  300 , as a matter of course. 
     Other Exemplary Embodiment 
     The present invention is not limited to the above exemplary embodiments, and can be modified as appropriate without departing from the scope of the invention. For example, since the optical phase variations can be added regardless of the order of variations, the locations of the lower-bit drive unit and the upper-bit drive unit can be replaced. Also, the order of locations of the D/A converters and drivers within the upper-bit drive unit can be arbitrarily changed. 
     In the above exemplary embodiments, the optical transmitters  100  and  200  are described as 4-bit optical transmitters and the optical transmitter  300  is described as a 5-bit optical transmitter, but these are illustrated by way of example only. That is, an optical transmitter capable of higher-order multilevel modulation can be configured by increasing the number of phase modulation regions (segmented electrodes), the number of D/A converters, and the number of levels. 
     The above exemplary embodiments illustrate an example in which differential output signals are supplied to the phase modulation regions, but this is illustrated by way of example only. For example, the value to be input to one of a pair of phase modulation regions may be fixed, and only the value to be input to the other phase modulation region may be changed. 
     While the present invention has been described with reference to exemplary embodiments, the present invention is not limited to the above-described exemplary embodiments. The configuration and details of the present invention can be modified in various manners which can be understood by those skilled in the art within the scope of the invention. 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2012-064767, filed on Mar. 22, 2012, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
           11 ,  31  OPTICAL MODULATORS 
           12 ,  32  DRIVE CIRCUITS 
           21  DECODING UNIT 
           100 ,  200 ,  300 ,  500  OPTICAL TRANSMITTERS 
           111 ,  112 ,  511 ,  512  OPTICAL WAVEGUIDES 
           113 ,  114 ,  513 ,  514  OPTICAL MULTIPLEXERS/DEMULTIPLEXERS 
           121  LOWER-BIT DRIVE UNIT 
           122 ,  322  UPPER-BIT DRIVE UNITS 
           123 ,  323 ,  324  BIT SPLITTING UNITS 
           400  OPTICAL TRANSMISSION/RECEPTION SYSTEM 
           401  OPTICAL RECEIVER 
           402  TRANSMISSION LINE 
           403  OPTICAL AMPLIFIER 
         DAC 1 , DAC 2 , DAC 51 -DAC 54  D/A CONVERTERS 
         DRV 1 -DRV 4  DRIVERS 
         L 1 , L 2  LIGHT 
         P 1 , P 2 , P 6 , P 7  INPUT PORTS 
         P 3 , P 4 , P 7 , P 8  OUTPUT PORTS 
         PM 11 _ 1 -PM 11 _ 4 , PM 12 _ 1 -PM 12 _ 4 , PM 31 _ 1 -PM 31 _ 3 , PM 32 _ 1 -PM 32 _ 3 , PM 51 _ 1 - 51 _ 4 , PM 52 _ 1 - 52 _ 4  PHASE MODULATION REGIONS